Motor Functions of the Spinal Cord ?????? ???????

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Motor Functions of the Spinal Cord ?????? ??????? Powered By Docstoc
					      Motor Functions of the Spinal Cord
                ‫المهام الحركية للنخاع/ الحبل الشوكي‬
                  ( 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 .

                               ‫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
 ‫العصبون الحسّي (وهو الوارد ‪ afferent‬للجهاز العصبي المركزي)‬
    ‫: ‪ُُُ Effferent ( Motor ) neuron‬‬
 ‫العصبون اآلمر ( حركي لعضلة أو إفرازي لغدة ، و هو الخارج‬
 ‫‪ Efferent‬من الجهاز العصبي المركزي‬
    ‫تعصيب : )‪Innervation ( Nerve Supply‬‬
    ‫مشبك : ‪Synapse‬‬
    ‫منعكس أحادي المشبك : ‪Monosynaptic reflex‬‬
    ‫منعكس متعدد المشابك : ‪Polysynaptic reflex‬‬

       What is a spinal reflex ?
 A spinal reflex is an
 automatic , involuntary
 neuromuscular action
 elicited by
a defined stimulus.                is Muscle

 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

        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 ).

    Afferent fiber


           Efferent fiber

       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 ).
        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
– Convergence ( e.g. on a motoneuron ) to
  facilitate spatial summation.

           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 .

                 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 ) .


   Alpha motor
   neuron to

neuron to
Fibers (inside
the muscle
spindle )

      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

  There are 3
Inputs to Alpha                                  DRG
 Motor Neurons                (1) Afferent
                           (sensory) neuron

                                     (2) Upper
  (3) Spinal interneuron

          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
                 ‫تعريفات هامة‬Important Definitions
  Reflex Time ‫: زمن األستجابة‬Time that elapses between
   application of the stimulus and appearance of the response .
                     ‫ الزمن الذي إنقضي بين إعطاء التحفيز و ظهور األستجابة‬
 ‫ و طبعا هو مجموع التأخير في العصبونين ( الوارد و الخارج ) + التأخير داخل‬
                                                 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 المشابك‬Time 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
 Number of synapses ‫= عدد المشلبك‬
                                          Central Delay /                 14

                                                             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 .
Stretch Reflexes are Monosynaptic

               Monosynaptic Stretch Reflex
   Postural adjustments ‫تظبيط وضعية الجسم‬
      Muscle tone
     knee-jerk   reflex
   Muscle spindles
     lengthdetectors
     Low threshold

     Activityin afferent from muscle spindle leads to:
     1) stimulation of extrafusal fibers in muscl
     2) Reciprocal Inhibition of antagonist muscle
 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 .

 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 البرازي‬erection reflex ‫المنعكس‬
   ‫. األنتصابي‬

            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.

                 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 )

The Muscle Spindle (3)

                   bag fiber

  Nuclear chain fiber
                              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 ) .

 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

         Mechanism of Action of the Stretch reflex
     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:
1.   Alpha Motoneurons , which send impulses to Extrafusal
     Muscle Fibres  contraction of the muscle bulk .
2.   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 .

     Dynamic Component of Stretch Reflex

1.       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
           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 ) .

   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
   – Asynchronous discharge resulting in
     mild sustained contraction of muscle as
     long as it is stretched
   – Basis of muscle tone

         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).

 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 .
 Higher control on Gamma Efferent Discharge 
and hence on the Stretch Reflex and Muscle Tone
3. Brain stem.
  – Facilitatory
        Pontine reticular formation
        Vestibular nuclei
  – Inhibitory:
        Medullary reticular area carrying
        impulses from inhibitory cortical areas,
        basal ganglia and red nucleus.

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,

         Summary of Factors Controlling
     /Regulating/Influencing the Stretch Reflex
Enhances ( ‫)تزيد من نشاطها‬              Inhibits
A/ Supraspinal influences :             A/Supraspinal :
1. Cortical :                               Cortical
(a) Primary motor area                  ( voluntary , conscious , mostly) .
(b) Corical/limbic :
    fear/apprehension , and anxiety ,
      pain                              2. Extrapyramidal
2. Brainstem :                          (Basal ganglia , Red Nucleus ) .
(a) Vestibular nuclei                   3. Brainstem :
(b) Pontine centers                     (a) Medulla .
3. Cerebellum                           (b) Reticular formation .
B/ Largely spinal mechanisms :          4, Cerebellum
related to noxious ( painful)
 stimuli .                              B/ Largely spinal mechanisms :
                                        1. Excessive muscle stretch
                                        ( stimulation of Golgi tendon organ ).
                                        2. Muscle contraction

      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

The Golgi tendon reflex
(inverse stretch reflex)

           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).

       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
  Withdrawal reflex is characterized by after
discharge, which prolongs the response and
further enhances the protective role of the reflex.

      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

         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.

             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.

            Spatial Facilitation
          ( Spatial Summation )
Input A      Input B

(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

        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

       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 after-
      discharge, the main type being reverberating
      circuit after-discharge.

    5. Reverberating circuits
 impulse from one neuron feed back to re-
    stimulate 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 .

Reverberating circus

       Properties of reflexes
6. Reciprocal innervation or reciprocal
  – 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
  – 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.

         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).

      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
  – The knee-jerk, which has the shortest
    response time, is a monosynaptic reflex.

       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
  –   Rebound is one of the important mechanisms
      for coordinating the rhythmic to and from
      movement required in walking and running.

    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.

Convergence of many neurons the Final
         common Pathway
        ( Anterior Horn Cell )

Centers of Some Deep Reflexes

  Deep Reflexes   Spinal center
  Biceps          C5-C6
  Triceps         C6-C7
  Knee            L2, L3, L4
  Ankle           S1, S2

  Centers of some Superficial
    and Visceral Reflexes
Superficial & visceral   Centers
Corneal                  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

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