The primary motor cortex is responsible for planning, initiating, and directing voluntary movements. It contains the precentral gyrus and posterior parts of the superior, middle, and inferior frontal gyri. The primary motor cortex encodes movement intentions and is involved in both voluntary movements and the expression of emotions through facial gestures and body posture. Brainstem motor centers help control posture and reflexes. The lateral corticospinal tract governs distal limb muscles for skilled movements while the ventromedial tract governs proximal muscles and posture. The motor cortex has a fractured somatotopic organization and single neurons can activate multiple muscles.

1. Primary motor cortex
Domina Petric, MD

2. Upper motor neurons
(descending systems):
a) Motor cortex (planing,
initiating and directing
voluntary movements)
b) Brainstem centers (basic
movements and postural
control)
Basal ganglia: gating
proper initiation of
movement
Cerebellum: sensory
motor coordination and
ongoing movement
Local circuit neurons:
lower motor neuron
integration
Motor neuron pools: lower
motor neurons
Sceletal
muscles
Sensory inputs

3. Motor cortex
• It is concerned with encoding the movement intention.
• Motor cortex includes precentral gyrus and posterior parts of superior,
middle and inferior frontal gyri.
• Motor cortex is mostly concerned with voluntary movements, but it is
also related to behaviors that are concerned with expression of
emotions.
• So, the motor cortex is also concerned with non-verbal communication
(facial gestures, body posture).
• Brainstem motor centers are concerned with adjustment of posture,
with setting the gain of segmental reflexes and it is important for
expressing the skill.

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Somatotopic
organisation of
the ventral horn

5. Spinal cord
• Lateral corticospinal tract provides axons that are mostly concerned with
governing the control of distal extremity muscles.
• Axons in the lateral white matter come from motor cortex.
• Lateral corticospinal tract is associated primarily with skills.
• Axons in the ventral and medial white matter of the spinal cord come from
the brainstem.
• These ventromedial spinal cord axons terminate in the medial parts of the
ventral horn so these axons are concerned with governing the control of the
proximal extremity muscles and posture.
• Ventromedial spinal cord white matter is mainly concerned with posture.

6. Upper motor
neurons in
cerebral cortex
Lateral white
matter of spinal
cord
Lower motor neurons
in lateral ventral horn
Distal limb muscles:
skilled movements
Upper motor
neurons in
brainstem
Anterior and medial white
matter of the spinal cord
Lower motor neurons in
medial ventral horn
Axial and proximal
limb muscles: posture
and balance
Cross the midline!
For contralateral limb!
Input is distributed
to both sides of the
spinal cord!

7. Volitional movement: descending pyramidal and
extrapyramidal projections from motor cortex and
brainstem
Lateral: fine control of
distal muscles of
extremities
Medial: posture and
proximal muscles of
extremities
Brainstem reticular
formation
Motor neuron pools
Motor neurons of cranial nerve nuclei
and ventral horn of the spinal cord
Autonomic preganglionic neurons
Muscle contraction and
movement
Activation of smooth
muscles and glands
Emotional expression: descending
extrapyramidal projections from limbic centers
of ventro-medial forebrain and hypothalamus
Medial: gain setting,
rhythmical reflexes
Lateral: specific
emotional behaviors
Pyramidal
pathway
Corticobulbar pathway

8. Motor cortex
Motor cortex is divided into:
• Primary motor cortex includes the precentral gyrus and
paracentral lobule.
• Premotor cortex includes posterior parts of the superior,
middle and inferior frontal gyri.
Thalamic nuclei that send inputs to the motor cortex are
VENTRAL ANTERIOR THALAMIC NUCLEUS and
VENTRAL LATERAL THALAMIC NUCLEUS (VA-VL
COMPLEX OF THE THALAMUS).

9. Motor cortex
Somatosensory cortex also sends inputs to the
motor cortex, both primary motor cortex and
premotor cortex.
˝Where˝ (magnocellular) visual pathway sends
inputs to the premotor cortex.

10. Primary motor cortex (Brodmann´s area 4)
Layer 5 in the primary motor cortex (Betz cells) gives
rise to spinal cord and brainstem projections:
corticospinal and corticobulbar tract.
Primary motor cortex has the lowest treshold for the
elicitation of movement and it is concerned with
those movements that are primarily concerning our
body and the space in front of the body.

11. Somatotopy
http://brainconnection.brainhq.com
BUT!
Somatotopy of the precentral gyrus
is rather fractured!

12. Fractured somatotopy
There are overlapping representations in the
precentral gyrus for flexor and extensor muscles.
There are multiple representations of the same body
region.
There is no internal somatotopy in the region for arm
in the precentral gyrus.

13. Fractured somatotopy
Single corticospinal axon supplies
multiple columns of lower motor
neurons in ventral horn.
Single spike in a corticospinal
axon activates many different
muscles in the forelimb.

14. Representations of the primary motor cortex
movements (not muscles) or movement intentions
multiple dimensions of movement (force, direction, amplitude)
movements that engage hand, lower face and hand to mouth
coordination
skilled manual behavior in central personal space
lesions impair fractionated (skilled) finger-facial movements

15. Literature
https://www.coursera.org/learn/medical-
neuroscience/lecture: Leonard E. White,
PhD, Duke University
www.studyblue.com
http://brainconnection.brainhq.com