The neurone The unit of the nervous system is the neurone. This may be defined as a nerve cell with all of its processes (protoplasmic outgrowths). The number of processes may be one, two or many. When single, the process divides into two and strictly speaking the neurone has two functional processes. The neurone is highly specialized for the conduction of impulses by the processes (nerve fibers) which are capable of conducting in both directions. Normally in the body, however, conduction takes place in one direction only, that is, there is polarity. This polarity is a function either of the nerve cell or of the point of contact between the processes of one neurone and those of the next in the chain. This junctional structure is termed the synapse. That there may be conduction in a forward direction, one or more of the processes of the nerve cell act as receiving antennae, as it were, and conduct impulses only toward the cell. These processes are known as dendrites or dendrones. Others propagate the impulses in a direction away from the cell body and are termed the axons. Their central axis cylinders contain the conducting neuro-fibrils. These neuro-fibrils terminate in an end brush and come in contact in the central nervous system with the dendrones or cell body of the next neurone of the chain, or peripherally terminate in some structure as the motor end plate of the muscle fiber, or on a glandular cell, or on some other structure. The size of the nerve cell as well as the length and diameter of a single nerve fiber varies considerably. In afferent nerve fibers whose cell bodies are to be found in ganglia which lie just outside of the central nervous system (dorsal root ganglia), the dendrones are usually longer than the axis cylinder and may reach the length of three feet or more, as in the case of a sensory nerve fiber from the toe whose cell body is situated in the root ganglion of the lower lumbar region near the level of the hips. On the other hand, the efferent fibers, whose nerve cells lie in the gray matter of the spinal cord, usually have short dendrones while their axis cylinders may reach a great length. All intermediate types are to be found. Nerve fibers do not usually exist singly, but are grouped together in larger or smaller bundles or nerve trunks, being held together by connective tissue, the epineurium. The single fibers are separated, one from another, by delicate sheets of connective tissue which are continuous with the epineurium (perineurium and endoneurium). Fibers serving quite diverse functions may be grouped together side by side in the same nerve trunk. Thus the nerves running to the arm contain motor fibers to the muscles and to the blood vessels, secretory fibers to the sweat glands, sensory and other afferent fibers from the muscles, skin, bone, and tendon. The characteristic features of the nerve cell are the presence of cytoplasmic granules (Nissl granules or bodies), which are readily stained with methylene blue and other basic dyes, and a clear globular nucleus. The neuro-fibrils of the nerve-cell body are continuous with those of the axis cylinder and end brush.		Home Classes of levers Varieties of muscle Smooth muscle The blood supply to muscles The neurone Cardiac muscle On the nomenclature of skeletal muscles The nerve supply to muscles Structure of a skeletal muscle Central nervous system The reflex arc and reflex action Physiologic properties of muscle Mechanical Changes The effects of temperature on muscular contraction Muscle glycogen, its source and fate The reaction of muscles Oxygen consumption of muscle Determination of the oxygen consumption of muscle The viscosity factor in muscle and muscle efficiency Significance of the recovery heat Production of Electricity