Muscle Spindles Flashcards
how many spindles per muscle?
40-60 spindles/muscle
what is the length of a muscle spindle?
4-10 mm
what are the 2 types of intrafusal fibers?
1: nuclear bag (DB1, SB2)
(2x the length & diameter of chain (8mm)
2: nuclear chain (long & short)
(1/2 the length (4mm)
how many nerve fibers go to the spindle?
more than 1/2
the muscle spindle lie_____ to the extrafusal fibers?
in parallel
gross movement muscles have _______ spindles than a muscle with finer movements?
less spindles
what are muscle spindles?
a length detector
what is a muscle spindles novel stimulus?
stretch
gets stretched bc its fibers lay parallel to the extrafusal fibers of the muscle
what are the 2 parts of the muscle spindle inside the capsule?
1: receptor (afferent)
2: associated striated muscle fibers (efferent)
what are extrafusal muscle fibers?
regular muscle fibers
controls where the limb is in space
what are intrafusal muscle fibers?
associated with spindle
skeletal muscle
has efferent motor component as well as afferent sensory component that controls contraction of intrafusal ms fiibers
controls the length of the spindle
what is the equatorial portion?
the nuclear bag?
what are the 2 types of intrafusal fibers?
1: nuclear bag
2: nuclear chain
nuclear chain fibers are more responsive to ___?
more responsive to slower changes in length
nuclear chain fibers line up in series; static; slow adapting
nuclear bag fibers are more responsive to ___?
more responsive to rapid changes in velocity
bulbous area where fibers stay in center; little longer; more type 1a
nuclear bag 1 responds to ___?
rapid changes in muscle length
nuclear bag 2 and nuclear chain responds to ___?
slow steady changes in muscle length
primary afferents respond to ___?
both the length of the muscle and the rate of change in length of the muscle
secondary afferents respond to ____?
responds chiefly to muscle length
bc secondary afferents are only in the nuclear chain
What is the nuclear bag fiber 1 response?
from resting conditions…
dynamic fusimotor stimulation causes local intrafusal contraction which stiffens the polar regions but has little effect on the primary region.
much of the external stretch is transmitted to the primary region which may extend by 15%
when stretch is maintained, the polar regions “creep”and the primary endings unload slightly. this is the basis of slow mechanical adaptation
dynamic bag fibers are sensitive to change in length and this is enhanced by dynamic fusimotor stimulation.
secondary afferents are rather less important
what is the primary action of nuclear bag 1 (dynamic) fibers?
increase length sensitivity
what is the static bag 2 fiber response
from resting conditions static fusimotor stimulation causes intrafusal contraction which imposes a significant stretch on the primary ending, which may extend by up to 20%
much of the external stretch is taken up at the polar regions, and the primary region may only extend by 5%
what is the primary action of nuclear bag 2 (static) fibers?
increase the discharge of primary sensory ending at any particular length without increasing the sensitivity to either the amplitude or the velocity of any length change
afferent input will increase and muscle force will be greater, but little change in gain of stretch reflex
What is the nuclear chain fiber response?
at resting conditions chain fibers are often kinked in the absence of fusimotor stimulation or external stretch
static fusimotor stimulation causes both primary & secondary endings to be stretched and increase discharge
when stretch is maintained there is no further mechanical change
what is the primary action of nuclear chain fibers?
increases the gain (sensitivity) of the stretch reflex when the muscle is at a constant length
(bag 1 is similar but only during movement)
mechanism would enable a muscle to support a heavy load
when the dynamic intrafusal system is active, the sensitivity to length changes of the primary endings is ____?
increased
when the static intrafusal system is active, the sensitivity to length changes of the primary endings is ____?
reduced
this may be a means of suppressing the stretch reflex during voluntary movement
what are the functions of a muscle spindle?
1: servo assistance of the motor command
2: muscle tone and rigidity
3: conscious appreciation of position- proprioception
4: modeling of planned movements
what is servo assistance of the motor command?
1: adjustment of spindle bias during movement:
spindle discharge is maintained following muscle shortening. Keeps spindle responsive to unexpected length changes resulting from fatigue, change in load or obstruction of movement (bag2)
2: abolition of spindle length sensitivity during movement:
decreases spindle discharge (holds constant) during muscle shortening without the need for modulation based on alpha drive (static drive of chain fibers). Apparent during low velocity normal unobstructed movements
3: Modulation of spindle length sensitivity during movement:
Dynamic system- movements that deviate from a desired trajectory need to be rapidly corrected. To do this a high length sensitivity is needed
muscle tone and rigidity:
low frequency spontaneous activity in sb2 fibers increases Ia discharge which reflexively causes a mild contraction
increase in gamma to chain fibers would increase muscle tone via Ia and II afferents
hypertonicity could be the result of over activity in either of these static intrafusal systems
hypo-tonicity=
no tone
no resistance to motion
no neurons firing to innervate those muscles
alpha motor neurons can’t fire
hyper-tonicity=
increased tone
no regulation of spindles and they go into a reflex
decreased ability to control these reflexes
what is muscle tone?
the sensation of resistance felt as one manipulates a joint through passive range of motion
limb inertia
mechanical- elastic properties of muscle and CT
reflexive
proprioception:
conscious appreciation of postion
Ia= change in length detector (velocity) II= length detector
unconscious proprioception:
rate and length but info will go to cerebellum (subconscious level) instead of cerebral cortex (conscious)
joint receptors only tell you the end of a movement
modeling of planned movements:
2 messages required:
1: a message to spindle signaling the desired trajectory of movement
2: a signal to extrafusal muscle that takes into account any load and fatigue (i.e. if load was unexpectedly large, spindle would not unload as much during shortening thereby assisting movement reflexively). Basis for alpha-gamma co-activation principle
what is a GTO?
Golgi tendon organ
lies in the tendons, in series with ms fibers
no efferents, only afferents
what is the novel stimulus for GTO?
force (amount of force muscle is applying to tendon)
passively stretching a muscle will increase force on tendon and cause Ib fibers to fire, however, fires more during volitional stretch
the more tension the faster the rate
Ib Inhibitory Interneuron
the more tension on Ib, the more inhibition of the alpha motor neuron; releases GABA, not glutamine
what are the 7 functions of the GTO?
1: regulate muscle force by bringing about changes in muscle tension
2: proprioception
3: clasp knife reflex
4: reciprocal excitation to antagonist
5: protection
6: muscle tone
7: contract-relax (stretching technique)
(using concept of autogenic inhibition)
