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Motor Systems II Loops and Tracts Reading: BCP Chapter 14 VA Motor System Intent: posterior parietal association cortex Plan of action: dorsolateral prefrontal association cortex Specific set of instructions: secondary motor cortex Execution: primary motor cortex Cortical Loops and Descending Tracts Two major cortical loops: • one through the basal ganglia and secondary motor cortex that selects and initiates action; • one through the cerebellum and primary motor cortex that modulates and sequences muscle contractions while a movement is in progress. Four major descending pathways (mainly from primary motor cortex): • two in the dorsolateral region of the spinal cord; and • two in the ventromedial region. VA The Basal Ganglia A collection of inter-connected, midline, nuclei located lateral to the thalamus Includes the striatum (caudate and putamen), globus pallidus (external and internal), subthalamic nucleus and substantia nigra External, GPe Internal, GPi Basal Ganglia Pathways Two major functional pathways through basal ganglia, direct and indirect/hyper-direct, with opposite net effects on thalamic targets. Proper function: balance Direct: excitation Facilitates motor (or cognitive) programs in the cerebral cortex that are adaptive for the present task Indirect/hyper-direct: inhibition Inhibits the execution of competing motor programs. VA Basal Ganglia: Parkinson’s Disease Parkinson’s disease is characterized by slowness or absence of movement (bradykinesia or akinesia), rigidity, and a resting tremor (hands, fingers) Cause: the loss of the dopaminergic neurons in the substantia nigra Direct pathway striatal neurons have D1 dopamine receptors, which cause depolarization, whereas indirect pathway striatal neurons have D2 dopamine receptors, which cause hyperpolarization. The nigrostriatal pathway thus produces net excitation of cortex in two ways. In Parkinson’s disease, balance is tipped in favor of the indirect inhibitory pathway. frontiersin.org Basal Ganglia: Huntington’s Disease The symptoms of Huntington’s disease are in many respects the opposite of the symptoms of Parkinson’s disease. Huntington’s disease is characterized by choreiform movements: involuntary, jerky movement of the body, especially of the extremities and face. Huntington’s disease results from the selective loss of striatal neurons in the indirect pathway. Thus, the balance between the direct and indirect pathways becomes tipped in favor of the direct pathway. Without their normal inhibitory inputs, thalamic neurons can fire randomly and inappropriately, causing the motor cortex to execute motor programs without proper control. Cerebellum Gross Anatomy: The visible part of the cerebellum is actually a single thin sheet of folded cortex. It is characterized by a series of shallow ridges call folia. Altogether, the cerebellum constitutes about 10% of brain mass, but more than 50% of its neurons • Cerebellar Cortex: Subdivided into lobes, lobules, vermis (midline region) and lateral hemispheres. • Cerebellar nuclei: Embedded deep in the white matter of the cerebellum; communicate cerebellar cortical output to other brain centers including motor cortex, descending motor pathways, and vestibular nuclei (balance) Anterior lobe Cerebellar cortex Posterior lobe Motor Loop through Cerebellum Layer V sensorimotor cortical cells (secondary and primary motor cortex, somatosensory cortex, and posterior parietal cortex) project to the pontine nuclei (in the pons). Pontine nuclei send massive input to cerebellar cortex. Lateral cerebellum projects to cortex via lateral nucleus of thalamus (VLc). Function: • To modulate and sequence muscle contractions for voluntary movements. • To evaluate disparities between intention and action. • To correct output of cortical and subcortical motor systems while movement is in progress. VA Sensorimotor Adaptation The cerebellum is important in reprogramming movements to compensate for sensory disturbances. – Example: Adaptation of motor output to altered visuospatial experience (e.g., ocular prisms). The cerebellum is also important for learning new motor skills; that is, practice improves efficiency, speed, and precision of motor performance. Cerebellar Disorders Ataxia: intention tremor disturbances in rate and regularity of alternating movements. Asynergia (dysynergia): prolonged reaction times “decomposition” of multi-joint movements. Dysmetria: Undershoot or overshoot of movements toward objects. Descending Motor Pathways Four major descending pathways: two in the dorsolateral region of the spinal cord; two in the ventromedial region. In each region, one is direct and one is indirect. Motor homunculus Upper motor neuron Lower motor neuron Dorsolateral tracts • corticospinal (direct) • corticorubrospinal Motor nuclei Medulla Ventromedial • corticospinal (direct) • cortico-brainstem-spinal Dorsolateral corticospinal Ventromedial corticospinal Dorsolateral Tracts Corticospinal tract: descends from primary motor cortex (e.g., Betz cells) through the medullary pyramids, then crosses the midline (decussates) and synapses directly in the cord. Wrist, hands, fingers, toes Corticorubrospinal tract: synapses at red nucleus, and crosses before medulla. Face, arms and legs. Dorsolateral tracts tend to synapse on small interneurons in the cord that, in turn, synapse on motor neurons of distal targets Ventromedial Tracts Corticospinal tract: descends ipsilaterally and directly to the spinal cord, then branches and innervates interneuron circuits bilaterally in multiple spinal segments Cortico-brainstem-spinal tract: synapses on various brainstem structures and descends bilaterally, carrying information from both hemispheres Ventromedial tracts synapse on small interneurons in the cord that, in turn, synapse on motor neurons of the trunk and proximal limb muscles Division of Labor Dorsolateral tracts • one direct tract, one that synapses in the brain stem • terminate in one contralateral spinal cord segment • distal muscles • limb movements Ventromedial tracts • one direct tract, one that synapses in the brain stem • terminate in multiple, bilateral spinal cord segments • proximal muscles • posture and whole body movements forumotion.net