21.2 Muscle Stretch Reflex

Question 1

What are the two types of proprioceptors and what is the function of each?

Answer 1

• Muscle spindles
  
  • Signal muscle stretch
  • Therefore involved in the muscle stretch reflex
• Golgi tendon organs
  
  • Signal tension produced by muscle contraction
  • Therefore are involved in negative feddback to prevent over-contraction

Question 2

Describe the structure of muscle spindles.

Answer 2

• Small (2-4mm long), encapsulated, spindle shaped
• Muscle spindles are made of intrafusal fibres that are in parallel with the extrafusal fibres that provide the main contractile force
• Each spindle has 3 main components:
  
  • Intrafusal muscle fibres that have a non-contractile middle part with a contractile part on each side
  • Large diameter sensory nerve fibres -> Wrap around the non-contractile part of the muscle fibres which is sensitive to stretch of the intrafusal fibre
  • Small diameter gamma motor nerve fibres -> Innervate the contractile ends of the intrafusal fibres

Question 3

Summarise the different fibres that innervate muscle spindles.

Answer 3

Afferent:

• Ia fibres:
  
  • Innervates all fibre types in muscle spindles.
  • Thus reports both muscle length and length changes.
• II fibres:
  
  • Innervates static fibres (static nuclear bag and nuclear chain fibres)
  • Thus primarily reports muscle length.

Efferent:

• Gamma (γ) motor nerves -> Supply the dynamic and the static intrafusal fibres to modulate the activity of these fibres

Question 4

Describe the structure of a Golgi tendon organ.

Answer 4

• Ib afferent fibres are interwoven with collagen fibres at the muscle-tendon junction
• Stretching the tendon compresses and stimulates the nerve

Question 5

What is the function of the Golgi tendon organs?

Answer 5

• The GTO effect was first thought to be purely protective to prevent over-contraction of muscles.
• However, GTOs are sensitive to small changes in tension so they are also involved in fine movement

Question 6

What are primary and secondary spindle afferent fibres?

[IMPORTANT]

Answer 6

• Primary -> Type Ia fibres
• Secondary -> Type II fibres

Question 7

What is the size principle of motor nerves?

Answer 7

The size principle states that motor units will be recruited in order of size from smallest to largest depending upon the intensity.

Question 8

Summary the patellar tendon reflex.

Answer 8

Question 9

What are some recurrent pathways (i.e. those that affect the muscle of origin) in the muscle stretch reflex?

[IMPORTANT]

Answer 9

Renshaw inhibitory interneurons inhibit the muscle that is firing to prevent fatigue (feedback inhibition).

Question 10

What are the different spinal interneurons you need to know about?

[IMPORTANT]

Answer 10

• Ia inhibitory interneurons -> Inhibit the motor neurons that innervate the antagonist in the muscle stretch reflex
• Ib inhibitory interneurons -> Inhibit the over-contraction muscle in the Golgi organ tendon reflex
• Renshaw interneurons -> Inhibit the muscle that is firing in the muscle stretch reflex to prevent fatigue

Question 11

What are the main descending control tracts influencing the muscle stretch reflex?

Answer 11

Corticospinal
Reticulospinal
Vestibulospinal

Question 12

Which spinal level coordinates the bicep tendon reflex?

Answer 12

C5-C6

Question 13

Which spinal level coordinates the patellar (knee jerk) reflex?

Answer 13

L3/L4

Question 14

Which spinal level coordinates the Triceps reflex?

Answer 14

C7

Question 15

Which spinal level coordinates the Achilles reflex?

Answer 15

S1-S2

Question 16

What is the orientation of muscle spindles and golgi tendon organs in relation to extrafusal fibres? Why?

Answer 16

Muscle spindles are in PARALELL to detect stretch and changes in length
Golgi tendon organs are loacated in SERIES to detect tension and force

Question 17

What makes muscle spindles important in proprioception?

Answer 17

Signal changes in the length in muscle which is closely associated with changes in the angles of joints that the muscles cross

Question 18

What is the stretch reflex? What is it important for?

Answer 18

Resists stretching of a muscle to maintain its length
When there is deviation of the intended position, this is detected and the muscle contracts to correct this

Question 19

How many synpases are made in the stretch reflex?

Answer 19

MONOSYNAPTIC
Ia afferent fibre synapses with the efferent motor neuron

Question 20

What is the function of the muscle stretch reflex?

Answer 20

• Important for posture (standing upright), holding a heavy object still in one hand, etc.
• When deviation from the intended position is detected, the muscle contracts to correct this.

Question 21

Describe what happens during the crossed extensor reflex

Answer 21

Flexion-withdrawal reflex activates divergent pathways:
-One excites motor neurons of ipsilateral flexor muscles of the limb (withdraw limb away from noxious stimuli)
-Inhibition of ipsilateral extensor muscles
-Excitation of motor neurons supplying contralateral extensors
-Inhibition of motor neurons supplying contralaterall flexors
(counteracts increased load caused by lifting the limb)

Question 22

Give an example of when the crossed extensor reflex would be useful

Answer 22

Enhances postural support when flexion-withdrawal of a foot from a painful stimulus occurs

Question 23

What are the contributions of the spindle and joint receptors to position sense?

Answer 23

*Muscle spindles - muscle length + any changes
*Golgi tendon organ - muscle tensionctive)