Revise Neural signalling for IB Biology (C2.2)

A neuron is a cell specialised for carrying signals over long distances. Signals arrive at the branching dendrites, which carry them toward the cell body. The cell body contains the nucleus and most of the organelles. A single long fibre, the axon, then carries the impulse away from the cell body toward its targets. In many neurons the axon is wrapped in a fatty myelin sheath that insulates it, interrupted by small gaps called nodes of Ranvier. At the far end the axon divides into axon terminals, which form synapses with the next neuron or with an effector such as a muscle. Knowing what each part does makes the rest of the topic fall into place: the resting and action potentials happen across the axon membrane, and the synapse is what the terminals form.

Before a neuron can send a signal it sits at a resting potential of about −70 mV, meaning the inside of the axon membrane is 70 millivolts more negative than the outside. This is not a passive situation — it is actively maintained. The sodium–potassium pump uses ATP to move three sodium ions (Na+) out of the cell for every two potassium ions (K+) it brings in. Because it exports more positive charge than it imports, and because it sets up a high Na+ concentration outside and a high K+ concentration inside, the interior of the axon is held negative. The membrane is also more permeable to K+ than to Na+ at rest, so a little K+ leaks back out down its gradient, adding to the negative interior. The result is a membrane that is like a charged battery, storing the potential difference that an action potential will briefly release.

An action potential is a rapid, temporary reversal of the membrane potential that carries the signal. It begins only if a stimulus depolarises the membrane to the threshold potential of about −55 mV. Below threshold nothing happens; at or above threshold a full-sized action potential always fires — this is what 'all-or-nothing' means. Once threshold is reached, voltage-gated Na+ channels open and Na+ floods into the cell down its electrochemical gradient, driving the potential up to about +30 mV: this is depolarisation. Almost immediately the Na+ channels close and voltage-gated K+ channels open, so K+ flows out of the cell and the potential falls back toward rest: this is repolarisation. K+ channels are slow to close, so the membrane briefly overshoots to below the resting potential before the sodium–potassium pump restores the resting state. Crucially, a stronger stimulus does not make a bigger action potential — it makes action potentials fire more frequently.

An action potential is only useful if it travels. When one patch of membrane depolarises, the local flow of ions depolarises the neighbouring patch to its threshold, triggering a fresh action potential there. This regeneration repeats point by point, so the impulse moves along the axon as a self-renewing wave rather than fading out. It travels in one direction only because the region just behind the impulse is in its refractory period and cannot be re-excited. Myelination changes the speed dramatically. Where the axon is wrapped in fatty myelin the membrane is insulated and cannot depolarise, and the voltage-gated channels are concentrated only at the nodes of Ranvier. The action potential therefore only has to be regenerated at the nodes, and effectively jumps from node to node — this is saltatory conduction. Because there are far fewer regeneration steps over the same distance, a myelinated axon conducts much faster, and it also uses less ATP because fewer ions need to be pumped back afterwards.

Where a neuron meets the next cell there is a synapse: a tiny gap called the synaptic cleft separates the pre-synaptic axon terminal from the post-synaptic membrane. An electrical impulse cannot cross this gap, so the signal is passed chemically. When the action potential reaches the terminal it causes voltage-gated calcium channels to open, and Ca2+ ions flow in. This influx triggers vesicles of neurotransmitter to fuse with the pre-synaptic membrane and release their contents into the cleft. The neurotransmitter diffuses across the cleft and binds to specific receptors on the post-synaptic membrane. Binding opens ion channels in the post-synaptic membrane, depolarising it; if enough channels open to reach threshold, a new action potential is triggered in the next neuron. Because neurotransmitter is stored only in the pre-synaptic terminal and the matching receptors sit only on the post-synaptic membrane, transmission can occur in one direction only — this is what makes the whole nervous system's signalling directional.

A reflex is a rapid, automatic response that does not depend on conscious thought, and it is carried by a simple pathway called the reflex arc. A receptor detects the stimulus and passes the signal along a sensory neuron into the central nervous system. There a relay neuron connects, through synapses, to a motor neuron, which carries the impulse out to an effector such as a muscle or gland that produces the response. Because the signal can trigger the effector directly through the spinal cord without waiting for the brain to process it, reflexes are fast — which is exactly what is needed for protective responses such as pulling your hand away from something sharp.

Explain-type questions in C2.2 are marked analytically: each distinct, valid point is worth one mark, up to the total available. Method marks (M) credit a correct named step in the mechanism, answer marks (A) credit the correct outcome, and error-carried-forward (ECF) means one missing step does not cost you the marks that follow. The key is to write in separate, creditable statements — name each ion, structure and step — rather than in a single vague sentence. Study how each mark below is tied to one specific idea.

The ideas in C2.2 are the foundation for the rest of the animal-physiology material. The resting-potential and action-potential machinery reappears wherever cells signal electrically, from sensory receptors to muscle fibres. The chemical synapse is where many drugs, toxins and neurotransmitters act, so understanding release–diffusion–receptor binding sets you up to analyse how substances excite or inhibit a post-synaptic neuron. And the one-directional logic — refractory period along the axon, receptors only on the post-synaptic side at the synapse — is the reason nervous signals carry reliable, directional information at all.