Comprehensive Exam Presentation Slides

Synaptic noise and motor unit discharge during voluntary contractions performed by young and old adults Michael Pascoe, M.S.

Outline • Motor unit anatomy • Motor unit physiology • Intrinsic properties • Discharge characteristics • Synaptic noise • Changes with healthy aging

Intrinsic properties

Synaptic input Noise

Motor unit discharge Characteristics

Muscle force

Outline • Motor unit anatomy • Motor unit physiology • Intrinsic properties • Discharge characteristics • Synaptic noise • Changes with healthy aging

Motor unit

Motor unit pool

range: 10-1,500

Innervation number

range: 10-3,000

Outline • 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

Input resistance I (nA)

Outward

+2

Inward

Vm (mV)

∆V = IRin

+1

-80

-70

-50

-40

-1 -2 Hyperpolarization Depolarization

∆V Rin = I

Current (nA)

Rheobase current 8 Rheobase

4 0

Membrane potential (mV)

0 -20 -40

Voltage threshold

-60

Resting membrane potential 0

20

40

60

Time (ms)

80

100

Accommodation 50 mV

5 nA 50 ms

4 nA/ms

44 nA 11 ms

50 ms

0.2 nA/ms

63 nA 306 ms Burke & Nelson 1971

Afterhyperpolarization period Membrane potential (mV)

50

0

Afterhyperpolarization period

-50

0

10 Times (ms)

100

Outline • Motor unit anatomy • Motor unit physiology • Intrinsic properties • Discharge characteristics • Synaptic noise • Changes with healthy aging

Control of muscle force Action potentials

A Force

B

(1) Recruitment

(2) Rate coding

Size principle ∆V = IRin

Interneuron

ISyn

Em

∆V

Rin

Diameter

Diameter

Em

Action potential

0 EPSP Rest

Rin

threshold

∆V

0 EPSP Rest

threshold

Minimal and maximal discharge rates

Force

Van Cutsem et al. 1997

Discharge rate variability

Outline • Motor unit anatomy • Motor unit physiology • Intrinsic properties • Discharge characteristics • Synaptic noise • Changes with healthy aging

Synaptic noise Input

ISI Motor neuron

Axon hillock

Vt Vm

Outline • Motor unit anatomy • Motor unit physiology • Intrinsic properties • Discharge characteristics • Synaptic noise • Changes with healthy aging

Healthy Aging

" Motor Unit

" Muscle Control

# Motor Performance

! Disability & Loss of Independence

Outline • Changes with healthy aging • Motor unit anatomy • Motor unit physiology • Intrinsic properties • Discharge characteristics

Limb motor neurons in thousands

Motor neuron number decreases with age 70 60 50 40 30 20 10 0 0

20

40

60

Age (years)

80

100

Tomlinson & Irving 1977

Motor unit remodeling

Motor unit remodeling

Motor unit remodeling Fewer motor units Larger innervation number

Innervation number increases with age Young Adults

EMG Amplitude (μV)

2,000

Old Adults

1,600 1,200 800 400 0 0

10

20

30

40

Recruitment Threshold (% Max force)

50

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

Summary • 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 (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