Synaptic transmission 1

The nervous system transmits impulses throughout the body. Neurones are linked with other neurones and with effectors through specialised junctions called synapses. Neurones do not actually make contact with their target cell. They are seperated by a narrow gap called the synaptic cleft.
Except for a few specialised electrical synapses, information passes across the synapse in the form of chemicals called neurotransmitters. Different neurones release different types of neurotransmitters, which may stimulate or inhibit the activity of the target cell.
Excitatory presynaptic cells release neurotransmitters that decrease the mebrane potential of the target cell, making it more excitable and, if it is another neurone, more likely to generate nerve impulses
Inhibitory presynaptic cells release neurotransitter that increase the membrane potential, making the target cell less excitable and less likely to generate nerve impulses
Excitatory synapses; a cholinegenic synapes uses acteylcholine as the neurotransmitter. Acetylcholine is synthesised within the synaptic bulb and stored in special organeles called synaptic vesicles. When an action potential reaches the presynaptic membrane it depolarises the membrane, that is, it makes the mebrane less negative than at rest. This depolarization triggers the opening of calcium ions diffuse into the synaptic bulb, causing the vesicales containing acetylecholine to migrate and fuse with the presynaptic membrane. Acetylechline is released into the synaptic cleft and diffuses across the synapse. Then it binds to specific protein receptor molecules on the postsynaptic membrane, a process known as receptor activation.
Inhibitory synapses. Receptor activation by inhibitary neurotransmitters causes other effects on postsynaptic membranes. Usually it opens chloride ion channels which makes the postsynaptic membrane more negative than normal and less likely to depolarise suffieciently to genrate an action potential
Recycling the neurotransmitter. After a neurotransmitter has affected a postsynaptic membrane, two important processes take place; 1. Enzymes break down the neurotransmitter molecules in the synaptic cleft, 2. Enzymes bring about the synthesis of neurotransmitter to refil the vesicles within the neurone from which they were orgianally released. the breakdown of neurotransmitter prevents further, unwanted effects. Acetylecholine, for example, dissociates from its receptor and is broken down by the enzyme acetylchlinesterase. The breakdown products diffuse back into the synaptic bulb where they are resynthesised into acetylecholine, using energy from ATP. The high density of mitrochondria in the synapitic bulbs ensures that there is plenty of ATP available for the synthesis of the neurotransmitter
A typical postsynaptic cell recieves information from 100's or even 1000's of presynatic neurones. The numerous synaptic connections allow the cell to combine different sources of information before responding. It reponses will depend on the sum of all the excitatory and inhibitory postsynaptic potentials produced by spatial and temporal summation
Spatial summation occurs when a single synapse does not release enough neurotransmitter to start an action potential on its own. But an action potential is fired when sufficient neurotransmitter builds up from several different synapses acting together. In this way the graded potentials produced by these several synapses can combine trigger an action potential
In temporal summation, a postsynaptic membrane fails to generate an action potential after a single impulse reaches the presynaptic membrane, but does so when two or more impulses arrive in quick succession from the same synapse. In this case, graded potentials produced by successive impulses add together, generating an action potential in the postsynapitic neurone
Transmission of nerve impulses through chemical synapses has several advantages; it enables information from different parts of the nervous system to be integrated, it provides a mechanism for filtering out trivial or non-essential information, it ensures nerve impulses are unidirectionals, passing only from presynaptic membranes to postsynaptic membranes, it allows the synapses to act as switches, so that nerve impulses can pass along one of the several seperate pathways in the nervous system

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Synaptic transmission 1

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