The human nervous system: neurones, reflexes and synapses

The nervous system uses electrical impulses travelling along neurones to coordinate fast, short-lived responses. It works alongside the slower hormonal system.

Organisation

The system splits into:

• Central nervous system (CNS) — the brain and spinal cord. Receives information, processes it and decides what to do.
• Peripheral nervous system — all the nerves connecting the CNS to the rest of the body.

Information always flows in the same order: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response.

Three types of neurone

1. Sensory neurones carry impulses from receptors (eyes, skin, ears…) to the CNS. Have a cell body part-way along the axon.
2. Relay (or interconnecting) neurones are short, found inside the CNS, and connect sensory neurones to motor neurones.
3. Motor neurones carry impulses from the CNS to effectors (muscles or glands). Have the cell body at one end and a long axon ending in motor end plates.

All neurones share key features: a cell body with a nucleus, dendrites that pick up signals, an axon that carries the impulse, and a myelin sheath that insulates the axon and speeds up the impulse.

Synapses

Where two neurones meet there is a tiny gap, the synapse. The electrical impulse cannot jump it directly. Instead:

1. The impulse arrives at the end of the first neurone.
2. Neurotransmitter chemicals are released into the synaptic gap.
3. They diffuse across and bind to receptors on the next neurone.
4. This triggers a new electrical impulse in the second neurone.

Synapses make signal transmission slightly slower (it's a chemical rather than electrical step) but they make the system flexible — they only allow signals to travel one way and they can be modulated.

Reflex arcs

A reflex action is a fast, automatic and protective response that does not involve conscious thought. The pathway, called a reflex arc, bypasses the brain — the relay neurone in the spinal cord directly connects sensory and motor neurones.

Classic example: hand on hot object.

• Stimulus — heat
• Receptor — temperature/pain receptor in the skin
• Sensory neurone — carries impulse to spinal cord
• Synapse → relay neurone → synapse
• Motor neurone — carries impulse to biceps
• Effector — biceps contracts
• Response — hand pulled away

The brain becomes aware afterwards, which is why you sometimes drop something before you "feel" it was hot.

Required practical: reaction time

Use the ruler-drop test. The partner holds a 30 cm ruler vertically. The subject's thumb and forefinger are level with the 0 mark. The partner releases without warning, and the subject catches it as fast as possible. Measure the distance the ruler fell, then either look up reaction time or use t = √(2d/g). Compare reaction time before and after a stimulus (e.g. caffeine).

Variables to control: same hand, same posture, no warning, multiple repeats and a mean.

Links

Connects to B5.1 (control systems), B5.3 (the brain) and B5.4 (the eye and pupil reflex).