C2.2 Neural signallingIB Biology HL: Revision notes

A neuron has a cell body (nucleus and cytoplasm), a single long axon and multiple shorter dendrites. The sodium–potassium pump uses ATP to pump 3 Na⁺ out and 2 K⁺ in, creating concentration gradients and a resting potential of about −70 mV. It is negative because more positive ions are pumped out than in, K⁺ leaks out more readily than Na⁺ leaks in, and there are negatively charged proteins inside.

A nerve impulse is an action potential: electrical because positive ions move. Speed increases with axon diameter (squid giant axons are fast) and is much greater in myelinated fibres. Conduction speed is positively correlated with axon diameter and negatively correlated with animal size. r measures the strength of a correlation (−1 to +1); R² gives the proportion of variation in one variable explained by the other.

At a synapse, depolarisation of the presynaptic membrane lets Ca²⁺ enter; calcium triggers exocytosis of neurotransmitter. Neurotransmitter diffuses across the cleft and binds to transmembrane receptors. Acetylcholine opens receptor channels so Na⁺ enters, producing an excitatory postsynaptic potential; it is used at many synapses including neuromuscular junctions. Signals pass in only one direction.

Depolarisation: a stimulus raises the potential to the threshold (about −55 mV); voltage-gated Na⁺ channels open, Na⁺ diffuses in and the potential rises to about +30 to +40 mV.

Repolarisation: Na⁺ channels close and voltage-gated K⁺ channels open; K⁺ diffuses out. K⁺ channels close slowly, causing a brief hyperpolarisation. The pump then restores the gradients.

Propagation: Na⁺ diffusing along inside the axon (and ions outside) form local currents that bring the next section to threshold.

Oscilloscope traces plot membrane potential against time. You can read the resting potential, threshold, peak and hyperpolarisation, and count impulses per second. A stronger stimulus increases frequency, not size: action potentials are all-or-nothing.

In myelinated fibres, ion pumps and channels are clustered at the nodes of Ranvier. Local currents flow from node to node under the myelin, so the action potential is propagated from node to node (saltatory conduction). This is much faster than continuous conduction and uses less energy for pumping.

Neonicotinoids (pesticides) bind to acetylcholine receptors in insects and are not broken down, blocking synaptic transmission. Cocaine blocks reuptake of dopamine, so it stays in the cleft and stimulation is prolonged.

Inhibitory neurotransmitters cause the postsynaptic membrane to become hyperpolarised, an inhibitory postsynaptic potential. A postsynaptic neuron sums the excitatory and inhibitory inputs from many presynaptic neurons; only if the threshold is reached is an action potential generated (all-or-nothing).

Free nerve endings in the skin have channels for positive ions that open in response to high temperature, acid or chemicals such as capsaicin. Positive ions enter, the threshold is reached, and impulses pass to the brain, where pain is perceived.

Consciousness is an emergent property: it arises from the interaction of vast numbers of individual neurons and cannot be found in any single neuron.