DEFINITIONS OF NEURAL STRUCTURES

Agonist: A muscle in a state of contraction, with reference to its opposing muscle, or antagonist.

Antagonist: Something opposing or resisting the action of another. One of two muscles which pull in nearly opposite directions.

Autonomic: The original meaning of the word referred to a system of neurons which were thought to be self-controlling, and independent of outside influences. Experimental research invalidated this idea. Instead, the "autonomic" nervous system regulates movements of the viscera in response to both external and internal stimuli. Therefore, like the somatic nervous system, peripheral autonomic nerves contain both sensory afferents and motor efferents to form reflexes of the spinal cord and brainstem. Like the reflexes of the somatic nervous system which are controlled by upper motor neurons in the cerebral cortex, autonomic reflexes are regulated by "upper motor neurons" located in the hypothalamus.

Axolemma: A cell membrane of an axon.

Axon: The part of the neuron which relays information transmitted through its soma and to the next neuron in the relay system.

Central Nervous System (CNS): Those neurons whose parts (dendrites, somata, and axons) lie entirely within the brain and spinal cord.

Dendrites: The part of the neuron which receives electrochemical information from a previous neuron in a chain of relays.

Dermatome: The area of skin innervated by sensory axons of a single dorsal root.

Dorsal primary ramus: A dorsal (posterior) division of a mixed spinal nerve that supplies the "true back" between the angles of the ribs.

Dorsal root: Contains sensory axons only, relaying pain, temperature and touch information from the body into the central nervous system. The cell bodies of these axons are in the dorsal root ganglion.

Effector: A peripheral organ that receives nerve impulses and reacts by movement (muscle) or secretion (gland).

Enteric nervous system: The nervous system of the gut, i.e., the neuronal networks of the submucosal and myenteric (Auerbach's) plexuses in the walls of the gut. A variety of experiments show that this nervous system can function essentially normally in the absence of autonomic inputs, whose effects are modulatory.

Gray matter: Collections within the central nervous system of nerve cell bodies.

Lower motor neuron: A neuron whose cell body lies in the central nervous system, and whose axon leaves the CNS through a foramen, and terminates on an effector.

Mixed spinal nerve: Contains both sensory and motor axons.

Motor neuron: Same as Lower Motor Neuron.

Myelin: A lipid material secreted in layers around axons by Schwann cells in the PNS and oligodendroglia in the CNS. This wrapping insulates electrically the axon from the surrounding extracellular environment, resulting in saltatory conduction between Nodes of Ranvier. This in turn increases axonal conduction velocity.

Myotome: All the muscles innervated by motor neurons of a single ventral root.

Nerve cell body (soma): Contains the metabolic machinery of the cell.

Nerve: A bundle of peripheral nerve fibers bound together into one or more fascicles by connective tissue.

Neurolemma: The outer sheath of a peripheral nerve fiber formed by a Schwann cell (i.e., its myelin wrapping).

Neuron: A cell specialized for rapid communication. It receives information in the form of a electrochemical signals from one neuron and relays it to the next in a sequence.

Node of Ranvier: The point along a peripheral nerve fiber at which the myelin

sheath is discontinuous and at which the neurolemma dips to the neuraxis.

 Nucleus: There are two definitions, denoting different structures.

1. The organelle within a nerve cell body containing chromatin and a nucleolus.
2. A group of nerve cell bodies within the central nervous system.

Peripheral Nervous System (PNS): Those neurons lying wholly or in part outside of the brain and spinal cord. The spinal and cranial nerves are examples.

Receptive Field: The set of points at which specific stimuli can influence the excitability of an individual neuron. For the somatosensory system it is the area of skin which can affect the afferent neuron supplying it. For the visual system it is the area within the visual field which influences an individual sensory neuron. For the auditory system it is the set of sound frequencies which alter the excitability of individual afferent auditory neurons. "The peripheral receptive fields of sensory units innervating a sensory surface vary greatly in size. The numbers of innervating nerve fibers per unit area of a body part vary also, usually inversely with the sizes of peripheral receptive fields. This peripheral innervation density of a body part or, for example, of the retina varies directly with the volume of central nervous tissue, e.g., of the cerebral cortex, devoted to the representation of that part, as indeed it does with the use and capacity of the part as a sentient organ." (Mountcastle, Medical Physiology, Vol II)

Receptor: An organ specialized to respond to a physical stimulus, such as depression of the skin (touch), light (vision), sound (hearing), or airborne or liquid-borne molecules (smell or taste). The receptor is contacted by a sensory neuron which relays information about the physical stimulus to the CNS.

Reflex: A reflex is a movement or activity of the body performed automatically and without conscious volition in consequence of activation of sensory neurons which transmit impulses toward the central nervous system, and which in turn activate motor axons which cause movements of muscles or glands. Reflexes may be purely somatic, purely autonomic or mixed somatic and autonomic.

Sensory Neuron: A neuron which relays information from a receptor into the CNS.

Somatic: In a general sense, pertaining to the body (soma). Usually, it is employed in the more specific sense of pertaining to the striated musculature of the body, in contrast to the viscera.

Synapse: The site at which one neuron chemically influences the next neuron in a chain or pathway of neurons.

Tract: A bundle of nerve fibers (i.e. axons) within the central nervous system.

Ventral primary ramus: A ventral (anterior) division of a mixed spinal nerve innervating the remainder of the body wall and the extremities. Ventral primary rami join to form nerve plexuses.

Upper motor neuron: A neuron which lies entirely within the CNS and causes movement because it terminates on a lower motor neuron.

Ventral primary ramus: A ventral (anterior) division of a mixed spinal nerve that supplies limbs and the anterolateral aspects of the trunk.

Ventral root: Contains motor axons and supplies effector organs. Their cell bodies are in the intermediate and ventral gray matter of the spinal cord.

Viscera: Pertaining to the large organs of the thoracic and abdominal cavities.

White matter: Collections of myelinated axons within the central nervous system.

White ramus communicans: Contains general visceral afferents and general visceral efferents (preganglionic sympathetic axons).