The composition of dendritic cytoplasm is similar to that of the perikaryon; however, dendrites are devoid of Golgi complexes. Nissl bodies and mitochondria are present except in very thin dendrites. Neurofilaments and microtubules, also found in axons, are more numerous in dendrites.
Dendrites are usually covered by a large number of spines, small dendritic projections that represent sites of synaptic contact.
Axon. Neurons have only one axon. An axon is a cylindrical process that varies in length and diameter according to the type of neuron. Although some neurons have short axons, axons are usually very long processes. For example, axons of the motor neurons of the spinal cord that innervate the foot muscles may have a length of up to 100 cm.
All axons originate from a short pyramid-shaped region, the axon hillock, that usually arises from the perikaryon. The axon hillock can be differentiated from dendrites by distinctive cytologic features. The rER and ribosomes found in perikaryons and dendrites not extend into the axonhillock; in the axon hillock, the microtubules are arranged in fascicles or bundles. The plasma membrane of the axon is called the axolemma; its contents as known as axoplasm.
In contrast to dendrites, axons have a constant diameter and do not branch profusely. Occasionally, the axon, shortly after its departure from the cell body, gives rise to right-angled branches that return to the area of the nerve cell body. These branches are known as collateral branches.
Axoplasm possesses a few mitochondria, microtubules, and neurofilaments and some cisternae of smooth endoplasmic reticulum. Because axons do not contain ribosomes, all the proteins in these processes are synthesized in the perikaryon and moved down the axon by an energy-requiring mechanism known as axonal transport.
Newly synthesized protein molecules migrate down the axons (anterograde transport) at several speeds, but there are 2 main rates:
• a fast rate (this mechanism moves membranes, mitochondria, and synaptic vesicles); and
• a slow rate (substances such as actin, tubulin, metabolic enzymes, and neurofilaments protein).
Transport of protein also occurs in a centripetal direction (retrograde transport) from the axon to the perikaryon. Thus, receptors and synaptic membrane proteins are removed from the periphery and shipped back to the cell body, where they can be reused.
Axonal transport is related to the presence of complex microtubule networks, and such activities are suspended if microtubules are disrupted. If an axon is severed, its peripheral parts degenerate and die.
SYNAPSES
Transmission of the nerve impulse depends on highly specialized structures called synapses.
Most central nervous system synapses are between an axon and a dendrite (axodendritic) or between an axon and a cell body (axosomatic). Many of the synapses occur on swellings of the dendrite (dendritic spines). But there are also synapses between between axons (axoaxonic).
Morphologically, several types of synapses can be identified. The axon terminal may form bulbous expansions, basketlike structures, or club-shaped terminations. These synaptic end bulbs are often called boutons terminaux. More often, the axon establishes several synapses along its course. In this case, there are enlargements along the axon, called boutons en passage.
The analysis of a synapse under the electron microscope shows that it is actually a specialized, localized region of contact between 2 cells (Fig.20). It is composed of a terminal membrane (presynaptic membrane), a region of extracellular space - a distance of 20-30 nm (synaptic cleft), and a postsynaptic membrane belonging to a dendrite, perikaryon, axon of another neuron, or membrane of a muscle or gland cell. The plasma membranes of the 2 neurons appear to be thicker at the presynaptic and postsynaptic areas of the synapse than elsewhere.
Дата добавления: 2015-12-07; просмотров: 1 | Нарушение авторских прав