Nervous System Chapter Notes | Biology Class 8 ICSE PDF Download

Introduction

The nervous system is like a control room in our body. It helps us think, move, feel, and understand the world around us. It works like a messenger system, sending signals to different parts of the body to do actions like walking, talking, or even feeling hungry. This chapter teaches us how the nervous system is made, how it works to manage our actions, and how it helps our body parts talk to each other. We will also learn about the types of nerves, how they carry messages, and what happens when we do things without thinking, like pulling our hand away from something hot.

Coordination

• We learn many life processes in earlier classes like digestion, respiration, excretion, etc., which keep going on in our body.
• These processes do not work independently but are linked to each other and to the needs of the whole body.
• Interaction of a living being with the outside world is also very important.
• Two types of coordination control these activities in the body: nervous coordination and chemical coordination.

(i) Nervous coordination

• This type of coordination is done by the nervous system.
• It is made of the brain, spinal cord, nerves, and sense organs.
• It controls how our body works and responds to the outside world.
• For example: How does our body let us know when we feel hungry? When we eat food, the act of eating involves a number of coordinated activities.
• The eyes look at the food placed on the table, the brain records this information, and the action starts.
• The hands get the message, and they are raised to pick up the food and push it into the mouth.
• The food goes down through the alimentary canal, and after a series of processes in the body, it reaches the blood stream to fulfill the requirement that had suddenly happened to come up.
• If someone suddenly applies the brakes in a running car, the driver quickly applies the brakes.
• In this situation, a fraction of a second goes to the brain as a message is sent to the leg muscles to press the brakes.

(ii) Chemical coordination

• This type of coordination is done through chemical messengers called hormones, which are a category of chemicals given by the body.
• For example: If you are suddenly attacked by a bull in the street, you try to run away from it.
• In such a situation, certain chemicals are released into the blood to provide you extra energy and strength to run.

Need of Nervous System

• The nervous system is very important for major functions in our body.
• It keeps us informed about the outside world through the sense organs like eyes, ears, nose, etc.
• It helps us to remember, think, and reason out things.
• It controls and brings together all voluntary muscle activities, like running, walking, or even holding a book in your hand while you are reading it.
• It regulates involuntary activities like breathing, heartbeat, etc., without us thinking about them.

Structural and Functional Unit of Nervous System – The Neuron

• The nervous system is made up of special cells called neurons or nerve cells.
• Neurons are made of two main parts: a main cell body called cyton and a long process called axon.
• A large body contains a nucleus, and many short processes called dendrites are present on the cyton.
• Dendrites are like tiny branches that receive messages from the body and send them to the cyton.
• The axon is a long part that carries the message away from the cyton to the next neuron.
• The end of the axon terminates in a number of branches called terminal branches.

Nerve

A nerve is formed of a bundle of axons (nerve fibres) enclosed in a tubular medullary sheath. This sheath over the axon acts like an insulation and prevents mixing of impulses in the adjacent fibres. There are three kinds of nerves based on the types of nerve fibres they have:

• Sensory nerves contain only sensory fibres, which bring impulses from the sense organs to the brain or spinal cord, e.g., optic nerve of the eye.
• Motor nerves contain only motor fibres, which carry impulses from the brain or spinal cord to the muscles or glands.
• Mixed nerves have both sensory and motor fibres, e.g., nerve going to the tongue.

Some Basics in the Functioning of Nervous System

To understand how the nervous system works, you should know some basic terms:

1. Stimulus

• Any change in the environment that causes a response in the body.
• For example: When you watch a programme on TV and you suddenly hear a loud sound, you press the remote to lower the sound—this sound is the stimulus.

2. Response

• The activity of the body due to the stimulus.
• For example: In the TV example, pressing the remote to lower the sound is the response.

3. Impulse

• A wave of electrical disturbance that runs through the nerves.
• For example: In the TV example, the nerve carries the message from the ear to the brain and then to the hand—this message is the impulse.

4. Receptors

• The sense organs which receive the stimulus.
• For example: In the TV example, the ear is the receptor because it hears the loud sound.

5. Effector

• Any muscle or gland where the response occurs.
• For example: In the TV example, the hand muscle that presses the remote is the effector.

The above actions can be understood by taking an example of sudden removal of hand on touching a hot object:

• Heat is the stimulus.
• The nerve cell in the skin that receives the stimulus is the receptor.
• The sensation of travelling through the nerve is the impulse.
• The muscle of the hand receiving the command from the brain is the effector.
• The contraction of the muscle for withdrawing the hand is the response.

Nervous System

The nervous system of human beings has three systems:

• The central nervous system: It consists of the brain and the spinal cord. The brain lies protected inside the skull, and the spinal cord lies within the vertebral column.
• The peripheral nervous system: It consists of the nerves passing to and from the central nervous system.
•  The autonomic nervous system: It consists of a pair of chains of ganglia (a mass of nerve cells) and nerves that control certain vital functions of the body, like the heartbeat, respiratory movements, and peristalsis.

The Brain

The brain has three main parts:

The cerebrum

• It is the largest part of the brain, divided into two (right and left) halves called the cerebral hemispheres.
• The outer surface is folded with ridges and grooves.
• Each hemisphere has an outer and an inner portion.
• The outer portion contains cell bodies of neurons and is called grey matter.
• It accommodates a large number of neurons (human cerebrum contains about 9 billion neurons).
• The inner portion is mainly made of axons and is called white matter.

The cerebellum

• The cerebellum is much smaller than the cerebrum.
• It is located under the cerebrum.
• It controls all voluntary activities like walking, standing, body balance, and coordination of muscular activities.
• The cerebellum decides the action, whereas the cerebrum implements the action.
• For example: If you get an idea to stand up and walk, it arises in the cerebrum, but the muscles involved in this process are controlled by the cerebellum.
• Cerebellum gets affected by alcohol, which is why an alcoholic person is unable to coordinate his muscular movements properly.

The medulla oblongata

• The medulla oblongata is located just below the cerebellum.
• It controls reflexes below the neck, like coughing, sneezing, etc.
• It conducts messages from the skin and muscles to the brain.
• It conducts commands from the brain to the muscles of the trunk and limbs.

The Spinal Cord

The spinal cord begins at the medulla of the brain and extends nearly the full length of the backbone. Unlike the brain, the spinal cord has grey matter on the inside, composed of nerve cell bodies, and white matter on the outside, made up of nerve fibers (axons).
Its primary roles include:

• Managing reflex actions below the neck.
• Transmitting sensory information from the skin and muscles to the brain.
• Carrying motor commands from the brain to the muscles of the trunk and limbs.

Peripheral Nervous System

The peripheral nervous system is made up of nerves that link the central nervous system to the rest of the body. It includes 12 pairs of cranial nerves that originate from the brain.

• First (Olfactory nerves): Connected to the nose, responsible for the sense of smell.
• Second (Optic nerves): Linked to the eyes, essential for vision.
• Seventh (Facial nerves): Involved in taste sensations from the tongue and movement of facial muscles.
• Eighth (Auditory nerves): Connected to the ears, responsible for hearing.

Spinal Nerves

The spinal cord gives rise to 31 pairs of spinal nerves. Each spinal nerve is a mixed nerve, containing:

• A sensory component that carries signals from sensory receptors like the skin to the spinal cord or brain.
• A motor component that sends instructions from the brain or spinal cord to muscles or glands. These nerves are involved in both voluntary and involuntary actions.

Autonomic Nervous System

This system includes a chain of nerves and ganglia on both sides of the spinal column. It regulates involuntary functions of the body, such as those related to internal organs and glands. The autonomic nervous system regulates the functions of internal organs without conscious control. the sympathetic and parasympathetic systems. These systems have opposite effects on the body. For instance, during fear or stress, the sympathetic system causes the pupils to widen, narrows the blood vessels in the skin, and increases the heart rate. In contrast, the parasympathetic system causes the pupils to shrink, expands the blood vessels, and slows the heart rate.

Reflexes (Involuntary Actions)

• Our body operates through two systems: the sympathetic and parasympathetic systems, which are antagonistic (opposite) in their actions.
• For example, the sympathetic nervous system dilates the pupil of the eyes, constricts the blood vessels of the skin, and accelerates the heartbeat.
• The parasympathetic nervous system is responsible for the constriction of the pupil of the eyes, dilation of blood vessels, and slowing down of the heartbeat.

There are two types of actions in our body: voluntary (performed consciously) and involuntary (performed unconsciously).

• Voluntary actions: Actions that we do knowingly.
  For example: When you watch a programme on TV and suddenly hear a loud sound, you press the remote to lower the sound—this is a voluntary action.
• Involuntary actions (occurring unknowingly): Actions that happen without us thinking about them.
  For example: If your eye has a particle and there is immediate flushing of tears, this is an involuntary action.

Reflex Action

The peripheral nervous system and spinal cord together control actions. When the brain is not involved, e.g., if you touch a hot object or get pricked by a pointed object, you instantly remove your hand—this is called a reflex action.

Types of Reflexes

1. Natural (inborn) reflexes

• Reflexes that we are born with and do not need previous experience or learning to do.
• For example: Blinking, coughing, sneezing—these are protective reflexes.
• Salivation, swallowing, and peristalsis—provide functional efficiency.
• Dilation of the pupil of the eyes to look in the dark and vice versa in light—controls movement.
• Pushing the food along the alimentary canal (muscular movement)—controls digestion.
• Sudden withdrawal of the leg if you step on a nail—protective reflex.
• Knee jerk response, in which if a tap is made below the knee cap, then the leg involuntarily extends—reflex to check body balance.

2. Conditioned (acquired) reflexes

• Reflexes that we develop during our lifetime through experience or learning.
• For example: Watering of the mouth (salivation) when you see or smell food—this is conditioned because you have learned to connect the sight or smell with eating.
• Watching the television—this is a learned action.
• Typing on a keyboard of a computer—this is also a learned action.
• Playing a musical instrument—this is a learned action.
• Salivation can occur as a natural reflex also, when you start eating food, but the sight or smell of food causing salivation is a conditioned reflex because it was learned through experience.
• Conditioned reflexes are not inborn and are thus also called acquired reflexes.

Fun Fact

Some examples of conditioned reflexes are:

• Watering of the mouth (salivation) when you see or smell food—you experience the sight or smell and connect it to eating, so your mouth waters.
• If you have not eaten that food earlier, your mouth would not water—this shows it’s a learned reflex.
• Watching the television—this is a learned action because you were taught how to do it.
• Typing on the keyboard of a computer—this is a learned action because you practiced it.
• Playing a musical instrument—this is a learned action because you were trained to do it.
• Salivation starts flowing down when you chew or eat food—this is a natural reflex because it happens automatically.
• But salivation due to the sight or smell of food was learned through experience, so it’s conditioned.
• Conditioned reflexes are learned through experience, while natural reflexes are inborn and do not need learning.