The membrane of the neuron functions as a receptive surface over its entire extent; however, specific inputs termed afferents from other cells are received primarily on the surface of the cell body and on the surface of the specialized processes known as dendrites. The dendritic processes may branch extensively and are often covered with projections known as dendritic spines.
Spines provide a tremendous increase in the surface area available for synaptic contacts. The dendritic processes and spines of neurons are essentially expansions of cytoplasm containing most of the organelles found in the cell body.
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Dendrites contain numerous orderly arrays of microtubules and fewer neurofilaments see below. The microtubule associated proteins MAPs in the dendrite have a higher molecular weight than those found in the axon.
Nervous System Cell
An example is MAP2. In addition, microtubules in dendrites have their positive ends toward the cell soma. Mitochondria are often arranged longitudinally. Rough endoplasmic reticulum and ribosomes are present in large but not small dendrites. The shape and extent of the "dendritic tree" of an individual neuron is indicative of the quantity and variety of information received and processed by that neuron.
The dendritic spines often contain microfilaments which is the cytoskeletal element responsible for changes in spine shape observed in some examples of synaptic plasticity. Information is received by the dendrite through an array of receptors on dendrite surface that react to transmitters released from the axon terminals of other neurons. Dendrites may consist of a single twig-like extension from the soma or a multi-branched network capable of receiving inputs from thousands of other cells.
For instance, an average spinal motor neuron with a moderate-sized dendritic tree, receives 10, contacts, with 2, of these on the soma and 8, on the dendrites. The cone-shaped region of the cell body where the axon originates is termed the axon hillock.
This area is free of ribosomes and most other cell organelles, with the exception of cytoskeletal elements and organelles that are being transported down the axon. The neurofilaments in the axon hillock become clustered together as fascicles. The region between the axon hillock and the beginning of the myelin sheath is known as the initial segment. In many cases, this region is the anatomical location for the initiation of the action potential.
The area under the axolemma in this region has material that stains darkly when viewed by EM. This region is shown in Figure 8.
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At the distal-most end of the axon and its collaterales are small branches whose tips are button-shaped cytoplasmic enlargements called terminal boutons or nerve endings. The other type of process in the idealized neuron is the axon. Each neuron has only one axon and it is usually straighter and smoother than the dendritic profiles. Axons also contain bundles of microtubules and neurofilaments and scattered mitochondria. The most MAPs in an axon have a lower molecular weight than those in the dendrite.
A predominant MAP in axons is tau. Microfilaments within the axon are usually associated with an area adjacent to the plasmalemma and often are the most dense at the nodes of Ranvier. Beyond the initial segments, the axoplasm lacks rough endoplasmic reticulum and free ribosomes. The branches of axons are known as axon collaterales.
The axon itself is often surrounded by a membranous material, called the myelin sheath, formed by glia cells. The myelin sheath acts to insulate the plasmalemma of the axon in a way that necessitates the more rapid spread of the depolarization of the plasmalemma and increases the speed of conduction of the nerve impulse see Chapter 3.
The part of the plasma membrane of the nerve ending that is specialized to form functional contacts with other cells is the synapse. View an EM of a nerve ending with spherical vesicles. When neurons interact with muscle fibers, the region of functional contact is called the neuromuscular junction or motor endplate Chapter 4. According to the classical definition of synapse, when a nerve ending synapses on a dendrite or soma of a second neuron it is termed either an axodendritic or an axosomatic synapse , respectively Chapter 7.
However, almost all possible combinations of pre- and postsynaptic elements have been found in the central nervous system. These different types of synapse are designated by combining the name of the structure of the presynaptic element with that of the postsynaptic structure. For example, when the transfer of information occurs from an axon to axon or from one terminal to another, the synapse involved is called an axoaxonic synapse.
Regions of functional contacts between neurons synapses have distinct morphological characteristics. Although a great deal of variation exists in the size and shape of boutons of individual neurons, synapses can be identified by the presence of the following:.
IX. Neurology. 1. Structure of the Nervous System. Gray, Henry. Anatomy of the Human Body.
View an EM of a nerve ending with flat vesicles. Numerous variations of the "model" neuron described above exist. An important modification, which occurs especially in receptor neurons, involves the designation of a neuronal process as a dendrite or as an axon. Classically, the axon has been identified as the myelinated or unmyelinated process that transmits signals away from the cell body.
The classical view of the dendrite is that of an unmyelinated tube of cytoplasm which carries information toward the cell body. However, this distinction does not hold for ALL neurons. Some cells have a myelinated process that transmits signals toward the cell body. Morphologically the "dendrite" and the "axon" may, therefore, be indistinguishable. Neither the position of the cell body nor the presence or absence of myelin is always a useful criterion for understanding the orientation of the neuron.
The region of impulse initiation is more reliable guide to understanding the functional focal point of the cell. This region is analogous to the initial segment of the model neuron, discussed above. Routinely the fiber or process, which contains the initial segment or trigger zone, is referred to as an axon.
Note, as shown in Figure 8. A number of conventions have evolved to classify and name neurons. Through this approach cells are classified as unipolar, bipolar and multipolar neurons as shown in Figure 8. Unipolar cells have only one cell process, and are primarily found in invertebrates. However, vertebrate sensory neurons are another form of this type of cell.
Because these cells start out developmentally as bipolar neurons and then become unipolar as they mature, they are called pseudo-unipolar cells. Bipolar cells are present in the retina and the olfactory bulb. Multipolar cells make up the remainder of neuronal types and are, consequently, the most numerous type.
These have been further sub-categorized into Golgi type II cells that are small neurons, usually interneurons, and Golgi type I cells that are large multipolar neurons. Cells are also named for their shape e. More recently, cells have been named for their function or the neurotransmitter they contain e. This description is possible because of the development of histochemical and immunocytochemical methods to specifically identify the neurotransmitter type used by neurons.
The nervous system
Two variations in cell morphology. On the left is the pyramidal cell named for its characteristic pyramid shape. This cell is prominent in the cerebral cortex. On the right is the cell soma and dendrites of the Purkinje cell found in the cerebellum and named for the scientist, Purkinje. Axolemma is the plasmalemma of the axon. Endoplasmic reticulum is a labyrinthine, membrane bounded compartment in the cytoplasm where lipids are synthesized and membrane bound proteins are made.
In some regions of the neuron ER is devoid of ribosomes and is termed smooth ER. Endosome is a membrane-bounded organelle that carries materials ingested by endocytosis and passes them to lysosomes and peroxisomes for degradation. It also functions in the nerve ending to recycle synaptic vesicles.
Golgi apparatus is a collection of stacked, smooth-surfaced membrane bound organelles where proteins and lipids made in the endoplasmic reticulum are modified and sorted. Lysosomes contain enzymes that digest compounds that originate inside or outside the cells. They are involved in converting proteins to amino acids and glycogen to glucose, the basic nutrient of neurons.
Their enzymes act at an acidic pH. The medullated fibers form the white part of the brain and medulla spinalis, and also the greater part of every cranial and spinal nerve, and give to these structures their opaque, white aspect. When perfectly fresh they appear to be homogeneous; but soon after removal from the body each fiber presents, when examined by transmitted light, a double outline or contour, as if consisting of two parts Fig. The central portion is named the axis-cylinder; around this is a sheath of fatty material, staining black with osmic acid, named the white substance of Schwann or medullary sheath, which gives to the fiber its double contour, and the whole is enclosed in a delicate membrane, the neurolemma, primitive sheath, or nucleated sheath of Schwann Fig.
The axis-cylinder is the essential part of the nerve fiber, and is always present; the medullary sheath and the neurolemma are occasionally absent, expecially at the origin and termination of the nerve fiber. The axis-cylinder undergoes no interruption from its origin in the nerve center to its peripheral termination, and must be regarded as a direct prolongation of a nerve cell. It constitutes about one-half or one-third of the nerve fiber, being greater in proportion in the fibers of the central organs than in those of the nerves.
It is quite transparent, and is therefore indistinguishable in a perfectly fresh and natural state of the nerve. It is made up of exceedingly fine fibrils, which stain darkly with gold chloride Fig. The axis-cylinder is said by some to be enveloped in a special reticular sheath, which separates it from the medullary sheath, and is composed of a substance called neurokeratin. The more common opinion is that this network or reticulum is contained in the white matter of Schwann, and by some it is believed to be produced by the action of the reagents employed to show it.
X Osmic acid. Nodes of Ranvier. The medullary sheath, or white matter of Schwann Fig. It varies in thickness, in some forming a layer of extreme thinness, so as to be scarcely distinguishable, in others forming about one-half the nerve fiber. The medullary sheath undergoes interruptions in its continuity at regular intervals, giving to the fiber the appearance of constriction at these points: these are known as the nodes of Ranvier Figs.