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Signalling between neurons - Coggle Diagram
Signalling between neurons
Multiple neurons communicate with one another in three-dimensional space and not just in a linear fashion. Each neuron can form up to 15 000 connections with other neurons.
Neuronal communication is extremely complex with vast amounts of processing occurring simultaneously.
The term presynaptic refers to the neuron that is sending out the information and postsynaptic refers to the neuron that receives the information. However, because neurons operate in a complex network, a neuron can generally be described as both presynaptic and postsynaptic depending on where it is in the chain of information.
The passage of information across the synapse is known as synaptic transmission.
Synapses
chemical synapses as these are the most common type of synapse in the CNS and are of most relevance to your understanding of mental health conditions.
When the action potential reaches the axon terminal it can do one of two things, depending on the nature of the synapse:
at electrical synapses, the electrical signal simply passes from one neuron to the next via special channels known as gap junctions
at chemical synapses, the action potential triggers the release of neurotransmitter that influences the postsynaptic neurons.
Axon terminal - Enlarged zone at the end of an axon that forms the synapse with the postsynaptic neuron
neurotransmitters are released from the axon terminals.
Synaptic vesicles
packages that contain molecules of neurotransmitter ready to be released in response to the arrival of an action potential at the axon terminal.
Synaptic cleft - also known as synaptic gap. a small gap between the presynaptic and postsynaptic neuron
the point at which one neuron will communicate with another via the release of neurotransmitters.
Neurotransmitter molecules
chemicals that are released from the axon terminal of a neuron into the synaptic cleft and bind to receptors on the postsynaptic neuron.
Postsynaptic receptors - proteins found on the membrane of the postsynaptic neuron
neurotransmitters bind to these receptors to effect a change in level of activity in the postsynaptic neuron.
Neurotransmitters
Neurotransmitters are synthesised either in the axon terminal or in the cell bodies of neurons after which they are transported to the axon terminal. They are then packaged into synaptic vesicles ready to be released in response to an action potential.
Neurons are often referred to by the neurotransmitter that they synthesise and release, for example a neuron that releases dopamine would be called a ‘dopaminergic neuron’.
Neurotransmitters are the key signalling molecules at a synapse, there are also chemicals that act as neuromodulators. These are also released into the synaptic cleft where they influence the responsiveness of postsynaptic neurons.
Neurotransmitters
Gamma-aminobutyric acid (GABA) and glutamate are the ‘workhorses’ of the nervous system. GABA always has an inhibitory effect on neurons whereas glutamate is nearly always excitatory.
The catecholamines, which include dopamine and noradrenalin, are another important category in the field of mental health.
Serotonin (sometimes known as 5-hydroxytryptamine or 5-HT) also features strongly in any discussion of mental health.
Synaptic transmission
Neurotransmitters are synthesised in neurons and packaged into synaptic vesicles in the axon terminal and that they can be released from the axon terminal in response to an action potential.
The size of the action potential never changes; rather the frequency of action potentials, or the number of action potentials produced in a given time period is all that can change. This is an important principle of the nervous system, that is, the change in the frequency of action potential firing is the ‘language’ of the nervous system.
Neurotransmitter molecules bind to receptors on the postsynaptic membrane and this results in electrical changes in the postsynaptic neuron which may have an excitatory or an inhibitory effect. These changes are mediated by the movement of ions through special channels, either directly or indirectly.
The effect on postsynaptic neurons is known as either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP). An EPSP means that it is more likely that an action potential will be triggered and an IPSP means it is less likely that an action potential will be triggered.
Whether a neurotransmitter results in an EPSP or an IPSP depends on the neurotransmitter and the properties of the receptor on the postsynaptic neuron. Some neurotransmitters, such as GABA, always have an inhibitory effect on the postsynaptic neuron. Other neurotransmitters, such as dopamine, can have an excitatory or an inhibitory effect. The reason for this is that there are different subtypes of dopamine receptors in different brain regions; some are excitatory, and some are inhibitory.
Synaptic integration
Synaptic integration - the postsynaptic neuron performs a type of computational processing of all the information by summing together the EPSPs and IPSPs. Based on this, the neuron may increase its firing rate of action potentials, decrease its firing rate of action potentials or there may be no change.
While neurotransmitters have an influence on the postsynaptic neuron, they can also influence the presynaptic neuron by binding to autoreceptors. These autoreceptors are found on the axon terminals and neurotransmitters bind to them in the same type of lock and key arrangement as that of the postsynaptic receptors. Binding of neurotransmitter to autoreceptors decreases the amount of neurotransmitter that is released and this therefore acts as a negative feedback loop. Neurotransmitters can therefore regulate their own release but there are also processes that help to quickly remove neurotransmitters from the synaptic cleft
Removal of neurotransmitter
There are various ways that neurotransmitter can be removed from synapses:
Neurotransmitters can be taken back up into the presynaptic axon terminal via reuptake channels in a process known as synaptic reuptake. At this point they are then broken down into their constituent molecules, which can be reused in the synthesis of new neurotransmitter molecules.
Enzymes are present in the synaptic cleft that can break down neurotransmitters in a process known as enzymatic degradation.
Neurotransmitters can also diffuse away from the synapse and be taken up into astrocytes where they are metabolised into their constituent molecules. These molecules are then shuttled back to the axon terminals of neurons, ready to be converted back into neurotransmitters.
These mechanisms are important, partly because excess neurotransmitter can be toxic to neurons, but also because signals between neurons need to be transient so that the brain can operate as a cohesive, continually responsive network.