Learning Objectives:
Describe the molecular events of
excitation-contraction coupling
Describe the Sliding Filament
theory and the role of regulatory
proteins in contraction and
relaxation
QUIZ/TEST REVIEW NOTES
SECTION 1 MUSCLE FIBER PHYSIOLOGY
[MUSCLES AND FIBERS]
1. SKELETAL MUSCLE ANATOMY
a. Introduction
1. Three forms of muscle
> Skeletal muscle (striated; attached to bones)
> Cardiac muscle (striated; heart muscle)
> Smooth muscle (internal organs and tubes)
b. Muscle Fiber Anatomy
2. Skeletal muscle is collection of muscle cells called muscle fibers
3. Groups of fibers bundled together into units called muscle fascicles
- Myofibrils are highly organized bundles of contractile and elastic proteins that carry out work of
contraction
- Sarcoplasmic Reticulum warps around each myofibril and contains longitudinal tubules which
release divalent Calcium ions
- Terminal cisternae sequester divalent Calcium ions from the Sarcoplasmic reticulum
- Transverse Tubules (t-tubules) closely associated with terminal cisternae; one t-tube with two
terminal cisternae is known as a triad
ORGANIZATIONAL LEVELS
1. Skeletal Muscle [whole muscle]
2. Muscle Fibers [muscle fascicles/cells containing sarcoplasm/multiple nuclei]
3. Myofibrils [contain SR/mitochondria/glycogen granules]
4. Myofilaments [repeating pattern forms Sarcomeres]
INNERVATION
One ________________ innervates one muscle
One neuron innervates ________________ fibers
One fiber is innervated by ________________
neuron
1
c. Myofibrils are contractile structures of Muscle Fiber
1. Myofibril is composed of several types of proteins
A. Contractile Myosin
a) Motor protein of myofibril
b) 250 myosin molecules joint to create thick filament
c) Myosin heads clustered at ends of filament
d) Central region is bundle of myosin tails
B. Contractile Actin
a) Composes think filament
b) Contains Titin
- Composed 25,000 amino acids
- Stabilizes position of contractile filaments
- Returns stretched muscles to their resting length
C. Regulatory Tropomyosin
a) Found on actin filaments
b) Elongated protein polymer that wraps around actin filament and partially
blocks myosin-binding sites
D. Regulatory Troponin
a) Found on actin filaments
b) Calcium binding protein that controls the position of tropomyosin
2. Cross Bridges
a) Connection of thick and thin filaments of myofibril
b) Form when myosin heads bind loosely to actin in thin filaments
2. CONTRACTION OF SINGLE MUSCLE FIBERS
a. Major steps leading up to skeletal muscle contraction
1. Events at neuromuscular junction [conversion chemical signal into electrical signal]
2. Excitation-contraction coupling
3. Divalent Cation Calcium Signal
4. Contraction-relaxation cycle [either muscle twitch or sliding filament theory]
2
b. STEP 1: Neuromuscular Junction
1. Contains motor end plates (series of folds looking like shallow gutters) on post-synaptic cell
2. Motor end plate contains ACh receptor channels
3. Synapse contains AChE to readily deactivate ACh into acetyl and choline
c. STEP 2-3: Excitation-Contraction Coupling & Divalent Cation Signal
1. Initiation of contraction at neuromuscular junction
2. Latent Period: Time between muscle action potential and the beginning of muscle tension
development; represents the time required for excitation-contraction coupling to take place
3. Twitch: A single contraction-relaxation cycle in a skeletal muscle fiber
4. STEPS
(a) ACh released from somatic motor neuron
(b) ACh initiates action potential in muscle fiber
- Allows Na and K to cross membrane
- Na influx exceeds K efflux
- Depolarizing cell causing end-plate potential EPP (EPP always reach threshold
and initiate muscle AP)
(c) Muscle action potential triggers calcium release from Sarcoplasmic reticulum
- AP conducted into t-tubules opens voltage-gated Na channels
- AP causes Ca2+ release from Sarcoplasmic reticulum; releasing Ca2+ into
cytosol
(d) Calcium combines with troponin and initiates contraction
- Ca2+ in cytosol binds with troponin, which signals tropomyosin to the “on”
position
d. STEP 4: Contraction-Relaxation Cycle
1. Relaxation
(a) Occurs when Sarcoplasmic reticulum pumps Ca2+ back into lumen using Ca2+ ATPase
(b) Ca2+ concentration decrease releases troponin causing tropomyosin to slide back into
myosin biding site causing fiber relaxation
3
2. SLIDING FILAMENT THEORY
A. Steps
1. ATP Binds
- ATP binds to its biding site on myosin head; releasing it from its resting position on
actin filament
2. Myosin Cleaves
- ATPase activity hydrolyzes ATP to ADP and Inorganic phosphate group
3. Myosin Binds
- Myosin head swings over and binds weakly to actin molecule forming cross-bridge;
creating a 90degree angle
4. Power Stroke
- Release of Inorganic Phosphate initiates the power stroke; myosin head rotates on its
hinge pushing the actin filament past it
5. ADP Released
- At end of power stroke the myosin head releases ADP and resumes cross-bridge rigor
state until new ATP binds to myosin
Rigor State: When no ATP or ADP occupies biding site on myosin head
leaving cell stock onto thick filament; period: short in living humans
4
5
6