Physiology of the peripheral motor system of...Physiology of the peripheral motor system Timothy J....
Transcript of Physiology of the peripheral motor system of...Physiology of the peripheral motor system Timothy J....
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Physiology of the peripheralmotor system
Timothy J. Doherty, MD, PhD, FRCPCDepts. of Clinical Neurological Sciences and Physical
Medicine and Rehabilitation
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Objectives
Review the basic anatomy of the peripheralnervous system
To understand the concept of the MU and knowbasic MU properties
To know Henneman’s size principle of MUrecruitment
To understand how the MU produces force To appreciate the impact of disease on the
motor system
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Anatomy of PNS
1. Somatic Nervous System – motor fibersto skeletal muscle, sensory fibers fromskin, viscera, muscle and tendonreceptors
2. Autonomic Nervous System – generallytravel with nerves or vessels and controlsweating and blood flow to skin andmuscle
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Anatomy of PNS
Dorsal rootletsDorsal root
Ventral rootletsDorsal root ganglion
Dorsal ramusVentral ramus
Ventral root
Gray and Whiterami communicantes
AnteriorHorn Cells
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Classes of PNS Efferent Fibers
A – alpha 12 – 20 um 65 – 120 m/s Extrafusal fibres
A – gamma 2 – 10 um 10 – 50 m/s Intrafusal fibres
B 1- 5 um 4 – 26 m/s Presynaptic auton.C 0.2 – 0.5 um 0.2 – 2.0 m/s Postsynaptic auton.
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Muscle Afferents
Ia afferents – nuclear bag and nuclear chain endings detect length and rate of change in length
Ib – golgi tendon organs, detect muscle tension II – detect muscle length, little rate sensitivity
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Classes of PNS AfferentsΙa 12 – 22 um 65 – 130 m/s muscle length and rate
of change in length
Ib 12 – 22 um 65 – 130 m/s muscle tension
II 5 – 15 um 20 – 90 m/s muscle length
III 2 – 10 um 12 – 45 m/s touch, pain, temperature(A-delta) free nerve endings
IV 0.2 – 1.5 um 0.2 – 2.0 m/s small pain fibers, muscle(C) pain, visceral receptors
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Normal Nerve
Large myelinatedaxons
Small Myelinatedaxons
Unmyelinated axons
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Large myelinatedAxon
Normal nerve
Small unmyelinatedAxons
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Motor system schematic
Upper MNCorticalNeuron &Projections(PyramidalTract)
Lower MN
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A little history
Sir Charles ScottSherrington
Described concept ofMU - reflex work oncats
“Final commonpathway” - 1929
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The Motor Unit
Anterior horn cell in ventralhorn of cord - lamina IX
Ventral Root
Myelinated Axon
Neuromuscular Junction
Muscle Fibers
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MU Organization
MUs arranged in columns
All muscles receiveinnervation from multiplecolumns (levels or“myotomes”)
Quads: L2, L3, L4Biceps: C5, C6TA: L4, L5
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The MU Territory
8 – 10 mm in human bicepsfrom EMG studies
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Glycogen depletion technique
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NormalMyosin ATPase
Fibre Type Grouping
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MU Numbers & Innervation Ratios
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Functional Organization of MUs
MUs are also organized into functionalgroups within a given muscle
E.g. in the biceps supination MUs aregrouped, and flexor MUs are grouped
This allows for more efficient forcegeneration and smoother motor control
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MU Types
MUs are often classified into differenttypes
Classification system is dependant onwhether physiologic, histochemical, orbiochemical features are measured
In general MUs are either slow and fatigueresistant or vice versa
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Motor Unit Types
I IIA IIBS FR FFSO FOG FG
Twitch CT Slow Fast FastTwitch Force Small Intermediate HighFatigue Low Low HighMyoglobin High High LowCapillary Supply High High LowMitochondria Many Many FewGlycogen Low High HighOxidative Enzymes High High LowGlycolytic Enzymes Low High HighMN cell body Small Moderate LargeAHP Long Moderate Short
Marathon 1500m Max Jump100 m
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MU Types
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MU Recruitment - Size PrincipleHenneman 1957
Henneman
• Excitatory Input (IA afferents)• Size of intracellular spike recording from motor neuron• Axonal CVs
Mainly in unanaesthetizeddecerebrate cats
Denny-Brown 1930’s
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Size Principle
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Size Principle
V = I x R (Ohm’s law)
Voltage of EPSP = current x resistance
The smaller the neuron, the larger the resistance due tothe increased channel density
Excitation
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Size Principle - Human
MUs with smallertwitch tensions andslower contraction timesare recruited beforelarger, faster MUs
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Rate Coding
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MU Firing Frequencies
Onset 5-7 Hz Min. sustained 8 – 10 Hz Maximum 20 – 40 Hz
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Recruitmet patterns of MUs
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Recruitment - decruitment
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Overall force production
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Is the recruitment order fixed?
Are ST units always recruited before FT? Cutaneous vibratory stimuli can lower the
threshold of FT MUs and raise the thresholdof ST MUs (Garnett and Stephens, 1980)
Varying the task changes the relativerecruitment threshold of MUs in a givenmuscle - eg. biceps supination vx flexion
Selective activation of FT MUs has beenshown in the cat, not consistently in humans
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Motor Unit Injury
Motor neuron ALS
Axon Trauma
Myelin Compression injury
Neuromuscular junction Myasthenia
Muscle Muscular dystrophy
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Motor Nerve Injury Neurapraxia
focal myelin injury conduction block
Axonotmesis axonal injury conduction block denervation
Neurotmesis injury to axon and
supportingconnective tissue
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Electrodiagnosis - Nerve ConductionStudies
M-potential sizeMotor nerve CV
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Motor nerve conduction studies
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Sensory Nerve CS
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Electrodiagnosis in nerve injury
Normal
Focal Demyelination
Generalized myelin injury
Axonal loss
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Conduction block
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Needle EMG
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Needle EMG
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Anterior Horn Cell or Axonal Injury
With complete nerve resection electrically inexcitablein 3 - 5 days
Needle EMG will show spontaneous “Fibrillations” and“Positive Sharp Waves” - Indicates denervation
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MU Remodeling
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MU Remodeling and the MUP
18 months
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MU Remodeling
Strength preserveddespite a decrease in
MU number
MU Remodeling
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Post-polio syndrome