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‫االسراء اية ‪58‬‬
Somatic Motor System
By
Dr. Abdel Aziz M. Hussein
Assist Prof of Medical Physiology
• It is a reflex relaxation (or lengthening) of a ms in
response to excessive stretch or contraction of
that ms.
Neural pathway:
• Stimulus: ↑ed ms tension by;
1. Overstretch or
2. Severe contraction
• Receptors: Golgi tendon organs
1) Site:
• tendons of skeletal ms in series
with ms fibers
2) Structure:
• Are encapsulated sensory receptor
• 6-20 elastic fibers
3) Innervations:
• Type Ib or A alpha afferent fibres
Receptors: GTOs
• Stimulated by ↑ed ms tension caused by
passive overstretch or active contraction
of the ms
Afferents:
• A alpha or Ib
Center :
a)inhibitory interneurons→ inhibit the α-MNs
supplying the same ms
b)excitatory interneurons→ excite the αMNs supplying the antagonistic ms
Response:
• Relaxation of the same ms
• Contraction of antagonistic group of ms.
Significance GTR:
a)Physiological significance:
• Protective reaction which prevent tearing of the ms or
avulsion of its tendon from its bony attachment when the
ms is overstretched.
Significance GTR:
b)Clinical significance: (clasp knife effect)
• Demonstrated clinically by passive flexion of a spastic limb
(e.g. in upper motor neuron lesions) at its main joint.
• As the limb is flexed, an initial resistance occurs due to
contraction of this ms a result of the stretch reflex.
• With persistent flexion, at a certain point, GTR is excited→
sudden disappearance of the initial resistance → the limb
flexes easily, as occurs during closing-of a pocket knife→
clasp knife effect.
• E.g. Flexion of knee and ankle
Def.,
• Rapid contraction followed by
relaxation of a ms due to
sudden stretching of that ms
by tapping on its tendon using
a medical hammer
Mechanism:
• It is a dynamic type of the
stretch reflex
To brain
6 Primary afferent
neuron stimulates
inhibitory interneuron
4 Primary afferent
neuron stimulates
alpha motor neuron
to extensor muscle
Alpha
MNs
1ry
endings
3 Primary afferent
neuron excited
Nuclear
bag
7 Interneuron inhibits
alpha motor neuron
to flexor muscle
5
Alpha motor neuron
stimulates extensor
muscle to contract
Ms
contraction
Sudden
stretch
2 Muscle spindle
stimulated
1 Extensor muscle
stretched
Flexor muscle
8 (antagonist) relaxes
• Cause of relaxation after contraction in the tendon
jerk:
a) Stoppage of discharge from the ms spindles.
b) Stimulation of the Golgi tendon organs.
c) Stimulation of the Renshaw’s Cells.
Jerk
Biceps
jerk
Triceps
jerk
Knee
jerk
Ankle
jerk
Jaw jerk
Center
Limb position
Tendon
Response
C5,6
The elbow
120°
is Tapping on biceps Flexion of
tendon
the forearm
C6,7
The
90°
is Tapping on triceps Extension of
tendon directly
the forearm
elbow
L2, 3 & 4 knee is semi Tapping
on Extension of
flexed
by patellar tendon
the knee
seating with the
leg to be tested
crossing
over
other
S1,2
feet
slightly Tapping
dorsiflexed
tendoachilles
Trigemin Mouth slightly Tapping on chin
al nerve opened
on Plantar
flexion.
Closure
mouth
of
Reinforcement of the tendon jerks
•
Response of the tendon jerks can be reinforced by
facilitating the spinal centers.
•
Can be done by either;
a) Jendrassik's maneuver → ask the patient to hook his
fingers or to clench his teeth→ send signals from
the contracted ms which stimulating γ-MNs.
b) Distracting patient’s attention→ prevents any
voluntary inhibition of the reflex.
1. Localization of spinal cord lesions:
• Loss of TJ means the lesion in its center e.g. ankle
jerk is lost in sacral region lesion.
2. Assessment of the ms tone :
• In hyperreflexia (exaggerated tendon jerks) →
hypertonia (↑ms tone).
• In hyporeflexia (↓ed tendon jerks) → hypotonia
(↓ms tone).
• In areflexia (lost tendon jerks) → atonia (lost ms
tone).
3. Assessment of the integrity of pathway of stretch reflex: so
areflexia or absent tendon jerk may be due to;
Site of lesion
Condition
•Afferent lesion
Tabes dorsalis
•Center (AHC) lesion
Poliomyelitis
•Efferent lesion
Trauma or neuritis
4. Assessment of the state of Supraspinal centers:
Hyperactive(exaggerated)
TJ
Hypoactive (decreased) TJ
Physiological
causes
Anxiety and nervousness
Sleep and anaesthesia
Pathological
causes
UMNL
Lesion in area 6
Tetany and
hyperthyroidism
Paleocerebellum lesion
LMNL
Lesion in area 4
Hypothyroidism
Neocerebellar syndrome
• Occurs in the neocerebellar syndrome and chorea.
• Characterized by hyporeflexia & hypotonia
• Knee jerk is weak than normal and during relaxation of the
quadriceps ms, the leg falls like a dead weight(due to
hypotonia) & swings for sometime like a pendulum before
resting.
Def.
• Alternating regular rhythmic contractions with
incomplete relaxations of a ms (its MNs is in a state
of facilitation) in response to sudden maintained
stretch.
Cause:
• UMNL
Types
1) Ankle Clonus:
• Produced by sudden maintained dorsiflexion of the
foot  leads to regular rhythmic planter flexions due
to rhythmic contractions of soleus and gastrocnemius
muscles.
2) Knee Clonus:
• Produced by the sudden downward displacement of
the patella  rhythmic oscillations of the patella.
Mechanism of clonus:
• Clonus is the result of a stretch reflex - inverse stretch
reflex sequence, which occurs as follows :
• Sudden stretch of the ms results in its contraction through the
stretch reflex.
• This is followed by relaxation due to;
a) Stoppage of impulse discharge from the ms spindles.
b) Initiation of an inverse stretch reflex due to stimulation of the
GTOs.
• As stretch is maintained, a new stretch reflex occurs (helped
by the state of excessive spinal facilitation), and the cycle is
repeated.
Higher Motor Centers in CC
Midbrain
Descending Motor Tracts
L
A
Lower MNs
In Brain Stem
Pons
Medulla
Motor centers in
brain stem
e.g. RF
Lower MNs in AHCS
Spinal Cord
• Consists of 2 sets of motor neurons;
1. Upper Motor neurons:
• Cell bodies are present in motor areas of cerebral
cortex and other motor centers in brain stem
• Axons form descending motor tracts
2. Lower motor neurons:
• Cell bodies are present in AHCs of spinal cord or
motor nuclei of cranial nerves
• Axons form peripheral nerves that supply skeletal
ms
Higher Motor Centers in CC
Midbrain
Upper Motor Neuron
L
A
Brain Stem Nuclei
Pons
Medulla
Motor centers in
brain stem
e.g. RF
AHCS Spinal Cord
Midbrain
Left
Internal
Capsule
L
A
Somatic Motor nuclei of
cranial nerves
Pons
Medulla
AHCs
Spinal Cord
• Initiation and performance of voluntary movements are
the result of motor commands signals from the motor
cortex to the lower motor centers via the descending
motor tracts
• Motor cortex is located in the frontal lobe, and comprises;
1. Primary motor area (area 4)
2. Premotor area (area 6)
3. Supplemental motor area
Area 4
Premotor
area
4
6
Site:
Precentral gyrus in frontal lobe
Precentral
gyrus
Central sulcus
Body representation:
1. Contralateral: i.e. Body ms from the Rt side of the body is
represented in the left hemisphere and Lt half of body is
represented in Rt hemisphere
N.B. some facial muscles
are represented bilaterally
Body representation:
2. Inverted: i.e. the ms of the
face are controlled by the
lowermost part of area 4 &
muscles of the lower limbs
controlled by the upper most
part of area 4
Body representation:
3. Ms involved in fine
voluntary movements
are represented by
relatively large areas
than those involved in
gross movements
Homunculus
Connections:
Premotor and
supplemental
motor areas.
a) Afferents:
Visual and
auditory areas
Prefrontal
area
Proprioceptors
(Sensory Feedback)
Basal ganglia
and
cerebellum.
Connections:
b) Efferents:
Area 4
on opposite side
Red nucleus and RF
(moderate No. of fibers)
Corticobulbospinal tract
(30%)
Basal ganglia and
cerebellum
(Large No. of fibers.
Functions:
1. Initiation of voluntary movements
done by the distal limb muscles
2. Facilitatory to the tone of the distal
muscles
Lesion:
1. loss of voluntary movements (paralysis) of
the distal limb muscles on opposite side
2. Hypotonia and inhibition of tendon jerks
3. Babinski’s sign with dorsiflexion of the big toe
only
Site:
•Frontal lobe in front of area 4
Premotor area
Body representation:
Contralateral and inverted
Connections:
Area 4
Supplemental
area
Basal ganglia
and
cerebellum.
Corticobulbospinal tract
(30%)
• Functions:
1. shares in planning of complex movements
together with area supplemental motor area
2. initiates the gross movements as those done by
the trunk
3. with B.G. involved in postural adjustment
during voluntary distal movements
4. inhibitory effect on muscle tone
5. with basal ganglia, initiate & control the automatic
associative movements
Functions: contains special areas in area 6
Broca’s area
Hand skills
Head rotation
Eye movements area
• Lesion:
1. impairment of complex movements or
paresis
2. Hypertonia and exaggeration of tendon jerks
3. Loss of automatic associative movements
4. Babinski’s sign with fanning of the lateral four
toes only
5. Motor aphasia
6. Motor apraxia
Site:
•Upper medial side of frontal lobe above area 6
supplemental area
Connections:
Area 4
Premotor area
Basal ganglia
and
cerebellum.
Corticobulbospinal tract
Functions:
1)Evokes complex movements which often
involve both sides of the body e.g.: causing
both hands to perform a motor act together.
2) With the premotor area (6) in providing
suitable background for the performance of
the fine skilled movements by hands and
fingers that are mediated by the C.B.S tract.
3)Shares in the planning and programming of
the complex movements with area 6.
Classified by 2 ways ;
a) Pyramidal and extrapyramidal tracts:
• If the tract passes through the pyramids of
the medulla→ pyramidal tract.
• If the tract does not pass through the
medullary pyramids→ extrapyramidal tract .
Pyramidal
Tract
Extrapyramidal
Tracts
Medial and Lateral Motor System
Medial motor system
Lateral motor system
• b) Medial and lateral motor system:
• i) Medial motor system→ composed of tracts that
terminate primarily on the ventromedial neurons →
innervate axial and girdle ms → concerned with
postural control.
• ii) Lateral motor system→ composed of tracts that
terminate primarily on the lateral neurons →
innervate distal ms of the limbs → concerned with
control of fine voluntary movements.
 5 important sets of descending motor tracts, named
according to the origin of their cell bodies and their final
destination:
1) Corticobulbospinal tract
(= Pyramidal tract)
2) Rubrospinal tract,
3) Reticulospinal tracts,
4) Vestibulospinal tracts
5) Tectospinal tract
Extrapyramidal tracts
Motor areas, somatic sensory areas
Corticonuclear tract
Midbrain
Left
Internal
Capsule
L
A
Corticobulbar tract
Pons
Medulla
Corticospinal tract
Spinal Cord
• Origin:
1. Primary motor area (area 4) → 30%
2. Premotor and supplemental areas → 30%
3. Somatic Sensory areas → 40%
• Origin:
Area 4
Premotor
area and
supplemental
motor area
Area 3,1,2 and
Area 5,7,
30 % 40 %
30 %
• Divisions:
• 1) Corticospinal tract : from cortex to spinal
cord
• 2) Corticonuclear tract: from cortex to cranial
nerves nuclei (3,4,6) that supply extraocular
muscles
• 3) Corticobulbular tract: from cortex to other
cranial nerves (5,7,9,10,11,12) of brain stem
M1
4
Genu
Anterior
Limb
Corticospinal Tract
Posterior
Limb
Left
Internal
Capsule
Midbrain
Pons
Pyramidal
Decussation
Mid
Medulla
Caudal
Medulla
Spinal Cord
• (a) 90% cross to the opposite side and descend in
the lateral column of the spinal cord as lateral
corticospinal tract→ terminate on lateral AHCs
→innervate the distal ms
• (b) 10% not cross → descend ipsilaterally as
ventral corticospinal tract.
• About 9% of these fibers cross and terminate on
medial AHCs → innervate the axial muscles of the
opposite side.
• Only 1% of the fibers terminate on the A.H.Cs of
the same side (bilateral innervations as respiratory
muscles).
• Pathway:
CC
Internal
capsule
Medulla
Lateral CST
Ventral or
medial CST
Spinal
cord
M1
Corticobulbar Tract
FA
L
Left
Internal
Capsule
VMidbrain
PONS
Upper Face
Lower Face
MEDULLA
F
Corticobulbar Tract is crossed and
uncrossed
EXCEPT to lower motor neurons which
control lower half of the face and tongue
are crossed only.
V
VII
Pons
VII
IX
IX
X
X
Mid
Medulla
LOWER
MOTOR
NEURON
NUCLEI
Caudal
Medulla
Spinal Cord
XII
Ipsilateral Side
XII
Contralateral Side
Area 8
Corticonuclear Tract
FA
L
Left
Internal
Capsule
Midbrain
PONS
III
Midbrain
IV
VI
F
III
IVPons
VI
LMN Supplying eye muscles
Mid
Medulla
Caudal
Medulla
Spinal Cord
Ipsilateral Side
Contralateral Side
• Functions:
• The C.B.S tract is primarily concerned with the
mediation of voluntary motor commands from the
cerebral cortex to lower motor centers:
• a- Corticonuclear tract controls the extraoccular ms.
• b- Corticobullar tract controls the ms of lower part
of the face and tongue which are involved in articulate
speech.
• c-Lateral corticospinal tract controls distal ms that
subserve skilled movements as those used in
manipulation by hands and fingers.
• Functions:
• 2) Medial C.B.S tract controls axial ms
concerned mainly with regulation of posture.
• 3) C.B.S tract is also facilitatory to the tone of
the distal limb muscles.
They are classified by 2 ways ;
a)Pyramidal and extrapyramidal tracts:
• If the descending motor tract passes
through the pyramids of the medulla→
pyramidal tract.
• If the descending motor tract does not pass
through the medullary pyramids→
extrapyramidal tract .
Pyramidal
Tract
Extrapyramidal
Tracts
AHCs
Medial motor system
Lateral motor system
• b) Medial and lateral motor system:
• i) Medial motor system→ composed of
tracts that terminate primarily on the
ventromedial neurons → innervate axial
and girdle ms → concerned with postural
control.
• ii) Lateral motor system→ composed of
tracts that terminate primarily on the lateral
neurons → innervate distal ms of the limbs
→ concerned with control of fine voluntary
movements.
There are 5 important sets of descending motor tracts,
named according to the origin of their cell bodies and their final
destination:
1) Corticobulbospinal tract
(= Pyramidal tract)
2) Rubrospinal tract,
3) Reticulospinal tracts,
4) Vestibulospinal tracts
5) Tectospinal tract
Extrapyramidal tracts
Motor areas, somatic sensory areas
Corticonuclear tract
Midbrain
Left
Internal
Capsule
L
A
Corticobulbar tract
Pons
Medulla
Corticospinal tract
Spinal Cord
• Origin:
1. Primary motor area (area 4) → 30%
2. Premotor and supplemental areas → 30%
3. Somatic Sensory areas → 40%
• Origin:
Area 4
Premotor
area and
supplemental
motor area
Area 3,1,2 and
Area 5,7,
30 % 40 %
30 %
• Divisions:
• 1) Corticospinal tract : from cortex to spinal
cord
• 2) Corticonuclear tract: from cortex to cranial
nerves nuclei (3,4,6) that supply extraocular
muscles
• 3) Corticobulbular tract: from cortex to other
cranial nerves (5,7,9,10,11,12) of brain stem
M1
4
Genu
Anterior
Limb
Corticospinal Tract
Posterior
Limb
Left
Internal
Capsule
Midbrain
Pons
Pyramidal
Decussation
Mid
Medulla
Caudal
Medulla
Spinal Cord
• (a) 90% cross to the opposite side and descend in
the lateral column of the spinal cord as lateral
corticospinal tract→ terminate on lateral AHCs
→innervate the distal ms
• (b) 10% not cross → descend ipsilaterally as
ventral corticospinal tract.
• About 9% of these fibers cross and terminate on
medial AHCs → innervate the axial muscles of the
opposite side.
• Only 1% of the fibers terminate on the A.H.Cs of
the same side (bilateral innervations as respiratory
muscles).
• Pathway:
CC
Internal
capsule
Medulla
Lateral CST
Ventral or
medial CST
Spinal
cord
M1
Corticobulbar Tract
FA
L
Left
Internal
Capsule
VMidbrain
PONS
Upper Face
Lower Face
MEDULLA
F
Corticobulbar Tract is crossed and
uncrossed
EXCEPT to lower motor neurons which
control lower half of the face and tongue
are crossed only.
V
VII
Pons
VII
IX
IX
X
X
Mid
Medulla
LOWER
MOTOR
NEURON
NUCLEI
Caudal
Medulla
Spinal Cord
XII
Ipsilateral Side
XII
Contralateral Side
Area 8
Corticonuclear Tract
FA
L
Left
Internal
Capsule
Midbrain
PONS
III
Midbrain
IV
VI
F
III
IVPons
VI
LMN Supplying eye muscles
Mid
Medulla
Caudal
Medulla
Spinal Cord
Ipsilateral Side
Contralateral Side
• Functions:
• The C.B.S tract is primarily concerned with the
mediation of voluntary motor commands from the
cerebral cortex to lower motor centers:
• a- Corticonuclear tract controls the extraoccular ms.
• b- Corticobullar tract controls the ms of lower part
of the face and tongue which are involved in articulate
speech.
• c-Lateral corticospinal tract controls distal ms that
subserve skilled movements as those used in
manipulation by hands and fingers.
• Functions:
• 2) Medial C.B.S tract controls axial ms
concerned mainly with regulation of posture.
• 3) C.B.S tract is also facilitatory to the tone of
the distal limb muscles.
THANKS