The nervous system and nerve impulses (SACE Stage 2 Biology)

What this dot point is asking

You need to describe neuron structure, explain how a nerve impulse (action potential) is generated and travels, and explain how signals cross synapses. This is the fast communication pathway used in homeostasis.

Neuron structure

A neuron (nerve cell) is specialised to carry electrical signals. Its main parts are:

• Dendrites - receive signals and carry them toward the cell body.
• Cell body - contains the nucleus.
• Axon - a long fibre that carries the impulse away from the cell body.
• Myelin sheath - a fatty insulating layer around many axons that speeds up impulse transmission.
• Axon terminals - pass the signal on at synapses.

The three functional types are sensory neurons (receptor to central nervous system), motor neurons (central nervous system to effector), and interneurons (relay neurons) that connect them.

The nerve impulse (action potential)

A nerve impulse is an electrical signal called an action potential.

• Resting potential. A resting neuron is polarised: the inside is negative relative to the outside, maintained by the sodium-potassium pump (active transport) moving ions across the membrane.
• Depolarisation. When a stimulus is strong enough to reach the threshold, sodium ions rush into the neuron, reversing the charge. This is the action potential.
• Propagation. The depolarisation triggers the next section of membrane, so the impulse travels along the axon as a wave.
• Repolarisation. Potassium ions move out and the pumps restore the resting potential, ready for the next impulse.

The action potential is all-or-nothing: if the threshold is reached the impulse fires fully; if not, nothing happens. A stronger stimulus produces more frequent impulses, not bigger ones. Myelinated axons conduct faster because the impulse jumps between gaps in the sheath.

The synapse

Neurons are not physically joined; there is a tiny gap called the synapse between them. The impulse cannot cross electrically, so transmission is chemical:

1. The action potential reaches the axon terminal.
2. Vesicles release neurotransmitter molecules into the synaptic gap.
3. The neurotransmitter diffuses across and binds to receptors on the next neuron.
4. If enough binds, a new action potential is triggered in that neuron.
5. The neurotransmitter is then broken down or reabsorbed so the synapse can reset.

Synapses ensure signals travel in one direction only, because only the terminal end releases neurotransmitter (the receptors are on the receiving neuron). Synapses are also where many drugs act, by mimicking a neurotransmitter, blocking its receptor, or preventing its breakdown or reuptake - which is why exam stimulus material often describes a drug's effect at the synapse and asks you to predict the consequence for signalling.

Why myelination speeds conduction

A common higher-mark point is to explain how the myelin sheath increases conduction speed. Myelin is an insulating layer interrupted by gaps (nodes). Because the membrane can only depolarise at the nodes, the action potential effectively jumps from node to node rather than moving smoothly along the whole axon. This saltatory conduction is much faster than continuous conduction in an unmyelinated axon, which is why myelinated motor neurons can trigger rapid responses. Diseases that destroy myelin slow or block conduction, illustrating the structure-function link.