PHYSIOLOGICAL BASIS OF CNS

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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
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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
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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.
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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.

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 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
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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.

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 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.
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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

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16. Muscle spindles monitor muscle length and prevent
overstretching
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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

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 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

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 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

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 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*.

20. Alpha motor
neuron
Gamma motor
neuron

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 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
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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.

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 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

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 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.

28. B) Load reflex (coactivation of alpha and gamma MNs)

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
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❖ 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.
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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.
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34. Stretch reflex
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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
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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
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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
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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.
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39. MAGDI AWAD SASI 39Figure 5.2

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
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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
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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.
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