Neural signalling transmits information through neuron structure, ion gradients, action potentials, synapses and chemical modulation of postsynaptic responses, linking cellular mechanisms to rapid communication in nervous systems.
| Neuron | Information route | Decisive structural pattern |
|---|---|---|
| sensory | receptor → CNS | long dendron toward an off-centre cell body; axon continues into CNS |
| relay | within CNS | many short dendrites and a short axon connect neurons locally |
| motor | CNS → muscle or gland | many dendrites around the cell body; one long axon reaches an effector |
Dendrites and dendrons carry impulses toward the cell body; an axon carries impulses away from it. A nerve is a bundle of many nerve fibres, not one neuron.
At rest, an axon's inside is about −70 mV relative to the outside. The membrane is polarized because ions and large negative solutes are distributed unequally across it.
The sodium–potassium pump hydrolyses ATP to move 3 Na⁺ out for every 2 K⁺ in, against both concentration gradients. It establishes and maintains high Na⁺ outside and high K⁺ inside.
At rest the membrane is much more permeable to K⁺ than to Na⁺. K⁺ leakage leaves behind impermeant negative charge, so the inside becomes negative. The pump maintains the gradients that this leakage continually uses.
An action potential is a brief reversal of membrane potential, typically from about −70 mV to about +40 mV. A nerve impulse is this action potential repeatedly regenerated along a nerve fibre.
Ion flow through open channels is passive, down electrochemical gradients. ATP is used by pumps to maintain and restore those gradients, not to push each ion through an open channel.
Transmission is normally one-way because release machinery is presynaptic while matching receptors are postsynaptic. Synapses can connect neuron to neuron, neuron to muscle, or neuron to a gland cell.
neurotransmitter diffuses across the cleft → binds a complementary postsynaptic receptor → ligand-gated cation channels open → Na⁺ entry depolarizes the membrane → an excitatory postsynaptic potential (EPSP) forms
An EPSP is graded: more transmitter can produce a larger local depolarization. It makes an action potential more likely, but only a summed depolarization reaching threshold triggers the all-or-none spike.
The signal must be removed. At cholinergic synapses, acetylcholinesterase hydrolyses acetylcholine; receptors close, and choline can be taken up for reuse. Other transmitters may be removed by reuptake or diffusion.