Functions of the various neuroglial cell types include:  physical support (protection) for neurons,  insulation for nerve cell bodies and processes that facilitates rapid transmission of nerve impulses,  repair of neuronal injury,  regulation of the internal fluid environment of the CNS,  clearance of neurotransmitters from synaptic clefts, and  metabolic exchange between the vascular system and the neurons of the nervous system
In addition to neurons and supporting cells, an extensive vasculature is present in both the CNS and the PNS. The blood vessels are separated from the nerve tissue by the basal laminae and variable amounts of connective tissue, depending on vessel size. The boundary between blood vessels and nerve tissue in the CNS excludes many substances that normally leave blood vessels to enter other tissues. This selective restriction of blood-borne substances in the CNS is called the blood–brain barrier, which is discussed on page 385.
The nervous system allows rapid response to external stimuli.
The nervous system evolved from the simple neuroeffector system of invertebrate animals. In primitive nervous systems, only simple receptor–effector reflex loops exist to respond to external stimuli. In higher animals and humans, the SNS retains the ability to respond to stimuli from the external environment through the action of effector cells (such as skeletal muscle), but the neuronal responses are infinitely more varied. They range from simple reflexes that require only the spinal cord to complex operations of the brain, including memory and learning.
The autonomic part of the nervous system regulates the function of internal organs.
The specific effectors in the internal organs that respond to the information carried by autonomic neurons include the following:  Smooth muscle. Contraction of smooth muscle modifies the diameter or shape of tubular or hollow viscera such as the blood vessels, gut, gallbladder, and urinary bladder.  Cardiac conducting cells (Purkinje fbers) located within the conductive system of the heart. The inherent frequency of Purkinje fiber depolarization regulates the rate of cardiac muscle contraction and can be modified by autonomic impulses. Glandular epithelium. The autonomic nervous system reg ulates the synthesis, composition, and release of secretions.
The regulation of the function of internal organs involves close cooperation between the nervous system and the endocrine system. Neurons in several parts of the brain and other sites behave as secretory cells and are referred to as neuroendocrine tissue. The varied roles of neurosecretions in regulating the functions of the endocrine, digestive, respiratory, urinary, and reproductive systems are described in subsequent chapters.
The neuron is the structural and functional unit of the nervous system.
The human nervous system contains more than 10 billion neurons. Although neurons show the greatest variation in size and shape of any group of cells in the body, they can be grouped into three general categories.
 Sensory neurons convey impulses from receptors to the CNS. Processes of these neurons are included in somatic afferent and visceral afferent nerve fibers.  Somatic afferent fbers convey sensations of pain, temperature, touch, and pressure from the body surface. In addition, these fibers convey pain and proprioception (nonconscious sensation) from organs within the body (e.g., muscles, tendons, and joints) to provide the brain with information related to the orientation of the body and limbs. Visceral afferent fbers transmit pain impulses and other sensations from internal organs, mucous membranes, glands, and blood vessels.
 Motor neurons convey impulses from the CNS or ganglia to effector cells. Processes of these neurons are included in somatic efferent and visceral efferent nerve fibers. Somatic efferent neurons send voluntary impulses to skeletal muscles. Visceral efferent neurons transmit involuntary impulses to smooth muscle, cardiac conducting cells (Purkinje fibers), and glands (Fig. 12.1).
 Interneurons, also called intercalated neurons, form a communicating and integrating network between the node of Ranvier