Hello, dear reader. This post is a sequel to my previous post on Neurotoxin, Nerves, Neurons and Impulses #2 where I discussed the action potential, depolarisation, repolarisation, and the disease of the nerves. In today’s post, I will extensively give an insight into the communication between neurons i.e talking about the synapse.
When an action potential reaches the end of an axon, it is passed on to the next neuron, or on to an effector cell such as a muscle or gland. The axon of one neuron does not usually make direct contact with the cell body of the next; the two cells are separated by a gap called a synapse.
The cell that carries a signal towards a synapse is a presynaptic cell; the cell carrying the signal away from the synapse is a postsynaptic cell. Presynaptic cells are always neurons but postsynaptic cells can be either neurons or effector cells.
The axon terminal of a presynaptic neuron is swollen, and is often called the synaptic bulb or synaptic knob. It meets the cell body of the next axon, leaving a gap or synaptic cleft of about 20 nm. This is small – you would have to split a hair 250 times to get it to fit sideways into this gap. Even so, synapses have a high electrical resistance, and this gap is too big to allow the action potential to simply jump from one neuron to the next.
So how does the action potential get across? It is relayed by chemicals that diffuse across the gap and initiate an action potential in the neuron at the other side. The synaptic bulb contains many mitochondria, which provide energy for the manufacture of chemicals called neurotransmitters.
Neurotransmitters are small molecules and they can diffuse easily across the synaptic cleft. Synaptic vesicles are temporary vacuoles (membrane-bound spheres) that store neurotransmitter chemicals, the most common being acetylcholine. Synapses that have acetylcholine as their transmitter are called cholinergic synapses.
Hundreds of neurotransmitters have been identified and there are certainly more to find. There are four main groups:
Acetylcholine also acts throughout the brain, modifying the activity of other neurotransmitters. Nerve pathways in which acetylcholine is a neurotransmitter seem to be involved in motivation and memory. A very fast-acting enzyme called acetylcholinesterase breaks down acetylcholine into acetic acid and choline. These substances are reabsorbed through the presynaptic membrane. ATP energy from mitochondria is used to resynthesise acetylcholine, which is then returned to the vesicles. The chemicals in some ‘nerve gases’ work by inhibiting acetylcholinesterase. Many drugs and poisons exert their effect because they interfere with the functioning of synapses. And, in some people, release of too much noradrenaline causes the heart to race. One way of treating this is to use drugs known as beta-blockers. These drugs have molecular shapes similar to noradrenaline. Nerve gases used in war contain organophosphates that block the enzyme acetylecholinesterase, disrupting transmission of nerve impulses at synapses. Many insecticides contain organophosphates, which is why they are so dangerous to people working in agriculture. If exposed to insecticides in large quantities, nerve and brain damage can result.
As you read the next section, follow the stages of chemical transmission at a synapse numbered in the figure below:
Does the arrival of an impulse at a synapse mean that an action potential is always generated on the postsynaptic neuron? The answer is no, because it would lead to chaos; all neurons would automatically pass on the signal to others. The significance of synapses is that they allow us to select particular pathways. Thus, at any one time, many more synapses need inhibiting than need stimulating. For this reason there are inhibitory synapses. Impulses arriving at these synapses make it more difficult for an action potential to be generated.
The neurotransmitters made by inhibitory synapses open potassium and chloride channels rather than sodium channels, and the resulting ion movement causes an IPSP – inhibitory postsynaptic potential – in which the membranes are hyperpolarised (to about -90mV) rather than depolarised. Usually, whether or not an impulse is generated in a particular nerve depends on the balance of inhibition and excitation that neurons receive at one moment. Receptor binding can also lead to the formation a second messenger (a transmitter substance) such as cyclic AMP (cAMP). This also changes the ionic permeability of the membrane, but it has a longer-lasting metabolic effect on the ion channels. Such long-term changes to brain neurons are thought to underlie memory.
Synapses have a vital role in information processing . Transmission of information across synapses is graded. They can amplify or damp down the information they receive. In many cases, they will not transmit it at all. Facilitation is a result of spatial summation. It is not a result of temporal summation, which is simply the accumulation of EPSPs arriving before the preceding EPSP has died down.
A neuron can be fed information by both excitatory synapses that produce EPSPs and inhibitory synapses that produce IPSPs. Whether or not the cell develops an action potential is determined by the sum of all the excitatory and inhibitory synapses at any particular moment. Put simply, impulses arriving at some synapses will ‘excite’ the cell, while others will calm it down’. Whether or not a neuron generates an action potential depends on the balance of the two types.
Imagine a synapse discharging its transmitter on to a postsynaptic neuron. This will set up an EPSP, but if it is not big enough to reach the threshold, no action potential is generated. However, if other synapses discharge their transmitter at the same time, or shortly after, the EPSPs will add up, or summate , until an action potential is generated.
Generally, there are two types of summation:
As a simple example of this idea, imagine the touch receptors from one area of skin feeding into one sensory neuron. An action impulse down just one receptor is almost certainly an insignificant stimulus, and can be ignored. It will not create an EPSP large enough to generate an action potential in the sensory nerve. However, if several touch receptors are stimulated at the same time, they will summate and produce a sensory impulse.
Synapses are important because they allow the transfer of information in nerve networks to be controlled. Synapses:
They allow us to select particular neural pathways. The process or learning is largely one of educating the synapses. People can play the violin or plano, or play tennis, because their synapses allow their brains to co-ordinate their senses and muscles in the right way. Your memories, too, have a basis in synapses choosing specific pathways. If you are asked, ‘What’s the capital of France?’ your synapses will (I hope) select a pathway of neurons in your brain which will lead you to the answer ‘Paris’.
I will pause here for now. But, in my next post, I will explain a special sort of synapse called the neuromuscular junction. I will also discuss the relationship between drugs and synapses.
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