Motor units have an “all-ornone” response
All of the fibers innervated by the
motor neuron contract
Or none of them do
1974 Hennenman’s Size
Principle
Smaller motor units are recruited first
The frequency of motor unit recruitment
(use/firing) is directly related to the size
and ease of triggering an action potential
in the soma (neuron cell body)
smaller cell bodies (slow motor units) will
be recruited first, and overall, most
frequently
Control of Movement
CNS: brain and spinal cord
PNS: all the nerves extending from the
brain and spinal cord
afferent nerves: direction of AP is toward
the spinal cord, usually involves sensory
information
efferent nerves: direction of AP is away
from the spinal cord, usually involves
motor information
reflexes are “all-or-none” contractions,
monosynaptic
Resting membrane potential:
~70 mV
EPSP: excitatory post-synaptic
potential
release of ACH increases post-synaptic
permeability to Na+, potential rises but
may not reach depolarizing threshold
level
IPSP: inhibitory post-synaptic
potential
another neurotransmitter is released which
decreases the Na+ permeability, reducing
the chance for potential reaching the
threshold
Activity in neuron within CNS
Depends on the sum total of EPSPs and
IPSPs
Occurring in a narrow range of time and
space
Muscle Spindles
Connective tissue capsules in the shape of
footballs that are filled with lymph and
specialized fibers, implanted between
muscle fibers
Afferent and efferent innervations
Provide information about absolute length
of muscle and rate of change in length of
the fiber
When muscle is contracted, activation of
the  motor neurons occurs, these efferent
-neurons cause contraction of the muscle
fibers within the muscle spindle
this “takes up the slack” within the fibers of
the capsule, allowing it to respond to
further stretch
Golgi Tendon Organs (GTOs):
respond to tension
exert inhibitory effects on the agonist and
facilitate the effects on the antagonist
muscles
is possible to disinhibit GTOs (minimizing
the effectiveness of the GTOs) with
training
allows athlete to push the limits of the
tissue
wrist wrestling: fractures and ruptures
muscles and tendons occur
Myoplasticity
Capacity of skeletal muscle for adaptive
change
Due to training, nutrition, endocrine milieu
Hypertrophy
Increase in fiber size
Hyperplasia
Increase in fiber number
Gender differences
men typically have greater mass crosssectional area than women
men are able to generate more absolute
strength than women
not true when strength is expressed
Str/BW or Str/FFW, gender differences
disappear
when examined for the ability to generate
force and power (normalized for crosssectional area) there are no gender
differences