The Nervous System

A Story by Luke

An expository essay elaborating on the CNS and PNS systems of the body.

The Nervous System

There are many types of cells in the body. Some of these include goblet cells which produce mucus, transitional epithelium which is exposed to much wear and tare, as well as blood cells which help transfer oxygen to the different regions of the body. All of these cells are different in their own way, and maybe the most different of all is the Neuron. The Neuron or nerve cell is an electrically charged cell whose purpose is to receive, process, and then transfer information.

The Neuron:

(http://www.allaboutspace.com/subjects/anatomy/brain/gifs/Neuron.GIF)

This exchange of information takes place within the different subsystems of the nervous system. The nervous system is broken up into two subsystems, the CNS and PNS. The CNS is the largest portion of the Nervous System and includes the brain and spinal chord. This system is covered in protective meninges which are composed of three layers; Dura, Arachnoid, and pea mater. Dedicated to processing information though a process called integration the CNS is a very powerful system (The Central Nervous System). Integration takes place as a sensory input is processed and a motor output is then formed. Yet even with this complex system the CNS cannot complete all the needed functions needed by the body. This is where the PNS or peripheral nervous system comes into the picture. This system exists outside the CNS and is not protected by bone. This means that is it more susceptible to physical injuries as well as toxins. Consisting of mostly nerves this system is divided. Branching into two systems known as the afferent division and the efferent divisions the PNS forms an intricate pathway of nueronic flow. Neurons are first received by sensory receptors in different organs of the body. These impulses, caused by an outside stimulus, are then sent through afferent pathways to the PNS thence forth to the CNS (The Peripheral Nervous System). At times, a response is needed to change the state of that organ or muscle. This change is made due to the efferent division of the PNS. In the motor, or efferent division, impulses are carried from the CNS to the PNS through efferent pathways to effectors in organs. This division is again dived into two subsystems deemed the somatic nervous system and the autonomic nervous system. The main responsibility of the somatic nervous system is to coordinate the body’s movements as well as allow people to voluntarily or consciously control their muscles. This differs much from the involuntary, or autonomic (ANS), system which controls the body’s activity of smooth and cardiac muscles. All of this control is a result of many subsystems and divisions working together, yet it would not be possible if not for the coordination of the Nervous System as an entirety. This delicate yet strong system is a glorious unity of control and the subconscious. Yet even in this vast collaboration of nerves and sensory organs, none would be possible without the simple neuron.

Anatomy of the neuron is general in all types the cell. The center of the cell is known as the cell body. This region is the metabolic center of the neuron. The cell body contains organelles, rough ER, and Nissl substance which help maintain cell shape. Protruding from the center of the cell are arm-like processes known as dendrites. These fibers convey incoming messages toward the cell body. At the other end of the neuron is the axon. This process generates and conducts nerve impulse away from the cell body. These impulses are made possible through the axon terminals at the end of the axon itself. These terminals contain many membranous sacs which contain chemicals called neurotransmitters. When a nerve cell receives an impulse from another neuron’s axon, its axon terminals are stimulated to release the neurotransmitters into the receiving cell’s extracellular space. Yet between the axon terminal of one cell and the other cell there lays a tiny gap. This area is known as the synaptic cleft. Neuron cells are never actually touching during this nuerotrasmitic exchange. These nerve cells are usually covered with a fatty material which is white in color. Myelin protects, insulates and increases the transmission rate of nerve impulse. The production of this sheath is done by specialized cells. In the CNS oligodendrocytes wrap themselves around the axon.

This jelly-roll union results in the myelin sheath. This process is duplicated in the PNS only with the assistance of Schwann cells. All of these different and specialized areas keep the simple nerve cell working like clockwork (The Central Nervous System). The neuron brings a means to the nervous system by which action maybe taken in response. Newton’s third law states, “For every action, there is an equal and opposite reaction (Henderson).” This is seen every time two neurons interact within the vast nervous system and its many subdivisions.

Even with its amazing functions and complex anatomy even the mighty neuron must differ to be able to provide adequate function for the body. Neuron classification is considered by structure and function. Structural classification is done by counting the number of dendrites that each neuron posses. Some neurons such as those found in PNS ganglia are unipolar. As prefixed, the word uni means one, or single, leaving these cells to poses a single process. This process is short and promptly branches into a central and a peripheral process. It is through these divisions that the cell is able to conduct nerve impulses to and from the cell. Yet in some cases neurons are bipolar, these neurons have only two processes. These processes are one axon and one dendrite. Bipolar neurons are rare in adults and are only found in some special sense organs. When a neuron posses multiple dendrites with a single axon it is deemed multipolar. These are structurally the most abundant of all the neuron types. Different structurally, yet functionally similar? The answer to this question is yes. When classified functionally these neuron types are divided into two subdivisions. The first of the two divisions are the sensory, or afferent, neurons. These cells carry impulses from sensory receptors to the CNS. These cells help the CNS stay informed constantly with the goings on of the body. These cells are similar to efferent neurons. These efferent, or motor, neurons differ in their direction of travel. Whereas afferent deliver impulses through afferent pathways, these motor neurons carry information from the CNS to viscera and muscles in the body. While many know human rights should not be ‘separate but equal’ this phrase is effective in the equality of the neuron classifications. Each of them is different, yet all work together within a system to fulfill the body’s demands.

As one delves into the aspect of neuron function one gains an insight into how these amazing cells actually carryout their functions. As anatomy reaches physiology it is portrayed how the different structures or the neuron interact and cooperate.

Neuron Function:

(http://www.sfn.org/skins/main/images/brainbriefings/neurotrophic.jpg)

The simple nerve cell has only to major functional properties. On one hand there is irritability; this being the ability to respond to a stimulus thus converting it into a nerve impulse. On the other hand one finds conductivity. This function allows the transmission of impulses to other neurons within the nervous system. When this transmission takes place many things must happen for the process to go smoothly. This communication between two cells occurs to generate an impulse. As the cell is stimulated and the balance of positively and negatively charged ions changes, the cell begins is process. This delicate balance is maintained by phagocytes or ion pumps, yet changes as the ions cross the cell membrane thus activating the cell.

Phagocytes (Ion Pump):

(http://nobelprize.org/nobel_prizes/chemistry/laureates/1997/illpres/cell.gif)

This forming impulse is an all or nothing response. This signal is either conducted over the entire axon or it ceases to happen all together. The cell would then restore the electrical conditions within its body in a process called repolarization. However when conductivity occurs this all or nothing response is fulfilled. The electrical impulse jumps from one axonal node to another creating conductivity. After jumping from node to node the action potential will then reach the axon terminals on the distal end of the neuron (Glodt). At this time tiny vesicles are release into a small gap between the two neurons called the synapse. These vesicles contain the neurotransmitter chemical, which attach to the membrane of the opposing neuron. If enough neurotransmitters bind to the membrane the whole process starts over again due to the change the ion balance in the opposing cell. Yet at times this process takes place without the chemical element. When gap junctions are created between two neurons, the electrical impulse is able to travel directly from one cell to the other thus creating an electrical synapse. These synapses along with impulses and action potential are among the smallest pieces of the nervous system’s vast puzzle of cells and synapses. They are the gears that allow neurons to carry their information which is always of paramount importance.

Yes the neuron is an amazing cell and performs many crucial functions within the body, but not even the neuron can do it alone. Like every great system the CNS has many parts and differing jobs for each cell type. The CNS is not only composed of neurons it is also contains many support cells and tissue. These cells are known as Neuralgia. These cells are meant to insulate, support and protect the fragile neurons. Nearly half of all neural tissue is made up of star shaped cells known as Astrocytes. These cells accomplish assistance of the nerve cells by anchoring them to their nutrient providing bloodlines. Astrocytes also act to protect the neurons from toxins in the blood by forming a living barrier between capillaries. But not all cells protect from blood related injury. Schwann cells (in the PNS) and their counterparts Oligodendrocytes (in the CNS) do more than just help protect the cell from harm. As these cells wrap their fatty processes around nerve fibers it gives the neuron much more than just a layer of fatty tissue. This insulating covering known as the myelin sheath actually increases the transmission rate of nerve impulse sent via the axon. These examples not only show many cells are interacting and helping each other, they prove that the nervous system is more diverse than some may think.

Fatty coverings may protect against some physical damage, but the system is flawed. All of the support cells in the nervous system give immense protection and serve the body very well, yet not all damage can be avoided. In some terrible cases physical damage can occur to the fragile and delicate system of nerves and cells (King). While the nerves and neurons in the CNS are protected in the dorsal cavity of the body, the structures such as the brain and spinal chord are open to injury if the conditions allow. In the cranial cavity resides the brain which is protected by three layers of meninges. Within this network of protection flows a constant supply of cerebrospinal fluid which offers even more protection for the brain. The skull offers a solid barrier between the brain and the outside world, encasing the brain with hard flat bones. Bones also provide protection for the spinal cord. The spine is made up of 24 segments; seven cervical vertebrae located in the neck, twelve thoracic vertebrae are found in the dorsal region of body, the abdominal portion of the spine is made up of five lumbar vertebrae. These irregularly shaped bones provide a strong system of protection for the spinal chord. Although this system is very formidable against physical ailments, it cannot save all. Brain damage is caused by physical destruction or degeneration of the brain cells.

This horrible fate can reach anyone who falls on their neck or back and injures the nervous system. These injuries can lead to paralysis which can be temporary or permanent. Treatment for these ailments vary with the type of doctor sought. Some doctors (neuropsychologists) seek to study the effects of this damage, whereas others (neurosurgeons) strive to treat and mitigate the damage. Treatment is not always a guarantee that one will make a recovery from a severe injury to the nervous system. This just proves that the nervous system and its subsystems are still very susceptible to injury even with its extensive support and protection system.

As with all ailments that afflict the body, the nervous system is attacked by more than just physical injury. Brain damage and damage to the spinal cord causes extensive damage to the function of the nervous system, but other disorders and diseases wreak havoc upon the CNS and PNS as well. These afflictions vary as do all diseases. The dementia causing Alzheimer’s disease affects western civilization every day (Brown). This disease, which affects more women than men, is caused by atrophy in the cerebral cortex and other areas of the forebrain. This disease causes short and long term memory loss in the afflicted individual. Along with memory loss, the individual may not be able to move or function without the assistance of others. Yet not all of these disorders are debilitating. Autism is a birth defect that is classified as a developmental disorder (Autism). This ailment results in impairment in social interaction, restricted communication, repetitive actions and specialized interests. Autism can be caused genetically or because of a virus in the pregnant woman’s first trimester. There is no cure and the afflicted person will live their entire life lacking adequate social and communication skills. Some disorders of the nervous system affect more than just behavior however. Multiple Sclerosis is an inflammatory disease that affects neuron cells. This disease destroys the Oligodendrocytes which make up the myelin sheath. After sustaining this damage the neurons can no longer conduct their electrical signals. This affects the person’s nervous system in various ways. Any neurological symptom can accompany this disease. As some diseases are caused through genetics, some are contagious though other means. Poliomyelitis, also known as Polio is a highly contagious disease which is passed by human-to-human contact. If this disease enters the nervous system it will destroy motor neurons and cause paralysis and a loss of muscle control. Since there is no cure for Polio, treatment is dedicated to lessening the symptoms. Long term rehabilitation as well as antibiotics help the afflicted person make a recovery. Even with these treatments physical control may still be affected. As with all injuries and afflictions these disorders make life painfully hard. Treatments exist, but few can ever escape the ravaging effects of these frightful diseases. All one can do is take the precautions to avoid injury to this system. If one does so, the system will continue to provide constant, reliable, and rapid neuron function.

Although the nervous system and its subdivisions are greatly protected, it is not perfect. Injury can be sustained during physical activity, as well as during pregnancy. The CNS and PNS are protected by an intricate network of support cells and ganglia. These protections allow the nervous system to work functionally on a daily basis. Divisions and subdivisions all specialize in accordance with the systems function, forming a system of systems. Cells known as neurons work together to deliver messages to and from the brain. The nervous system’s super computer control system is a complex formation of structures from the cerebellum to the spinal chord. In its entirety, the nervous system is a harmony of fast moving signals and electrical impulses. Each part and section of this vast system works together for the full functionality of the body. As the great Benjamin Franklin once wrote, “We must all hang together, or assuredly, we shall all hang separately.” This statement only emphasizes the importance of communication and cooperation. This couple can be seen clearly within the nervous system. From the smallest cell, the neuron, to the vast network of peripheral nerves the nervous system is the perfect example of neural collaboration.