Muscle stretch reflexes and proprioception Flashcards
Types of muscle receptors
Muscle spindles and Golgi tendon organs
Muscle spindles signal
Stretch
Golgi tendon organs signal
Tension produced by muscle contraction
Joint mechanoreceptors consist
Larger fibres signal joint position
Smaller Aδ fibres are most active at the extremes of movement and are protective.
Muscle spindles are situated
In the fleshy part of muscles
Golgi tendon organs are situated
In tendons at the ends of a muscle
Are muscle spindles encapsulated?
Yes
How do muscle spindles lie with extrafusal fibres?
Specialised intrafusal fibre in parallel with extrafusal fibres
Muscle spindle structure and innervation
Specialised intrafusal muscle fibers the central part of which is non-contractile and contains the nuclei.
Large diameter myelinated sensory nerve fibres wrapped around the noncontractile part of the muscle the ends of which are sensitive to stretch of the intrafusal fibre
Small diameter gamma motor nerve fibres that innervate the contractile ends of the intrafusal fibres
Two types structural types of intrafusal fibres
Nuclear chain fibres
Nuclear bag fibres
Nuclear chain fibres abundance and structure
Nuclear chain fibres (variable numbers per spindle), named because their nuclei are aligned in a single row
(chain) in the centre of the fibre
Nuclear bag fibres abundance and structure
Nuclear bag fibres (2-3 per spindle) - named because their nuclei are collected in a bundle in the middle of the fibre.
Two functional types of intrafusal fibers and their function
Static: Non-adapting (or very slowly adapting); these measure the length of the spindle at any instant.
Dynamic: Rapidly (though not completely adapting); these measure the rate of change of length
Nuclear bag fibre function
Nuclear bag fibres can be static
Only nuclear bag fibres are dynamic, the contractile ends of the dynamic bag fibres are more viscous so stretching occurs in the central nuclear part.
Nuclear chain fibre function
All nuclear chain fibres are static
Main sensory nerves sending afferents from stretch receptors in the muscle
Myelinated A-alpha and A-beta fibres.
A-alpha/type I afferent (fastest fibre) records
From the centre of all fibres.
These afferents report dynamic, rate of change, as well as static information
Gamma (γ) motor nerves supply
The dynamic and the static intrafusal fibres to modulate the activity of these fibres
A-beta/type II afferent records
Only from static bag and chain fibres
Therefore primarily reports spindle/ muscle length.
Type Ia and Type II afferents in a state of rest
Both fibres fire at regular intervals
Activity in type Ia and II fibres when muscle stretches
When antagonistic muscle contracts (thus analysed muscle is stretched), a large volley of action potentials is sent through the Ia fibres.
There is also a slight increase in static II fibres as the static fibres reach a new level of stretch
Activity in type Ia and II when stretch is maintained
When the muscle remains in state of stretch, soon the Ia fibres adapt and firing reduces to the same as the static fibres.
Basis of the stretch reflex
The Ia afferent makes excitatory monosynaptic connections with the alpha motor neurons to the muscle.
Knee jerk reflex spinal level
Quadriceps (knee jerk) L2-4 patella tendon
Function of the stretch reflex
Resists stretching of a muscle and maintains its length.
This is 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
How is the stability of he stretch reflex improved ?
The stretch reflex recruits and inhibits other motor neurons to improve the stability of the motor response.
The stretch reflex doesn’t only act on the same muscle but across several motor pools to ensure a locally co-ordinated motor response
In addition to stimulating the homonymous muscle, the Ia afferent stimulates the motor neurons supplying the synergist muscles.
Inhibits (via Ia inhibitory interneuron) the motor neurons supplying the antagonist muscles. This is called “reciprocal inhibition”.
The myotactic (stretch) reflex is a purely monosynaptic reflex
False (other muscles activated and inhibited)
How do descending and recurrent pathways affect the stretch reflex?
Corticospinal tract: signalling voluntary movement, therefore inhibits the stretch reflexes
Vestibulospinal tract: primarily engages the extensor anti-gravity muscles
Reticulospinal tract: modulates the intensity of reflex via gamma motor neurons
How is fatigue prevented in a recurrently stimulated muscle?
Recurrent processes engage the Renshaw inhibitory neurons which inhibit the firing muscle to prevent fatigue.
Role of gamma motor nerves in modulation of stretch
When muscle is stretched (sustained), there is tension in the muscle and thus Ia firing from the spindles.
Without gamma motor neurons when muscle contracts, unload the spindle, and this leads to absence of firing via the spindle
When gamma motor neuron stimulated: innervate the contractile ends of the intrafusal fibre, contracts in parallel with the extrafusal fibres so continual reporting of contractile information (feedback from Ia)
The stretch reflex short latency component
The monosynaptic connection predominates in the short latency component (M1) and is involved primarily with axial and proximal muscle control.
The stretch reflex long latency component
The long latency (M2) is slower than monosynaptic but shorter than voluntary reaction time.
It involves the cerebral cortex and is involved mostly in fine voluntary distal limb movements.
Jaw jerk reflex
(CN V)
Biceps reflex
Brachioradialis reflex
C5/C6
C6
Extensor digitorum reflex
Triceps reflex
C6/C7
Ankle jerk reflex
S1/S2
Golgi tendon structure
The body of the organ is made up of braided strands of collagen (intrafusal fasciculi) that are less compact than elsewhere in the tendon and are encapsulated.
The capsule is connected in series with a group of muscle fibers at one end, and merge into the tendon proper at the other.
Each capsule is about 1 mm long, has a diameter of about 0.1 mm,
Golgi tendon organ innervation and activation
Perforated by one or more afferent type Ib sensory nerve fibers (Aɑ fiber).
Stretching the tendon compresses and stimulates the nerve
Ib afferent response when activated
The Ib afferent provides feedback inhibition onto the homonymous motorneurons via a spinal interneuron (i.e. the opposite to the muscle spindle stretch reflex)
The muscle contraction stretches the GTO
If excessive load is placed on the muscle, the GTO reflex causes relaxation - thereby protecting the muscle.
In locomotion when are flexors and extensors recruited?
Extensors – recruited during stance phase, ‘anti-gravity’ function
Flexors – recruited during swing phase, voluntary movement
Proprioceptive ascending pathways terminate in
Area 3a of S1.
