Samacheer Class 10 Science - Nervous System

‘Stimulus’ refers to the changes in the environmental condition, that are detected by receptors present in the body. Relevant changes in the activities of organisms to a particular stimuli are called their reactions or responses.

It is formed of three parts cerebellum, pons and medulla oblongata.

The brain is covered by three connective tissue membranes or meninges:
(i) Outer Duramater
(ii) Middle arachnoid
(iii) Inner piamater
It protects the brain from mechanical injury.

Playing a harmonium by striking a particular key on seeing a music note is an example of conditioned reflex.

Hypothalamus of the fore brain acts as a link between nervous and endocrine system.

The pathway taken by nerve impulse to accomplish reflex action is called reflex arc.

Voluntary actions

Refer to textbook.

involuntary actions

Refer to textbook.

Refer to textbook.

Medullated nerve fibre
1, The axon of the neuron is covered with myelin sheath.

Non-medullated nerve fibre

Refer to textbook.

Refer to textbook.

A neuron typically consists of three basic parts: Cyton, Dendrites and Axon.
(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 human brain is formed of three main parts:
(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.

If some one pricks our hand with a needle, we will immediately pull our hand to protect ourselves. This is an example of a reflex action.
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.

Spinal Cord :
(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.

(i) All the information from the environment are detected by the receptors located in our sense organs such as the eyes, the nose, the skin etc.
(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.

The neurons may be of different types based on their structure and functions.
Structurally the neurons may be of the following types:
(i) Unipolar neurons : Only one nerve process arises from the cyton which actsas both axon and dendron.
(ii) Bipolar neurons : The cyton gives rise to two nerve processes of which one acts as an axon while another as a dendron.
(iii) Multipolar neurons : The cyton gives rise to many dendrons and an axon.
Types of Nerve Fibres : Nerve fibres are of two types based on the presence or absence of myelin sheath.
(i) Myelinated nerve fibre : The axon is covered with myelin sheath
(ii) Non-myelinated nerve fibre : The axon is not covered by myelin sheath.
Myelinated and non-myelinated nerve fibres form the white matter and grey matter of the brain.

(i) A refers to the spinal cord.
(ii) (a) Bony cage 'B' refers to the vertebral column.
(b) 'C' refers to meninges.
(iii) D - 12 pairs of nerves.

'L' refers to neurons.
(ii) 'M' - Dendrites, 'N' - Axon
(iii) 'O' - Synapse
(iv) The chemical substance 'P' is acetylcholine
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