Neural control &
recruitment
Motor
Units
Germann & Stanfield, fig 10.14
“an a-motoneuron and all the muscle
fibres it innervates”
• Size
– Small motor units
• Provide fine control
• eg: eye (100 x 10 fibre units)
– Medium motor units
• eg: hand
(100 x 300 fibre units)
– Large motor units
• Gross control (strength)
• eg: gastrocnemius
(600 x 2000 fibre units)
• Provide the basis of the:
“All or none principle”
Stimulation of an a-motoneuron
will cause contraction of
every innervated muscle fibre
1
Larger muscles, larger numbers
of motor neurons
Motor axons: size and velocity
• Motor axons vary in diameter (cat, 10-20
μm)
• Motor axons differ in their conduction
velocities (cat, 40-100 m/s)
• Motor axons to slow muscles have lower
conduction velocities and smaller in
diameter
• Large diameter axons have large cell
bodies in the ventral horn of spinal cord
2
Control of motor units
• Denny-Brown (1929) muscles with more slow
units are activated preferentially in tonic
contractions, muscles that have more fast
units activated in when rapid contraction is
needed.
• Henneman (1960s onwards) studied this at
the level of the motor unit
• Motor Unit Recruitment
– The primary mechanism whereby a whole
skeletal muscle can vary force output
Jones, Round & Haan,
Haan, fig 5.3
Henneman’s Size Principle
“Motor Units are recruited
in order of their size
from small low force units
to large high force units”
• If motor unit recruitment
was the only mechanism to
alter force, expect:
– Low forces
• Small stepwise increments
• Consistent with fine control
– Higher forces
• Larger force increments
• Less precise movements
Henneman et al. (1974). Rank
order of motoneurons within
a pool: law of combination . J
Neurophysiol 37: 1338-1349..
3
Wilmore & Costill,
Costill, fig 1.15
Henneman’s Size Principle
• Different
proportion of the
fibre types used
dependent on force
requirements
Henneman Size principle
• Recruitment of motor units is
SO > FOG (Fatigue Resistant) > FG (Fatigable)
ie (I > IIa > IIb)
• Small motor neurones, with small units are the
most easy to activate. These units receive the
most tonic activation
• Sizes of units vary in a muscle. Therefore, the
size principle gives an automatic gradation of
force
4
Motor Unit Characteristics
• Small motor units
– Slow contracting
– Low excitation threshold
(i.e. easily excitable)
– Easily recruited
– Fatigue resistant
– Utilised for prolonged
daily activities
• Posture control, walking
• Large motor units
– Fast contracting
– High excitation threshold
(i.e. less easily excitable)
– Less easily recruited
– Rapidly fatigable
– Utilised for high force
contractions
• sprinting, jumping etc
• Recruitment order is small → large
• Provided by the Henneman Size Principle
Distribution of innervation number across
motorneuron pool comprising 120 motor units
• Cumulative sum of the
number of muscle fibres in
successive motor units
•Only a small number of
the fastest fibres
R.M. Enoka and A.J. Fuglevand 2001.
5
Force production
• The motor unit represents the final common path by
which the CNS sends motor commands to the muscle
(Liddell & Sherrington, 1925)
• Fibres in a single unit are distributed throughout the
muscle and spread the load
• Gradation of force by frequency occurs largely in
fast motor units
• Recruitment is orderly: If the same contraction is
performed several times, motor units are activated in
a relatively fixed order: Denny-Brown & Pennybacker
(1938)
• De-recruitment occurs in a fixed order: the motor
unit recruited is the first to be derecruited
Recruitment of motor units during a
voluntary contraction
Needle
electrodes
• Recruitment of two
motor units (Unit 1 with
a lower recruitment
threshold)
• Average force response
of each motor unit to its
action potential. Unit 1
is weaker and has a
longer time to peak
force
6
Recruitment and the CNS
• The sequence of motor unit recruitment is
determined by spinal mechanisms, not
specified by the brain
• A motor command from the brain does not
contain information related to which
motor units should be activated
• Not possible to activate motor units
selectively by stimulation at brain
• “Upper motorneuron” disorders rarely
cause alterations in recruitment
Neural Factors in adaptation:
increases and decreases
•
•
•
•
Changes in neural drive to muscles
Coordination of muscles
Plasticity in the spinal cord
So, peripheral and central changes:
different levels of the movement
hierarchy can be effected
• Some effects difficult to separate from
muscular changes
7
Neuromuscular stimulation
•Electrical stimulation over skin,
generates a.p. in intramuscular
nerve branches
•Large-diameter fibres more
easily excited by imposed fields
• A peripheral change can occur:
impacts on recruitment order
• Neuromuscular stimulation also
generates action potentials in
sensory receptors. This feedback
reaches primary somatosensory
cortex, and may lead to central
changes
Time (days)
To calf muscles, enough to
produce up to MVC
What happens when homologous muscles
in two limbs are activated concurrently?
• Decline in force during MVCs – A bilateral
deficit (Howard & Enoka 1991)
• E.g. A task with contractions of triceps
brachii muscles with elbow at a right angle
• Maximum force by each arm reduced
• EMG reduced at the extensor of each arm
8
Maximal force and EMG of elbow extensor muscles
(left and right triceps) during maximal voluntary
contractions
Reduction
Reduction
Right Only
Right and Left
Left Only
But, effect of training
• Bi-lateral interactions are modified with
training (Secher, 1975)
• Howard & Enoka (1991) Comparing MVC
during one and 2 limb knee extensor and
found:
• Bi-lateral deficit: Untrained, Cyclists
• Bi-lateral facilitation: Weightlifters
9
Plasticity in the Spinal Cord, increasing strength:
Effects of 12 weeks of training
•Training was rapid
contractions against a
moderate load
Torque
• Changes in torque
development, EMG and
motor unit discharge during
rapid submaximal
contractions
EMG
Increase
Motor Units
Increase
Decrease in strength
•
•
•
•
Ageing
De-nervation
Immobilization
Unloading
10
Jones, Round & Haan,
Haan, fig 14.8
Ageing and decline in strength, motor
neurons and motor units
Rutherford & Jones (1999).
The relationship of muscle
and bone loss and activity
levels with age in women.
Age and Ageing, 21: 286293.
Dennervation changes fibre properties
• If the nerve to a muscle is cut target cells –
the motor fibres – change:
• Dennervation atrophy: decrease in size (3
days +, all fibres)
• Necrosis (several fibres, months)
• Lowered enzyme activity
• Decline in contractile properties
11
Limb immobilization effect (arm)
• Healthy
humans, with arm immobilized in a cast
(Semmler et al 2000)
•Measurements of EMG for 24 h periods before and
during immobilization
- EMG activity of biceps brachii declined by 38% and EMG of
brachioradialis decreased by 29%
• Different studies show different effects, different
relationships with decline in muscle mass, sometimes male
vs female differences
Changes in motor units after
joint immobolization
AFTER:
•Increase in
recruitment
threshold
•Reduction in
force
•Reduction in
firing rate
Each point is a single motor unit
12
Relation between decline in EMG and
reduction in muscle mass?
(rat hindlimb unloading)
3 weeks
S-L= soleus,
long length
S-S= soleus,
short length
S-N= soleus,
neutral length
M-S=Medial gastrocnemius,
short length
Summary: Sites of Neural Adaptations
For strength :
(1) Enhanced output from
supraspinal centres – imagined
contractions
(2) Reduced co-activation of
antagonist muscles
IN= interneuron
(3)Greater activation of agonist and
synergist muscles
(4) Enhancing coupling among
interneurons
(5)Changes in descending drive
reducing bilateral deficit
MN= motor
neuron
(Extensor,
Flexor
(6)Shared input to motor neurons
(7)Greater EMG
(8)Heighted excitability onto
motoneurons
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