Skeletal Muscle-Lecture 14 & 15 Exam 3 Flashcards
Skeletal Muscle Characteristics
-Striated
-Attached to bones, controls body movement
-Contract in response to a somatic motor neuron
(voluntary)
Cardiac Muscle Characteristics
-Striated
-Located in the heart, moves blood through the circulatory system
-Contracts spontaneously (involuntary)
-Influenced by ANS
Smooth Muscle Characteristics
-No striations
-In internal organs & tubes, move material within the body
-Controlled by the ANS or spontaneous contraction (involuntary)
Components within a Skeletal Muscle Fiber
-Sarcolemma
-Sarcoplasm
-Transverse Tubules
-Sarcoplasmic Reticulum
-Myofibrils
-Thick (myosin) Filaments
-Thin (actin) Filaments
Sarcolemma
Plasma Membrane
Sarcoplasm
Cytosol of the fiber (cell)
Sarcoplasmic Reticulum
Membranous channels similar to E.R., functions in Ca2+ release
T-Tubules (transverse tubules)
A tubular structure that joins with the sarcolemma & conducts action potentials into the muscle fiber
Myofibrils
-Bundles of contractile proteins (thick & thin filaments)
-Organized into sarcomeres
Thick (myosin) Filaments
-Comprised of many myosin proteins
-Myosin heads are clustered at each end of the filament
-Central region of the filament is a bundle of myosin tails
-Myosin heads have two sites:
-Binding site for actin
-A site that hydrolyzes ATP to ADP + Pi (myosin ATPase)
Thin (myosin) Filaments
-Comprised of actin molecules
-One actin molecule is a globular protein (G-actin)
-Each actin molecule has a binding site for myosin heads
-Multiple G-actin molecules form long filaments (F-actin)
-Two F-actin polymers twist together to create thin filaments
Sarcomere
-Repeating contractile structures
-One segment of a myofibril, from Z-disc to Z-disc
A Band
Dark area of thick filaments, some overlap with thin filaments
I Band
Light area around Z-disk containing actin filaments
H Zone
Area of thick filaments that do not overlap with actin filaments
Z Disk
Border of sarcomere, thin filaments attachment site
M Line
Middle of the sarcomere, thick filament attachment site
The Sliding Filament Theory of Skeletal Muscle Contraction
-When muscles contract, they decrease in length
-Due to the shortening of the sarcomere as a whole, not the shortening of the thick & thin filaments
Consider What Aspects of the Sarcomere (A-band, I-band, H-zone) Increase in Size, Decrease in
size, or Do not change during contraction
-A band does not change
-I band decreases in size
-H Zone decreases in size
Cross-Bridge
Form when myosin heads bind to actin
Sequence of Events of a Skeletal Muscle Contraction/Cross-bridge Cycling
-Rigor state
-Myosin heads are tightly bound to G-actin
-No ATP or ADP is bound to myosin
1. ATP binds to the myosin head, and myosin unbinds from G-actin
2. ATPase from the myosin head causes ATP hydrolysis: ATP -> ADP + Pi The myosin head rotates & weakly binds to the next G-actin molecule
3. The myosin releases the Pi & initiates a power stroke: the myosin head pulls the actin filament
4. Myosin releases ADP at the end of the power stroke & myosin heads tightly bind to G-actin (rigor state)
Muscle Relaxation
-Ca++ in the cytosol is low
-Tropomyosin blocks binding sites on actin
-Crossbridge cycling cannot occur
Initiation of Muscle Contraction
-Ca++ levels increase in cytosol
-Ca++ binds to troponin
-Troponin-Ca++ complex pulls tropomyosin away from actins myosin-binding site
-Myosin binds strongly to actin and completes a power stroke
Muscle Contraction & Relaxation Overview
- Initiation of muscle action potential 2. Excitation-contraction coupling
- Muscle relaxation
- Initiation of Muscle Action Potential
-Somatic motor neuron releases
ACh at the neuromuscular junction
-ACh binds to nicotinic receptors
on the motor end plate
-Net influx of Na+ enters the
muscle fiber and initiates an end
plate potential (EPP)
-EPP’s are always excitable and
depolarize the muscle fiber, generating an action potential
- Excitation-contraction Coupling
-T-tubules carry action potential deep into the muscle fiber
-DHP receptors on the T-tubule sense the depolarization & open RyR Ca2+
channels on the sarcoplasmic reticulum
-Ca2+ is released from the SR into the sarcoplasm
-Ca2+ binds to troponin which moves tropomyosin out of the way to reveal
actin binding sites
-Myosin heads bind to actin
-Myosin head undergoes a power stroke pulling thin (actin) filaments over thick (myosin) filaments
-Fresh ATP allows the myosin heads to detach from actin & crossbridge
cycling repeats if Ca2+ remains attached to troponin
- Relaxation
-When APs stop being produced, Ca2+ is pumped back into the SR by
primary active transport (Ca2+-ATPase)
-Ca2+ no longer binds to troponin & tropomyosin moves to cover the
myosin binding site on actin
-Skeletal muscle relaxes
