The anatomy of the nerves and muscles is referred to frequently throughout this book. The overview presented here is intended to provide a quick reference for the reader. More specific information is included with each topic as needed.
Three fairly distinct components make up the nervous system in the human body: the central nervous system, which is somewhat analogous to the main processing unit of a computer; the peripheral nervous system (PNS), which links the CNS to the muscles; and the sympathetic nervous system, which links the CNS to the internal organs (see figure). The CNS has two major parts, the brain and spinal cord, which in turn have several subdivisions, each of which plays a unique role in regulating the functions of the body.
The portion of the brain referred to as the cerebrum acts as a master control system and is responsible for initiating all thought and movement. Memory, personality, vision, hearing, touch, and muscle tone all are housed within the cerebrum. Behind the cerebrum is the cerebellum, which coordinates movement and "smoothes" muscle activity. The proper functioning of this region of the brain controls balance during walking and the smooth use of your hands and arms.
Beneath the cerebrum and cerebellum is the brain stem, which contains the nerves that control eye movements and the vital centers that are involved in functions such as breathing and heart rate. Extending downward from the brain stem is the spinal cord, which functions very much like a large electrical cord that carries messages between the brain centers and all other parts of the body. Although numerous biochemical reactions occur in the brain and spinal cord, their major role is to produce electrical activity that stimulates and regulates various bodily activities. These messages
are delivered to the target structures very efficiently and effectively because the entire system is well insulated and shielded by the myelin that surrounds the conducting systems and allows the electrical nervous impulses to move through the pathways with little loss of information. The myelin in the brain and spinal cord is produced by a specific type of cell called an oligodendrocyte (oligo). Both oligos and myelin appear to be injured in MS. When they are injured, the nerve beneath the myelin sometimes also is injured (axonal damage). Oligos disappear as the affected myelin becomes hardened and scarred, forming what is called a plaque and causing a short-circuiting of electrical transmission.
The peripheral nervous system (PNS) is responsible for transmitting electrical messages between the spinal cord and the muscles, including those of the arms and legs. This system also contains myelin, although it is made by a different cell type than the oligo, a cell that does not appear to be affected by MS. Thus, although it is not uncommon to find leg or arm weakness in MS, the problem lies in the central conduction system (the brain and spinal cord), not in the peripheral nerves that lead from the spinal cord.
The autonomic nervous system has two divisions, the sympathetic and the parasympathetic. These systems are responsible for automatic types of function such as the beating of your heart, perspiration, etc. This system also contains myelin but, like the PNS, it is not directly affected by MS.
Although MS directly affects only the CNS, the disease has indirect effects on other systems and their functions because all components of the nervous system communicate with each other.
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