Synaptic noise and motor unit discharge during voluntary contractions

performed by young and old adults

Michael Pascoe, M.S.

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

• Synaptic noise

• Changes with healthy aging

Outline

Motor unit discharge

Intrinsic properties

Synaptic input

Muscle force

Characteristics

Noise

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

• Synaptic noise

• Changes with healthy aging

Outline

Motor unit

Motor unit pool

range: 10-1,500

Innervation number

range: 10-3,000

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

• Synaptic noise

• Changes with healthy aging

Outline

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

• Synaptic noise

• Changes with healthy aging

Outline

Input resistanceI (nA)

+2

+1

-80 -70 -50 -40

-1

-2

Vm (mV)

Hyperpolarization Depolarization

Inw

ard

Out

war

d

∆V = IRin

Rin = I∆V

Rheobase current

Time (ms)

0 20 40 60 80 100

Voltage threshold

Mem

bran

e po

tent

ial (

mV

)

0

-20

-40

-60Resting membrane

potential

0

4

8

Cur

rent

(nA

)Rheobase

Accommodation

Burke & Nelson 1971

50 mV

5 nA

50 ms 50 ms

4 nA/ms44 nA11 ms

0.2 nA/ms63 nA

306 ms

Afterhyperpolarization period

50

0

-50Mem

bran

e po

tent

ial (

mV)

Afterhyperpolarizationperiod

Times (ms)0 10 100

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

• Synaptic noise

• Changes with healthy aging

Outline

Control of muscle force

(1) Recruitment

Action potentials

ForceA

B

(2) Rate coding

Size principle

∆V = IRin

EPSP thresholdRest

0

Em

∆V

Interneuron

ISyn

Diameter

Rin

Diameter

Rin

Em

0

Rest

EPSP

Actionpotential

threshold∆V

Minimal and maximal discharge rates

Force

Van Cutsem et al. 1997

Discharge rate variability

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

• Synaptic noise

• Changes with healthy aging

Outline

Synaptic noise

Motor neuron

Axon hillock

InputISI

Vt

Vm

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

• Synaptic noise

• Changes with healthy aging

Outline

Healthy Aging

! Disability & Loss of

Independence

" Motor Unit

# Motor Performance

" Muscle Control

Outline

• Changes with healthy aging

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

Motor neuron number decreases with age

0

10

20

30

40

50

60

70

0 20 40 60 80 100

Age (years)

Lim

b m

otor

neu

rons

in t

hous

ands

Tomlinson & Irving 1977

Motor unit remodeling

Motor unit remodeling

Motor unit remodeling

Fewer motor unitsLarger innervation number

Innervation number increases with age

0

400

800

1,200

1,600

2,000

0 10 20 30 40 50

Young Adults Old Adults

EMG

Am

plitu

de (μV

)

Recruitment Threshold (% Max force)Masakado et al. 1994

Outline

• Changes with healthy aging

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

Input resistance increases with age

Study Young Old ∆

Chase et al. 1985 1.0 ± 0.6 1.6 ± 1.0 + 37%

Morales et al. 1987 1.0 ± 0.6 1.4 ± 0.8 + 29%

Kalmar et al. 2008 1.5 ± 0.7 2.3 ± 1.3 + 50%

values given in MΩ

Rheobase current decreases with age

Study Young Old ∆

Morales et al. 1987 13.7 ± 8.9 11.0 ± 7.7 - 20%

Kalmar et al. 2008 11.2 ± 5.0 8.04 ± 3.9 - 29%

values given in nA

Afterhyperpolarization period

Study Model ∆ w/ Age

Morales et al. 1987 Cat lumbar ↔

Engelhardt et al. 1989 Cat lumbar ↑

Cameron et al. 1991 Cat phrenic ↓

Kalmar et al. 2008 Rat lumbar ↑

Piotrkiewicz et al. 2007 Human biceps brachii ↑

Outline

• Changes with healthy aging

• Motor unit anatomy

• Motor unit physiology

• Intrinsic properties

• Discharge characteristics

Recruitment and rate coding with age

• Greater proportion of lower recruitment threshold motor units

• Upper limit is reduced

• (90% in young to 64% in old)

• Size principle still holds

Klass et al. 2002, 2008

Recruitment and rate coding with age

Muscle force (%Max)Barry et al. 2007

Discharge rate variability

• Greater in old adults

• Similar in young and old

• When individual motor units are tracked there are no differences

• These studies used brief contractions and perhaps differences can be seen during longer duration contractions

• Movement is produced by varying the number of active motor units and the rate at which they discharge action potentials

• Features of motor unit anatomy, physiology, and synaptic noise influence the output of motor units

Summary

Summary (cont’d)• With healthy aging there are reductions in the

performance of the motor system that imply changes in motor units

• Anatomy: reduced number of motor units, and an increased innervation number

• Intrinsic properties: increased input resistance and decreased rheobase current

• Discharge characteristics: greater proportion of lower threshold motor units, a reduced upper limit of recruitment, and reduced maximal discharge rates

Conclusions

• Changes in motor unit properties are associated with reductions in muscle force control

• Reduced ability to control muscle force can lead to an increased disability and loss of independence

Thank you