Transcript 01-Spinal Reflexes Student`s Copy

Motor Functions of the Spinal Cord
‫ الحبل الشوكي‬/‫المهام الحركية للنخاع‬
( Spinal Reflexes)
‫المنعكسات النخاعية‬
Dr. Taha Sadig Ahmed,
MB, BS ( England ) , MAANEM ( USA)
Consultant , Clinical Neurophysiology ( Saudi Council of
Health Specialities )
Consultant ,Clinical Neurophysiology ( King Abdulaziz
University Hospital , Riyadh
Associate Professor , College of medicine , Riyadh .
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‫مصطلحات طبيّة‬
‫النخاع‪ /‬الحبل الشوكي ‪Spine / Spinal cord‬‬
‫عصبون )‪Neuron/ Nerve fiber ( one nerve cell‬‬
‫عصب ‪Nerve :‬‬
‫العصب يتكون من عشرات اآلالف أو مئات اآلالف من العصبونات‬
‫‪e.g., Sciatic nerve , median nerve , ulnar nerve‬‬
‫المهام الحركية للنخاع الشوكي ‪Motor Functions of the Spinal Cord‬‬
‫المنعكسات النخاعية ( المنعكسات التي مركزها في الحبل الشوكي ) ‪Spinal Reflexes‬‬
‫‪Afferent ( sensory ) neuron :‬‬
‫سي (وهو الوارد ‪ afferent‬للجهاز العصبي المركزي)‬
‫العصبون الح ّ‬
‫‪ُُُ Effferent ( Motor ) neuron :‬‬
‫العصبون اآلمر ( حركي لعضلة أو إفرازي لغدّة ‪ ،‬و هو الخارج‬
‫‪ Efferent‬من الجهاز العصبي المركزي‬
‫تعصيب ‪Innervation ( Nerve Supply) :‬‬
‫مشبك ‪Synapse :‬‬
‫منعكس أحادي المشبك ‪Monosynaptic reflex :‬‬
‫منعكس متعدد المشابك ‪Polysynaptic reflex :‬‬
‫‪2‬‬
What is a spinal reflex ?
A spinal reflex is an
automatic , involuntary
neuromuscular action
elicited by
a defined stimulus.
The basic unit of a
reflex is the reflex arc .
The controlling center
of the spinal reflex is
located in one or more
spinal cord segments
Receptor
is Muscle
Spindle
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The Spinal Reflex Arc
Consists of :
1. Sense organ (receptor).
2. Afferent ( sensory ) neuron.
3. Centre ( in spinal cord ) with one synapse in
case of monosynaptic reflexes , and more
than one synapse in case of polysynaptic
reflexes ).
4. Efferent ( motor ) neuron .
5. Effector (muscle or gland ).
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Afferent fiber
DRG
Efferent fiber
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The Afferent Neurons (1)
The afferent (sensory ) neuron is a pseudobipolar
cell , having its cell-body in the dorsal root
ganglion ( DRG) .
Its afferent fiber enters the spinal cord via the
posterior spinal root , and :
(1) In case of monosynaptic reflexes  synapses
directly on the anterior horn cell
(2) in case of polysynaptic reflexes  synapses on
a posterior horn cell , which can be
(a) at the same spinal segment where the afferent
fiber entered the spinal cord , or
(b) at an adjacent , higher or lower spinal segment (
after the afferent fiber ascends or descends for a
few higher or lower spinal levels ).
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The Afferent Neurons (2)
In case of polysynaptic reflexes one or more
interneurn will connect the posterior horn cell
to the anterior horn cell .
Afferent neurons can undergo:
– Divergence to spread the effect of a
single stimulus to more motoneurons in
the same spinal segment , or to adjacent
segments,
– Convergence ( e.g. on a motoneuron ) to
facilitate spatial summation.
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The Interneurons
(in case of Polysynaptic reflexes)
Interneurons are small excitable cells in the grey
matter of spinal cord , connecting posterior ( dorsal )
horn cells to the anterior horn cells .
They can be single or multiple .
Together , they may constitute local spinal circuits
Some of them could be excitatory and others could be
inhibitory .
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The Efferent Neurons


These are motor neurons ( motoneurons ) .
That are situated in the anterior horn of the spinal cord ( i.e.,
they are anterior horn cells, AHCs):
 They are 2 types:
(1) Large alpha motor neurons : Innervates the large extrafusal
muscle fibers . ( Extrafusal fibers are the regular contractile
units of the muscle , which constitute the muscle bulk , and
which are responsible for the actual shortening of the
muscle ).
(2) Small gamma motor neurons : Innervate the intrafusal fibers .
( Intrafusal fibers are the tiny fibers that are present inside the
muscle spindle , which is the muscle receptor ) .
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Muscle
Receptor
Alpha motor
neuron to
Extrafusal
Fibers
Gamma
motor
neuron to
Intrafusal
Fibers (inside
the muscle
spindle )
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Input to Alpha Motor Neurons
3 sources ‫ثالثة مصادر‬
1. DRG neurons
– sensory neurons (proprioception)
2. Upper motor neurons
– primarily from M1 (Cortical Motor Area))
3. Spinal interneurons
– Can be excitatory or inhibitory
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There are 3
Inputs to Alpha
Motor Neurons
DRG
(1) Afferent
(sensory) neuron
(2) Upper
motor
(3) Spinal interneuron
neurons
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Important Definitions
Reflex ( Response) Time = Time that elapses between
application of the stimulus and appearance of the
response .
Minimal Synaptic delay ( time taken in one synapse)
~ 0.5 ms.
Central Delay : Time taken in spinal cord synapses
Central Dealy = Total Reflex time –Time spent in conduction
of impulses along the afferent and efferent nerves.
Number of synapses =
Central Delay
/0.5 ms
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‫تعريفات هامة‬Important Definitions
Reflex Time ‫ زمن األستجابة‬:Time that elapses between
application of the stimulus and appearance of the response .
‫ الزمن الذي إنقضي بين إعطاء التحفيز و ظهور األستجابة‬
‫ التأخير داخل‬+ ) ‫ و طبعا هو مجموع التأخير في العصبونين ( الوارد و الخارج‬
Central Delay ‫المشابك‬
‫ يعني الوقت الذي استغرقته الرحلة في العصبونات زائدا الوقت الذي استغرققه‬
)‫التأخير داخل المشابك (التي هي بين العصبونات‬
 Central Delay ‫التأخير داخل مجموع المشابك‬: Time taken in spinal
cord synapses
 i.e., Reflex Time = Central Delay + Time spent in conduction
of impulses along the afferent and efferent nerves.
 Minimal Synaptic delay : ‫ (التأخير داخل المشبك الواحد‬time taken in
one synapse) ~ 0.5 ms.
 Central Dealy = Total Reflex time –Time spent in conduction
of impulses along the afferent and efferent nerves.
‫ ألنه لو طرحنا الوقت الذي استغرقته الرحلة في العصبونات من كل وقت التأخير‬
‫المركزي نتوصل إلي التأخير داخل المشابك التي هي بين العصبونات‬
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Central Delay /
 Number of synapses ‫= عدد المشلبك‬
0.5 ms

Classification of Reflexes According to the
Number of Synapses Present in the Reflex Arc
(1) Monosynaptic Reflexes ‫المنعكسات أحادية المشبك‬:
have one synapse only : The sensory ( afferent ) axon
synapse directly on the anterior horn cell.
Therefore , the reflex arc does not contain
interneurons .
Examples : The Stretch ( Tendon ) reflexes
( also called Tendon Jerks ).


Polysynaptic reflxes ‫المنعكسات متعددة المشابك‬:
Have more than one synapse , therefore contain
interneuron(s) .
Examples : Abdominal Reflexes , withdarwal reflex ,
Plantar response .

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Stretch Reflexes are Monosynaptic
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Monosynaptic Stretch Reflex

Postural adjustments ‫تظبيط وضعية الجسم‬
 Muscle tone
 knee-jerk

reflex
Muscle spindles
 length
detectors
 Low threshold
 Activity
in afferent from muscle spindle leads to:
 1) stimulation of extrafusal fibers in muscl
 2) Reciprocal Inhibition of antagonist muscle
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Classification of Reflexes According to
the Location of the Receptor (1)
(1) Superficial Reflexes :
The receptor is in the skin ( cutaneous receptor) e.g.,
abdominal reflexes , plantar reflex , anal reflex .
(2) Deep reflexes : The receptor is located in muscle or
tendon e.g.,
a) Stretch Reflexes (Tendon jerks ) : such as the biceps
jerk , triceps jerk , knee-jerk ( patellar reflex ) , ankle jerk.
The receptor for all these is called muscle spindle , and
is located within the muscle itself .
b) Inverse Stretch Reflex ( Golgi Tendon Organ reflex ) :
The receptor is called Golgi Tendon Organ , and is
present in the muscle tendon .
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Classification of Reflexes According to
the Location of the Receptor (2)
(3) Visceral Reflexes ‫ منعكسات حشوية‬:
Reflexes involving the autonomic reflex arc , receptors are
located in the viscera e.g., micturition reflex ‫ المنعكس البولي‬,
Defecation reflex ‫ المنعكس البرازي‬, erection reflex ‫المنعكس‬
‫ األنتصابي‬.
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The Muscle Spindle (1)
‫المغزل العضلي‬
Each spindle consists of 3-12 tiny ( small)
muscle fibers , called Intrafusal Fibres.
They lie parallel to the large Extrafusal Fibres
( which constitute the muscle bulk ) and are
attached to them or to the tendon.
Each intrafusal fibre consists of:
– Central non-contractile area (receptor),
– Peripheral contractile parts.
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The Muscle Spindle (2)
Types of Intrafusal Fibers
There are two types of intrafusal fibers:
– Nuclear bag fibers : contain many nuclei in
a dilated central area ( “ bag ” ) . Typically there are 2
nuclear bag fibers per spindle .
– Nuclear chain fibers : thinner and shorter than
nuclear bag fibers , and have one line of nuclei spread
in a chain along the receptor area . There are 4 or
more nuclear chain fibers per spindle ( 4 – 9 usually )
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The Muscle Spindle (3)
Nuclear
bag fiber
Nuclear chain fiber
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The muscle spindle(5)
Sensory Innervation of the Muscle Spindle (2)
B/ Secondary ( Flower-spray ) Afferents :
 Innervate ONLY the nuclear chain receptor
– Discharge at an increased rate throughout the period during which the
muscle is being stretched, directly proportion to the degree of stretch
(measure only muscle length).
– This response is known as the Static Response
– ‫ألنها تسجل فقط التغير المطلق في الطول و لكنها ال تسجل سرعة التغير في طول العضل‬
Nuclear bag fibres are supplied by primary endings only 
responsible for the dynamic response ( e.g. in case of knee-jerk )
Nuclear chain fibres are supplied by both primary and secondary
endings  responsible mainly for the static response ( and to a
much lesser extent , dynamic response ) .
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Motor innervations of the muscle
spindle (1)
Efferent fibres to spindle
gamma motor neurons supply the
peripheral contractile parts of the
intrafusal muscle fibres and are of two
functional types:
– Dynamic gamma efferent end mainly on
the nuclear bag fibres
– Static gamma efferent end mainly on
nuclear chain fibres
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Mechanism of Action of the Stretch reflex
1.
2.
Stretching the muscle bulk ( extrafusal fibers ) also
stretches the receptor ( muscle spindle ) , because the
muscle spindle intrafusal fibers lie in parallel with the
extrafusal fibers of the muscle bulk .  stretch receptor
in intrfusal fibre  stimulation of sensory endings .
Afferent impulses  go to spinal cord  stimulate:
Alpha Motoneurons , which send impulses to Extrafusal
Muscle Fibres  contraction of the muscle bulk .
Gamma Efferent Motoneurons , which send motor
impulses to the Intrafusal Fibers inside the muscle
spindle  leading to shortening of the peripheral
contractile parts of the intrafusal fibres  increase the
sensitivity of the receptor to muscle stretch .
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Dynamic Component of Stretch Reflex
Dynamic stretch reflex
Sudden rapid stretch of a muscle  stimulates the
receptor  which sends synchronous strong
excitatory impulses from the primary ( annulospiral )
endings to the alpha motor neurons in the anterior
horn of the spinal cord .
The stimulated alpha motoneurons send excitatory
impulses to the extrafusal fibers causing contraction
of the muscle bulk  this results in a jerky movement
As the muscle shortens  the spindle becomes lax
1.



–
 and ceases to discharge  no more stimulation of alpha
motoneuron  no more excitatory impulses from alpha
motoneuron to the extrafusal fibers  muscle relaxes
This is the basis of tendon jerks( stretch reflexes ) .
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Static Component of Stretch Reflex
2. Static stretch reflex
– Maintained stretch of muscle
– Impulses from muscle spindle travel
through secondary sensory nerve to
alpha motor neuron causing muscle
contraction
– Asynchronous discharge resulting in
mild sustained contraction of muscle as
long as it is stretched
– Basis of muscle tone
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Higher control on gamma
efferent discharge (1)
1.
Cortical areas.
–
The primary motor area (motor area 4) is
facilitatory to gamma motor neurons.
lesions of motor area 4 or pyramids in the
medulla oblongata  hypotonia.
–
Lesions of motor cortex, such as strokes
(which damage both facilitatory and
inhibitory areas), are characterized by
hypertonia (spasticity).
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Higher control on Gamma Efferent Discharge
 and hence on the Stretch Reflex and Muscle
Tone (2)
2. Basal ganglia (BG) ( Extrapyramidal Areas )
–
–
–
Generally ( in most instances ) Inhibitory to
Gamma Efferents ( and consequently to the
muscle tone and stretch reflex ) .
Indirect effect ( there is no direct BG to spinal
cord tract ) . They exert their effect by
stimulating a small area in the reticular
formation of the brain stem (medullary reticular
formation). The latter sends inhibitory impulses
to gamma motor neurons via the lateral
reticulospinal tracts.
Loss of BG inhibitory influence ( e.g. in
Parkinson’s disease ) result in  hypertonia 
muscle rigidity .
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Higher control on Gamma Efferent Discharge 
and hence on the Stretch Reflex and Muscle Tone
(3)
3. Brain stem.
– Facilitatory
Pontine reticular formation
Vestibular nuclei
– Inhibitory:
Medullary reticular area carrying
impulses from inhibitory cortical areas,
basal ganglia and red nucleus.
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Higher control on Gamma Efferent Discharge
 and hence on the Stretch Reflex and Muscle
Tone (4)
4. The cerebellum.
– Cerebellar cortex exerts an inhibitory
influence on the stretch reflex.
– Deep cerebellar nuclei (DCN) are
excitatory via lateral vestibular nuclei.
– Cerebellar lesions in humans
characterized by hypotonia, due to deep
cerebellar nuclei lesion,
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Summary of Factors Controlling
/Regulating/Influencing the Stretch Reflex
Enhances ( ‫)تزيد من نشاطها‬
Inhibits
A/ Supraspinal influences :
1. Cortical :
(a) Primary motor area
(b) Corical/limbic :
fear/apprehension , and anxiety ,
pain
2. Brainstem :
(a) Vestibular nuclei
(b) Pontine centers
3. Cerebellum
B/ Largely spinal mechanisms :
related to noxious ( painful)
stimuli .
A/Supraspinal :
Cortical
( voluntary , conscious , mostly) .
2. Extrapyramidal
(Basal ganglia , Red Nucleus ) .
3. Brainstem :
(a) Medulla .
(b) Reticular formation .
4, Cerebellum
B/ Largely spinal mechanisms :
1. Excessive muscle stretch
( stimulation of Golgi tendon organ ).
2. Muscle contraction
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The Golgi tendon reflex
(inverse stretch reflex)
Inverse stretch reflex: excessive tension on
the muscle (passive stretch of tendon or active
muscle contraction) >> muscle relaxes
opposite response to stretch reflex.
The receptors are Golgi tendon organs in
muscle tendons stimulated >> muscle contract
and pulled on the tendon (tension)
stimulate golgi organ>> A fibers > spinal
cord > excitation of inhibitory interneuron>>
inhibit alpha motor neuron > muscle relaxation
Protect muscle from rupture
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The Golgi tendon reflex
(inverse stretch reflex)
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Withdrawal reflex (1)
(flexor reflex/respnse)
Stimulation of pain receptors in a limb ( e.g.,
hand or foot )>> impulses to spinal cord via A or C
fibres >> interneurons >> anterior horn cells >>
stimulate hand flexor muscles >>move the hand
away from the injurious stimulus.
a polysynaptic reflexe.
stimulation of flexors muscle accompanied by
inhibition of extensors.
Inhibitory inter neurons synapse with extensor
motor neurons known as reciprocal innervations
(reciprocal inhibition).
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Withdrawal reflex (2)
Flexion and withdrawal of the stimulated limb >>
extension of the opposite limb >> occurs with
strong stimulus (crossed extensor reflex)
Reciprocal innervations occurs in extensor
reflex >> flexors are inhibited while extensors
are excited >> pushes the entire body away from
the injurious agent and supports the body
weight.
Withdrawal reflex is characterized by after
discharge, which prolongs the response and
further enhances the protective role of the reflex.
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Properties of reflexes
1. Adequate stimulus
– Each reflex has a precise stimulus called
the adequate stimulus. For example, a
painful stimulus to the sole of the foot
produces the flexor withdrawal reflex,
while deep pressure applied to the same
area produces the positive supportive
reaction.
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Properties of Reflexes
2. Irradiation and recruitment ( spatial ):
–
–
–
The extent of the response in a reflex depends on the
intensity of the stimulus. The more intense the stimulus
is, the greater is the spread of activity in the spinal cord,
involving and recruiting more and more other motor
neurons .
when the sole of the foot is stimulated by a weak painful
stimulus, only the big toe is flexed. A stronger stimulus
will cause reflex flexion of the big toe , other toes , plus
the ankle. The strongest stimulus will cause withdrawal of
the whole leg by causing reflex flexion of the big toe,
ankle, knee and hip .
Impulses may also cross to the other side of the spinal
cord to cause extension of the other leg.
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Properties of reflexes
3. Summation
– A/ In spatial summation: two or more stimuli,
through different sensory routes, which by themselves
are insufficient to elicit a reflex contraction, may elicit a
contraction when given together.
Summation of subliminal fringes
–
–
–
The tension obtained in a certain reflex by weak (but
threshold) stimulation of two adjacent afferents
simultaneously is more than the sum of tension
developed when each is stimulated separately.
Stimulation of each afferent separately causes discharge
of some neurons but only facilitates some nearby neurons
(excites them but not enough to discharge).
Those facilitated neurons are said to lie in the subliminal
fringe zone of those discharging.
40
Spatial Facilitation
( Spatial Summation )
Input A
Input B
41
(1) If neuron B fires, neuron X is
excited to threshold and fires AP
(4) Neuron Y is in
Subliminal Fringe Zone. If
BOTH B and C fire at the
same time  Y threshold of
is reached  it fires AP
(3) If B alone OR C alone fire , they only
increase excitability of neurons in the area
within the square (called Subliminal Fringe
zone ) , but membranes of neurons in
Subliminal Fringe zone t do not reach the
firing level to fire .
(2) If neuron c fires, neuron z is
excited to threshold and fires AP
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Summation ( continued )
– Simultaneous stimulation of
the two afferents causes
overlapping of the subliminal
fringe zones and raises the
excitability of the neurons
(within the subliminal zones)
to discharge.
– B/ In temporal summation:
– two or more stimuli given at
different times, which by
themselves are insufficient to
elicit a reflex contraction, may
elicit a contraction if given
within a short time of each
other (15 ms).
Temporal Summation
43
Properties of reflexes
4. Recruitment and after-discharge
–
–
If a repetitive stimulus is maintained, the
strength of the reflex contraction slowly
increases to a final level. The slow build-up is
due to gradual activation of more motor neurons
(recruitment , temporal ).
The reflex response may continue some time
after cessation of the stimulus, due to afterdischarge, the main type being reverberating
circuit after-discharge.
44
5. Reverberating circuits
impulse from one neuron feed back to restimulate itself for long time, this 
(1) prolongs the response so that it outlives the
stimulus for a considerable time .
(2) augments ( reinforces ) the response .
It eventually stops or wanes out due to fatigue
or due to inhibitory impulses from other
parts of CNS with inhibitory interneurons .
45
Reverberating circus
46
Properties of reflexes
6. Reciprocal innervation or reciprocal
inhibition
– Reflex contraction of an agonist muscle is
accompanied by inhibition of the antagonist.
– When the biceps is involved in a stretch reflex,
the triceps must relax or no movement will
occur.
– This is a reciprocal inhibition >> impulses enter
the spinal cord will stimulate
the motor neurons supplying the stimulated muscle
an inhibitory interneuron >> inhibiting motor neurons
supplying the antagonist muscle.
47
48
7. The Renshaw cell
Is located in anterior horn in close
association with motor neurons.
it is an inhibitory cell excited by
collaterals from an alpha motor neuron to
project back and inhibit the same motor
neuron (negative feedback fashion).
49
50
Properties of reflexes
8. Response time
– Response time is an indicator of the
number of synapses in the reflex arc.
Very short response times indicate a
smaller number of synapses in reflex
path.
– The knee-jerk, which has the shortest
response time, is a monosynaptic reflex.
51
Properties of reflexes
9. Rebound phenomenon
–
–
This is the exaggeration of a reflex after a
temporary period of inhibition. For example, a
flexor withdrawal reflex in one limb involves
stimulation of flexors and inhibition of
extensors. If this is followed by another reflex
which involves stimulation of extensors of the
same limb the extensor response will be greatly
exaggerated.
Rebound is one of the important mechanisms
for coordinating the rhythmic to and from
movement required in walking and running.
52
10. One Spinal Common Output for
Motor Activity ( Final Common Pathway
)
– Many sensory inputs and interneurons converge on the motor
neuron, because motor neurons are the only efferent pathways
from the spinal cord .
–
–
–
Movement is the result of contraction of skeletal muscle stimulated by
motor neurons , which are the final common efferent pathway through
which all spinal and supraspinal inputs must converge & pass to
influence muscle activity .
Each lower motor neuron ( motoneuron ) controls a group of muscle
fibers . The name given to a single motor neuron and all the muscle
fibers it controls is “ Motor Unit ’’ ( remember Nerve & muscle course ).
In the spinal cord motoneurons are arranged in 2 groups : medial and
lateral :
The medial group innervate (1) trunk muscles (2) muscles of the
proximal parts of the limbs  thereby controlling posture and
gross movements
The lateral group innervate muscles of the most distal parts of the
limbs concerned with fine skilled movements.
53
Convergence of many neurons the Final
common Pathway
( Anterior Horn Cell )
54
Centers of Some Deep
Reflexes
Deep Reflexes
Biceps
Spinal center
C5-C6
Triceps
C6-C7
Knee
L2, L3, L4
Ankle
S1, S2
55
Centers of some Superficial
and Visceral Reflexes
Superficial & visceral
reflexes
Corneal
Centers
Pons
Upper abdominal
T8 - T10
Lower abdominal
T10 - T12
Planter
L5 - S1
Anal
S3 - S4
Micturation ( visceral ) S2, S3, S4
Defecation ( visceral ) S2, S3, S4
56