Samacheer Class 11 Bio Zoology - Neural Control and Coordination

Slightly below the posterior pole of the eye, the optic nerve and the retinal blood vessels enter the eye. This region is devoid of both rods and cones, the photoreceptor cells responsible for vision. Since there are no photoreceptors present in this area, light falling on this region cannot be detected or processed, resulting in a gap in the visual field. Hence, this region is called the blind spot. Despite the presence of this blind spot in each eye, we do not normally perceive a blind area in our vision because the blind spot of one eye is covered by the visual field of the other eye, and the brain also compensates for this gap through neural processing and interpretation of the surrounding visual information.

• The increase in intraocular pressure leads to the disease called Glaucoma.
• Any block in the canal of Schlemm increases the intraocular pressure of aqueous humor and leads to ‘Glaucoma’ where the optic nerve and the retina are compressed due to pressure.

The principle involved is called the All-or-None Law or All-or-Nothing Principle. According to this principle, a nerve fiber responds to a stimulus in an all-or-none manner, meaning that if the stimulus reaches or exceeds the threshold value, the nerve fiber will generate a complete action potential and conduct an impulse. However, if the stimulus is below the threshold value, no action potential is generated and no impulse is conducted. There is no partial or graded response; the response is either complete or absent. This principle applies to individual nerve fibers and muscle cells, ensuring that once a threshold stimulus is reached, a maximal response is triggered, whereas sub-threshold stimuli fail to initiate any response whatsoever.

When Ravi smelled the pleasant aroma of food, several parts of the brain were involved in identifying the food and generating emotional responses. The olfactory nerve carries the sense of smell from the nasal epithelium to the olfactory bulb in the brain. From there, the olfactory information is transmitted to the sensory areas present in the parietal lobe of the cerebrum, where the smell is consciously identified and recognized. The stimuli of smell also reaches the mammillary bodies present in the hypothalamus, which are involved in olfactory reflexes and emotional responses to odor. The hypothalamus acts as a center for appetite, thirst, and heat regulation, and it processes the olfactory information to generate emotional and physiological responses such as hunger and the urge to eat. Additionally, the limbic system, which includes structures like the amygdala and hippocampus, plays a role in associating the pleasant smell with emotional responses and memory formation.

The choroid plexus, located within the ventricles of the brain, is responsible for the secretion of cerebrospinal fluid (CSF). This fluid plays several vital roles in the protection and functioning of the central nervous system. Firstly, it provides buoyancy to the brain and spinal cord, effectively reducing their weight and preventing them from being crushed under their own mass. Secondly, CSF acts as a shock absorber, cushioning the delicate neural tissues against mechanical injury from impacts or sudden movements. Thirdly, it plays a crucial role in nourishing brain cells by transporting essential nutrients and oxygen from the blood to the neural tissue. Fourthly, CSF aids in the removal of metabolic waste products and toxins from the brain, carrying them back to the bloodstream for elimination. Finally, it helps to maintain a constant intracranial pressure, which is essential for optimal brain function.

The limbic system is called the emotional brain because it plays a primary role in the regulation of pleasure, pain, anger, fear, sexual feeling, affection, and memory. The limbic system is a set of components located on both sides of the thalamus present in the inner part of the cerebral hemisphere. It includes several important structures: the olfactory bulbs, which process smell; the cingulate gyrus, which is involved in emotion processing and regulation; the mammillary body, which is associated with olfactory reflexes and emotional responses; the amygdala, which processes emotions especially fear and aggression; the hippocampus, which is crucial for memory formation and emotional memory; and the hypothalamus, which regulates emotional responses and basic drives. Together, these structures form an integrated system that generates, processes, and regulates emotional experiences and their associated physiological responses, making the limbic system the emotional center of the brain.

Receptors can be classified based on the type of stimuli they respond to. Mechanoreceptors respond to pressure and vibration stimuli and are present in the cochlea of the inner ear, the semicircular canals, and the utriculus, where they detect sound waves and changes in body position. Chemoreceptors respond to chemical stimuli and are located in the taste buds on the tongue and in the nasal epithelium, allowing us to detect taste and smell. Thermoreceptors respond to temperature stimuli and are distributed in the skin, enabling us to sense hot and cold. Photoreceptors respond to light stimuli and are located in the rod and cone cells of the retina in the eye, allowing us to see. Each type of receptor is specialized to detect specific types of stimuli and convert them into nerve impulses that are transmitted to the central nervous system for processing and interpretation.

The first five cranial nerves are as follows: The Olfactory nerve, cranial nerve I, is sensory in nature and carries the sense of smell from the nasal epithelium to the brain. The Optic nerve, cranial nerve II, is also sensory and carries the sense of sight from the retina of the eye to the visual cortex. The Oculomotor nerve, cranial nerve III, is motor in nature and controls the movement of the eye muscles, allowing for eye movement in various directions. The Trochlear nerve, cranial nerve IV, is motor in nature and is responsible for the rotation of the eyeball, particularly controlling the superior oblique muscle. The Trigeminal nerve, cranial nerve V, is mixed in nature, containing both sensory and motor fibers, and is involved in the sensation of the face, including touch, pain, and temperature, as well as the functioning of the muscles of mastication used in chewing.

Neurons are the structural and functional units of the nervous system. They are specialized cells responsible for detecting, receiving, processing, and transmitting various kinds of stimuli throughout the body. Neurons perform three main functional roles: sensory neurons detect stimuli from the environment and internal organs and transmit signals toward the central nervous system; motor neurons carry signals from the central nervous system to muscles and glands to produce responses; and autonomic neurons regulate involuntary functions of internal organs and maintain homeostasis. Each neuron consists of a cell body containing the nucleus, dendrites that receive signals, and an axon that transmits signals to other neurons or effector organs. Through their interconnected networks, neurons enable communication between different parts of the body and coordinate all nervous system activities.

Neuroglia, also known as glial cells, are specialized non-nervous cells that provide structural and functional support to neurons in the nervous tissue. They are far more numerous than neurons and play crucial roles in maintaining the health and function of the nervous system. Their functions include providing nourishment and metabolic support to neurons, which are highly energy-dependent cells. Neuroglia also play a vital role in repairing injured nervous tissue by clearing debris and promoting regeneration. Some types of neuroglia, like microglia, act as phagocytic cells, engulfing and removing foreign particles, pathogens, and cellular debris, thereby protecting the brain from infection and damage. Furthermore, certain glial cells are involved in the formation of the myelin sheath, which insulates axons and speeds up nerve impulse transmission. They also contribute to the regulation of the extracellular environment around neurons, which is essential for proper neuronal activity.

• They provide nourishment to the surrounding neurons.
• They involve in the memory process.
• They repair the injured tissue due to their dividing and regenerating capacity.
• They engulf the foreign particles at the time of any injury to the brain.

Exteroceptors are sensory receptors located at or near the surface of the body that are sensitive to external stimuli from the environment. These receptors detect changes in the external world and transmit sensory impulses to the central nervous system. Exteroceptors are responsible for the special senses including vision through photoreceptors in the eye, hearing through mechanoreceptors in the ear, touch and pressure through tactile receptors in the skin, taste through chemoreceptors on the tongue, and smell through olfactory receptors in the nasal epithelium. They provide crucial information about the external environment, enabling organisms to perceive and respond appropriately to their surroundings.

• When a very quick response is needed the spinal cord can effect motor initiation as the brain and brings about an effect.
• This rapid action by the spinal cord is called reflex action.

A preganglionic neuron is the first neuron in an autonomic reflex arc that originates from the central nervous system. Its cell body is located in the brain or spinal cord, and it extends a myelinated axon that exits the central nervous system as part of a cranial or spinal nerve. The axon of the preganglionic neuron travels to and terminates in an autonomic ganglion, where it forms synapses with the cell body of a postganglionic neuron. The preganglionic neuron is responsible for transmitting signals from the central nervous system to the autonomic ganglia, where the signal is then relayed to the postganglionic neuron for delivery to visceral effector organs.

The cochlea, a spiral-shaped cavity in the bony labyrinth of the inner ear, is divided into three distinct fluid-filled chambers. These chambers are the scala vestibuli, the scala tympani, and the scala media. The scala vestibuli begins at the oval window and extends into the apex of the cochlea. The scala tympani starts at the round window and also extends towards the apex. The scala media, also known as the cochlear duct, is a triangular chamber situated between the scala vestibuli and the scala tympani. These chambers are separated by specific membranes. The scala vestibuli is separated from the scala media by Reissner's membrane, a thin, delicate membrane. The scala media is separated from the scala tympani by the basilar membrane, which is wider at the apex and narrower at the base and contains the organ of Corti, the sensory organ for hearing.

Ampullae are swollen, enlarged regions located at one end of each semicircular canal in the inner ear. Each ampulla contains sensory structures called crista ampullaris, which are composed of sensory hair cells and supporting cells embedded in a gelatinous structure called the cupula. The ampullae are specialized to detect rotational movements and angular acceleration of the head in three-dimensional space. When the head rotates, the fluid within the semicircular canals moves, causing the cupula and hair cells within the ampulla to bend, which stimulates the hair cells and generates nerve impulses that are transmitted to the brain. This information allows the brain to perceive and respond to rotational movements of the head, contributing to balance, equilibrium, and the coordination of eye movements with head movements.

• The intensity of sound is measured in decibels. (dB)
• 0-50 dB is the threshold of hearing for normal ear.
• Prolonged exposure to sound with intensities greater than 90dB causes hearing loss.

Taste buds are specialized sensory organs responsible for the detection of taste. They are primarily located on the tongue within small projections called papillae, but can also be found on the soft palate, pharynx, and epiglottis. Each taste bud is composed of a cluster of gustatory epithelial cells, including gustatory cells (receptor cells) and supporting cells. These cells are constantly subjected to wear and tear due to the mechanical friction from chewing food and the thermal stress from hot or cold foods. Consequently, taste bud cells are among the most dynamic cells in the body, undergoing a rapid cycle of regeneration. They are typically replaced every seven to ten days, ensuring continuous sensory function and the ability to detect a wide range of tastes.

The nervous system performs three primary functions: sensory input, integration, and motor output. Sensory functions involve receiving stimuli from both the internal and external environments. Sensory receptors detect changes such as light, sound, temperature, and pain, and transmit this information as nerve impulses to the central nervous system (CNS). Integration is the process by which the CNS processes and interprets this sensory information, making decisions about appropriate responses. Motor functions involve transmitting commands from the CNS to effector organs, such as muscles and glands. This allows the body to respond to the integrated information. Autonomic functions, a subset of motor functions, specifically control involuntary actions like reflex actions, heart rate, and digestion, ensuring the body's internal balance is maintained.

• Afferent neurons – That take sensory impulses to the central nervous system from the sensory organs.
• Efferent neurons – That carry motor impulses from CNS to the effector organ.
• Inter neurons – That lie entirely within the CNS between the afferent and efferent neurons.

• Inner to the axolemma the cytoplasm contains the infra cellular fluid with large amounts of potassium and magnesium phosphate with negatively charged proteins and other organic molecules.
• Outside the axolemma contains large amounts of sodium chloride bicarbonates CO 2 and metabolic wastes
• Due to the charged particles present in the inner and outside of the axolemma are responsible for the conduction of nervous impulses.

• When the axolemma reaches the spike potential the sodium voltage – gate closes and potassium – voltage gate opens.
• It checks influx of Na + ions and initiation the efflux of K ions which lowers the number of positive ions within the cell. Thus the potential falls back towards the resting potential.
• The reversal of membrane potential inside the axolemma to negative occurs clue to efflux of K + ions. This is called repolarisation.

• If repolarisation becomes more negative than the resting potential -70 mV to about -90 mV.
• It is called hyper polarisation. During this K + ion, gate is more permeable to K + even after reaching the threshold level as it closes slowly hence called lazy gates.
• The membrane potential return to its original resting state when K + ion channel close completely.
• During hyper polarization the Na + voltage gate remains closed.

Saltatory conduction is a highly efficient mechanism for nerve impulse transmission that occurs in myelinated axons. Unlike in non-myelinated axons where the action potential propagates continuously along the entire length of the axon membrane, saltatory conduction involves the impulse 'jumping' from one gap in the myelin sheath to the next. The myelin sheath, formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system, acts as an electrical insulator. The voltage-gated sodium (Na+) and potassium (K+) channels, which are essential for generating action potentials, are concentrated at these gaps, known as the nodes of Ranvier. When an action potential is generated at one node, the resulting electrical current flows to the next node, triggering a new action potential there. This discontinuous mode of conduction significantly increases the speed of impulse transmission compared to continuous conduction in unmyelinated axons, allowing for faster communication within the nervous system.

The brain is protected by three layers of membranes, collectively known as the cranial meninges. These protective layers surround the brain and spinal cord, providing cushioning and support. The outermost and thickest layer is the dura mater, which is a tough, fibrous membrane that lines the inner surface of the cranial cavity. Beneath the dura mater is the arachnoid mater, a thin, web-like membrane. The space between the dura mater and the arachnoid mater is called the subdural space, which normally contains a small amount of serous fluid. The innermost layer, which is closely adhered to the surface of the brain and spinal cord, is the pia mater. The pia mater is a delicate and highly vascularized membrane. The arachnoid mater and the pia mater are separated by the subarachnoid space, which is filled with cerebrospinal fluid (CSF). This fluid acts as a shock absorber, further protecting the brain from injury.

• It serves as a relay centre for impulses between the spinal and brain and cerebrum.
• Information is sorted and edited.
• It plays a key role in learning and memory.
• It is a co-ordinating centre for sensory and motor signaling.

• The midbrain is located between the diencephalon and the pons.
• The lower portion of the mid-brain consists of a pair of longitudinal bands of nervous tissue called cerebral peduncles.
• This relay impulses back and forth between cerebrum cerebellum pons and medulla.
• The dorsal position of the mid Brain consist of four rounded bodies called corpora quadrigemina which acts as a reflex center for vision and hearing.

• 150ml of cerebro spinal fluid is secreted in an adult.
• 500ml of cerebro spinal fluid is secreted in a day.
• Every 8 hours this fluid is rejuvenated.
• The choroid plexus carries harmful metabolic wastes from the brain to the blood.

Receptors can be classified based on their anatomical position in the body into three main categories. Exteroceptors are sensory receptors located at or near the surface of the body. They detect external stimuli and are responsible for the senses of hearing, vision, touch, taste, and smell. These receptors allow us to perceive and interact with our external environment. Interoceptors, also called visceroceptors, are located within the visceral organs, blood vessels, and internal body cavities. They are sensitive to internal stimuli such as changes in blood pressure, oxygen levels, carbon dioxide levels, and the chemical composition of body fluids. These receptors monitor the internal state of the body and help maintain homeostasis. Proprioceptors are specialized receptors located in muscles, tendons, joints, and the inner ear. They provide information about the position, movement, and orientation of the body in space. This sensory information is crucial for maintaining balance, posture, and coordinating voluntary movements. Together, these three types of receptors provide comprehensive sensory information about both the external environment and the internal state of the body.

• The sympathetic and parasympathetic neural system are is mainly involved.
• On seeing his friend out of the happiness his autonomic nervous system is stimulated and he gets tears in his eyes.
• After sometime the parasympathetic stops the secretion of tears.

• Depression is a functional deficiency of serotonin and nor epinephrine.
• This disorder is characterized by a pervasive negative mood, loss of interest an inability to experience pleasure and suicidal tendencies.

When dust falls on our eyes and the eyelids close immediately, or when we touch a hot pan and withdraw our hand rapidly, these are examples of reflex actions. A reflex action is a rapid, automatic response to a stimulus that occurs without conscious thought or waiting for brain involvement. This happens through a reflex arc, which is a neural pathway that allows the spinal cord to directly process sensory information and initiate a motor response. When a stimulus is detected by sensory receptors, the sensory neuron transmits the signal to the spinal cord. In the spinal cord, the sensory neuron synapses directly with a motor neuron, bypassing the brain. The motor neuron then immediately sends a signal to the appropriate muscles, causing them to contract and produce the reflex response. This direct pathway through the spinal cord allows for extremely fast responses because the signal does not have to travel all the way to the brain and back. The brain receives information about the reflex action only after it has already occurred. This mechanism is vital for survival as it allows us to quickly withdraw from harmful stimuli such as heat or sharp objects before we even consciously realize the danger.

Unconditioned reflexes and conditioned reflexes are two types of reflex actions that differ in their origin and how they are acquired. An unconditioned reflex is an inborn, automatic response to a specific stimulus that is present from birth and does not require any prior learning or experience. These reflexes are genetically determined and are the same in all individuals of a species. Examples include the blinking of the eye when a dust particle is about to fall into it, the withdrawal of the hand from a hot surface, and the pupillary reflex where the pupil constricts in bright light. A conditioned reflex, on the other hand, is a learned response to a stimulus that is acquired through experience, training, and association. Conditioned reflexes do not naturally exist in animals but develop when a neutral stimulus is repeatedly paired with an unconditioned stimulus. Over time, the neutral stimulus alone can trigger the response that was originally triggered only by the unconditioned stimulus. A classic example is the salivation of dogs in response to the sound of a bell if the bell has been repeatedly paired with the presentation of food. Another example is the excitement and secretion of salivary glands when seeing food, which is learned through repeated association between the sight of food and eating. In summary, unconditioned reflexes are innate and require no learning, while conditioned reflexes are acquired through learning and experience.

Exteroceptors and interoceptors are two categories of sensory receptors that differ in their location and the type of stimuli they detect. Exteroceptors are sensory receptors located at or near the surface of the body, including the skin, eyes, ears, nose, and tongue. These receptors are stimulated by external stimuli from the environment and are responsible for the five special senses: hearing, vision, touch, taste, and smell. They allow us to perceive and respond to external environmental changes. Interoceptors, also called visceroceptors, are sensory receptors located deep within the body in the visceral organs such as the stomach, intestines, and heart, as well as in blood vessels and other internal structures. These receptors are sensitive to internal stimuli and monitor the internal environment of the body. They detect changes in blood pressure, oxygen and carbon dioxide levels, blood pH, osmotic pressure, and the chemical composition of body fluids. Interoceptors provide information that helps maintain homeostasis by detecting deviations from normal internal conditions and triggering appropriate physiological responses. While exteroceptors keep us aware of the external world, interoceptors monitor our internal physiological state, and together they provide comprehensive sensory information for both external awareness and internal regulation.

• The lens is a transparent biconvex structure made up of slender columnar epithelial cells.
• These cells are called as lens fibre.
• These cells are formed of crystaline protein.

The position of the eyeball within the orbit is maintained and controlled by a set of six extrinsic muscles. These muscles are responsible for the precise movements of the eye, allowing for visual tracking and focusing. The rectus muscles are four in number: the superior rectus muscle, which elevates the eyeball; the inferior rectus muscle, which depresses the eyeball; the lateral rectus muscle, which moves the eyeball laterally (away from the midline); and the medial rectus muscle, which moves the eyeball medially (towards the midline). In addition to the rectus muscles, there are two oblique muscles. The superior oblique muscle helps to depress, abduct, and internally rotate the eyeball, while the inferior oblique muscle assists in elevating, abducting, and externally rotating the eyeball. Together, these six extrinsic muscles coordinate their actions to ensure the stability and mobility of the eye.

• Eye lashes and the eye brows
• They help to protect the eyeballs from foreign objects, perspiration and from direct sunrays.
• Sebaceous glands or ciliary glands.
• They secrete a lubricating fluid.
• Lacrymal glands.
• Secrete tears.
• Tears contain salts mucus and lysozyme enzyme to destroy bacteria.

• It supplies nutrients and oxygen to the lens cornea and retinal cells.
• It is produced and drained at same rate.
• It maintains a constant infra ocular pressure of about 16 mmHg.

Glaucoma is a serious eye disease characterized by increased intraocular pressure that can lead to permanent vision loss if left untreated. The eye contains a clear fluid called aqueous humor that maintains the shape of the eye and provides nutrition to the lens and cornea. This fluid is continuously produced and drained through a network of tiny channels called the canal of Schlemm. In glaucoma, any blockage or obstruction in the canal of Schlemm prevents the normal drainage of aqueous humor, causing the intraocular pressure to increase abnormally. This elevated pressure puts excessive stress on the optic nerve and the retina, the light-sensitive tissue at the back of the eye. The increased pressure gradually damages the nerve fibers in the optic nerve, leading to a progressive loss of vision. If the condition is not detected and treated early, the optic nerve damage becomes irreversible and can result in permanent blindness. Glaucoma is often called the silent thief of sight because it typically progresses without noticeable symptoms in its early stages. Regular eye examinations and early detection are crucial for preventing vision loss. Treatment options include medications to reduce intraocular pressure, laser therapy, or surgical procedures to improve aqueous humor drainage.

• Redcones. It contains visual pigment erythropsin which is sensitive to long wavelength close to 560 nm.
• Green cones – It has a pigment chloropsin which is sensitive to medium wavelength of 530 nm.
• Blue cones – It has a pigment which is sensitive to short wavelength of 420 nm.

Conductive hearing loss and sensorineural hearing loss are two distinct types of hearing impairment that differ in their causes and the part of the auditory system affected. Conductive hearing loss occurs when sound waves cannot be efficiently conducted through the external ear canal, eardrum, or middle ear to reach the inner ear. This type of hearing loss can result from blockage of the ear canal with earwax, rupture or perforation of the eardrum, middle ear infections with fluid accumulation, or restriction of the movement of the tiny bones called ossicles in the middle ear. These conditions prevent sound from being properly transmitted to the inner ear, resulting in reduced hearing ability. Sensorineural hearing loss, also called neuro-sensory hearing loss, occurs when there is damage to the inner ear structures or the auditory nerve pathways. The defect may be in the organ of Corti, which contains the sensory hair cells responsible for converting sound vibrations into electrical signals, or in the auditory nerve itself. Damage can also occur in the ascending auditory pathways in the brainstem or in the auditory cortex of the brain. Sensorineural hearing loss can result from aging, loud noise exposure, infections, head trauma, or genetic factors. Unlike conductive hearing loss, sensorineural hearing loss is often permanent and more difficult to treat. Conductive hearing loss is often reversible with medical or surgical treatment, while sensorineural hearing loss typically requires hearing aids or cochlear implants for management.

• Melanocytes synthesize melanin.
• It gives colour to skin and protects it from the sun.
• Vitiligo is a condition in which the melanin pigment is lost from the areas of the skin causing white patches.
• The leukoderma appears when melanocytes fails to synthesis melanin pigment.

Taste buds
* Taste buds are subjected to huge amounts of friction because of their location and are routinely burned by hot foods.
* These cells are replaced every 7-10 days.

These are small light pressure receptors found just beneath the epidermis in the dermal papillae.
* They are numerous in hairless skin areas such as finger tips and soles of the feat.
(5 Marks)
IV. Brief Answers

Neuron is composed of three region.
* Cell body
* Dendrites
* axon.
Cell body:
The cell body is spherical. There is no centriole.
The plasma membrane covering the neuron is called neurilemma and axon is axolemma.
Dendrites:
The repeatedly branched short fibres coming out of the cell bod dendrites which transmit impulses towards the cell body in it’s cytoplasm Nissl’s granules are present.
Axon:
It is a long fibre that arises from a cone shaped area of the cell body. There is no golgi bodies and Nissis granules in its cytoplasm.
The axon of peripheral nerves is surrounded by Schwann’s cells to form myelin sheath. The myelin sheath is not continuous. There are gaps in the myelin sheath between an adjacent Schwann cells called nodes of Ranvier.
Each branch at the distal end of the axon terminates into a knob like structure called synaptic knob which possesses synaptic vesicles filled with transmitters. The axon transmits nerve impulses away from the body to neuro muscular junction. The myelinated nerve cell transmits impulses faster than non – myelinated nerve cells.

Neurons, the fundamental units of the nervous system, are classified based on the number of processes extending from the cell body. Multipolar neurons possess one axon and two or more dendrites, making them the most common type in the central nervous system, involved in motor control and complex processing. Bipolar neurons have a single axon and a single dendrite, typically found in specialized sensory pathways such as the retina of the eye, the inner ear, and the olfactory epithelium, relaying sensory information. Unipolar neurons, also known as pseudounipolar neurons, have a single short process that bifurcates into a peripheral and a central branch, acting as an axon; these are primarily involved in transmitting sensory information from the body to the central nervous system.

Ionic channels:
* Leakage channels
* Ligand – gated channels
* Voltage-gated channels.
1. Leakage channels:
*
* These channels are always remain open.
* K + leakage channels are more in number than the Na + leakage channels.
* Sarco lemma has greater permeability to k + icons than Na + icons
* These icons keep moving continuously to maintain the potential continuously to maintain the
* potential difference across the axo lemma.
2. Ligand – gated channels:
These are chemic called gated channels which open or close in response to a chemical stimuli.
* They are located between the pre synaptic membrane of the first axon and post synaptic membrane of the second.
* The neurotransmitter acetylcholine opens ligand channels that allow Na + and Ca ++ ions diffuse inward and K + icon diffuse outward.
Voltage-gated channels:
This channels open in response to a physical stimulus in the form of vibration such as touch and pressure.
* Sodium Voltage gated channels.
* Potassium Voltage gated channels.

The transmission of impulse involves two main phases; Resting membrane potential and Action membrane potential. Resting membrane Potential: The electrical potential difference across the plasma membrane of a resting neuron is called the resting potential during which the interior of the cell is negative due to greater efflux of K + outside the cell than Na + influx into the cell.
When the axon is not conducting any impulses i.e. in resting condition, the axon membrane is more permeable to K + and less permeable to Na + ions, whereas it remains impermeable to negatively charge protein ions. The axoplasm contains high concentration of K + and negatively charged proteins and low concentration of Na + ions.
In contrast, fluid outside the axon (ECF) contains a low concentration of K+ and a high concentration of Na+, and this forms a concentration gradient. This ionic gradient across the resting membrane is maintained by ATP driven Sodium-Potassium pump, which exchanges 3Na+ outwards for 2K+ into the cells.
In this state, the cell membrane is said to be polarized. In neurons, the resting membrane potential ranges from -40 mV to -90 mV, and its normal value is -70 mV. The minus sign indicates that the inside of the cell is negative with respect to the outside.
Action membrane potential:
An action potential occurs when a neuron sends information down an axon, away from the cell body. It includes the following phases, depolarization, repolarization, and hyperpolarization.
Depolarization – Reversal of polarity:
When a nerve fibre is stimulated, sodium voltage-gated opens and makes the axolemma permeable to Na+ ions; meanwhile the potassium voltage-gated closes. As a result, the rate of flow of Na + ions into the axoplasm exceeds the rate of flow of K + ions to the outside fluid [ECF]. Therefore, the axolemma becomes positively charged inside and negatively charged outside.
This reversal of electrical charge is called Depolarization. During depolarization, when enough Na + ions enter the cell, the action potential reaches a certain level, called threshold potential [-55 mV], The particular stimulus which is able to bring the membrane potential to the threshold is called threshold stimulus.
The action potential occurs in response to a threshold stimulus but does not occur at subthreshold stimuli. This is called the all or none principle. Due to the rapid influx of Na + ions, the membrane potential shoots rapidly up to + 45 mV which is called the Spike potential.
Repolarisation [Falling Phase]: When the membrane reaches the spike potential, the sodium voltage-gated closes, and the potassium voltage-gated opens. It checks influx of Na + ions and initiates the efflux of K + ions which lowers the number of positive ions within the cell.’Thus,.the potential falls back towards the resting potential. The reversal of membrane potential inside the axolemma to negative occurs due to the efflux of K+ ions. This is called Repolarisation.
Hyperpolarization:
If repolarization becomes more negative than the resting potential -70 mV to about -90 mV, it is called Hyperpolarization. During this, K + ion gates are more permeable to K+ even after reaching the threshold level as it closes slowly; hence called Lazy gates. The membrane potential returns to its original resting state when K + ion channels close completely. During hyperpolarization, the Na + voltage gate remains closed.
Conduction Speed of a nerve impulse: The conduction speed of a nerve impulse depends on the diameter of axon. The greater the axon’s diameter, the faster is the conduction.. The myelinated axon conducts the impulse faster than the non-myelinated axon.
The voltage-gated Na + and K + channels are concentrated at the nodes of Ranvier. As a result, the impulse jumps node to node, rather than traveling the entire length of the nerve fibre. This mechanism of conduction is called Saltatory Conduction. Nerve impulses travel at the speed of 1-300 m/s.

a) An action potential occurs when a neuron sends information down an axon away from the cell body.
b) When a nerve fibre is stimulated sodium voltage gate opens and makes the axo lemma permeable to Na ++ and the potassium voltage gate closes and potassium is getting out of the axo lemma. The concentration reduces
* The axo lemma becomes positively charged inside and negatively charged outside.
* This reversal of electrical charge is called depolarisation potential reaches level called threshold potential (-55mV)
* The stimulus which bring this threshold potential is called threshold stimulus.
* The action potential occur in response to a threshold stimulus but does not occur at subthreshold stimuli. This is called all or none principle.
* Due to the rapid influx of Na + ions the membrane potential shoots rapidly up to +45mV which is called the spike potential.

• The junction between two neurons is called a synapses through which a nerve impulse is transmitted,
• The first neuron involved in the synapse forms the pre synaptic neuron and the second neuron is the post – synaptic neuron
• A small gap between the two neuron is synaptic cleft.
• The axon terminals contain synaptic vesicles filled with neurotransmitters.
• When an impulse arrives at the axon terminals it depolorizes the pre – synaptic membrane opening the voltage gated calcium channels.
• Influx of calcium ions stimulates the synaptic vesicles toward the pre – synaptic membrane and fuse with it.
• In the neurilemma the vesicles release their neurotransmitters into the synaptic cleft by exocytosis.
• The released neurotransmitters bind to their specific receptors.
• The entry of the ions can generate a new potential in the post synaptic neuron.
• This excitatory post – synaptic potential causes depolarisation and in inhibitory post – synaptic potential causes hyperpolarisation.

The brain is located in the cranial cavity and it is covered by three cranial membranes.
* The outer layer — durameter the inner most layer piameter and the median thin layer arachnoid. The brain is divided into three major regions. Fore brain, Mid-brain, Hind
* It comprises of cerebrum and diencephalon.
* Tire cerebral cortex is composed of grey and unmyelinated nerve cells.
* The medulla is composed of white mater.
* The surface of the cerebrum shows many convolutions and grooves. The folds are called gyri and the shallow groove is sulci.
There are eight lobes in cerebrum.
A pair of frontals, parietals temporals and occipital lobes.
The longitudinal fissure divides the cerebrum longitudinally into two hemispheres The hemispheres are connected by a tract of nerve fibres called corpus callosum.
Cerebral cortex has three functional areas
Sensory areas:
It occurs in the parietal temporal and occipital lobes of the cortex.
Motor areas:
This controls voluntary muscular movement which lies in the posterior part of the frontal iobes.
Association area:
It lies in between cortex and diencephalan This involves in memory communication learning and reasoning.

The forebrain comprises the following regions: Cerebrum and Diencephalon. The cerebrum is the ‘seat of intelligence’ and forms the major part of the brain. The cerebrum consists of an outer cortex, inner medulla and basal nuclei.
The superficial region of the cerebrum is called the cerebral cortex, which looks grey due to the presence of unmyelinated nerve cells. Cerebral cortex – consists of the neuronal cell body, dendrites, associated glial and blood vessels.
The surface of the cerebrum shows many convolutions (folds) and grooves. The folds are called gyri, the shallow grooves between the gyri are called sulci and deep grooves are called fissures. These sulci and gyri increase the surface area of the cerebral cortex. Several sulci divide the cerebrum into eight lobes; a pair of frontals, parietals, temporals and occipital lobes.
A median longitudinal fissure divides the cerebrum longitudinally into two cerebral hemispheres. A transverse fissure separates the cerebral hemispheres from the cerebellum.
The hemispheres are connected by a tract of nerve fibres called corpus callosum. Cerebral cortex has three functional areas namely sensory areas occur in the parietal, temporal and occipital lobes of the cortex. They receive and interpret the sensory impulses.
Motor area of the cortex which controls voluntary muscular movements lies in the posterior part of the frontal lobes. The areas other than sensory and motor areas are called Association areas that deal with integrative functions such as memory, communications, learning and reasoning. Inner to the cortex is medulla which is white in colour and acts as a nerve tract between the cortex and the diencephalon.
Diencephalon consists largely of following three paired structures. The epithalamus forms the roof of the diencephalon and it is a non-nervous tissue. The anterior part of the epithalamus is vascular and folded to form the choroid plexus. Just behind the choroid plexus, the epithalamus forms a short stalk that ends in a rounded body called pineal body which secretes the hormone, melatonin which regulates the sleep and wake cycle.
Thalamus is composed of grey matter which serves as a relay centre for impulses between the spinal cord, brain stem and cerebrum. Within the thalamus, information is sorted and edited and plays a key role in learning and memory. It is a major coordinating centre for sensory and motor signaling.
Hypothalamus forms the floor of the diencephalon. The downward extension of the hypothalamus, the infundibulum connects the hypothalamus with the pituitary gland. The hypothalamus contains a pair of small rounded body called mammillary bodies that are involved in olfactory reflexes and emotional responses to odour.
Hypothalamus maintains homeostasis and has many centres which control the body temperature, urge for eating and drinking. It also contains a group of neurosecretory cells which secrete the hypothalamic hormones. Hypothalamus also acts as the satiety centre.
Limbic system: The inner part of the cerebral hemisphere constitutes the limbic system. The main components of limbic system are olfactory bulbs, cingulate gyrus, mammillary body, amygdala, hippocampus and hypothalamus.
The limbic system is called the ‘emotional brain’ because it plays a primary role in the regulation of pleasure, pain, anger, fear, sexual feeling and affection. The hippocampus and amygdala also play a role in memory. Brain stem is the part of the brain between the spinal cord and the diencephalon. It consists of mid-brain, pons varolii and medulla oblongata.

Epithalamus:
* It is a non – nervous tissue.
* The anterior part of epithalamus is vascular and folded to form the anterior choroid plexus.
* The epithalamus forms a short stalk which ends in a rounded body called pineal body.
* This secretes the hormone melotonin which regulates sleep and wake cycle.
Thalamus:
* It is formed of grey matter
* It serves as a relay centre for impulses between the spinal cord brain stem and cerebrum.
* It plays a key role in learning and memory.
* It is a major co-ordinating centre for sensory and motor signalling.
Hypothalamus:
* It forms the floor of the diencephalon.
* It has a pair of small rounded body called mammillary bodies
* It involves in olfactory reflexes and emotional responses to odour.
* It maintains homeostasis.
* It controls the body temperature.
* Urge for eating and drinking.
* It also contains a group of neuro secretory cells which secrete the hypothalamic hormones.
* It also acts as the satiety centre.

Rhombencephalon forms the hindbrain. It comprises of cerebellum, pons varolii and medulla oblongata. The cerebellum is the second largest part of the brain. It consists of two cerebellar hemispheres and a central worm-shaped part, the vermis. The cerebellum controls and coordinates muscular movements and body equilibrium. Any damage to the cerebellum often results in uncoordinated voluntary muscle movements.
Pons varoli lies in front of the cerebellum between the midbrain and the medulla oblongata. The nerve fibres in the pons varolii form a bridge between the two cerebellar hemispheres and connect the medulla oblongata with the other region of the brain. The respiratory nuclei found in the pons cooperate with the medulla to control respiration.
Medulla oblongata forms the posterior-most part of the brain. It connects the spinal cord with various parts of the brain. It receives and integrates signals from spinal cord and sends it to the cerebellum and thalamus. Medulla contains vital centres that control cardiovascular reflexes, respiration and gastric secretions.

Conditioned stimulus and unconditioned stimulus are two different types of stimuli that play distinct roles in reflex and learned behavior. An unconditioned stimulus is a stimulus that naturally and automatically triggers a specific response without any prior learning or conditioning. The response to an unconditioned stimulus is inborn and does not require any past experience or training. For example, the presence of food naturally triggers salivation in the mouth, and a bright light naturally causes the pupil of the eye to constrict. These responses occur automatically and are the same in all individuals of a species. A conditioned stimulus is a neutral stimulus that initially does not produce any particular response, but through repeated pairing with an unconditioned stimulus, it becomes associated with the unconditioned stimulus and eventually triggers the same response as the unconditioned stimulus. A conditioned stimulus is acquired through learning and experience. For example, if a bell is repeatedly rung just before food is presented, the bell becomes a conditioned stimulus. Eventually, the sound of the bell alone will trigger salivation, even without the presence of food. Another example is the secretion of salivary glands in response to seeing food, which is a conditioned response learned through repeated association between the sight of food and eating. In summary, an unconditioned stimulus is a natural trigger that requires no learning, while a conditioned stimulus is a learned trigger that has been associated with an unconditioned stimulus through repeated pairing.

Cranial nerves
Nature of nerve
Function
I Olfactory nerve
Sensory
Sense of smell
II Optic nerve
Sensory
Sense of sight
III Oculomotor nerve
Motor
Movement of the eye
IV Trochlear nerve
Motor
Rotation of the eye ball
V Trigeminal nerve
Sensory and motor (mixed)
Functioning of facial parts
VI Abducens nerve
Motor
Rotation of the eye ball
VII Facial nerve
Mixed
Functioning of facial parts
VIII Auditory/ Vestibulocochlear nerve
Sensory
Maintains the equilibrium of the body/ Auditory function
IX Glossopharyngeal nerve
Mixed
Taste and touch
X Vagus
Mixed
Regulation of the visceral organs
XI Spinal accessory
Motor
Muscular movement of pharynx, larynx, neck and shoulder
XII Hypoglossal
Motor
Speech and swallowing

The sympathetic and parasympathetic nervous systems are two divisions of the autonomic nervous system that have opposing effects on various body functions, allowing for precise regulation of physiological processes. The sympathetic nervous system is responsible for the fight-or-flight response and prepares the body for action and stress. It dilates the pupil of the eye to allow more light to enter, inhibits the secretion of saliva, increases the heart rate and force of contraction to pump more blood, dilates the bronchi to allow more air into the lungs, inhibits digestion to conserve energy, increases the release of glucose from the liver for energy, stimulates the release of epinephrine and norepinephrine from the adrenal glands, inhibits peristalsis and secretion in the digestive tract, and relaxes the bladder. The parasympathetic nervous system is responsible for the rest-and-digest response and promotes relaxation and recovery. It constricts the pupil of the eye, stimulates the secretion of saliva for digestion, reduces the heart rate, constricts the bronchi, stimulates digestion by increasing peristalsis and secretion of digestive juices, stimulates the release of bile from the gallbladder, reduces the release of epinephrine and norepinephrine, stimulates peristalsis and secretion in the digestive tract, and contracts the bladder for urination. These opposing actions allow the two systems to work together to maintain homeostasis and enable the body to respond appropriately to different situations and demands.

• There are two indentations the posterior median sulcus and the anterior median fissure.
• In the spinal cord the grey matter forms an inner butterfly-shaped region surrounded by the outer white matter.
• The grey matter consists of dendrites inter neurons and guai cells.
• White matter consists of bundles of nerve fibres.
• Each half of the grey matter is divided into a
• Dorsal horn – Cell bodies of inter neurons.
• Ventral horn – Efferent motor neurons supplying the skeletal muscle.
• Lateral horn – Nerves supply to heart smooth muscles exocrine glands. Originate from the cell bodies.
• Ascending tract – This carry sensory impulses to the brain.
• Descending tract – This carry motor impulses to brain.

Sensory receptor – It is a sensory structure that responds to specific stimulus.
Sensory neuron – This neuron takes the sensory impulse to the grey (afferent) matter of the
spinal cord through the dorsal root of the spinal cord.
Inter neuron – It may serve to transmit the impulses from the sensory neuron to the motor neuron.
Motor neuron – It transmits impulse from CNS to the effector organ
Effector neuron – It may be a muscle or gland which responds to the impulse received.
* Lacrymal glands located in the upper lateral region of each orbit secrete tears.
* Tears are secreted at the rate of 1 ml/day.
* Tears contain salt mucus and lysozyme enzyme to destroy bacteria.
* The protective mucus membrane present in the outer surface of the eye ball.
* The eye has two compartments anterior and posterior compartments filled with aqueous humour and vitreous humour respectively.
* The eye ball is consists of three layers sclera, vascular choroid and sensory retina.
Sclera:
* It consists of anterior cornea and the posterior sclera.
* Cornea is composed of stratified squamous epithelium. Sclera forms the white of the eye and protects the eyeball.
* At the junction of the sclera and the cornea is a channel called canal of schlemm which continuously drains out the excess of aqueous humour.
Choroid layer:
It is highly vascularised pigmented layer.

Sympathetic Neural System (SNS)
Parasympathetic Neural System (PNS)
SNS originates in the thoracic and lumbar region of the spinal cord.
PNS originates in the cranial region of the brain and the sacral region of the spinal cord.
Sympathetic ganglia are linked up to form a chain.
Its ganglia remain isolated
Preganglionic fibres are short and the postganglionic fibres are long.
Preganglionic fibres are long and the postganglionic fibres are short.
Noradrenaline is produced at the terminal ends of the postganglionic fibres at the effector organs. Hence the system is adrenergic.
Acetylcholine is produced at the terminal ends of the postganglionic fibres at the effector organs. Hence the system is cholinergic.
Active during stressful conditions preparing the body to face them.
Active during relaxing times restoring normal activity after a stress.
The overall effect is excitatory and stimulating.
The overall effect is inhibitory.
It is considered as the flight or fight system.
It is considered as “The rest and Digest System” or “The Feed and Breed System”.

• All nervous tissue outside the central nervous system is the peripheral neural system.
• It includes nerves ganglia enteric plexuses and sensory receptors.
• Ganglia are Sinai masses of nervous tissue.
• The neurons of these plexuses help in regulating the digestive system.
• The specialised structure that helps to respond to changes in the environment are called sensory receptor.
• This triggers nerve impulses along the afferent fibres to CNS.
• PNS comprises 12 pairs of cranial nerves and 31 pairs of the spinal nerve.
• The neural retina layer consists of cones and rods.
• The yellow flat spot at the centre of the posterior region of the retina is called macula lutea.
• A small depression present in the centre of the

The autonomic neural system is auto-functioning and self-governed. It is a part of the peripheral neural system that innervates smooth muscles, glands, and cardiac muscle. This system controls and coordinates the involuntary activities of various organs. ANS controlling centre is in the hypothalamus.
An autonomic neural system comprises the following components:
* A preganglionic neuron whose cell body is in the brain or spinal cord; its myelinated axon exits the CNS as part of cranial or spinal nerve and ends in an autonomic ganglion.
* Autonomic ganglion consists of an axon of preganglionic neurons and cell bodies of the postganglionic neurons.
* Postganglionic neuron conveys nerve impulses from autonomic ganglia to visceral effector organs.
* The autonomic neural system consists of the Sympathetic neural system and the Parasympathetic neural system.

• When light enters the eye it gets refracted by the cornea, aqueous humor and lens and it is focused on the retina and excites the rod and cone cells.
• The rods and cones contain the retinal a derivative of vitamin A and the photo pigment opsin.
• Light induces dissociation of retinal from opsin and causes the structural changes in opsin.
• This generates an action potential in the photo receptor cells.
• It is transmitted by the optic nerves to the visual cortex of the brain via the optic nerves for the perception of vision.

Myopia
* The affected person cannot see distant objects.
* As the eyeball is elongated or thickened lens the image of distant object is formed in front of the yellow spot.
* This error can be corrected using a concave lens.
Hypermetropia
* Affected person cannot see the nearby objects.
* This is due to a shortened eyeball and thin lens. So the image of closest object is converged behind the retina.
* This defect can be corrected by using convex lens.
Presbyopia
* Due to aging, the lens loses elasticity and the power of accommodation.
* Convex lenses are used to correct this defect.
Astigmatism
* It is due to the rough curvature of the cornea or lens.
* Cylindrical glasses are used to correct this error.

Anatomically the ear is divided into three regions external ear middle ear and inner ear.
External ear:
* It consists of pinna external auditory meatus and ear drum.
* The pinna collects the sound waves.
* The external auditory meatus extends up to the tympanic membrane.
* The tympanic membrane is covered with skin outside and with mucus membrane inside.
Middle ear:
* It is an air-filled cavity in the temporal bone.
* The middle ear contains three ossicles malleus incus and stapes.
* The malleus is attached to the tympanic membrane and its head articulates with the incus.
* The stapes is attached to the oval window in the inner ear.
* The Eustachian tube connects the middle ear cavity with the pharynx
Inner ear:-
* It is the fluid-filled cavity consisting of two parts the bony labyrinth and the membranous labyrinths.
* The bony labyrinth consists of three areas, cochlea, vestibule, and semicircular canals.
a Cochlea is a coiled portion consists of 3 chambers namely.
* Scala vestibule
* Scala media
* Scala tympanii.
Scala vestibule and scala tympani are filled with peri lymph.
The scala media is filled with endo lymph.

• Organ of corti is a sensory ridge located on the top of the basilar membrane.
• Basilar membrane contains numerous hair cells that are arranged in four rows.
• Protruding from the apical part of each hair cell is hair like structures known as stereocilia.
• On the organ of cortii a stiff gel membrane called tectorial membrane is situated.
• During the conduction of sound wave sterocilia makes contact with tectorial membrane.

Myopia (nearsightedness): The affected person can see the nearby objects but not the distant objects. This condition may result due to an elongated eyeball or thickened lens; so that the image of a distant object is formed in front of the yellow spot. This error can be corrected using a concave lens that diverges the entering light rays and focuses them on the retina.
Hypermetropia (long-sightedness): the affected person can see only the distant objects clearly but not the objects nearby. This condition results due to a shortened eyeball and thin lens; so the image of the closest object is converged behind the retina. This defect can be overcome by using a convex lens that converges the entering light rays on the retina.
Presbyopia:
Due to aging, the lens loses elasticity and the power of accommodation. Convex lenses are used to correct this defect. Astigmatism is due to the rough (irregular) curvature of the cornea or lens. Cylindrical glasses are used to correct this error.
Cataract:
Due to the changes in the nature of the protein, the lens becomes opaque. It can be corrected by surgical procedures.

Rods and cones are two types of photoreceptor cells in the retina of the eye that differ in their structure, function, and distribution. Rods are specialized for vision in dim light and are responsible for black-and-white vision and night vision. They contain a visual pigment called rhodopsin, which is formed by the combination of the protein opsin and the vitamin A derivative retinal or retinol aldehyde. Rhodopsin is highly sensitive to light and allows rods to detect even small amounts of light. There are approximately 120 million rods distributed across the retina, and they are particularly abundant in the peripheral regions of the retina, away from the fovea. Cones are specialized for color perception and vision in bright light. They contain a visual pigment called photopsin, which is also formed by the combination of the protein opsin and retinal. There are three types of cones, each containing a different form of photopsin that is sensitive to different wavelengths of light, allowing us to perceive red, green, and blue colors. There are approximately 6 to 7 million cone cells in the retina, and they are richly concentrated in the fovea centralis, the area of sharpest vision in the center of the retina. Because rods are more sensitive to light but cannot distinguish colors, they are useful for night vision and detecting movement in dim conditions. Cones require more light to function but provide detailed color vision and sharp visual acuity. The distribution of rods and cones across the retina allows us to have both sensitive peripheral vision and detailed central vision.

The skin senses this skin is the largest sense organ.
* All over the skin sensory receptors of pressure heat cold and pain.
* Following are the sensory receptors of the skin.
Tactile Merkel disc
* It is a light touch receptor lying in the deeper layer of the epidermis.
* Hair follicle receptors. These are light-touch receptors lying around the hair follicles.
Meissner’s corpuscles
* These are small light pressure receptors found just beneath the epidermis in the dermal papillae.
* They are numerous in hair less skin areas such as finger tips and soles of the feet.
Pacinian corpuscles
* These are the large egg-shaped receptors found scattered in the dermis.
* It detects different textures temperature hardness and pain.
Ruffini endings
This lie in the dermis responds to continuous pressure Krause end bulbs are thermo receptors that sense temperature.

• Deafness may be temporary or permanent
• Conductive deafness
• Possible causes
• The blockage of the ear canal with ear wax.
• Rupture of the ear drum.
• Middle ear infection with fluid accumulation,
• Restriction of ossicular movement.
• Sensory-neural deafness
• The defect may be in the organ of cortii or the auditory nerve or in the ascending auditory pathways or auditory cortex.

• Sound waves entering the external auditory meatus fall on the tympanic membrane and get vibrated.
• These vibrations are transmitted to the oval window through the three ear ossicles.
• Since the tympani membrane is 17-20 times larger than the oval windows. This pressure exerted on the oval window is about 20 times more than that on the tympanic membrane.
• This pressure causes the round window to alternately bulge outward and inward meanwhile the basilar membrane along with the organ of Corti moves up and down.
• These movements of hair alternately open and close the mechanically gated ion channel causes action potential.
• This is propagated to the brain as a sound sensation through the cochlear nerve.

Skin is the sensory organ of touch and is also the largest sense organ. This sensation conies from millions of microscopic sensory receptors located all over the skin and associated with the general sensations of contact, pressure, heat, cold and pain. Some parts of the body, such as the fingertips have a large number of these receptors, making them more sensitive. Some of the sensory receptors present in the skin are:
* Tactile Merkel disc is a light touch receptor lying in the deeper layer of the epidermis.
* Hair follicle receptors are light-touch receptors lying around the hair follicles.
* Meissner’s corpuscles are small light pressure receptors found just beneath the epidermis in the dermal papillae. They are numerous in hairless skin areas such as fingertips and soles of the feet.
* Pacinian corpuscles are the large egg-shaped receptors found scattered deep in the dermis and monitoring vibration due to pressure. It allows detecting different textures, temperatures, hardness, and pain.
* Ruffini endings which lie in the dermis respond to continuous pressure.
* Krause end bulbs are thermoreceptors that sense temperature.

• Taste buds are flask-shaped. There are two major types.
• Gustatory epithelial cells or taste cells.
• Basal epithelial cells or repairing cells.
• Long micro villi called gustatory hairs project from the tip of the gustatory cells and extends through a taste pore to the surface of the epithelium.
• Gustatory hairs are tire sensitive portion of the gustatory cells and they have sensory dendrites which send the signal to the brain.
• The basal cells that act as stem cells divide and differentiate into new gustatory cells.

The sense of taste is considered to be the most pleasurable of all senses.
* The tongue is provided with many small projections called papillae.
* Taste buds are located mainly on the papillae.
* Taste buds are flask-shaped.
There are two major types.
* Gustatory epithelial cells or taste cells.
* Basal epithelial cells or repairing cells.
* Long microvilli called gustatory hairs project from the tip of the gustatory cells and extend through a taste pore to the surface of the epithelium.
* Gustatory hairs are the sensitive portion of the gustatory cells and they have sensory dendrites which send the signal to the brain.
* The basal cells that act as stem cells divide and differentiate into new gustatory cells.

The smell receptors are excited by air-borne chemicals that dissolve in fluids. The yellow coloured patches of olfactory epithelium form the olfactory organs that are located on the roof of the nasal cavity.
The olfactory epithelium is covered by a thin coat of mucus layer below and olfactory glands bounded connective tissues, above. It contains three types of cells: supporting cells, Basal cells and millions of pin-shaped olfactory receptor cells (which are unusual bipolar cells).
The olfactory glands and the supporting cells secrete the mucus. The unmyelinated axons of the olfactory receptor cells are gathered to form the filaments of the olfactory nerve [cranial nerve-I] which synapses with cells of the olfactory bulb.
The impulse, through the olfactory nerves, is transmitted to the frontal lobe of the brain for identification of smell and the limbic system for the emotional responses to odour.
Part-II.
11th Bio Zoology Guide Neural Control and Coordination Additional Important Questions and Answers
I. Choose The Correct Answer