ICSE 8 Biology Nervous System Basic

1.0 The Nervous System: Control and Coordination

The nervous system is the body's internal communication network. it receives information from the surroundings, interprets it, and generates an appropriate response. Unlike the endocrine system, the nervous system uses electrical impulses for rapid action.

The Unit of Life

Neuron (Nerve Cell): The structural and functional unit of the nervous system. It is specialized to carry messages in the form of electrical impulses.

2.0 Structure of a Neuron

Main Parts of a Neuron

• 🌟 Cyton (Cell Body): Contains the nucleus and cytoplasm. It is the metabolic center of the neuron.
• 🌿 Dendrites: Short, branched projections that receive impulses from other neurons.
• 🛤️ Axon: A long, single fiber that carries impulses away from the cyton.
• 🛡️ Myelin Sheath: An insulating fatty layer around the axon that increases the speed of impulse transmission.
• 🔌 Axon Terminals: The end of the axon where impulses are passed to the next neuron via a Synapse.

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Fig 7.1: Structure of a Multipoloar Neuron

Types of Neurons

• Sensory Neurons: Carry impulses from sense organs to the Brain/Spinal Cord.
• Motor Neurons: Carry impulses from the Brain/Spinal Cord to muscles or glands (Effectors).
• Association (Relay) Neurons: Located in the CNS; they link sensory and motor neurons.

⚠️ Important Note:

Neurons are the only cells in the human body that cannot divide once they are fully formed. This is because they lack centrioles.

💡 Did You Know?

Information travels through your nerves at speeds up to 120 meters per second! That's faster than a high-speed racing car.

3.0 The Central Nervous System (CNS): The Brain

The CNS acts as the control center for the entire body. It consists of the Brain and the Spinal Cord. The brain is highly protected by the skull (cranium) and three layers of membranes called Meninges.

Main Parts of the Brain

The human brain is divided into three major regions, each with distinct responsibilities:

• 🧠 Cerebrum (Forebrain): The largest part. It is the seat of intelligence, memory, and reasoning. It controls all voluntary actions and interprets sensations like sight and sound.
• ⚖️ Cerebellum (Hindbrain): Located at the base of the cerebrum. Its main function is to maintain body balance, posture, and coordinate muscular activities.
• 🔌 Medulla Oblongata: The lowest part that continues into the spinal cord. It controls involuntary actions like heartbeat, breathing, and blood pressure.

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Fig 7.2: Different Regions of the Human Brain

Cerebrum vs. Cerebellum

Feature	Cerebrum	Cerebellum
Size	Largest part (80% of brain).	Much smaller.
Function	Thinking, Memory, Voluntary actions.	Balance, Posture, Muscle coordination.

⚠️ Exam Tip:

An injury to the Medulla Oblongata is usually fatal. Why? Because it controls life-sustaining involuntary actions like heartbeat and respiration. Students are often asked this "Give Reason" question.

💡 Did You Know?

Alcohol primarily affects the Cerebellum. This is why a person under the influence of alcohol struggles to walk in a straight line or maintain balance.

4.0 The Spinal Cord and Reflex Actions

The Spinal Cord is a long, tubular structure that extends from the medulla oblongata down through the vertebral column. It conducts impulses to and from the brain and acts as the center for Reflex Actions.

Definition

Reflex Action: An automatic, quick, and involuntary response of the body to a stimulus, without the conscious involvement of the brain.

The Reflex Arc

The shortest pathway taken by an impulse to complete a reflex action is called a Reflex Arc. It involves five essential components:

1. Receptor: The sense organ (like skin) that receives the stimulus.
2. Sensory Neuron: Carries the impulse to the spinal cord.
3. Association Neuron (Relay): Processes the signal inside the spinal cord.
4. Motor Neuron: Carries the command back to the muscle.
5. Effector: The muscle or gland that performs the action (e.g., pulling your hand back).

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Fig 7.3: Pathway of a Reflex Arc (Withdrawal Reflex)

⚠️ Important Note:

Inborn (Natural) Reflexes: Blinking of eyes, coughing, or sneezing. These don't need to be learned.
Conditioned Reflexes: Salivation at the smell of favorite food or typing on a keyboard. These are developed through experience and learning.

💡 Did You Know?

Your brain only finds out about a reflex action after it has happened! The spinal cord handles the emergency first to save time and prevent injury.

5.0 Peripheral and Autonomic Nervous Systems

The Peripheral Nervous System (PNS) consists of all the nerves that emerge from the brain and spinal cord, linking the CNS to the rest of the body.

Types of Peripheral Nerves

• 🧠 Cranial Nerves: 12 pairs of nerves that emerge directly from the Brain. They reach parts like the eyes, ears, and nose.
• 🦴 Spinal Nerves: 31 pairs of nerves that emerge from the Spinal Cord and spread throughout the trunk and limbs.

The Autonomic Nervous System (ANS)

The ANS controls the involuntary activities of internal organs (like the heart, lungs, and stomach) without our conscious effort. It is divided into two opposing parts:

System	Role	Effect on Body
Sympathetic	Prepares body for action (Stress).	Dilates pupils, increases heart rate.
Parasympathetic	Calms the body (Rest/Digest).	Constricts pupils, slows heart rate.

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Fig 7.4: Comparison of Sympathetic and Parasympathetic Actions

⚠️ Exam Tip:

Memorize the exact number of nerve pairs! It is a very common fill-in-the-blank question: 12 pairs of Cranial nerves and 31 pairs of Spinal nerves.

🎓 Chapter Summary

The nervous system ensures that all organs work in harmony. While the brain is the CEO, the nerves are the high-speed fiber optics that keep the "Body Corporation" running smoothly!