Nervous activity involves the spread of electrical charge via the generation and propagation of action potentials. Communication between neurones is predominantly via chemical transmission at synapses, in which an impulse in one fibre leads to the release of neurotransmitter, which acts on an adjacent neurone. In the autonomic nervous system a presynaptic fibre may synapse with a single postsynaptic fibre, which then releases neurotransmitters to control the target tissue. Synaptic transmission ensures that nervous conduction is orthodromic, i.e. unidirectional. In the central nervous system (CNS) a neurone may make up to 10 000 synapses with other neurones, reflecting the complexity and integration of connections with the CNS.
General principles
The arrival of the action potential at the presynaptic terminal leads to depolarisation, which activates voltage-sensitive Ca2+ channels resulting in Ca2+ influx. The rise in intracellular Ca2+ then leads to exocytosis of neurotransmitter vesicles and the release of neurotransmitter into the synaptic cleft. The neurotransmitter then binds to receptors on the postsynaptic terminal, leading to activation or inhibition of the neurone. The activation often involves depolarisation, leading to an EPSP (excitatory postsynaptic potential), and inhibition often leads to hyperpolarisation with an IPSP (inhibitory postsynaptic potential). The neurotransmitter is then degraded or taken up by transport mechanisms (Figure 20.1).
Temporal and spatial summation of impulses
The post-synaptic events, whether excitatory or inhibitory, may be additive. For example, if two excitatory impulses are close in time (i.e. temporally close) then the EPSPs in the post-synaptic cell may combine, leading to greater depolarisation and so increasing the probability of the cell reaching threshold and initiating an action potential. This leads to an enhancement of excitation, termed temporal summation (Figure 20.2).
For a post-synaptic neurone that has multiple presynaptic inputs, then the EPSPs (or IPSPs) derived from different connections can summate (or act in opposition) and so determine whether the cell reaches threshold and sets up an action potential. This is termed spatial summation (Figure 20.2).
The consequence of these properties of summation mean that information may be reinforced by high-frequency stimulation and that there is integration information from the different inputs.
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