Function of the Neuron

The function of a nerve cell is to pass on an electrical impulse. A neuron at rest has a membrane potential of -60mV, this is set by a specific balance of ion gradients and ion permeabilities which is largely controlled by voltage gated sodium and potassium channels, in a resting cell these are usually closed. An electrical stimulus causes the sodium channels to open which makes the membrane ten times more permeable to sodium than to potassium, this causes the sodium to rush into the cell making it positive on the inside instead of negative, the membrane potential peaks at 40 mV. This is called depolarization, once triggered it propagates down the axon toward the axon terminal. Once the membrane potential has peaked, it quickly repolarizes, this happens because the sodium channels are inactivated and remain closed preventing any sodium from entering the cell, and also the potassium channels open so potassium can leave the cell. This eventually restores the membrane potential to the resting -60mV. The wave of depolarization propagates down the axon till it reaches the axon terminal where it causes the terminal buds to release a neurotransmitter which diffuses into the synaptic gap, the space between one neurons axons and another neurons dendrites, and then either causes an inhibitory or excitatory response in the postsynaptic neuron that influences whether that neuron will go through depolarization. The glial cells aid in the propagation of the wave of depolarization because when they wrap around axons they leave a space between each myelin sheath known as nodes of nodes of Ranvier. Myelination decreases the ability of the neuronal membrane to retain electrical charge which permits a depolarization event to spread further and faster than it would along a nonmyelinated axon. This is important in times where a very fast response is beneficial (Becker et al, 2002).

The Central Nervous System

The Central nervous system consists of cerebrum, cerebellum and the spinal cord, it has little connective tissue and is a soft gel-like organ (Junqueira and Carneiro, 2005).
When stained, they show regions of white matter and regions of grey matter, the white is composed of myelinated axons and the myelin producing oligodendrocytes (Junqueira and Carneiro, 2005). The grey matter contains neuronal cell bodies, dendrites and the unmyelinated portions of the axons and glial cells. The grey matter is prevalent at the surface of the cerebrum and the cerebellum, as compared to the white matter which is present in more central regions (Junqueira and Carneiro, 2005).

In the cerebral cortex, the grey matter has six layers of cells of all different forms and sizes. There are both sensory neurons and motor neurons in the different regions of the cerebral cortex which control voluntary movements. The cells of the cerebral cortex have functions related to the integration of sensory information and the initiation of voluntary motor responses, see figure 1-7(Junqueira and Carneiro, 2005).

Figure 1-8 Silver-stained section of cerebral cortex showing many pyrimad-shaped neurons with their processes and a few glial cells. Medium magnification (Junqueira and Carneiro, 2005)

There are three layers of the cerebellum: a molecular layer, a Purkinje layer and a granular layer.The molecular layer is the outermost layer, it cells are less dense than those in the granular layer, it also has the dendrites of the Purkinje cells that occupy the Purkinje layer. The Purkinje layer composed of large Purkinje cells, these have a conspicuous cell body and have highly developed dendrites. The Granular layer is the inner layer, it is formed by small neurons which are very compacted together (Junqueira and Carneiro, 2005). The cerebellum and its layers can be seen in figures 1-8 and 1-9.

Figure 1-8 Photomicrograph of the cerebellum. The staining procedure used (H&E) does not reveal the unusually large dendritic aborization of the Purkinje cell. Low magnification (Junqueira and Carneiro, 2005).

Figure 1-9 Section of the cerebellum with dintinct Purkinje cells. H&E stain. Medium magnification. (Junqueira and Carneiro, 2005).

The Spinal cord has white matter on the outside and grey matter on the inside usually forming an H shape and has large and multipolar neurons The horizontal bar in the H of the grey matter forms the central canal, the legs of the H form the anterior horns and the arms of the H form the anterior horns.The central canal is remnat of the luman of the embryonic neural tube and the anterior horns have the motor neurons whose axons make up the ventral roots of spinal nerves, the anterior horns receive sensory fibers from neurons in the spinal ganglia (Junqueira and Carneiro, 2005). A cross section of the spinal cord can be seen in figure 1-10.

Figure 1-10 Cross Section of the spinal cord in the transition between grey matter (Below) and white matter (Above). PT Stain. Medium Magnification (Junqueira and Carneiro, 2005)

The skull and the vertebral column protect the central nervous system, but they are aided by a membrane of connective tissue called meninges. The meninges are the dura matter, arachnoid and pia matter. Dura matter is the external layer and is composed of dense connective tissue with the periosteum of the skull. The arachnoid has two compartments: a layer that connects with the dura matter and a layer of trabeculae connecting to the pia matter. The Pia matter is loose connective tissue that contains the blood vessels (Junqueira and Carneiro).

Pathology

There are many different diseases of the nervous system, including Parkinson’s, tumors of the nervous system, Alzheimer’s and multiple sclerosis, to only name a few.
Multiple sclerosis, also known as MS, is the most common cause of neurologic disability, it is a autoimmune disease, which happens when the body mistakes its own self cells as antigens and attacks them. In the case of MS the body attacks the central nervous system. It is usually diagnosed between the ages of 20 and 40 and some symptoms are mild including numbness in the limbs or very severe like paralysis or loss of vision. People who have the disease produce autoreactive T lymphocytes that participate in the formation of inflammatory lesions along the myelin sheath of nerve fibers. The cerebrospinal fluid of patients with active MS contain activated T lymphocytes that infiltrate the brain and cause the characteristic inflammatory lesions which destroys the myelin. This breakdown in the myelin sheath leads to numerous neurological problems (Kindt Goldsby and Osborne, 2007).
Like most autoimmune diseases, the cause of MS is not well known, though some studies have proposed a link between MS and infection by certain viruses. It is known that some viruses can cause demyelinating diseases so it is easy to speculate that a viral infection plays a role in the development of MS (Kindt et al, 2007).

References

Becker Wayne M. , Kleinsmith Lewis J. , and Hardin, Jeff. The World of the Cell fitfh edition. Benjimen Cummings. San Francisco, CA. 2002.

Hill Richard W. , Wyse Gordon A. , and Anderson Margaret. Animal Physiology. Sinauer Associates, Inc. Massachusetts USA. 2004.

Junqueira Luiz Carlos, and Carneiro Jose. Basic Histology text and atlas eleventh edition. McGraw-Hill. USA. 2005.

Kindt Thomas J. , Goldsby Richard A. , and Osborne Barbara A. Kuby Immunology sixth edition. W.H Freeman and Company. New York. 2007.