5. Describe the difference between afferent and efferent innervation. Distinguish between the types of afferent and efferent fibers in the nervous system.

Most of the nerves have both afferent and efferent fibers packaged together. In fact if not explicitly stated as a purely afferent or efferent nerve, assume that any named nerve has both types of fibers.

When thinking of afferent (sensory) innervation, think carrying information away from an organ or sensory receptor or information arriving  at the central nervous system (CNS).

Types of afferent (sensory) fibers:

• Somatic afferent

  • Most commonly discussed afferent fiber

  • Sensation from skin

    • Pain, touch, pressure, and temperature

    • Dermatome maps depict the different conceptual areas of skin afferently innervated by a single cranial or spinal nerve.

  • Proprioception

    • Conveys information to CNS regarding positioning of a joint or the degree of tension in muscle tendons

    • When discussing afferent innervation of a muscle, this is what one is describing

• Visceral afferent

  • Conveys information from hollow viscera (organs) and blood vessels

    • Pain or reflex sensations, such as pressure (e.g. blood) or organ distension

• Special afferent (typically described as special sensory)

  • Associated with special senses (vision, olfaction, hearing, taste, & equilibrium)

  • Fibers of certain cranial nerves

With efferent (motor) innervation, think of carrying information from the CNS to an effector organ, to create an effect (such as muscle contraction or glandular secretion).

Types of efferent (motor) fibers:

• Somatic efferent

  • Impulses from the CNS to skeletal muscle (can lead to contraction)

  • When discussing what nerve innervates a muscle, often considering this aspect of innervation

    • However, most named, somatic nerves have both efferent and afferent fibers, particularly those derived from plexuses.

• Visceral efferent

  • Impulses from the CNS to glands and smooth muscle

  • Associated with autonomic nervous system