Lecture13_motorsystems - Motor systems cerebellar eyeblink...

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Motor systems & cerebellar eyeblink conditioning
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There are 206 bones in the human body! These bones form the skeleton Many bones are connected together by flexible joints The way these joints bend determines what movements are possible for the body to make Skeletal movements are controlled by skeletal muscles Joints and Movements
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Muscles are connected to bone by tendons Each muscle is a large bundle of many smaller muscle fibers Muscles exert force by contracting (shortening of the fibers); they can only PULL a limb, not PUSH it! Many skeletal joints are controlled by a pair of two antagonistic muscles: 1) Flexors : contraction bends the joint (example: bicep) 2) Extensors: contraction straightens the joint (example: tricep) Skeletal Muscles: Flexors and Extensors
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Each muscle fiber is a single large cell with many nuclei (very unusual!!!) Running through the center of the fiber is a bundle of protein filaments called a myofibril The myofibril is made mostly of two kinds of proteins: 1) Actin : A cytoskeletal protein 2) Myosin: A motor protein that grabs actin and “walks” along the actin filament Muscle Fibers and Myofibrils
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The muscle fiber is divided into segments called sarcomeres Myosin heads can shorten the sarcomere by grabbing actin filaments and drawing them together Shortening the sarcomeres causes the muscle to contract But what tells the myosin to grab the actin and start pulling on it?? Muscle Contraction Movement of actin filament Direction of myosin walk
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Motor neurons that projects from the spinal cord to the muscle have branches near the end of their axons Each of the branches forms a synapse onto a different muscle fiber, so when the motor neuron fires an action potential, it will cause more than one fiber to contract A motor unit is a single motor neuron plus all of the muscle fibers that it contracts Each motoneuron contacts several muscle fibers, but each muscle fiber receives input from only one motoneuron (in the ADULT…)
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Motor neurons in the spinal cord send their axons to muscle fibers, where they make synapses called neuromuscular junctions (NMJs) When the motor neuron fires an action potential, ACh is released at the NMJ ACh binds to nicotinic receptors on the muscle fiber, causing Na+ to enter Voltage-gated Ca+ channels are activated and the muscle fiber fires an action potential ! Ca 2+ enters the fiber, and this causes myosin to pull actin filaments together
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Spinal Nerves : 31 pairs of nerves (left/right, dorsal/ventral) emanate from the spinal cord. Ventral roots carry motor commands from brain to body. Each pair of ventral roots sends axons to different muscle groups (similar to dermatomes) Motoneurons reside in the gray matter of the ventral horn of the spinal cord Spinal Motor Nuclei
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Brainstem Motor Nuclei Cranial motor neuron cell bodies reside in the brainstem motor nuclei Motor axons exit the brainstem through the cranial nerves to contact muscles in the head and face
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What activates the motor neurons?
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