Proprioceptors Flashcards
What do type Ia and type II afferents do?
- Type Ia - detect rate of change of muscle length
- Type II - detect changes in muscle length
Describe the basic structure of a muscle spindle.
- Fusiform (spindle-shaped) capsule connected to extrafusal muscle fibres by connective tissue
- Contains miniature, highly specialised intrafusal fibres which stretch along with the capsule
- The intrafusal fibres have a stretchy, non-contractile central portion to which the sensory afferents (proprioceptors) attach
Explain what happens when a muscle is passively stretched.
- Extrafusal and intrafusal fibres are stretched
- Type Ia and II afferents detect the stretch and excite the alpha motor neurons of that muscle and any synergist muscles
- This causes reflex contraction
Explain the role of gamma motor neurons in coordination of the stretch reflex.
- Gamma motor neurons innervate the contractile poles of the muscle spindle
- They regulate the sensitivity of the stretch reflex by shortening or relaxing the poles
- Excitation of gamma motor neurons causes the poles to shorten
- Simultaneous activation of alpha and gamma motor neurons ensures that the central portion of the muscle spindle remains the same length - no afferents will be activated and the movement will not be opposed by the stretch reflex
What is reciprocal inhibition?
- Type Ia afferents inhibit the alpha motor neurons of antagonist muscles via reciprocal inhibitory interneurons
- These interneurons release glycine which acts at ligand-gated ion channels on the alpha motor neuron, causing hyperpolarisation
- Thus reciprocal inhibition produces a rapid but transient effect
- The motor neuron is less responsive to any input, so this inhibition is non-selective
Explain how presynaptic inhibition by type Ia afferents is controlled and describe its effects.
- Type Ia afferents suppress activity in the alpha motor neurons of the antagonist muscle via GABAergic inhibitory interneurons
- These presynaptic inhibitory interneurons terminate on the presynaptic boutons of the antagonist muscle’s type Ia afferents and act via GABAB receptors - GPCRs
- They therefore reduce the release of NT from these afferents, and hence selectively inhibit the type Ia stretch reflex of the antagonist muscle
- The effect is long-lasting as it is mediated via GPCRs
What treatment is used to relieve type Ia mediated hyper-reflexia (i.e. spasticity and hypertonia)?
Local application of GABAB agonists, which damp down the stretch reflex by acting on metabotropic GABAB receptors on the presynaptic boutons of type Ia afferents
When someone performs a controlled, voluntary movement, why doesn’t the stretch reflex oppose it?
Upper motor neurons activate reciprocal and presynaptic inhibitory interneurons, which suppresses activity in the alpha motor neurons of the antagonist muscle
Why are the muscle spindle afferents stimulated by unexpected changes in muscle length but not intentional changes?
- During voluntary movements the gamma motor neurons fire at the same time as the alpha motor neurons
- This shortens the intrafusal fibres in parallel with the shortening of the muscle as a whole
- The central portions of the spindles will remain taut at the same length, so the afferents will remain sensitive to stretch but will not be activated
Explain how muscle spindles help to correct a movement when there is an unexpected load.
- The extrafusal fibres shorten more slowly than expected
- However, the intrafusal fibres shorten at the intended rate, meaning there is a mismatch between the spindle length and muscle length
- The central regions with stretch and activate the type Ia and type II afferents
- Activity in the alpha motor neurons will be increased until the whole muscle “catches up” with the muscle spindles
Hypertonia, resistance to low-velocity stretch, is mediated primarily by which afferents?
Type II afferents
Explain the role of nuclear bag intrafusal fibres.
- Nuclear bag fibres are associated with type Ia afferents
- When stretched slowly, the poles remain very flexible and the central portion is unaffected
- When stretched rapidly, the poles will not change length and the central portion will be stretched, strongly activating the Ia afferents
- This is the monosynaptic stretch reflex, which is velocity-dependent
- The role of this pathway is to provide feedback regarding rapid unintended changes in muscle length
Explain the role of nuclear chain intrafusal fibres.
- Nuclear chain fibres are associated with type II afferents
- Type II afferents are activated in proportion to the amount of stretch of the central portion, and hence the length of the intrafusal fibres as a whole
- The sensitivity of this reponse is modified by the activity in gamma-static motor neurons
- Type II afferents mediate the disynaptic stretch reflex, which is not velocity-dependent
- This pathway provides feedback regarding slow unintended changes in muscle length, and is therefore responsible for maintenance of muscle tone and posture
