{"id":2180,"date":"2014-10-30T21:11:57","date_gmt":"2014-10-30T21:11:57","guid":{"rendered":"https:\/\/courses.candelalearning.com\/apvccs\/?post_type=chapter&p=2180"},"modified":"2019-08-11T20:17:52","modified_gmt":"2019-08-11T20:17:52","slug":"communication-between-neurons","status":"web-only","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-dutchess-anatomy-physiology\/chapter\/communication-between-neurons\/","title":{"raw":"Communication Between Neurons","rendered":"Communication Between Neurons"},"content":{"raw":"
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Introduction to Synapses<\/h2>\r\nSeveral neurons are generally needed to transmit a signal from one place in the body to another. So how does the signal pass from one neuron to the next along a neural pathway? The signal must be transmitted across the interface between successive neurons and we will learn how that is accomplished in this next section.\r\n\r\n\"Synapse\r\n\r\n
Synapse cells. By Miserlou at en.wikipedia (http:\/\/upload.wikimedia.org\/wikipedia\/commons\/3\/3e\/Neurons_big1.jpg)<\/a><\/address>
\u00a0<\/address>The term synapse means \u201ccoming together.\u201d Where two structures or entities come together, they form a synapse. Although one can use the word synapse to mean any cellular junction, in physiology we traditionally limit its usage to: the junction of two neurons, the junction between a neuron and a target cell (ex. the neuromuscular junction), or the interface between adjacent cardiac muscle cells or adjacent smooth muscle cells. In the nervous system, a synapse is the structure that allows a neuron to pass an electrical or chemical signal to another cell.\r\n

Synapse Cells<\/h2>\r\nThe cell that delivers the signal to the synapse is the presynaptic<\/strong> cell. The cell that will receive the signal once it crosses the synapse is the postsynaptic<\/strong> cell. Since most neural pathways contain several neurons, a postsynaptic neuron at one synapse may become the presynaptic neuron for another cell downstream.\r\n\r\nA presynaptic neuron can form one of three types of synapses with a postsynaptic neuron. The most common type of synapse is an axodendritic synapse, where the axon of the presynaptic neuron synapses with a dendrite of the postsynaptic neuron. If the presynpatic neuron synapses with the soma of the postsynaptic neuron it is called an axosomatic synapse, and if it synapses with the axon of the postsynaptic cell it is an axoaxonic synapse. Although our illustration shows a single synapse, neurons typically have many (even 10,000 or more) synapses.\r\n

Synapses Types<\/h2>\r\nThere are two types of synapses found in your body: electrical and chemical (Figure 1). Electrical synapses<\/strong> allow the direct passage of ions and signaling molecules from cell to cell. In contrast, chemical synapses<\/strong> do not pass the signal directly from the presynaptic cell to the postsynaptic cell. In a chemical synapse, an action potential in the presynaptic neuron leads to the release of a chemical messenger called a neurotransmitter. The neurotransmitter then diffuses across the synapse and binds to receptors on the postsynaptic cell. Binding of the neurotransmitter leads to the production of an electrical signal in the postsynaptic cell.\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"699\"]\"Structure Figure 1.<\/strong>\u00a0A) Electrical synapses allow ions to diffuse through gap junctions from the pre to the postsynaptic cell. B) At a chemical synapse, a presynaptic neuron releases neurotransmitter towards the postsynaptic cell.[\/caption]\r\n\r\n
Structure of a chemical synapse. (http:\/\/creativecommons.org\/licenses\/by\/3.0\/us\/<\/a>).<\/address>Why does the body have two types of synapses? Each type of synapse has functional advantages and disadvantages. An electrical synapse passes the signal very quickly, which allows groups of cells to act in unison. A chemical synapse takes much longer to transmit the signal from one cell to the next; however, chemical synapses allow neurons to integrate information from multiple presynaptic neurons, determining whether or not the postsynaptic cell will continue to propagate the signal. Neurons respond differently based on information transmitted by multiple chemical synapses. Let\u2019s take a closer look at the structure and function of each type of synapse.\r\n\r\nElectrical synapses transmit action potentials via the direct flow of electrical current at gap junctions (Figure 2). Gap junctions are formed when two adjacent cells have transmembrane pores that align. The membranes of the two cells are linked together and the aligned pores form a passage between the cells. Consequently, several types of molecules and ions are allowed to pass between the cells. Due to the direct flow of ions and molecules from one cell to another, electrical synapses allow bidirectional flow of information between cells. Gap junctions are crucial to the functioning of the cardiac myocytes and smooth muscles.\r\n\r\n[caption id=\"attachment_4379\" align=\"alignnone\" width=\"700\"]\"\"<\/a> Figure 2.<\/strong> The intracellular fluid of the pre and postsynaptic cells are linked by protein channels called connexons.[\/caption]\r\n\r\n
Structure of an electrical synapse (gap junction). By (https:\/\/commons.wikimedia.org\/wiki\/File:Gap_cell_junction-en.svg<\/a>).<\/address>
\u00a0<\/address>Chemical synapses comprise most of the synapses in your body (Figure 3). In a chemical synapse, a synaptic gap or cleft separates the pre- and the postsynaptic cells. An action potential propagated to the axon terminal results in the secretion of chemical messengers, called neurotransmitters, from the axon terminals. The neurotransmitter molecules diffuse across the synaptic cleft and bind to receptor proteins on the cell membrane of the postsynaptic cell. Binding of the neurotransmitter to the receptors on the postsynaptic cell leads to a transient change in the postsynaptic cell\u2019s membrane potential.\r\n\r\n[caption id=\"attachment_4378\" align=\"alignnone\" width=\"700\"]\"Two<\/a> Figure 3.<\/strong> A presynaptic neuron stimulates a postsynaptic neuron at a synapse.[\/caption]\r\n\r\n
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\u00a0<\/address>An example of a chemical synapse is the neuromuscular junction (NMJ) described in the chapter on muscle tissue. In the nervous system, there are many more synapses that are essentially the same as the NMJ. All synapses have common characteristics, which can be summarized in this list:\r\n