CASE 6-1

A 46-year-old man was admitted with complaints of lower back pain that radiated down to his foot over the last 2 months. The pain was not relieved with medical therapy. What is your diagnosis?

Relevant Physical Exam Findings

●Absent right ankle jerk

●Weakness of dorsiflexion and plantar flexion

●Decreased pinprick over the dorsum of the foot

Relevant Lab Findings

●An anteroposterior myelogram demonstrated compression of the first sacral nerve root on the left at the level of the L5-S1 vertebrae.

Diagnosis

● Lumbar intervertebral disc herniation

Tracts of the Spinal Cord

IPosterior (Dorsal) Column—Medial Lemniscus

Pathway (Figures 6-5 and 6-6)

Figure 6-5 The major ascending and descending pathways of the spinal cord. The ascending tracts are shown on the left, and the descending tracts are shown on the right.

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.

IIAnterolateral 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 fastand 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.

IIILateral Corticospinal Tract (Figures 6-5 and 6-8)

A.Function. The lateral corticospinal tract mediates voluntary motor activity, primarily of the upper limbs.