Question

Sensory systems allow us to perceive different types of stimuli in the environment. Compare similarities and differences of sensory transduction processes in the visual, hearing, vestibular and somatosensory systems. Discuss common causes, molecular mechanisms of sensory transduction dysfunction and potential treatment approaches for correcting sensory transduction in the aforementioned sensory systems

Answer 1

The Visual System:

The sensory system and the development of the individual senses occur in the afferent and efferent motion perception. The afferent motion is the movement of the objects pertaining to the environment; whereas, efferent is consecutive to movements to the eyes, body or head (Kapoula & Thuan, 2006). The afferent motion perception consists of two visual systems: focal and ambient. The focal system also known as central vision, specializes in object motion perception and object recognition; whereas, ambient or peripheral vision is sensitive to movement scene and is thought to dominate both perception of self-motion and postural control. The retinal slip, a part of the afferent motion perception, is related to a person’s displacement by the central nervous system (CNS), and is used as feedback for compensatory sway (Guerraz & Bronstein, 2008).

Although it is a known fact that vision is the primary sensory system used in balance (Poole, 1991; Merla & Spaulding, 1997; Uchiyama & Demura, 2009); it must be noted that one can stand in the dark and remain upright. However, research has shown spontaneous lateral body oscillations are largely reduced when standing objects fixate a small light emitting diode (LED) in an otherwise darkened environment (Guerraz & Bronstein, 2008). Therefore, postural stability increases with the improvement of the visual environment. There are also other contributing parameters that affect visual control of posture such as object size and localization, binocular disparity, visual motion, visual acuity, depth of field, and spatial frequency. The peripheral vision on postural control deserves some recognition. The peripheral vision rather than the central vision plays an essential role in maintaining stable quiet stance. A study conducted by Berencsi, Ishihara, & Inanaka (2005), showed visual stimulation of the peripheral visual field decrease postural sway in the direction of the observed visual stimulus to the antero-posterior rather than medial-lateral. The authors concluded peripheral vision operates in a viewer-centered frame of reference. Therefore, “peripheral vision is used either for visual stabilization of spontaneous body sway or visually-induced body sway is more likely due to the size of stimulated field manipulated than to functional specialization of the peripheral vision for postural control” (Guerraz & Bronstein, 2008, p. 394).

There are two hypotheses that attempt to explain how individuals maintain stability despite eye movements: inflow and outflow theory. The inflow theory proports proprioceptive receptors (e.g., muscle spindles) of the extraocular muscle provide the information about the position and displacement of the eyes in the orbit. Whereas, the outflow theory states the branches of the neural outflow (e.g., corollary discharge) or an efference copy (e.g., signals about the eye movements) informs the CNS to maintain visual consistency (Guerraz & Bronstein, 2008).

The Vestibular System:

The vestibular system is unique from other systems because it becomes immediately multisensory and multimodal. For example, the vestibular system interacts with the proprioceptive system coupled with corollary discharge of a motor plan allowing the brain to distinguish actively generated from passive head movements (Angelaki & Cullen, 2008). Also, both visual and proprioceptive systems interact with the vestibular system throughout the central vestibular pathways and are essential for gaze and postural control.

The brain stem contains premotor neurons and second-order sensory neurons that receive afferent input and send it directly to the motoneurons, making it a streamlined circuitry of short latencies. “Simple pathways also mediate the vestibulo-spinal reflexes that are important for maintaining posture and balance”(Angelaki & Cullen, 2008, p. 127).

The interaction of multisensory and multimodal pathways is important for higher level of function such as self-motion perception and spatial orientation and it is largely due to inherent complexity.

The Somatosensory System:

To maintain normal quiet, stance and to safely accomplish the majority of activities of daily living, individuals rely primarily on proprioceptive and cutaneous input.

The CNS processes multimodal afferent input and integrates it at various levels, resulting in efferent processing for coordinated firing of multi alpha motoreurons and their corresponding muscle fibers (Shaffer & Harrison, 2007).

The muscle spindles play an important role in proprioception. It is mechanoreceptors that provide the nervous system with information about the muscle’s length and velocity of contraction, thus contributing to the individual’s ability to discern joint movement and position sense (Shaffer & Harrison, 2007). The muscle spindles also provide afferent feedback that translates it to appropriate reflexive and voluntary movements.

Another organ that contributes to proprioceptive information is the golgi tendon organ (GTO). The GTO located at the muscle tendon interface relays information about tensile forces, and is sensitive to very slight changes (Shaffer & Harrison, 2007). When GTO is activated, the afferent neuron synapses in the spinal cord interneurons, which inhibit the alpha motoneuron of the muscle resulting in decreased tension within the muscle and the tendon.