Exam 2 week 8 ppt 3 Muscle Spindles Flashcards
what are muscle spindles?
•Complex encapsulated somatic receptor scattered throughout striated muscles
a type of mechanoreceptor
a type of proprioreceptor
where are muscle spindles found in higher density?
•Complex encapsulated somatic receptor scattered throughout striated muscles
Do muscle spindles have a complex or simple structure?
complex!
Details on the complex structure of Muscle Spindles (5 things)
- –Connective tissue capsule surrounding specialized muscle cells (intrafusal fibers)
- –Intrafusal fibers have contractile mechanism only in ends (polar) which is innervated by smaller g-motor neurons (fusimotor)
- –End contraction stretches center
- –Central (equatorial) non-contractile part of intrafusal fiber is encircled by sensory nerve axon (annulospiral or 1° ending)
- –Junction between equatorial & polar regions (juxta-equatorial) also has sensory nerve axons (flowerspray or 2° ending)
As sensory receptors go, they have a relatively Complex structure that begins with a Connective tissue capsule (fusum) surrounding specialize muscle cells called intrafusal fibers because they are inside this fusum.
These Intrafusal muscle fibers have contractile mechanism only in ends (polar) which is innervated by smaller gamma-motor neurons (fusimotor motor neurons). The contraction of the ends of the intrafusal muscle fiber pulls on and stretches the center of the intrafusal fiber. Central (equatorial) non-contractile part of intrafusal fiber is encircled by a sensory nerve axon called an annulospiral or 1° muscle spindle ending. They are called this because the spiral around the intrafusal muscle fiber which passes through the annulus in the middle of the spiraling axonal ending. Each intrafusal fiber has additional sensory axon endings at Junction between the center region (called equatorial region) and the ends (called the polar regions). This junction region is called the juxta-equatorial region of the muscle spindle intrafusal fiber. These additional sensory endings are called flowerspray or 2° endings. These are called flowerspray endingings because the ending reminded anatomists of a small bouquet of flowers. As shown in this picture the 1° endings are of the Group Ia (or A-alpha) caliber and the 2° endings are of the group II (A-beta) caliber sensory axons.
what surrounds the msucle spindle?
connective tissue capsule
What kind of fibers are inside of the musucle spindle
intrafusal fibers (specialized muscle cells)
where are intrafusal fibers contractile?
noncontractile?
Intrafusal fibers have contractile mechanism only in ends (polar)
The central part of the intrafusal fiber is called the (equatorial) non-contractile part
what is the name for the ends of intrafusal fibers?
polar parts
what is the name of the central part of the intrafusal fibers?
equatorial part
what type of nerve fibers innervate the polar part of the intrafusal fibers?
gamma motor neurons
what type of nerve fibers encircles the equatorial part of the intrafusal fiber?
name the four ways to describe them
sensory group Ia afferent fibers
A-alpha
annulospiral endings
1 degree endings
what is the junction between the equatorial and polar regions of the intrafusal fibers called?
juxta-equatorial region
what type of nerve fibers are found at the jusxta-equatorial region of the intrafusal fibers?
name the four ways to describe them
group II endings
A-beta endings
flowerspray endings
2 degree endings
what are extrafusal muscle fibers?
The normal contractile muscle fibers we usually talk about as skeletal muscle
how are intrafusal muscle fibers oriented in relationship to extrafusal fibers?
they are pareallel with each other
what stimulates the muscle spindle receptor?
how does this work?
stretching or lengthening of the muscle and hence the muslce spindle receptor
This stretches both the 1 degree and 2 degree endings and produces an increased firing rate alonb both of these sensory axons.
how many muscle spindles will be activated in a muscle when it stretches?
all of them
what is the muscle spindle designed to signal?
ongoing status about static muscle length and change in muscle length
this is also an indrect measure of joint angle and joint motion
So the function of the muscle spindle involves Stretching the ______ which stretches the spindle
So the function of the muscle spindle involves Stretching the muscle which stretches the spindle
how do the sensory nerve fibers respond when the spindle stretches (generally)
- §Stretching spindle stretches 1° (Ia) & 2° (II) afferent endings
- §1° & 2° afferent endings respond by producing action potentials whose frequency is proportional to amount of stretching and stretch respectively
Describe the function of 1 degree afferent endings in more detail
- –Responds briskly to lengthening more than change in length (Dynamic response)
- –Rapidly adapting and signal rate of stretching rather than amount of stretch
As we can see in this illustration, 1° afferent endings Responds briskly to lengthening more than change in length and have what we call a Dynamic response. They are rapidly adapting and so signal rate of stretching rather than amount of stretch.
Describe the function of 2 degree afferent endings in more detail
•Function - 2° afferent endings
- –Respond less briskly to stretching but maintains firing during stretch (static response)
- –Signals amount of stretch rather than rate
Conversely, 2° afferent endings Respond less briskly to stretching but maintains firing during stretch which we call a static response. So this signals amount of stretch much better than it signals the rate of stretching.
How do the gamma efferents interact with the intrafusal fibers
- –Contract the ends of the intrafusal fibers
- –Stretches the center of the intrafusal fiber
As a reminder, the Gamma motor neuron efferents end on the ends of the contractile ends of the intrafusal fibers. So when activated they Contract the ends of the intrafusal fibers which Stretches the center of the intrafusal fiber where the 1° afferent endings and 2° afferent endings are. So these would be activated when gamma motor neurons are excited.
.
does the gamma efferents___________ (increase/decrease) the _______________ (static and/or dynamic) responsiveness of the spindle?
how?
Gamma efferents increases both static and dynamic responsiveness of the spindle
§ncreases action potential frequency when the spindle is lengthening (dynamic response) & when lengthened (static response)
So the activation of Gamma (g) motor neuronal efferents increases the firing of both 1° afferent endings and 2° afferent endings. This increases both static response of the spindle due to enhanced activation of the 2° afferent endings
& dynamic responsiveness of spindle due to enhanced activation of 1° afferent endings. This Increases action potential frequency when the spindle is lengthening (dynamic response) & when lengthened (static response). In other words at any given rate of lengthening, gamma motor neuronal activity will enhance the dynamic response of the muscle spindle so it will seem to the central nervous system that the muscle is lengthening at a greater rate than it actually is. Likewise, the muscle will seem more elongated than it actually is to the CNS because of the increased static firing frequency.
List the two main functions of gamma efferents
- –Enhance sensitivity of muscle spindle
- –Prevent “unloading” (reduced activity) of spindle afferent firing with muscle shorten-ing – Alpha (a)-Gamma (g) co-activation
Two main Functions
Gamma motor neuronal efferents will Enhance sensitivity of muscle spindle as we see in this illustration – greater firing during stretching and at a level of stretch which makes the muscle spindle a more sensitive receptor. As we will see in later lectures this makes the muscle spindle a better proprioceptor in balance and will enhance our balance response
But gamma motor neuronal efferent activity will also Prevent “unloading” or the reduced activity of spindle afferent activity when muscle shortens –due to a process known as Alpha (a)-Gamma (g) coactivation
what do spindle afferents activate? explain
- –Ia & II afferent fibers from a muscle activate the alpha motor neurons to that same muscle
- –This causes reflex contraction of that muscle
when extrafusal muscle fibers lengthen, what happens to the muscle spindle/intrafusal fibers?
•When extrafusal muscle fibers lengthen (a), the muscle spindle lengthens
when extrafusal muscle fibers shorten, what happens to the muscle spindle/intrafusal fibers?
when extrafusal muscle fibers lengthen, what happens to the muscle spindle/intrafusal fibers?
when the extrafusal musle fibers lengthen, what happens to the firing rate from the muscle spindle afferents?
What results?
•When extrafusal muscle fibers lengthen, firing rate from muscle spindle afferent fiber is increased (a) so increased muscle activation
what is alpha-gamma co-activation?
the activation of both alpha motor neurons and gamma motor neruons during muscle contraction
this keeps the muscle spindle relatively taut firing during muscle contraction
when the extrafusal musle fibers shorten, what would happen to the firing rate from the muscle spindle afferents if there was no alpha-gamma co-activation?
What results?
•when extrafusal muscle fibers shorten, firing rate from muscle spindle afferent goes down (b) so decreased muscle activation
what is the purpose of alpha-gamma co-activation?
•Gamma-motor neuron activity can maintain muscle spindle stretch, maintaining muscle spindle afferent firing rate even while the extrafusal muscle is shortened (c)
