Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle...

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Motor Systems

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Transcript of Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle...

Page 1: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

Motor Systems

Page 2: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.
Page 3: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

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”

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sarcomere

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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.

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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.

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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.

Page 10: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.
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Co-activation of α-motoneuronsand γ-motoneurons

at restα-motoneuron activation

α-, γ-motoneuron co-activation

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Meissner’s Merkel’s

Pacinian Ruffini’s Free nerve ending

Page 17: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

Monosynaptic stretch reflex

Golgi tendon organ reflex

Page 18: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

somatosensoryafferents

motor afferents

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(Aδ)

(C)

Page 20: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

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)

Page 21: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

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

Page 22: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

Reciprocal inhibition between motorunits of antagonistic muscles

Page 23: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

Crossed extensor reflex

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oscillators = “legos” of movementCPGs = what you can build with legos

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Central Pattern Generators vs. Reflexes

Central Pattern GeneratorsFast

Act as “clocks”Flexible--<100 oscillators necessary

ReflexesRefine timing

Respond to the unexpectedAcquisition of new skills

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Descending motor pathwaysLateral and ventromedial pathways

Pyramidal and extra-pyramidal systems

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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

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Cortex

Cerebellum

Substantia

Nigra

Caudate-

putamen

(stria

tum)

GlobusPallidus

Thalamusand STN

RedNucleus(nucleu

sruber)

Reticulospinal tract

Rubrospinal tract

Page 41: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

Cortex

Cerebellum

Substantia

Nigra

Caudate-

putamen

(stria

tum)

GlobusPallidus

Thalamusand STN

RedNucleus(nucleu

sruber)

Reticulospinal tract

Rubrospinal tract

cortex

pontine nuclei

cerebellum thalamus

Page 42: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

Cortex

Cerebellum

Substantia

Nigra

Caudate-

putamen

(stria

tum)

GlobusPallidus

Thalamusand STN

RedNucleus(nucleu

sruber)

Reticulospinal tract

Rubrospinal tract

cortex

striatum

globuspallidus thalamus

Page 43: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

Cortex

Cerebellum

Substantia

Nigra

Caudate-

putamen

(stria

tum)

GlobusPallidus

Thalamusand STN

RedNucleus(nucleu

sruber)

Reticulospinal tract

Rubrospinal tract

cortex

striatum

substantianigra thalamus

Page 44: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

Cortex

Cerebellum

Substantia

Nigra

Caudate-

putamenGlobu

sPallidus

Thalamusand STN

RedNucleus(nucleu

sruber)

Reticulospinal tract

Rubrospinal tract

degenerate in Huntington’s disease

Page 45: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.
Page 46: Motor Systems. Motor Unit Motoneuron + muscle fibers it innervates Range in size from a few muscle fibers (e.g. extraocular muscles) To hundreds of.

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

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Subthalamus pedunculopontine nucleus (PPN)reticulospinal tractspinal CPGs

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ALS: Upper motoneuron diseasePolio: Spinal motoneuron diseaseApraxias: Frontal cortical damage


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ESS 303 – Biomechanics The Neurological System. Motor Units A motor nerve and ALL the muscle fibers (cells) it innervates All or nothing – force and unit.

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Fig 12.1 P. 327 Each somatic neuron together with all the muscle fibers it innervates. Each muscle fiber receives a single axon terminal from a somatic.

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