Visceral Afferents
- Primary general visceral afferents have cell bodies in the dorsal root gan-
glia and terminate in the dorsal horn. Ascending secondary neurons make
abundant reflex connections with autonomic and somatic pathways and
terminate in the reticular formation and intralaminar thalamic nuclei. - Central processes of primary general visceral afferents associated with
cranial nerves VII, IX, and X enter the solitary fasciculus and terminate
in the nucleus of the solitary tract. Secondary fibers make reflex connec-
tions with visceral motor nuclei. Taste is represented in the thalamus in
a region medial to the VPM nucleus.
DESCENDING (MOTOR) PATHWAYS- In the brain, the cell bodies of general somatic efferent neurons are located
in columns ventral to the cerebral aqueduct and fourth ventricle and ven-
trolateral to the central canal. Special visceral efferents (associated with
branchial archderived muscle) are located lateral and ventral to the general
somatic efferents. In the spinal cord, they originate in the ventral horn. - These are lower motor neurons, or the “final common pathway.” Total, or
flaccid, paralysis results from destruction of peripheral nerves or motor
nuclei. Destruction of upper motor neurons (from higher centers)
results in spastic paralysis: initally hyporeflexia and later hyperreflexia.
Cerebellar Pathways
- The dentate nucleus receives fibers from the Purkinje neurons of the cerebellum
and projects via the superior peduncle to the reticular formation (descending
limb) and to the basal ganglia/thalamus-motor cortex (ascending limb). - The cerebellum is involved with coordination of fine movements.
- Lesions to the cerebellum or superior peduncle result in ataxia, hypoto-
nia, hyporeflexia, and/or intention tremor on the same side as the lesion.
CORTICOSPINAL (PYRAMIDAL) PATHWAYS- Fibers arise from pyramidal neurons in layer 5 of the precentral gyrus
and premotor areas and descend through the internal capsule and basis
pedunculi, cross at the spinomedullary junction and form the lateral cor-
ticospinal tract in the lateral funiculus of the spinal cord. They terminate
on lower motor neurons in the ventral horn or on interneurons. - Most muscles are represented in the contralateral motor cortex. How-
ever, some (such as the muscles of the upper face and the muscles of
mastication and muscles of the larynx) are represented bilaterally.
High-Yield Facts 41