General Organization

At the most general level of organization, the nervous system consists of the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and the spinal cord, and contains the centers for reaction to environmental stimuli. The PNS is divided into sensory and motor branches. The sensory branch is known as the afferent nervous system, and consists of all the neurons that bring information from receptors to the CNS.

The motor branch of the PNS, the efferent nervous system, is further divided into somatic and autonomic branches. The somatic nervous system carries information from the CNS to the skeletal muscles, and is generally associated with initiation of voluntary action. An individual impulse travels from the CNS to an effector organ along the axon of a single motor neuron. The autonomic nervous system carries information from the CNS to smooth muscles and glands, and is generally associated with involuntary action. An individual impulse travels from the CNS to an effector organ through a chain of at least two motor neurons.

The autonomic nervous system is divided into sympathetic and parasympathetic branches. The sympathetic nervous system is largely concerned with the diversion of energy to the skeletal muscles, and is generally associated with so called “fight or flight” mechanisms. The parasympathetic nervous system is largely concerned with the diversion of energy to the viscera, and is generally associated with “rest and digest” activities.

Bundles of axons outside the CNS are called nerves, and usually contain both afferent and efferent fibers. Bundles of axons within the CNS are called tracts. All of the fibers of a particular tract are usually carrying information in the same direction, either toward or away from the brain.

Collections of cell bodies outside the CNS are referred to as ganglia; collections of cell bodies within the CNS are called nuclei. Ganglia of the afferent nervous system are located just outside the spinal cord along the dorsal root of each spinal nerve (dorsal root ganglia). Sympathetic nervous system ganglia are located in side branches of spinal nerves close to the spinal cord (prevertebral or sympathetic chain ganglia). Parasympathetic nervous system ganglia are close to or within the tissues of effector organs (terminal ganglia).

Within the CNS are areas of white and gray matter. White matter consists of myelinated axons and their associated neuroglia, usually organized into tracts. Gray matter consists of cell bodies, dendrites, and unmyelinated axons along with their associated neuroglia. Collections of cell bodies within the CNS are referred to as nuclei. Nuclei are centers of activity where information is either sent to other parts of the brain or spinal cord or acted on directly.

ORGANIZATION OF SENSORY RECEPTORS

Our sensory receptors can be grouped into two categories: special sensory receptors that only exist in one or two places in the body, and general sensory receptors, which can be found many places throughout the body. Special sensory receptors include photoreceptors for vision (the rods and cones of the retina: rods for light intensity and cones for color), mechanoreceptors for hearing and equilibrium, and chemoreceptors specialized for smell and taste.

General sensory receptors include the stretch receptors (also called muscle spindles) and the Golgi tendon organs, the two primary proprioceptors. The tactile receptors spread throughout the body include all of receptor that help create your sense of touch, including free nerve endings. In areas of the body covered with hair, your sense of touch is aided by hair root plexus receptors, which translate the movement of hair into neural signals. Thermoreceptors code for core temperature and innocuous changes in environmental temperature, while nociceptors encode pain signals related to damaging heat, damaging cold, and other stimuli that may be dangerous to tissue. The interior state of the body is also monitored by baroreceptors that are sensitive to pressure, and chemoreceptors specialized for sensing pH and levels of oxygen, carbon dioxide, toxins, drugs, and hormones.

Touch sensation transmitted from the skin in specific dermatomal patterns that are largely consistent across individuals. The entire surface of the body can be divided into dermatomes: small bilateral regions for which all of the region’s sensation is transmitted to the CNS by a single pair of spinal nerves. This means that impingement of or damage to individual spinal nerves can impair sensation to just the skin in the corresponding dermatome.