Nervous System Histology

The Central Nervous System

The central nervous system (CNS) is formed by the brain and spinal cord. These elements are enclosed within the skull and spinal vertebral canal. They are covered by the meninges, the dura, arachnoid and pia. Cerebrospinal fluid flows over the surface and fills the chambers (ventricles, central canal of the spinal cord). Two primary cell types make up the CNS - the neurons, and the glia.

Neurons
Neurons are stellate cells with projections extending from the cell body. The projections come into contact with other neurons or outside the CNS, with other cell types such as muscle. There are many types of neuron based on the size and shape of the cell body and the arrangement of the processes. A silver stain developed by Golgi was used for many years to assign neurons to categories. Based on their staining neurons could be seen to be unipolar, bipolar or multipolar. Most of the neurons within the CNS are multipolar. The processes extending from the cell body are either axons or dendrites. Neurons usually have only one axon but many dendrites. The neuronal cell body Neurons are unique in the body with respect to cell size. Although the cell body may only be about 50um in diameter, the axon may be several feet long. This feature of neurons is related to the organization of the organelles in the cell body. Neurons have an extensive cytoplasm and surface membrane to maintain, and must therefore have the machinery to synthesize the components. It is not surprising therefore that the most prominent organelle is the rough endoplasmic reticulum. RER is formed into large bodies in the neuronal cell cytoplasm. By light microscopy these bodies are clearly visible and are termed Nissl bodies, after the discoverer. The cell bodies of neurons also contain extensive neurofilaments and microtubules. These organelles are involved in the transport of materials synthesized in the cell body out into the processes. Other organelles present are common to most cells. The nucleus of many neurons shows a nucleolus, a dense ball of chromatin.

The axon Each neuronal cell body gives rise to a single axon (although there may be two). The axon projects through the neuropil (the mass of cell processes) or through a peripheral nerve to make contact with other cells. The axon contains cytoplasm, microtubules and neurofilaments but no Nissl substance. There is therefore little

ability in the axon to synthesize protein and the requirement for protein substrate must be met by axoplasmic transport from the cell body. Within the CNS axons terminate at synapses on neuronal cell processes or cell bodies. At a synapse the axon is expanded and contains vesicles containing neurotransmitter substances. The neurotransmitters are stored within vesicles. The transmitters are released when the membrane of the vesicle fuses with the presynaptic membrane at the synapse. The vesicle membrane is not released and may be reused. The vesicles may store a range of transmitter molecules either separately or within the same vesicle. Molecules released from synaptic vesicles can bind with either the presynaptic or the post-synaptic membrane, and may inhibit or enhance the activity of other transmitter molecules. Synapses are described according to the two elements involved: axo-axonic, axodendritic, or axosomatic. The pre- and post-synaptic membranes may show increased density, indicating the active site. Some axons terminate close to blood vessels. The axon terminals in this case release their products as neurosecretions. Examples of these axons are found in the posterior lobe of the pituitary gland, where oxytocin and vasopressin are released. The spread of depolarisation from the cell body to the axon terminal begins at the axon hillock, at the base of the axon. The rate of conduction of the impulse depends on axon diameter and the presence of myelin. The rate of conduction of the action potential increases as the diameter of the axon increases. The presence of a myelin sheath around the axon also markedly increases the speed of conduction. The dendrite Most neurons have many dendrites emanating from the cell body. In some neurons the dendrites are organised as apical or basal giving the cell a kind of polarity. The denditic cytoplasm is a reflection of the cytoplasm of the cell body. Dendrites are therefore able to synthesise much of what is synthesised in the cell body. The surface of dendrites is specialised to receive axon terminals. In most cells, dendritic spines are present over the entire surface, e.g. about one million spines are present on the dendrites of each cerebellar Purkinje cell. Neuronal organization Neurons are organized within the CNS as sheets, or as nuclei. In the brain, the cerebral hemispheres have layers of neurons (the grey matter) close to the surface. In the middle of the brain, neurons are grouped into nuclei in the parts of the diencephalon. In the spinal cord the neurons form a central core with the fibres (the white matter) on the outside.

Glial cells
The neuroglia, or nerve glue, have a close relationship with the neurons and act to support the neurons both structurally and functionally. Astrocytes Two types of astrocytes are found in the CNS. Fibrous astrocytes form a type of scaffolding throughout the grey matter. Their cell bodies give off many thin processes which pass through and contribute to the neuropil. Protoplasmic astrocytes have shorter, thicker processes and are usually found encircling blood vessels where they form a continuous covering. Both types of astrocyte have extensive cytoplasmic filaments, the glial fibrillary acid protein cytoskeleton. Both types of astrocyte are able to divide in response to injury. In the presence of appropriate stimuli, astrocytes become phagocytic. Although these cells are not the blood brain barrier, they induce the endothelial cells of the blood vessels to form tight junctions with each other, forming the functional blood brain barrier. Oligodendrocytes The oligodendroglia form the myelin sheaths around central axons. Each cell is able to send out processes which wrap around up to twelve axons. Each process myelinates a short section of axon. Nodes of Ranvier are the short naked axon segments between neighbouring myelinated sections. Myelin is formed by an oligodendrocyte process wrapping around an axon until several layers have formed. As the process of myelination develops, the cytoplasm is squeezed out of the layers until the cell membranes touch. By the fusion of inner and outer leaflets of the cell thick and thin lies alternate in the pattern characteristic of myelin. In some places the cytoplasms does not get completely squeezed out, leaving swellings called incisures of Schmidt-Lantermann. Myelin insulates the parts of the axon it covers. This encourages saltatory conduction in which the impulse jumps from node to node, increasing the speed of conduction many times over. Myelin of axons can be removed and replaced. In some disease processes myelin is destroyed and impulse conduction fails. Microglia In addition to those large, or macroglial, cells just described, small or, microglial cells are also present. These cells are quiescent under normal circumstances. With age they slowly accumulate pigment. In some disease or injury states the microglia become active and proliferate. The active cells are phagocytic and engulf cellular debris.

The Peripheral Nervous System Ganglia

Some neurons exist in ganglia, outside of the central nervous system. The neurons in the ganglia have the same features as those within the central nervous system, except for their relationship to glial cells. Satellite cells usually surround the cell bodies of ganglion cells. Myelination of axons is provided by Schwann cells.

Peripheral nerves
In peripheral nerves the peripheral processes of neurons are formed into bundles by connective tissue. Each axon may be myelinated or unmyelinated. Myelin is produced by Schwann cells, which unlike oligodendrocytes myelinate only one section of one axon. The connective tissue is formed by fibroblasts and lies between axons as the endoneurium, as the wrapping around bundles of fibres the perineurium. Epineurium forms the connective tissue which bundles all of the fibre bundles into a nerve. Since two types of tissue are present in peripheral nerve, there are basement membranes between them. Schwann cells are able to divide in response to injury and become phagocytic. Injury to peripheral axons may be followed by regrowth in which axon sprouts are able to grow back through the connective tissue skeleton of the nerve.

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