Describe nervous control, including the structure of a neuron, the generation of action potentials, synaptic transmission and the reflex arc

What this dot point is asking

QCAA wants you to describe how neurons signal: their structure, how an action potential travels along an axon, how the signal crosses a synapse, and how a reflex arc gives a rapid involuntary response. Diagrams of neurons and reflex arcs are common stimulus.

The answer

The nervous system is the body's fastest control system. It uses electrical signals along neurons and chemical signals across synapses to coordinate rapid responses.

Structure of a neuron

A neuron is a specialised cell built for long-distance signalling.

• Dendrites. Branching extensions that receive signals from other neurons.
• Cell body (soma). Contains the nucleus and most organelles. Integrates incoming signals.
• Axon. Long extension carrying the action potential away from the soma. Up to a metre long in motor neurons supplying the foot.
• Axon terminals (synaptic knobs). End of the axon, where the signal is passed to the next cell via a synapse.
• Myelin sheath. Lipid-rich layer wrapped around the axon by Schwann cells (peripheral nervous system) or oligodendrocytes (central nervous system). Insulates the axon.
• Nodes of Ranvier. Gaps in the myelin sheath where the action potential is regenerated.

Three functional types of neuron:

• Sensory neurons. Carry signals from receptors to the CNS.
• Interneurons. Within the CNS; integrate signals.
• Motor neurons. Carry signals from the CNS to effectors (muscles, glands).

The resting potential

At rest, the inside of the neuron is around 70 mV more negative than the outside.

• The sodium-potassium pump exports 3 Na+ for every 2 K+ imported, using ATP.
• The membrane is more permeable to K+ at rest, and K+ leaks out down its gradient, leaving the inside negative.
• Large negatively charged proteins inside the cell also contribute.

The action potential

If a stimulus depolarises the membrane past the threshold (around minus 55 mV), an action potential is triggered. The response is all-or-nothing: once threshold is crossed, the action potential goes ahead at full size; below threshold, nothing happens.

1. Depolarisation. Voltage-gated sodium channels open. Na+ rushes in, raising the membrane potential to about plus 30 mV.
2. Repolarisation. Voltage-gated sodium channels inactivate. Voltage-gated potassium channels open. K+ flows out, bringing the potential back toward minus 70 mV.
3. Hyperpolarisation. Potassium channels close slowly. The membrane briefly overshoots below the resting potential.
4. Restoration. The sodium-potassium pump re-establishes the ion gradients (over milliseconds).

The action potential propagates along the axon because depolarisation at one point triggers depolarisation of the adjacent membrane. In myelinated axons, the action potential jumps from node to node (saltatory conduction), greatly increasing speed (up to 120 m per second in large myelinated fibres).

The refractory period is the brief interval immediately after an action potential when the membrane cannot fire another one. It ensures one-way travel along the axon.

Synaptic transmission

A synapse is the junction between two neurons (or between a neuron and an effector). Most synapses in mammals are chemical.

1. The action potential arrives at the axon terminal.
2. Voltage-gated calcium channels open. Ca2+ enters the terminal.
3. Synaptic vesicles fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft by exocytosis.
4. Neurotransmitter molecules diffuse across the cleft and bind receptors on the postsynaptic membrane.
5. The postsynaptic membrane depolarises (excitatory) or hyperpolarises (inhibitory) depending on the receptor type.
6. Neurotransmitter is removed (reuptake, enzymatic breakdown or diffusion) to end the signal.

Examples of neurotransmitters: acetylcholine (neuromuscular junction), glutamate (main excitatory in CNS), GABA (main inhibitory in CNS), dopamine, serotonin, noradrenaline.

The reflex arc

A reflex arc is the neural pathway responsible for a reflex: a rapid, involuntary response to a specific stimulus. The classic example is the patellar (knee-jerk) reflex.

Five components.

1. Receptor. Detects the stimulus (muscle spindle in the quadriceps for the knee-jerk).
2. Sensory neuron. Carries the signal from the receptor to the spinal cord.
3. Integration centre. A single synapse in the spinal cord for monosynaptic reflexes (knee-jerk); typically an interneuron in more complex reflexes (withdrawal from a hot object).
4. Motor neuron. Carries the signal from the spinal cord to the effector.
5. Effector. The muscle or gland that produces the response (quadriceps contracts and the leg kicks).

Reflexes are faster than conscious actions because the brain is bypassed; the spinal cord directly initiates the response. The brain is informed of the action only after it has begun.

Common traps

Treating action potentials as graded. They are all-or-nothing once threshold is crossed.

Forgetting calcium at the synapse. Calcium entry is the trigger for neurotransmitter release; sodium and potassium drive the action potential, but calcium handles the synapse.

Calling the synapse electrical. Most vertebrate synapses are chemical; electrical synapses exist but are uncommon and rarely assessed at QCE level.

Skipping the receptor or effector in a reflex arc. Mark schemes require all five components.

Cross-link to Year 12 assessment

The neuron and reflex arc framework is the canonical "rapid control system" complement to the slower endocrine system in Unit 2. The action potential mechanism reappears as the example in EA short-response questions on excitable cells; receptor-mediated signalling foreshadows the cell signalling required for innate and adaptive immunity (see innate and adaptive immunity).

In one sentence

Neurons signal by propagating all-or-nothing action potentials (Na+ in, then K+ out) along their axons, releasing neurotransmitter at chemical synapses to excite or inhibit the next cell, and reflex arcs use a receptor, sensory neuron, integration centre in the spinal cord, motor neuron and effector to produce rapid involuntary responses that bypass the brain.