Control of Muscle Length Flashcards
What are the three types of movement?
1 - Involuntary / reflex (simplest).
2 - Voluntary (most complex).
3 - Rhythmic (a combination of reflex and voluntary).
Describe the pathway of output from the brain to motoneurones.
1 - Initiation of motor act.
2 - Association cortex.
3 - Basal ganglia and cerebellum.
4 - Thalamus.
5 - Primary motor cortex.
6 - Brainstem.
7 - Spinal neurones.
8 - Motoneurones.
List 4 sources of sensory feedback for motor movements.
1 - Visual system.
2 - Vestibular system.
3 - Skin receptors.
4 - Proprioceptors.
Which of the sources of sensory feedback for motor movements are responsible for somatic sensation?
Skin receptors and proprioceptors.
Which of the sources of sensory feedback for motor movements are responsible for the vestibulo-ocular reflex and optokinetic reflex?
The visual and vestibular systems.
Which proprioceptors are found in series with / parallel to muscle fibres?
1 - Muscle spindle (in parallel with fibres, composed of intrafusal muscle fibres).
2 - Golgi tendon organs (NOT in the muscle but in tendons, in series with fibres).
What do proprioceptors in the muscle help control?
Relex of muscle length and tension.
What regions of the muscle spindle are there?
- Central and polar regions.
- Central regions have primary and secondary sensory afferent endings.
- Polar regions adjust the sensitivity of spindle.
What are the polar regions of muscle spindle innervated by?
Gamma motoneurones.
What are the functions of the regions of the muscle spindle?
Describe the innervation of these regions.
- Central regions are innervated by primary and secondary sensory afferents:
Primary afferents respond to velocity of lengthening.
Secondary afferents respond to static length.
- Polar regions adjust the sensitivity of the spindle.
- Polar regions are innervated by gamma motoneurones.
How are the primary and secondary afferents of the spindle distributed?
Primary afferents innervate the whole central region whereas secondary afferents innervate the ends of the central region.
Where does integration of tonic muscle contraction occur?
In the spinal cord.
What is the difference between intrafusal and extrafusal muscle fibres?
Intrafusal muscle fibres are sensory organs, whereas extrafusal muscle fibres contract.
List the stages of the stretch reflex.
1 - Muscle stretches.
2 - Increased afferent signals generated by gamma motoneurones of intrafusal fibres.
3 - Afferent signals travel to the spinal cord where integration occurs.
4 - Alpha motor neurones carry efferent signals to the extrafusal muscle fibre.
5 - Muscle contracts.
Explain the role of gamma efferents in reflexes.
- Passive stretch activates gamma efferents, which can initiate corrective reflexes.
- If an alpha motor neurone is activated without firing of gamma efferents, the spindle loses activity when the muscle contracts, so no corrective reflexes can occur.
What is the purpose of alpha-gamma co-activation?
To maintain spindle function when a muscle contracts.
Why does tapping the patellar tendon cause the patellar / knee jerk reflex?
- Tapping the tendon stretches the muscle spindle of the quadriceps, initiating a stretch.
- The quadriceps contract and the hamstrings relax.
List 2 functions of reflexes.
1 - To maintain stability.
2 - To avoid dangerous stimuli.
How are reflexes able to be context dependent?
Gamma motoneurones can control feedback gain of spinal loops via feedforward mechanisms from higer motor centres.
When a painful stimulus activates a nociceptor, why are both the flexion reflex and the crossed extensor reflex activated?
- The flexion reflex moves the limb away from the painful stimulus (causing a weight shift).
- The crossed extensor reflex contracts muscles to support the weight shift.
What is the function of golgi tendon organs?
To respond to tension changes in the muscle.
Which neurones innervate golgi tendon organs?
Myelinated type IIb fibres.
What does activation of myelinated type IIb fibres innervating the golgi tendon organs result in?
- Myelinated type IIb fibres inhibit glycinergic (type IIb) alpha motoneurones (opposite effect of spindle).
- Causes inhibitory reflex in the efferent signal, so reduction in contraction strength.
Where in the spinal cord do myelinated type IIb fibres from golgi tendon organs synapse with interneurones?
In the ventral horn.
How does the threshold at which myelinated type IIb fibres (from golgi tendon organs) activate interneurones differ from that of gamma motoneurones (from muscle spindle)?
The threshold for myelinated type IIb fibres is higher than for gamma motoneurones.
What is the purpose of the inhibitory reflexes generated by golgi tendon organs?
- Initially believed to prevent overstretching.
- Now believes to assist in fine control of muscle length (playing a role in posture).
How might golgi tendon organs respond to fatigue?
- Fatigue reduces force in muscle.
- The golgi tendon organ inhibition will reduce to increase the force in the muscle.
