vrijdag 23 september 2016

An introduction to the sensorimotor system

Introduction
Contrast sensory and motor tracts. Information travels to the brain via ascending tracts. Ascending tracts within the CNS are (somato)sensory. Descending motor tracts transmit information from the brain to the periphery.

Involved in the major sensory tracts (posterior column, spinothalamic (sensory, from spinal cord to thalamus) and spinocerebellar (from cord to cerebellum) are three divisions of neuronal systems:
Neurons of the 1st order transmit sensory information to the CNS via dorsal or cranial (root) ganglion;
Neurons of the 2nd order act on soma in CNS;
Neurons of the 3rd order transmit information from the thalamus to the cerebral cortex.

Gnostic sensibility pathways
As for gnostic sensibility (fine touch, vibration and proprioception), sensory projections enter the spinal cord via dorsal root ganglion cells. The axon transmits information via the posterior column of white matter, ascending to the brain. A first synapse is made in the caudal medullary gracile nucleus (all levels inferior to the sixth thoracic vertebra, T6) or cuneate nucleus (levels superior to T6); the medullary axon crosses the medial lemniscus, up to the ventral posterior nuclei in thalamus, where it ascends to the somatosensory cortex.

Take notice of the medial-lateral rule: information stemming from mechanosensory receptors from the lower body, entering the lumbar spinal cord, is transmitted through medial pathways within the spinal cord (that is, before the medullary crossover); mechanosensory receptors from the upper body, entering the cervical spinal cord, transmit information via lateral pathways in the spinal cord.

Vital sensibility pathways
Vital sensibility sums up nociception, temperature and crude touch sensations. Pain and temperature sensations, however, travel through antolateral or spinothalamic tracts (tractus spinothalamicus lateralis). Pain ( Aδ fibers, myelinated, fast conducting, or C fibers, unmyelinated slowly conducting) fibers and temperature afferent fibers send their axons to the dorsal root ganglia to enter the posterior gray horn (1st order); these axons synapse onto spinal neurons (this is the crossover site, 2nd order), to project across the medial line, entering the lateral spinothalamic tract, ascending via the ventral posterolateral thalamic nuclei (3rd order), up to S1, on the side opposite to the receptive field.

Within the anterior spinothalamic tracts, nerve fibers conducting information on crude touch and pressure, send their axons to enter the dorsal root ganglia, in order to enter the posterior gray horn (1st order); the second-order site of neuronal cell bodies is the posterior gray horn, but in contrast to the lateral spinothalamic tract as described above, the axons enter the anterior spinothalamic tract on the opposite, that is, the second order is a ventral pathway up to the ventral posterolateral thalamic nuceli (3rd order). Like the antolateral tract, the destination is S1, opposite to the receptive field.














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