Motor control Flashcards
Is activation of muscle fibres gradual?
No; all or none
how is skeletal muscle attached to bone
tendon
describe the structure of skeletal muscle
potein filaments make up myofibrils that make up muscle fibres that make up muscle fasiculus which make up muscle
what is a motor unit
an alpha motor unit (LMN) and all the extrafusal skeletal muscle fibres it innervates
what is the effect of less fibres innervated by a motor neurone
greater variation of movement
what happens when an alpha motor neurone depolarises
causes contraction of all fibres in that unit
where are alpha MN that control distal muscles located
laterally in spinal cord
where are alpha MN that control proximal muscles located
medially in spinal cord
what is the motor plate
region of muscle fibre plasma membrane that lies directly under terminal portion of axon
what is a neuromuscular junction
junction of an axon terminal with motor end plate
what do stretch receptors do
monitor muscle length and rate of change of muscle length
describe the structure of stretch receptors
peripheral endings of afferent nerve fibres wrapped around modified muscle fibres - whole apparatus = muscle spindle
what do muscle spindles detect
change in muscle resistance to stretch (contraction AND relaxation) regardless of current muscle length
describe the structure of intrafusal fibres;
middle 1/3; non, contractile, supplied by fast type 1a afferent sensory nerve
end 2/3s; contractile, gamma MN attached, supplied by slow conducting type 2 afferent
how are muscle spindles attached to extrafusal muscle fibres
by connective tissue, in parallel
where are extrafusal fibres found, what do they do and what innervates them
skeletal muscle, muscle contraction, alpha MN
where are intrafusal fibres found, what do they do and what innervates them
muscle spindle, proprioception, gamma MN
what happens if alpha MN alone is activated
increased sensitivity of muscle to stretch
what happens if an alpha AND gamma MN activated
prevents central 1/3 of spindle going slack during muscle contraction
- so info about muscle length will be continuously available so can adjust accoridngly
what happens when an external force is applied to an intrafusal fibre
pulls on intrafusal, receptor ends activated; the more/ faster muscle is stretched the more receptors fire
what happens when an AP arrives at a MN
extrafusal contraction so muscle shortens so tension removed from spindle so slows rate of firing of stretch receptor so reduction in sensory information
what does tension depend on
muscle length
load on muscle
degree of muscle fatigue
what do GTO do
detect changes in muscle tension
where are GTO found
junction of skeletal muscle/ tendon
what are GTO
endings of afferent fibres that wrap around collagen bundles in tendons near their junction with the muscles
what are the afferent fibres that lead from GTO to spinal cord
1b fibres that run to anterior horn of spinal cord
what happens when muscle is stretched/ attached extrafusal muscle fibre contracts (regarding GTO)
tension exterted on tendon so straightens the collagen bundle so distorts GTO receptor endings so activating them
what do 1b fibres do
cause inhibition of a MN of contracting muscle and its synergists
why are 1b fibres important
regulate muscle tension at normal range and protect it from overload
what is the output of GTO
proportional to muscle tension
are GTO afferent fibres faster or slower than muscle spindles
slower
what does 1b input do
excites alpha MN
what does the afferent from GTO do
inhibits alpha MN
what is muscle tone
degree of contraction of muscle/ proportion of Motor units that are active at any time
what does high muscle tone look like
feels firm/ rigid, resists passive stretch
what does low muscle tone look like
feels soft/ flaccid, offers little resistance to passive stretch
what is the contribution of alpha MN activity in a relaxed person
doesn’t make significant contribution to resistance to stretch
what is the contribution of alpha MN activity when someone is increasingly alert
more activation of alpha MN so muscle tone increases
what is hypertonia
high muscle tone
what causes hypertonia
UMN disorder; disorders of descending pathways which normally inhibit MN
how can you demonstrate hypertonia and what is seen
when joint is moved passively at high speed; increased resistance is due to increased level of alpha MN (LMN) activity which keeps muscle contracted despite attempt to relax it
what is spasticity
type of hypertonia
muscles don’t develop increased tone until they are stretched a bit, then after a brief increase in tone contraction subsides after a short time
what is rigidity
type of hypertonia
increased muscle contraction is continual and resistance to passive stretch is constant
what is hypotonia
low muscle tone accompanied by weakness, atrophy and decreased/ absent reflex response
what causes hypotonia
LMN lesion, neuromuscular junctions, muscles themselves
commonly accompanies disorders of alpha MN
what is clasp-knife phenomenon
type of spasticity (hypertonia)
period of ‘give’ occurring after time of resistance - especially when someone bends limb of patient; initially some resistance but after some time resistance falls dramatically
what causes clasp-knife phenomenen
1b afferent fibres from GTO inhibiting alpha MN once GTO detect tension
- example of inverse stretch (myotatic) reflex
when would you expect to see clasp-knife phenomenon
UMN lesion
