Some profiles of dendrites are electron dense and shrunken ( Fig. Profiles of some dendrites in the aging cerebral cortex have membrane bound vacuoles in their cytoplasm (Figs. That these profiles belong to dendrites is shown by the fact that their bounding plasma membrane can be involved in the formation of synapses with axon terminals.ģ. 2.2 may be an early stage in degeneration, leading to the formation of profiles of dendrites that are almost completely devoid of organelles ( Fig. The profiles of dendrites such as shown in Fig. Profiles such as these are common in layer 1 of the aging cortex, a layer which becomes thinner with age as the apical tufts of pyramidal neurons retract and degenerate.Ģ. The profiles of some dendrites in the cortices of old monkeys show a cytoplasm devoid of organelles beyond a few microtubules beneath the plasma membrane ( Fig. Nevertheless, unusual images of dendrites that probably reflect the effects of age are sometimes encountered and some of these images are as follows:ġ. Although no systematic studies have been made of the effects of age on the fine structure of dendrites, most profiles of dendrites in electron micrographs show no or few obvious age changes. It is known that with age dendrites, especially those in the apical tufts of pyramidal cells, retract and degenerate. Consequently profiles of their dendrites have more regular shapes and these dendrites receive most of their synapses on their shafts. In contrast to the dendrites of pyramidal neurons, the dendrites of nonpyramidal, inhibitory neurons have smoother outlines, with few or no dendritic spines. The dendritic spines receive the majority of axon terminals synapsing with dendrites of pyramidal and spiny stellate neurons, but electron microscopic images that show the entire lengths of dendritic spines are not commonly encountered and have to be sought. The dendrites of the excitatory pyramidal cells are also profusely decorated with dendritic spines so that profiles of their dendrites tend to have somewhat irregular outlines. They travel singly through the neuropil and have a cytoplasm that contains regularly spaced microtubules, ribosomes and rough endoplasmic reticulum, mitochondria, and long cisternae of smooth endoplasmic reticulum ( Figs. Together, can innervate multiple parts of the brain and generate thousands of synaptic terminals.Dendrites are processes that extend from the cell bodies of cortical neurons. A single axon, with all its branches taken Other synapses appear as terminals at the ends Junctions appear partway along an axon as it extends these are called en passant The target cell, and special molecular structures transmitĮlectrical or electrochemical signals across the gap. Most axons branch, in some cases alot.Īxons make contact with other cells usually other neurons but sometimes muscle or gland cells at junctions called a synapse.Īt a synapse, the membrane of the axon closely adjoins the membrane of Several regions that function more or less independently of each other. Invertebrates such as insects or leeches the axon sometimes consists of No neuron ever has more than one axon however in Some types of neurons have no axons and transmit signals from theirĭendrites. Such as those for touch and warmth, the electrical impulse travelsĪlong an axon from the cell to the cell body, and from the cellīody to the spinal cord along another branch of the same axon.Īn axon is one of two types of protoplasmic protrusions that extrude from the cell body of a neuron, the other type being dendrites (see above). The function of the axon is to transmit information to different neurons, muscles and glands. An axon also known as a nerve fibre is a long, slender projection of a neuron, that typically conducts electrical impulses.
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