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Voluntary Movement From Ch. 38 “Principles of Neural Science”, 4th Ed. Kandel et al Voluntary movement • Voluntary movements are organized in cortex • Sensory feed back – Visual information – Proprioceptive information – Sounds and somatosensory information • Goal of movement – Vary in response to the same stimulus depending on behavioral task (precision vs. power grip) • Improves with learning/ experience • Can be generated in response to external stimuli or internally Cortical organization • Hierarchical organization of motor control and task features – Populations of neurons encode motor parameters e.g. force, direction, spatial patterns – The summed activity in a population determines kinematic details of movement – Voluntary movement is highly adaptable • Novel behavior requires processing in several motor and parietal areas as it is continuously monitored for errors and then modified – Primary motor cortex • • Fires shortly before and during movement Fires only with certain tasks and patterns of muscle activation – Premotor areas encode global features of movement • Set-related neurons – Sensorimotor transformations (external environment integrated into motor programs) – Delayed response Motor cortex • Primary motor cortex – – – Activated directly by peripheral stimulation Executes movements Adapt movements to new conditions • Premotor areas (Motor planning) – Dorsal premotor area (dPMA) • Selection of action; Sensorimotor transformations; Externally triggered movements; external cues that do not contain spatial information – Ventral premotor area (vPMA) • Conforming the hand to shape of objects; Mirror neurons; Selection of action; Sensorimotor transformations; Externally triggered movements – Supplementary motor area (SMA) • Preparation of motor sequence from memory (internally not in response to external information) – Pre-supplementary motor area (pre-SMA) • Motor sequence learning – Cingulate motor area (CMA) • • Dorsal and ventral portions of caudal and roastral CMA (along the cingulate sulcus) Functions: to be determined Somatotopical organization Sequence in human and monkey M1 similar Face and finger representations are much bigger than others Greater motor control required for face and fingers Motor cortex stimulation Historical perspective • • • • • • • • 1870 Discovery of electrical excitability of cortex in the dog; first brain maps (Fritsh and Hitzig) 1875 First motor map of the primate brain (Ferrier) 1926 Recording of extracellular spike activity of a nerve fiber (Adrian) 1937 First experimentally derived human motor map (Penfield and Boldrey) 1957 Microelectrode recordings to map primary somatosensory area (Mountcastle et al.) 1958 First recordings from neurons in awake monkeys (Jasper) 1967 Intracortical microstimulation for mapping of cortical motor output (Asanuma) 1985 TMS is used to activate motor cortex noninvasively (Barker et al.) Transcranial stimulation • TES – transcranial electrical stimulation (Merton and Morton 1980) – – – • TMS – transcranial magnetic stimulation (Barker 1985) – – • Discharge of large capacitive currents (5-10kA, 2-300us) through a coil producing high magnetic field (1-2T). Stimulus site depends on coil design, coil orientation and stimulus intensity Non-invasive techniques to study – – – • High voltage (1-2kV), short duration pulses (10-50us), low resistance electrodes. Direct stimulation occurs at the anode Current passes through skin and scalp (resistance) before reaching cortex. Structure-function relationship (e.g. rTMS virtual lesion) Map brain motor output (typically averaged EMG as output =MEP) Measure conduction velocity TMS has advantages over TES – – – No discomfort (no current passes through skin and high current densities can be avoided) No attenuation of field when passing through tissue No skin preparation (conduction gel) Transcranial magnetic stimulation Principles of TMS Coil design Motor cortex stimulation Latency difference • Movements can be evoked by direct stimulation of motor cortex • Activates corticospinal fibers – Direct from motor cortex to spinal motor neurons or interneurons • Evokes a short latency EMG response in contralateral muscles • Latency depends on corticospinal distance impulses have to travel Cortex-muscle connections Shoulder muscle Wrist muscle Maps can be generated by intracortical microstimulation Sites controlling individual muscles are distributed over a wide area of motor cortex Muscle representations overlap in cortex Stimulation of single sites activates several muscles (diverging innervation) Many motor cortical neurons contribute to multijointed movements Cortical projections • Premotor cortex and primary motor cortex has reciprocal connections • Parietal projections to premotor areas (sensorimotor transformations) • Prefrontal projections to some premotor areas (cognitive-affective control and learning) • Premotor areas and primary motor areas have direct projections to spinal motor neurons Other projections • Inputs from cerebellum – Do not project directly to spinal cord • Inputs from basal ganglia – Do not project directly to spinal cord • Cortico-striatal pathways – Motor loops – Motor cortex => striatum => globus pallidus => Thalamus => motor cortex Motor cortex plasticity • The functional organization of M1 changes after transection of facial nerve Practiced movements • M1 representation becomes more dense with practice PET data Pyramidal tract • Bilateral sectioning of the pyramidal tract removes the ability of fine movements • Successive cortical stimuli result in progressively larger EPSP in spinal motor neurons • Make it possible to make individual movement of digits and isolated movements of proximal joints – Direct corticospinal control is necessary for fine control of digits Ia spinal circuits • Type Ia sensory fibers are primary afferent fibers – – – • Spinal Ia neurons are inhibitory interneurons – – – – • Agonist muscle: generates specific movement Antagonist muscle: acts opposite the specific movement Proprioceptor Component of the muscle spindle Conveys information about the velocity of stretch and change in muscle length Can respond directly to changes in somatosensory input Cortical centers do not need to respond to minor changes Sends inhibitory signals to antagonist motor neurons when muscle spindles in the agonist muscle are activated Spinal Ia neurons also inhibits spinal reflexes Spinal circuits are used as components of complex behaviors Direction of movement Activity in individual neurons in M1 is related to muscle force and not joint displacement Flexor muscle: decreases joint angle Extensor muscle: increases joint angle Increased activity with load Wrist displacement constant but load is different Postspike facilitation • Cortical motor neuron – – EPSP have fixed latency One EPSP increases the probability of spinal motor neuron firing. It does not fully depolarize the motor neuron • The EMG is the sum of spike trains of a population of motor units within a muscle • The EMG is an indicator of firing of spinal motor neuron • Spike-triggered averaging – – – – Averaging the EMG profile over thousands of discharges from a single cortical neuron Cancels out random noise Peak in EMG profile at 6ms latency = postspike facilitation Indicator of connectivity between cortical neuron and the motor neuron M1 and force • Two types of cortical motor neurons – – Isometric wrist torques: torque level is reached and held Phasic-tonic: initial dynamic burst Tonic: tonic high level • Linear relationship between M1 firing rate and force generation • In both types of neurons activity increases with torque Direction of movement Single neuron response to 8 directions Population vector Many neurons with different preferred direction Major response: 90- 225 deg Predicted from vector Actual movement Direction of movement is encoded by a population of neurons Motor cortical neurons are broadly tuned to directions but have a preferred direction Direction of movement M1 encoding of force required to maintain a direction Single 8 directions of movement Arm movements without and with external loads (a) Unloaded: preferred direction to the upper left (b) Loaded: opposite, preferred direction to the lower right A cells firing rate increases if a load opposes movement in preferred direction and decreases if load pulls in preferred direction Activity of a single motor neuron Length of vector = discharge magnitude Activity depends on motor task Precision grip: same activity whether force is light or heavy Power grip: No activity, but EMG activity the same Complexity of movement Internal and external information Influence of visual cue and prior training in motor cortex 16 trials Before movement After movement Task: press 3 buttons in a sequence either guided by (a) light or (b) previously learned Motor preparation Laterality specific response • Dorsal premotor area is active during preparation • Fires according to different delay times • Fires during the whole period of anticipation Visuomotor transformations • Separate but parallel fronto-parietal projections Ventral premotor cortex Specific hand tasks activate vPMC Mirror neurons Ventral premotor area • Observed movement • Observed human movement • Self-performed movement Summary • Hierarchical organization of motor control and task features – Populations of neurons encode motor parameters e.g. force, direction, spatial patterns – The summed activity in a population determines kinematic details of movement – Voluntary movement is highly adaptable • – Primary motor cortex • • – Novel behavior requires processing in several motor and parietal areas as it is continuously monitored for errors and then modified Fires shortly before and during movement Fires only with certain tasks and patterns of muscle activation Premotor areas encode global features of movement • Set-related neurons – – Sensorimotor transformations (external environment integrated into motor programs) Delayed response