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EXTRA PYRAMIDAL SYSTEM
DR.TAYYABA AZAR
• The term extrapyramidal motor system is
denote all those portions of the brain and
brain stem that contribute to motor control
but are not part of the direct corticospinalpyramidal system.
• These include pathways through the basal
ganglia, the reticular formation of the brain
stem, the vestibular nuclei, and often the red
nuclei.
Excitation of the Spinal Cord Motor
Control Areas by the Primary Motor
Cortex and Red Nucleus
• The cells of the motor cortex are organized in
vertical columns a fraction of a millimeter in
diameter, with thousands of neurons in each
column. Each column of cells functions as a
unit, usually stimulating a group of synergistic
muscles, but sometimes stimulating just a
single muscle.
• Each column has six distinct layers of cells, as
is true throughout nearly all the cerebral
cortex. The pyramidal cells that give rise to the
corticospinal fibers all lie in the fifth layer of
cells from the cortical surface.
• Conversely, the input signals all enter by way
of layers 2 through 4. And the sixth layer gives
rise mainly to fibers that communicate with
other regions of the cerebral cortex itself.
Function of Each Column of Neurons.
• The neurons of each column operate as an
integrative processing system.
• Each column can function as an amplifying
system to stimulate large numbers of pyramidal
fibers to the same muscle or to synergistic
muscles simultaneously.
• This is important, because stimulation of a single
pyramidal cell can seldom excite a muscle.
Usually, 50 to 100 pyramidal cells need to be
excited simultaneously or in rapid succession to
achieve definitive muscle contraction.
Dynamic and Static Signals Transmitted by the
Pyramidal Neurons
• If a strong signal is sent to a muscle to cause
initial rapid contraction.
• Each column of cells excites two populations
of pyramidal cell neurons, one called dynamic
neurons and the other static neurons.
• The dynamic neurons are excessively excited
for a short period at the beginning of a
contraction, causing the initial rapid
development of force.
• Then the static neurons fire at a much slower
rate, but they continue firing at this slow rate
to maintain the force of contraction as long as
the contraction is required.
RED NUCLEUS
• The neurons of the red nucleus have similar
dynamic and static characteristics, except that a
greater percentage of dynamic neurons is in
the red nucleus and a greater percentage of
static neurons is in the primary motor cortex.
cerebellum plays an important role in rapid
initiation of muscle contraction,.
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Somatosensory Feedback to the Motor
Cortex Helps Control the Precision of
Muscle Contraction
Nerve signals from the motor cortex cause a
muscle to contract, somatosensory signals
return all the way from the activated region of
the body to the neurons in the motor cortex
that are initiating the action.
somatosensory signals arise:
1:Muscle spindles,
2:Muscle tendons,
• 3:Tactile receptors of the skin overlying the
muscles.
• somatic signals often cause positive feedback
enhancement of the muscle contraction.e.g:
In muscle spindles, if fusimotor muscle fibers in
the spindles contract more than the large skeletal
muscle fibers contract, the central portions of the
spindles become stretched and, therefore,
excited.
• Signals from these spindles then return rapidly to
the pyramidal cells in the motor cortex to signal
them that the large muscle fibers have not
contracted enough. The pyramidal cells further
excite the muscle, helping its contraction to catch
up with the contraction of the muscle spindles.
• In the case of the tactile receptors, if the
muscle contraction causes compression of the
skin against an object, such as compression of
the fingers around an object being grasped,
the signals from the skin receptors can, if
necessary, cause further excitation of the
muscles and, therefore, increase the tightness
of the hand grasp.
RUBROSPINAL TRACT
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it originates from the red nucleus which is present in the
tegmentum of mid brain.
Then these cross over to the opposite side and descend through the
pons, medulla oblongata to enter the lateral white column of spinal
cord and it terminates onto the motor neurons in the ventral horn
of spinal cord at various level of spinal cord.
Termaintion is indirect through interneurons.
Red nucleus has connections with cerebral cortex and cerebellum.
FUNCTIONS
Rubrospinal tract forms an alternate or accessory pathway through
which cerebral cortex and cerebellum can control the motor
activity.
Rubrospinal tract is fascillitatory for flexers and inhibitory for
extensors or antigravity muscles.
TACTOSPINAL TRACT
• It originates from the superior colliculus in the tactum
of mid brain.
• Then it descends without crossing over, descend
through the pons and medulla oblongata to reach
spinal cord where it terminates on to the motor
neurons in the ventral horn in the upper cervical
segments of the spinal cord.
• FUNCTIONS
• This tract is involved in the control of reflex postural
movement of head and neck in response to visual
stimulation.
• Here from superior colliculus impulses also go to the
extraocular muscles to control the movement of eyes.
RETICULOSPINAL TRACT
• It arises from reticular formation.
• Reticular formation consists of groups of
scattered neurons along with nerve fibers. It is
present in midbrain, pons and medulla.
Superiorly reticular formation is connected to
cerebral cortex and inferiorly to the spinal
cord. One component of reticular formation is
called reticular activating system.
Components of reticulospinal tract
• Pontine reticulo spinal tract:
• Arise from the reticular formation of pons. Most of the
fibers remain uncrossed. Tract enters the anterior
white column of spinal cord. Then it terminates onto
the motor neurons in the ventral horn at various levels.
• FUNCTIONS
• This tract is fascillitatry to the extensors or the
antigravity muscles.
• It controls the activity of both alpha and gamma motor
neurons.
• In the control of muscle tone it plays an important
role.
• Medullary reticulospinal tract:
• It originates from the reticular formation of
medulla oblongata. Most of the fibers cross
over to the opposite side and these descend
to enter the lateral white column of spinal
cord.
• FUNCTIONS
• This tract is inhibitory to the extensors.
VESTIBULOSPINAL TRACT
• Vestibular nuclei are present in the floor of
4trh ventricle in the lower pons and upper
medulla oblongata. Vestibular nuclei receive
fibers from the inner ear and also cerebellum.
There are two components of this tract.
• The major component is lateral
vestibulospinal tract and minor component is
medial vestibulospinal tract.
• LATERAL PART:
• Remains uncrossed and terminate on the motor
neurons of spinal cord.
• Facilliatatory to the extensors of the body.
• Medial part:
• It arises from the medial vestibular nucleus. This
nucleus.
• After the origin, the tract fibers remain uncrossed and
they terminate onto the motor neurons in the cervical
segments of the spinal cord. This tract is also
facilitatory to the extensors
OLIVOSPINAL TRACT
• It originates from the inferior olivary nucleus
in the medulla oblongata.
• After the origin, it crosses over to the opposite
side and descends into the lateral white
column of spinal cord.
• FUNCTIONS
• This tract is also involved in the control of
motor activity.