Motor System

MOTOR
SYSTEM
DR SARAN AJAY
DEPT. OF PHYSIOLOGY, GMCM

DEPT. OF PHYSIOLOGY, GMCM 3
Specific Learning Objectives
• Introduction
• Organization of Motor System
• Muscle and its connections
• Structure of Muscle Spindle
• Effect of various conditions on mucle spindle discharge

Movements are the major way in which humans
interact with the world.
ˈ

• Contraction of skeletal muscle fibers are responsible
for the movement of the body.
• Motor function of nervous system is the control of
skeletal muscle activity.

Motor System plays a role in inducing voluntary activity, to
adjust body posture, and to make movements smooth
and precise.

• Introduction

Organization of Motor System
1. Muscle and its connections
2. Segmental circuit in spinal cord
3. Brainstem controlling centres
4. Basal ganglia, cerebellum
5. Thalamus
6. Cerebral cortex

STRETCH REFLEX

Stretch Reflex
• When a skeletal muscle with an intact nerve supply is
stretched, it contracts.
• This response is called the stretch reflex.
• Sense organ – muscle spindle

Muscle spindle

1. Stimulus - stretch of the muscle
2. Sense organ / receptor - muscle spindle (intrafusal)
3. Afferent nerve – group Ia fibre
4. Centre – spinal cord (alpha motor neuron)

5. Efferent nerve – axons of alpha motor neuron
6. Effector organ – extrafusal muscle fibres
7. Effect- contraction of same muscle

• Introduction

1. Muscle and its connections

Muscle has two types of fibers
a. Extrafusal fibers
• Regular striated contractile units of muscle
• Motor Function

b. Intrafusal fibers / Muscle spindle
• Spindle shaped.
• Less distinct striations.
• Do not contribute to overall contractile force, serves
a pure sensory function.

• Introduction

1. Each spindle consists up to 10 intrafusal muscle
fibres (3-12 fibres).
2. Enclosed in a connective tissue capsule, attached
to glycocalyx of surrounding large extrafusal skeletal
muscle fibres.
Muscle Spindle

3. Has contractile polar ends and non-contractile centre
(receptor portion).

4. Fibers lie parallel to extrafusal fibers.
Hence, transmits information about muscle length or
rate of change of length.
5. Since changes in muscle length are associated with
changes in joint angle, muscle spindle provides
information about position (proprioception).

6. Each muscle spindle has 3 elements
a. Intrafusal Fibers
b. Afferent Neurons
c. Efferent Neurons

a. Intrafusal Fibers
2 types of intrafusal fibers
i. Nuclear bag fibers
• Contains many nuclei in a dilated central area
• 1-3 in each spindle
• 2 subtypes: Dynamic and Static nuclear bag fibers

ii. Nuclear chain fibers
• Nuclei aligned in a chain throughout the receptor area
• Lacks a definite bag
• Thinner and shorter fibers
• 3-9 in each spindle

b. Afferent Neurons
• Group Ia and II fibers
• Two types of endings
• Primary/ Annulospiral endings
• Secondary/ Flower Spray endings

i. Primary (annulospiral) endings
1. Terminations of rapidly conducting group Ia
afferent fibers
2. Wraps around center of both dynamic and static
nuclear bag fibers as well as nuclear chain fibers

3. Respond to
• velocity of changes in length → dynamic response
• respond to sustained stretch → static response

ii. Secondary (flower-spray) endings
1. Termination of group II afferent fibers
2. Innervates static nuclear bag fibers as well as nuclear
chain fibers

3. Respond to sustained stretch → static response

Response to stretch – 2 types
1. Static response
• Impulses transmitted from both 1º and 2º endings
• Increases directly in proportion to degree of stretching
• Discharge occurs throughout the period of stretching

• Respond to length of receptor.
• Responsible for muscle tone.

2. Dynamic response
• Impulses transmitted from 1º endings in nuclear bag
region.
• Discharge rapidly when muscle is stretched.
• Respond to rate of change of receptor length during
stretch.

• Provide information about the speed of movement.
• Allow for quick corrective movements that oppose
sudden changes in muscle length.

c. Efferent Neurons
• γ motor neurons
• Supply the contractile ends of muscle spindle

Histologically,
• Plate endings → motor end plates in nuclear bag fibers
• Trail endings → motor end plates in nuclear chain fibers

2 types of γ efferents:
i. Dynamic γ efferents (gamma-d)
• Stimulates dynamic nuclear bag fibers
• Increases sensitivity of Ia afferents → dynamic response
of Ia fiber is markedly enhanced

ii. Static γ efferents (gamma-s)
• Stimulates static nuclear bag fibers and nuclear
chain fibers
• Increases tonic activity in Ia and II fibers

• Introduction

1. Stretch of muscle
Stretch of muscle → stretch of muscle spindle →
sensory endings get distorted →increased sensory
output of muscle spindle (Loading of the muscle
spindle) → POSITIVE signal

Action potentials are generated at a frequency proportional
to degree of stretching.

2. When muscle contracts → spindle afferents stop
firing (Unloading of the muscle spindle) →
NEGATIVE signal.

3. Stimulation of γ Motor Neuron
Stimulation of γMN → contractile ends of muscle
spindle shorten → stretches nuclear bag region →
increased impulses in sensory fibers → increases
sensitivity of muscle spindle to stretch.

What are the different ways through which
a muscle can be contracted?
Q

1.Stretch of the muscle – Stretch Reflex
2.Descending Motor Pathways - Corticospinal Tract
3.Gamma Efferent to Muscle Spindle

• Introduction

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MOTOR
SYSTEM 2
DR SARAN AJAY

• α- γ linkage (α- γ coactivation)
• Higher Control of Stretch Reflex
• Functions of Muscle Spindle
• Segmental Circuit in Spinal Cord
• Other areas in motor system hierarchy

α- γ linkage (α- γ coactivation)
Increased γ efferent discharge along with increased
discharge of α motor neuron.

• Descending pathways send signals to both αMN as
well as to γ MN.
• αMN and γ MN are stimulated simultaneously.
• Because of this linkage, intrafusal and extrafusal
fibres contract together (spindle shortens with the
muscle)

Two effects
• Keeps the length of the receptor portion of muscle
spindle from changing during course of whole muscle
contraction.
1

2
• Spindle remain capable of responding to stretch and
reflexly adjust motor neuron discharge.
• So, helps to maintain proper damping function of the
muscle spindle, regardless of any change in muscle length.

Higher Control of Stretch Reflex
1. Facilitates stretch reflex by increasing γ efferent
discharge
i. Facilitatory reticular formation in the brain stem
ii. Vestibular nucleus

2. Inhibit stretch reflex by decreasing γ efferent discharge
i. Cerebral cortex
ii. Cerebellum, basal ganglia
iii. Inhibitory reticular formation

1- Cortex, 2- Basal Ganglia, 3- Cerebellum, 4- Medullary Reticular
Formation, 5- Pontine Reticular Formation, 6- Vestibular Nucleus

Factors influencing γ MN discharge
1. Anxiety
• Increased γMN →
anxious people
2. Unexpected movements
• →

3. Stimulation of skin by noxious agents
• Increased γMN activity to ipsilateral flexor muscle spindles
while decreasing that to extensors

4. Jendrassik’s Maneuver
Done by asking the subject to
make a strong voluntary muscle
contraction and simultaneously
eliciting the deep tendon reflex.

• Eg. Pull hands apart when the flexed fingers are hooked
together.
• Reinforcement of tendon jerks maybe due to increased
γ MN discharge initiated by afferent impulses from
hands
• Thus increases the sensitivity of muscle spindle to
stretch.

Functions of Muscle Spindle
1. Acts as a feedback mechanism to maintain muscle length
2. Damping function of dynamic and static stretch reflex
3. Maintenance of muscle tone
4. Maintenance of posture

1. Feedback mechanism to maintain muscle length
• Stimulated by stretching of muscles
• Provides a feedback mechanism
• Operates to maintain muscle length

When muscle is stretched
↓
Increased spindle discharge
↓
Afferents pass through type Ia fibers
↓
Enter spinal cord through dorsal root
↓
Synapses with anterior motor neurons
supplying same muscle

↓
Reflex shortening of muscle by contraction
of extrafusal fibers
↓
Decreased stimulation of spindle
Muscle relaxation

Two type of responses
1. Dynamic response / phasic response
2. Static response / tonic response

2. Damping function of dynamic and static stretch
reflexes
• Signals from spinal cord are often transmitted to a muscle
in an unsmooth form.
• If the muscle spindle is not functioning properly muscle
contractions will be jerky.

Curve A → normal muscle
Curve B → muscle whose muscle spindles were denervated by section of the posterior roots of the cord 82 days previously

“Signal averaging” .

• Marked dynamic response helps to dampen oscillations
caused by conduction delays in feedback loop regulating
muscle length.
• Normally a small oscillation occur in this feedback loop -
Physiological tremor.

Physiological Tremor
• Normal phenomenon
• Low amplitude, Frequency-10 Hz
• Barely visible to the naked eye
• Become exaggerated when we are anxious or tired or
because of drug toxicity

3. Maintenance of muscle tone
• Tone is the resistance offered by a muscle to passive
stretch.
• It is a state of partial contraction found in muscles at
rest.

• Static response of muscle spindles are responsible for
tone.
• Helps to maintain posture.

Normal tone is ill defined area somewhere between
flaccidity and spasticity.

Low frequency asynchronous discharge of gamma motor
neuron causes slight contraction of muscle under resting
state.

Hypotonic occurs when rate of γ motor neuron discharge
is low and hypertonic when it is high.

4. Maintenance of posture
Muscle spindle stabilizes body position during tense
motor action.

• Spindles of muscles on both sides of each joint are
activated at the same time → reflex contraction of the
muscles occur.
• This stabilizes the major joints.
• Aids tremendously in performing the additional fine
voluntary movements of fingers or other body parts.

2. Segmental Circuit in Spinal Cord

A. Anterior Motor Neurons
Alpha motor neuron
Gamma motor neuron
B. Interneurons
C. Renshaw Cells

A. Anterior Motor Neurons
• Located in each segment of the anterior horns of spinal
cord gray matter.
• Give rise to nerve fibers that leave the cord by way of
anterior roots and directly innervate the skeletal muscle
fibers.

2 types
• Alpha motor neurons and
• Gamma motor neurons

Motor root or
Anterior root or
Ventral root

Alpha motor neurons
Axons : Aα fibres
14-15 µm in diameter
Innervate the large skeletal
muscle fibers
Gamma motor neuron
Axons : Aγ fibres
5 µm in diameter
Supply intrafusal muscle fibres
Alpha v/s Gamma

Functions of inputs converging on Alpha motor
neuron
1. Bring about voluntary activity
2. Adjust body posture to provide stable background for
movement
3. Coordinate various movements to make movements
smooth and precise

Levels of inputs to the Alpha motor neuron
• From same spinal segment
• From supra segmental levels in the spinal cord
• From brain stem
• From cerebral cortex, basal ganglia and cerebellum

B. Interneurons
• Present in all areas of the cord gray matter—in the dorsal
horns, the anterior horns, and the intermediate areas
• 30 times as numerous as the anterior motor neurons

Interconnections among interneurons and anterior motor
neurons are responsible for most of the integrative
functions of spinal cord.

C. Renshaw cells
• Inhibitory cells
• Transmit inhibitory signals to surrounding motor neurons
• Stimulation of each motor neuron tends to inhibit adjacent
→ lateral inhibition
• Focus or sharpen signals

3. Motor Cortex
• Corticospinal tract / pyramidal tract
• Corticobulbar projections →
projections arise
• From sensory cortex to motor cortex - Sensory motor
coordination

4. Brainstem controlling centers
• →
motor nuclei to αMN
• Reticulospinal tract
• Vestibulospinal tract
• Mainly concerned with postural movements

5. Basal ganglia
• Subcortical structure
• No direct sensory input from spinal cord
• Project to motor cortex via thalamus
• Involved in initiation, smoothening and coordination of
movements.

6. Cerebellum
• Receives inputs from all sensory modalities
• Project to brainstem motor nuclei and motor cortex
• Control almost all aspects of movement - planning,
programming, initiation, termination and coordination

7. Thalamus
• Major sensory relay station
• Receives inputs from cerebellum and basal ganglia
• Plays an important role in sensory motor coordination.

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MOTOR
SYSTEM 3
DR SARAN AJAY

INPUT OUTPUT

MOTOR OUTPUT
VOLUNTARY MOVEMENTS
REFLEXES
RHYTHMIC MOVEMENTS

• Voluntary Movements
• Cortical Motor Areas
• Descending Tracts
• Pyramidal Tract

VOLUNTARY MOVEMENTS

• Commands for voluntary movements originate in
cortical association areas.
• Planning and organization of movements → by cortex,
basal ganglia and lateral portion of cerebellum

• Plan is projected to the motor and premotor cortex.
• Commands are sent to muscle → via corticospinal and
corticobulbar tracts.
• Feedback information that adjusts and smoothens
movement relayed to motor cortex and spinocerebellum.

• Pyramidal Tract

Cortical Motor Areas
• Control voluntary movement
• Comprises of
1. Primary Motor Cortex
2. Premotor Area
3. Supplementary Motor Area
4. Posterior Parietal Cortex
5. Primary Somatosensory Area

Primary Motor Cortex
• M1, Brodmann area 4
• Located in precentral gyrus of frontal lobe.

1. Primary Motor Cortex
• M1, Brodmann area 4
• Located in precentral gyrus of frontal lobe.
• Begins laterally in the sylvian fissure, spreads superiorly
to the uppermost portion of the brain.
• Then dips deep into the longitudinal fissure.

• Concerned with execution of movements.
• Generates signals that control the execution of
discrete, individual movements rather than one
specific muscle
• Topographical Representation – Motor Homunculus

Motor Homunculus
• Figurative representation of body map encoded in
primary motor cortex.
• Mapped by Penfield and Rasmussen.

• Each side of the body is represented on the opposite
side in the brain.
• Inverted map → feet at the top and face at the bottom
• Facial area is represented bilaterally.
• Area involved in speech and hand movements → large
representation in the cortex.

• Axial musculature and proximal portions of limb
represented along the anterior edge of precentral gyrus.
• Distal part of limb along the posterior edge.

Cortical representation of each body part is proportional
in size to the skill with which the part is used in fine
voluntary movement.

• Motor system "learns by doing" and performance
improves with repetition → cortical plasticity.
• Maps of motor cortex are therefore not immutable.

2. Premotor Area
• Brodmann’s area 6
• Lies immediately anterior to primary motor cortex –
extending inferiorly to Sylvian fissure and superiorly to
longitudinal fissure

Contains a somatotopic map that is roughly same as that
of primary motor cortex

• Complex “patterns” of movement.
• Concerned with setting posture at the start of a planned
movement and getting the individual to move.
• Most involved in control of proximal limb muscles
needed to orient the body for movement.

Premotor area sends signals
1. Either directly to primary motor cortex to excite
specific muscles
2. Or by way of basal ganglia and thalamus back to
primary motor cortex

Mirror Neurons?

• Special class of neurons - mirror neurons present.
• Transform sensory representations of acts that are heard
or seen into motor representations of these acts.

• Special class of neurons - mirror neurons present.
• Transform sensory representations of acts that are heard
or seen into motor representations of these acts.
• Important for understanding the actions of other people
and for learning new skills by imitation.

Special areas in Premotor cortex
1. Broca’s area (Motor Speech Area) – related to speech
2. Voluntary eye movement field
1. For moving eyes toward different objects
2. Also controls eyelid movements such as blinking

3. Head rotation area
• Directs the head toward different objects
• Closely associated with the eye movement field

4. Area for hand skills
• Lies immediately anterior to the primary motor
cortex for the hands and fingers
• Lesions cause hand movements become un-
coordinated and non-purposeful - Motor apraxia

3. Supplementary motor area
• Situated on and above the superior bank of cingulate
sulcus.
• This area project to motor cortex.

• Involved in programming motor sequences – when
movements performed are complex and need planning.
• Lesions produce inability to perform complex action

4. Posterior Parietal cortex
• Two areas: area 5 and area 7
• Provide fibers to corticospinal and corticobulbar tracts
• Project to premotor cortex

• Neurons in area 5 are concerned with aiming the
hands towards an object and manipulating it.
• Neurons in area 7 are concerned with hand eye
coordination.

5. Primary somatosensory cortex
• Area 3, 1, 2
• Projects to premotor cortex.
• Lesion of somatosensory area affects learned sequence
of movements eg. Eating with knife and fork.

• Pyramidal Tract

Descending tracts or Motor pathways
1. Pyramidal tract or Corticospinal tract and
Corticobulbar or Corticonuclear tract
2. Extra pyramidal pathways
• Reticulospinal, Vestibulospinal, Rubrospinal,
Tectospinal

• Pyramidal Tract

Corticospinal tract or Pyramidal pathway
• Primary pathway for initiation of skilled voluntary
movements.
• Longest tract
• Becomes myelinated in the first 2 years of life.

• Corticospinal tract + corticobulbar tract
• 1 million fibers in each corticospinal tract

A. Origin
1. 30% from Primary motor cortex
2. 30% from Premotor cortex and Supplementary motor area
3. 40% from Somatosensory area posterior to central sulcus

Cells of origin
• Giant pyramidal cells of Betz → 3%
• Small pyramidal cells → 97%

Betz cells
• Betz in 1874 described the giant pyramidal cells in 5th
layer of primary motor cortex.
• Only 3% of CST fibers arise from Betz cells - large cell,
velocity-70m/sec.

B. Course
Cerebral cortex-various areas
↓
Corona radiata
↓
Internal capsule – genu and anterior 2/3rd of
posterior limb (head region anteriorly, feet posteriorly)

B. Course
Cerebral cortex-various areas
↓
Corona radiata
↓
Internal capsule – genu and anterior 2/3rd of
posterior limb (head region anteriorly, feet posteriorly)
↓
Midbrain – middle 3/5th of crus cerebri
(head medially, feet laterally)

↓
Pons (broken up to discrete bundles by pontine nuclei)
At the lower border collected into a compact bundle

↓
Pons (broken up to discrete bundles by pontine nuclei)
At the lower border collected into a compact bundle
↓
Medulla – seen as Pyramid

At the lower border of medulla,
• 80% cross to opposite side – crossed Pyramidal tract or
Lateral Corticospinal tract
• 20% uncrossed fibers – Anterior or Ventral Cortico-
spinal tract

Lateral Corticospinal Tract
• 80% of pyramidal fibers cross to opposite side
• Descend down in lateral funiculus of spinal cord

C. Termination of Lateral CST
• Terminates at all spinal cord levels directly on αMNs.
• Lateral CST – make monosynaptic direct
connections to motor neurons on opposite side

• Controls distal limb muscles → concerned with fine
skilled movements

Anterior Corticospinal Tract
• About 20% fibers do not cross in medulla
• Descend down in anterior funiculus of spinal cord

C. Termination of Anterior CST
• Most of fibers cross at the level of spinal cord where
they terminate, but some fibers remain uncrossed.
• Anterior CST – connect with interneuron that make
connection with α motor neuron on both sides of body

• Controls muscles of trunk and proximal muscles of
limbs → concerned with postural adjustments and
gross movements.

Within the brainstem and spinal cord,
• Pathways and neurons concerned with control of axial
muscles & proximal limb muscles are located medially
or ventrally.

Within the brainstem and spinal cord,
• Pathways & neurons that are concerned with control of
muscles in distal portions of the limbs are located
laterally.

C. Termination of CST
• Synapse with α motor neuron in anterior horn directly or
indirectly through interneuron .
• Few terminate on sensory relay neurons in dorsal horn

• Lateral CST – make monosynaptic direct connections
to motor neurons on opposite side (esp. for skilled
movements)
• Anterior CST – connect with interneuron that make
connection with α motor neuron on both sides of body

*Draw this diagram for exam

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MOTOR
SYSTEM 4
DR SARAN AJAY

• Corticobulbar / Corticonuclear Tracts
• Concept of LMN and UMN
• Functions of Pyramidal Tract
• Extrapyramidal System
• Functions of Extrapyramidal System

Corticobulbar or Corticonuclear tracts
Through out the brain stem, fibers are given off from
pyramidal tract to the nuclei of motor cranial nerves.

Corticobulbar neurons end either directly on the cranial
nerve nuclei or on their antecedent interneurons within
the brainstem.

• Midbrain - 3rd and 4th cranial nerve nuclei
• Pons - 5th, 6th and 7th cranial nerve nuclei
• Medulla - 9th,10th,11th and 12th cranial nerve

1. In midbrain, corticobulbar fibers terminate in motor
nuclei of CN III and IV bilaterally.
2. In pons, corticobulbar fibers to motor nuclei to CN V
and VI bilaterally

• Corticobulbar fibers to CN VII → to upper and lower part of
contralateral motor nucleus and only upper part of
ipsilateral motor nucleus.
3. In medulla, corticobulbar fibers to motor nuclei of CN IX,
X, XI bilaterally and unilaterally to contralateral motor
nucleus CN XII.

Concept of LMN and UMN

Lower motor neuron (LMN)
• Spinal and cranial motor neurons that directly innervate
muscles.
• Alpha motor neuron and the motor part of cranial
nerves.

Upper motor neuron (UMN)
• Neurons in brain and spinal cord that activate or inhibit
the alpha motor neuron or corresponding cranial nerve
nuclei through the descending tracts.
• Pyramidal tract and extra pyramidal tracts

Functions of pyramidal tract
1. Voluntary motor function
A. Lateral corticospinal tract
• Main control of movements of distal limb muscles
• Initiation of skilled voluntary movements

B. Anterior corticospinal tract
• Movement of trunk and proximal limb muscles
• Postural adjustments and gross movements
C. Corticobulbar fibers
• Supply muscles at face, eyes, tongue, larynx and
pharynx.

2. Forms pathway for superficial reflexes (abdominal
reflex, plantar reflex)
3. Some fibers transmit information from brain to
afferent neuron, so can modify afferent inputs –
sensory motor co-ordination.

The Extrapyramidal System
Parts of nervous system excluding motor cortex and
corticospinal pathway which are concerned with
movement and posture.

Consists of
1. Basal ganglia
2. Cerebellar Nuclie
3. Reticular formation
4. Vestibular nuclei
5. Red nuclei
6. Extrapyramidal tracts
• Tectospinal tract
• Pontine and medullary
reticulospinal tracts
• Vestibulospinal tract
• Rubrospinal tract

Extrapyramidal tracts
All descending motor pathways other than pyramidal
tract, concerned with control of muscle tone, posture
and equilibrium.

EXTRAPYRAMIDAL TRACTS
LATERAL BRAINSTEM PATHWAYS
MEDIAL BRAINSTEM PATHWAYS
• Descend in ipsilateral
anterior funiculus.
• Synapse at medial part of
anterior horn.
• Control axial and
proximal muscles.
• Descend in lateral
funiculus.
• Synapse at lateral part of
anterior horn.
• Control distal limb
muscles.

EXTRAPYRAMIDAL TRACTS
VESTIBULOSPINAL
RETICULOSPINAL
TECTOSPINAL
RUBROSPINAL

1. Rubrospinal tract
• Arise from magnocellular portion of red nucleus in
midbrain.
• Fibers cross to the opposite side as Forel’s decussation.
• → Reticular formation of Pons → Medulla → Spinal cord →
Anterior horn cell

• Involved in regulation of posture and coordination
• Concerned with adjustments of distal limb muscles -
Influence αMN that controls distal limb muscles on
contralateral side of body
• Excites flexor muscles and inhibits extensor muscles

2. Tectospinal Pathway
• Fibers arise from tectum (superior colliculus) of mid brain
• Cross to opposite side → Reticular formation of Pons →
Medulla → Spinal cord → Anterior horn cell
• Receives mainly visual input.
• Control movements of head and eyes- regulates head
movements in response to visual stimuli.

3. Vestibulospinal tract
• Originates from vestibular nucleus
• Stimulates αMN
• Function in association with pontine reticular nuclei to control
antigravity muscles.

Lateral Vestibulospinal
• Arises in the lower pons in the lateral vestibular nucleus
• Descend in the spinal cord in the anterior funiculus
anterior to rubrospinal tract
• Fibers are uncrossed

• Projects ipsilaterally to neurons that activate antigravity
muscles at all spinal levels.
• Mediates body postural adjustments after angular and
linear accelerations of head.

Medial Vestibulospinal
• Arises from lower pons in medial and inferior vestibular
nuclei
• Descend in the anterior funiculus – crossed & uncrossed

• Projects bilaterally to cervical spinal motor neurons
that control neck musculature.
• Mediates adjustments in head position in response to
angular acceleration.

4. Reticulospinal – Medial & Lateral
• Arise from reticular formation of pons and medulla
• Project to all spinal levels
• Terminate both on alpha motor neuron and gamma
motor neuron
• Involved in maintenance of posture and modulation
of muscle tone.

Pontine (medial) reticulospinal tract
• Uncrossed tract
• Pontine reticular formation is spontaneously active.
• In addition, they receive strong excitatory signals from the
vestibular nuclei, as well as from deep nuclei of cerebellum.

• Fibers of pontine reticulospinal tract terminate on
medial anterior motor neurons that excite axial
• Stimulate extensor γMN.

Medullary (lateral) reticulospinal tract
• Crossed tract
• Medullary reticular nuclei receive strong input collaterals
from corticospinal tract + rubrospinal tract + other motor
pathways.
• Counterbalances the excitatory signals from the pontine
reticular system.

• Terminate on anterior motor neurons that control
• Inhibit extensor γMN.

Functions of Extrapyramidal system
1. Facilitate or inhibit voluntary movements
2. Control of posture and equilibrium
3. Control coordinated movements of body and limbs –
coordinated movements of arms and legs during sitting,
walking, running etc.

4. Influence γ motor neuron discharge : control of muscle tone
1. They exert tonic inhibitory control over lower centers
2. Lesion cause increased tone → Rigidity of muscles
5. Cause alterations in respiration, blood pressure

Q

Tracts concerned with adjustments of trunk and proximal
muscles (postural adjustments and gross movements)
1. Ventral corticospinal tract
2. Other medial descending tracts from brainstem-
tectospinal, reticulospinal, vestibulospinal

Tracts concerned with distal limb muscles (fine, skilled
voluntary movement)
1. Lateral corticospinal tract
2. Rubrospinal tracts

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MOTOR
SYSTEM 5
DR SARAN AJAY

• Lesions of Corticospinal Tract
• Lesions of the extrapyramidal tract
• Reveiw Questions

Lesions of Corticospinal Pathway
• Loss of ability to initiate voluntary movements
• When lateral CST is specifically damaged → loss of
ability to carry out fine movements.
• When ventral CST is damaged → inability to produce
gross movements like walking, climbing etc.

Most lesions of corticospinal system damage the extra
pyramidal system also.

• Lesion above decussation → loss of voluntary movement
on opposite side of body.
• At Cerebral Cortex → Monoplegia (localized paralysis
affecting one limb).

• At Internal Capsule → Hemiplegia (paralysis of one half
of body) - because the fibers are closely packed
• At Brainstem → Crossed Hemiplegia one or more
cranial nerves may be affected on the side of lesion &
signs of UMN lesion on opposite side.

• At Spinal Cord → Corticospinal tract may be affected
bilaterally.
• The level of lesion is usually delineated by accompanying
LMN lesion signs.

Depending on the level of spinal cord lesion
• Paraplegia – both lower limbs are paralyzed.
• Quadriplegia – All four limbs are paralyzed.

Hemiplegia
1. Paralysis of one half of body
2. Lesion of Pyramidal tract
3. Site of lesion – Internal capsule
4. Usually caused by thrombosis or hemorrhage in
lenticulo striate branch of middle cerebral
artery.

5. UMN type of lesion
6. In acute state usually there are signs of shock -
Hypotonia, no reflex movements
7. After 2-3 weeks signs of typical UMN lesion
appear.

Clinical Features
1. Power
• Unilateral paralysis – one half of the body – Hemiplegia
• Sometime only weakness is seen – Hemiparesis
• Lower part of face is involved
• Mouth deviates to opposite side of lesion
• Upper part of face escapes – bilateral representation

2. Tone
• In pure pyramidal tract lesion hypotonia is seen.
• Usually Pyramidal and Extrapyramidal tracts are damaged
resulting in spasticity – Spastic Paralysis
• Spasticity → Clasp knife effect
• Due to operation of stretch reflex and then inverse stretch
reflex.

3. Deep tendon reflexes:
• Exaggerated (Hyperreflexia)
• Clonus may be present
4. Superficial reflexes:
• Usually absent
• Plantar reflex – Positive Babinski sign

Positive Babinski sign
• Extensor plantar response
• It is a flexor withdrawal reflex that is normally held in
check by the lateral corticospinal system.

• Damage to the lateral corticospinal tract produces the
Positive Babinski sign in response to this stimulation.
• Dorsiflexion of the great toe and fanning of the other
toes.

5. Bulk:
• Gross muscle wasting is absent
• Only slight disuse atrophy
6. Speech:
• Dysarthria – due to weakness or incoordination of the
muscle of face, pharynx, lips, tongue or palate.

7. Gait
• Pyramidal gait/ Hemiplegic gait
• Circumduction

Crossed Hemiplegia
Crossed hemiplegia – here paralysis of muscles
supplied by the cranial nerves on same side and
hemiplegia on opposite side.

• Midbrain – 3rd & 4th cranial nerves are damaged (LMN
lesion) → paralysis of ocular muscles on same side.
• Pons – cranial nerves 5,6 & 7 are affected. (LMN lesion)
• Medulla – 9,10,11 &12 cranial nerves are affected → vital
centers may get affected → death

Spinal cord lesion
• Pyramidal tract on both sides are damaged
• Usually paralysis of both lower limbs - Paraplegia
• If at the level of cervical spine – Quadriplegia/ Tetraplegia

Cerebral Palsy
• Nonprogressive neurologic disorder.
• Occur before or during childbirth or during early childhood.
• Exposure of developing brain to hypoxia, infections, or
toxins.
• More common in premature babies.

1. Motor deficits
• Spasticity, ataxia
• Deficits in fine motor control
• Abnormal gait (crouched or “scissored gait”)

2. Sensory deficits
• Loss of vision and hearing
3. Learning difficulties and seizures

Diseases affecting Extrapyramidal system
• Characterized by difficulty in initiating voluntary movements.
• Appearance of involuntary movements.
• Impairment of balancing and orienting reflexes.
• Alteration of muscle tone.
• Muscle strength is usually unaffected

KUHS 2021

KUHS 2020

KUHS 2014

KUHS 2013

KUHS 2021

One-word answers
1. The receptor for inverse stretch reflex is ……. 2021
2. In spinal cord the dorsal root is sensory and the ventral root is motor, this
law is called ……… 2021
Short Essay (5 marks)
1. Draw a diagram to show the origin, course and termination of Corticospinal
tract 2019

Physiological basis
1. Clasp Knife Rigidity. 2014
2. Babinski’s Sign in newborn. 2015
3. Abnormal plantar in neurological diseases. 2016
4. Cogwheel Rigidity. 2017
5. UMN Lesions produce hypertonia. 2018
6. Pyramidal Tract Lesions produces exaggerated deep tendon reflexes. 2019

Answer briefly
1. Stretch Reflex 2013
Draw and label:
1. Stretch and Inverse Stretch Reflex. 2012
2. Corticospinal/ Pyramidal Tract. 2011, 2016, 2019, 2020
3. Functional areas of cerebral cortex. 2014, 2020

1

Motor System

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