Материал: Kaplan USMLE-1 (2013) - Anatomy

MEDICAL 379

Section IV • Neuroscience

Occlusion ofthe anteriorspinal artery

This artery lies in the anterior median sulcus of the spinal cord. Occlusion ofthe anterior spinal artery interrupts blood supplyto the ventrolateral parts ofthe cord, including the corticospinal tracts and spinothalamic tracts. Below the level ofthe lesion, the patient exhibits a bilateral spastic paresis and abilateral loss ofpain and temperature (Figure IV-4-18).

Syringomyelia

Syringomyeliais adisease characterizedbyprogressive cavitation ofthe central canal, usually in the cervical spinal cord but may involve other cord regions or the medulla. Early in the disease, there is a bilateral loss ofpain and temperature sensation in the hands and forearms as a result ofthe destruction ofspinothalamic fibers crossing in the anteriorwhite commissure. When the cavitation expands, lower motoneurons in theventralhorns are compressed, resulting inbilateral flaccid paralysis ofupperlimb muscles. A late manifestation ofcavitation is Homer syndrome, which occurs as a result of involvement ofdescending hypothalamic fibers innervating preganglionic sympathetic neurons inthe Tl throughT4 cord segments. Horner syndrome consists ofrniosis (pupillary constriction), ptosis (drooping eyelids), and anhidrosis (lack of sweating) in the face (Figure IV-4-19).

Tabes dorsalis

Tabes dorsalis is one possible manifestation of neurosyphilis. It is caused by bi­ lateral degeneration of the dorsal roots and secondary degeneration of the dor­ sal columns. There maybe impaired vibration and position sense, astereognosis, paroxysmal pains, and ataxia, as well as diminished stretch reflexes or inconti­ nence. Owing to the loss ofproprioceptive pathways, individuals with tabes dor­ salis are unsure ofwhere the ground is and walk with a characteristic and almost diagnostic "high-step stride" (Figure IV-4-18). Tabetic patients may also present with abnormal pupillary responses (Argyll Robertson pupils).

Subacute combined degeneration

Subacute combined degeneration is seen most commonlyin cases ofvitamin B12 deficiency, sometimes related to pernicious anemia. The disease is characterized by patchy losses ofmyelin in the dorsal columns and lateral corticospinal tracts, resulting in a bilateral spastic paresis and a bilateral alteration oftouch, vibration, and pressure sensations below the lesion sites (Figure IV-4-19). Myelin in both CNS and PNS is affected.

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Chapter 4 • The Spinal Cord

ChapterSummary

•The spinal cord is internally divided into 31 segments that give rise to 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Each segment is divided into an inner gray matter containing neuron cell bodies. The ventral horn of gray contains alpha and gamma motoneurons, the intermediate horn contains preganglionic neurons and Clarke nucleus, and the dorsal horn contains sensory neurons. The outer partofthe spinal cord is the white matter containing ascending and descending axons that form tracts located within funiculi.

Motor Pathways

•The corticospinal tract is involved in the voluntary contraction of skeletal muscle, especially in the distal extremities. This pathway consists of

2 neurons, an upper motor neuron, and a lower motor neuron. Most of the upper motor neurons have their cell bodies in the primary motor cortex and premotor cortex of the frontal lobe. These axons leave the cerebral

hemispheres through the posterior limb of the internal capsule and descend medially through the midbrain, pons, and medulla. In the medulla, 80-90% of these fibers decussate at the pyramids and then descend in

the spinal cord as the lateral corticospinal tract in the lateral funiculus of the white matter. These enter the ventral horn of gray at each cord segment and synapse upon the lower motor neurons. Axons ofthe lower neurons (final common pathway) leave via the ventral root ofthe spinal nerves and innervate the skeletal muscles. Lesions above the decussation (in the brain stem or cortex) produce contralateral deficits, and lesions below the decussation (in the spinal cord) produce ipsilateral findings. Patients with upper motor neuron lesions present with spastic paralysis, hyperreflexia, a clasp-knife reflex, and a positive Babinski. Lower motor neuron lesions present with flaccid paralysis, areflexia, atonia, muscle atrophy, and fasciculations.

Sensory Pathways

•Most sensory systems use 3 neurons to project sensory modalities to the cerebral cortex. The first neuron (primary afferent neuron) has its cell body in the dorsal root ganglion ofthe spinal nerve. This axon enters the spinal cord and either synapses in the spinal cord or the brain stem. The second neuron will decussate and project to the thalamus. The third neuron then projects from the thalamus to the somatosensory cortex of the parietal lobe.

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MEDICAL 381