Focus on the control of voluntary movements of the hand and arm in primates. Cortical networks that control voluntary movement, the role of the primary motor cortex in the generation of motor commands.

Control of voluntary movement involves more than generating a particular pattern of msucle activity - involves sensory, perceptual, ad cognitive processes not rigidly compartmentalized in neural structures.

Woosley and Penfield: recognition of the motor cortex (rostral to the central sulcus) and the “cortical homunculus”. Area of arm and hand concentrated in the “Fundus”.

Naive understanding of voluntary movements says that voluntary motor control appears to be strictly serial processing, with only the neurons related to the last processing stage connecting to the spinal cord. This is not correct, as the brain does not even have a single, unified perceptual representation of the world.

Two main areas: Primary Motor Cortex and Premotor Cortex, which lies directly rostral of primary motor cortex. The medial part of the premotor cortex is the Supplementary motor area. However, these three areas can further be functionally organized into more areas, especially the parts of the premotor cortex.

The supplementary motor area, dorsal premotor cortex (PMd), and ventral premotor cortex (PMv) have somatotopically reciprocal connections with the primary motor cortex and with each other. Those and the primary motor cortex (M1) also have somatotopically inputs from the primary somatosensory cortex and the rostral parietal cortex (sensory areas).

Pre-supplementary and pre-dorsal premotor areas do not project to the primary motor cortex or anything more rostral. They receive higher-order cognitive information through the prefrontal cortex.

Several cortical regions project in multiple parallel tracts to subcortical areas of teh brain as well as the spinal cord. Therefore the theory of the primary motor cortex as the “final common path” from the cortex to spinal cord is incorrect, and multiple cortical regions contribute to voluntary movements.

Corticomotoneurons are corticospinal axons that extend into the ventral horn of the spinal cord and contact the spinal motor neurons. The axons of these neurons become a bigger part of the corticospinal tract moving higher in primate phylogeny. This may explain why lesions of the primary motor cortex have bigger effect on motor control in humans compared to lower mammals. Pyramidal tract neurons is the aggregate of upper motor neuron nerve fibers that travel from the cortex and terminate either in the brainstem or spinal cord. Nerve fibers usually descend down the brain in columns.

Motor commands are population encodings (Georgopolos studies) - “further studies have confirmed that similar poopulation-coding mechnisms are used in all cortical motor areas”.

The motor cortex encodes both the kinematics and kinetics of movement. Experiments in which a load is applied to either oppose or assist some arm motion found the population and single neuron activity either increased or decreased accordingly, corresponding to increased or decreased muscle activity, confirming kinetics encoding. Studies in which the activity of some corticomotoneurons does not always correlate with the contraction of their target muscles, but instead correlate with carefully controlled or powerful movements hint that they may also encode kinematics. Signals about both the desired kinematics and required kinetics of movements may be generated simultaneously in different, or possibly even overlapping, populations of primary motor cortex neurons.

Hand and finger movements are directly controlled by the motor cortex. Specifically, cortical neurons controlling the hand and digits occupy the large central core of the pirmary motor cortex motor maps but also overlap extensively with populations of neurons controlling more proximial parts of the arm. We can imagine mapping the movements of the hand and digits into a component neuron space, where each neuron controls a combination of muscle activations. This is contrasted by the highly ordered representation of tactile sensory inputs from different parts of the hand and digits in the somatosensory cortex.

The motor map is dynamic and adaptable, and can experience functional reorganization. Learning a motor skill can induce reorganization, which can also decay when “out of practice” possibly due to horizontal connections and local inhibitory circuits (John Donoghue). Bizzi 2001 demonstrated 4 different motor cortex neurons during motor skill adaptation and washout - kinematic neurons (tuning does not change), dynamic neurons (tuning change in both adapation and washout), and memory neurons (change either during adaptation or washout only).

Studies found that adaptive changes in motor cortex activity lag the improvement in motor performance by several trials during adaptation. This suggests that leraning-related adjustments to motor commands are initially made elsewhere, with the cerebellum as one strong candidate. The primary motor cortex may thusb e more strongly involved in the slower process of long-term retention and recall of motor skills rather than the initial phase of learning a new skill.

The primary motor cortex is part of a distributed network of cortical motor areas, each with its own role in voluntary motor control. The primary motor cortex should be regarded as a dynamic computational map whose internal organization and spinal connections convert central signals about motor intentions and sensory feedback about the current state of the limb into motor output commands, rather than as a static map of specific muscles or movements of body parts. The tmoro cortex also provides a substrate for adatpve alterations during the acqustion of motor skills and the recovery of function after lesions.