| # | Statement (Answer in bold) |
|---|---|
| 1 | Neuron is the longest cell in our body. |
| 2 | Impulses travel rapidly in myelinated neurons. |
| 3 | A change in the environment that causes an animal to react is called stimulus. |
| 4 | Dendrite carries the impulse towards the cell body. |
| 5 | The two antagonistic component of autonomic nervous system are sympathetic nerves and para sympathetic nerves. |
| 6 | A neuron contains all cell organelles except centrioles |
| 7 | cerebro spinal fluid maintains the constant pressure inside the cranium. |
| 8 | sulci and gyri increases the surface area of cerebrum. |
| 9 | The part of human brain which acts as relay center is thalamus. |
| # | Statement | Answer | Correction (if False) |
|---|---|---|---|
| 1 | Dendrons are the longest fibres that conducts impulses away from the cell body. | False | Dendrone are the longest fibres and they conduct impulses towards cell body. |
| 2 | Sympathetic nervous system is a part of central nervous system. | False | Sympathetic nervous system is a part of autonomic nervous system. |
| 3 | Hypothalamus is the thermoregulatory centre of human body. | True | — |
| 4 | Cerebrum controls the voluntary actions of our body. | True | — |
| 5 | In the central nervous system myelinated fibres form the white matter. | True | — |
| 6 | All the nerves in the body are covered and protected by meninges. | False | The brain and spinal cord are covered and protected by meninges. |
| 7 | Cerebrospinal fluid provides nutrition to brain. | True | — |
| 8 | Reflex arc allows the rapid response of the body to a stimulus. | True | — |
| 9 | Pons helps in regulating respiration. | True | — |
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The brain, the central control organ of the nervous system, is exceptionally well-protected by a multi-layered system. The outermost and most robust protective structure is the skull, also known as the cranium, a bony case that encases and shields the brain from physical trauma. Beneath the skull lie the meninges, a set of three protective membranes: the dura mater (outermost), the arachnoid mater (middle), and the pia mater (innermost). These membranes provide an additional layer of cushioning and support. Suspended within the space between the arachnoid mater and the pia mater is the cerebrospinal fluid (CSF). This fluid acts as a shock absorber, cushioning the brain against impacts, and also helps to nourish the brain and remove waste products.
A reflex arc is the neural pathway that mediates a reflex action. It is the fundamental unit of the nervous system's involuntary response system. When a sensory receptor detects a stimulus, it generates a nerve impulse that travels along a sensory neuron to the spinal cord or brainstem. In the central nervous system, the impulse is processed, often by an interneuron, and then transmitted to a motor neuron. The motor neuron carries the impulse to an effector, such as a muscle or gland, which then produces the appropriate response. This entire pathway, from stimulus detection to response execution, constitutes the reflex arc.
Actions performed by organisms can be broadly categorized into voluntary and involuntary types based on conscious control. Voluntary actions are those that are carried out with conscious thought and deliberate intention, allowing for control over their execution. Examples include activities like walking, writing, speaking, or playing a musical instrument, all of which are primarily governed by the cerebrum. Involuntary actions, on the other hand, occur automatically and are not under conscious control. These are managed by the autonomic nervous system or through reflex arcs and include essential life-sustaining processes such as the beating of the heart, digestion of food, and involuntary reflexes like blinking.
Voluntary actions are defined as those movements and behaviors that are performed with conscious awareness, intention, and control. These actions originate from the cerebrum, the largest part of the brain, which is responsible for higher-level cognitive functions, including planning and executing deliberate movements. Examples of voluntary actions include everyday activities such as writing, walking, picking up an object, or speaking. The ability to perform these actions relies on the brain's capacity to process sensory information, make decisions, and send precise signals through motor pathways to the muscles involved.
Nerve fibres can be classified into two main types based on the presence or absence of a myelin sheath. Medullated (or myelinated) nerve fibres are those where the axon is covered by a segmented insulating layer called the myelin sheath. In contrast, non-medullated (or unmyelinated) nerve fibres are those where the axon lacks this myelin sheath covering.
(i) Cyton:
(a) Cyton is also called cell body or perikaryon. It has a central nucleus with abundant cytoplasm called neuroplasm.
(b) The cytoplasm has large granular body called Nissl’s granules and the other cell organelles like mitochondria, ribosomes, lysosomes, and endoplasmic recticulum.
(c) Neurons do not have the ability to divide.
(ii) Dendrites :
(a) These are the numerous branched cytoplasmic processes that project from the surface of the cell body. They conduct nerve impulses towards the cyton.
(b) The branched projections increase the surface area for receiving the signals from other nerve cells.
(iii) Axon :
(a) The axon is a single, elongated, slender projection. The end of axon terminates as fine branches which terminate into knob like swellings called synaptic knob.
(b) The plasma membrane of axon is called axolemma, while the cytoplasm is called axoplasm. It carries impulses away from the cyton.
(c) The axons may be covered by a protective sheath called myelin sheath which is further covered by a layer of Schwann cells called neurilemma.
(d) Myelin sheath breaks at intervals by depressions called Nodes of Ranvier. The region between the nodes is called as internode. Myelin sheath acts as insulator and ensures rapid transmission of nerve impulses.
(iv) Synapse:
(a) A junction between synaptic knob of axon of one neuron and dendron of next neuron is called synaptic junction.
(b) Information from one neuron can pass to another neuron through these junctions with the release of chemicals known as neurotransmitters from the synaptic knob.
(a) forebrain (b) midbrain and (c) hindbrain.
Forebrain : The forebrain is formed of cerebrum and diencephalon. The latter consists of dorsal thalamus and ventral hypothalamus.
Cerebrum :
(i) It is the largest portion forming nearly two-third of the brain. The cerebrum is longitudinally divided into two halves called cerebral hemispheres by a deep cleft which are interconnected by thick band of nerve fibres called corpus callosum.
(ii) The outer portion of each cerebral hemisphere is formed of grey matter and is called cerebral cortex. The inner or deeper part is formed of white matter and is called cerebral medulla.
(iii) The cortex is extremely folded forming elevations called gyri with depressions between them termed as sulci that increase its surface area.
(iv) The cerebrum is also responsible for the thinking, intelligence, consciousness, memory, imagination, reasoning and willpower.
Thalamus : Thalamus present in cerebral medulla is a major conducting centre for sensory and motor signalling. It acts as a relay centre.
Hypothalamus :
(i) It lies at the base of the thalamus.
(ii) It controls involuntary functions like hunger, thirst, sleep, sweating, sexual desire, anger, fear, water balance, blood pressure etc.
(iii) It acts as a thermoregulatory (temperature control) center of the body.
(iv) It controls the secretion of hormones from anterior pituitary gland and is an important link between nervous system and endocrine system.
Midbrain :
(i) It is located between thalamus and hind brain.
(ii) It consists of four rounded bodies called corpora quadrigemina that control visual and auditory (hearing) reflexes.
Hindbrain :
It is formed of three parts cerebellum, pons and medulla oblongata.
Cerebellum : It is second largest part of the brain formed of two large sized hemispheres and middle vermis. It coordinates voluntary movements and also maintains body balance.
Pons : It is a bridge of nerve fibre that connects the lobes of cerebellum. It relay signals between the cerebellum, spinal cord, midbrain and cerebrum. It controls respiration and sleep cycle.
Medulla Oblongata :
(i) Medulla oblongata is the posterior most part of the brain that connects spinal cord and various parts of brain.
(ii) It has cardiac centres, respiratory centres, vasomotor centres to control heart beat, respiration and contractions of blood vessels respectively. It also regulates vomiting and salivation.
Reflex action : A reflex is any response that occurs automatically without consciousness. Most of the reflex actions are monitored and controlled by the spinal cord. Hence they are also known as spinal reflexes.
Reflex arc: The pathway taken by nerve impulse to accomplish reflex action is called reflex arc.
(i) When we are pricked by a needle, the stimulus is the prick which is sensed by receptors in our hand. This stimulus (pain) inturn triggers an impulse in sensory neuron.
(ii) The sensory neuron transmits or conveys the message to the spinal cord.
(iii) Spinal cord interprets the stimulus and the impulse is passed on to the relay neuron which inturn transmits it to a motor neuron.
(iv) Motor neurons carry command from spinal cord to our arm.
(v) Muscle in our arm contracts and we withdraw our hand immediately from the needle.
In this example, muscle is an effector organ which has responded to the pain.
(i) Spinal cord is a cylindrical structure lying in the neural canal of the vertebral column. It is also covered by meninges. It extends from the lower end of medulla oblongata to the first lumbar vertebra.
(ii) The posterior most region of spinal cord tapers into a thin fibrous thread like structure called filum terminale.
(iii) Internally, the spinal cord contains a cerebrospinal fluid filled cavity known as the central canal.
(iv) The grey matter of spinal cord is ‘H’ shaped. The upper end of letter 'H' forms posterior horns and lower end forms anterior horns.
(v) A bundle of fibres pass into the posterior horn forming dorsal or afferent root. Fibres pass outward from the anterior horn forming ventral or efferent root. These two roots joins to form spinal nerves.
(vi) The white matter is external and have bundle of nerve tracts.
(vii) Spinal cord conducts sensory and motor impulses to and from the brain. It controls reflex actions of the body.
(ii) Information from the receptors is transmitted as electrical impulse and is received by the dendritic tips of the neuron.
(iii) This impulse travels from the dendrite to the cell body and then along the axon to its terminal end.
(iv) On reaching the axonal end, it causes the nerve endings to release a chemical (neurotransmitter) which diffuses across a synapse and starts a similar electrical impulse in the dendrites of the next neuron, then to their cell body to be carried along the axon.
(v) In this way, the electrical signal reaches the brain or spinal cord.
(vi) The response from brain (or spinal cord) is similarly passed on to the effector organs such as the muscle or gland cell, that undergoes the desired response.
(vii) The flow of nerve impulses from axonal end of one neuron to dendrite of another neuron through a synapse is called synaptic transmission.
(viii) Neurotransmitters : Neurotransmitters are the chemicals which allow the transmission of nerve impulse from the axon terminal of one neuron to the dendron of another neuron or to an effector organ. The important neurotransmitter released by neurons is called Acetylcholine.
(i) The cells referred to as L, which are the longest cells in the body and are fundamental to the nervous system, are neurons. (ii) M represents the axon, which is the long branch extending from the neuron's cell body, responsible for transmitting nerve impulses away from the cell. N represents the dendrites, which are the shorter, branching extensions that receive nerve impulses from other neurons and transmit them towards the cell body. (iii) The gap O between two neurons, across which nerve impulses are transmitted, is called a synapse. (iv) The chemical substance P released at the synapse to transfer the nerve impulse is a neurotransmitter, specifically acetylcholine in many cases.
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