Motor Cortex
The motor cortex is a region in the frontal lobe that consists of the Primary Motor Cortex (M1), Premotor Cortex, and Supplementary Motor Area. Electrical stimulation of these areas induces specific movements in various body parts. M1, located in the precentral gyrus, triggers simple movements with low-level stimulation, while the premotor cortex requires higher current levels and produces more complex movements. The premotor and supplementary motor areas are considered higher-level regions responsible for encoding intricate motor patterns and selecting appropriate plans for desired outcomes.
Similar to the somatosensory cortex, M1 exhibits somatotopic organization, representing body parts with precise movements disproportionately larger than those with coarse movements. This organization extends to the premotor and supplementary motor areas. While individual muscle movements are associated with widespread activity in M1, small regions of the motor cortex can elicit movements requiring coordination from multiple muscles. Consequently, the primary motor cortex homunculus does not represent individual muscle activity but rather the coordinated movements of specific body parts involving various muscle groups.
Motor Cortex Cytoarhcitcture
The primary motor cortex, like other parts of the neocortex, consists of six layers. However, it differs from primary sensory areas by lacking a cell-packed granular layer IV. Instead, its most distinctive layer is the descending layer output (layer V), which contains giant Betz cells.
Around 30% of the fibers in the corticospinal tract, responsible for motor control, originate from the pyramidal cells and other projection neurons in the primary motor cortex. The remaining fibers come from the premotor cortex and supplementary motor areas (another 30%), the somatosensory cortex (about 30%), and the parietal cortex (approximately 10%). These various inputs contribute to the complex network of signals that coordinate motor functions in th
Cortical Afferents and Efferents
The motor cortex controls muscles through various pathways. Besides directly connecting to alpha motor neurons through the corticospinal tract, it influences other descending pathways:
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Corticofugal Tract: This allows the cortex to adjust the rubrospinal tract.
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Corticotectal Tract: It enables the cortex to modulate the tectospinal tract.
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Corticoreticular Tract: This pathway lets the cortex influence the reticulospinal tracts.
The cortex also affects side loops in the motor hierarchy:
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Corticostriate Tract: Connects to the basal ganglia's caudate nucleus and putamen.
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Corticopontine and Cortico-Olivary Tracts: Innervate the cerebellum, providing important inputs.
Additionally, through cortico-cortical and corticothalamic pathways, different cortical areas influence each other. It's crucial to note that these pathways are often bidirectional. The motor cortex not only sends signals but also receives input from other cortical areas, both directly and indirectly through the thalamus. Furthermore, it receives input from the cerebellum and basal ganglia, always routed through the thalamus.
ENCODING OF THE MOVEMENT BY MOTOR CORTEX
Primary Motor Cortex
The primary motor cortex doesn't directly control individual muscles but is involved in orchestrating specific movements or sequences of movements. It influences alpha motor neurons in the spinal cord, which encode the force of contraction in muscle groups using a rate code and the size principle.
Observations from experimental animals performing various motor tasks suggest that the primary motor cortex encodes parameters defining movement or simple movement sequences. Primary motor neurons typically fire 5-100 milliseconds before movement onset, conveying motor commands to alpha motor neurons, initiating muscle contractions.
The primary motor cortex encodes the force of movement, with many neurons specifying the force needed for a particular movement (differentiating between Movement Force and Muscle Force). While a minority encode individual muscle force, most focus on the force required for a specific movement.
Direction of movement is encoded, as demonstrated by neurons selective for particular movement directions. For instance, a cell might activate strongly when the hand moves left but inhibit when moving right. The extent of movement is also encoded, with some neurons correlating with the distance of movement or the interaction of distance and direction in monkeys.
Speed of movement is another aspect encoded by primary motor cortex neurons. Movements generally follow a bell-shaped curve in velocity, with acceleration in the first half, peak velocity around halfway to the target, and deceleration until reaching the target. This comprehensive encoding in the primary motor cortex contributes to the precise control of voluntary movements.
Functions of the Basal Ganglia
The basal ganglia seem to play a role in facilitating practiced motor acts and regulating the initiation of voluntary movements. They achieve this by modulating motor programs stored in various parts of the motor hierarchy, including the motor cortex. While voluntary movements are not directly initiated in the basal ganglia, their proper functioning is crucial for the motor cortex to transmit the appropriate motor commands to lower levels of the motor hierarchy. In essence, the basal ganglia act as a regulatory system, influencing and enabling the execution of learned motor acts and controlling the initiation of voluntary movements at higher hierarchical levels.
Premotor Cortex
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Axonal Connections of Premotor Cortex:
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Sends axons to both the primary motor cortex and the spinal cord directly.
2. Complex Task-Related Processing:
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Performs more complex task-related processing than the primary motor cortex.
3. Involvement in Motor Planning:
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Involved in the selection of appropriate motor plans for voluntary movements.
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Contrasts with the primary motor cortex, which is more focused on executing voluntary movements.
4. Neuronal Signaling for Movement Preparation:
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Premotor cortex neurons signal the preparation for movement.
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Many neurons fire selectively in the delay interval before movement onset.
5. Motor-Set Neurons:
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Certain premotor neurons, known as motor-set neurons, fire during movement preparation but do not cause the movement itself.
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They prepare the organism for the correct movement when the "GO" signal is given.
6. Mirror Neurons:
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Some premotor neurons, termed "mirror neurons," respond not only to a particular action but also to the sight and sound of that action.
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For example, firing when a monkey breaks a peanut or observes/hears the peanut being broken.
7. Behavioral Context Sensitivity:
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Premotor cortex is sensitive to the behavioral context of a movement.
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Neurons are responsive to inferred intentions of a movement, not just the movement itself.
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For instance, firing may differ when observing a video of a full cup and plate versus empty dishes.
8. Signal for Correct and Incorrect Actions:
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Premotor cortex signals correct and incorrect actions.
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In human fMRI studies, bilateral activation during correct action trials and movement error trials was observed.
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Object error trials activated only the left hemisphere's premotor cortex.
Supplementary Motor Area
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Function of Supplementary Motor Areas (SMA):
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Involved in programming complex sequences of movements and coordinating bilateral movements.
2. Contrast with Premotor Cortex:
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While the premotor cortex is involved in selecting motor programs based on visual stimuli or abstract associations, the supplementary motor area focuses on selecting movements based on the sequence of movements.
3. Response to Sequence and Mental Rehearsal:
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SMA responds to the sequence of movements and mental rehearsal of movement sequences.
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Brain activity measured using a PET scanner showed SMA activation during both simple and complex sequences of movements.
4. Activation Patterns in Simple vs. Complex Movements:
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Contralateral primary motor cortex and somatosensory cortex activated during simple repetitive movements.
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Bilateral SMA activated, along with contralateral primary motor cortex and somatosensory cortex, during complex sequences of finger movements.
5. SMA Activity During Mental Rehearsal:
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SMA remains active during mental rehearsal of complex movement sequences, even when primary motor and somatosensory cortex are silent.
6. Transformation of Kinematic to Dynamic Information:
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SMA is involved in the transformation of kinematic to dynamic information in movement planning.
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Movements can be defined in terms of kinematics (distance and angles) and dynamics (force required).
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SMA neurons shift their firing correlates from kinematic to dynamic representations, suggesting a role in this transformation.
7. Role in Bilateral Movements and Mental Rehearsal:
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SMA appears to play a vital role in bilateral movements and in the mental rehearsal of movements.
Association Cortex
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Fourth Level of Motor Hierarchy:
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The association cortex, specifically the prefrontal cortex and posterior parietal cortex, constitutes the fourth level of the motor hierarchy.
2. Non-Motor Nature of Prefrontal and Parietal Cortex:
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Prefrontal and posterior parietal cortex are not strictly motor areas; stimulation doesn't directly lead to motor output.
3. Role in Adaptive Movements:
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Essential for ensuring adaptive movements aligned with the organism's needs and appropriate to the behavioral context.
4. Posterior Parietal Cortex Function:
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Involved in accurate targeting of movements to external objects.
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Processes and constructs spatial relationships of objects, independent of the observer's eye or body position.
5. Consequences of Damage to Parietal Cortex:
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Damage can result in apraxias, characterized by an inability to make complex, coordinated movements.
6. Prefrontal Cortex Function:
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Involved in selecting appropriate actions for specific behaviors.
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Evaluates consequences of a chosen course of action.
7. Effects of Prefrontal Cortex Damage:
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Damage can lead to executive processing problems.
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Patients may make inappropriate behavioral decisions and struggle to anticipate consequences.
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Display impulsive behavior and have difficulty delaying instant gratification for a larger, long-term reward.