Muscle III

Muscle III - Muscle III: Control of Movement Hierarchy of...

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Muscle III: Control of Movement Hierarchy of Muscle Control Sensorimuscle cortex is the middle level of motor control and includes the parts of the cerebral cortex that act together to control skeletal muscle activity – premotor area, primary motor cortex, supplementary motor cortex, somatosensory cortex, parietal-lobe association cortex. Other middle levels of motor control are the basal nuclei (part of subcortical nuclei), thalamus, brainstem, and cerebellum. Local levels of motor control – brainstem and spinal cord, receptors, muscle fibers 2 pathways from brain/CNS to spinal cord Corticospinal pathway – long axons, most cross in the brainstem and go down opposite side of spinal cord – crosstalk Brainstem Pathway – mostly uncrossed, go straight down spinal cord Each muscle is connected to only one motor neuron – it doesn’t need any other nerves But neurons are biforlated – each neuron regulates several muscle cells A motor unit is a motor neuron and all of its attached muscle fibers A complete muscle is a mixture of motor units Motor units are controlled according to their size – cross section and pass from the cortex to the spinal cord 1. Tiny motor units – slow-oxidative fibers, recruited first so they fire first 2. Intermediate motor units – fast-oxidative fibers, recruited after slow- oxidative, so they fire next. 3. Gigantic motor units – fast-glycolytic fibers – recruited last, fire last and die off quickly Varying degrees of muscle tension for different sizes of motor units Small motor units can’t cause much rise in muscle tension, intermediates cause a little more, and the fast-glycolytic fibers cause the greatest rise because they have the largest-diameter fibers and have the most number of fibers per motor unit. The size of the motor unit is directly related to the size of the muscle fibers and the number of the muscle fibers that the motor neuron is connected to, as well as the size of the motor neuron Frequency of the AP determines which size motor units activated Small frequency = small motor units activated, as the frequency increases, larger motor units are activated The other two types of motor units keep firing even when the large motor units are activated (the slow-oxidative never gives up – keeps firing throughout) Small motor units are activated first because they have smaller neurons, which means that the spike initiation zone is smaller. In larger motor units with larger neurons, the spike initiation zone is larger, so the synaptic current is more spread out – requiring a larger frequency.
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This note was uploaded on 05/01/2008 for the course BIOL 34 taught by Professor Velez during the Spring '07 term at Dartmouth.

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Muscle III - Muscle III: Control of Movement Hierarchy of...

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