IB Biology SL C2.2 Neural signalling Notes

Neurons carry electrical impulses in the nervous system. Motor, sensory, and relay neurons differ in axon, dendrite, dendron, and cell body arrangement, and those differences fit whether the cell carries signals from receptors, between neurons, or to effectors.

Resting potential is an maintained ion-gradient state, not the action potential itself. The sodium-potassium pump uses ATP to move 3 Na+ out and 2 K+ in, keeping the inside of the axon about -70 mV relative to outside.

A nerve impulse is a propagated action potential along a nerve fibre. It starts when threshold opens voltage-gated sodium channels; sodium enters, the membrane depolarizes, and that local voltage change helps trigger the next region of membrane.

Larger axon diameter lowers resistance and increases impulse speed because current spreads more easily through the axoplasm; squid giant axons are the classic unmyelinated comparison. Myelin sheaths and nodes of Ranvier enable faster saltatory conduction because depolarization is regenerated only at the gaps.

Synapses connect neurons to neurons, muscles, or glands. Chemical synapses transmit one way across a narrow synaptic cleft because the presynaptic terminal releases transmitter and the postsynaptic membrane carries the receptors that respond.

At the presynaptic terminal, action potentials open voltage-gated Ca²⁺ channels. Ca²⁺ causes vesicle fusion and neurotransmitter exocytosis into the cleft, converting the electrical arrival signal into chemical release.

After release, neurotransmitters such as acetylcholine diffuse and bind transmembrane receptors on the postsynaptic membrane. EPSPs depolarize the membrane and make threshold more likely, so each excitatory synapse adds a small step toward firing.