BIOL 350 Lecture 7: Motor Systems PDF

Lecture 7: Motor Systems Organization -​ motor systems are those areas of the nervous system that are primarily responsible for controlling movement -​ self expressive/ specific/ goal-directed movement e.g. situation of need -​ very planned movements...

Lecture 7: Motor Systems Organization -​ motor systems are those areas of the nervous system that are primarily responsible for controlling movement -​ self expressive/ specific/ goal-directed movement e.g. situation of need -​ very planned movements e.g. doing surgery -​ highest level of movement, all movement must be planned -​ stereotypic repetitive/ automatic movements, e.g. walking & swimming -​ reflexes e.g. controlled at spinal/ higher levels depending of type of reflex -​ the movement can either be: -​ closed-loop (or reflex-controlled): always have feedback from the input, guided by inputs from sensory system → ensures precise and adaptive movements by continuously adjusting to changes in the environment or body position -​ open-loop: triggered by some internal desire to move rather than continuous feedback during the movement, control & planned Premotor and supplementary motor area -​ the premotor area aids in controlling movements of the core muscles to maintain posture during movement -​ the supplementary motor area is hypothesized to be responsible for planning and coordinating movement -​ upper motor neurons start at this point or 1st order -​ closer to the prefrontal cortex -​ start in PMC = 1st order, then 2nd order -​ sensory other way around ​ Lower Motor Neuron -​ aka 2nd order, defined as the neuron whose cell body lies in the anterior horn of the spinal cord or cranial nerve nuclei of the brainstem and which directly innervated the muscle via its axon -​ all motor systems are anterior ​ Control of Movement -​ the motor system includes cortical and subcortical areas -​ afferent inputs (influences the motor system): closed loop -​ feedback from sensory systems (somatosensory cortex) -​ cerebellar afferents -​ proprioception: sensing where muscles are -​ highest -​ efferent outputs (descending tracts): -​ corticobulbar tract: cortex → medulla -​ corticospinal tract: cortex → spinal cord -​ pyramidal: voluntary movement  -​ corticopontine tract: cortex → pons -​ rubrospinal tract: midbrain → spinal cord -​ reticulospinal tract (pons + medulla): control of awakeness -​ vestibulospinal tract: brain stem → spinal cord -​ tectospinal tract: eyes → spinal cord -​ extrapyramidal: involuntary movement Major Motor Systems -​ corticospinal and corticobulbar tracts -​ origin: from sensorimotor cortex around the central sulcus, (55% PMC and 35% PSSC, 10% other regions of frontal + parietal) -​ pathway: cortex → posterior limb of internal system (IC) → brainstem (CB tract) → CS tract continues and makes the pyramids at the medulla → decussation → lateral column of the SC -​ internal capsule is tiny hollow space everything passes through -​ 10% of pf pyramidal tracts do not decussate but go towards the aimed region = gross movements and trunk muscles -​ pyramids are are anterior -​ green neurons = interneurons, lower motor neuron -​ ventral/anterior horns = motor Extrapyramidal Motor System -​ involuntary control -​ this is a set of subcortical circuits and pathways which includes: -​ the corpus striatum (caudate nucleus, putamen, globus pallidus) -​ subthalamic nucleus, substantia nigra, red nucleus -​ brainstem reticular formation: feedback in how we do do voluntary control -​ practice fine tuning the feedback loops ​ Basal ganglia -​ inhibition of motor control -​ direct and indirect pathway: the striatum (caudate and putamen) is the major input to the basal ganglia from the cortex → excitatory input -​ striatum (inhibitory GABAergic axons) → inner part of globus pallidus → these provide a strong inhibitory input to the globus pallidus -​ direct pathway = more movement -​ excitatory → inhibiting inhibitory → more signals -​ indirect pathway = less movement -​ excitatory → inhibiting inhibitory → excitatory → inhibitory → less signals ​ Striatonigral projection -​ 2nd major outflow of basal ganglia is the substantia nigra (midbrain) -​ dopaminergic neurons (both excitatory and inhibitory) in the pars compacta of the substantia nigra → the striatum (nigrostriatal projection) → form inhibitory  synapses on the striatal neurons that have D2 dopamine receptors and excitatory synapses in neurons that have D1 dopamine receptors -​ extrapyramidal system -​ direct pathway striatal neurons have D1 dopamine receptors (depolarize the cell in response to dopamine) = excitation of the direct pathway = more movement -​ indirect pathway striatal neurons have D2 dopamine receptors (hyperpolarize the cell in response to dopamine) = inhibiting the indirect pathway = more movement -​ more practice = better connection to the associative planning -​ the nigrostriatal pathway thus has the dual effect of exciting the direct pathway while also inhibiting the indirect pathway -​ an upset of the balance between direct and indirect pathways results in the motor dysfunctions that characterize the extrapyramidal syndrome -​ substantia nigra = damage = Parksinsons -​ Parkinsons, drag feet, cannot raise them Descending motor pathways -​ descending motor pathways can be classified according to: -​ their site of origin: pyramidal or extrapyramidal tracts, location within the cord and the muscles they ultimately innervate -​ descending motor pathways synapse on interneurons (both motor and interneurons) within the spinal cord -​ damage to structures within the spinal cord affects descending motor pathways. ascending sensory pathway, and lower motor neurons