Transcript Motor systems

Motor Systems
Motor Unit
Motoneuron + 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 control
a 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.
at rest
α-motoneuron
activation
α-, γ-motoneuron
co-activation
Co-activation of α-motoneurons
and γ-motoneurons
Meissner’s
Pacinian
Merkel’s
Ruffini’s
Free nerve ending
Monosynaptic stretch reflex
Golgi tendon organ reflex
somatosensory
afferents
motor afferents
(Aδ)
(C)
Muscle Spindles (intrafusal fibers)
respond to muscle length, or stretch
insensitive to tension
1a afferents for dynamic spindles
Group II afferents for postural spindles
monosynatic stretch reflex (excitatory)
Golgi Tendon Organs
respond to muscle tension
insensitive to muscle length, or stretch
1b afferents
disynaptic reflex (inhibitory)
Problem of Locomotion
Degrees of freedom problem (Bernstein)
Dozens of joints controlled by hundreds
of muscles
Muscles can contract in any combination,
in any order, for any duration, and with
any force.
Possibilities are virtually limitless, making
control of complex movements
at the level of individual muscles
impossible
Reciprocal inhibition between motor
units of antagonistic muscles
Crossed extensor reflex
oscillators = “legos” of movement
CPGs = what you can build with legos
Central Pattern Generators vs. Reflexes
Central Pattern Generators
Fast
Act as “clocks”
Flexible--<100 oscillators necessary
Reflexes
Refine timing
Respond to the unexpected
Acquisition of new skills
Descending motor pathways
Lateral and ventromedial pathways
Pyramidal and extra-pyramidal systems
Supplementary Motor Area (SMA)
important for instituting motor programs
activated prior to voluntary movement
activated during mental rehearsal
Premotor cortex
sensory guided movement
cooperatively with basal ganglia
mirror neurons
Pre-frontal cortex
decisions for actions
long-term planning
Cortex
Globus
Pallidus
Thalamus
and STN
Rubrospinal
tract
Substantia
Nigra
Reticular Formation
Red
Nucleus
(nucleus
ruber)
Cerebellum
Reticulospinal
tract
Cortex
cortex
Thalamus
and STN
Rubrospinal
tract
cerebellum
Substantia
Nigra
Pontine nuclei and
Reticular Formation
Red
Nucleus
(nucleus
ruber)
pontine nuclei
Globus
Pallidus
Reticulospinal
tract
Cerebellum
thalamus
Cortex
cortex
Thalamus
and STN
Rubrospinal
tract
globus
pallidus
Substantia
Nigra
Pontine nuclei and
Reticular Formation
Red
Nucleus
(nucleus
ruber)
striatum
Globus
Pallidus
Reticulospinal
tract
Cerebellum
thalamus
Cortex
cortex
Thalamus
and STN
Rubrospinal
tract
substantia
nigra
Substantia
Nigra
Pontine nuclei and
Reticular Formation
Red
Nucleus
(nucleus
ruber)
striatum
Globus
Pallidus
Reticulospinal
tract
Cerebellum
thalamus
Cortex
Globus
Pallidus
Thalamus
and STN
Rubrospinal
tract
Substantia
Nigra
Reticular Formation
Red
Nucleus
(nucleus
ruber)
degenerate in
Huntington’s disease
Cerebellum
Reticulospinal
tract
Cortex
Globus
Pallidus
Thalamus
and STN
Rubrospinal
tract
Treatments:
1. l-DOPA to compensate
for lost DA projections
Substantia
Nigra
Reticular Formation
Red
Nucleus
(nucleus
ruber)
degenerates in
Parkinson’s disease
Cerebellum
2. Deep brain stimulation
of striato-pallidal
projections
3. Surgical destruction of
cells in medial pallidum
Reticulospinal
tract
Subthalamus pedunculopontine nucleus (PPN)
reticulospinal tractspinal CPGs
ALS: Upper motoneuron disease
Polio: Spinal motoneuron disease
Apraxias: Frontal cortical damage