motor systems. motor unit motoneuron + muscle fibers it innervates range in size from a few muscle...
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Motor Systems
Motor UnitMotoneuron + muscle fibers it innervates
Range in size from a few muscle fibers(e.g. extraocular muscles)
To hundreds of fibers(e.g. digits)
To thousands of fibers(e.g. trunk and major limb segments)
Smaller motor units yield more refined controla motor “fovea”
sarcomere
Excitation of a Muscle Fiber•An action potential in an alpha motoneuron axon.
•ACh is released by the axon terminal at the neuromuscular junction (synapse between motoneuron and muscle.
•Nicotinic receptor channels open, Na+ enters and the membrane depolarizes (EPSP).
•Voltage-gated Na+ channels open and an action potential is propagated down the muscle fiber.
•Depolarization produces Ca2+ release from intracellular stores.
Contraction of a Muscle Fiber•Ca2+ binds to tropinin.
•Myosin binding sites are exposed.
•Myosin heads bind actin.
•Myosin cross-bridges pivot.
•Myosin heads disengage at the expense (ATP-dependent).
•Recycle as long as Ca2+ and ATP are present.
Relaxation of a Muscle Fiber•As EPSPs end, membrane returns to resting potential.
•Free Ca2+ is sequestered by metabolically driven pump.
•Myosin binding sites covered.
Co-activation of α-motoneuronsand γ-motoneurons
at restα-motoneuron activation
α-, γ-motoneuron co-activation
Meissner’s Merkel’s
Pacinian Ruffini’s Free nerve ending
Monosynaptic stretch reflex
Golgi tendon organ reflex
somatosensoryafferents
motor afferents
(Aδ)
(C)
Muscle Spindles (intrafusal fibers)
Golgi Tendon Organs
respond to muscle length, or stretch
insensitive to tension
1a afferents for dynamic spindles
Group II afferents for postural spindles
monosynatic stretch reflex (excitatory)
respond to muscle tension
insensitive to muscle length, or stretch
1b afferents
disynaptic reflex (inhibitory)
Problem of LocomotionDegrees of freedom problem (Bernstein)
Dozens of joints controlled by hundredsof muscles
Muscles can contract in any combination,in any order, for any duration, and withany force.
Possibilities are virtually limitless, makingcontrol of complex movementsat the level of individual musclesimpossible
Reciprocal inhibition between motorunits of antagonistic muscles
Crossed extensor reflex
oscillators = “legos” of movementCPGs = what you can build with legos
Central Pattern Generators vs. Reflexes
Central Pattern GeneratorsFast
Act as “clocks”Flexible--<100 oscillators necessary
ReflexesRefine timing
Respond to the unexpectedAcquisition of new skills
Descending motor pathwaysLateral and ventromedial pathways
Pyramidal and extra-pyramidal systems
Supplementary Motor Area (SMA)important for instituting motor programsactivated prior to voluntary movementactivated during mental rehearsal
Premotor cortexsensory guided movementcooperatively with basal gangliamirror neurons
Pre-frontal cortexdecisions for actionslong-term planning
Cortex
Cerebellum
Substantia
Nigra
Caudate-
putamen
(stria
tum)
GlobusPallidus
Thalamusand STN
RedNucleus(nucleu
sruber)
Reticulospinal tract
Rubrospinal tract
Cortex
Cerebellum
Substantia
Nigra
Caudate-
putamen
(stria
tum)
GlobusPallidus
Thalamusand STN
RedNucleus(nucleu
sruber)
Reticulospinal tract
Rubrospinal tract
cortex
pontine nuclei
cerebellum thalamus
Cortex
Cerebellum
Substantia
Nigra
Caudate-
putamen
(stria
tum)
GlobusPallidus
Thalamusand STN
RedNucleus(nucleu
sruber)
Reticulospinal tract
Rubrospinal tract
cortex
striatum
globuspallidus thalamus
Cortex
Cerebellum
Substantia
Nigra
Caudate-
putamen
(stria
tum)
GlobusPallidus
Thalamusand STN
RedNucleus(nucleu
sruber)
Reticulospinal tract
Rubrospinal tract
cortex
striatum
substantianigra thalamus
Cortex
Cerebellum
Substantia
Nigra
Caudate-
putamenGlobu
sPallidus
Thalamusand STN
RedNucleus(nucleu
sruber)
Reticulospinal tract
Rubrospinal tract
degenerate in Huntington’s disease
Cortex
Cerebellum
Substantia
Nigra
Caudate-
putamenGlobu
sPallidus
Thalamusand STN
RedNucleus(nucleu
sruber)
Reticulospinal tract
Rubrospinal tract
degenerates in Parkinson’s disease
Treatments:
1. l-DOPA to compensate for lost DA projections
2. Deep brain stimulation
of striato-pallidal projections
3. Surgical destruction of
cells in medial pallidum
Subthalamus pedunculopontine nucleus (PPN)reticulospinal tractspinal CPGs
ALS: Upper motoneuron diseasePolio: Spinal motoneuron diseaseApraxias: Frontal cortical damage