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Cambridge IGCSE Biology · 0610

Chapter 14: Coordination and response (Part 1)

The nervous system

Role
The nervous system coordinates and regulates body functions by sending electrical signals known as nerve impulses along specialized nerve cells called neurones.
Central nervous system (CNS)
Consists of the brain and spinal cord.
Peripheral nervous system (PNS)
Consists of the nerves outside the brain and spinal cord that carry impulses to and from the CNS.

Exam Traps

  • Do not include peripheral nerves in the CNS — only brain and spinal cord form the CNS.

Types of neurones

  1. Sensory: Carries impulses from a receptor to the CNS.
  2. Relay (connector): Carries impulses between different parts of the CNS.
  3. Motor (effector): Carries impulses from the CNS to an effector (e.g., a muscle or gland).
Structure of sensory, relay, and motor neurones showing cell body, axon, and dendrites
Diagram 1: Types of neurones. Sensory, relay, and motor neurones with the cell body, axon, and dendrites labelled for each type.

Exam Traps

  • Do not say motor neurones carry impulses from receptors — sensory neurones do that.
  • Avoid placing the relay neurone outside the CNS in a spinal reflex — it is in the spinal cord.

Reflexes

Reflex action
An automatic and rapid means of integrating and coordinating stimuli with the responses of effectors to protect the organism from danger.
Reflex arc
The pathway taken by an impulse: Stimulus → Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector → Response.
A simple reflex arc showing spinal cord cross-section, receptor, sensory neurone, relay neurone, motor neurone, and effector muscle
Diagram 2: A simple reflex arc. A cross-section of the spinal cord with a receptor in the skin, sensory neurone entering the dorsal root, a relay neurone in the grey matter, and a motor neurone exiting via the ventral root to an effector muscle.

Exam Traps

  • Do not reverse the order — effector comes after motor neurone, not before receptor.
  • Avoid saying all reflexes are slow voluntary responses planned in the cerebrum.

Synapses

Definition
A junction between two neurones.
Structure
Includes vesicles containing neurotransmitter molecules in the presynaptic neurone, a synaptic gap, and receptor proteins on the postsynaptic neurone.
Transmission
  1. An impulse stimulates the release of neurotransmitters from vesicles into the gap.
  2. Neurotransmitters diffuse across the gap.
  3. They bind with receptor proteins on the next neurone, triggering a new impulse.
Unidirectionality
Synapses ensure impulses travel in one direction only because vesicles are only in the presynaptic neurone and receptors are only on the postsynaptic side.

Exam Traps

  • Do not say impulses jump electrically across the synapse — neurotransmitters carry the signal chemically.
  • Avoid claiming impulses can travel backwards at synapses — direction is fixed by vesicle and receptor positions.

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