Файл: Kaplan USMLE-1 (2013) - Anatomy.pdf

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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Section IV • Neuroscience

ChapterSummary(Conrd)

Dorsal Column-Medial Lemniscal System

•This pathway conducts sensory information for touch, proprioception, vibration, and pressure. The primary afferent neurons ofthis pathway have their cell bodies in the dorsal root ganglia. Their axons enter the spinal cord and ascend in the dorsal columns of the white matter as the fasciculus gracilis (from lower limb) or the fasciculus cuneatus (from upper limb). They synapse with the second neuron in the same named nuclei in the lower medulla. Axons ofthe second neuron decussate (internal arcuate fibers) and ascend the midline of the brain stem in the medial lemniscus to reach the ventral posterolateral (VPL) nucleus of the thalamus. The third neuron then projects through the posterior limb ofthe internal capsule to the somatosensory cortex. Lesions above the decussation (in the brain stem or cortex) produce contralateral loss of joint position, vibration, and touch, whereas lesions below decussations (in the spinal cord) produce ipsilateral deficits below the level of the lesion. A positive Romberg test indicates lesions of the sensory input to the cerebellum.

Anterolateral System

•The anterolateral pathway carries pain and temperature sensations. The first neuron fibers enter the spinal cord and synapse in the dorsal horn with the second neurons. The first neuron often ascends or descends one or 2 segments before they synapse. The second neuron axons then

decussate (ventral white commissure) and ascend the spinal cord and form the spinothalamic tract in the lateral funiculus of the white matter. The spinothalamic tract ascends the lateral aspect ofthe brain stem and synapses in the VPL nucleus ofthe thalamus where the third neuron projects to the cortex. All lesions ofthe spinothalamic tract in the spinal cord, brain stem, or cortex produce contralateral loss of pain and temperature below the lesion. Note that a central cord lesion at the spinal canal (syringomyelia) produces bilateral loss of pain and temperature at the level ofthe lesion.

•Lesions ofthe spinal cord that involve the above-mentioned tracts include poliomyelitis, tabes dorsalis, amyotrophic lateral sclerosis, anterior spinal artery occlusion, subacute combined degeneration, syringomyelia, and Brown-Sequard syndrome.

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The brain stem is divisible into 3 continuous parts: the midbrain, the pons, and the medulla. The midbrain is most rostral and begins just below the diencephalon. The pons is in the middle and is overlain by the cerebellum.

The medulla is caudal to the pons and is continuous with the spinal cord.

The brain stem is the home of the origins or sites of termination of fibers in 9 of the 12 cranial nerves (CNs).

CRANIAL NERVES

Two cranial nerves, the oculomotor and trochlear (CN III and IV), arise from the midbrain (Figure IV-5- 1).

Four cranial nerves-the trigeminal, abducens, facial, and vestibulocochlear nerves (CN V, VI, VII, and VIII)-enter or exit from the pons.

Three cranial nerves-the glossopharyngeal, vagus, and hypoglossal nerves (CN IX, X, and XII)- enter or exit from the medulla. Fibers ofthe accessory nerve arise from the cervical spinal cord.

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Section IV • Neuroscience

SENSORYAND MOTOR NEURAL SYSTEMS

Each of the following 5 ascending or descending neural tracts, fibers, or fasciculi courses through the brain stem and will be found at every transverse sectional level.

Medial Lemniscus

The medial lemniscus (ML) contains the axons from cellbodies found in the dor­ sal column nuclei (gracilis and cuneatus) in the caudal medulla and represents the second neuron in the pathway to the thalamus and cortex for discriminative touch, vibration, pressure, and conscious proprioception. The axons in the ML cross the midline ofthe medulla immediately after emerging from the dorsal col­ umn nuclei. Lesions in the ML, in any part of the brain stem, result in a loss of discriminative touch, vibration, pressure, and conscious proprioception from the contralateral side of the body.

Spinothalamic Tract (Part ofAnterolateral System)

The spinothalamic tract has its cells of origin in the spinal cord and represents the crossed axons ofthe second neuron in the pathway conveying pain and tempera­ ture to the thalamus and cortex. Lesions of the spinothalamic tract, in any part of the brain stem, results in a loss of pain and temperature sensations from the contralateral side of the body.

CorticospinalTract

The corticospinal tract controls the activity oflower motoneurons, and interneu­ ron pools for lower motoneurons course through the brain stem on their way to the spinal cord. Lesions of this tract produce a spastic paresis in skeletal muscles of the body contralateral to the lesion site in the brain stem.

Descending Hypothalamic Fibers

The descending hypothalamic fibers arise in the hypothalamus and course with­ out crossing through the brain stem to terminate on preganglionic sympathetic neurons in the spinal cord. Lesions ofthis pathway produce an ipsilateral Horner syndrome. Horner syndrome consists of miosis (pupillary constriction), ptosis (drooping eyelid), and anhidrosis (lack of sweating) in the face ipsilateral to the side of the lesion.

Descending hypothalamic fibers course with the spinothalamic fibers in the lat­ eral part of the brain stem. Therefore, brain stem lesions producing Horner syn­ drome may also result in a contralateral loss of pain and temperature sensations from the limbs and body.

Medial Longitudinal Fasciculus

The medial longitudinal fasciculus is a fiber bundle interconnecting centers for hor­ izontal gaze, the vestibular nuclei, and the nerve nuclei of CN III, rv,and VI, which

innervate skeletal muscles that move the eyeball. This fiber bundle courses close to the dorsal midline ofthe brain stem and also contains vestibulospinalfibers, which

course through the medulla to the spinal cord. Lesions of the fasciculus produce internuclear ophthalmoplegia and disrupt the vestibulo-ocular reflex.

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