Slide 1The motor systemThe Somatic motor systemThe Somatic motor systemlower motor neuronslower motor neuronslower motor neuronsAlpha motor neuronsAlpha motor neuronsAlpha motor neuronsAlpha motor neuronsmotor Unitsmotor Unitsmotor Unitsmotor UnitsMuscle contractionMuscle contractionMuscle contractionMuscle contractionMuscle contractionMuscle contractionMuscle contractionAlpha motor neuronsProprioception from muscle spindlesProprioception from muscle spindlesProprioception from muscle spindlesProprioception from muscle spindlesProprioception from muscle spindlesProprioception from Golgi tendon organsProprioception from Golgi tendon organsProprioception from other sourcesRecall…Spinal interneuronsSpinal interneuronsSpinal interneuronsSpinal interneuronsCentral pattern generatorsCentral pattern generatorsCentral pattern generatorsCentral pattern generatorsCentral pattern generatorsSlide 42SPINAL CONTROL OF MOVEMENTNEUR 3000Dr. Joseph J. NormandinTHE 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 cordTHE 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 & have contractile properties•Each muscle fiber is innervated by a single axon branch from the CNSTHE SOMATIC MOTOR SYSTEMLOWER 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 rootLOWER MOTOR NEURONSLOWER 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 laterALPHA MOTOR NEURONSALPHA 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 NEURONSALPHA 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 neuronsMOTOR 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 fibersMOTOR 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•These channels interact with calcium release channels in the SR•AP  voltage-gated calcium channel  influx of calcium  calcium release channel  influx of calcium•Calcium then acts as an intracellular messengerMUSCLE CONTRACTIONMUSCLE 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 ATPMUSCLE 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 CONTRACTIONMUSCLE CONTRACTIONMUSCLE CONTRACTION•The muscle