06/09/2017
The human nervous system consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS contains the brain and spinal cord. The PNS comprises the nerve fibers that connect the CNS to every other part of the body. The PNS includes the motor neurons that are responsible for mediating voluntary movement. The PNS also includes the autonomic nervous system which encompasses the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. The sympathetic and parasympathetic nervous systems are tasked with the regulation of all involuntary activities. The enteric nervous system is unique in that it represents a semi-independent part of the nervous system whose function is to control processes specific to the gastrointestinal system. The nervous systems of the body are composed of two primary types of cell: the neurons that carry the chemical signals of nerve transmission, and the glial cells that serve to support and protect the neurons.
Two important concepts relate to the functioning of the nervous system. These terms are efferent and afferent. Efferent connections in the nervous system refer to those that send signals from the CNS to the effector cells of the body such as muscles and glands. Efferent nerves are, therefore, also referred to as motor neurons. Afferent connections refer to those that send signals from sense organs to the CNS. For this reason these nerves are commonly referred to as sensory neurons.
Another important cellular structure in nervous systems are the ganglia. The term ganglion refers to a bundle (mass) of nerve cell bodies. In the context of the nervous system, ganglia are composed of soma (cell bodies) and dendritic structures. The dendritic trees of most ganglia are interconnected to other dendritic trees resulting in the formation of a plexus. In the human nervous system there are two main groups of ganglia. The dorsal root ganglia, which is also referred to as the spinal ganglia, contains the cell bodies of the sensory nerves. The autonomic ganglia contain the cell bodies of the nerves of the autonomic nervous system. Nerves that project from the CNS to autonomic ganglia are referred to as preganglionic nerves (or fibers). Conversely, nerves projecting from ganglia to effector organs are referred to as postganglionic nerves (or fibers). Generally the term ganglion relates to the peripheral nervous system. However, the term basal ganglia (also basal nuclei) is used commonly to describe the neuroanatomical region of the brain that connects the hypothalamus, cerebral cortex, and the brainstem.
