GSU NEUR 3000 - NEUR 3000 - Chapter 13 (42 pages)

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NEUR 3000 - Chapter 13



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NEUR 3000 - Chapter 13

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Pages:
42
School:
Georgia State University
Course:
Neur 3000 - Hon Principles of Neuroscience
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SPINAL CONTROL OF MOVEMENT NEUR 3000 Dr Joseph J Normandin THE MOTOR SYSTEM Consists of all of the muscles of the body and the neurons that control them Produces behavior through highly coordinated neuromuscular activity Much of this activity is coordinated by circuitry within the spinal cord Command and control of the muscles The brain interacts with the spinal cord to execute and modify spinal cord command Command and control of the spinal cord THE SOMATIC MOTOR SYSTEM The somatic motor system includes all of the skeletal muscles of the body and the neurons that control them This excludes smooth muscle cardiac muscle the visceral motor system Skeletal muscles are connected to bone by tendons and exert force on bones to move body parts Muscles are made of muscle cells also called muscle fibers which are excitable THE SOMATIC MOTOR SYSTEM LOWER MOTOR NEURONS The somatic musculature is innervated by somatic motor neurons Also called lower motor neurons Cell bodies are located in the ventral horn of the spinal cord Axons exit through the ventral root LOWER MOTOR NEURONS LOWER MOTOR NEURONS There are two types of lower motor neurons Alpha motor neurons Directly trigger the generation of force on muscles that we think of as movement Gama motor neurons Will be discussed later ALPHA MOTOR NEURONS ALPHA MOTOR NEURONS Alpha motor neurons are able to control muscle contraction in a graded fashion The synapse between a motor neuron and a muscle is the neuromuscular junction Acetylcholine and the nicotinic acetylcholine receptor are used here One motor neuron AP muscle fiber End Plate Potential EPP twitch contract relax Sustained contraction requires a continual barrage of APs EPPs are summed graded contraction More alpha motor neurons more contraction further graded ALPHA MOTOR NEURONS ALPHA MOTOR NEURONS Alpha motor neurons are able to control muscle contraction in a graded fashion The proportion of innervation by each motor neuron can result in fine or gross control One small motor neuron to a few muscle fibers in the fingers fine One large motor neuron to a 1000 muscle fibers in the leg gross For most muscles there is a mix of such innervation size principle When you need fine control use a small motor neuron When you need a lot of force use small and large motor neurons MOTOR UNITS Types of motor units Muscle fibers come in two types Rapid contraction powerful fatigue quickly Few mitochondria anaerobic metabolism Arm muscles White meat Slow contraction fatigue resistant Many mitochondria aerobic metabolism Antigravity muscles of the legs Dark meat Most muscles contain a mix of these fibers MOTOR UNITS Types of motor units Each motor unit contains only fibers of a single type Fast motor units Rapidly fatiguing white fibers Innervated by large fast conducting alpha motor neurons High frequency bursts of APs 30 60 s Slow motor units Fatigue resistant dark fibers Innervated by small slow conducting alpha motor neurons Low frequency steady APs 10 20 s MOTOR UNITS Types of motor units There is an intimate relationship between motor neurons and the muscles they innervate If you experimentally alter the innervation of a slow muscle with a fast motor neuron the muscle changes to a fast muscle and vice versa MOTOR UNITS Types of motor units There is an intimate relationship between motor neurons and the muscles they innervate If you experimentally alter the activity of a slow motor neuron to make it fast the muscle also becomes fast Could learning and memory alter such neural patterns to change behavior If you change the absolute amount of activity of a neuron muscles change as well Lots of motor neuron activity exercise muscle gets larger hypertrophy Not much activity slacker muscle gets smaller atrophy MUSCLE CONTRACTION Excitation contraction coupling Ach Nicotinic Ach receptor influx of sodium EPP voltage gated sodium channels influx of sodium AP This AP in the muscle is excitation and leads to contraction Muscle fibers have an excitable cell membrane sarcolemma Myofibrils are the contractile structures They are surrounded by the sarcoplasmic reticulum that contains calcium T tubules form pores in the sarcolemma and a network of tubes around the sarcoplasmic reticulum SR The T tubule membrane has voltage gated calcium channels MUSCLE CONTRACTION MUSCLE CONTRACTION Calcium and the sarcomere Sarcomeres are an arrangement of myosin thick and actin thin filaments in myofibrils Myosin binds to actin and changes conformation to bring the actin filaments closer together Contraction Dependent on calcium and ATP MUSCLE CONTRACTION Calcium and the sarcomere The myosin binding site on actin is covered by troponin The binding site is uncovered when calcium binds troponin Myosin binds to actin and myosin head pivots contraction Binding of ATP to myosin releases myosin from actin Repeats as long as calcium and ATP are present When are calcium and ATP present MUSCLE CONTRACTION MUSCLE CONTRACTION MUSCLE CONTRACTION The muscle fiber at rest Back to resting membrane potential Calcium is pumped back into SR ATP dependent Myosin binding site is covered by troponin ALPHA MOTOR NEURONS Control of alpha motor neurons Three sources of synapses with alpha motor neurons in the ventral horn Dorsal root ganglion cells sensory from muscle spindles A special reflex to maintain muscle tone Interneurons within spinal cord May be excitatory or inhibitory to control muscle movement Involved in reflexes Upper motor neurons from the PROPRIOCEPTION FROM MUSCLE SPINDLES When a muscle is pulled on stretched it pulls back contraction Myotatic reflex or stretch reflex Important for maintaining muscle tone Requires a sensor for stretch and an output for contraction PROPRIOCEPTION FROM MUSCLE SPINDLES Deep inside most skeletal muscles are muscle spindles stretch receptors Special muscle fibers intrafusal Surrounded by Ia sensory neuron axons Enclosed in a capsule Detects muscle length or stretch Proprioception Body sense Body PROPRIOCEPTION FROM MUSCLE SPINDLES Ia sensory neuron axons project to spinal cord via dorsal root Activated by mechanically gated cation channels Cell bodies in dorsal root ganglia Forms synapses with interneurons and alpha motor neurons of ventral horn One Ia sensory neuron may innervate ALL alpha motor neurons in that muscle s motor neuron pool Myotatic reflex Requires a sensor for stretch and an output for contraction Muscle spindle Ia sensory neurons detect stretch and alpha motor neurons contract the


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