Muscle Spindles and Proprioception

What does proprioception include?

1. static limb position

2. kinesthesia (ability to detect movement)

What mediates proprioception?

receptors in joints, muscles, and skin

where are muscle spindles located and how are they arranged

located within muscle belly and are “in parallel” with skeletal muscle fibers

What do muscle spindles do?

provide CNS with info about length and rate of change in muscles

Do muscle spindles vary in # and density?

yes - fine motor is required

How many fibers are in a spindle?

2-12 of modified striated muscle

difference between infrafusal and extrafusal muscle fibers

intra = buried in muscle, afferent receptors for stretch

extrafusal = bulk of muscle, force generating

Name 2 different intrafusal fiber types

nuclear bag: longest and largest

nuclear chain: shorter and thinner

what are the 2 innervations for a muscle spindle and what are they composed of?

afferent: sensory; Ia and II nerve fibers

efferent: motor; gamme motor neurons

Ia fibers (annulospiral) - what do they do

respond to phasic (mov’t initiation) and tonic (position)

Ia fibers - phasic

max output during quick stretch (tendon tap)

Ia fibers - tonic

sustained during constant stretch

II fibers - end where and what response

end on nuclear chain fibers

ONLY tonic response (position)

What is the afferent response to a stretch

initiating passive mov’t of joint

• during movement (dynamic) = Ia fibers active

• at position (tonic) = BOTH Ia and II fibers active

Alpha MN are _____ and innervate _____ muscle fibrs

largest; extrafusal

List what can stimulate Alpha MN (3 things)

Ia and II mm spindles (reflex)

corticospinal tract fibers (voluntary)

brain stem motor reflex

Gamma MN innervate _____ muscle fibers

intrafusal

What do gamma MN NOT innervate?

extrafusal mm fibers

What go gamma MN receive input from?

reticular formation, vestibular system, cerebellum, basal ganglia

contraction causes _____ of both extrafusal and intrafusal mm

shortening

Why are Gamma MN activated?

keep mm spindles same length as the extrafusal mm fibers

(prevent slack in receptors)

Voluntary movements are typically activated by what of alpha and gamma MN?

co-activation

What does co-activation do?

prevent “unloading” of mm spindle fibers, allow mm to provide info to CNS

Phasic stretch reflex

FAST and QUICK

tendon-jerk, deep tendon reflex

Stimulus (phasic stretch reflex)

quick stretch to muscle (usually tendon tap)

sensory afferent (phasic stretch reflex)

muscle spindle, group Ia (and II) sensory axons

synaptic connection (phasic stretch reflex)

monosynpatic, excitatory (very fast)

motor (phasic stretch reflex)

alpha MN to same muscle

What is activated at the same time stimulus hits Ia sensory afferent?

Ia inhibitory neuron — cause inhibition of antagonist muscle (relax)

Tonic stretch reflex stimulus does what?

maintained muscle stretch (static length)

where do tonic stretch reflexes synapse to

II axons synapse on alpha MN that innervate same muscle which spindle is located

synapse to synergist

inhibit antagonist

What does a tonic stretch reflex create resistance to

muscle stretch

Where are GTO located?

musculotendinous junctionho

How are GTO arranged?

in series

What are GTO sensitive to?

change in muscle contraction

Autogenic inhibition - what is the stimulus?

muscle contraction (weak or strong) OR muscle stretch (ROM extreme only)

Autogenic inhibition - what is the receptor?

GTO

Is Autogenic inhibition afferent or efferent?

afferent - group Ib axon

what is the response of Autogenic inhibition

inhibition of homonymous muscle and its synergists

facilitation of antagonist

**fires IPSP in same muscle fiber info came from

Does articular hyaline cartilage in joint receptors have innervation?

NO - anneural

Where are golgi ligament endings located

ligament/bone junction

found in ALL joints (except C spine)W

What do golgi ligament endings sense?

extreme stretch tension on ligament (ROM)

Where are ruffini endings located

fibrous capsule found in proximal joints (few distal)

What do ruffini endings sense?

maintained stretch tension on joint capsule at extreme ROM

• direction and amount of joint stretch

• rate/speed of joint stretch

• sensitive to swelling of capsule

Pacinian corpuscle are located where?

in fibrous joint capsule @ synovium/fibrous interface and bone periostium

• present more in distal joints than prox

What do pacinian corpuscle sense?

movement onset and termination

• low firing threshold, respond to very small deformation

Where are golgi-mazzoni corpuscles located

fibrous joint capsule, perpendicular to capsule surface

What do golgi-mazzoni sense?

compression - NOT stretch of capsule

very sensitive

Where are free nerve endings located?

in ligaments - high concentration

also in fibrous capsule, synovium, intra-articular fat pads

found in ALL joints

what do free nerve endings sense?

mechanical deformations (mov’t —> capsule stretch)

chemicals released from injured tissues

pain

Proprioception integrates info from where?

muscle, joint, cutaneous receptors w

What does prorioception dependent on?

activation of muscle receptors

muscle contraction (in mid range) is reliant on

GTO

GTO is activated through ____ on a tendon

stretch

If a muscle is passively stretch, what is activated

muscle spindles and GTO

• GTO at extreme ROM

What informs CNS of exceeding normal ROM of a joint?

joint receptors

The dorsal column medial lemniscus tract mediates what? for conscious proprioception

limb position

motion (active and passive)

muscle tension

effort

lower extremities and trunk are what tracts?

ventral spinocerebellar tract

dorsal spinocerebellar tract

upper extremity are what tracts?

cuneocerebellar tract

rostral spinocerebellar tract

motor control

• projected up to cerebellum (site for coordination)

motor learning

• cerebellum also responsible for motor learning

  • stores motor memory “feels right”

posture and balance control

• cerebellum controls these via reflexes

• balance-correcting movements