Muscle receptors

Define proprioception

sense of the body’s position in space based on specialised receptors that reside in the muscles and tendons

What 2 things can proprioception be?

• static - joint position sense

• dynamic - sense of limb movement

Proprioceptors are integrated with mechanoreceptors - what do larger and smaller fibres signal?

• larger = joint position

• smaller a delta = protective and active at extremes of movement

What do muscle spindles signal and how are they situated?

• stretch

• in parallel w extrafusal fibres

What do golgi tendon organs signal and how are they situated?

• tension produced by muscle contraction

• in series w extrafusal fibres

Basic muscle spindle info:

• specialised intrafusal fibre in parallel with extrafusal fibres

• attached to muscle connective tissue

3 main components of spindles?

• intrafusal muscle fibres in central part which is non-contractile and contains the nuclei

• large diameter myelinated sensory nerve fibres wrapped around the non-contractile part sensitive to stretch of intrafusal fibre

• gamma motor nerve fibres innervating contractile ends of intrafusal fibres

What are the two types of intrafusal fibre?

• nuclear chain fibre

• nuclear bag fibre

Static fibres what and who?

• non or slow adapting and measure length of spindle at any instant

• all chain and some bag

Dynamic fibres what and who??

• rapidly adapting and measure rate of change of length

• only bag

Type of sensory nerve fibres involved in muscle spindles?

• 1a afferent

  • reports dynamic and static from centre of all fibres

• II afferent

  • reports length only fom static bag and chain

• gamma motor nerves

  • supply the dynamic and static intrafusal fibres to modulate activity of these fibres

SKIPPED SOME STUFF FROM EARLIER

LOOK AT POWERPOINT

READING HAS USEFUL STUDIES TOO

WANT TO MAKE USEFUL PLAN/SOMETHING CONNECTING IT ALL

What does the 1a afferent connect to?

excitatory monosynaptic connections with the alpha motor neurons to the muscle

What is an example of the stretch reflex?

the knee jerk reaction

• (muscle contracts to resist the stretch normally but assuming here it does too much hence the movement?)

Who demonstrated that the knee jerk reaction was not an intrinsic property of the muscle but rather required sensory feedback by cutting dorsal or ventral roots?

Sherrington 1906

Other evidence for stretch reflex (think I mention in reading bit)

lloyd and eccles etc

What is the function of the stretch reflex?

• resists stretching of a muscle and maintains its length

• important for posture, holding heavy object still etc

Stretch reflex basically??

deviation from intended position detected and so muscle contracts to correct this

Stretch reflex more detailed?

• muscle spindles stretch

• stretching of equatorial region causes depolarisation of 1a axon ending due to opening og mechanosesnitive ion channels

• increase AP discharge of 1a axons synaptically depolarises the alpha motor neurons

• alpha neurons AP freq increased

• muscle contracts and shortens

What else does the stretch reflex do?

• recruits and inhibits other motorneurons to improve the stability of the motor response

  • 1a afferent stimulates the motor neurons supplying the synergistic muscles as well as the homonymous muscle

  • inhibits via 1a inhibitory interneurons the motor neurons supplying the antagonist muscles

    • RECIPROCAL INHIBITION

What is reciprocal innervation also the basis of?

spinal motor programmes that generate many repeitive movements eg walking

Mechanics of reciprocal inhibition?

• steady input excites 2 interneurons that connect to the motor neurons controlling the flexors and extensors

• acitivty alternates because of inhibitory neurons which cause a burst of one interneuron to strongly inhibit the other, creating rhythmic pattern

What can affect the stretch reflex?

descending and recurrent pathways

What does the corticospinal tract impact?

signals voluntary movement inhibiting stretch reflexes

What does the vestibulospinal tract do?

primarily engages extensor antigravity muscles

What does the reticulospinal tract impact?

modulates intensity of reflex activity via gamma motor neurons

Why do we have the gamma feedback loop?

• when muscle contracted the stretch of the spindles is lost and so no signals from the 1a fibres

• bad as no longer aware of surroundings due to lack of proprioception

What does the gamma feedback loop do?

• gamma fibres connected to ends of the muscle spindle

• these activate causing contraction of the two poles

• this pulls on the non-contractile equatorial regions

• keeps 1a axons active

What different types of gamma motor neurons are there?

• dynamic - predominates in slow predictable movements

• static - predominates in rapid unpredictable movements

Pearson KG 2004?

What does the stretch reflex contain?

short and long latency component

What is the short latency component like?

predominated by monosynaptic connection with initial spike M1 and involved primarily with axial and proximal muscle control

What is the long latency component like?

slower than monosynaptic but shorter than voluntary reaction time, contains spike M2 and involved the cerebral cortex and involved mostly in fine voluntary distal limb movements

What is Klippel-Feil syndrome?

something wrong with loop so goes to contralateral side (search up)

What does the stretch reflex contribute to?

postural adjustments and muscle tone

What was shown in Sherringtons classic experiments on cats where brainstem sectioned between superior and inferior colliculus (removes cortical but not brainstem locomotor influences)?

• limbs of animal showed increased reflexes and spastic paralysis - hypertonus

• small stimuli to the lower limbs produced strong contraction of extensor muscles

• also happens in soem human lesions aka strokes

What fibre is involved in golgi tendon organs?

1b afferent fibre

• which interweaves with the collagen fibres at the muscle tendon junction

• stretchng the tendon compresses and stimulates the nerve

Purpose of the GTO effect?

• protective

• also sensitive to small changes in tension and involved in fine movement as well as protection from overstretch

Detailed function of GTO for execution of fine motor acts eg pouring water?

• 1b axons enter spinal cord and branch repeatedly and synapse on 1b inhibitory interneurons

  • which receive input from other sensory receptors and descending pathways too

• 1b interneurons form inhibitory connections with the alpha motor neurons, innervating the same muscle

• regulates muscle tension within an optimal range so, as muscle tension increases, the inhibition of the alpha motor neuron slows muscle contraction

• and vice versa

What does the 1b afferent provide?

• feedback inhibition onto the homonymous motorneurons via a spinal interneuron

• opposite to stretch reflex

• muscle contraactioon stretches GTO and if excessive load then causes relaxation protecting muscle

What are reflexes subject to?

MODULATION

• relfexes are stereotyped but highly adaptable to control movements

• influenced by descending tracts and modulatory inputs especially when reliant on intervening interneurons as more synapses = more modulation

• observed in lesions of spinal cord and higher

What is the effect of 1b afferents?

inhibit extensor motor neurons at rest but to excite them during locomotion (state dependent reflex reversal)

What is the state dependent reflex reversal?

during locomotion, the afferent must recruit an alternative excitatory interneuron pathway???

What is the protective reflex pathway (involving the flexor withdrawal reflex and contralteral crossed extensor supporting reflex)?

• painful stimulus activates nociceptor

• primary sensory neuron enters spinal cord and diverges

  • one ascending collateral activates ascending pathways for sensation (pain) and postural adjustment

  • withdrawal reflex pulls foot away from stimulus

  • crossed extensor reflex supports body as weight shifts away from painful stimulus

What can be used to assess whether higher control is intact?

Babinski response

• normal response is flexion of toes to push stimulus away

• abnormal is extension and fanning showing signs of upper motor neuron lesion

• exception is newborns who extend due to immaturity of descending tracts

Anatomical differences?

• parallel vs series

• multiple types vs not

• size

• afferent fibre type

• contents (intrafusal muscle fibres vs collagen fibrils)

Functional differences?

• stretch vs tension

• monosynaptic vs polysynaptic

Lloyd, 1946

• Evidence that the stretch reflex is monosynaptic

• Electrical stimulation of muscle afferents produced direct EPSPs in α-motoneurons recorded from ventral roots

• Too short to involve interneurons - 0.7msec

• Argued due to timings not actual intracellular data

Eccles et al, 1950s

• Proved intracellularly that monosynaptic

• Directly recorded the synapse, confirming that Ia afferents make monosynaptic excitatory connections onto homonymous α-motoneurons.

Aman et al, 2015

• Proprioceptive training eg passive and active motor training/balance

• Can yield meaningful improvements in somatosensory and sensorimotor function

• Stroke, Parkinson, healthy etc patients

Bullinger KL et al, 2011 - permanent central…

• Reorganisation of spinal circuits after nerve injury helps us to understand why some movement disorders persist despite regeneration of axons

  • Shows the importance of sensory axons from the muscle spindles in motor coordination, each bit is important, reinnervation is not enough

• Motor and sensory proprioceptive axons reinnervate muscles after peripheral nerve transections yet motor coordination remains abnormal and stretch reflexes absent

• In conjunction with the companion electrophysiological study (Bullinger et al., 2011), we conclude that peripheral nerve injuries cause a permanent retraction of IA afferent synaptic varicosities from lamina IX and disconnection with motoneurons that is not recovered after peripheral regeneration and reinnervation of muscle by sensory and motor axons.

Grillner et al, 1995

• Investigated the rhythmic patterns of activity which underline locomotion using lamprey fish

• Showed that the activation of NMDA receptors on spinal interneurons was sufficient to generate this locomotor activity