The Human Nervous System (B5.2)

Overview

The nervous system allows rapid coordination of the body's response to stimuli. It consists of:

• Central Nervous System (CNS): brain + spinal cord — the control centre.
• Peripheral Nervous System (PNS): all other nerves connecting the CNS to the rest of the body.

Signal pathway:
Stimulus → Receptor → Sensory neurone → CNS → Motor neurone → Effector → Response

Neurones

Structure of a motor neurone:

• Cell body (nucleus, cytoplasm)
• Dendrites — receive impulses from other neurones
• Axon — long fibre that carries impulse away from cell body
• Myelin sheath — fatty insulation; speeds up impulse conduction
• Synaptic knobs at the end — release neurotransmitters

Synapses

A synapse is the junction between two neurones. The gap is called the synaptic cleft. Electrical impulses cannot cross — instead:

1. Impulse reaches synaptic knob.
2. Vesicles release neurotransmitter (e.g. acetylcholine) into the cleft.
3. Neurotransmitter diffuses across and binds to receptors on the post-synaptic membrane.
4. New impulse generated in the next neurone.
5. Neurotransmitter is broken down (by enzymes) or reabsorbed.

Synapses allow signals to pass in one direction only and permit complex signal integration.

Reflex arc

Reflexes are automatic, rapid responses that do not involve conscious thought. They protect the body.

Pathway:
Receptor → Sensory neurone → Relay neurone (in spinal cord) → Motor neurone → Effector

Example — touching a hot object:

1. Thermoreceptors in skin detect heat.
2. Sensory neurone carries impulse to spinal cord.
3. Relay neurone passes signal.
4. Motor neurone carries impulse to bicep muscle.
5. Muscle contracts — hand pulled away.

The brain is informed after the reflex occurs (but did not initiate it).

Common exam errors

1. Saying the brain controls reflexes — the relay neurone in the spinal cord does. The brain is informed later.
2. Drawing the reflex arc in the wrong direction.
3. Forgetting the function of the synapse is to transmit signals via chemical neurotransmitters (not electrically).