Synapses are an essential and fascinating part of communication within the central nervous system. They are the transmitters of chemical and electrical messages that cause us to see, feel, move and much more. The brain consists of around 100 billion neurons, each of which has around 7,000 synaptic connections to other neurons. It has been estimated that a three year old child has 1,000 trillion synapses, and since number of synapses decreases with age until it stabilises in adulthood it is estimated the average adult has between 100 and 500 trillion synapses.(Wikipedia contributors (2006). When looking at the brain in this context, you can appreciate the sheer complexity of it and that looking at the functioning of a single synapse is a mammoth achievement of science.
In order to look at synapses in detail, it is necessary to understand the structure and components of the cell and those it attaches to. Figure 1 shows a neuron, as indicated on the diagram; the dendrites are the receivers of information for that cell. The dendrites receive neurotransmitters from the synapses that connect to it and if enough are present, an action potential is caused which then travels down the axon to the terminal buttons also known as synapses. There are electrical and chemical synapses in the body although the vast majority are chemical. Electrical synapses are located in the retina and in some invertebrates such as crayfish (Pocock, G. & Richards, C, D 2004), since chemical transmissions are more prevalent, these will be covered below.
Figure 1: Neuron, http://www.answers.com/topic/neuron
A synapse forms a junction between which nerve impulses and electricity in some cases pass through a gap (of approximately twenty nanometres) known as the synaptic cleft from the axon terminal to the target cell. This junction consists of the presynaptic neuron being linked to the post synaptic cell by a series of filaments thus keeping them in place; this target cell may be another neuron and therefore a dendrite or it may be a muscle or gland cell. (Pocock, G. & Richards, C, D 2004) The synapse is a complex part of a neuron and consists of many components. To begin with, the axon ends in many terminal buttons, also known as synaptic knobs. At the end of the Synaptic knob is the presynaptic membrane which is opposite the postsynaptic membrane on the postsynaptic neuron which is what receives the message. Between these is the synaptic cleft, as mentioned above. The synaptic cleft consists of extracellular fluid which is fluid from the lymph system and allows the diffusion of neurotransmitters. A. (Carlson, N, R. 2004)
Inside the presynaptic cell is naturally cytoplasm, along with mitochondria, synaptic vesicles, and a cisterna which is part of the Golgi apparatus. The mitochondria produce ATP, Adenosine triphosphate (Wikipedia contributors 2006), which is a type of energy the body utilizes; they produce this via oxidative phosphorylation. Synaptic vesicles are spherical shaped carriers of neurotransmitters and are made of a phospholipidbilayer (approximately 4.2 nanno metres in diameter) containing proteins imbedded in the bilayer called synaptotagmin to which calcium ions bond which is an important part of exocytosis as will be discussed later. Finally the cisterna is a series of membranes comprising the Golgi apparatus. The cisterna is where all vesicles in the body are produces, and in this case, the synaptic vesicles. The Golgi apparatus (cisterna) packs the neurotransmitters in spheres of spare membrane to be transported to the presynaptic membrane. Imbedded in this membrane exists proteins which allows reuptake of neurotransmitters from the synaptic cleft for reuse later. The postsynaptic membrane has multiple neurotransmitter receptors imbedded in it, in this context; it will have acetylcholine receptors. The synaptic cleft has...
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