THE DESCENDING MOTOR SYSTEMS
 For voluntary movements to occur, one needs 2 neurons:
1- Upper motor neurons
 whose cell bodies lie in the higher
motor centers in the brain and brain
stem, and their axons constitute the
descending motor pathways.
2- Lower motor neurons
 whose cell bodies lie in the spinal
ventral horns or the
cranial
motor
corresponding
nuclei , and include
both -and -MNs . Axons
of the
lower motor neurons proceed through
the
peripheral
somatic
innervate skeletal muscles.
nerves
to
The Primary Motor Area
Location
in the precentral gyrus of the frontal lobe, and corresponds to Brodmann’s
area 4 .
Body Representation
1) contralateral and inverted. However, several facial muscles are
represented bilaterally .
2) organized in a somatotopic manner with the feet at the upper medial
region of the gyrus and the face at the lower lateral region
3) Area of representation is proportional with the complexity of function
done by the muscle. So, muscles of hands and tongue occupies 50% of this
area
Neural Connections
A) Afferents
sensory feed-back input from the muscle and
joint proprioceptors to provide information about
1)Thalamus &Somatic
Sensory Area
the actual motor performance
of the same side providing higher control of its
activity.
regulate and coordinate its motor activity.
providing it with information about the spatial
relations
of
the
body
to
the
external
2) The premotor &
supplemental motor
3) The basal ganglia &
cerebellum
4)The visual & auditory
cortices
environment.
appropriate course of motor action suitable with
5) The prefrontal area
the surrounding environment.
coordinating bilateral motor activities performed
by both sides of the body .
6) The motor areas of the
opposite hemisphere
B)
Efferents
1) About 30% of the axons of the corticobulbospinal tract.
2) large number of fibers project onto the basal ganglia, establishing a neural
pathway for planning and programming of motor actions .
3) tremendous number of fibers project to the cerebellum, establishing
pathways between the motor cortex and the cerebellum for coordination and
regulation of movements.
4) moderate number of fibers that pass to the red nucleus in the midbrain,
and also to the reticular formation in the brain stem for controlling their
activity
Role in Movements
1) discharges the descending motor commands that produce voluntary
movements. It controls mainly the distal muscles
2) facilitatory to the tone of distal muscles, particularly flexor muscles.
Premotor Area
Location
The premotor area lies immediately anterior to the lateral regions of the
primary motor area
 It occupies a large portion of area 6, and is bounded superiorly by the
supplemental motor area.
Body Representation
Contralateral and inverted
Neural Connections
With the Primary & supplemental motor areas:
With the Cerebellum
With the Basal Ganglia
Contributes to cortico-bulbo-spinal tract.
Role in Movements
1) Enhancing the primary motor area to commence its activity.
2) Adjusting posture during performance of voluntary movements.
3) In association with the supplemental motor area, establishing the motor
programs necessary for execution of complex movements.
4) In association with the B.G., initiates the automatic associative movements
which occur at subconscious level; as swinging of arms during walking.
5) Inhibit grasp reflex
6) Influence autonomic activity as heart rate & arterial blood
pressure.
7) Inhibit muscle tone
8) A few highly specialized motor centers have been found in the premotor
areas of the human cerebral cortex :
Broca’s Area for Speech
The Frontal Eye Movements Area
located above Broca’s area in the frontal
lobe
controls voluntary movements of the eyes
toward different objects in the visual field.
Head Rotation Area
located just above the eye movement area in the motor cortex .
directing the head toward different visual objects .
Area for Hand Skills
(Exner’s Area)
Supplemental motor area
Location
I n the upper medial side of the frontal lobe just above the premotor area.
Neural Connections
i) It has connections with the primary motor and premotor areas.
ii) It has connections with basal ganglia and cerebellum.
iii) It contributes to the CBS tract.
Role in Movements
1) It evokes complex movements which often involve both sides of the
body e.g.: causing both hands to perform a motor act together.
2) It functions with the premotor area 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) It shares in the planning and programming of the complex
movements with area 6.