Cell Biology

SENDING PATHWAYS

Cells in the spinal cord receive inputs from ipsilateral structures. Pathways

to the thalamus cross to terminate on the contralateral side.

The major ascending pathways are the dorsal (posterior) columns and

the anterolateral system.

Pain,Temperature, and Tactile

• Simple receptors, unmyelinated, or poorly myelinated fibers.

• Enter via dorsal root and may ascend or descend a few segments.

• Secondary fibers cross the midline in the ventral commissure and ascend

in ventral and lateral funiculi (ventral and lateral spinothalamic tracts).

• Terminate in ventral posterior lateral nucleus of thalamus.

• Tertiary fibers project via the internal capsule to terminate in the postcentral gyrus.

• Injury to the spinothalamic tracts results in loss of pain and temperature

sensation on the opposite side of the body.

• Syringomyelia interrupts pain and temperature fibers crossing in the

ventral white commissure and thus results in bilateral sensory deficit.

Proprioception,Tactile Discrimination, and Stereognosis

• Primary fibers arising from more complicated receptors are generally

well myelinated.

• Afferents enter the spinal cord via the dorsal root and ascend in the dorsal funiculus. The dorsal funiculus divides into a medial fasciculus gracilis (sacral, lumbar, and lower thoracic inputs) and a lateral fasciculus

cuneatus (upper thoracic and cervical inputs). Both fasciculi terminate in

corresponding nuclei in the medulla.

• Secondary fibers from the nucleus gracilis and nucleus cuneatus cross

the midline and ascend in the medial lemniscus to terminate in the ventral posterior lateral nucleus of the thalamus.

• Tertiary fibers terminate in the postcentral gyrus.

• Muscle spindle information is sent to the cerebellum via two major

pathways. The dorsal spinocerebellar tract originates from Clarke’s

nucleus in the thoracic cord and enters the cerebellum via the inferior

cerebellar peduncle. The ventral spinocerebellar tract originates from

spinal cord gray matter and enters the cerebellum via the superior cerebellar peduncle.

• Interruption of primary fibers in the dorsal funiculus will cause loss of

proprioception, and so forth, on the same side of the body as the lesion.

• Interruption of secondary fibers in the medial lemniscus will give rise to

contralateral deficits.

• Tabes dorsalis and pernicious anemia attack the dorsal funiculi.

Trigeminal Pathways

• Primary trigeminal fibers enter at the level of the pons.

• Primary afferents of the descending root terminate in the spinal trigeminal nucleus.

• Secondary fibers ascend through the medulla and pons as the trigeminal

lemniscus to terminate in the ventral posterior medial (VPM) nucleus of

the thalamus.

• The ascending root primary tactile afferents terminate in the main sensory nucleus of CN V.

• Secondary fibers ascend in the trigeminal lemniscus to the VPM.

• The cell bodies of the primary proprioceptive afferents from the muscles

of mastication are located in mesencephalic nucleus of V, and thus are

“like” dorsal root ganglion cells embedded in the brain. They project to

the motor nucleus of V for a monosynaptic jaw jerk reflex.

• Lesion of the descending root of V and the adjacent lateral spinothalamic

tract on one side of the medulla will result in pain and temperature

deficits on the contralateral side of the body and the ipsilateral side of the

head Vestibular Pathways

• Primary afferents terminate in the vestibular nuclei and in the cerebellum

on the same side.

• Secondary fibers ascend or descend in the medial longitudinal fasciculus

or the ventral funiculus of the spinal cord.

• Unilateral lesions of the vestibular system result in movement of the

head, body, and eyes (nystagmus) to the affected (ipsilateral) side. Symptoms include vertigo, nausea, and a tendency to fall to the affected side Visceral Afferents

• Primary general visceral afferents have cell bodies in the dorsal root ganglia 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.