Nervous Coordination: Nerve Impulses and Synapses

Nervous coordination

The nervous system detects stimuli and coordinates rapid responses via electrical impulses along neurones. A reflex arc (receptor → sensory neurone → relay → motor neurone → effector) gives fast, protective responses. At A-Level you need the detail of the nerve impulse and the synapse.

The resting potential

A resting neurone is polarised at about −70 mV inside relative to outside. This is maintained by:

• the sodium–potassium pump actively moving 3 Na⁺ out for every 2 K⁺ in;
• the membrane being more permeable to K⁺ than Na⁺ (K⁺ leaks back out).

The inside is therefore negative compared with the outside.

The action potential

When a stimulus is large enough to reach the threshold, an action potential fires:

1. Depolarisation: voltage-gated Na⁺ channels open; Na⁺ floods in; the inside becomes positive (~+40 mV).

2. Repolarisation: Na⁺ channels close, K⁺ channels open; K⁺ leaves; the inside becomes negative again.

3. Hyperpolarisation then a return to resting potential (pump restores the gradients).

• All-or-nothing: an action potential either fires fully or not at all; a stronger stimulus increases the frequency, not the size.
• Refractory period: a short recovery when no new impulse can fire — ensures impulses travel one way and limits frequency.

Speeding up conduction

• Myelination: the myelin sheath insulates the axon; the impulse "jumps" between gaps (nodes of Ranvier) — saltatory conduction — which is much faster.
• Greater axon diameter and higher temperature also increase speed.

The synapse

Where two neurones meet there is a gap (synaptic cleft). Transmission is chemical:

1. The action potential reaches the presynaptic knob; Ca²⁺ channels open.

2. Vesicles of neurotransmitter (e.g. acetylcholine) fuse with the membrane and release it by exocytosis.

3. It diffuses across the cleft and binds to receptors on the postsynaptic membrane, opening Na⁺ channels and (if threshold is reached) triggering a new action potential.

4. The neurotransmitter is broken down/removed so the response stops.

Synapses ensure one-way transmission and allow signals to be combined (summation).

Worked example

Why does a stronger stimulus not produce a bigger action potential?

• Action potentials are all-or-nothing — once threshold is reached the size is fixed. A stronger stimulus is coded by a higher frequency of action potentials, not a larger one. ✓

Common mistakes

• Getting depolarisation/repolarisation ions wrong: Na⁺ in (depolarise), K⁺ out (repolarise).
• Forgetting the refractory period ensures one-way, discrete impulses.
• Saying impulses cross synapses electrically — they cross as chemical neurotransmitters.

Exam tips

• Learn the resting potential mechanism (Na⁺/K⁺ pump, 3 out : 2 in).
• Describe the action potential in ordered stages with the ion movements.
• Explain saltatory conduction and the steps of synaptic transmission.

Key facts to remember

• Resting potential (~−70 mV) maintained by the Na⁺/K⁺ pump; action potential = Na⁺ in (depolarise) → K⁺ out (repolarise), all-or-nothing, with a refractory period.
• Myelination gives fast saltatory conduction (jumping between nodes of Ranvier).
• Synapses transmit chemically (neurotransmitter → receptors → new impulse), ensuring one-way transmission.