Ascending Sensory Pathways

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Clinical case

A 60-year-old woman complains of falls, imbalance, and numbness and tingling in her hands and legs. There is also some incoordination of hand use and she has difficulty manipulating small items such as buttons. She is unable to play the piano now since she cannot position her fingers correctly on the piano keys. She thinks the strength in her arms and legs is adequate. Symptoms started with very slight tingling sensations, which she noticed about 5 years ago. The falls and difficulty walking have been present for about 2 months. Higher order cognitive functions are intact according to her husband. Her vision is normal.

On examination, the patient shows normal mental status. Strength seems essentially normal throughout. Sensation, particularly to vibration and joint position, is severely diminished in the distal upper and lower limbs (arms, legs, hands, and feet). Tendon reflexes are normal in the arms, but somewhat brisk in the legs at the knees and ankles. Gait is moderately ataxic and she has to reach out for support by touching the walls of the hallway at times. Fine movements of the fingers are performed poorly, even though finger and wrist strength seems normal. 

A variety of sensory receptors scattered throughout the body can become activated by exteroceptive, interoceptive, or proprioceptive input. Exteroceptive input relays sensory information about the body’s interaction with the external environment. Interoceptive input relays information about the body’s internal state, whereas proprioceptive input conveys information about position sense from the body and its component parts. Each receptor is specialized to detect mechanical, chemical, nociceptive (L. nocere, “to injure,” “painful”), or thermal stimuli. Activation of a sensory receptor is converted into nerve impulses and this sensory input is then conveyed via the fibers of the cranial or spinal nerves to their respective relay nuclei in the central nervous system (CNS). The sensory information is then further processed as it progresses, via the ascending sensory systems (pathways), to the cerebral cortex or to the cerebellum. Sensory information is also relayed to other parts of the CNS where it may function to elicit a reflex response, or may be integrated into pattern-generating circuitry. The ascending sensory pathways are classified according to the functional components (modalities) they carry as well as by their anatomical localization. The two functional categories are the general somatic afferent (GSA) system, which transmits sensory information such as touch, pressure, vibration, pain, temperature, stretch, and position sense from somatic structures; and the general visceral afferent (GVA) system, which transmits sensory information such as pressure, pain, and other visceral sensation from visceral structures.