Motor Functions of the Nervous System

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Part 4 Motor Functions of the Nervous System I
Motor Unit and Final Common Pathway
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1. Motor Unit
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• Every striated muscle has encapsulated muscle
  fibers scattered throughout the muscle called
  muscle spindles.
• Extrafusal and intrafusal fibers

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The extrafusal muscle fibers are innervated by
Alpha motor neuron The intrafusal muscle fibers
are innervated by Gamma motor neurons
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Motor units

• A motor unit is a single motor neuron (a motor)
  and all (extrafusal) muscle fibers it innervates
• Motor units are the physiological functional unit
  in muscle (not the cell)
• All cells in motor unit contract synchronously

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Motor units and innervation ratio
Innervation ratio Fibers per motor
neuron Extraocular muscle 31 Gastrocnemius 20001
Purves Fig. 16.4
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• The muscle cells of a motor unit are not grouped,
  but are interspersed among cells from other motor
  units
• The coordinated movement needs the activation of
  several motors

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organization of motor subsystems
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Overview - organization of motor systems
Motor Cortex
Brain Stem
?-motor neuron
Final common pathway
Skeletal muscle
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Final common path - ?-motor neuron
(-)
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II Motor Functions of the Spinal Cord Spinal
Reflex
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Spinal Reflexes

• Somatic reflexes mediated by the spinal cord are
  called spinal reflexes
• These reflexes may occur without the involvement
  of higher brain centers
• Additionally, the brain can facilitate or inhibit
  them

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1. Stretch Reflex
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(1) Anatomy of Muscle Spindle

• The muscle spindles detect change in the length
  of the muscle
• -- stretch receptors that report the stretching
  of the muscle to the spine.
• Each spindle consists of 3-10 intrafusal muscle
  fibers enclosed in a connective tissue capsule
• These fibers are less than one quarter of the
  size of extrafusal muscle fibers (effector fibers)

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Anatomy of Muscle Spindle

• The central region of the intrafusal fibers which
  lack myofilaments are noncontractile,
• serving as the receptive surface of the spindle
  (sensory receptor)

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Anatomy of Muscle Spindle

• Intrafusal fibers are wrapped by two types of
  afferent endings that send sensory inputs to the
  CNS
• Primary sensory endings
• Type Ia fibers
• Innervate the center of the spindle
• Secondary sensory endings
• Type II fibers
• Associated with the ends of the spindle

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Components of muscle spindle
Dynamic intrafusal fiber
Static intrafusal fibers
Static intrafusal fibers
Primary ending

Ia
Afferent axons
II
Secondary ending

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Anatomy of Muscle Spindle

• Primary sensory endings
• Type Ia fibers
• Stimulated by both the rate and amount of stretch

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Anatomy of Muscle Spindle

• Secondary sensory endings
• Type II fibers
• stimulated only by degree of stretch

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Anatomy of Muscle Spindle

• The contractile region of the intrafusal muscle
  fibers are limited to their ends as only these
  areas contain actin and myosin filaments
• These regions are innervated by gamma (?)
  efferent fibers

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Muscle stretch reflex
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(2) Muscle stretch reflex Definition Whenever a
muscle is stretched, excitation of the spindles
causes reflexive contraction of the same muscle
from which the signal originated and also of
closely allied synergistic muscle. The basic
circuit Spindle Proprioceptor nerve fiber
dorsal root of the spinal cord synapses with
anterior motor neurons ? -motor N. F.
the same M. from whence the M. spindle fiber
originated.
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Circuit of the Strength Reflex
Muscle spindle
Dorsal root
Muscle fiber
?-mn
Ventral root
Tendon
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The Stretch Reflex

• Exciting a muscle spindle occurs in two ways
• Applying a force that lengthens the entire muscle
• Activating the ? motor neurons that stimulate the
  distal ends of the intrafusal fibers to contact,
• thus stretching the mid-portion of the spindle
  (internal stretch)

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The Stretch Reflex

• Whatever the stimulus, when the spindles are
  activated
• their associated sensory neurons transmit
  impulses at a higher frequency to the spinal cord

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The Stretch Reflex

• At spinal cord sensory neurons synapse directly
  (mono- synaptically) with the ? motor neurons
  which rapidly excite the extrafusal muscle fibers
  of stretched muscle

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The Stretch Reflex

• The reflexive muscle contraction that follows (an
  example of serial processing) resists further
  stretching of the muscle

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The Stretch Reflex

• Branches of the afferent fibers also synapse with
  inter- neurons that inhibit motor neurons
  controlling the antagonistic muscles

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• Inhibition of the antagonistic muscles is called
  reciprocal inhibition
• In essence, the stretch stimulus causes the
  antagonists to relax so that they cannot resist
  the shortening of the stretched muscle caused
  by the main reflex arc

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The types of the Stretch Flex

• 1)   Tendon reflex (dynamic stretch reflex)
• ? Caused by rapid stretch of the muscle, as
  knee-jerk reflex
• ? Transmitted from the IA sensory ending of the
  M. S.
• ? Causes an instantaneous, strong reflexive
  contraction of the same muscle
• ? Opposing sudden changes in length of the M.
• A monosynaptic pathway
• being over within 0.7 ms

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The types of the Stretch Flex
2)   Muscle tonus (static stretch reflex) ?
Caused by a weaker and continues stretch of the
muscle, ? Transmitted from the IA and II sensory
ending of the M. S. ? Multiple synaptic pathway,
continues for a prolonged period. ?
Non-synchronized contraction, ? M. C. for at
least many seconds or minutes, maintaining the
posture of the body.
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The Stretch Reflex

• The stretch reflex is most important in large
  extensor muscles which sustain upright posture
• Contractions of the postural muscles of the spine
  are almost continuously regulated by stretch
  reflexes

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(3) Gamma impact on afferent response
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Muscle spindle motor innervation

• Gamma motoneurons
• Innervate the poles of the fibers.

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WHAT IS THE g-LOOP?
Muscle spindle
g
Activation of the g-loop results in increased
muscle tone
MUSCLE
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Functional significance of gamma impact on
spindle activity

• The tension of intrafusal fibers is maintained
  during active contraction by gamma activity.
• The system is informed about very small changes
  in muscle length.

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2. The Deep Tendon Reflex (1) Structure and
Innervation of Golgi Organ
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Golgi tendon organ structure

• Located in the muscle tendon junction.
• Connective tissue encapsulating collagen fibers
  and nerve endings.
• Attached to 10-20 muscle fibers and several MUs.
• Ib afferent fiber.
• sensitive to tension

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(2) Golgi tendon organ response properties

• Less frequent than muscle spindle.

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Golgi tendon organ response properties (cont)

• Sensitive to the change of tension caused by the
  passive stretch or active contraction

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(3) The Deep Tendon Reflex

• When muscle tension increases moderately during
  muscle contraction or passive stretching,
• GTO receptors are activated and afferent impulses
  are transmitted to the spinal cord

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The Deep Tendon Reflex

• Upon reaching the spinal cord, informa- tion is
  sent to the cerebellum, where it is used to
  adjust muscle tension
• Simultaneously, motor neurons in the spinal cord
  supplying the contracting muscle are inhibited
  and antagonistic muscle are activated (activation)

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The Deep Tendon Reflex

• Deep tendon reflexes cause muscle relaxation and
  lengthening in response to the muscles
  contraction
• This effect is opposite of those elicited by
  stretch reflexes
• Golgi tendon organs help ensure smooth onset and
  termination of muscle contraction
• Particularly important in activities involving
  rapid switching between flexion and extension
  such as in running

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Compare spindle and golgi
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Compare spindle and golgi
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3. The Crossed Extensor Reflex

• The reflex occur when you step on a sharp object
• There is a rapid lifting of the affected foot
  (ipsilateral withdrawal reflex ),
• while the contralateral response activates the
  extensor muscles of the opposite leg
  (contralateral extensor reflex)
• support the weight shifted to it

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4. Superficial Reflexes

• Superficial reflexes are elicited by gentle
  cutaneous stimulation
• These reflexes are dependent upon functional
  upper motor pathways and spinal cord reflex arcs
• Babinski reflex

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Babinski reflex - an UMN sign

• Adult response - plantar flexion of the big toe
  and adduction of the smaller toes
• Pathological (Infant) response - dorsoflexion
  (extension) of the big toe and fanning of the
  other toes
• Indicative of upper motor neuron damage

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5. Spinal cord transection and spinal shock
(1) Concept When the spinal cord is suddenly
transected in the upper neck, essentially all
cord functions, including the cord reflexes,
immediately become depressed to the point of
total silence. (spinal animal)
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(2) During spinal shock complete loss of all
reflexes, no tone, paralysis, complete
anaesthesia, no peristalsis, bladder and rectal
reflexes absent (no defecation and micturition
) no sweating arterial blood Pressure
decrease(40mmHg),
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(3) the reason The normal activity of the spinal
cord neurons depends to a great extent on
continual tonic excitation from higher centers
(the reticulospinal-, vestibulospinal-
corticospinal tracts). (4) The recovery of spinal
neurons excitability.
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III. Role of the brain stem Support of the Body
Against Gravity Roles of the Reticular and
Vestibular nuclei
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Facilitated and inhibitory area
Areas in the cat brain where stimulation produces
facilitation () or inhibition (-) of stretch
reflexes. 1. motor cortex 2. Basal ganglia 3.
Cerebellum 4. Reticular inhibitory area 5.
Reticular facilitated area 6. Vestibular nuclei.
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1. Facilitated arearoles of the reticular and
vestibular nuclei. (1) The pontine reticular
nuclei ? Located slightly posteriorly and
laterally in the pons and extending to the
mesencephalon, ? Transmit excitatory signals
downward into the cord (the pontine
reticulospinal tract)

• motor cortex
• 2. Basal ganglia
• 3. Cerebellum
• 4. Reticular inhibitory area
• 5. Reticular facilitated area
• 6. Vestibular nuclei.

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(2) The vestibular nuclei ? selectively control
the excitatory signals to the different
antigravity M. to maintain equilibrium in
response to signals from the vestibular apparatus.

• motor cortex
• 2. Basal ganglia
• 3. Cerebellum
• 4. Reticular inhibitory area
• 5. Reticular facilitated area
• 6. Vestibular nuclei.

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MOTOR TRACTS LOWER MOTOR NEURON
MOTOR CORTEX
MIDBRAIN RED NUCLEUS (Rubrospinal Tract)
VESTIBULAR NUCLEI (Vestibulospinal Tract)
PONS MEDULLA RETICULAR FORMATION (Reticulospinal
Tracts)
LOWER (ALPHA) MOTOR NEURON THE FINAL COMMON
PATHWAY
SKELETAL MUSCLE
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Properties of the Facilitated Area
? Terminate on the motor neurons that exciting
antigravity M. of the body (the M. of vertebral
column and the extensor M. of the limbs). ? Have
a high degree of natural (spontaneous)
excitability. ? Receive especially strong
excitatory signals from vestibular nuclei and the
deep nuclei of the cerebellum. ? Cause powerful
excitation of the antigravity M throughout the
body (facilitate a standing position), supporting
the body against gravity.
1. motor cortex 2. Basal ganglia 3. Cerebellum
4. Reticular inhibitory area 5. Reticular
facilitated area 6. Vestibular nuclei.
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2. Inhibitory area medullary reticular system
(1) Extend the entire extent to the medulla,
lying ventrally and medially near the middle.
(2) Transmit inhibitory signals to the same
antigravity anterior motor neurons (medullary
reticulospinal tract).
1. motor cortex 2. Basal ganglia 3. Cerebellum
4. Reticular inhibitory area 5. Reticular
facilitated area 6. Vestibular nuclei.
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MOTOR TRACTS LOWER MOTOR NEURON
MOTOR CORTEX
MIDBRAIN RED NUCLEUS (Rubrospinal Tract)
VESTIBULAR NUCLEI (Vestibulospinal Tract)
PONS MEDULLA RETICULAR FORMATION (Reticulospinal
Tracts)
LOWER (ALPHA) MOTOR NEURON THE FINAL COMMON
PATHWAY
SKELETAL MUSCLE
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(3) Receive collaterals from the corticospinal
tract the rubrospinal tracts and other motor
pathways. These collaterals activate the
medullary reticular inhibitory system to balance
the excitatory signals from the P.R.S., so that
under normal conditions, the body M. are normally
tense.
1. motor cortex 2. Basal ganglia 3. Cerebellum
4. Reticular inhibitory area 5. Reticular
facilitated area 6. Vestibular nuclei.
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Areas in the cat brain where stimulation produces
facilitation () or inhibition (-) of stretch
reflexes. 1. motor cortex 2. Basal ganglia 3.
Cerebellum 4. Reticular inhibitory area 5.
Reticular facilitated area 6. Vestibular nuclei.
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Decerebrate Rigidity

• Decerebrate Rigidity transection of the
  brainstem at midbrain level (above vestibular
  nuclei and below red nucleus)
• Symptoms include
• extensor rigidity or posturing in both upper and
  lower limbs

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• Results from
• loss of input from inhibitory medullary RF
  (activity of this center is dependent on input
  from higher centers).
• active facilitation from pontine RF
  (intrinsically active, and receives afferent
  input from spinal cord).

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• The extensor rigidity is g-loop dependent
• section the dorsal roots interrupts the g-loop,
  and the rigidity is relieved. This is g-rigidity.

THE g-LOOP?
Muscle spindle
g
Activation of the g-loop results in increased
muscle tone
MUSCLE
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IV. The cerebellum and its motor functions
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Cerebellar Input/Output Circuit
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Based on cerebral intent and external conditions
The cerebellum tracks and modifies
millisecond-to-millisecond muscle contractions,
to produce smooth, reproducible movements
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Without normal cerebellar function, movements
appear jerky and uncontrolled
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Functional Divisions-cerebellum

• Vestibulocerebellum (flocculonodular lobe)

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The vestibulocerebellum
input-vestibular nuclei output-vestibular nuclei
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The vestibulocerebellum

• Function
• The control of the equilibrium and postural
  movements.
• Especially important in controlling the balance
  between agonist and antagonist M. contractions of
  the spine, hips, and shoulders during rapid
  changes in body positions.
• Method
• Calculate the rates and direction where the
  different parts of body will be during the next
  few ms.
• The results of these calculations are the key to
  the brainss progression to the next sequential
  movement.

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• Spinocerebellum (vermis intermediate)

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• Spinocerebellum (vermis intermediate)
• input-periphery spinal cord
• output-cortex

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• Spinocerebellum (vermis intermediate)

Functions -- Provide the circuitry for
coordinating mainly the movements of the distal
portions of the limbs, especially the hands and
fingers -- Compared the intentions from the
motor cortex and red nucleus, with the
performance from the peripheral parts of the
limbs, --Send corrective output signals to the
motor neurons in the anterior horn of spinal cord
that control the distal parts of the limbs (hands
and fingers) --Provides smooth, coordinate
movements of the agonist and antagonist M. of the
distal limbs for the performance of acute
purposeful patterned movements.
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• Cerebrocerebellum (lateral zone)

input-pontine N. output-pre motor cortex
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• Cerebrocerebellum (lateral zone)

• ? Receives all its input from the motor cortex,
  adjacent pre-motor and somatic sensory cortices
  of the brain. Transmits its output information
  back to the brain.
• Functions in a feedback manner with all of the
  cortical sensory-motor system to plan sequential
  voluntary body and limb movements,
• Planning these as much as tenths of a second in
  advance of the actual movements (mental rehearsal
  of complex motor actions)

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• Vestibulocerebellum (flocculonodular lobe)
• Balance and body equilibrium
• Spinocerebellum (vermis intermediate)
• Rectify voluntary movement
• Cerebrocerebellum (lateral zone)
• Plan voluntary movement

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V The motor functions of basal ganglia
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Components of Basal Ganglia
Caudate
Putamen
GPe
1. Corpus Striatum
Striatum ----- Caudate Nucleus Putamen
Pallidum ----- Globus Pallidus (GP)
GPi
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Components of Basal Ganglia
2. Substantia Nigra Pars Compacta (SNc)
Pars Reticulata (SNr)
STN
3. Subthalamic Nucleus (STN)
SN (r c)
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Basal Ganglia Connections

• Circuit of connections
• cortex to basal ganglia to thalamus to cortex
• Helps to program automatic movement sequences
  (walking and arm swinging or laughing at a joke)
• Output from basal ganglia to reticular formation
• reduces muscle tone
• damage produces rigidity of Parkinsons disease

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cortex to basal ganglia to thalamus to cortex
D1 D2 Dopamine receptors
GPe/i Globus pallidus internal/external STN
Subthalamus Nucleus SNc Pars Compacta (part of
substantia Nigra)
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• Direct Pathway
• Disinhibition of the thalamus facilitates
  cortically mediated behaviors

D1 D2 Dopamine receptors
GPe/i Globus pallidus internal/external STN
Subthalamus Nucleus SNc Pars Compacta (part of
substantia nigra))
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• Indirect pathway
• Inhibition of the thalamus inhibits cortically
  mediated behaviors

D1 D2 Dopamine receptors
GPe/i Globus pallidus internal/external STN
Subthalamus Nucleus SNc Pars Compacta (part of
substantia nigra)
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Medical Remarks
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• Hypokinetic disorders result from overactivity in
  the indirect pathway.
• example Decreased level of dopamine supply
  in nigrostriatal pathway results in akinesia,
  bradykinesia, and rigidity in Parkinsons disease
  (PD).

D1 D2 Dopamine receptors
GPe/i Globus pallidus internal/external STN
Subthalamus Nucleus SNc Pars Compacta (part of
substantia nigra)
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Parkinsons Disease
PD
Disease of mesostriatal dopaminergic system
normal
Muhammad Ali in Alanta Olympic
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Parkinsons Disease
Substantia Nigra, Pars Compacta
(SNc) DOPAminergic Neuron
Clinical Feature (1)
Slowness of Movement - Difficulty in Initiation
and Cessation of Movement
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Parkinsons Disease
Clinical Feature (2)
Resting Tremor Parkinsonian Posture Rigidity-Cogwh
eel Rigidity
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• Hyperkinetic disorders result from underactivity
  in the indirect pathway.
• example Lesions of STN result in Ballism.
  Damage to the pathway from Putamen to GPe
  results in Chorea, both of them are involuntary
  limb movements.

D1 D2 Dopamine receptors
GPe/i Globus pallidus internal/external STN
Subthalamus Nucleus SNc Pars Compacta (part of
substantia nigra)
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SYDENHAMS CHOREA
Clinical Feature
- Fine, disorganized , and random movements
of extremities, face and tongue - Accompanied
by Muscular Hypotonia - Typical exaggeration
of associated movements during voluntary
activity - Usually recovers spontaneously in
1 to 4 months
Principal Pathologic Lesion Corpus Striatum
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HUNTINGTONS CHOREA
Clinical Feature
- Predominantly autosomal dominantly inherited
chronic fatal disease (Gene chromosome 4) -
Insidious onset Usually 40-50 - Choreic
movements in onset - Frequently associated with
emotional disturbances - Ultimately, grotesque
gait and sever dysarthria, progressive
dementia ensues.
Principal Pathologic Lesion Corpus Striatum
(esp. caudate nucleus) and Cerebral Cortex
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HEMIBALLISM
Clinical Feature
- Usually results from CVA (Cerebrovascular
Accident) involving subthalamic nucleus -
sudden onset - Violent, writhing, involuntary
movements of wide excursion confined to one
half of the body - The movements are continuous
and often exhausting but cease during sleep -
Sometimes fatal due to exhaustion - Could be
controlled by phenothiazines and stereotaxic
surgery
Lesion Subthalamic Nucleus
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VI Control of muscle function by the motor cortex

• Two principal components
• Primary Motor Cortex
• Premotor Areas

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The primary motor cortex
The topographical representations of the
different muscle areas of the body in the primary
motor cortex
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Characteristics of the PMC 1, It has predominant
influence on the opposite side of the body
(except some portions of the face)
2. It is organized in a homunculus pattern with
inversed order 3. The degree of representation is
proportional to the discreteness (number of motor
unit) of movement required of the respective part
of the body. (Face and fingers have large
representative) 4. Stimulation of a certain part
of PMC can cause very specific muscle
contractions but not coordinate movement.
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• Projects directly
• to the spinal cord to regulate movement
• Via the Corticospinal Tract
• The pyramidal system
• Projects indirectly
• Via the Brain stem to regulate movement
• extrapyramidal system

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Descending Spinal Pathways pyramidal system

• Direct
• Control muscle tone and conscious skilled
  movements
• Direct synapse of upper motor neurons of cerebral
  cortex with lower motor neurons in brainstem or
  spinal cord

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Descending Spinal Pathwaysextrapyramidal system

• Indirect
• coordination of head eye movements,
• coordinated function of trunk extremity
  musculature to maintaining posture and balance
• Synapse in some intermediate nucleus rather than
  directly with lower motor neurons

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• Premotor area composed of supplementary motor
  area and lateral Premotor area

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• Premotor Areas
• Receive information from parietal and prefrontal
  areas
• Project to primary motor cortex and spinal cord
• For planning and coordination of complex planned
  movements