2. The muscle spindle composition and function
Gamma motor neuron
The destribution of muscle tone and supraspinal control functions.
Stretch reflex ,types in response to stretch
Characters of stretch reflex
Inverse stretch reflex
MAGDI AWAD SASI 2
3. Muscle spindle
◦ Response to stretch
Within muscle fibers as intrafusal fibrer
Automonic with gamma motor neurons
Golgi tendon organ
◦ Muscle tension especially during isometric
Relaxation reflex - protective
Joint receptors
◦ Are found in capsules and ligaments around joints
MAGDI AWAD SASI 3
4. For skeletal muscle activity to be smoothly
coordinated, proprioceptor input is necessary.
Muscle spindles inform the nervous system of the
length of the muscle.
Golgi tendon organs inform the brain as to the
amount of tension in the muscle and tendons.
5. The Striated Muscle
◦ Anatomy - Two types of fibers
Extrafusal Fiber: these fibers do the work of the muscle.
Intrafusal Fiber or Muscle Spindle: controls muscle tone and
provides important sensory information.
◦ Contraction
Controlled by the nervous system
Muscles only shorten (i.e., only go one direction)
Organized in opposition pairs.
MAGDI AWAD SASI 5
6. A group of intrafusal fibers parallel to the surrounding
extrafusal fibers (1)p:697ch55
The central part of it has no actin or myocin so it
doesn't contract.(receptor)
The peripheral part contract in response to type A
gamma motor neuron.
7. MAGDI AWAD SASI 7
Extrafusal fibers are ordinary skeletal muscle cells(contractile) .
Muscle spindles, composed of 3–10 small bundles of encapsulated
intrafusal fibers, are dispersed throughout gross muscle.
These are dynamic stretch receptors that continuously check for
changes in muscle length.
8. 2 parts:
1- Central (receptor) 2- peripheral contractile area
Types of intrafusal fibers:
1- Nuclear bag fibers (the dynamic response)
2- Nuclear chain fibers (the static response)
-Nuclear chain is more abundant than the nuclear bag
fibers
MAGDI AWAD SASI 8
9. MAGDI AWAD SASI 9
Intrafusal fibers
Noncontractile in their central regions (lack myofilaments)
Wrapped with two types of afferent endings: primary sensory
endings of type Ia fibers and secondary sensory endings of type II
fibers.
Contractile end regions are innervated by gamma () efferent
fibers that maintain spindle sensitivity.
10. MAGDI AWAD SASI 10
Both nuclear bag and nuclear chain fibers possess a central,
noncontractile region housing multiple nuclei, and a skeletal muscle
(myofibril-containing) contractile portion at each end of the central
region.
The nuclear bag fibers are larger, and their multiple nuclei are
clustered in the “baglike” dilated central region of the fiber.
The nuclear chain fibers are smaller and consist of multiple nuclei
arranged sequentially, as in a “chain” of pearls, in the central region
of the fiber.
11. Sensory Endings in Muscles
◦ Anulospiral: wraps around muscle spindle, senses dynamic
changes in muscle length.
◦ Flower spray: looks like little flowers, sense static changes in
muscle length, helps determine position.
MAGDI AWAD SASI 11
12. MAGDI AWAD SASI 12
Excited in two ways:
1. External stretch of muscle and muscle spindle
2. Internal stretch of muscle spindle:
Activating the motor neurons stimulates the ends to
contract, thereby stretching the spindle
Stretch causes an increased rate of impulses in Ia fibers
13. Figure 13.15
Secondary sensory
endings (type II fiber)
Efferent (motor)
fiber to muscle spindle
Primary sensory
endings (type Ia
fiber)
Connective
tissue capsule
Muscle spindle
Tendon
Sensory fiber
Golgi tendon
organ
Efferent (motor)
fiber to extrafusal
muscle fibers
Extrafusal muscle
fiber
Intrafusal muscle
fibers
14. Figure 13.16a, b
(a) Unstretched
muscle. Action
potentials (APs)
are generated at
a constant rate in
the associated
sensory (la) fiber.
Muscle
spindle
Intrafusal
muscle fiber
Primary
sensory (la)
nerve fiber
Extrafusal
muscle fiber
Time
(b) Stretched
muscle. Stretching
activates the muscle
spindle, increasing
the rate of APs.
Time
16. Muscle spindles monitor muscle length and prevent
overstretching
MAGDI AWAD SASI 16
Figure 13-4a
Sensory neuron
endings
Intrafusal fibers
of muscle spindle
(a) Spindles are firing even when muscle is relaxed.
Spinal cord
Sensory
neuron
Alpha motor
neuron
Extrafusal muscle
fibers at resting length
Sensory neuron is tonically active.
Spinal cord integrates function.
Alpha motor neurons to extrafusal
fibers receive tonic input from
muscle spindles.
Extrafusal fibers maintain a certain
level of tension even at rest.
1
2
3
4
5
1
2
3
4
5
17. MAGDI AWAD SASI 17
1-Keep CNS informed about the muscle length and the
rate of velocity of change in the muscle length and
provide information about position (proprioception)
2- Send either: positive and negative signals regarding
the stretching of the muscles.
3- Maintain muscle length against rupture
18. MAGDI AWAD SASI 18
1/ Annulo-spiral (primary ending), supplies the nuclear
bag fiber(mainly) and the nuclear chain fibers
Type I a nerve fibers—A alpha
Responsible for the rapid and dynamic response
(velocity of change in the muscle length)
Example: tendon jerk
Very high velocity a 70 to 120 m/s
19. MAGDI AWAD SASI 19
2/ Flower spray (secondary ending), supplies the
nuclear chain fibers only
Responsible for the sustained and static response
Type II nerve fibers- A beta
Example: muscle tone
-The brain receives inputs about the state of the
skeletal muscles enabling it to coordinate the
muscular movements and allow conscious awareness
by sensory neurons *by sending collaterals to the
brain*.
21. MAGDI AWAD SASI 21
Efferent:
1/ Alpha motor fiber s, supplies extrafusal muscle fibers (70%)
2/ Gamma motor fibers , supplies intrafusal muscle fibers *the
peripheral contractile parts* (30)%
- Gamma motor fiber has 2 types:
1- Dynamic gamma (plate ending), supplies nuclear bag
(enhances the dynamic response)
2- Static gamma (trail ending), supplies nuclear chain (enhances
the static response)
-In general, the gamma motor fibers increase the sensitivity of
the muscle spindles by contraction of the peripheral part of the
intrafusal fiber
22. Present in the tendons.
Thick myelinated Ib afferent nerve
fibers –A alpha .
Stimulated by passive stretch and
active contraction of the muscle,
slowly adapting.
Tension receptors, not under nerve
control.
Axons from Golgi tendon organs
inhibit neurons in the spinal cord that
synapse with the a motor neurons
that innervate the same muscle.
Protects muscle from large forces.
Maintains steady level of muscle
force
MAGDI AWAD SASI
22
24. MAGDI AWAD SASI 24
Site:
γ-MNs are thin myelinated motors neurons
Represent 30% of AHCs.
The axons (about 4 u)
Supply the peripheral parts of intrafusal ms fibers.
Types:
There are 2 types of γ-MNs
1. Dynamic or d- γ-MNs→ supply nuclear bag ms fibers.
2. Static or s- γ-MNs→ supply nuclear chain ms fibers.
26. MAGDI AWAD SASI 26
Function:
Stimulation leads to stretch of the central parts of the muscle spindles ,which
increase the sensitivity of the muscles to stretch and may result in reflex muscle
contraction.
Control :
The activity is controlled by signals discharged from :
1. Several supraspinal areas –
2. The skin ---noxios stimulation
3. The skeletal muscles
27. MAGDI AWAD SASI 27
They adjust ms spindle sensitivity
↑ γ-MNs cause contraction of the peripheral parts of
intrafusal fibers→ stretch of central parts of ms spindle
→ ↑es the sensitivity of the ms spindle to stretch
Vice versa.
29. Muscle tone is the general state of contraction of the muscles.
If you have low muscle tone the muscles are flaccid (relaxed).
If high muscle tone, the muscle is contracted.
Muscle tone changes over time, e.g. during a step
◦ Muscle tone is low as we pick up our leg.
◦ Muscle tone is high on all leg muscles as we prepare to put it back down and it
has to support our weight
MAGDI AWAD SASI 29
30. MAGDI AWAD SASI 30
❖ Type: monosynaptic (sensory neuron synapse with motor
neuron) and deep
❖ Stimulus: Stretching of the muscles
❖ Response: Contraction of the muscles
❖ Aim of the response: to prevent tearing of the muscles by
activating the extrafusal muscle fibers
❖ Receptor: Muscle spindle (located deep in the muscles):
which consists of 3-12 intrafusal fibers.
31. Stimulus: Stretch of the muscle
Afferents: Fast conducting Ia afferents
Receptors: Muscle spindles (intrafusal fibers)
Center: Spinal cord/Brain stem
Efferents: A- alpha fibers
Effector structure: Extrafusal (regular contractile) fibers of the
muscle that is stretched
Response: Contraction of the muscle
Example: Knee jerk
32. Functions:
1. Maintenance of the erect posture
2. Helps both the venous and lymph flow –LL
3. The abdominal muscles prevents visceral ptosis.
4. It is an important source of heat production ,so it
markedly increases on exposure to cold.
MAGDI AWAD SASI 32
33. Distribution:
Present In all skeletal muscles, especially antigravity
muscles B/C they are subjected to more stretch by
the force of gravity.
They include:
1. Lower limb extensors
2. Upper limbs flexors
3. Back , back of the neck muscles.
4. The elevators of lower jaw
5. The anterior abdominal wall muscles.
MAGDI AWAD SASI 33
35. SR is normally subjected to facilitation and inhibition
by signals discharged via the descending tracts from
any supraspinal centers which affect the activity of
both alpha and gamma motor neurons
MAGDI AWAD SASI 35
36. 1. The primary cortical motor area –4—alpha motor neuron
2. The neocerebellum
3. F.Pontine reticualr formation:
4. The lateral vestibular nucleus -:
In medulla , activate the PontineRF.
Tract---- vestibulospinal
MAGDI AWAD SASI 36
37. Certain cortical areas –premotor 6& 4---supressor area
The inhibitory (medullary)reticular formation:
No intrinsic activity ,medulla
It is activited by signals from other inhibitory areas
It reach the spinal cord via the lateral medullary reticulospinal
tract where they inhibit mainly the gamma motor neurons
The red nucleus—mid brain
Discharging direct I signals to the alpha motor neuron
The lenticular (lentiform)nucleus of basal ganglia
Inhibits the medullary RF
MAGDI AWAD SASI 37
38. For skeletal muscles to perform normally:
I. The Golgi tendon organs (proprioceptors)
must constantly inform the brain as to the
state of of the muscle.
II. Stretch reflexes initiated by muscle spindles
must maintain healthy muscle tone.
MAGDI AWAD SASI 38
40. STATICDYNAMIC
Maintained stretch --
gravity
Sudden stretch –
tendon tapping
Stimulus
Type II fibersType I a fibersAfferent
Maintained smooth
contraction
Brisk contraction
&rapid relaxation
Response
Production of muscle
tone
Has clinical
significance
Function
Slowly adaptingRapidly adaptingAdaptation
Slowly fatiguedRapidly fatiguedFatiguability
Not enhanced by itJendrassiks maneuverenhancement
the gamma-s & beta-s
nerve fibers
the gamma d&beta-d
nerve fibers
enhanced
MAGDI AWAD SASI 40
41. It is a reflex relaxation of a muscle in response to
excessive stretch.
It is an inhibitory reflex .
Response ---muscle relaxation
It is a protective reaction against tearing of the muscle or
avulsion of the tendon from its bony attachment.
How? Golgi tendon organs Activity
MAGDI AWAD SASI 41
43. 1. Spinal deep reflex, servo assist, damping effect
2. It is monosynaptic
3. Reaction time is short– 19-24ms
4. Highly localized
5. No after discharge
6. Controlled by supra-spinal center
7. The contraction is jerky in dynamic type but is smooth in the
static type B/C the spinal motor neurons discharge at a low
frequency and alternate their activity.
8. The static type is present in all muscles –antigravity.
MAGDI AWAD SASI 43