Ch. 20 Flashcards
Define striated muscle
Muscle that consists of cells in which the thick myosin and thin actin filaments are arranged in sarcomeres (in vertebrates, skeletal and cardiac muscle)
Name and define the three muscle types
Skeletal - produces locomotory movements or other external movements of the body, attached to the endoskeleton within vertebrates
Smooth - consists of small, spindle-shaped uninucleate cells without any striations (myosin and actin present, but not organized into sarcomeres), found in hollow organs such as the intestine
Cardiac - forms the wall of the heart, consists of striated cells connected by intercalated discs
Define myofibril
Longitudinal component of a striated muscle cell that consists of a series of sarcomeres and extends the length of the cell
Define sacromere
The contractile unit of striated muscle that consists of contractile, regulatory, and cytoskeletal proteins running in series separated by Z discs
Define thin filament
Polymers of G-actin monomers in muscle cells
Define thick filament
Polymers of myosin molecules in muscle cells
Define cross bridges
The head of a myosin molecule interacting with actin molecules to produce muscle contraction
Define excitation-contraction coupling
The linkage of electrical excitation of the cell membrane with contractile activity facilitated by available Ca2+ in the cytoplasm
Define isometric contraction
A contraction in which a muscle does not shorten significantly as it exerts force (tension) against a load it cannot move
Define isotonic contraction
A contraction in which muscle force remains constant as the muscle changes length, rarely occurring in nature
Define intercalated disc
An intercellular contact between adjacent cardiac muscle fibers of vertebrates that contains desmosomes and gap junctions
Define motor unit
A motor neuron and all the muscle fibers it innervates
Name the order of muscle structure from largest to smallest.
- Muscle
- Muscle fiber
- Myofibril
- Sarcomere
- Myofilaments (actin and myosin)
What are Z lines?
Indicators of the start and finish of a single sarcomere
What are I bands?
Contain only thin filaments
What are A bands?
Contain thin and thick filaments
What is the H zone?
Contains only thick filaments
What is the M line?
Splits the H zone in half
What are the three key proteins within thin filaments?
Actin (g-actin), troponin, and tropomyosin
How many binding sites does the myosin cross-bridge contain?
Two; one for ATP and one for actin
Which bands shorten during a contraction?
I and H bands
Define rigor
A state of constant contraction that occurs when there is no ATP available to release myosin from actin
Describe the step at which the cross-bridge cycle is typically found.
ATP has been hydrolyzed, so myosin is loosely bound to actin and waiting for Ca2+ to continue the cycle
What would happen to a muscle without ATP?
Stuck in rigor without ATP releasing cross bridge
What would happen to a muscle without ATPase?
Stuck in relaxed state without ATP hydrolysis
What would happen to a muscle without calcium ions?
Stuck in relaxed state without Ca2+ to initiate power stroke
What does tropomyosin do?
Blocks myosin-binding sites in g-actin until moved by troponin-Ca2+
What does troponin do?
When bound to Ca2+, it shifts tropomyosin to expose myosin-binding sites to initiate the power stroke
________ is released by motor neurons in an action potential.
Acetylcholine
How does initial sarcolemma depolarization occur?
Acetylcholine binds to its receptors, which opens Na+/K+ channels to depolarize the membrane
How is DHPR activated?
DHPR is voltage-sensitive, so depolarization of the sarcolemma continues into the t-tubules to change its conformation
How are RyR calcium channels opened in the SR?
Activated DHPR goes through a conformation change, which allows RyRs to open and release calcium ions into the cytoplasm
Muscle contraction _________________ (uses/produces) a lot of energy.
uses
What metabolic pathways are used to supply ATP for muscle contraction? (3)
- Creatine phosphate to creatine
- Glycolysis
- Oxidative phosphorylation
Define tonic muscles
Postural muscles that can stay “on” for long periods of time, also found in fish, no action potentials
Define a twitch
The mechanical response of a muscle to a single action potential
Name the three types of twitch muscles
- Slow oxidative
- Fast oxidative glycolytic
- Fast glycolytic
Define muscle antagonism
One muscle shortens and an opposed muscle lengthens
Define load
The object muscle exerts force onto
Define tension
The force exerted on a load by muscle
Describe isometric contraction
Elastic components remain the same, and sarcomeres contract
Describe isotonic concentric contraction
Elastic components stretch inward, shortening sarcomeres
Describe isotonic eccentric contraction
Elastic components stretch outward to prevent damage from heavier loads
Describe load scaling
The same amount of force is not going to be used for the same two objects (i.e., 200-pound weight vs phone)
Cardiac muscle cells are separated by what?
intercalated disc
