Communication Between Neurons - Synaptic Transmission
Anatomy & Physiology I
By the end of this section, the student will be able to:
Neurons can communicate with three types of cells: muscle cells, glandular cells, and other neurons. These cells that neurons communicate with are called effector cells. Some ways that the cells communicate with each other are either through chemical synapses or electrical synapses called gap junctions, which send signals faster since they are connected by their cytoplasm. Gap junctions as you may recall are formed by proteins called connexons that make a gaping hole in the cell membranes of adjacent cells. Because of this opening, the cytoplasm between the two cells can be shared directly. The action potential can spread much more rapidly as the ions flow from one cell to the other through the gap junction. This can be seen in the heart muscle cells, which have to rapidly signal each other in order to work together as a unit in what is termed a syncytium. Visceral smooth muscle cells have gap junctions as does the developing embryo.
Electrical synapses need to communicate through their watery cytoplasm, directly sending ions over to the adjacent cell. In most cases, however, the neuron is separated from its effector cell by an actual space called the synaptic cleft. This cleft is very important because it allows for one-way conduction of impulses only. For example, if two neurons were actually in contact with one another, when one neuron fired an impulse, the other neuron would also fire an impulse because their membranes were in contact. If you touch an electric fence and touch a friend at the same time, both you and your friend feel the effect. However, if you touch an electric fence and are not in direct contact with your friend, only you get shocked, because there is no connection between the two of you. The synaptic cleft between cells allows the presynaptic cell to communicate with its effector only and not vice versa, by bridging the gap with a chemical signal.
Let's think about how a signal at an open space like this cleft would have only one-way signaling. On which membrane, presynaptic or post-synaptic, would the receptors be for the ACh neurotransmitter? Now you can explain why the signal only goes one way and not back the other way from where the ACh was released.
Video 1. Excitation-Contraction Coupling video (opens in new window).
Synaptic transmission is referred to as chemical transmission because it involves the use of chemicals called neurotransmitters to relay the message from the neuron to its effector cell. These neurotransmitters are synthesized in the soma and stored in synaptic vesicles in the presynaptic neuron's synaptic end-bulb. When the presynaptic neuron fires an action potential, this potential spreads across the neurolemma. When the neurolemma of the synaptic end-bulb depolarizes, special voltage-gated calcium gates are triggered to open. When the calcium gates open, calcium ions diffuse into the synaptic end-bulb. These calcium ions trigger the exocytosis of the neurotransmitter substance from the presynaptic neuron's synaptic vesicles. The neurotransmitter substance diffuses across the synaptic cleft and binds with specific receptors in the cell membrane of the effector cell forming a neurotransmitter-receptor complex. The formation of the neurotransmitter-receptor complex causes a change in ion gates in the plasma membrane of the effector cell. The neurotransmitter is quickly taken up by the presynaptic neuron or broken down by specific enzymes in the cleft, so the effector cell is only stimulated once per presynaptic action potential. Another way that the neurotransmitter becomes less able to stimulate receptors is that it simply diffuses out from the cleft.
The effect neurotransmission has on the effector cell depends on the type of neurotransmitter substance used and the specific type of effector cell. For example, the neurotransmitter acetylcholine (ACh) has an excitatory effect on skeletal muscle cells. This means the formation of the ACh-receptor complex in the sarcolemma causes sodium gates and sodium ions diffuse into the sarcoplasm, depolarizing the muscle cell. However, ACh has an opposite effect on cardiac muscle tissue. In cardiac muscle tissue, the formation of the ACh-receptor complex causes potassium gates to open and the cardiac muscle cell hyperpolarizes. Another neurotransmitter substance, norepinephrine, and causes the opposite effect on the heart.
There are several systems of neurotransmitters that are found at various synapses in the nervous system. These groups refer to the chemicals that are the neurotransmitters, and within the groups are specific systems.
The first group, which is a neurotransmitter system of its own, is the cholinergic system. It is the system based on acetylcholine. This includes the NMJ as an example of a cholinergic synapse, but cholinergic synapses are found in other parts of the nervous system. They are in the autonomic nervous system, as well as distributed throughout the brain.
The cholinergic system has two types of receptors, the nicotinic receptor is found in the NMJ as well as other synapses. There is also an acetylcholine receptor known as the muscarinic receptor. Both of these receptors are named for drugs that interact with the receptor in addition to acetylcholine. Nicotine will bind to the nicotinic receptor and activate it similar to acetylcholine. Muscarine, a product of certain mushrooms, will bind to the muscarinic receptor. However, nicotine will not bind to the muscarinic receptor and muscarine will not bind to the nicotinic receptor.
Another group of neurotransmitters are amino acids. This includes glutamate (Glu), GABA (gamma-aminobutyric acid, a derivative of glutamate), and glycine (Gly). These amino acids have an amino group and a carboxyl group in their chemical structures. Glutamate is one of the 20 amino acids that are used to make proteins. Each amino acid neurotransmitter would be part of its own system, namely the glutamatergic, GABAergic, and glycinergic systems. They each have their own receptors and do not interact with each other. Amino acid neurotransmitters are eliminated from the synapse by reuptake. A pump in the cell membrane of the presynaptic element, or sometimes a neighboring glial cell, will clear the amino acid from the synaptic cleft so that it can be recycled, repackaged in vesicles, and released again.
Another class of neurotransmitter is the biogenic amine, a group of neurotransmitters that are enzymatically made from amino acids. They have amino groups in them, but no longer have carboxyl groups and are therefore no longer classified as amino acids. Serotonin is made from tryptophan. It is the basis of the serotonergic system, which has its own specific receptors. Serotonin is transported back into the presynaptic cell for repackaging.
Other biogenic amines are made from tyrosine, and include dopamine, norepinephrine, and epinephrine. Dopamine is part of its own system, the dopaminergic system, which has dopamine receptors. Dopamine is removed from the synapse by transport proteins in the presynaptic cell membrane. Norepinephrine and epinephrine belong to the adrenergic neurotransmitter system. The two molecules are very similar and bind to the same receptors, which are referred to as alpha and beta receptors. Norepinephrine and epinephrine are also transported back into the presynaptic cell. The chemical epinephrine (epi- = "on"; "-nephrine" = kidney) is also known as adrenaline (renal = "kidney"), and norepinephrine is sometimes referred to as noradrenaline. The adrenal gland produces epinephrine and norepinephrine to be released into the blood stream as hormones.
A neuropeptide is a neurotransmitter molecule made up of chains of amino acids connected by peptide bonds. Neuropeptides are often released at synapses in combination with another neurotransmitter, and they often act as hormones in other systems of the body, such as vasoactive intestinal peptide (VIP) or substance P.
The effect of a neurotransmitter on the postsynaptic element is entirely dependent on the receptor protein. First, if there is no receptor protein in the membrane of the postsynaptic element, then the neurotransmitter has no effect. The depolarizing or hyperpolarizing effect is also dependent on the receptor. When acetylcholine binds to the nicotinic receptor, the postsynaptic cell is depolarized. This is because the receptor is a cation channel and positively charged Na+ will rush into the cell. However, when acetylcholine binds to the muscarinic receptor, of which there are several variants, it might cause depolarization or hyperpolarization of the target cell.
The amino acid neurotransmitters, glutamate, glycine, and GABA, are almost exclusively associated with just one effect. Glutamate is considered an excitatory amino acid, but only because Glu receptors in the adult cause depolarization of the postsynaptic cell. Glycine and GABA are considered inhibitory amino acids, again because their receptors cause hyperpolarization.
A very important synapse in animal physiology is called the neuromuscular junction, This is the junction between motor neurons and skeletal muscle, so it is also called the myoneural junction. The following list outlines the sequence of events involved with synaptic transmission at the neuromuscular junction (seen in depth in the muscle chapter).
Synapses are the contacts between neurons, which can either be chemical or electrical in nature. Chemical synapses are far more common. At a chemical synapse, neurotransmitter is released from the presynaptic element and diffuses across the synaptic cleft. The neurotransmitter binds to a receptor protein and causes a change in the postsynaptic membrane (the PSP). The neurotransmitter must be inactivated or removed from the synaptic cleft so that the stimulus is limited in time.
The particular characteristics of a synapse vary based on the neurotransmitter system produced by that neuron. The cholinergic system is found at the neuromuscular junction and in certain places within the nervous system. Amino acids, such as glutamate, glycine, and gamma-aminobutyric acid (GABA) are used as neurotransmitters. Other neurotransmitters are the result of amino acids being enzymatically changed, as in the biogenic amines, or being covalently bonded together, as in the neuropeptides.
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Text from Michael Ayers, M.S, for c3bc.
Other text from BioBook licensed under CC BY NC SA and Boundless Biology Open Textbook licensed under CC BY SA.
Other text from OpenStaxCollege licensed under CC BY 3.0. Modified by Alice Rudolph, M.A. and Andrea Doub, M.S. for c3bc.
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