Skeletal Muscle
• Classification of muscle fiber types
• Sarcomere’s length and muscle tension
during contraction
Skeletal muscle fibers are
classified
• According to contraction speed and
resistance to fatigue during repeated
stimulation.
• 1. Fast-twitch glycolytic fibers
• 2. Fast-twitch oxidative fibers
• 3. Slow-twitch (oxidative) fibers
Fast-twitch fibers
• Fast speed of contraction
• Fast myosin ATPase isoform
Fast-twitch fibers
• Twitch has a short duration
• Ca++ is quickly removed from the cytosol
during relaxation
• Fast Ca++ ATPase on SR
• Where? Hands (typing, piano), Eyelids
(blinking).
Slow-twitch fibers
• For posture, standing, walking
• Lift heavy loads
• Contractions last a long time
Fast glycolytic fibers
• Fatigue quickly
• Anaerobic glycolysis to generate ATP
• acidosis
Slow-twitch fibers
• Fatigue slowly
• Oxidative phosphorylation for ATP
production
• Blood vessels bring oxygen
• Myoglobin inside the fibers. It has a high
binding affinity of oxygen.
• Small diameter of fibers, short distance for
oxygen diffusion
Fast-twitch glycolytic fibers
• Fewer blood vessels
• Larger muscle fibers
• Quickly run out of oxygen
Fast twitch oxidative
• Have some myoglobin
• Intermediate size
• Use oxidative and glycolytic metabolism
Sarcomere resting length
• The length of a sarcomere in the body
• This is called the optimal length
• A contraction of optimal force occurs when
the sarcomere is at optimal length
The sliding filament theory of contraction predicts optimal length for muscle tension.
Skeletal muscle physiology
• Muscle contraction in the body
– Summation of muscle twitches
– Motor units
– Muscle contraction and loads
• Friday: ch. 14 Cardiavascular Physiology
(After ch. 14, we’ll cover smooth muscle
located at the end of ch. 12)
A twitch is a single contraction/relaxation cycle
The muscle does not relax completely before the second stimulus occurs, resulting
In a more powerful contraction: summation
Stimulus
Stimulus
Stimulus
Motor unit
• One somatic motor unit and the muscle
fibers that it innervates
A muscle may have
many motor units of
many fiber types.
Recruitment of motor unit activity
during muscle activity
• Each motor neuron has its own threshold
potential for firing action potentials
• Each motor unit has its own threshold
• A muscle’s activity varies with the number
of motor units that are recruited
• Recruitment is controlled by the nervous
system
Recruitment
• Weak stimulus activates low threshold
neurons. These regulate slow fatigue
resistant fibers.
• Stronger stimuli  fatigue resistant
oxidative fast twitch fibers
• Very strong stimuli  glycolytic fast twitch
fibers
Isotonic contraction
– any contraction that creates a force and moves a load
Isometric contraction
- Create force without movement
How can a muscle generate force
without changing its length?
• Each muscle elastic elements:
– Tendons
– Intracellular cytoskeletal proteins with elastic
properties
– Contractile proteins themselves can stretch
• These are included in the term: series
elastic elements.