ICSE 7 Biology Nervous Basic

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Table of Contents

    The Nervous System

    Basic Study Notes

    1.0 Introduction

    The Nervous System is the master coordinating system of the body. it controls, directs, and coordinates all the activities of our body, both voluntary and involuntary.

    DEFINITION

    Stimulus: Any change in the environment (external or internal) that results in a reaction or response in an organism. Example: Light, Sound, Heat.

    2.0 The Neuron (Nerve Cell)

    Definition: A neuron is the basic structural and functional unit of the nervous system. It is specialized to carry messages in the form of electrical impulses.

    Main Parts of a Neuron:

    • Cyton (Cell Body): Contains a well-defined nucleus and cytoplasm.
    • Dendrites: Small, branched cytoplasmic extensions that receive impulses and carry them toward the cyton.
    • Axon: A single, long fiber that carries impulses away from the cyton.
    • Myelin Sheath: An insulating fatty covering around the axon that helps in faster transmission of impulses.
    • Axon Terminals: The branched ends of an axon that transmit the signal to the next neuron.

    3.0 Synapse

    DEFINITION

    Synapse: The microscopic gap between the axon terminal of one neuron and the dendrites of the next neuron through which impulses are transmitted.

    🔎 Did you know? Nerve cells are the longest cells in the human body. Some neurons, like those in your legs, can be over a meter long!

    4.0 Types of Neurons

    Neurons are classified into three types based on the direction in which they carry impulses:

    1. Sensory Neurons (Afferent)

    Function: They carry impulses from the sense organs (receptors) toward the Brain or Spinal Cord.

    2. Motor Neurons (Efferent)

    Function: They carry impulses from the Brain or Spinal Cord toward the muscles or glands (effectors).

    3. Association Neurons (Interneurons)

    Function: They are located within the CNS and serve as a link between sensory and motor neurons.

    5.0 The Central Nervous System: The Brain

    DEFINITION

    Meninges: Three protective membranes that surround and protect the brain and spinal cord.

    The Brain is divided into three main parts:

    • 🧠 Cerebrum: The largest part. Responsible for intelligence, memory, reasoning, and voluntary actions.
    • 🧠 Cerebellum: Located at the base of the cerebrum. It maintains balance and posture and coordinates muscular activities.
    • 🧠 Medulla Oblongata: The brain stem. It controls involuntary actions like heartbeat, breathing, and blood pressure.
    🔎 Clinical Tip: Alcohol primarily affects the Cerebellum, which is why a person under the influence loses their balance and cannot walk in a straight line!

    6.0 The Central Nervous System: Spinal Cord

    The Spinal Cord is a cylindrical structure that extends downwards from the Medulla Oblongata through the Vertebral Column (backbone).

    Functions of the Spinal Cord:

    • Conduction: It carries sensory impulses from the skin and muscles to the brain.
    • Response: It carries motor impulses from the brain to the muscles of the trunk and limbs.
    • Reflex Center: It acts as the center for Reflex Actions (automatic responses) below the neck.

    7.0 Peripheral Nervous System (PNS)

    The PNS consists of all the nerves that emerge from the brain and spinal cord and spread throughout the body. It is divided into:

    Type of Nerve Origin Number of Pairs
    Cranial Nerves Emerge from the Brain 12 Pairs
    Spinal Nerves Emerge from the Spinal Cord 31 Pairs

    8.0 Types of Actions

    Voluntary Actions

    Actions performed consciously with our will. Controlled by the Cerebrum.

    Example: Reading, Walking, Dancing.

    Involuntary Actions

    Actions performed without our will or conscious thought. Controlled by Medulla.

    Example: Heartbeat, Digestion, Breathing.

    DEFINITION

    Reflex Action: An immediate, automatic, and involuntary response to a stimulus, controlled primarily by the spinal cord without involving the brain.

    🔎 Did you know? Even though reflexes don't require the brain to act, the brain is informed *after* the action has happened so it can store the memory of the event!

    9.0 Mechanism of Reflex Action: The Reflex Arc

    A reflex action is incredibly fast because the impulse bypasses the brain for the initial decision. The shortest pathway traveled by a nerve impulse from a receptor to an effector is called the Reflex Arc.

    DEFINITION

    Effector: A muscle or a gland that responds to the motor impulse sent by the nervous system.

    The 5 Components of a Reflex Arc:

    1. Receptor: Receives the stimulus (e.g., skin sensing heat).
    2. Sensory Neuron: Carries the impulse to the spinal cord.
    3. Association Neuron: Located in the spinal cord; acts as a processing link.
    4. Motor Neuron: Carries the response impulse to the muscle.
    5. Effector: The muscle that contracts to perform the action.
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    10.0 Common Reflex Examples

    In the ICSE curriculum, these examples are essential to remember:

    • ðŸ”Ĩ Withdrawal Reflex: Quickly pulling your hand away after touching a hot object.
    • 👁️ Blinking Reflex: Closing eyes suddenly when an object is thrown toward them.
    • ðŸĶī Knee-Jerk Reflex: The sudden kicking movement of the lower leg when the tendon below the kneecap is tapped.
    • 🍋 Salivation: Watering of the mouth at the sight or smell of favorite food (this is a conditioned reflex).

    11.0 Control Centers Summary

    Action Type Main Control Center
    Voluntary Actions (Walking, Talking) Cerebrum (Brain)
    Involuntary Actions (Breathing, Heartbeat) Medulla Oblongata (Brain)
    Reflex Actions (Withdrawal from Heat) Spinal Cord

    CHAPTER COMPLETED: THE NERVOUS SYSTEM

    Neurons, Brain Structure, and Reflex Mechanisms covered.

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