BY4: Synapses
Neurones are not in direct contact with each other but are separated by tiny gaps known as synapses. The main role of the synapse is to convey action potentials between neurones. A range of drugs function by interfering with the neurotransmitters involved in synaptic transmission.
The structure of the synapse
Most junctions between neurones take the form of chemical synapses. Branches of axons lie close to dendrites of other neurones but do not touch; there is a gap of about 20um between them. When impulses are transmitted, this gap is crossed by the secretion of a neurotransmitter from the axon membrane (pre-synaptic membrane), which diffuses across the space to stimulate the dendritic membrane (post-synaptic membrane).
The arrival of an action potential at the pre-synaptic knob (pre-synaptic bulb) stimulates voltage gated Calcium (Ca2+) ion channels to open, allowing Ca2+ ions to enter the pre-synaptic bulb. The influx of calcium ions causes the synaptic vesicles to fuse with the pre-synaptic membrane so releasing the neurotransmitter (e.g. acetylcholine) into the synaptic cleft. When the neurotransmitter diffuses across the gap (synaptic cleft), it attaches to a receptor site on the post-synaptic membrane, depolarising it and so initiating an action potential and nerve impulse in the next neurone. The postsynaptic membrane contains specific protein receptors with which the neurotransmitter molecules combine. Once combined, Na+ ion channels open in the membrane, allowing Na+ ions to diffuse from the cleft into the postsynaptic neurone. If the membrane becomes sufficiently depolarised, an action potential is initiated in the axon of the postsynaptic neurone (an action potential will generated at the "axon hillock" where the cell body extends to form the axon, if enough Na+ ions diffuse to this point from the cell body to depolarise the membrane potential to -55mV threshold: all the Na+ ion channels at this point will then open allowing rapid influx of Na+ ions into the axon and generating an action potential = ALL OR NOTHING law).
Acetylcholine, when released, is quickly destroyed by enzymes in the synaptic cleft, so its effect is limited and the merging of impulses is prevented. If insufficient acetylcholine is released, the postsynaptic membrane will not be stimulated. The enzyme which destroys acetylcholine is called cholinesterase. The resulting choline and ethanoic acid diffuse back across the synaptic cleft to reform acetylcholine inside the pre-synaptic bulb. ATP is required to re-form transmitter molecules and store them in vesicles.
Another transmitter substance is noradrenaline. It occurs, together with acetylcholine, in the involuntary nervous system.
The function of the synapse is to convey action potentials between neurones.
• Transmit information between neurones
• Synapses act as junctions where nerve impulses pass in one direction only (from pre-synaptic neurone to post-synaptic neurone): since synaptic vesicles are present only in the bulb of the presynaptic neurone and receptors for neurotransmitters are located only on the post-synaptic membrane
• Filter out low level stimuli. That is, remove ‘background noise’ from the nervous system. If not enough neurotransmitter is released from the pre-synaptic bulb into the synaptic cleft (then not enough neurotransmitter will bind to the receptors on the post-synaptic membrane and not enough Na+ ion channels will open to allow the membrane potential to get to -55mV). These low level stimuli do not cause nerve impulses to progress to the post-synaptic neurone.
• To protect the response system from over-stimulation
Interesting video (not on spec but worth a listen):