21.2 Muscle Stretch Reflex Flashcards
What are the two types of proprioceptors and what is the function of each?
- 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
Describe the structure of muscle spindles.
- 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
Summarise the different fibres that innervate muscle spindles.
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
Describe the structure of a Golgi tendon organ.
- Ib afferent fibres are interwoven with collagen fibres at the muscle-tendon junction
- Stretching the tendon compresses and stimulates the nerve
What is the function of the Golgi tendon organs?
- 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
What are primary and secondary spindle afferent fibres?
[IMPORTANT]
- Primary -> Type Ia fibres
- Secondary -> Type II fibres
What is the size principle of motor nerves?
The size principle states that motor units will be recruited in order of size from smallest to largest depending upon the intensity.
Summary the patellar tendon reflex.
What are some recurrent pathways (i.e. those that affect the muscle of origin) in the muscle stretch reflex?
[IMPORTANT]
Renshaw inhibitory interneurons inhibit the muscle that is firing to prevent fatigue (feedback inhibition).
What are the different spinal interneurons you need to know about?
[IMPORTANT]
- 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
What are the main descending control tracts influencing the muscle stretch reflex?
Corticospinal
Reticulospinal
Vestibulospinal
Which spinal level coordinates the bicep tendon reflex?
C5-C6
Which spinal level coordinates the patellar (knee jerk) reflex?
L3/L4
Which spinal level coordinates the Triceps reflex?
C7
Which spinal level coordinates the Achilles reflex?
S1-S2
What is the orientation of muscle spindles and golgi tendon organs in relation to extrafusal fibres? Why?
Muscle spindles are in PARALELL to detect stretch and changes in length
Golgi tendon organs are loacated in SERIES to detect tension and force
What makes muscle spindles important in proprioception?
Signal changes in the length in muscle which is closely associated with changes in the angles of joints that the muscles cross
What is the stretch reflex? What is it important for?
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
How many synpases are made in the stretch reflex?
MONOSYNAPTIC
Ia afferent fibre synapses with the efferent motor neuron
What is the function of the muscle stretch reflex?
- 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.
Describe what happens during the crossed extensor reflex
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)
Give an example of when the crossed extensor reflex would be useful
Enhances postural support when flexion-withdrawal of a foot from a painful stimulus occurs
What are the contributions of the spindle and joint receptors to position sense?
*Muscle spindles - muscle length + any changes
*Golgi tendon organ - muscle tensionctive)
