![]() Next, when an action potential does arrive at the terminal, the neurotransmitter must be quickly and efficiently released from the terminal and into the synaptic cleft. First, the neurotransmitter must be synthesized and stored in vesicles so that when an action potential arrives at the nerve ending, the cell is ready to pass it along to the next neuron. The process by which this information is communicated is called synaptic transmission and can be broken down into four steps. ![]() Others are inhibitory, causing the membrane of the next cell to hyperpolarize, thus decreasing the probability of that the next neuron will fire an action potential. Some neurotransmitters are excitatory and depolarize the next cell, increasing the probability that an action potential will be fired. Instead, chemicals called neurotransmitters are used to communicate the signal from one cell to the next. Such cells are separated by a space called a synaptic cleft and thus cannot transmit action potentials directly. Most cells, however, communicate via chemical synapses. In such cases, the action potential simply travels from one cell to the next through specialized channels, called gap junctions, which connect the two cells. Some cells communicate this information via electrical synapses. Each axon terminal is highly specialized to pass along action potentials to adjacent neurons, or target tissue, in the neural pathway. The end of the axon branches off into several terminals. It is in these uninsulated areas that the actual flow of ions along the axon takes place. In between each sheath of myelin is an exposed portion of the axon called a node of Ranvier. Most axons are covered by myelin, a fatty substance that serves as an insulator and thus greatly enhances the speed of an action potential. This wave of depolarization along the axon is called an action potential. Regenerating itself, this electrical signal travels down the cell's axon, a specialized extension from the cell body which ranges from a few hundred micrometers in some nerve cells, to over a meter in length in others. If the dendrites receive a strong enough signal from a neighboring nerve cell, or from several neighboring nerve cells, the resting electrical potential of the receptor cell's membrane becomes depolarized. Extending from the cell membrane, however, is a system of dendritic branches which serve as receptor sites for information sent from other neurons. The cell body, or soma, of a neuron is like that of any other cell, containing mitochondria, ribosomes, a nucleus, and other essential organelles. In this section we will investigate the way in which the unique morphology and biochemistry of neurons makes such communication possible. Efficient communication between these cells is crucial to the normal functioning of the central and peripheral nervous systems. The nervous system is composed of billions of specialized cells called neurons. Synaptic Transmission: A Four Step Process ![]()
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