Neuroscience Analysis of Occupational Performance (OCTH 455)
and
Neuroscience (PTRS 850)
Sensory Systems Review: Question #5

The Scenerio

• .... you turn your head to look at the patch of ice as you begin to fall, keeping your eyes on that one spot as your head and body twist and turn wildly!

  Diagram: Vestibulo-ocular response
  

  1. Input #1: vestibular receptor activity --> (cnVIII) --> vestibular nuclei

  2. Output #1: vestibular nuclei --> (medial longitudinal fasciculus) --> brainstem nuclei of c.n. III, IV and VI --> (c.n. III, IV and VI) --> extraocular muscles
     or
     Output #2: vestibular nuclei --> (medial longitudinal fasciculus then medial vestibulospinal tracts) --> postural muscles of head & neck

  3. Input #2 (feedback): retina --> (optic nerve/chiasm/tract) --> superior colliculus --> brainstem nuclei of c.n. III, IV and VI
     (note that in this situation, maintaining eye position is reflexive so there is no need to involve the cortex)

  Output behaviors would include coordinated activity among the extraocular muscles of both eyes (via c.n. III, IV and VI on the left and right sides) and postural adjustments in head and neck position (via the left and right medial vestibulospinal tracts). These outputs may be independent of other, ongoing activity in other pats of the body.

• .... you extend your arms to each side, your free leg kicks out, your back arches, and your head turns - all before you are able to plant your supporting foot firmly on the ground.

  Diagram: Rapid postural adjustments
  

  1. Input #1: vestibular receptor activity --> (cnVIII) --> vestibular nuclei (& cerebellum)

  2. Output #1: vestibular nuclei --> (lateral vestibulospinal tracts) --> large postural muscles of trunk & muscles of legs
     or
     Output #2: vestibular nuclei --> cerebellum --> (cerebellar peduncles) --> red nucleus --> (rubrospinal pathway) --> large muscles of trunk & legs

  3. Input #2 (feedback):proprioceptive receptors --> (spinocerebellar tracts then cerebellar peduncles) --> cerebellum

  Output behaviors involve facilitation of those muscles used to counteract gravity (a mixture of flexors & extensors in the arms & legs) and postural adjustments at the level of the spinal cord & cerebellum (fast & reflexive rather than voluntary & therefore too slow to be of help) involving the large muscles of the legs & trunk. The lateral vestibulospinal, reticulospinal, and rubrospinal tracts on both sides of the body would play a large role in these behaviors. Spinal reflex arcs in multiple regions would help to quickly adjust muscle tone in the involved muscles, providing reflexive reciprocal innervation of agonist and antagonist muscles on either side of the joints involved with these postural adjustments. Postural adjustments of the head and neck would involve the left and right medial vestibulospinal tracts.

1. What neurological deficits would impair your ability to execute these behaviors?
   • damage involving proprioceptive inputs, dorsal roots, or dorsal horn of the spinal cord
   • damage to the ascending proprioceptive pathways (e.g., spinocerebellar tracts or cerebellar peduncles)
   • damage to the cerebellum, lesions of cerebellar output pathways (cerebellar peduncles)
   • damage to motor output pathways (rubrospinal, vestibulospinal, reticulospinal tracts; note that these are all are upper motor neurons)
   • damage to either the alpha (muscle contraction) or gamma (spindle length) motor neurons (both are lower motor neurons).
   • damage to inputs from other sensory systems (e.g., vision, vestibular) used to providing feedback about your performance.

2. What clinical assessments would be appropriate? One of the more simple ones would be the Romberg test; remember that a positive Romberg test (swaying) indicates there is a problem. Other tests are available (tilt tests, sway tables, etc.) which evaluate the function of specific elements of the behavior such a posture adjustment, vestibular sensitivity, etc.

3. What other behavioral changes or problems would you be likely to experience? Some degree of gravitational insecurity, wide-based stance & gait (ataxia), increased caution when using stairways (particularly when going down the stairs), avoidance of stairs entirely, some degree of 'clumsiness', increased risk for injury (sprained ankles, etc.).

Go to the Practice questions page for this course.