Skeletal muscle
microscopically
Skeletal Muscle Fiber
• Long, cylindrical in shape
• Multinucleate
• Larger than normal body cells
• Results from the fusion of many
embryonic cells
The major players
• Sarcolemma – plasma membrane
• Sarcoplasm – like cytoplasm
a. myoglobin – red pigment that stores oxygen
b. Glycosomes – stored glycogen
• Sarcoplasmic reticulum- like the smooth ER
and regulates ionic calcium levels
• T- tubules – connect sarcolemma to fiber
interior
Sliding Filament Theory of Contraction
• Thin filaments slide past thick filaments
• Actin and myosin overlap to a greater
degree
• Cross bridges generate tension
• Occurs in sarcomeres simultaneously
• Muscle fiber shortens
Physiology of Skeletal Muscle Fiber
• Excitation-contraction
coupling
• Propagates an action
potential – electrical
current
Neuromuscular
junction
•Axons travel
to muscle fiber
•Each branch
meets a
muscle fiber
•Located
midway along
the fiber
Action potential
• Polarization – resting sarcolemma – inside
membrane is negative compared to outside
• Depolarization – charge differentiation
changes and the inside become more positive
and the outside more negative due to K+ and
Na+ exchange – Na+ goes in
• Repolarization – K+ goes out
• Refractory period – time before stimulation
can occur again
Excitation Contraction Coupling
• Sequence of events that lead
from action potential to the
sliding myofilaments
• Latent period – between
action potential and beginning
of mechanical activity
So What Happened?
1. Axon of neuron meets with muscle fiber
at neuromuscular junction
2. ACh is released across the synaptic cleft
3. ACh causes the flow of Na+ ions across
the sarcolemma.
4. The cell becomes less negative and
causes a potential voltage difference.
5. This causes an action potential-an electrical current that propagates along the
Sarcolemma
6. Then K+ moves out of the cell and
The charge difference is restored.
7. Eventually the Na+ and K+ channels
Restore the concentrations of Na+ and K+
extracellularly and intracellularly
Polarized – resting state sarcolemma
With more K+ intracellular and more
Na+ extracellularly. Intra is more (-)
Depolarized – less negative inside
Repolarized – returned to more
(-) inside
Refractory period – a muscle cell
cannot be stimulated again until
repolarization is complete
The Units, the Twitch, the
Responses, and the Tone
A motor unit is a motor neuron
and the muscle fibers it supplies
•Some small – fine motor
•Some large – large motor
Twitch
Response of a motor unit
to a single action potential
a. Latent period – no
response seen on the
myogram
b. Contraction – cross
bridges occur
c. Relaxation – Ca+ moves
back to sarcolemma
and muscle returned to
initial length
• Wave summation – when
one twitch is followed by a
2nd twitch
• Incomplete tetanus- more
contraction
• Complete tetanus – the
plateau of the muscle
contracation