muscle spindle samar

8/8/2019 Muscle Spindle Samar

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Functions of


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Are composed of a few intrafusal muscle fibers that lack actin

and myosin in their central regions, are noncontractile, and

serve as receptive surfaces. Muscle spindles are wrapped with two types of afferent

endings: primary sensory endings of type Ia fibers and

secondary sensory endings of type II fibers.

These regions are innervated by gamma efferent fibers.

Note: contractile muscle fibers are extrafusal fibers and are

innervated by alpha efferent fibers.


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Two main functions:

1.Act as receptor organ for stretch reflex.

2.Plays an important role in maintaining the muscle tone.


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A. Stretch Reflex:

When a muscle is stretched it contracts reflexly. This is called the Stretch Reflex. It is a monosynaptic reflex and the quickest of all. The extensor muscles, particularly the antigravity muscles exhibit a severe and

prolong contraction during stretch reflex.

B. Role of Muscle spindle:

The intrafusal fibers,which form the muscle spindle are situated parallel to theextrafusal fibers of the skeletal muscles

These fibers are attached to the tendon of the muscle by means of capsule. When the muscle is stretched, the muscle spindle is also stretched and stimulated. The sensory impulses are discharged from muscle spindle and are transmitted via

primary and secondary nerve fibers to the spinal cord. Now the impulses from the alpha motor neuron causes contraction of extrafusal

fibers.


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Dynamic Response . Static Response .

1. Dynamic Response :

When there is change in the length of the muscle by stretching, theprimary sensory nerve fibers from the muscle spindle startdischarging impulses very rapidly

But the discharge become less or nil during continuous stretching of

the muscle . The discharge of impulses start only if there is change in the degree

of stretching of the muscle. This is called the dynamic response and is carried out by nerve

fibers supplying nuclear bag fibers.


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2.Static Response:

In this, the impulses are discharged continuously from the primary andsecondary nerve fibers, through out the period of muscle stretch. Socalled static response.

It is carried by the nuclear chain fibers, that are innervated by bothprimary and secondary nerve fibers.

Thus the muscle spindle gives response to change in length of the muscleas well as the rate of change of the length.

Negative stretch Reflex:o

Muscle suddenly shortened, exactly opposite effects occur because ofdecrease nerve impulses from the spindles.o If the muscle is already taut, any sudden release of load on the muscle

that allows it to shorten elicits both dynamic and static reflex muscleinhibition rather than reflex excitation.

o This is called the negative stretch reflex.


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Signals from spinal cord and other parts of the nervous system

are often transmitted to the muscle in an unsmooth form.

When the muscle spindle is not functioning properly, themuscle contraction is jerky during the course of such a signal.

Also called a signal averaging function of muscle spindle

reflex.


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When signals are transmitted from the motor cortex or from

any other area of brain to alpha motor neurons, the gamma

motor neurons are stimulated simultaneously, an effect called

coactivation of alpha and gamma motor neurons.

This causes both the extrafusal and intrafusal fibers to contract

at the same time.


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The gamma efferent system is excited by signals

from the

I. Bulboreticular facilitatory region of the brainstemII. Bulboreticular area from:

1. cerebellum.

2. Basal ganglia.3. Cerebral cortex.


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Muscle tone is due to the continuous discharge of impulses from gamma

motor neurons.

The gamma motor neurons innervate the intrafusal fibers.The impulses

from gamma motor neurons causes contraction of end portion of intrafusal

fibers. So, the central portion of the intrafusal fibers is stretched and

activated.

This leads to discharge of impulses from the primary sensory nerve

endings.

The impulses stimulate the alpha motor neurons of the spinal cord.

The alpha motor neurons, inturn send impulses to extrafusal fibers andcauses contraction of the muscle fibers.

When the frequency of the discharge from gamma motor neuron increases,

the activity of the muscle spindle is increased and the muscle tone also

increases.


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Stretching the muscles activates the muscle spindle. There is

an increased rate of action potential in Ia fibers.

Contracting the muscle reduces tension on the muscle

spindle.There is a decreased rate of action potential on Ia

fibers


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Stretching the muscle activates the muscle spindle.

Excited motor neurons of the spindle cause the

stretched muscle to contract. Afferent impulses from the spindle result in

inhibition of the antagonist.

Example: patellar reflex.

Tapping the patellar tendon stretches thequadriceps and starts the reflex action.

The quadriceps contract and the

antagonistic hamstrings relax.


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Simplest reflex because it has only 1 synapse in the

path of its arc.

Muscle spindles contain the sensory receptors for the

stretch reflex.

Each spindle contains modified muscle fibers calledspindle or intrafusal fibers (inside spindle),

innervated by efferent fibers.


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The middle segment of each spindle fiber acts as a

mechanical stretch receptor that is connected to a

sensory afferent nerve to the spinal cord.

Stretching of the muscle stretches the spindle fibers

activating the muscle spindle stretch receptors and

the associated sensory fibers.


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The terminals of the spindle sensory fibers make

direct excitatory synaptic contact with alpha motor

neurons serving the ordinary muscle fiber (extrafusalfibers).

Contraction of these fibers shorten the muscle and

relaxes the spindle fibers terminating the stretchreflex and muscle contraction.


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1. Passive stretch of a muscle (produced by tapping its tendon)

stretches the spindle (intrafusal) fibers.

2. Stretching of a spindle distorts its central (chain) region,

which stimulates dendritic endings of sensory nerves. 3. Action potentials are conducted by afferent (sensory) fibers

into the spinal cord on the dorsal roots of spinal nerves.

4. Axons of sensory neurons synapse with dendrites and cell

bodies of somatic motor neurons located in the ventral horn

gray matter of the spinal cord.

5. Efferent impulses in the axons of somatic motor neurons

(which form the ventral roots of the spinal nerves) are

conducted to the ordinary (extrafusal) muscle fibers. These

neurons are (alpha) motor neurons.


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6. Release of Ach from the endings of alpha motor neuron

stimulates the contraction of extrafusal fibers, and thus the

whole muscle.

7. Contraction of the muscle relieves the stretch of itsspindles, thus decreasing electrical activity in the afferent

nerve fibers, and relaxes the spindle fiber and terminates the

stretch reflex and muscle contraction.

Note: By sending command to the motor neurons, the brain

set a muscles length. The stretch reflex makes sure the

muscle stay at that length. The stretch reflex is therefore

important for maintaining muscle tone and upright posture.


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Thank You.

muscle spindle samar

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