THE MOTOR SYSTEM, part I
SOMATIC MOTOR SYSTEM
Muscles and neurons that control muscles
Role: Generation of coordinated movements
Parts of motor control
Spinal cordÆ coordinated muscle
contraction
BrainÆ motor programs in spinal cord
SOMATIC MOTOR SYSTEM
Types of Muscles
Smooth: digestive tract, arteries, related structures
Striated: 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 Musculature
Axial muscles: Trunk movement
Proximal muscles: Shoulder, elbow, pelvis, knee movement
Distal muscles: Hands, feet, digits (fingers and toes) movement
Antagonist
Synergist
Flexors
Extensors
THE SPINAL CORD
The Lower Motor Neuron
Lower motor neuron: Innervated by ventral
horn of spinal cord
Upper motor neuron: Supplies input to the
spinal cord
Ventral root
Spinal
nerve
Lower motor
neuron
Muscle fiber
li
Ventral horn
THE SPINAL CORD
Alpha Motor Neurons
Two lower motor neurons: Alpha and Gamma
Alpha Motor Neurons directly trigger the contraction
of the muscle
Motor Unit: muscle fibers + 1 alpha motor neuron
Motor neuron pool: all alpha motor neuron that
innervate a single muscle
Graded Control of Muscle Contraction by
Alpha Motor Neurons
Varying 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 Neurons
1) Information about muscle lenght
2) Voluntary control of movement
3) Excitatory or inhibitory in order to generate a spinal motor program
3
1
2
THE MOTOR UNITS
Types of Motor Units
Red muscle fibers: Large number of mitochondria and enzymes, slow to contract, can sustain contraction
White muscle fibers: Few mitochondria, anaerobic metabolism, contract and fatigue rapidly
Fast motor units: Rapidly fatiguing white fibers
Slow motor units: Slowly fatiguing red fibers
Hypertrophy: Exaggerated growth of muscle fibers
Atrophy: Degeneration of muscle fibers
Normal
innervation
Crossed
innervation
slow
fast
slow
fast
slow
fast
Fast like
Slow like
THE MOTOR UNITS
Muscle fiber structure
Sarcolemma: external membrane
Myofibrils: cylinders that contract after an AP
Sarcoplasmic 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 Contraction
Z lines: Division of myofibril into segments by disks
Sarcomere: Two Z lines and myofibril
Thin filaments: Series of bristles. Contains actin
Thick 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 contraction
Alpha motor neurons release ACh
ACh produces large EPSP in muscle fibers (via
nicotinic ACh receptors)
EPSP evokes action potential.
Action potential triggers Ca2+ release, leads to
fiber contraction
Relaxation, Ca2+ levels lowered by organelle
reuptake
Excitation: Action potential, ACh release, EPSP,
action potential in muscle fiber, depolarization
Contraction: Ca2+, myosin binds actin, myosin
pivots and disengages, cycle continues until
Ca2+ and ATP present
Relaxation: EPSP end, resting potential, Ca2+ by
ATP driven pump, myosin binding actin covered
SPINAL CONTROL
Muscle spindles: specialized structures inside the skeletal muscle. They inform
about the sensory state of the muscle (proprioception)
SPINAL CONTROL
The Myotatic Reflex
Stretch reflex: Muscle pulledÆ tendency to pull back
Feedback loop. Monosynaptic
Discharge rate of sensory axons: Related to muscle length
Example: knee-jerk reflex (stretching the quadriceps and consequent contraction)
SPINAL CONTROL
Intrafusal fibers: gamma motor neuron
Extrafusal fibers: alpha motor neuron
Gamma Loop
Provides additional control of alpha motor
neurons and muscle contraction
Circuit: Gamma motor neuronÆ intrafusal
muscle fiberÆ Ia afferent axonÆ alpha
SPINAL CONTROL
Proprioception from Golgi Tendon Organ.
In series with the muscle fibers. Information about the tension applied to the muscle
Reverse myotatic reflex function: Regulate muscle tension within optimal range
Golgi
Tendon
Organ
SPINAL CONTROL
Spinal Interneurons
Synaptic inputs
1)Primary sensory axons
2)Descending axons from brain
3)Collaterals of lower motor neuron axons
Synaptic 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