Detail The Components Of A Synapse And Describe The Sequence Of Events At A Synapse When Information Is Transmitted.

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 enzymes in it to breakdown the acetylcholine into acetyl and choline, this enzyme is called acetylcholinesterase. Figure 2 shows most of these parts as described above. (Rosenzweig, M, R. Breedlove, S, M. & Watson, N, V. 2005)

Figure 2: http://www.iworx.com/company2/WebToolsCD/Illustrations/synapse/synapse_web2.jpg

Electrical synapses work faster than chemical synapses with almost no time delay involved. This is because in electrical synapses, the synaptic cleft is only between 2 and 4 nanometres as compared to the 20 to 40 nanometre cleft in a chemical synapse. Also the presynaptic and postsynaptic membranes of the electrical synapses have larger channels fixed in them allowing ions to travel directly from one cell to the other without having to pass through the synaptic cleft, and the electrical current can travel between presynaptic and postsynaptic membranes with practically no time delay. This is opposed to the delay of around a millisecond caused by passage through the synaptic cleft in a chemical synapse, which although is very quick, for neurons is relatively slow. (Rosenzweig, M, R. Breedlove, S, M. & Watson, N, V. 2005)

In order for information to be transmitted from the presynaptic neuron to the postsynaptic cell, a series of events needs to take place. This begins when a nerve impulse i.e the result of an action potential reaches the presynaptic axon terminal or synaptic knob. This results in the synaptic knob becoming depolarized, that is, more negatively charged thus activating the voltage-gated calcium channels into opening. Positively charged calcium ions rush in to the synaptic knob due to diffusion, there are fewer inside than outside so they are attracted in, and also through electrostatic force since the polarity inside is negative thus attracting the positive charge of the calcium ions. If the frequency of action potentials is large, there will be a greater influx of calcium ions drawn into the synapse. Some of these calcium ions will bind to the synaptotagmin protein on the synaptic vesicles, those vesicles now bound with calcium that are located in the active zone of the synapse are drawn towards the presynaptic membrane by protein complexes known as SNAREs . The more calcium ions in the synapse, the more fuse to the vesicles thus causing more to be drawn towards the membrane. These vesicles then fuse with the membrane which is known as endocytosis and release the neurotransmitters they were carrying into the synaptic cleft. Some of these neurotransmitter molecules then bind to transmitter receptors of the postsynaptic membrane and either directly or indirectly causes the ion channel to open. All of the synaptic