Composition of Nerves
The human nervous system consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain, spinal cord and nerves. It acts as the central control region of the human nervous system, integrating information and issuing commands. The PNS includes the somatic motor nervous system and the sensory nervous system, which connect the CNS to every other part of the body. The PNS consists mainly of nerves that transmit signals from the brain which are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent. Most nerves serve both functions and are called mixed nerves.
Main Categories of Cells in the Nervous System
The nervous system contains two main categories of cells: neurons and glial cells. Neurons generate and propagate electrical and chemical signals, whereas glia function mainly to modulate neuron function and signaling .
Neurons are the basic information-processing units of the nervous system (see Fig. 6.13) . The main portion of the neuron is the cell body which contains a nucleus and cytoplasm. Over most of the cell there are numerous projections that branch out into still finer extensions. These branching threads are called dendrites. At one end of an elongated structure is a branching mass that is the axon. Along the axon are a number of protuberances labeled as myelin sheaths. The axon and pre-
Fig. 6.13 Features of a generalized neuron. A neuron comprises the soma (cell body), dendrites, and axon. The soma gives rise to numerous dendrites which receive synaptic signals from other neurons. The soma has only one long thin axon which is covered by the myelin sheath and transmits signals to other neurons  (Copyright permission from Elsevier Science Ltd) synaptic nerve terminal contain a large population of mRNAs and an active protein synthetic system which play a critical role in the development, function, and longterm viability of the neuron . Mutations in RNA-binding proteins or perturbations of the mRNA metabolism can cause neurodegenerative disorders .
Glial cells are non-neuronal cells. There are many types of glia that perform various functions . For example, glia form myelin, participate in signal transmission, maintain homeostasis, and provide support and nutrition in the nervous system . In the human brain, the total number of glia is roughly estimated to equal the number of neurons . Glial cells have various functions since they have unique protein compositions that can change depending on activity . The main functions of glial cells are to destroy pathogens and remove dead neurons; to insulate neurons electrically; to supply nutrients to neurons; to support neurons; and to provide guidance cues directing the axons of neurons to their targets .