Overriding Reflexes

Question 1

How can you override the stretch reflex?

Answer 1

A strong descending inhibition.
Alpha motoneurons are inhibited.

Question 2

How can you enhance the stretch reflex?

Answer 2

Jendrassik manoeuvre - interlock fingers and pull.
Upper motoneurons have high activity.
Lower motoneurons are depolarised.
This overcomes descending inhibition.

Question 3

The _____ of a stretch reflex is not evidence of _____ _____. _____ _____ is highly _____ and only affects _____ _____ at one or two _____ _____.

Answer 3

1 - absence.
2 - nerve damage.
3 - spindle input.
4 - localised.
5 - alpha motoneurons.
6 - spinal segments.

Question 4

How can you override the inverse stretch reflex?

Answer 4

A voluntary excitation of alpha motoneurons overrides GTO inhibition and the muscle stays contracted.

Question 5

Describe how alpha motoneurons integrate all their signals.

Answer 5

The net effect at the cell body is summed (excitation minus inhibition).
Distant dendrites have less effect on membrane potential changes than those nearer the cell body.

Question 6

How can you override the flexor reflex?

Answer 6

Anticipating pain can prevent the flexor reflex.

Question 7

Pain input diffuses over…
More powerful pain causes…

Answer 7

Several spinal segments.
Greater spread, more muscles recruited, larger response.

Question 8

What is the clinical relevance of reflexes?

Answer 8

Assesses integrity of the spinal cord.
Reflexes above a given level may localise a problem.

Question 9

Describe facilitation.

Answer 9

Enhances the effectiveness of sensory inputs. Inputs lead to excitation of nearby motoneurons.

Question 10

Explain how facilitation works.

Answer 10

Motoneurons next to inputs are depolarised. If enough inputs occur, nearby motoneurons reach threshold.