Fibers: composed of myofibrils
Myofibrils composed of
protein filaments:
1. Myosin: thick
filaments
2. Actin: thin filaments
Muscle structure:
Actin
Muscle
Muscle fiber
Myofibril
Myosin
Muscle Structure
Sarcomere: the muscle contraciton
unit
Sarcomere Components
1. A Band: area of myosin filaments
2. I Band: area between the “A” bands – both
actin and myosin
3. H Zone: thickened area of myosin
4. Z line: center of the I band
5. Z to Z: equals one sarcomere
Muscle Contraction Components
1. Motor Neurons: cells of the nervous system
– pass impulse from brain or spinal cord
Motor Unit: muscle fiber and motor neuron
2. Sarcolemma: the cell membrane of
muscle fibers
3. Transverse Tubule: deep invaginations,
enfoldings of the sarcolemma
4. Sarcoplasmic Reticulum: network of muscle
contractions tubules that carry the muscle impulse
5. Mitochondria: cellular organelle which
synthesizes adenosine triphosphate (ATP)
6. Tropomyosin: a protein attached to the actin
filament which blocks myosin-binding sites
7. Troponin: a protein complex
located on the tropomyosin
-Receives calcium from the sarcoplasmic
reticulum
8. Myosin Cross Bridge: portion of the myosin
filament which binds to the actin binding site
9. Calcium: ions released from sarcoplasm –
bind to troponin – clear the myosin binding sites
10. Acetylcholine (ACh): the neurotransmitter
released by the motor neurons
Neuromuscular Junction
Sliding Filament Contraction Theory
General Sequence of Events:
-neural response is sent to the sarcoplasmic
reticulum
-thin filament (actin) slides past the thick
filament (myosin)
- “I” band becomes smaller
- Actin/myosin slide back and forth
Sliding Filament Contraction Theory
Detailed Sequence of Events: 8 steps
1. Neural impulse (polarization/depolarization)
from motor neuron moves to the synaptic
knob and releases acetylcholine (ACh)
2. ACh carries the impulse across the synaptic
cleft to the sarcolemma
Sliding Filament Contraction Theory
3. ACh carries the impulse along the
sarcolemma to the transverse tubules. The
ACh enters the transverse tubule to the
sarcoplasmic reticulum.
4. ACh activates the sarcoplasmic reticulum to
release Ca++ (calcium ions)
5. Ca++ binds to the troponin, moving the
tropomyosin off of the binding sites on actin
Sliding Filament Contraction Theory
6. Cross bridge of myosin connects to actin
7. The cross bridge performs a “power stroke”
– which shortens the sarcomere
-ATP  ADP is the energy source
8. Myosin grabs actin and pulls – the muscle
contracts
Relaxation
1. Impulse stops – no ACh
2. ACh in tubules breaks down by acetylcholine
esterase (AChE)
3. Ca++ reabsorbed by sarcoplasmic reticulum
4. Sarcomere lengthens and relaxes
Smooth Muscle:
FX:
1. move food through
intestine
2. adjust the eye for
light
3. move blood through
vessels
4. moves babies from
uterus through birth
canal
Characteristics/Location
-
Muscle in hollow organs (except heart)
Digestive, urinary, circulatory and reproductive
One nucleus per myocyte
No striations/sarcomeres/ “T” tubules
Fusiform shape
lay in sheets at right angles
Alternating contraction/relaxation of sheets
causes elongation – peristalsis
- Ca++ comes from extra-cellular fluid
Cardiac Muscle:
-makes up most of the
heart
-has striations like skeletal
muscle
-myocytes branched and
interconnecting
-each myocyte stimulates
its neighbor – chain
reaction
Cardiac Muscle:
- Large T-tubules admit
more Ca++ for
contraction
- Sino-Atrial node:
bundle of nervous
tissue that stimulates
contraction