CH 6: Muscular System
graded responses
- Muscle fiber contraction is "all-or-none," meaning it will contract to its fullest when stimulated adequately - Within a whole skeletal muscle, not all fibers may be stimulated during the same interval - Different combinations of muscle fiber contractions may give differing responses - Graded responses—different degrees of skeletal muscle shortening
The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge?
A myosin head bound to actin As soon as the activated myosin head forms a cross bridge with actin, the power stroke begins.
only energy source that can be used to directly power muscle contraction stored in muscle fibers in small amounts that are quickly used up after this initial time, other pathways must be utilized to produce ATP
ATP
After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment?
ATP binds to the myosin head. The binding of ATP to the myosin head weakens the bond between myosin and actin, forcing the myosin head to detach. ATP also provides the energy for the next power stroke.
neurotransmitter that stimulates skeletal muscle contraction.
Acetylcholine
a type of chemically-gated ion channel located on the junctional folds of the muscle fiber.
Acetylcholine receptor
enzyme located in the synaptic cleft that breaks down acetylcholine.
Acetylcholinesterase
Thin filaments
Actin
What causes the release of calcium ions into the sarcoplasm from the terminal cisterns?
An action potential traveling along the t tubule In response to an action potential traveling along the t tubule, voltage-sensitive proteins change shape, opening calcium channels in the adjacent terminal cisterns of the sarcoplasmic reticulum.
- sheetlike structures - Attach muscles indirectly to bones, cartilages, or connective tissue coverings
Aponeuroses
Action potential arrives at axon terminal Calcium ions enter the axon terminal Synaptic vesicles fuse to membrane of axon terminal Acetylcholine is released into the synaptic cleft Acetylcholine binds to its receptors on the junctional folds Junctional folds become depolarized Action potential is initiated on the sarcolemma
Arrange the sequence of events at the NMJ from first to last.
graded responses can be produced in two ways
By changing the frequency of muscle stimulation By changing the number of muscle cells being stimulated at one time
a type of voltage-gated ion channel located on the axon terminal.
Calcium channel
what causes filaments to slide?
Calcium ions (Ca2+) bind regulatory proteins on thin filaments and expose myosin-binding sites, allowing the myosin heads on the thick filaments to attach Each cross bridge pivots, causing the thin filaments to slide toward the center of the sarcomere Contraction occurs, and the cell shortens During a contraction, a cross bridge attaches and detaches several times ATP provides the energy for the sliding process, which continues as long as calcium ions are present
Which of the following is responsible for muscle relaxation?
Calcium ions are removed from the sarcoplasm by active transport. Removal of calcium ions lowers the calcium ion concentration below the threshold for muscle contraction.
Calcium ions couple excitation of a skeletal muscle fiber to contraction of the fiber. Where are calcium ions stored within the fiber?
Calcium ions are stored in the sarcoplasmic reticulum. Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle fibers. The sarcoplasmic reticulum is very elaborate in skeletal muscle fibers, allowing for significant storage of calcium ions.
What specific event triggers the uncovering of the myosin binding site on actin?
Calcium ions bind to troponin and change its shape. The shape change caused by the binding of calcium to troponin shifts tropomyosin away from the myosin binding sites on actin.
Walls of the heart Branching chains of cells; one nucleus, striations; intercalated discs Endomysium attached to the fibrous skeleton of the heart INVOLUNTARY
Cardiac muscle
When does cross bridge cycling end?
Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin. The sarcoplasmic reticulum contains Ca2+-ATPases that actively transport Ca2+ into the SR. Without Ca2+, troponin returns to its resting shape, and tropomyosin glides over and covers the myosin binding sites on actin.
Length of I band Length of H zone Distance between Z discs
Decreases in length or distance
What is the type of chemical reaction used to rebuild ADP into ATP?
Dehydration synthesis
Rank the sequence of events in excitation-contraction coupling from first to last. Do not overlap any events.
Depolarization of the junctional folds triggers an action potential Action potential travels along the sarcolemma Action potential along the T tubules opens calcium channels Calcium ions flood the sarcoplasm Calcium ions bind to troponin Troponin changes shape Tropomyosin moves off of actin binding sites Myosin heads form cross bridges with actin
muscle cells store creatine phosphate, a high energy moluecule after ATP is depleted, ADP remains
Direct phosphorylation
Encloses a single muscle fiber
Endomysium
Which of the following is NOT a role of ATP in muscle contraction?
Exposing myosin binding sites on actin Exposure of myosin binding sites does not utilize ATP. During excitation-contraction coupling, calcium ions released from the terminal cisterns of the sarcoplasmic reticulum flood into the sarcoplasm. Calcium ions bind to troponin, which changes its conformation to slide tropomyosin away from the myosin binding sites. Therefore, calcium ions, not ATP, are responsible for exposure of the myosin binding sites on actin.
Which of the following processes produces molecules of ATP and has two pyruvic acid molecules as end products?
Glycolysis
EGTA is a substance that binds calcium ions. Imagine an experimental setup with a motor neuron and a muscle fiber. Stimulation of the motor neuron causes contraction of the muscle fiber through activity at the neuromuscular junction and excitation-contraction coupling. Now, inject the muscle fiber with EGTA. Which of the following effects would EGTA have on excitation-contraction coupling after the neuron releases acetylcholine at the neuromuscular junction?
It would prevent myosin from forming cross bridges with actin.
Receives stimulus from the motor neuron
Junctional folds
Which of the following processes produces 36 ATP?
Krebs cycle and oxidative phosphorylation
What structure most directly stimulates a skeletal muscle fiber to contract?
Motor neuron
Long organelles inside muscle cell Light (I) bands and dark (A) bands give the muscle its striated appearance
Myofibrils
Thick filaments
Myosin
Rank the sequence of cross bridge cycling, starting with the myosin-binding sites being exposed and ending with relaxation due to cross bridge cycling ending.
Myosin head forms cross bridge with actin Power stroke moves thin filament ATP attaches to myosin head Cross bridges detach from actin Myosin head is re-energized Calcium ions pumped into the sarcoplasmic reticulum Calcium ion concentration decreases below the threshold for binding to troponin Myosin binding sites covered
Na+ diffusion Calcium channels open Less negative charges inside sarcolemma Chemically-gated channels open ACh in synaptic cleft
Neuromuscular Junction Active
No Na+ diffusion Calcium channels closed Chemically-gated channels closed ACh in synaptic vesicles More negative charges inside sarcolemma
Neuromuscular Junction at Rest
Length of A band Length of thick filament Length of thin filament
No change in length or distance
The "rest and recovery" period, where the muscle restores depleted reserves, includes all of the following processes EXCEPT __________.
Pyruvic acid is converted back to lactic acid.
Inadequate calcium in the neuromuscular junction would directly affect which of the following processes?
Release of acetylcholine from the synaptic vesicles
Refers to the changes in muscle that occur after death
Rigor mortis
Specialized plasma membrane
Sarcolemma
Contractile unit of a muscle fiber
Sarcomere
Specialized smooth ER Surrounds myofibril Stores and releases calcium
Sarcoplasmic reticulum
Attached to bones or, for some facial muscles, to skin Single, very long, cylindrical multinucleate cells with very obvious striations Epimysium, perimysium, and endomysium VOLUNTARY
Skeletal muscles
Maintain posture and body position Stabilize joints Generate heat
Skeletal muscles
What are the three basic muscle types found in the body?
Skeletal, cardiac and smooth muscle
Mostly in walls of hollow visceral organs (other than the heart) Single, fusiform, one nucleus; no striations Endomysium INVOLUNTARY
Smooth muscle
a type of voltage-gated ion channel located on the sarcolemma of the muscle fiber.
Sodium channel
the space between the axon terminal and junctional folds.
Synaptic cleft
membranous sac located in the axon terminal that contains neurotransmitter.
Synaptic vesicle
Conducts action potentials throughout the interior of the muscle fiber
T tubule
Releases calcium ions into the sarcoplasm
Terminal Cistern
How does the myosin head obtain the energy required for activation?
The energy comes from the hydrolysis of ATP. Myosin is a large, complex protein with a binding site for actin. It also contains an ATPase. The energy released during the hydrolysis of ATP activates the myosin head.
What structure is the functional unit of contraction in a skeletal muscle fiber?
The sarcomere A sarcomere is a regular arrangement of thin and thick myofilaments that extends from one Z disc to the next. A myofibril consists of a series of sarcomeres.
Blocks binding sites on actin
Tropomyosin
Changes shape upon binding with calcium ions
Troponin
Tetanic contraction
When a muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sustained, the muscle is said to be in tetanus or in tetanic contraction. Tetanic contractions of muscles are smooth and prolonged muscle contractions, which can be slight or vigorous depending on what work has to be done.
Myasthenia gravis is a disease resulting from an autoimmune attack on the ACh receptors of the motor end plate. Binding of antibodies to the ACh receptors results in generalized muscle weakness that progresses as more ACh receptors are destroyed. Which of the following medications would help alleviate the muscle weakness?
a drug that binds to and inactivates acetylcholinesterase (neostigmine)
results in stronger, more flexible muscles with greater resistance to fatigue biking, jogging makes body metabolism more efficient improves digestion, coordination
aerobic (endurance) exercise
supplies ATP at rest and during light/moderate exercise slower reaction that requires continuous delivery of oxygen and nutrients
aerobic respiration
reaction breaks down glucose without oxygen glucose is broken down to pyruvic acid to produce about 2 ATP
anaerobic glycolysis and lactic acid formation
1. potassium ions (K+) diffuse out of the cell 2. sodium-potassium pump moves sodium and potassium ions back to their original positions
cell returns to its resting state when:
Conduction of an action potential along the sarcolemma depends upon ___________.
diffusion of sodium ions through voltage-gated channels
covers the entire skeletal muscle
epimysium
muscle size, strength, and endurance
exercise increases
on the outside of the epimysium
fascia
muscles that are concerned with bracing actions are called
fixators
muscle filaments are trying to slide but the muscle is pitted against an immovable object tension increases but muscles do not shorten ex: pushing your palms together in front of you
isometric contractions
myofilaments are able to slide past each other during contractions muscle shortens and movement occurs ex: bending the knee, lifting weight, smiling
isotonic contractions
The nuclei of skeletal muscle cells are pushed aside by long, ribbonlike organelles called _______, which nearly fill the cytoplasm.
myofibrils
associate site of axon terminal of the motor neuron and sarcolemma of a muscle
neuromuscular junction
chemical released by the nerve upon arrival of nerve impulse in the axon terminal acetylcholine (ACh) is the neurotransmitter that stimulates skeletal muscle
neurotransmitter
series of metabolic pathways use oxygen and occur in mitochondria
oxidative phosphorylation
wraps around a fascicle (bundle) of muscle fibers
perimysium
increases muscle size and strength weight lifting individual muscle fibers enlarge
resistance (isometric) exercise
The plasma membrane of a muscle cell is usually termed the _______.
sarcolemma
the plasma membrane of a muscle cell is most usually termed a
sarcolemma
The endomysium is a delicate connective tissue sheath that surrounds a/an _______.
skeletal muscle fiber
irritability: the ability to respond to a stimulus such as a nerve impulse contractibility: ability to forcibly shorten when an adequate stimulus is received extensibility: ability of muscle cells to be stretched elasticity: ability to recoil and resume resting length after stretching
special functional properties of skeletal muscles
gap between nerve and muscle filled with interstitial fluid although very close, the nerve and muscle do not make contact
synaptic cleft
1. Direct phosphorylation of ADP by creatine phosphate 2. Aerobic respiration 3. Anaerobic glycolysis and lactic acid formation
three pathways to regenerate ATP
Which type of muscle fiber has a large quantity of glycogen and mainly uses glycolysis to synthesize ATP?
white fast twitch fibers
