Primary Motor Cortex

PRIMARY MOTOR CORTEX (also known as M-1 or Brodmann’s Area 4; part of the neocortex, “gray matter” – cell bodies and unmyelinated axons; neocortex about 2.5 mm thick)

A. General Characteristics

1. Located in precentral gyrus area of the frontal lobe of the cerebral cortex

Note: the primary motor cortex extends onto the medial cortical surface within the longitudinal fissure

2. Role: initiation of voluntary muscle contraction; essential for normal movement; evidence:

a. electrical stimulation of a particular area of M-1 leads to contraction of a particular skeletal muscle or group of muscles (stimulation of other cortical areas can cause contraction but their excitation threshold is much higher and the resulting contraction is less discrete and precise)

b. lesions of M-1 or its descending pathways (e.g. stroke) lead to either paralysis or paresis and loss of fine movement control (apraxia)

3. Organization: somatotopic: small, distorted, discontinuous map of the body (homunculus), with larger areas devoted to body regions characterized by fine or complex movements and smaller areas to body regions characterized by gross movements involving few muscles

4. Hand, face, intraoral and, to some extent, foot muscles are particularly well represented on M-1

5. Somatotopic representation is mainly contralateral; exception: several cranial nerves subserve bilateral (both ipsilateral and contralateral) control

Note:in humans, the effect of primary motor cortex lesions or interruptions of the corticospinal tract is immediate loss or reduction of voluntary function in contralateral muscles corresponding to the region of injury and spastic paralysis; frequently there is some recovery of function with time

Note:  the same is true for cortical or corticobulbar tract lesions to cranial nerves innervating only contralateral or only ipsilateral muscles

B. Cellular Organization

1. The neocortex (most of the cerebral cortex) consists of six layers of histologically and functionally distinct cells cells (particularly pyramidal neurons and interneurons);  also glia (supporting cells)

Note:  the relative thickness of each layer varies with the function of the region of the cortex (e.g. sensory vs. motor)

2. Each region of M-1 (and the remainder of the neocortex) is organized as units of interconnected columns of several thousand neurons arranged perpendicular to the cortical surface and including all six cortical layers

3. Stimulation of a given motor column may activate a single muscle; more commonly stimulation of  a column activates several muscles to produce a coordinated movement

Note: this is sometimes expressed as "The motor cortex thinks in terms of movements, not muscles"

4. Groups of columns control groups of alpha motoneurons to determine the force and direction and velocity of movement

5. Descending axons forming the cortical output from M-1 (and from the cortex in general) arise from the pyramidal cells in cortical layer 5

6. Pyramidal cells in layers 2 and 3 send axons to other regions of the ipsilateral cortex (layer 2) and to corresponding areas of the contralateral cortex via the corpus callosum (layer 3)

Note: Axial regions of the body are well represented in corpus callosal axons connecting corresponding contralateral regions of the motor cortex but distal regions are less well represented.

C. Descending Pathways from the Motor Cortex

1. Corticospinal tract:

a. axons pass through the internal capsule (a frequent site of stroke injury), then through the ipsilateral diencephalon, midbrain, pons, and into the medulla, where they form the medullary pyamids

Note: the corticospinal tract is also termed the “pyramidal tract” (named for the medullary pyramids, not for the cortical pyramidal cells)

b. as the axons pass from the medulla into the spinal cord, most (80-90%) decussate (cross to the opposite side) and descend in the white matter of the contralateral cord, forming the lateral corticospinal tract, until they reach the region containing the alpha motoneurons of the muscles they represent; the majority of these motoneurons innervate flexors

c. the non-decussating axons descend in the ipsilateral cord, forming the ventral corticospinal tract, until they reach the level of region of the body they represent, and then they decussate also; the majority of these motoneurons control extensors

Note: these decussations are the reason M-1 somatotopic representation is mainly contralateral

d. in the spinal cord, some descending axons terminate directly on the alpha motoneurons of the muscles they control but most descending axons terminate on interneurons which eventually innervate alpha motoneurons

Note: direct innervation of alpha motoneurons by pyramidal tract axons is mainly to distal muscles and is associated with the ability to execute fine, precise movements; direct pyramidal tract innervation is more-or-less limited to primates

Note: functional corticospinal innervation to alpha-motoneurons innervating the lower extremities is required to suppress the Babinski sign

e.  Effect of Lesions (e.g. stroke)

1)  Weakness
2)  inability to perform learned skilled movements ("apraxia")

2. Corticobulbar tract:

a. axons pass through the internal capsule and terminate either directly on alpha motoneurons or on interneurons innervating alpha motoneurons in the brainstem controlling somatic motor activity in the head (e.g. muscles controlling eye movement, muscles of mastication, muscles of facial expression)

b. axons innervating some somatic motor nerve cranial nuclei decussate before their termination and so control contralateral muscles; corticobulbar tracts controlling other motor nuclei split, some axons decussating and others descending ipsilateral, resulting in bilateral innervation

c. cranial nerve innervation is either direct or through interneurons, as in the corticospinal tract

3. Cortico-rubar and Cortico-reticular tracts

a. consist of axons originating In M-1 and terminating in the red nucleus (midbrain) and the reticular formation (throughout the brainstem), respectively

b. output from the red nucleus (rubrospinal tract) and the reticular formation (reticulospinal tract) includes descending fibers affecting alpha and gamma motoneurons (rubro-spinal and reticulo-spinal tracts) and influencing muscle tone, body posture, and reflex sensitivity; these brainstem nuclei can also support limited crude voluntary movement

Note: the cortico-rubar and cortico-reticular tracts and their descending connections are constituents of what is sometimes termed the extrapyramidal system

Note: an additional descending tract innervating alpha motoneurons and contributing to voluntary actions is the vestibulo-spinal tract, but the vestibular nucleus does not receive direct projections from the motor cortex.

Note: another motor tract is the tectospinal tract. This originates in the tectum (the roof of the midbrain). Tectum nuclei receive input from the visual and auditory systems. The function of the tectospinal tract is rapid reorientation of the head in response to visual and auditory stimuli.