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THE MOTOR SYSTEM, part I

SOMATIC MOTOR SYSTEM

Muscles and neurons that control musclesRole: Generation of coordinated movements

Parts of motor controlSpinal cord coordinated muscle contractionBrain motor programs in spinal cord

SOMATIC MOTOR SYSTEM

Types of MusclesSmooth: digestive tract, arteries, related structuresStriated: Cardiac (heart) and skeletal (bulk of body muscle mass)In each muscle there are 100 of muscle fibers innervated by a single axon from the CNS

muscle fibers

Axon from CNS

muscle

SOMATIC MOTOR SYSTEM

Somatic MusculatureAxial muscles: Trunk movementProximal muscles: Shoulder, elbow, pelvis, knee movementDistal muscles: Hands, feet, digits (fingers and toes) movement

Flexors

Extensors

SynergistAntagonist

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Ventral hornLower motor neuron

Ventral root

Muscle fiberSpinal nerve

The Lower Motor NeuronLower motor neuron: Innervated by ventral horn of spinal cordUpper motor neuron: Supplies input to the spinal cord

THE SPINAL CORD

Alpha Motor NeuronsTwo lower motor neurons: Alpha and GammaAlpha Motor Neurons directly trigger the contraction of the muscleMotor Unit: muscle fibers + 1 alpha motor neuronMotor neuron pool: all alpha motor neuron that innervate a single muscle

Graded Control of Muscle Contraction by Alpha Motor NeuronsVarying firing rate of motor neurons (temporal summation)Recruit additional synergistic motor units. More motor units in a muscle allow for finely controlled movement by the CNS

THE SPINAL CORD

Inputs to Alpha Motor Neurons1) Information about muscle lenght2) Voluntary control of movement 3) Excitatory or inhibitory in order to generate a spinal motor program

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THE SPINAL CORD

THE MOTOR UNITS

Types of Motor UnitsRed muscle fibers: Large number of mitochondria and enzymes, slow to contract, can sustain contractionWhite muscle fibers: Few mitochondria, anaerobic metabolism, contract and fatigue rapidlyFast motor units: Rapidly fatiguing white fibersSlow motor units: Slowly fatiguing red fibers

Normal innervation

Crossedinnervation

slow fast slow fast

slow fast Slow likeFast like

Hypertrophy: Exaggerated growth of muscle fibersAtrophy: Degeneration of muscle fibers

Muscle fiber structure Sarcolemma: external membraneMyofibrils: cylinders that contract after an APSarcoplasmic reticulum: reach of Ca2+T tubules: network that allow the AP to go through

Mitochondria Myofibrils

T tubules

Sarcoplasmic reticulum

Opening of T tubules

Sarcolemma

THE MOTOR UNITS

The Molecular Basis of Muscle ContractionZ lines: Division of myofibril into segments by disksSarcomere: Two Z lines and myofibrilThin filaments: Series of bristles. Contains actinThick filaments: Between and among thin filaments. Contains myosin

Sliding-filament model: Binding of Ca2+ to troponin causes myosin to bind to actin. Myosin heads pivot, cause filaments to slide

THE MOTOR UNITS

Muscle contractionAlpha motor neurons release AChACh produces large EPSP in muscle fibers (via nicotinic ACh receptors)EPSP evokes action potential. Action potential triggers Ca2+ release, leads to fiber contractionRelaxation, Ca2+ levels lowered by organelle reuptake

THE MOTOR UNITS

Excitation: Action potential, ACh release, EPSP, action potential in muscle fiber, depolarizationContraction: Ca2+, myosin binds actin, myosin pivots and disengages, cycle continues until Ca2+ and ATP presentRelaxation: EPSP end, resting potential, Ca2+ by ATP driven pump, myosin binding actin covered

SPINAL CONTROL

Muscle spindles: specialized structures inside the skeletal muscle. They informabout the sensory state of the muscle (proprioception)

SPINAL CONTROLThe Myotatic ReflexStretch reflex: Muscle pulled tendency to pull backFeedback loop. MonosynapticDischarge rate of sensory axons: Related to muscle lengthExample: knee-jerk reflex (stretching the quadriceps and consequent contraction)

SPINAL CONTROL

Intrafusal fibers: gamma motor neuron Extrafusal fibers: alpha motor neuron

Gamma LoopProvides additional control of alpha motor neurons and muscle contractionCircuit: Gamma motor neuron intrafusal muscle fiber Ia afferent axon alpha

SPINAL CONTROLProprioception from Golgi Tendon Organ.In series with the muscle fibers. Information about the tension applied to the muscleReverse myotatic reflex function: Regulate muscle tension within optimal range

Golgi Tendon Organ

SPINAL CONTROLSpinal InterneuronsSynaptic inputs1)Primary sensory axons2)Descending axons from brain3)Collaterals of lower motor neuron axonsSynaptic outputs: alpha motor neuron

Reciprocal inhibition: Contraction of one muscle set accompanied by relaxation of antagonist muscle Example: Myotatic reflex

Crossed-extensor reflex: Activation of extensor muscles and inhibition of flexors on opposite side

flex flex

extend extend

MOTOR PROGRAM