Neural control & recruitment
Muscle and Nerve Part II
Lecture 2
Types of neural fibres
  Fibres with the
lowest threshold
have the greatest
conduction velocity
  Large diameter
fibres have greatest
conduction velocity
Motor
Units
“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
Larger muscles, larger numbers
of motor neurons
Topographical organization of motor nuclei
• a.k.a. motor neuron pools
•  Flexor-Extensor rule
–  ventral: extensors
–  dorsal: flexors
•  Proximal-distal rule
–  medial: proximal muscles
–  lateral: distal muscles
F
P
•  Parallel control systems
D
E
Ventral horn
–  proximal: postural
–  distal : manipulative
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
Control of motor units
•  Denny-Brown (1929) muscles with more slow
units are activated preferentially in tonic
contractions, muscles that have more fast
units are activated 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
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’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
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.
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
Twitch & Tetanic Contractions
•  Twitch : single stimulation
•  Tetanus : high-frequency stimulation
–  Complete force summation
–  Frequency dependant
Rate Coding
•  Stimulation frequencies
–  Initial recruitment 8-12 Hz
–  Steady high-force 20-50 Hz
–  Ballistic actions
•  Initial high force generation
•  May use 150 Hz for 2-3 s
•  Maintained at lower Hz
•  Small muscles (panel A)
eg: adductor pollicis (hand)
–  Up to 50% Fmax: motor unit recruitment
–  >50% Fmax: increase F via rate coding
–  Enables fine control
•  Large muscles (panel B)
eg: biceps brachii, quadriceps
–  Motor unit recruitment for 90% Fmax
Motor unit
recruitment
Rate coding
Motor unit
recruitment
Rate coding
Comparing muscles further
First dorsal interosseus (~120 units, hand)
- Small units recruited and fire 9Hz
-  As force increased, units increased firing frequency and
new, larger units recruited and fire at 9Hz. When 40%
MVC, all motor units recruited and force modulation
provided by firing frequency changes up to 40Hz
Deltoid (~1000 units, shoulder)
-  New units were recruited up to 80 % MVC with initial
firing at 13 Hz
-  Rate modulation not showing much involvement, frequency
of units only raised to 25 Hz after initial recruitment
-  MISMATCH of MVC and maximal available force. This is
not seen in small muscles
-  Differences dependent on the size and function of the
muscle
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
What determines contractile
force?
•
•
•
•
Motor unit size (larger motor units, more fibres)
Motor unit fibre type (fast fibres larger)
Number of motor units (spatial summation)
Frequency of stimulation (fusion of tension)
Next Lecture:
•  Length: force length relationship
•  Velocity: force velocity curve
•
Series compliance (tendon & cross-bridge)