Synapses: Junctions between the Nerves
When an action potential passes down an axon to its conclusion, it must indulge a small pause, the synapse (Gr. Synapsis, contact, union), identifying it from some other nerve cell or an effector organ. Two distinct kinds of synapses are known: electrical and chemical.
Electrical synapses, although much less common than chemical synapses, have been produced in both invertebrate and vertebrate groups. Electrical synapses are points at which ionic currents flow at once across a narrow gap junction (see Figure) from one heart cell to another. Electrical synapses show no time lag and therefore are important for escape reactions. They likewise have been noted in other excitable cell types, and form an important method of communication between cardiac muscle cells of the facial expression and smooth muscle cells (for example, the uterus,).
A great deal more complex than electrical synapses are chemical synapses, which contain packets of specialized chemicals called neurotransmitters. Neurons bringing impulses toward chemical synapses are called presynaptic neurons; those carrying impulses away are postsynaptic neurons. At a synapse, membranes are separated by a minute gap, the synaptic cleft, having a width of about 20 millimeter.
The axon of most neurons divides at its terminal into many parts, each of which has a synaptic knob that sits along the dendrites or cell body of the next nerve cell (Figure 35-7A). Because a single impulse coming down a nerve axon is transmitted along these many branches and synaptic endings on the next neuron, many impulses converge on the cell body at one minute. In essence, the axon terminations of many neurons may almost cover a nerve cell body and its dendrites with thousands of synapses. The 20 mm fluid-filled gap between presynaptic and postsynaptic membranes prevents action potentials from spreading like a shot to the postsynaptic neuron. Instead the synaptic knobs secrete a specific neurotransmitter that communicates chemically with the postsynaptic cell. One of the most common neurotransmitters of the peripheral neural system is acetylcholine, which illustrates typical synaptic transmission. Deep down the synaptic knobs are numerous tiny synaptic vesicles, each comprising several thousand molecules of acetylcholine. Evidence indicates that when an impulse arrives at a terminal knob a sequence of cases occurs as portrayed in Figure. The action potential has an inward movement of calcium (Ca+) ions through channels in the synaptic knob membrane and this induces exocytosis of some neurotransmitter-filled synaptic vesicles. Acetylcholine molecules diffuse across the interruption in a fraction of a millisecond and blend briefly to receptor molecules on ion channels in the postsynaptic membrane. This makes a possible change in the postsynaptic membrane. Whether the potential change is big enough to trigger a postsynaptic potential depends on how many acetylcholine molecules are liberated and how many ducts are clear. Acetylcholine is rapidly put down by the enzyme acetylcholinesterase, which converts acetylcholine into acetate and choline. If not inactivated in this manner, the neurotransmitter would continue to stimulate indefinitely. Organophosphate insecticides (such as malathion) and certain military nerve gases are poisonous for precisely this reason; they block acetylcholinesterase. The last step in the sequence is absorbed of choline into the presynaptic terminal, resynthesis of acetylcholine and its stored in synaptic vesicles, ready to react to another impulse.
Many different chemical neurotransmitters have been placed in both vertebrate and invertebrate nervous systems. Some, such as acetylcholine, norepinephrine, and glutamate, depolarize postsynaptic membranes; they are released at excitatory synapses. Other neurotransmitters, such as gamma aminobutyric acid (GABA), hyperpolarize postsynaptic membranes; thereby stabilizing them against depolarization. These neurotransmitters are released at inhibitory synapses. Nerve cells in the cardinal nervous system have both excitatory and inhibitory synapses among the hundreds or thousands of synaptic knobs on the dendrites and cell body of each nerve cell.
The net remainder of all excitatory and inhibitory inputs received by a postsynaptic cell determines whether it gets an action potential. If many excitatory impulses are picked up at one time, they can bring down the resting membrane potential enough in the postsynaptic membrane to elicit an action potential. Inhibitory impulses, however, stabilize the postsynaptic membrane, making it less likely that an action potential will be bred. The synapse is an indispensable part of the decision-making equipment of the central nervous system, modulating the flow of information from single nerve cell to the next.
Source :Eplant Science