Nervous tissue is designed to transmit electrical impulses:
Nervous tissue consists of nerve cells and associated supporting cells. All cells exhibit electrical properties, but nerve cells, also called neurons, are specifically designed to transmit electrical impulses from one site in the body to another, and to receive and process information. Supporting cells are non-conducting cells that are in intimate physical contact with neurons. They provide physical support, electrical insulation and metabolic exchange with the vascular system.What makes it a nerve cell?:
Nerve cells are very variable in appearance, but all neurons have a cell body, also called soma or perikarion, and processes. The cell body contains the nucleus and organelles that maintain the nerve cell. The processes extend from the nerve cell to communicate with other cells. There are two types of processes: dendrites that receive impulses and axons that transmit impulses. All nerve cells have an axon (usually only one), which is usually the longest process that extends from the cell. Most nerve cells have dendrites, usually many, and these are generally shorter and thicker than the axon. Nerve cells can also receive impulses right on the nerve cell body, and some nerve cells have no dendrites. The junction where a nerve cell communicates with another nerve cell or an effector cell (eg. muscle fibre) is called a synapse. The terminal part of the axon releases a chemical called a neurotransmitter which acts on the membrane of the other cell.
Many axons are wrapped in a lipid-rich covering called myelin. This myelin sheath insulates the axon from the surrounding extracellular component and increases the rate of electrical conduction. The myelin sheath is discontinuous at intervals called the nodes of Ranvier. The areas covered with myelin are called internodal areas. In myelinated axons, the voltage reversal (that is, the impulse propagation) can occur only at the nodes, and the impulse "jumps" from node to node. This is called saltatory conduction. In unmyelinated axons, the impulse is conducted more slowly, moving as a wave of voltage reversal along the axon.Brief overview of the organization of the nervous system:
The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is limited to the brain and spinal cord, all other nervous tissue belongs to the peripheral nervous system. An individual nerve cell can have components in both the CNS and the PNS. For example, the cell body of a motor neuron (one that causes skeletal muscle to move) lies in the CNS, but its axon leaves the CNS to travel in a peripheral nerve to reach the muscle it innervates. In many instances nerve cell bodies are found together in clusters. Clusters of nerve cell bodies in the CNS are called nuclei (sing. nucleus, not to be confused with the nucleus of every cell that houses its DNA etc.). Clusters of nerve cell bodies in the peripheral nervous system are called ganglia (sing. ganglion).
In the central nervous system, areas containing nerve cell bodies, their myelinated and unmyelinated processes and supporting (glial) cells are called grey matter. Areas containing predominantly myelinated axons but also some unmyelinated axons and glial cells are referred to as white matter, because in fresh preparations the myelinated tissue looks white. In the brain, the grey matter is exterior to the white matter, the reverse is the case in the spinal cord.
In the CNS, there is a selective restriction of blood-borne substances called the blood-brain barrier. This barrier is formed by special tight junctions in the endothelial cells of the blood vessels. Certain specific parts of the brain are "leakier" than the rest - otherwise (for example) hormones couldn't act on the brain to influence behavior.
The peripheral nervous system has motor components called efferent neurons (because they arise in the CNS and head away from it to the periphery), and sensory components called afferent neurons (because the impulse is initiated in a specialized receptor in the PNS and heads toward the CNS). A special component of the peripheral nervous system is the autonomic nervous system (ANS). This is the portion of the PNS that conducts impulses to smooth muscle, cardiac muscle and glandular epithelium. It is classified into three divisions: the sympathetic, parasympathetic and enteric divisions.
You will be learning more about the structure of nerve cells and the organization of the nervous system in the Homeostasis and Development Block and in the Neurology Block. The histology of the nervous system is discussed in Chapter 11 of Ross et al.Support cells play a vital role:
Support cells are essential to the function and survival of nerve cells. The CNS and PNS each have their own specific types of support cells.
Support cells in the CNS:
The general term for support cells in the CNS is glia or neuroglia (a.k.a. glial cells, neuroglial cells). There are three types of neuroglial cells. (1) Oligodendrocytes, the myelin-secreting cells of the CNS. (2) Astrocytes, which provide physical and metabolic support for nerve cells. (3) Microglia, or microglial cells (a.k.a. mesoglia), which are the phagocytes of the CNS.
Oligodendrocytes. As their name implies, oligodendrocytes have few processes. They are often found in rows between axons. The myelin sheath around axons is formed by concentric layers of oligodendrocytes plasma membrane. Each oligodendrocyte gives off several tongue-like processes that find their way to the axon, where each process wraps itself around a portion of the axon, forming an internodal segment of myelin. Each process appears to spiral around its segment of the axon in a centripetal manner, with the continued insinuation of the leading edge between the inner surface of newly formed myelin and the axon. One oligodendrocyte may myelinate one axon or several. The nucleus-containing region may be at some distance from the axon(s) it is myelinating. In the CNS, nodes of Ranvier (between myelinated regions) are larger than those of the PNS, and the larger amount of exposed axolemma makes saltatory conduction more efficient.
Unmyelinated axons in the CNS are truly bare, that is they are not embedded in any glial cell process. (In contrast to the situation in the PNS, described below.)
Astrocytes. Astrocytes are the largest of the neuroglial cells. They have elaborate processes that extend between neurons and blood vessels. The ends of the processes expand to form end feet, which cover large areas of the outer surface of the blood vessel or axolemma. Astrocytes are believed to play a role in the movement of metabolites and wastes to and from neurons, and in regulating ionic concentrations within the neurons. They may be involved in regulating the tight junctions in the capillaries that form the blood-brain barrier. Astrocytes also cover the bare areas of neurons, at nodes of Ranvier and synapses. They may act to confine neurotransmitters to the synaptic cleft and to remove excess neurotransmitters.
Two kinds of astrocytes are identified, protoplasmic and fibrous astrocytes. Both types contain prominent bundles of intermediate filaments, but the filaments are more numerous in fibrous astrocytes. Fibrous astrocytes are more prevalent in white matter, protoplasmic ones in grey matter.
Microglia. These are the smallest of the glial cells, with short twisted processes. They are the phagocytes of the CNS, considered part of the mononuclear phagocytic system (see pg 110 in Ross et al.). They are believed to originate in bone marrow and enter the CNS from the blood. In the adult CNS, they are present only in small numbers, but proliferate and become actively phagocytic in disease and injury. Their alternate name, mesoglia, reflects their embyonic origin from mesoderm (the rest of the nervous system, including the other glial cells, is of neuroectodermal or neural crest origin).
In routine histological preparations of the CNS, only the nuclei of glial cells can be identified. In the slides in your collection, you will not be able to attribute any individual glial cell nucleus to any specific type of glial cell. You will however be able to distinguish the neurons from glial cell nuclei. Special preparations are needed to dentify glial cells, as seen in some of the images shown below.
Support cells in the PNS:
The support cells of the PNS are called satellite cells and Schwann cells.
Satellite cells. Satellite cells surround the cell bodies of the neurons in ganglia (ganglion cells). These small cuboidal cells form a complete layer around the nerve cell body, but only their nuclei are visible in routine preparations. They help maintain a controlled microenvironment around the nerve cell body, providing electrical insulation and a pathway for metabolic exchange. In paravertebral and peripheral ganglia, nerve cell processes must penetrate between satellite cells to establish a synapse.
Schwann cells. Schwann cells are responsible for the myelination of axons in the PNS. A Schwann cell wraps itself, jelly roll-fashion, in a spiral around a short segment of an axon. During the wrapping, cytoplasm is squeezed out of the Schwann cell and the leaflets of plasma membrance of the concentric layers of the Schwann cell fuse, forming the layers of the myelin sheath. An axon's myelin sheath is segmented because it is formed by numerous Schwann cells arrayed in sequence along the axon. The junction where two Schwann cells meet has no myelin and is called (as you know) the node of Ranvier (the areas covered by Schwann cells being the internodal regions).
The lack of Schwann cell cytoplasm in the concentric rings of the myelin sheath is what makes it lipid-rich. Schwann cell cytoplasm is however found in several locations. There is an inner collar of Schwann cell cytoplasm between the axon and the myelin, and an outer collar around the myelin. The outer collar is also called the sheath of Schwann or neurilemma, and contains the nucleus and most of the organelles of the Schwann cell. The node of Ranvier is also covered with Schwann cell cytoplasm, and this is the area where the plasma membranes of adjacent Schwann cells meet. (These adjacent plasma membranes are not tightly apposed at the node, so that extracellular fluid has free acess to the neuronal plasma membrane.) Finally, small islands of Schwann cell cytoplasm persist within successive layers of the myelin sheath, these islands are called Schmidt-Lanterman clefts. The process of myelination is described in greater detail on pg 264-268 of Ross et al.
Not all nerve fibres is the PNS are covered in myelin, some axons are unmyelinated. In contrast to the situation in the CNS, unmyelinated fibres in the PNS are not completely bare, but are enveloped in Schwann cell cytoplasm. The Schwann cells are elongated in parallel to the long axis of the axons, which fit into grooves on the surface of the Schwann cell. One axon or a group of axons may be enclosed in a single groove. Schwann cells may have only one or up to twenty grooves. Single grooves are more common in the autonomic nervous system.And here's what nervous tissue looks like:
Neurons come in many shapes (and sizes):
Figure 1 shows the cell body of a motor neuron from the spinal cord. The nucleus is pale and a very prominent nucleolus is evident. (There is a Barr body, indicating female sex, at the top of the nucleolus.) The dark-staining bodies in the cytoplasm are the Nissl bodies, each of which corresponds to a stack of rough endoplasmic reticulum. Nissl bodies and some other organelles (seen with EM) extend into the dendrites but never beyond the axon hillock, the conical projection of the cell body from which the axon arises. This absence of Nissl helps to distinguish the axon from the dendrites. Here, it is possible to distinguish the axon as the process at the lower left. However, it is not possible to do so in every instance. The wispy strands in the background are neuropil, the processes of nerve cell bodies. The small dark bodies in the neuropil are glial cell nuclei.
Figure 2 shows another nerve cell body from a whole mount of a spinal cord smear. Generally, the processes of a neuron are cut during sectioning, but here many can be seen extending for a considerable distance from the nerve cell body. The pale nucleus with prominent nucleolus can be identified although cytological detail is not as good as in Figure 1. The smaller "dots" in the background are the nuclei of glial cells, the wispy strands are the neuropil.
Figure 3 shows a nerve cell body surrounded by neuropil in the spinal cord. This section was prepared with a silver stain. While cytological detail is lost, this stain reveals the abundance of nerve cell processes not readily apparent in routine preparations. Glial cell nuclei appear as small dark spots.
Figure 4 shows the Purkinje cells of the cerebellum prepared with a Golgi stain. These cells have an extraordinary, finely branching dendritic tree (D) at one end, and a single axon (A) at the other. This stain reveals the shape of the cell (the axon is not identifiable in standard preparations) but obscures cytological detail (Purkinje cells have a nucleus with nucleoli just like other nerve cells). [The Purkinje neurons of the cerebellum have nothing to do with the Purkinje fibres of the heart, other than being named after the same person.]
Figure 5 shows a pyramidal cell from the cerebral cortex (Golgi stain). Dendrites (D) arise from the apex and the side, a single axon (A) extends from the base.
Figure 6 shows nerve cell bodies in an autonomic ganglion between smooth muscle layers of the stomach. (In contrast to the above examples from the CNS, this image is from the PNS.) The round nerve cell bodies are reddish purple. The large, pale blue nuclei are clearly outlined, their nucleoli appear as small dark dots. Not all nerve cell bodies are sectioned through both the cytoplasm and the nucleus. The most extensive section is through the nerve cell body on the left. Satellite cells are not readily seen in this section. Nerve processes and Schwann cell nuclei are seen toward the top of the figure. Smooth muscle tissue in cross section is seen at the bottom of the image.
Nervous tissue in the CNS:
Figure 7 shows a low power view of the spinal cord. When the whole spinal cord is viewed under low power, the inner grey matter is described as having an H or butterfly shape.
The grey matter has dorsal and ventral horns. The ventral horns are shorter and thicker. The two sides of the grey matter are connected by a commissure inside which is the central canal lined with ependymal cells. These secrete the cerebrospinal fluid (CSF). The neurons inside the grey matter are variable in size and shape but are larger than the neuroglial cell nuclei. (Figure 6 is taken from slide #24 of your collection.)
Figure 8 shows a high power view of a standard H&E (hematoxylin and eosin) preparation of the grey matter of the spinal cord. Several nerve cell bodies with Nissl and emerging processes can be seen. The one at the centre is sectioned through the nucleus and the nucleolus is prominent. Its processes at the left and the bottom right can be followed for some distance. The background matting is the neuropil (nerve cell processes). The smaller dots are the nuclei of glial cells.
Figure 9 shows a high power view of the spinal cord stained with silver. The grey matter is on the right and the white matter on the left. A nerve cell body is seen in the grey matter among the dense neuropil and glial cell nuclei. At the top left, a bundle of fibres is seen penetrating into the white matter. There is little staining in the white matter, which shows mainly myelinated nerve fibres cut in cross-section. The axons appears as dots, surrounded by the white space of what had been their myelin covering (removed during preparation). The other material that stains in the white matter consists of unmyelinated fibres (small patches of wavy lines) and glial cell nuclei.
Neuroglial cells up close:
Figure 10 shows a silver-stained oligodendrocyte in the brain.
Figure 11 shows a silver-stained fibrous astrocyte in the brain.
Figure 12 shows an iron hematoxylin-stained microglial cell (with processes) in the brain.
Some nervous tissue in the PNS:
Figure 13 shows a teased myelinated nerve fibre prepared with osmium tetroxide to stain the myelin. A node of Ranvier is seen toward the middle of the fibre. The darker staining myelin sheath is interrupted at the node, where the fibre is covered only by the lighter Schwann cell cytoplasm (best seen at left). The boundary between adjacent Schwann cells is indistinguishible.
Figure 14 shows a high power view of part of a myelinated nerve stained with silver. The fatty myelin has been removed during processing. Axons form a bullseye at the centre of the spaces previously occupied by myelin. The spaces are surrounded by the remnants of the neurilemma (outer collar of Schwann cell cytoplasm) and the connective tissue of the endoneurium. Endoneurium consists of collagen fibrils associated with individual nerve fibres and running in parallel with them. The collagen fibrils are secreted mainly by the Schwann cells, as fibroblasts are fairly sparse around nerve fibres. In some of the nerve fibres (such as at the lower right), one sees two rings around an axon. This means the section has passed through a Schmidt-Lanterman cleft (island of Schwann cell cytoplasm within myelin sheath).
Figure 15 shows a lower power view of a myelinated nerve. Obviously, more of the nerve is in the field of view, and here we see some perineurium (P), and part of the epineurium (E). Perineurium is the connective tissue that binds a bundle of nerve fibres together into a fascicle. Its cells are contractile (which is why cut nerves contract) and also serve as a semipermeable barrier. (Thus they have a myoid and epithelioid function, as well as connective tissue-secreting function.) In light microscopy, the unusual nature of the perineurium is not evident. The epineurium is the dense connective tissue that surrounds the outer part of a nerve, binding all the fascicles together. It is typical dense connective tissue and contains the blood vessels that supply the nerve.
Figure 16 shows a small unmyelinated nerve lying in the dermis of the skin. (It is taken from slide #25 of your collection.) Nerves that you see in typical tissue sections look like wavy bundles. They follow a zigzag course which allows stretching during movement. They generally consist of a single fascicle surrounded by perineurium, an epineurium separate from the surrounding connective tissue cannot usually be identified. Most of the nuclei belong to Schwann cells, some to fibroblasts.
Motor end plates:
Figure 17 shows the terminal part of an axon of a motor neuron dividing into several branches, each of which terminates on a separate muscle fibre as a motor end plate. One motor neuron may innervate a few to over a thousand muscle fibres, depending on the fineness of the movement.
An example of a sensory receptor:
Figure 18 shows a Pacinian corpuscle. Pacinian corpuscles are encapsulated sensory receptors sensitive to pressure and course touch, vibration and tension. At their centre is a single, unmyelinated nerve fibre, which becomes myelinated when it leaves the corpuscle. Surrounding this are many concentric layers of flattened cells separated by interstitial fluid spaces, giving the corpuscle the appearance of an onion.
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