on 15-Aug-2020 (Sat)

A striking characteristic of the primary motor cortex in humans is that over half of it is associated with the motor activity of the hands, tongue, lips, and larynx, reflect- ing the manual dexterity and ability for speech that humans possess.

Nerve cells in the primary motor cortex are organized into groups, each group sending its axons to the cranial nerve motor nuclei, or the reticular formation in the brainstem, or the spinal cord gray matter, where they control the motor activity of a single muscle

The total cortical area that mediates motor activity of a particular body region is proportional to the complexity of the motor activity produced in that region

Premotor cortical areas

The premotor cortical areas consist of four regions: the premotor cortex, supplementary motor cortex, frontal eye field, and the cingulate motor areas, all of which reside in the frontal lobe. The premotor cortical areas function in the programing of complex motor activity, which is then relayed to the primary motor cortex where the execution of motor activity is initiated

In addition to the motor cortical areas located in the frontal lobe, there is one cortical area, the posterior parietal area which is involved in movement and is located in the parietal lobe

Thus, the majority of nerve signals that are generated in the premotor cortical areas cause complex movements produced by groups of muscles performing a task, unlike the discrete muscle contractions elicited by stimulation of the primary motor cortex

Experimental studies have shown that stimulation of the premotor cortical areas seldom generates a movement, but instead, interrupts the movement in progress as well as any impending movements. Stimulation of the premotor cortical areas interferes with the planning phase of movement

The supplementary motor cortex (area) (SMA) lies in Brodmann’s area 6. This area has important functions in the programing phase of the patterns and sequences of elaborate movements, produced by groups of muscles and coordina- tion of movements occurring on the two sides of the body. This cortical area is associated with muscle contractions of the axial (trunk) and proximal limb (girdle) musculature (i.e., muscles controlling movement of the arm and thigh). It also functions in guiding or turning the body or limbs toward a desired (or appropriate) direction

The premotor cortex (PMC) resides in most of Brod- mann’s area 6, on the lateral aspect of the frontal lobe. This cortical area is the recipient of projections from sensory areas of the parietal lobe. The PMC in turn projects to the primary motor cortex, the reticular formation (which is the origin of the reticulospinal tracts), and the spinal cord

Once an intended movement has begun and is in prog- ress, activity in the PMC decreases, reflecting its key function in the planning phase of motor activity

The frontal eye field (FEF) occupies Brodmann’s area 8. This region is located rostral to the premotor cortex, on the frontal lobe. The FEF functions in the coordination of eye movements, particularly movements mediating voluntary visual tracking of a moving object

The posterior parietal area resides in the superior pari- etal lobule which corresponds to Brodmann’s areas 5 and 7. Area 5 receives input from the somatosensory cortex and is involved in tactile discrimination (the ability to perceive a subtle distinction by the sense of touch) and stereognosis (the recognition of the three-dimensional shape of an object by the sense of touch). Area 7 is involved with movements that require visual guidance. Thus, when one reaches for a glass of cold water, visual guidance (turning the body and aim- ing the upper limb in the direction of the glass), and tactile sensation (which in this case helps to realize that the glass is slippery and must be grasped firmly), both play a role in ac- complishing a desired motor task by their projections to the supplementary and premotor cortices.

The cingulate motor cortex which corresponds to Brod- mann’s areas 6, 23, and 24, is located in the superior and inferior banks of the cingulate sulcus. Similar to the other premotor cortical areas it also projects to the primary motor cortex. Since the cingulate gyrus is part of the limbic system, this motor cortical area may be associated with the motiva- tional or emotional aspects of movement

The cerebral cortex consists The internal pyramidal layer of six histologically distinct (layer V) of the motor cortex layers (Fig. 13.4). The inter- contains the cell bodies of the nal pyramidal layer (layer V), pyramidal cells, constituting the main output neurons of the cortical the most conspicuous layer descending motor pathways of the motor cortex, contains the somata of the pyramidal cells. These cells constitute the main output neurons, which contribute to the cortical descending motor pathways, the corticospinal and corticonuclear (corticobulbar) tracts that terminate in the spinal cord and brainstem, respectively

All of the pyramidal cell fibers release glutamate, an excitatory neurotransmitter, that stimulates excitatory or inhibitory interneurons, or less commonly, lower motoneurons, directly

There are seven descending motor pathways that ultimately exert their influence on muscle activity (Table 13.1). Three of these pathways, the lateral corticospinal, the anterior cortico- spinal, and the corticonuclear tracts derive their fibers from the sensorimotor cortex, whereas the other four tracts, the tectospi- nal, rubrospinal, reticulospinal, and vestibulospinal tracts, derive their fibers from the brainstem. All of the descending tracts terminate in the spinal cord with the exception of the cor- ticonuclear tract, which terminates in the brainstem