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Free STEP1 Exam Braindumps (page: 24)

Page 23 of 213
PDF with 845 Questions

Which of the following thalamic nuclei subserves a motor function?

  1. dorsomedial nucleus
  2. lateral geniculate nucleus
  3. medical geniculate nucleus
  4. ventral lateral nucleus
  5. ventral posterior medial nucleus

Answer(s): D

Explanation:

The ventral lateral nucleus of the thalamus receives projections from the deep cerebellar nuclei and, in turn, sends projections to the motor cortex. It is thus functionally defined as a thalamic motor nucleus. The dorsomedial nucleus (choice A) is considered a limbic nucleus of the thalamus as it receives projections from the amygdala and olfactory cortex, and projects to the prefrontal cortex and hypothalamus. The lateral (choice B) and medial geniculate (choice C) nuclei and the ventral posterior medial nucleus (choice D) are considered sensory nuclei as they receive information mediating vision, audition, and somatosensory signals from the face, respectively. In turn, these three brain areas project to the somatosensory cortex.



A neurology resident is testing the jaw-jerk reflex in a patient by tapping gently on the right masseter muscle and observing elevation of the mandible. What is the location of the neuronal cell bodies of the proprioceptive fibers mediating the jaw-jerk reflex?

  1. mesencephalic trigeminal nucleus
  2. motor trigeminal nucleus
  3. principal (main) trigeminal nucleus
  4. spinal trigeminal nucleus
  5. trigeminal (gasserian) ganglion

Answer(s): A

Explanation:

The jaw-jerk reflex is a monosynaptic (stretch) reflex for the masseter muscle. Proprioceptive fibers from the muscle travel by way of the trigeminal nerve back to their cell bodies in he mesencephalic trigeminal nucleus. Projections from this nucleus synapse on motor neurons of the motor trigeminal nucleus (choice B) which elicits contraction of the masseter muscle. The principal (main) trigeminal nucleus (choice C) receives light touch sensory information from the face, whereas the spinal trigeminal nucleus (choice D) receives pain and temperature sensations. The trigeminal (gasserian) ganglion (choice E) contains the cell bodies of all sensory neurons projecting to the principal (main) and spinal trigeminal nuclei.



Which of the following statements concerning muscle spindles is correct?

  1. Activation of type Ia sensory fibers from a given spindle leads to inhibition of the muscle in which that spindle is located.
  2. Alpha motoneurons synapse directly with intrafusal muscle fibers.
  3. Each intrafusal fiber is innervated by two different gamma motoneurons.
  4. Only one type of intrafusal muscle fiber (cell) is present in most muscle spindles.
  5. Type Ia sensory fibers from a spindle form direct synaptic contact with alpha motoneurons in the spinal cord.

Answer(s): E

Explanation:

The type Ia sensory fibers from a spindle form direct excitatory synapses with alpha motoneurons.
Activation of type Ia sensory fibers (choice A) leads to excitation of the muscle in which that spindle is located. Alpha motoneurons (choice B) synapse with extrafusal muscle fibers, whereas gamma motoneurons synapse with intrafusal muscle fibers. Each intrafusal muscle fiber (choice C) is innervated by only one gamma motoneuron. Each muscle spindle contains a mixture of both nuclear bag and nuclear chain intrafusal fibers, not just one type as indicated in choice D.



Which of the following is characteristic of damage to the corticospinal (pyramidal) system?

  1. Babinski's sign
  2. flaccid paralysis and hypotonia
  3. immediate muscle degeneration and atrophy
  4. intention tremor
  5. loss of deep tendon reflexes

Answer(s): A

Explanation:

The Babinski sign--dorsiflexion of the great toe in response to stroking the plantar aspect of the foot-- is a characteristic sign of pyramidal tract involvement. Signs and symptoms of corticospinal tract injury that are nearly always apparent to some degree include spastic paralysis, hypertonia, loss of deep tendon reflexes, and hyperactive abdominal and cremasteric reflexes. Flaccid paralysis and hypotonia (choice B) are commonly seen following lower motoneuron injury, as is loss of deep tendon reflexes (choice E). Muscle degeneration and atrophy (choice C) are not characteristic symptoms of corticospinal tract damage. The presence of an intention tremor (choice D) is a sign of cerebellar damage, and is not seen with corticospinal tract lesions.






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