Chemical synapses

Most of the synapses in the nervous system are chemical synapses in which the presynaptic neuron and the postsynaptic neuron are not in direct contact but instead are separated by a narrow (0.01 to 0.02 mm) space called the synaptic cleft. This space prevents the direct spread of the electrical impulse from one cell to the next. Instead, a chemical referred to as a neurotransmitter is released from the presynaptic neuron. The neurotransmitter diffuses across the synaptic cleft, binds to its specific receptor, and alters electrical activity of the postsynaptic neuron.

The mechanism of action of a chemical synapse is shown in Figure 5.1. The axon terminal broadens to form a swelling referred to as the synaptic knob. Within the synaptic knob are many synaptic vesicles that store the preformed neurotransmitter. Also found in the membrane of the synaptic knob are voltage-gated Ca++ channels. When the electrical impulse, or action potential, has been transmitted along the length of the axon and reaches the axon terminal, the accompanying change in voltage causes the voltage-gated Ca++ channels to open. Because calcium is in greater concentration in the extracellular fluid compared to the intracellular fluid, Ca++ ions enter the cell down their concentration gradient. The Ca++ ions then induce the release of the neurotransmitter from synaptic vesicles into the synaptic cleft by causing the vesicles to fuse with the presynaptic membrane, thereby facilitating the process of exocytosis. The neurotransmitter molecules diffuse across the cleft and bind to specific receptors on the membrane of the postsynaptic neuron.

This binding of the neurotransmitter alters permeability of the postsyn-aptic neuron to one or more ions. As always, a change in ion permeability results in a change in the membrane potential of the cell. This change at the synapse is in the form of a graded potential only. At any given synapse, the change in membrane potential is not great enough to reach threshold and generate an action potential. Instead, many graded potentials generated at one or more synapses are conducted over the cell membrane toward the axon hillock. If the depolarization caused by multiple graded potentials added together is sufficient for the axon hillock to reach threshold, then an action potential is generated here.

The two types of synapses are:

• Excitatory synapses

• Inhibitory synapses

At an excitatory synapse, binding of the neurotransmitter to its receptor increases permeability of the membrane to Na+ ions and K+ ions through chemical messenger-gated channels closely associated with the receptor. As a result, Na+ ions enter the cell down their concentration and electrical gradients and K+ ions leave the cell down their concentration gradient only. Because two forces cause inward diffusion of sodium and only one force causes outward diffusion of potassium, the influx of Na+ ions is significantly greater than the efflux of K+ ions. This greater movement of (+) charges into the cell results in a small depolarization of the neuron referred to as an excitatory postsynaptic potential (EPSP), which is a graded potential only. A single action potential occurring at a single excitatory synapse opens too few

POSTSYNAPTIC NEURON CELL BODY

Figure 5.1 Mechanism of action at a chemical synapse. The arrival of an action potential at the axon terminal causes voltage-gated Ca++ channels to open. The resulting increase in concentration of Ca++ ions in the intracellular fluid facilitates exocytosis of the neurotransmitter into the synaptic cleft. Binding of the neurotransmitter to its specific receptor on the postsynaptic neuron alters the permeability of the membrane to one or more ions, thus causing a change in the membrane potential and generation of a graded potential in this neuron.

POSTSYNAPTIC NEURON CELL BODY

Figure 5.1 Mechanism of action at a chemical synapse. The arrival of an action potential at the axon terminal causes voltage-gated Ca++ channels to open. The resulting increase in concentration of Ca++ ions in the intracellular fluid facilitates exocytosis of the neurotransmitter into the synaptic cleft. Binding of the neurotransmitter to its specific receptor on the postsynaptic neuron alters the permeability of the membrane to one or more ions, thus causing a change in the membrane potential and generation of a graded potential in this neuron.

Na+ channels to depolarize the membrane all the way to threshold; however, it brings the membrane potential closer toward it. This increases the likelihood that subsequent stimuli will continue depolarization to threshold and that an action potential will be generated by the postsynaptic neuron.

At an inhibitory synapse, binding of the neurotransmitter to its receptor increases permeability of the membrane to K+ ions or to Cl- ions through chemical messenger-gated channels. As a result, K+ ions may leave the cell down their concentration gradient carrying (+) charges outward or Cl- ions may enter the cell down their concentration gradient carrying (-) charges inward. In either case, the neuron becomes more negative inside relative to the outside and the membrane is now hyperpolarized. This small hyperpo-larization is referred to as an inhibitory postsynaptic potential (IPSP). The movement of the membrane potential further away from threshold decreases the likelihood that an action potential will be generated by the postsynaptic neuron.

Almost invariably, a neuron is genetically programmed to synthesize and release only a single type of neurotransmitter. Therefore, a given synapse is either always excitatory or always inhibitory. Once a neurotransmitter has bound to its receptor on the postsynaptic neuron and has caused its effect, it is important to inactivate or remove it from the synapse in order to prevent its continuing activity indefinitely. Several mechanisms to carry this out have been identified:

• Passive diffusion of the neurotransmitter away from the synaptic cleft

• Destruction of the neurotransmitter by enzymes located in the syn-aptic cleft or in the plasma membranes of presynaptic or postsynaptic neurons

• Active reuptake of the neurotransmitter into the synaptic knob of the presynaptic neuron for reuse or enzymatic destruction

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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