The Nervous System: Synapses and Action Potentials
The Nervous System
The nervous system allows rapid communication between different parts of the body. It consists of the central nervous system (CNS) — the brain and spinal cord — and the peripheral nervous system (PNS) — sensory and motor neurones connecting the CNS to receptors and effectors.
Neurone Structure
A typical motor neurone has:
- Cell body — contains the nucleus and most organelles; located in the CNS
- Dendrites — short, branching extensions that receive impulses from other neurones
- Axon — a long, thin extension that carries impulses away from the cell body toward the effector
- Myelin sheath — a fatty insulating layer formed by Schwann cells wrapping around the axon. Myelin is composed of multiple layers of Schwann cell membrane (rich in lipid). Between adjacent Schwann cells are gaps called nodes of Ranvier
- Axon terminal (synaptic knob) — the swollen end of the axon that forms a synapse with the next cell
The Resting Potential
When a neurone is not transmitting an impulse, a resting potential of approximately -70 mV exists across its membrane (the inside is negative relative to the outside). This is maintained by:
1. The sodium-potassium pump (Na⁺/K⁺ ATPase) — actively transports 3 Na⁺ out and 2 K⁺ in for each ATP hydrolysed. This creates a net movement of positive charge OUT of the cell.
2. Potassium leak channels — the membrane is much more permeable to K⁺ than to Na⁺ at rest. K⁺ diffuses out of the cell down its concentration gradient (there is more K⁺ inside), making the inside more negative.
3. Large organic anions (proteins, nucleic acids) inside the cell contribute negative charge and cannot leave.
The result is a polarised membrane — the inside is negative, the outside is positive.
The Action Potential
An action potential is a rapid, temporary reversal of the membrane potential that travels along the axon as a nerve impulse.
Stages of the Action Potential
1. Resting state (-70 mV):
- Voltage-gated Na⁺ and K⁺ channels are closed
- The Na⁺/K⁺ pump maintains the resting potential
2. Depolarisation (stimulus):
- A stimulus (e.g. from a receptor or another neurone) causes some Na⁺ channels to open
- Na⁺ ions rush into the cell down their electrochemical gradient
- The membrane potential becomes less negative (moves toward 0 mV)
3. Threshold (-55 mV):
- If depolarisation reaches the threshold potential (~-55 mV), many more voltage-gated Na⁺ channels open in a positive feedback loop
- This is the all-or-nothing principle: if threshold is reached, a full action potential fires; if not, no impulse is generated. There is no partial response.
4. Rapid depolarisation (to +40 mV):
- Massive influx of Na⁺ → the membrane potential rapidly reverses, reaching approximately +40 mV
- The inside of the cell becomes positive relative to the outside
5. Repolarisation:
- Na⁺ channels close (inactivate) after ~1 ms
- Voltage-gated K⁺ channels open (they open more slowly than Na⁺ channels)
- K⁺ ions rush out of the cell, restoring the negative internal charge
- The membrane potential returns toward -70 mV
6. Hyperpolarisation (undershoot):
- K⁺ channels are slow to close, so slightly too much K⁺ leaves
- The membrane potential briefly dips below -70 mV (to about -80 mV)
- This is the hyperpolarisation or undershoot
7. Restoration of resting potential:
- K⁺ channels close
- The Na⁺/K⁺ pump restores the ion concentrations to their resting levels
- The membrane returns to -70 mV
Refractory Periods
| Period | Duration | Na⁺ channels | Can another AP fire? | Significance |
|---|---|---|---|---|
| Absolute refractory | ~1-2 ms | Inactivated (cannot reopen) | No — no matter how strong the stimulus | Ensures action potentials are discrete events; limits maximum firing frequency; ensures unidirectional propagation |
| Relative refractory | ~3-4 ms | Recovering; some can open | Yes — but only with a stronger-than-normal stimulus | Allows frequency coding of stimulus intensity |
Propagation of the Action Potential
In Unmyelinated Neurones
The action potential at one point on the membrane depolarises the adjacent region, triggering voltage-gated Na⁺ channels to open there. This creates a wave of depolarisation that travels along the axon. The refractory period behind the active zone ensures the impulse travels in one direction only.
Speed: approximately 0.5-2 m/s
In Myelinated Neurones: Saltatory Conduction
The myelin sheath is an electrical insulator — ions cannot flow across the membrane where myelin is present. Voltage-gated Na⁺ channels are concentrated at the nodes of Ranvier (gaps between Schwann cells, ~1 μm wide, spaced ~1 mm apart).
The action potential "jumps" from one node to the next — this is called saltatory conduction (from the Latin saltare, to jump).
Advantages:
- Much faster — up to 120 m/s (compared to 0.5-2 m/s in unmyelinated fibres)
- More energy efficient — fewer ions cross the membrane (only at nodes), so the Na⁺/K⁺ pump uses less ATP
Multiple sclerosis (MS) is a disease in which the immune system attacks the myelin sheath, disrupting saltatory conduction and impairing nerve function.
Factors Affecting Speed of Conduction
| Factor | Effect | Explanation |
|---|---|---|
| Myelination | Faster | Saltatory conduction |
| Axon diameter | Larger = faster | Less electrical resistance to ion flow inside the axon |
| Temperature | Higher = faster (up to a point) | Increased kinetic energy of ions; faster enzyme activity (Na⁺/K⁺ pump) |
Synapses
A synapse is the junction between two neurones (or between a neurone and an effector). Most synapses are chemical synapses — they use a neurotransmitter to transmit the signal across a gap (the synaptic cleft, ~20-40 nm wide).
Structure of a Cholinergic Synapse
A cholinergic synapse uses acetylcholine (ACh) as the neurotransmitter.
- Presynaptic knob — contains many synaptic vesicles filled with ACh, plus many mitochondria (for ATP production) and voltage-gated Ca²⁺ channels
- Synaptic cleft — the gap between pre- and postsynaptic membranes
- Postsynaptic membrane — contains nicotinic ACh receptors (which are also Na⁺ ion channels) and the enzyme acetylcholinesterase (AChE) bound to the membrane
Synaptic Transmission
1. An action potential arrives at the presynaptic knob
2. Depolarisation opens voltage-gated Ca²⁺ channels in the presynaptic membrane
3. Ca²⁺ ions diffuse into the presynaptic knob down their concentration gradient
4. Ca²⁺ causes synaptic vesicles to move to and fuse with the presynaptic membrane (exocytosis)
5. ACh is released into the synaptic cleft
6. ACh diffuses across the cleft and binds to nicotinic receptors on the postsynaptic membrane
7. These receptors are ligand-gated Na⁺ channels — ACh binding causes them to open
8. Na⁺ ions flood into the postsynaptic cell, causing depolarisation (an excitatory postsynaptic potential, EPSP)
9. If the EPSP reaches threshold, an action potential is generated in the postsynaptic neurone
Removal of Neurotransmitter
ACh must be rapidly removed to prevent continuous stimulation:
- Acetylcholinesterase (AChE) in the synaptic cleft hydrolyses ACh into choline and acetyl (ethanoic acid)
- Choline is taken back into the presynaptic knob by active transport and recycled to make new ACh (using acetyl CoA and choline acetyltransferase)
- This ensures the synapse is reset and ready for the next impulse
Summation
A single vesicle release may not produce enough depolarisation to reach threshold. Summation can amplify the signal:
- Temporal summation — many impulses arrive at the same presynaptic knob in rapid succession; the EPSPs add together before they decay
- Spatial summation — impulses arrive simultaneously at several presynaptic knobs synapsing on the same postsynaptic neurone; the EPSPs from different synapses add together
Inhibitory Synapses
Some synapses are inhibitory — the neurotransmitter (e.g. GABA) opens Cl⁻ channels or K⁺ channels on the postsynaptic membrane, making the inside more negative (hyperpolarised). This produces an inhibitory postsynaptic potential (IPSP), making it harder for the cell to reach threshold.
The postsynaptic neurone integrates all EPSPs and IPSPs arriving simultaneously. An action potential fires only if the net effect reaches threshold.
Exam Tips
- AQA expects you to describe the stages of an action potential in order, with correct voltage values (resting -70 mV, threshold -55 mV, peak +40 mV)
- Always explain the all-or-nothing principle — the size of the action potential is always the same; stimulus intensity is coded by frequency of action potentials
- For synaptic transmission, remember the sequence: AP arrives → Ca²⁺ enters → vesicles fuse → ACh released → binds receptors → Na⁺ channels open → depolarisation
- Explain why synapses are unidirectional — receptors are only on the postsynaptic membrane; vesicles are only in the presynaptic knob
- Know that drugs and toxins affect synapses — e.g. nerve agents inhibit AChE (ACh accumulates, continuous stimulation); curare blocks ACh receptors (paralysis)