High-Yield Neuroanatomy 5e

Spinal Cord 51
A. Function. The posterior column—medial lemniscus pathway mediates fine touch, conscious proprioception,
and vibratory sense.
B. Receptors include Pacinian and Meissner corpuscles, joint receptors, muscle spindles, and Golgi
tendon organs.
C. First-order Neurons are located in the spinal ganglia at all levels. Central processes project to
the spinal cord through the medial root entry zone. First-order neurons give rise to the:
1. Gracile fasciculus from the lower extremity.
2. Cuneate fasciculus from the upper extremity.
3. Collaterals for spinal reflexes (e.g., myotatic reflex).
D. Second-order Neurons are located in the gracile and cuneate nuclei of the caudal medulla.
They give rise to axons and internal arcuate fibers that decussate and form a compact fiber bundle (i.e.,
medial lemniscus). The medial lemniscus ascends through the contralateral brain stem and terminates
in the ventral posterolateral (VPL) nucleus of the thalamus.
E. Third-order Neurons are located in the VPL nucleus of the thalamus. They project through
the posterior limb of the internal capsule to the postcentral gyrus—the primary somatosensory cortex
(Brodmann areas 3, 1, and 2).
F. Transection of the Posterior Column—Medial Lemniscus Tract
1. Superior to the sensory decussation, transection results in contralateral loss of the posterior
column modalities.
2. In the spinal cord, transection results in ipsilateral loss of the posterior column modalities.
II Anterolateral System (Figures 6-5 and 6-7)
A. Function. The anterolateral system is comprised of three main pathways: the spinothalamic tracts
(lateral and medial) that mediate pain and temperature and crude touch, respectively; the spinoreticular
tract, which carries pain to the reticular formation for arousal; and the spinotectal tract that mediates
auditory and visual reflex orientation of the head and neck.
B. Receptors are free nerve endings. Pain ascends on both fast- and slow-conducting pain fibers (i.e.,
A-δ and C, respectively).
C. First-order Neurons are found in the spinal ganglia at all levels. Central projections enter the
spinal cord through the posterolateral tract (of Lissauer) (lateral root entry zone) to second-order neurons.
D. Second-order Neurons are found in the posterior horn. They give rise to axons that decussate
in the anterior white commissure and ascend in anterior aspect of the contralateral lateral funiculus.
Their axons terminate in the VPL nucleus of the thalamus.
E. Third-order Neurons are found in the VPL nucleus of the thalamus. They project through the
posterior limb of the internal capsule to the primary somatosensory cortex (Brodmann areas 3, 1, and 2).
F. Transection of the Lateral Spinothalamic Tract of the anterolateral system results in
contralateral loss of pain and temperature inferior to the level of the lesion.
III Lateral Corticospinal Tract (Figures 6-5 and 6-8)
A. Function. The lateral corticospinal tract mediates voluntary motor activity, primarily of the upper
limbs.