Synaptic transmission

Where it happens

Synaptic transmission takes place at a synapse, the junction between the end of one neurone and the next neurone, or between a neurone and an effector such as a muscle. A tiny gap, the synaptic cleft, separates the two membranes.

The synaptic knob is a swollen ending of the axon packed with mitochondria and synaptic vesicles. The mitochondria supply the ATP needed to make neurotransmitter and to reload the vesicles after each impulse.

The membrane facing the cleft is the presynaptic membrane; the membrane of the next cell, carrying the receptor proteins, is the postsynaptic membrane. When the postsynaptic cell is a muscle fibre, the junction is called a neuromuscular junction and works the same way, with the muscle contracting instead of a new impulse being fired.

Inputs and outputs

• Input: a nerve impulse (action potential) arriving at the synaptic knob of the presynaptic neurone.
• Input: calcium ions, which diffuse into the knob when the impulse arrives and trigger vesicle movement.
• Input: neurotransmitter, such as acetylcholine, stored in synaptic vesicles.
• Input: ATP from the mitochondria in the knob, used to make neurotransmitter and refill vesicles.
• Output: neurotransmitter released into the synaptic cleft and bound to receptors on the postsynaptic membrane.
• Output: a new impulse in the postsynaptic neurone, or contraction of a muscle fibre; the neurotransmitter is then broken down and its fragments reabsorbed.

The steps

1. A nerve impulse travels along the axon and arrives at the synaptic knob of the presynaptic neurone.
2. The impulse makes the presynaptic membrane more permeable to calcium ions, which diffuse into the knob.
3. The calcium ions cause synaptic vesicles to move towards and fuse with the presynaptic membrane.
4. The vesicles release neurotransmitter into the synaptic cleft by exocytosis.
5. The neurotransmitter diffuses across the cleft, a journey that takes a fraction of a millisecond.
6. Neurotransmitter molecules bind to specific receptor proteins on the postsynaptic membrane.
7. Binding opens ion channels, changing the charge across the postsynaptic membrane; if enough neurotransmitter binds, a new impulse is generated in the postsynaptic neurone.
8. An enzyme in the cleft breaks the neurotransmitter down; the fragments are reabsorbed into the synaptic knob and rebuilt into neurotransmitter using ATP, so the synapse is ready for the next impulse.

Why it matters and how it is controlled

Synaptic transmission lets impulses pass between separate neurones that are not physically joined, and allows one neurone to branch and connect to a whole network of other neurones. Because only the presynaptic membrane has vesicles of neurotransmitter and only the postsynaptic membrane has receptors, an impulse can only cross a synapse in one direction.

Synapses are the control points of the nervous system. Because the postsynaptic neurone fires only when enough neurotransmitter binds, weak or stray impulses are filtered out.

Impulses from different neurones arriving at the same synapse can add together (summation) to reach the threshold, and some neurotransmitters make the postsynaptic neurone less likely to fire rather than more, which is how the brain stops a reflex when it chooses to. The quick breakdown of neurotransmitter by enzyme keeps each signal short and prevents the next neurone from firing repeatedly from a single impulse.

Drugs and toxins that block receptors or the enzyme show how tightly this step is regulated.

How it is examined

You may be asked to describe the sequence of events at a synapse using a labelled diagram, to explain why transmission across a synapse is one-way, or to explain why synaptic transmission is slightly slower than transmission along an axon.

The diagram question is the classic form: a labelled synapse with the synaptic knob, vesicles, mitochondria, cleft and postsynaptic membrane, and you must name the parts and describe the sequence in order. A second form gives data, for example, the effect of removing calcium ions from the surrounding fluid, and asks you to predict and explain what happens to transmission.

Comparison questions set synaptic transmission against transmission along the axon, expecting you to state that one is chemical and slower, the other electrical and faster.

Common misconceptions

Worked exam-style question

Question. The diagram shows a synapse between two neurones. Structure X is a membrane-bound sac inside the synaptic knob; structure Y is a bean-shaped organelle beside it.

(a) Name structures X and Y and state the function of each. (b) Describe how an impulse arriving at the synaptic knob leads to a new impulse in the next neurone.

(c) A toxin prevents the enzyme in the synaptic cleft from working. Explain the effect on the postsynaptic neurone.

(d) Explain why transmission across a synapse can only occur in one direction.

Model answer. (a) X is a synaptic vesicle, which stores neurotransmitter; Y is a mitochondrion, which supplies ATP for making neurotransmitter and refilling vesicles. (b) The impulse causes calcium ions to enter the knob; vesicles move to and fuse with the presynaptic membrane, releasing neurotransmitter by exocytosis.

The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane, opening ion channels and generating a new impulse. (c) Neurotransmitter is not broken down, so it stays bound to the receptors and the postsynaptic neurone keeps firing repeatedly; if the effector is a muscle, it stays contracted.

(d) Only the presynaptic knob has vesicles of neurotransmitter and only the postsynaptic membrane has receptors, so the chemical signal can pass in one direction only.

Source:SRC-DSKP-EN

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