Physiology, Vibratory Sense

Excerpt

Our somatosensory system has three basic types of sensory receptors that detect different types of external stimuli. These include mechanoreceptors that detect light touch, vibration, pressure, and texture; nociceptors that detect pain; and thermoreceptors that detect temperature. Mechanoreceptors further divide into Merkel disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscles based on the specific type of mechanical stimuli they perceive. The perception of vibratory sensation is by two main types of mechanoreceptors, Meissner corpuscles (MC) and Pacinian corpuscles (PC). MCs are large myelinated fibers that detect low-frequency vibration and are present in glabrous (smooth, hairless) skin on fingertips and eyelids. PCs contain large myelinated A-beta fibers which are rapidly adapting and are present in the deeper layers of skin, ligaments, and joints where their function is to detect high-frequency vibration and deep pressure.

The vibratory sensations transmit via the dorsal column of the spinal cord to the primary somatosensory area of cortex. The sensory signals from Meissner and Pacinian corpuscles terminate in the cell body of dorsal root ganglion (pseudounipolar neurons) and constitute the first-order neuron of the dorsal column. The axonal processes leave the dorsal horn gray matter and enter the dorsal funiculus to constitute either the fasciculus gracilis or the fasciculus cuneatus. Fasciculus gracilis carries sensory information from the lower limbs and synapses with the nucleus gracilis in the caudal medulla. It is located medially in the dorsal column as compared to fasciculus cuneatus which is located laterally and carries sensory information from the upper extremities and synapses with nucleus cuneatus in caudal medulla. That is why fasciculus cuneatus is only present at spinal level T6 and above. Nucleus gracilis and nucleus cuneatus constitute the second-order neurons of this pathway. The fibers from both sides decussate in caudal medulla as internal arcuate fibers and ascend contralaterally as medial lemniscus. The second synapse occurs in the ventral posterolateral nucleus (VPL) of the thalamus, which constitutes the third-order neurons of the pathway. Axons from VPL nucleus travel through the posterior limb of the internal capsule and terminate in the primary somatosensory cortex, located in the postcentral gyrus.