Neurons are the cells that send electrical messages around the nervous system. The brain alone has about 86 billion of them, plus a similar number of glial support cells. Three types matter for GCSE.
Three types of neuron
- Sensory neurons — long dendrites, short axons. Carry signals from receptors (skin, eye, ear) to the CNS.
- Relay neurons — short dendrites, short axons. Sit inside the CNS and connect sensory to motor neurons (or other relay neurons).
- Motor neurons — short dendrites, long axons. Carry signals from the CNS to effectors (muscles, glands).
A simple reflex arc (P2.B.1) uses all three: receptor → sensory → relay → motor → effector.
Anatomy of a neuron
- Cell body (soma) — contains the nucleus.
- Dendrites — branch-like extensions that receive signals from other neurons.
- Axon — the long fibre that carries the signal away from the cell body.
- Myelin sheath — fatty insulation in segments along the axon, with nodes of Ranvier between them. Speeds up the signal by allowing it to "jump" from node to node (saltatory conduction).
- Axon terminals (terminal buttons) — the ends of the axon, which release neurotransmitter into the synapse.
The action potential
At rest, the neuron is more negative inside than outside (about −70 mV). When sufficiently stimulated, sodium ions rush in, the inside becomes positive — this depolarisation is the action potential. It travels down the axon as a wave. The neuron then repolarises (potassium ions out) and briefly hyperpolarises during the refractory period before returning to rest.
Synaptic transmission
Neurons don't physically touch — they're separated by a tiny gap called the synaptic cleft (about 20 nm). Transmission across this gap is chemical:
- The action potential reaches the presynaptic terminal.
- Voltage-gated calcium channels open; calcium flows in.
- Synaptic vesicles containing neurotransmitter fuse with the membrane and release the neurotransmitter into the synaptic cleft.
- The neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic neuron.
- The receptor opens ion channels, generating a new electrical signal in the postsynaptic neuron.
- The neurotransmitter is removed by reuptake (back into the presynaptic terminal) or enzymatic breakdown.
Key point: synaptic transmission is one-way (presynaptic → postsynaptic) because only the presynaptic neuron has vesicles.
Excitation vs inhibition
Neurotransmitters can be excitatory (e.g. glutamate, adrenaline) — they make the postsynaptic neuron more likely to fire — or inhibitory (e.g. GABA, serotonin in some pathways) — they make firing less likely. The postsynaptic neuron sums all incoming signals; if the net excitation crosses threshold, it fires its own action potential. This is summation.
Why it matters
Drugs work by altering synaptic transmission: SSRIs for depression block reuptake of serotonin, leaving more in the synapse; stimulants like amphetamines increase dopamine release; opioids mimic endorphins. Understanding the synapse explains why mental health treatments take effect.
⚠Common mistakes— Common errors
- Saying neurons "touch" each other — they don't; the synapse is a chemical gap.
- Confusing the direction (signals travel down the axon, across the synapse, then down the next axon).
- Forgetting that summation determines whether the next neuron fires — a single signal usually isn't enough.
AI-generated · claude-opus-4-7 · v3-deep-psychology