afferents

An afferent is a nerve fiber or pathway that transmits information from peripheral receptors toward the central nervous system. Afferents are part of the sensory division and are contrasted with efferents, which carry commands from the CNS to muscles and glands. Broadly, afferents are classified as general (somatic or visceral) and special (the senses that occur via dedicated receptors). General somatic afferents convey touch, pressure, temperature, pain, and proprioception from the skin, muscles, and joints, while general visceral afferents carry information from internal organs such as fullness, nausea, and internal pain. Special afferents include the receptors of vision, hearing, balance, taste, and smell.

Anatomically, most sensory neurons are pseudounipolar. Their peripheral processes terminate in receptors and their central processes enter the CNS via dorsal roots in the spinal cord or through cranial nerves with sensory ganglia (for example, the trigeminal ganglion). The cell bodies reside in dorsal root or cranial sensory ganglia.

Within the CNS, afferent fibers terminate in specific nuclei or tracts. Somatic afferents reach the dorsal horn and ascend in pathways such as the dorsal column–medial lemniscus for fine touch and proprioception, or the spinothalamic tract for pain and temperature. Visceral afferents project to brainstem nuclei, including the solitary nucleus, and can influence autonomic responses or reach higher centers via thalamic and brainstem relays.

Clinically, assessing afferent pathways is central to neurologic examination, with tests for light touch, vibration, proprioception, and nociception. Damage to afferent pathways can result in sensory loss or paresthesias and may contribute to referred pain patterns and autonomic disturbances.