PoH: Reflexes

Question 1

Define stretch reflex/myotatic reflex

Answer 1

The contraction of a muscle in response to its passive stretching

Question 2

When a muscle is stretched, the stretch reflex regulates the length of the muscle automatically by increasing its ____

Answer 2

Contractility
(as long as the stretch is within physiological limits)

Question 3

Define muscle spindle, and give the location

Answer 3

Stretch receptor that signal the length and changes in length of a muscle

They’re located in an independent capsule, parallel to the main muscle

Question 4

Why do we have a stretch reflex?

Answer 4

Protective mechanism to prevent overstretching

Question 5

What nerve is activated by a stretch reflex? What does it do?

Answer 5

1a afferent sensory nerves

It increases the number of action potentials in other 1a afferents, projecting through the dorsal horn into the spinal cord

Question 6

Name the 3 types of connections of spindle sensory afferent nerves

Answer 6

1. Directly contacting a motoneurons
2. Reciprocal inhibition - indirectly connecting with antagonist muscles
3. Travelling up dorsal column to thalamus and somatosensory cortex

Question 7

What reflex is caused by 1b afferent nerves?

Answer 7

Inverse Stretch Reflex/Golgi-Tendon Organ/Clasp-Knife Reflex

Question 8

What reflex is caused by 1a afferent nerves?

Answer 8

Stretch reflex

Question 9

Explain the Inverse Stretch Reflex

Answer 9

Muscle contracts and shortens, pulling on tendon

Sensory 1b afferent nerves from the Golgi-Tendon Organ increases firing of action potentials

Activation of inhibitory interneurones to the agonist muscle. They decrease in contraction strength

Activation of excitatory interneurones to antagonist muscles

Information about muscle tension ascends the dorsal column to the somatosensory cortex

Question 10

What sensory receptor does the flexor-withdrawal reflex use? What 3 places are they found in the body?

Answer 10

Nociceptors in skin, muscles and joints

Question 11

What does the flexor-withdrawal reflex lead to?

Answer 11

Withdrawal from painful stimulus and towards the body

Question 12

If the flexor-withdrawal reflex is caused by a flame to the left foot, which foot responds?

Answer 12

Both.

Ipsilateral flexion in response to pain

To stop us falling over, the contralateral limb extends via excitatory interneurons crossing the spinal cord, to excite the contralateral extensors

Question 13

In a flexor-withdrawal reflex, how does sensory information ascend to the brain?

Answer 13

Contralateral spinothalmic tract

Question 14

Define interneurone (in the context of reflexes)

Answer 14

A neurone in between a sensory and motor neurone. They coordinate the motor response needed to address a stimulus and communicate with the brain

Question 15

Are the following reflexes ipsilateral or contralateral: Stretch Reflex, Inverse Stretch Reflex, Flexor-Withdrawal Reflex

Answer 15

SR - ipsilateral
ISR - ipsilateral
FWR - contralateral

Question 16

Where in the brain does info about muscle tension in reflexes go?

Answer 16

Somatosensory cortex

Question 17

Explain the role of higher centres in reflexes?

Answer 17

The brain can modify (e.g. intensity and timing) or inhibit reflex actions.

Question 18

Can the stretch reflex be overridden?

Answer 18

Yes

Question 19

What method can doctors use to test the stretch reflex if the patient voluntarily prevents the reflex? Why does it work?

Answer 19

The Jendrassik Manoevre - interlock your fingers in front of your chest and pull hard, trying to pull your hands apart. While doing that, doctor performs a knee-jerk reflex

It works because high activity in upper motorneurones spreads to and depolarises lower level motorneurones, overcoming the descending inhibition

Question 20

In a withdrawal reflex, more pain causes what? And what’s this concept called?

Answer 20

More powerful pain

Greater spinal segmental spread

More muscles are recruited

Larger response

This is called facilitation