Nervous System | ICSE Class 8 Biology Notes

This note covers the human nervous system, nerve cells and messages, types of nerves, the brain and spinal cord, protection of nervous tissue, reflex actions, voluntary and involuntary actions, muscle responses and learning through association.

Why does the body need a nervous system?

The nervous system is the body's network for receiving information, processing it and directing responses. It includes the brain, spinal cord and nerves. Coordination means different organs working together so that their activities support one another.

A stimulus is a change that can be detected and produce a reaction. The reaction is called a response. Touching a hot object provides a clear example: heat is the stimulus, and withdrawing the hand is the response.

How is a change detected?

A receptor is a specialised structure that detects a stimulus. Receptors are usually located in sense organs. These receptors are specialised tips of some nerve cells. Gustatory receptors detect taste, while olfactory receptors detect smell.

A neuron, or nerve cell, is a specialised cell that receives and transmits information. Nervous tissue consists of an organised network of neurons. A tissue is a group of cells working together to perform a function.

An impulse is an electrical message travelling through a nerve cell. Messages carry information from receptors to processing centres and instructions from these centres to responding organs. An effector is a muscle or gland that carries out a response.

How does detection lead to action?

The nervous system connects the detection of a change with an appropriate response. A muscle can move a body part; a gland can release a substance. A gland is a structure that produces and releases a secretion, such as saliva.

Detection and response are different jobs. The receptor detects heat, whereas the muscles move the hand. The messages between them allow the response to be coordinated. Nervous tissue carries information; muscular tissue carries out the movement.

Definition: Nervous coordination is the control and linking of body activities through messages carried by neurons.

What are the main parts of a neuron?

A neuron has three main parts: the cell body, which contains the cell's main contents; dendrites, which are branching extensions receiving messages; and an axon, which is a long extension carrying messages away from the cell body.

The cell body contains the nucleus, the structure containing the cell's inherited instructions, and cytoplasm, the semi-fluid material inside the cell surrounding the nucleus. Dendrites project from the cell body and branch repeatedly, while the far end of the axon also branches.

A myelin sheath is the covering formed around some axons by cells called Schwann cells. An axon with this covering is called myelinated. The gaps between adjacent portions of the sheath are called nodes of Ranvier.

How does each part contribute?

A motor neuron carries instructions towards an effector. In a movement response, its message reaches a muscle. Knowing the roles of dendrites, cell body, axon and endings helps explain how the message moves through such a neuron.

The drawing shows a cell body containing a nucleus, several branching dendrites, a long axon and branching nerve endings. The labels identify the nucleus, dendrite, cell body, axon and nerve ending.

See Fig. 6.1a in your NCERT textbook

How should the labelled structure be read?

Follow the neuron from its receiving branches towards its transmitting end. The branching dendrites and branching axon endings occupy different positions and perform different roles. Dendrites lead towards the cell body; the axon leads away from it towards another cell.

The neuron is a single cell. A nerve is a bundle of nerve fibres. A nerve fibre is an axon with its associated coverings where present. Distinguishing a cell from a bundle prevents confusion between a neuron and a nerve.

How does a nerve message pass from one cell to another?

When information is received at a neuron's dendritic tip, a chemical reaction produces an electrical impulse. The message travels through the neuron in a definite sequence. It passes from the dendrite to the cell body and then along the axon towards its end.

A synapse is the junction through which a neuron communicates with another cell. At a chemical synapse, a tiny gap separates the communicating cells. Chemicals released from an axon ending carry the message across this gap.

What is the sequence of transmission?

1. A stimulus is detected at a receiving tip, setting off a chemical reaction that produces an electrical impulse.
2. The impulse travels along the dendrite towards the cell body of the neuron.
3. It continues along the axon to the axon's terminal branches.
4. Arrival at the axon ending causes the release of chemical messengers called neurotransmitters.
5. These chemicals cross the gap and can start a new electrical impulse in the receiving neuron.

This sequence combines electrical transmission within a neuron with chemical transmission at a chemical synapse. It does not mean that a single electrical impulse simply jumps through an empty gap unchanged. The receiving cell produces its own electrical response.

Can the final message reach a muscle?

The receiving cell need not be another neuron. A similar junction allows a neuron to communicate with a muscle cell or a gland. A neuromuscular junction is the junction where a motor neuron communicates with a muscle fibre, which is a muscle cell.

Electrical impulses provide rapid communication, but they reach only cells connected by nervous tissue. After generating and transmitting an impulse, a cell needs time to reset before producing another. It cannot continually generate and transmit impulses without this resetting.

Note: Keep the two stages distinct: an electrical impulse travels along the neuron, while chemical messengers carry information across a chemical synapse.

How do sensory, motor, mixed, cranial and spinal nerves differ?

Nerves can be described according to the direction in which their fibres carry information or according to where they arise. These are different ways of classifying them. A functional name describes the message carried; an origin name identifies its connection.

What do sensory, motor and mixed mean?

A sensory nerve carries impulses from receptors towards the brain or spinal cord. Its messages provide information about a detected stimulus. The word afferent describes fibres carrying information towards the central nervous system, the brain and spinal cord together.

A motor nerve carries instructions from the brain or spinal cord towards effectors. The word efferent describes fibres carrying messages away from the central nervous system. A muscle responding to an instruction receives this outgoing information.

A mixed nerve contains both sensory and motor fibres. Its sensory fibres carry incoming information, and its motor fibres carry outgoing instructions. These functions belong to different fibres within the bundle.

What do cranial and spinal mean?

Cranial nerves arise from the brain. Humans have 12 pairs of cranial nerves. Spinal nerves arise from the spinal cord. Humans have 31 pairs of spinal nerves. These counts refer to pairs, rather than individual nerves.

The peripheral nervous system, abbreviated PNS, comprises the nerves connecting the central nervous system with other parts of the body. Cranial and spinal nerves provide these connections. The central nervous system is abbreviated CNS.

In the hot-object example, information has to reach the spinal cord and instructions have to reach the arm muscles. The sensory and motor pathways therefore have complementary roles. Neither the incoming message alone nor the outgoing instruction alone describes the complete response.

What makes up the central nervous system, and how is it protected?

The brain is the body's main coordinating centre. The spinal cord is the nervous structure running within the backbone and communicating with the brain. Together they form the CNS, where information is received, processed and linked with responses.

The central and peripheral nervous systems work together. Peripheral nerves carry information towards the CNS and instructions away from it. The brain and spinal cord receive information from different parts of the body and integrate it, meaning they bring it together for processing.

How is the brain protected?

The skull, including the bony brain case called the cranium, encloses the brain. Within this bony protection, membranes and fluid provide further protection. The fluid surrounding the brain helps absorb shocks.

The protective membranes around the brain are called meninges. Their three layers are the dura mater, the outer layer; the arachnoid, the thin middle layer; and the pia mater, the inner layer in contact with brain tissue.

How is the spinal cord protected?

The vertebral column, or backbone, protects the spinal cord. The spinal cord and backbone are different structures: the spinal cord belongs to the nervous system, while the backbone forms its bony protection.

This distinction matters when describing a pathway. An impulse involved in a spinal reflex is processed through nervous tissue in the spinal cord. It does not travel through the bones of the backbone as its nervous pathway.

The CNS has both communication and control functions. It receives information, participates in selecting or organising responses and sends instructions onwards. Protecting these delicate structures helps preserve the connections needed for sensation, movement and coordination throughout the body.

What are the main parts of the brain and their functions?

The brain has three major regions: the forebrain, the front region containing the main thinking centres; the midbrain, between the forebrain and hindbrain; and the hindbrain, which includes centres involved in balance and involuntary functions.

What does the forebrain do?

The forebrain includes the cerebrum, which forms the major part of the brain; the thalamus, a centre coordinating sensory and motor signals; and the hypothalamus, which contains centres controlling body temperature and urges to eat and drink.

The cerebrum is divided into left and right halves called cerebral hemispheres. Its outer layer, the cerebral cortex, is folded. It contains sensory areas receiving information, motor areas directing voluntary movement and association areas linking different kinds of information.

Association areas help connect incoming information with other sensations and information already stored. This supports memory and communication. A voluntary action is an action performed through conscious choice, such as deciding to move a chair.

How can information lead to a chosen movement?

1. Receptors provide information, and sensory impulses reach receiving areas of the forebrain.
2. Association areas interpret this information alongside other incoming information and stored information.
3. A decision about the response is made, linking the detected situation with a chosen action.
4. Motor areas send instructions that control the movement of voluntary muscles.

What do the midbrain and hindbrain do?

Involuntary actions occur without conscious control. Many are controlled by the midbrain and hindbrain. The hindbrain comprises the pons, which contains connecting nerve tracts; the cerebellum, important for precision and balance; and the medulla oblongata, which contains centres for vital involuntary functions.

The cerebellum helps maintain posture, the position in which the body is held, and balance. It helps make voluntary movements precise. Walking in a straight line, riding a bicycle and picking up a pencil illustrate the coordination it supports.

The medulla controls breathing and functions including salivation, vomiting and blood pressure. Salivation means the release of saliva; blood pressure is the pressure exerted by blood on blood-vessel walls. The medulla is connected to the spinal cord.

The side-section drawing shows the large folded cerebrum, a smaller folded cerebellum towards the lower back, and the spinal cord continuing downwards. Labels identify the forebrain, midbrain, pons, medulla, cerebellum, hypothalamus and cranium.

See Fig. 6.3 in your NCERT textbook

How does a reflex arc produce a quick response?

A reflex action is a rapid, automatic response to a stimulus without first making a conscious decision. Withdrawing the hand from a hot object is an example. The connecting pathway followed by the nervous message is called a reflex arc.

Thinking through a dangerous situation involves complex interactions among many neurons. A simpler connection between the incoming sensory pathway and outgoing motor pathway allows a quick response. In the withdrawal example, this connection is made within the spinal cord.

What happens when the hand touches a hot object?

1. Reception: Receptors in the skin detect the heat or painful stimulus. The incoming information starts a nervous message.
2. Sensory transmission: A sensory neuron carries the impulse from the skin towards the spinal cord.
3. Relay: Within the spinal cord, a relay neuron, a neuron connecting other neurons in the pathway, links the sensory pathway with the motor pathway.
4. Motor transmission: A motor neuron carries the outgoing impulse from the spinal cord towards the arm muscle.
5. Response: The muscle acts as the effector. It contracts, meaning it shortens, and the hand is withdrawn.

The stimulus, receptor, message and response are different parts of this explanation. Heat is the stimulus; the receptor detects it; an impulse carries the information; and withdrawal is the response. Calling the muscle a receptor would confuse detection with action.

The drawing shows a hand touching a hot vessel, skin receptors, a sensory neuron leading to the spinal cord, a relay neuron and a motor neuron leading to an arm muscle. An additional arrow is labelled as a message to the brain.

See Fig. 6.2 in your NCERT textbook

Does the brain receive information?

The information also reaches the brain. A spinal reflex can organise the immediate withdrawal without waiting for a conscious decision, but this does not mean that the brain is disconnected from the event. The illustrated pathway includes communication towards the brain.

The spinal cord therefore has more than a reflex role. It also carries information involved in communication with the brain. Describing it simply as a reflex centre leaves out this important connection between the body and the brain.

Other familiar reflex responses include closing the eyes when a hand moves in front of the face and the forward movement of the foot when the knee is tapped while sitting. These examples show a stimulus followed by an automatic response.

How do voluntary, involuntary and reflex actions compare?

Actions can be compared by whether they involve conscious choice and by how the response is organised. Writing, talking, moving a chair and clapping after a programme are examples of voluntary actions. They involve decisions about what to do.

Involuntary activities continue without having to think about each occurrence. Heartbeat, salivation and digestive movements illustrate such control. Many involuntary actions are controlled by the midbrain and hindbrain. It would be incorrect to place all automatic activity in the spinal cord.

What is distinctive about a reflex?

A reflex is an automatic response linked to a stimulus. Pulling a hand away from a hot object is different from choosing to move a chair. The withdrawal response uses a reflex pathway, while moving the chair involves a conscious decision.

Reflex actions are involuntary in the sense that they do not require conscious choice. However, the term reflex draws attention to the stimulus and response pathway. Involuntary control also includes continuing activities such as heartbeat.

Why should examples be described precisely?

Seeing movement does not by itself tell us whether it is voluntary. Ask what initiated the movement and whether a conscious decision preceded it. Both voluntary movements and withdrawal reflexes involve muscles, so the presence of muscle action does not distinguish them.

Likewise, lack of conscious choice does not prove that the spinal cord controls an activity. Involuntary centres also occur in the brain. The kind of action and the nervous structures involved must be identified together.

How do muscles turn nervous instructions into movement?

A nervous message is information; movement requires an effector to act on that information. In the withdrawal reflex, the effector is an arm muscle. The neuron carries the instruction to the muscle, and the muscle performs the movement.

What happens within the muscle cell?

A muscle fibre changes its shape by shortening. Muscle cells contain special proteins, large molecules that carry out many structural and working roles in cells. In response to nervous electrical impulses, these muscle proteins change their shape and arrangement.

The altered arrangement gives the muscle cell a shorter form. This shortening is contraction. It links the arrival of a nervous instruction to the action of muscular tissue. The final movement depends on the muscle's response to the message.

A description of a reflex should therefore end with the action of the effector. Stopping at “the impulse reaches the motor neuron” leaves the response unexplained. In the hot-object example, contraction of the arm muscle produces withdrawal of the hand.

How do nervous and muscular tissues share the work?

Nervous tissue detects or conveys information and carries instructions. Muscular tissue produces movement by changing the shape of its cells. These are complementary functions in the same coordinated response, rather than two names for the same process.

The final receiving cell can also be a gland. In that case, the response concerns a secretion rather than movement of a limb. This is why the general term effector includes both muscles and glands.

What does Pavlov's experiment show about learned responses?

Conditioning is learning through an association. In classical conditioning, one stimulus becomes a signal for another and begins to produce a learned response. Ivan P. Pavlov investigated this process while studying digestion in dogs.

Food naturally causes salivation. A response that occurs without learning the association is called an unconditioned response. A response acquired through the association is called a conditioned response. The distinction concerns how the response comes to be triggered.

What sequence established the association?

1. Pavlov sounded a bell and then immediately presented food to a hungry dog.
2. The dog ate the food and salivated in response to it.
3. On repeated trials, presentation of the food continued to be preceded by the bell.
4. In a test trial, the bell was sounded without food following it, and the dog still salivated.

The bell had become associated with the arrival of food. Food was the unconditioned stimulus, meaning the stimulus producing the unlearned response. After learning, the bell was the conditioned stimulus, meaning the signal producing the learned response.

What does the result mean?

The learned connection explains why the sound could produce salivation when no food followed it during the test. The sound had become a signal for food. The important change was the stimulus capable of setting off the response.

This example distinguishes an unlearned response to food from a learned response to a signal associated with food. Both involve salivation, but the history of the stimulus matters. The bell produces the conditioned response after the association has been established.

Glossary

• Stimulus — A detectable change that can lead to a reaction by an organism.
• Response — The reaction of an organism or part of it to a detected stimulus.
• Receptor — A specialised structure that detects a stimulus and initiates incoming information.
• Effector — A muscle or gland that carries out a response to an instruction.
• Neuron — A specialised nerve cell that receives and transmits information through nervous messages.
• Dendrite — A branching extension of a neuron carrying impulses towards its cell body.
• Axon — A long neuronal extension carrying impulses away from the cell body.
• Impulse — An electrical message transmitted through a neuron as part of nervous communication.
• Synapse — The communicating junction between a neuron and another neuron or responding cell.
• Sensory nerve — A nerve carrying impulses from receptors towards the brain or spinal cord.
• Motor nerve — A nerve carrying instructions from the central nervous system towards effectors.
• Mixed nerve — A nerve containing sensory and motor fibres that carry messages in their respective directions.
• Reflex arc — The nervous pathway through which an automatic response to a stimulus is organised.
• Cerebellum — A hindbrain part supporting precise voluntary movements and maintenance of posture and balance.
• Conditioned response — A learned response to a stimulus that has become associated with another stimulus.

Common errors and misconceptions

• Misconception: A nerve and a neuron are identical. Correct: A neuron is a cell, whereas a nerve is a bundle of nerve fibres.
• Misconception: Dendrites carry impulses away from the cell body. Correct: Dendrites carry impulses towards it; the axon carries impulses away.
• Misconception: The receptor performs the movement in a reflex. Correct: The receptor detects the stimulus, and the muscle effector performs the movement.
• Misconception: A spinal reflex sends no information to the brain. Correct: Information also reaches the brain, although immediate withdrawal does not wait for conscious decision-making.
• Misconception: The backbone and spinal cord are the same structure. Correct: The backbone is the bony protection around the spinal cord's nervous tissue.
• Misconception: All automatic actions are controlled by the spinal cord. Correct: Many involuntary actions are controlled by the midbrain and hindbrain.
• Misconception: Humans have 12 cranial nerves and 31 spinal nerves in total. Correct: These numbers count pairs: 12 pairs of cranial nerves and 31 pairs of spinal nerves.
• Misconception: Pavlov's bell caused salivation before any association with food was learned. Correct: Salivation to the bell was acquired after repeated pairing of the bell with food.

Exam-style questions with model answers

Q1. A person touches a hot object and immediately withdraws the hand. Identify the stimulus and the response. [2 marks]

1. The stimulus is the heat from the hot object detected by receptors in the skin.
2. The response is withdrawal of the hand, produced by the action of the arm muscles.

Q2. In a neuron, a message passes along a receiving branch towards the cell body and then along a long extension to its endings. Name these two extensions and explain their directions of transmission. [2 marks]

1. The receiving branch is a dendrite, which carries the impulse towards the cell body.
2. The long extension is the axon, which carries the impulse away from the cell body towards its endings.

Q3. A nerve carries incoming information from receptors; another carries outgoing instructions to muscles; a third contains fibres for both functions. Identify and explain the three functional types in that order. [3 marks]

1. The first is a sensory nerve. It carries incoming impulses from receptors towards the brain or spinal cord for processing.
2. The second is a motor nerve. It carries instructions away from the brain or spinal cord towards responding muscles.
3. The third is a mixed nerve. Its sensory and motor fibres provide incoming and outgoing communication through different fibres in the same bundle.

Q4. Explain four protective features or structures associated with the brain and spinal cord: the skull, meninges, surrounding fluid and vertebral column. [4 marks]

1. The skull forms a bony enclosure around the brain, protecting this delicate organ within the head.
2. The meninges are protective membranes around the brain, with outer dura mater, middle arachnoid and inner pia mater.
3. The fluid surrounding the brain provides further protection by helping absorb shocks within the bony enclosure.
4. The vertebral column, or backbone, protects the spinal cord. It is a different structure from the nervous tissue it encloses.

Q5. In withdrawal from a hot object, the pathway includes skin receptors, a sensory neuron, a relay neuron in the spinal cord, a motor neuron and an arm muscle. Explain these five stages in order, and state whether information also reaches the brain. [5 marks]

1. Skin receptors detect the heat or painful stimulus from the hot object. This detection begins the incoming nervous message.
2. A sensory neuron carries the impulse towards the spinal cord, bringing information from the stimulated region of the skin.
3. A relay neuron in the spinal cord links the sensory pathway with the motor pathway. Information also travels towards the brain.
4. A motor neuron carries the outgoing instruction from the spinal cord towards the arm muscle, completing the outgoing nervous pathway.
5. The arm muscle acts as the effector and contracts, withdrawing the hand. This immediate response does not wait for a conscious decision.

Q6. Give one principal function of each named brain part: cerebrum, hypothalamus, cerebellum and medulla oblongata. [4 marks]

1. The cerebrum helps process sensory information and directs voluntary actions through its sensory, association and motor areas.
2. The hypothalamus contains centres involved in regulating body temperature, helping coordinate this internal function of the body.
3. The cerebellum helps maintain posture and balance, supporting accurate coordination when the body performs voluntary movements.
4. The medulla oblongata contains centres controlling vital involuntary activities such as breathing, which continue without a conscious decision for each occurrence.

Q7. In Pavlov's experiment, a hungry dog salivated when given food. A bell repeatedly sounded immediately before food was presented. Later, the bell sounded without food, and the dog salivated. Identify the original stimulus, the learned signal and the evidence for conditioning. [3 marks]

1. Food was the original, unconditioned stimulus because it produced salivation without the dog first learning an association with the bell.
2. The bell became the conditioned stimulus after repeated presentations in which its sound was followed immediately by food.
3. Salivation when the bell sounded without food was the evidence for conditioning. This learned response showed that the bell had become associated with food.

Q8. A person deliberately moves a chair, withdraws a hand automatically after touching a hot object, and has a heartbeat without consciously directing each beat. Classify each activity and explain the basis of the classification. [3 marks]

1. Moving the chair is a voluntary action because the person deliberately decides to perform the movement and muscles carry out that decision.
2. Withdrawing the hand is a reflex action because the hot stimulus produces an automatic response without waiting for a conscious decision.
3. Heartbeat is an involuntary activity because each beat occurs without the person consciously deciding that it should happen.

Key takeaways

• The nervous system coordinates detection, processing and response through the linked activities of receptors, neurons and effectors.
• Dendrites carry impulses towards a neuron's cell body, while its axon carries impulses away towards other cells.
• Sensory nerves bring information to the central nervous system; motor nerves carry instructions to muscles or glands.
• The brain and spinal cord form the central nervous system; peripheral nerves connect them with other body parts.
• The cerebellum supports precise movements, posture and balance, while the medulla contains centres controlling important involuntary functions.
• A spinal reflex organises a rapid response without awaiting conscious choice, while information also reaches the brain.
• The skull and protective membranes protect the brain, surrounding fluid absorbs shocks, and the backbone protects the spinal cord.
• Pavlov's experiment shows how a signal repeatedly associated with food can acquire the ability to produce salivation.

Test yourself

What is the difference between a stimulus and a response?

A stimulus is a detected change; a response is the reaction produced after that change is detected.

Which parts of a neuron receive information and carry it away from the cell body?

Dendrites receive information and carry impulses towards the cell body. The axon carries impulses away from it.

What carries information across a chemical synapse?

Chemical messengers released at the axon ending cross the gap and act on the receiving cell.

How many pairs of cranial and spinal nerves occur in humans?

Humans have 12 pairs of cranial nerves arising from the brain and 31 pairs of spinal nerves arising from the spinal cord.

Why is a muscle called an effector in the withdrawal reflex?

It carries out the response by contracting and producing withdrawal of the hand from the hot object.

Which brain part supports posture and balance?

The cerebellum helps maintain posture and balance and improves the precision of voluntary actions.

Does a spinal withdrawal reflex mean the brain receives no information?

No. Information also reaches the brain, although the immediate response does not wait for a conscious decision.

What made the bell a conditioned stimulus in Pavlov's experiment?

Repeated pairing with food established an association, after which the bell alone could produce salivation.