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physiology
Lec. Date: 11/5/2011
in this lecture we are going to talk about the sensory receptors of the
muscle which includes, the muscle spindle. Also we are going to discuss
the types of reflexes with their characteristics.
The muscle spindle:
we said that there is a lot of muscle spindles inside the muscle fibers,
and the density of the muscle spindle are more In the antigravity
muscles.
The muscle spindle consist of 2 kind of fibers : the nuclear chain and the
nuclear bag
in the nuclear bag there are a lot of nuclei in the centre sometimes they
are called primary endings because they respond to the dynamic part of
the movement which meant they sense the rate of change
The nuclear bag is sometimes called static because they sense the
position in the static way not in the dynamic.
The primary endings are supplied by type 1a fiber sensory neurons
(primary efferent), but the nuclear chain are supplied by type 2 fibers
which means that type 1a is faster conducting fiber than type
2(secondary efferent).
Inside the muscle spindle we have the contractile fibers at the end,
which are called the intrafusal fibers. And they are important for keeping
the central part of the muscle spindle stretched.
the muscle spindle receptors they respond to stretch, if there is
stretch(lengthening) of the muscle which means that there is stretch in
the centre the respond by sending impulses to 1a and 2 fibers.
The intrafusal fibers are supplied by gamma(y) motor neurons, but the
extrafusal muscle fibers are supplied by alpha (a) motor neurons.
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the importance of the gama motor neurons:
when there are stimulation in gama they cause contraction of the
intrafusal muscle fibers, when the intrafusal fibers contract they move
laterally so they will cause stretch of the centre of the muscle, the
stretch is considered stimulus, it will go to the spinal cord, then back to
the alpha motor neurons to contract the extrafusal muscle fibers, for
example: in the knee jerk reflex, when u hit the tendon, u stretch the
central part of the muscle spindle, this stimulus goes to the spinal cord
back to the alpha motor neurons and causes contraction of the muscle.
So the importance of the gama motor neurons is to keep the central part
of the muscle spindle stimulated to maintain contraction.
*remember: we said that the muscle spindle is located in parallel to the
muscle so it's sensitive to length, but the golgi tendon is located in series
to the muscle so it's sensitive to tension.
-dynamic efferent: it's the primary sensory of the gama fibers that
supplies the nuclear bag.
-static efferent: the gama fibers that supplies nuclear chain.
The static response is sensitive to the degree of stretch. And the
dynamic is sensitive to the rate of stretch.
so in the dynamic response since it measures the rate of stretch, if u
have the distance u can calculate the time, so u can stop wherever your
destination will be.
Function of the muscle spindle:
We'll talk about one of them , if u don't have muscle spindle the
movement will be erratic meaning that, if u elongate the muscle the
central part will be stimulated and sends impulses to the alpha motor
neurons and causes contraction of the muscle, here if the muscle
contract the central part will be inhibited (relaxation) because there is
no muscle spindle. so there will be contraction then relaxation.
but if there was muscle spindle, and the gama motor neurons are
contracted at the same time, it keeps the central part of the muscle
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spindle excited (sends impulses to the alpha motor neurons which keeps
the muscle contracted).
Now if there is stretch in the muscle:
in the 1a fibers which is dynamic there is a high rate during the stretch
But in the 2 fibers which is the static, the rate is increasing but the
increase is slower, so it tells the system about the degree of stretch.
now if the stretch has finished:
in the 1a fibers , the rate will decrease rapidly , so this tills the system
that there is change and how fast is this change( what is the speed of
this change),and this gives the system The Predictive function meaning
that u can predict by this rate after some time where is your motion will
be. Also this informs the system about the rate of release (how fast u r
releasing the stretch).
In the 2 fibers there will be a decrease but it is slower that the dynamic
one.
To sum up the system will be informed by the position and the rate of
change in position, by the static and the dynamic fibers.
The basal rate:
Both 1a fibers and (2) fibers have a basal rate of firing which out any
stretch.
basal rate of firing is imp. For negative(-) and positive(+) control. For
example: in the dynamic response when we removed the stimulus there
was a rapid drop In the rate, so we sensed a (-) change.
the effect of gama motor fibers (dynamic ,static):
1- if no stimulation of the gama, there will be extension, but the impulse
rate will decrease slowly.
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2- stimulation of the static fibers, the impulse rate is increasing slowly
and continuous.
3- stimulation if the dynamic fibers, there is increase in the number of
fibers during the stretch, very high rate during extension.
REFLEXES:
The idea of these reflexes is to stretch the muscle, if u stretch the
muscle u stretch the central part of the muscle spindle, comes back
to the spinal cord, and going back to the extrafusal muscle fibers
through the alpha motor neurons and this will contract the muscle
which is stretched.
There are many kinds of reflexes:
Knee jerk reflex(stretch reflex), golgi tendon reflex, withdrawal
reflex(flexor reflex), crossed extensor reflex.
Reflex means: rapid involuntary movements due to a stimulus.
So for this stimulus we should have receptor (afferent neurons), they
send impulses to the spinal cord, to the integrating
centre(interneuron), then it will excite alpha motor neurons and the
alpha will go to the effectors or to a gland sometime.
These steps are called receptor arch components.
First-The knee jerk reflex (stretch reflex):
-stretching the patellar tendon with a hammer stretches the
quadriceps, therefore stretching the muscle spindle inside it, this
sends impulses to the spinal cord back to the alpha motor neurons
and this will cause contraction of the quadriceps causing movement
of the knee anteriorly.
-The stretch reflex can be done with almost any muscle.
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The circuit of the stretch reflex: Stretching the muscle will stimulate
the sensory receptors
1. The sensory neurons will be excited and sends impulses
2. These impulses will go to the spinal cord and to the
cerebellum(dorsal and ventral spinocerebellar tract)
3. Synapse with the alpha motor neurons goes to the extrafusal
muscle fibers and causes contraction.
Note: the receptors are found on the same muscle being
stimulated.
Note: -fibers go to the brain, tells the brain about your actual
position(during walking for ex)
-The cerebellum compares the actual with the intention that
comes from the corticopontocerebellar fibers.
Characteristics of the stretch reflex:
1- it is an excitatory reflex
2- Monosynaptic
3- the receptors are found on the same muscle being stimulated
4- ipsilateral.
*A student asked the doctor that the fibers goes to 2 muscles
why?
The doc. Answers that the second fiber is interneuron and it is
inhibitory, for the hamstring muscle for example. When I want to
stretch the quadriceps(the flexor) I need to relax the
hamstring(the extensor) and this is done during walking.
Second-the tendon reflex:
It is protective reflex how?
If u stretches the muscle, contract it, if the contraction was too
strong this will tear the tendon or cause avulsion of the tendon, to
protect this, there is another receptor that response to the
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amount of tension on this tendon, this receptor sends impulses to
the spinal cord and cerebellum back to to tendon where there is
an INHIBITORY INTERNEURON causing inhibition of the alpha
motor neuron, causing relaxation of the muscle.
Note: tendone reflex has type 1B fibers which are fast fibers.
Characteristics of the tendon reflex:
12345-
Ipsilateral
Disynaptic (interneuron)
Receptors found on the same muscle that is stimulated
Inhibitory
It is used for walking
Third-withdrawal reflex:
If u hit on a painful nail, or fire, or hot objects (pain, temperature,
noseceptive, thermal stimuli to the skin) u flex your leg, u flex not
just 1 muscle, the whole group of muscles are stimulated, so the
impulse from the spinal cord goes to the whole group of muscles
which have the same function (agonist), at the same time u inhibit
the extensors on the same side, so there is reciprocal
innervations(stimulation of the agonist and inhibition of the
antagonist).
So stepping on a painful thing, fibers will go to the spinal cord
through the anterio-lateral spinothalamic tract which is slow fibers
(A-delta or C fibers).
Other characteristics of the spinothalamic tract:
When it enters the spinal cord it ascends 1 or 2 segment up and 1
or 2 segments down (propriosinal)
then after spinal cord the impulses will go to the group of muscles
and contract it.
Characteristics of the withdrawal reflex:
1- Ipsilateral
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23456-
Polysynaptic
Receptors found on the skin
It is transmitted very slow
Is involves group if muscles
There is reciprocal innervations
Fourth-crossed extensor reflex:
When u step of a painful thing u flex 1 leg, but at the same time u extend
the other leg to support yourself.
The characteristics of this reflex:
123456-
Contralateral
Polysynaptic
Involves group of muscles
It is much slower (too many synapses)
It goes 1 or 2 segments up and 1 or 2 segments down
Reciprocal innervations
Myograms of flexor and crossed extensor reflex:
-Synaptic After discharge:
When the withdrawal reflex begins, it develops after discharge.
The crossed extensor reflex will take very long after discharge
because it has too many synapses. Also it takes long latent period.
But the flexor reflex takes short after discharge with sorter latent
period. All that depends on the number of synapses.
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