Key Points

• Transmission of sensory information to the central nervous system is triggered when receptors reach the threshold required to initiate action potentials; the number of action potentials reflect the strength of the triggering stimulus.

• Somatic sensory neurons release glutamate at their axon terminals in order to stimulate postsynaptic neurons within the central soma-tosensory pathway.

• Different modalities and submodalities of somatosensory sensation project in discrete pathways within the dorsal and lateral portions of the spinal cord.

• Centrally projecting somatosensory fibers are somatotopicallyarranged, with cells in each dorsal root ganglion responsible for sensations from one area of the body, defined as a dermatome.

• Neuronal cell bodies occupy the central core of the spinal cord and are surrounded by ascending (sensory) fibers in the dorsal and lateral portions of the cord and by descending (motor) fibers in the ventral regions.

• The sensory pathway carrying information about discriminatory touch and proprioception ascends in the dorsal columns of the spinal cord, crosses the midline in the lower medulla, and projects to the thalamus before reaching the somatosensory cortex.

• Information about pain and crude touch is transmitted through fibers that decussate in the spinal cord, ascend in the lateral columns of the spinal cord, and synapse in the thalamus before joining other somatosensory projections in the primary sensory cortex.

• Somatosensory information from the head region is carried by cranial nerve V.

• Subcortical regions that receive somatosensory information include: (1) the cerebellum, which uses proprioceptive information to coordinate movements and posture; and (2) motor nuclei in the spinal cord and brain stem, which control

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