motor units

the motor unit

• motor neuron and muscle fibres it innervates

  • basic functional unit of motor control

• two kinds of somatic MN

  • alpha MN - extrafusal fibres

    • contractile units

  • gamma MN - intrafusal fibres

    • dynamic MN - activate dynamic bag or bag 1

    • static MN - activate both static bag or bag 2 and chain muscle fibres

• motor units come in different sizes and types

  • small motor units - small # of fibres per MN

    • fine movements eg. eye muscles

      • more precision

  • large motor units - large # of fibres per MN

    • gross movements eg. leg muscles

• innervation ratio = number for fibres per MN

muscle spindles

• static motor neuron

  • innervated by everything

• only dynamic gamma motor neuron innervated dynamic bag fibres

  • can test spasticity

• muscle spindle increases stimulus to contract and stretch muscle to shorten muscle spindle at same rate as muscle shortening

• alpha gamma co-acitvation

  • as muscle shortens so does spindle

  • increases overall force production

    • gamma motor neuron provides expected stimulus to contract

      • dynamic - magnitude and rate

      • static - magnitude of stretch

motor units as we age

• innervation ration

  • under 10 in muscle controlling eye movements

  • over 1000 in large muscle participating in postural control

• with age, number of motor neurons decreases

  • process of re-innervation takes place

  • leads to an increase in size of individual motor units

  • corresponding increase in innervation ration

    • as we age muscle wasting and neuron degeneration occurs

      • one neuron for many muscle fibres

      • more gross motor movements and less fine motor units

physiological classification of muscle fibre types

• type of training helps to convert fibres

• can be classified by

  • myosin heavy chain proteins

  • energy source

  • speed of contraction

  • fatigue resistance

    • can be adaptable b/w types as needed

• type 1 - small and weak

• type IIa - purely glycolytic

  • 2nd to convert

• type IIx

  • either oxygen of glucose

    • convert between for primary source of energy

• Type IIb fibres

  • unlikely to convert

    • strong and fast

fibres within a MU

• same physiological type

  • similar metabolic and contractile properties

    • eg. fast or slow twitch

• MN species muscle fibre type

  • transformation

  • local contractile events will also determine fiber properties

  • heredity is major determinant of fiber distribution

• adaptations

  • training, spaceflight, cross-innervation, immobilization

  • neuromodulation?

Motor neuron size in different fibre types

• large muscle fibre increases motor units

• type II (a,x,b) alpha motor neurons

  • large cell bodies, large diameter axons

  • innervate many large fibres

  • conduct action potentials at high velocities

  • more capacity for larger contraction d/t conduction velocity

    • more force produced

      • less need to regulate

• type I alpha motor neurons

  • smaller cell bodies nad axons

  • innervate fewer fibres

  • slower conduction velocities

• denervation

  • nerve atrophied no longer function

    • now innervated by larger ratio

    • if absorbed by different type will act like same type

    • demyelination occurs decreased signal conductance

muscle force activation

• muscle force varied by

  1. recruitment - changes number of active MU

     • type 1 → type 2a → type 2x → type 2b

       • increase size

  2. rate coding - change firing rate of different MU

     1. temporal summation

• force is increased by adding more active MUs and by increasing the firing rate of active units

  • firing rates can range dorm 10-60Hz

    • typically 80% activation

• orderly recruitment of the 3 basic MU types

  • type 1

    • slow oxidative; slow twitch

  • type IIa

    • fast oxidative-glycolytic, fast twitch

  • type IIx/b

    • fast glycolytic, fast twitch

      • Henneman’s size principle

motor unit recruitment

• controlled by the nervous system

  • stereotyped!

    • not all myosin heads contract at the same time

      • consistent and stable force for conduction

      • stable resistance to fatigue

• stimulus or command to activate a pool of MUs activates them in a specific order

  • smallest units recruited first

  • with stronger stimulus or more forceful contraction

• fibre type affects fatigue profile of the MUs and thus the whole muscle

  • asynchronous firing of MUs avoids fatigue

• EMG should increase and decrease force w/ fatigue

  • only fatigue resistant fibres available

• takes 6-8 contractions to fatigue type 2 muscle fibres

  • increased neural drive to everything available

• More active motor unites = more muscle fibres

• asynchronous firing allows for more consistent

  • fatigue reisitant force

    • synchronous is stronger but less reliable

• force-frequency relation of muscle

force-frequency

• fiber type affects force-frequency relationship

• fast twitch produce more force at all firing rates

• slow twitch produce a greater percentage of their max force at all firing rates

Henneman’s size principle

• recruitment related to basic biophysical principle

  • current - neural drive

  • resistance - resistance to flow

    • permeability and diameter

• regardless of neuron size same abosolute change in voltage required to reach threshold

  • resistance of small neuron > large neuron

  • applies to intracellular stimulation

• need to effect a threshold change in voltage in MN

  • during contraction synaptic drive to MU pool is the same for all units

    • ie flow of current is the same

• some synaptic currents Brin small diameter MN to threshold for AP generation while sub threshold in leader neuron

motor unit recruitment

• assuming constant synaptic drive to MU pool

  • MU will be recruited from smallest to largest

• increase synaptic drive increased maximum motor unit recruits next fibre size as they reach max activation threshold

  • smooth recruitment to better regulates force contraction

rheobase

• minimum current required to recruit and maintain firing of a MU

  • must be above value to maintain contraction

  • neural synaptic drive to contract

rate coding and recruitment during ramp contraction

• instantaneous firing rate

• firing rates increased

  • once recruited will maintain

• larger units are recruited

MU’s and recruitment threshold relationship

• MU thresholds often determined by force recruitment threshold

  • D has more small motor units

    • more dexterity

  • ND is stronger with more high threshold motor units

• more low thresold (type 1) than high threshold units in dominant hand

  • usage of a muscle can alter this

    • eg. dominant (D) and non-dominant (ND) had

      • more adaptability of force production in D but less ability to produce force

        • similar effect shin down

• not in all populations - this is an untrained group

• recruitment thresholds, firing rates and force fluctuations are different between the hands

violations of size principle

• electrical stimulation

  • extracellular stimulation reverses order

    • stimulation of whole nerve trunk

    • relies of large resistant of current with electrical stimulation resistant is not limiting factors as we try to shove as much as possible down

• cat “paw-shake” response

  • high threshold units preferentially recruited for maximal velocity rpepertive, cyclic movements

  • selective innervation of type 2 and inhibition of type 1 to move really quickly

    • used in many reflex responses - localized response

• eccentric contractions

  • some evidence that during rapid eccentric contraction larger MUs are preferentially activated

    • recently have several studies rejecting this theory

  • mechanism: injury prevention - protective mechanism

  • there is reverse activation of muscle

    • all neurons depolarize at same time but type 2 contract 1st as neurons gets there 1st

    • largest fibres contract 1st due to conduction velocity

MU recruitment with electrical stimulation

• larger cells recruited 1st

• unlike intracellular stimulation, current flow ≠ in 2 different sized neurons

  • current flow directly related ot neuronal diameter

• larger cell have increased I flow so larger change in V occurs

  • the effect od R is less important than current flow

• target cell is thus excited before the smaller cell

  • recruitment reversed compared to synaptic activation

alterations in control of MU firing

• MU synchronization

• MU firing frequency

• MU doublet firing

• fatigue and “muscle wisdom”

  • can change contractile properties by taking advantage of muscle properties

motor unit synchronization

• MUs fire asynchronously

  • smooth contraction

  • less fatigue

• Can have MU synchronization

  • at high force levels, in fatigue, possibly after training, after spinal cord injury

    • neural infuse tends ot stimulate everything

  • PRO: synchronized discharge leads to higher force contraction more quickly

  • CON: fatigue and fine motor control is compromised

• can be measured with single MU records and cross-correlograms

• temporal patter of firing rate for MU with respect to another

  • MU 2 vs. MU 1

  • cross-correlation histogram

    • peak at time zero indicates synchorny

    • can be narrow or broad peak

      • different mechanisms

mechanisms of synchronization

• common presynaptic inputs

  • either to MN themselves (short-term) or to INs before the MN (broad peak)

    • narrow peak still involved with some coordination but broader more so just increase force

    • direct synapse with 2+ motor neurons

  a. inputs to spinal (alpha) motor neurons

  • synergist and common pre-synaptic input to distinct pool

  b. distribution of common inputs to spinal motor neurons

strength training increases synchronization

• specialized training leads ot increased synchrony and more synchronous dominant hand

  • in specifically trained populations we observed different patterns

• more control/confidence of D hand is used more effectively

  • higher training stimulus

• oringialy found that “weight lifters” has more control than untrained controls

• if consistently producing larger force body needs to find efficient way to continue

• 6 weeks ST in finger muscles

  • increased synchrony

  • persistent 6 weeks later

  • no effect on other hand

    • even in D vs ND same effect in trained hand

specificity of synchrony

• chronic, habitual activity changes MU synchronization

  • D vs ND hands

• musician: synchrony lower in both

• weightlifters: both are higher

• untrained: lowest in D

  • fine movement required

    • want asynchronous firing for increased control

motor unit firing rate

• distinct measurable change in firing rate of 1.5x

  • increase neural drive to make muscle work higher

• max MU discharge ration 15%/49% higher for young/adults respectively

• no change in discharge rates during submit contractions

doublet firing

• 30.2% increase in strength

• doublet discharges increased from 5.2% to 32.7% after training

  • send multiple action potentials close in time

    • more often in facial and neck muscles

• earlier motor unit activation

• enhanced maximal firing rate

  • doublet firing at onset of contraction to increase control

• doublet

  • depolarization in relative refractory period

synchrony and motor control

• “steadiness” of force is decreased by MU synchronization

  • increase standard deviation

• may be functionally useful

  • loose stability but gain punch of force

• increase max firing rate at decreased precision and metabolically expensive

distonia

• pathological outcome of synchronization

  • common in musicians

• increase muscle co-acitvation that impairs coordination

• “smearing in somatosensory cortex

• increase synchronous discharge of MU

  • impairs movement

  • may be able to be retained

fatigue

• causes wide spread changes altering metabolic properties, tissue changes, contractile elements etc.

  • systemic

    • increase fatigue causes decreased ability to produce force

      • strongest fibres are fibres that fatigue first

      • lower intensity - longer we can resist fatigue

• acute impairment of performance due to physical activity

• quantified as a decrease in maximal force that a muscle can exert

  • voluntary, involuntary, maximal and submaximal

    • neuron looses resources as SK re-sequesters Ca

MU discharge rates

• typically reduced through a fatiguing contraction

• increased variability and pattern of discharge

neuromuscular activation during fatigue

• M-wave amplitudes decline during fatigue

  • at max all MU activated - will all depolarize

  • rapidly recover - within 10min completely recovered

    • instead of increased stimulation we got to vialational fatigue

      • participant is over it

  • if Ca consistently dumps into muscle it will never replenish

muscle wisdom

• interaction between muscle-level and neural properties

  • muscle depolarizes at same level during refractory period

    • catch same amount of AP

• during fatigue

  • discharge of MU is relied

  • contractile properties of muscle are changed

    • slower contraction, decreased rate of relaxation

• the ability of muscle to reduce discharge rate of its motor neurons to match the change in reception of its relaxation rate

  • more force for same discharge rate in non fatigue state

• increase summation and more economical contraction

  • shift in force-frequency relation

• the same amount of force can be generated with a reduced frequency of activation

  • doesn’t occur in all muscles in all people

    • useful for perchance under fatigue

      • no muscle wisdom in fast twitch fibres

  • often see in postural muscles

    • ability to run long distances - evolutionary development

  • if expressible will occur each time

1. intrinsic membrane properties

   • same synaptic input results in fewer APs

2. increased feedback

   • from group III and IV afferents (CV and V reflex responses) and disfacilaition of Ia afferents

3. reduction in central output

   • descending command to MN is rescued in fatigue

• relative contribution is task-dependent

  • multifactorial