Sensivity and co ordination: the synapse
Synaptic transmission is the process by which an action potential in a presynaptic neurone leads to the transmission of a signal to a postsynaptic cell. Since action potentials cannot jump across the synaptic cleft, the signal is passed across by a chemical neurotransmitter. In outline, an action potential arriving at the presynaptic neurone causes it to release transmitter substance into the synaptic cleft. These molecules then diffuse across the cleft and may set up an action potential in the postsynaptic neurone.
The detailed sequence of events in synaptic transmission is as follows:
An action potential arrives at the synaptic bulb of the presynaptic neurone.
The depolarisation caused by the action potential opens voltage-gated calcium ion channels in the presynaptic membrane.
Calcium ions diffuse into the cytoplasm of the synaptic bulb down their electrochemical gradient.
The influx of calcium ions triggers the movement of synaptic vesicles containing neurotransmitter (such as acetylcholine in a cholinergic synapse) towards the presynaptic membrane.
These vesicles fuse with the presynaptic membrane and release their contents (the neurotransmitter) into the synaptic cleft by exocytosis.
The neurotransmitter molecules diffuse across the synaptic cleft. This diffusion is rapid because the distance is very small, usually less than 0.5 ms.
On the postsynaptic membrane, there are receptor proteins with a complementary shape to the neurotransmitter molecules. The neurotransmitter binds to these receptors, causing a change in the shape of the receptor protein.
In the case of a cholinergic synapse, the binding of acetylcholine opens chemical-gated sodium ion channels in the postsynaptic membrane.
Sodium ions rush into the cytoplasm of the postsynaptic neurone, causing depolarisation of the membrane.
If this depolarisation is large enough to reach the threshold potential, it will trigger an action potential in the postsynaptic neurone.
To prevent continuous firing, the neurotransmitter is quickly removed from the synaptic cleft. In cholinergic synapses, the enzyme acetylcholinesterase (AChE) in the synaptic cleft breaks down acetylcholine into acetate and choline.
The choline is reabsorbed into the presynaptic neurone, where it is used to resynthesise acetylcholine, requiring energy from ATP supplied by mitochondria. The acetylcholine is then transported back into synaptic vesicles.
The structure of a cholinergic synapse includes the following key components:
Presynaptic neurone: The neurone that transmits the action potential towards the synapse. Its axon terminal ends in a swelling called a synaptic bulb. The presynaptic membrane of this bulb contains voltage-gated calcium ion channels. The cytoplasm of the synaptic bulb contains mitochondria (to provide ATP) and synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).
Synaptic cleft: A very small gap, usually about 20 nm wide, between the presynaptic and postsynaptic neurones. This cleft contains the enzyme acetylcholinesterase.
Postsynaptic neurone: The neurone that receives the signal. Its membrane, adjacent to the synaptic cleft, is called the postsynaptic membrane. This membrane contains receptor proteins that are specific for acetylcholine, and these receptors are linked to chemical-gated sodium ion channels. In a neuromuscular junction, the postsynaptic membrane is the plasma membrane of a muscle fibre.
Synapses play several crucial roles in the nervous system:
Ensuring one-way transmission: Signals can only pass in one direction at synapses, from the presynaptic to the postsynaptic neurone. This is because only the presynaptic neurone releases neurotransmitter, and only the postsynaptic neurone has the receptors for it. This directionality ensures that signals are directed along specific pathways rather than spreading randomly.
Interconnecting nerve pathways: Synapses allow for a wider range of behaviour by enabling convergence and divergence of neural pathways.
Convergence: Many different presynaptic neurones can converge on a single postsynaptic neurone. Whether the postsynaptic neurone fires an action potential depends on the summation of all the signals it receives.
Divergence (Dispersal of impulses): The axon of a presynaptic neurone may branch to synapse with many postsynaptic neurones, allowing a signal to spread to multiple destinations.
Neural summation: Synapses allow for the cumulative effect of multiple impulses. A postsynaptic neurone may not reach the threshold and fire an action potential if it receives only a weak or infrequent signal from a single presynaptic neurone. However, if it receives simultaneous or rapidly successive signals from multiple presynaptic neurones (spatial summation and temporal summation), the resulting depolarisation can be sufficient to trigger an action potential.
Inhibition: Some synapses are inhibitory. They release neurotransmitters that cause hyperpolarisation of the postsynaptic membrane, making it less likely to generate an action potential. This allows for fine control of neural activity.
Filtering out impulses: Synapses can filter out low-frequency or weak stimuli, preventing the central nervous system from being overwhelmed by unimportant sensory information.
Memory and learning: Synapses are believed to be involved in memory and learning. The strength and number of synaptic connections can change over time in response to experience, leading to the formation of new neural pathways and the modification of existing ones.
Modulation by drugs and toxins: The process of synaptic transmission can be affected by various drugs and toxins, which can mimic or block the action of neurotransmitters, affect their release or breakdown, or alter the sensitivity of postsynaptic receptors.