Motor Functions of the Spinal Cord
‫المهام الحركية للحبل الشوكي‬
Dr. Taha Sadig Ahmed
‫طه صادق أحمد‬
•
•
•
•
•
•
• Objectives
At the end of this lecture the student should :
(1) appreciate the two-way trafiic along the spinal cord .
(2) describe the reflex arc .
(3) classify reflexes into superficial and deep ; monosynaptic & polysynaptic , give
examples of them , and show how they differ from each other .
(4) describe the general properties of reflexes and their synaptic pools such as
convergence , divergence , irradiation , recruitment , reverberating circuits ,afterdischarge , minimal synaptic delay, central delay and reflex time .,
(5) be able to describe the spinal centers of biceps , triceps , knee , ankle ,
abdominal and plantar reflexes .
• Refernce Book
• Ganong’s Review of Medical Physiology , 23rd edition . Barrett KE,
Barman SM, Boitano S, Brooks HL , edotors . Mc Graw Hill, Boston 2010 .
• Pages 157-165 .
‫صطلحات طبيّة هامة‬
‫الحبل الشوكي ‪• 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
‫منعكس الحبل الشوكي ‪• Spinal Reflex‬‬
‫‪• Afferent ( sensory ) neuron :‬‬
‫سي (\الوارد إلي الحبل الشوكي جالبا األحساسات )‬
‫العصبون الح ّ‬
‫العصبون اآلمر ( الحركي) (الخارج ‪• Efferent ( Motor ) neuron :‬‬
‫( اآلمر لعضلة لتنقبض) من الحبل الشوكي‬
‫تعصيب ‪• Innervation ( Nerve Supply) :‬‬
‫مشبك ‪• Synapse :‬‬
‫منعكس أحادي المشبك ‪• Monosynaptic reflex :‬‬
‫منعكس متعدد المشابك ‪• Polysynaptic reflex :‬‬
‫العصبون الحركي العلوي ( )‪Upper motor neuron ( UMN‬‬
‫العصبون الحركي السفلي ( )‪Lower motor neuron (LMN‬‬
‫شناج ( بضم الشين ) ‪ :‬فرط التوتر التشنجي ‪Spasticity‬‬
‫‪Spastic‬‬
‫شناجي ‪ ،‬تشنجي‬
‫‪ Stretch reflex = Tendon jerk‬درجة التوتر العضلي ‪Muscle tone‬‬
‫منعكس الشد ‪ ،‬األنتفاضة العضلية‬
‫) ‪ muscle spindle‬و مستقبله المغزل العضلي )‬
‫منعكس قولجي الوتري (و مستقبله عضو قولجي الوتري ‪Golgi tendon Reflex‬‬
‫‪Ascending tracts‬‬
‫سية )‬
‫السبل الصاعدة ( ح ّ‬
‫السبل ‪Descending tracts‬‬
‫‪4‬‬
‫•‬
‫•‬
‫•‬
‫•‬
‫•‬
‫•‬
‫•‬
‫•‬
‫•‬
The dorsal rootcontains afferent (sensory) nerves coming from receptors .
The cell body of these neurons is located ‫ موجود‬in dorsal ( posterior ) root
ganglion ( DRG)
The ventral root carries efferent (motor) fibers
The cell-body of these motor fibers (AHC, Lower Motor Neuron) is located
in the anterior horn of the spinal cord .
Reflex Arc
AHC ( Lower Motor
Neuron , LMN)
Final Common
Pathway)
Consists of :
(1) Sense organ (receptor)
(2) Afferent ( sensory ) neuron.
(3) Motor ( Efferent ) neuron ,
in the anterior horn of spinal
cord  Hence the spinal motor
neuron ( or homologous cranial
nerve motor neuron ) is called
Anterior Horn Cell (AHC) or
Lower Motor Neuron ( LMN)
The “ center ” of the reflex
comprises the part of the reflex arc
inside the spinal cord .

In case of monosynaptic reflexes
the afferent neuron synapses directlly
on the AHC ; & in case of polysynaptic
reflexes , one or more interneuron
connects the afferent & efferent neurons
.

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 reflex ‫ ( منعكس الشد‬also
called Tendon Jerk ).
(2) Polysynaptic reflxes ‫المنعكسات متعددة المشابك‬:
– Have more than one synapse , therefore contain
interneuron(s) between the afferent nerve & AHC .
–Examples : Abdominal Reflexes , withdarwal reflex ,
Plantar response .
7
Classification of Reflexes According to the Location of
the Receptor
(A) Superficial Reflexes :
Are polysynaptic reflexes . The receptor is in the skin .
Examples are abdominal reflexes and plantar
reflex ,
(B) Deep reflexes : The receptor is located in muscle or
tendon Examples :
(1) Stretch Reflexes (Tendon jerks ‫ )منعكسات الوتر‬,
monosynaptic : such as knee-jerk ( patellar reflex ) and
ankle jerk .
The receptor for all these is the muscle spindle ( which is
located within the muscle itself .
(2) Inverse Stretch Reflex ( Golgi Tendon organ reflex
‫ )منعكس قولجي الوتري‬, polysynaptic : The receptor is called
Golgi Tendon Organ , and is8present in the muscle tendon
.
Types of Muscle Fibers
• (1) Extrafusal
Lower Motor
Neuron (AHC)
(2) Intrafusal fibers :
are tiny , microscopic fibers ,
present within the muscle spindle
(the muscle length detector )
innervated by Gamma motor neurons



9
fibers :
• are the contractile
units of the muscle
, which constitute
the muscle bulk ,
• and which are
responsible for the
actual shortening
and force
generation by the
muscle
• Innervated by
Alpha motor
neurons .
Types of AHC :
 (1) Large ones , called Alpha motor neurons  supply
extrafusal fibers
 Also called Lower Motor Neuron ( LMN)
 (2) Small ones , called Gamma motor neurons 
supply intrafusal fibers


Inputs to theAHC ( LMN)
3 sources
(1) Primary Afferent ( sensory ) neurons
(2) Spinal interneurons
(3) Upper motor neurons ( UMN) , ( from Brain )

10




Q : What is the Final Common
Pathway
It is the Alpha motor neuron (AHC)
It constitutes he only output of
CNS on muscle i.e.,
All spinal & supraspinal influences
converge on ithe AHC  up to
10000 synapses can be present on
one alpha motoneuron .
Q : What is “ Motor Unit ’’ ?
Motor unit comprises 
(1) alpha Motor neuron ( LMN) +
 (2) all muscle fibers it innervates
 ( remember musculoskeletal block
lectures ).



11
Irradiation & Recruitment

The extent of response
( strength of muscle
contraction ) depends on the
intensity ( strength ) of the
stimulus .
This is because 
(1) Increased stimulation
intensity  irradiation to
other segments of the
spinal cord
(2) Progressive recruitment of
more and more motor units)
 stronger contraction
12
Example of a Superficial , Polysynaptic Reflex :
Withdrawal reflex
(flexor reflex/respnse )
13
Withdrawal reflex (flexor reflex/respnse )
It is a superficial , polysynaptic , protective
reflex
Stimulation of pain receptors in a limb ( e.g., hand
or sole of foot ) 
 impulses to spinal cord via A or C fibres 
 interneurons 
 anterior horn cells stimulate limb flexor muscles
  withdrawal of limb ( moving it away from the
injurious agent ) .
stimulation of flexors muscle accompanied by
inhibition of extensors.via inhibitory interneurons 
Reciprocal Inhibition, based on Reciprocal
Innervation ).

Crossed Extensor Reflex
If a stronger stimulus ( than
that needed to elicit the
Withdrawal Reflex) is delivered

 Flexion withdrawal of the
stimulated limb will be
accompanied by extension of the
opposite limb 
the latter response is called
Crossed Extensor Reflex
(1) Pushing the entire body away
from the injurious agent and
(2) supporting the body weight
against gravity  There fore it
is an Antigravity Reflex
 Reciprocal innervations occurs
also in extensor reflex : flexors
in the opposite limb are inhibited
while extensors are excited 

15
Sustained
After-Discharge in Reverberating
Circuits ‫ دوائر الصدي‬prolongs the response
Withdrawal reflex is characterized by after
discharge, which prolongs the response and further
enhances the protective role of this reflex.
 In short, Withdrawal reflex and Crossed Extensor
reflex are polysynaptic and show the properties of
reciprocal innervation , motor unit recruitment ,
irradiation and after-discharge .
16
Important Definitions ‫تعريفات هامة‬






17
Reflex Time :Time that elapses between application
of the stimulus and appearance of the response .
Central Delay : 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 : time taken in one synapse
~ 0.5 ms.
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
Thanks
!
18