Muscle Action, Recruitment,
and Energy Sources
Muscle Activation
► Caused
by message
from brain
► Message
sent along
motor neuron (nerve
cell attached to
muscle)
► M.n.
cleft
ends in synaptic
Synapse
► Synaptic
cleft terminates
in sarcolemma (outside
of muscle cell)
► Releases
acetylcholine,
which causes
sarcolemma to release
Ca+ ions into muscle cell
► Begins
muscle
contraction
Action Potential
► Influx
of Ca+ ions
causes an electrical
current called action
potential
► Once
sarcolemma is
activated, begins
chain reaction along
entire muscle cell
Remember
► Each
muscle is composed of hundreds of
muscle cells stacked into a bundle
► So
far we have only activated ONE muscle
fiber
► When
a single muscle fiber is activated, it is
an “all-or-nothing” response
Muscle recruitment
► If
more force is
needed, nerve will
activate more muscle
fibers in the fascicle
(muscle recruitment)
Another way to provide more force
► Nerve
can also
increase rate of
stimulation, causing
muscle to contract
faster
► Can
reach complete
tetanus (full
contraction with no
relaxation)
Other forms of contraction
► Twitch-
short, weak
signal causes brief
contraction
► Summation-
contraction
rate increases (no
complete relaxation)
► Incomplete
tetanus –
faster rate; contractions
closer together
Complete Tetanus
► Continuous
contraction
with no relaxation
► Evident
exercise
in isometric
► Also
happens as a
result of some diseases
and infections
 Rabies
 C. tetani
In review
► Two
ways to increase force
 Recruit more muscles
 Increase rate of contraction
Energy for Contraction
►3
possibilities
 1) Aerobic respiration
 2)Lactic acid
fermentation
 3)Direct
phosphorylation
(fastest, but only lasts
about 20 seconds)
Direct Phosphorylation
► ADP
encounters
Creatine Phosphate
(CP)
► Takes
phosphate
molecule to form ATP,
leaves creatine
► CP
only present in
muscle cells