Set 5 (Part I) Flashcards
How is skeletal muscle attached to bones?
By tendons
Differentiate flexor and extensor muscles.
- Flexor: brings bones together
- Extensor: moves bones away
What are antagonistic muscle groups?
Flexor-extensor pairs
What are the characteristics of skeletal muscle?
- Striations
- Multinucleations
- Fibers arranged in sarcomeres
What is the epimysium?
Coarse sheath covering the muscle as a whole (connective tissue)
What is the perimysium?
Tough connective tissue binding fascicles together
What is the endomysium?
Connective tissue membrane that covers skeletal muscle fibers
What are thick filaments? What are thin filaments?
- Thick: myosin
- Thin: actin
What is each muscle fiber made of?
Many cells fused into fibers
What is the sarcolemma?
Plasma membrane of muscle fibers
What is the sarcoplasmic reticulum?
A type of smooth endoplasmic reticulum
What structure plays a role in Ca2+ regulation during contraction?
Sarcoplasmic reticulum
What is the structure of the T-tubules? How are they formed?
- Extend across the cell
- Formed from the inward extension of the sarcolemma
What is the chief function of T-tubules? What is it required for?
- Allow electrical signals traveling along the sarcolemma to move deeper into the cell (moves FASTER)
- Required for contraction
What is the triad of skeletal muscle tissue?
- One T-tubule
- Two flanking terminal cisternae (sarcoplasmic reticulum) surrounding the T-tubule
What allows a signal traveling along the T-tubule to stimulate adjacent membranes on the SR?
The triad of skeletal muscle tissue
Why do muscle fibers contain many mitochondria and several nuclei?
As they require a lot of ATP
What is a sarcomere?
- Contractile unit of fibers
- Segment of myofibril between two successive Z lines
Each myofibril consists of many ___________
sarcomeres
What is the contractile unit of muscle?
Sarcomere
What separates one sarcomere from the next and serves as an anchor for the myofibrils?
Z-disc
Which element is where the thick and thin filaments overlap? How does it appear?
- The A-band
- Dark (dense region due to overlapping)
What stimulates a skeletal muscle?
Motor neuron
What stimulates smooth muscle?
Endocrine or paracrine signal
_________ are a continuation of the sarcolemma, and are functionally linked to the sarcoplasmic reticulum
T-tubules
What are the four different kinds of protein molecules that make up myofilaments?
- Myosin
- Actin
- Tropomyosin
- Troponin
Which filaments do not attach to the Z-lines?
- Myosin does NOT attach
- Only actin
Which protein molecule makes up almost all the thick filament?
Myosin
Which protein molecule makes up the bulk of the thin filament?
Actin
What is the overall function of tropomyosin?
Protein that blocks the active sites on actin molecules
What is the overall function of troponin?
Protein that holds tropomyosin molecules in place
What kind of signal is released at a neuromuscular junction?
Acetylcholine
What is the neuromuscular junction?
- Motor neurons connect to the sarcolemma at the motor endplate
- Synapse where neurotransmitter molecules transmit signals
The distal end of the motor neuron forms a _______ with the muscle fiber, and makes contact with the sarcolemma
synapse
Where the motor neuron makes contact with the muscle fiber is called what?
Motor endplate
What does the motor neuron release? What does it cause?
- Releases acetylcholine
- Binds to receptors on the muscle fiber and depolarizes the cytoplasmic membrane (sarcoplasm)
How does action potential travel deep to reach into the muscle fiber and begin contraction?
Traveling down T-tubules
What does the voltage-changed brought by T-tubules trigger?
- Triggers the opening of the voltage-gated Ca2+ channels
- Allows passive diffusion of the Ca2+ out of the SR, where it triggers the contraction process
What initiates a muscle action potential?
Net entry of Na+ through Ach
What does the action potential in T-tubule alter?
The conformation of the DHP receptor
When the DHP receptor has an altered conformation, what does it do?
- Opens RyR Ca2+ release channels in sarcoplasmic reticulum
- Ca2+ enters the cytoplasm
When do myosin cross-bridges form?
When the myosin heads of thick filaments bind to actin in the thin filaments
A Z disk runs through the middle of every _ band, so each half of an _ band belongs to a different sarcomere.
I
Which zone is occupied by thick filaments only? It is found in the center of which band?
- H-zone
- A-band
Which band is occupied solely by thin filaments?
I band
Which band represents proteins that form the attachment site for thick filaments
M-line
Which band serves as attachment sites for thin filaments?
Z-disc
The __ line divides the A band in half
M
What proteins assures the proper alignment of filaments within a sarcomere?
Titin and nebulin
Where is a titin molecule positioned within the organization of the sarcomere?
One Z disc to the neighboring M line
What is the primary function of titin?
- Stabilizes MYOSIN
- Its elasticity returns stretched muscles to their resting length
Where is nebulin positioned within the organization of the sarcomere? What is its overall function?
- Lies alongside thin filaments and attaches to the Z disk
- Nebulin helps align the ACTIN filaments of the sarcomere
What are the three anatomical elements of a neuromuscular junction?
- One somatic motor neuron
- Synpatic cleft
- Motor end plate on the muscle fiber
What is the chemical signal at the neuromuscular junction?
Acetylcholine
Which regions shorten and which remain constant during muscle contraction?
- H zone and I band both shorten
- A band remains constant
Each myosin molecule has binding sites for what molecules?
- Actin
- ATP (myosin ATPase)
Name an elastic fiber in the sarcomere that aids in relaxation.
Titin
In the sliding filament theory of contraction, what prevents the filaments from sliding back to their original position each time a myosin head releases to bind to the next actin binding site?
The crossbridges do not all unlink at one time, so while some myosin heads are free and swivelling, others are still tightly bound
Where are DHP receptors found?
Only in skeletal muscle
What is a twitch?
A single contraction-relaxation cycle in a skeletal fiber
What causes the short delay (latent period) between the muscle action potential and the beginning of muscle tension development?
The time required for calcium release and binding to troponin
Which part of contraction requires ATP? Does relaxation require ATP?
- The release of myosin heads from actin requires ATP binding
- Energy from ATP is required for the power stroke
- Relaxation does not directly require ATP, but relaxation cannot occur unless Ca2+ is pumped back into the sarcoplasmic reticulum using a Ca2+ ATPase
Which muscle is known as the striated involuntary muscle?
Cardiac muscle
Why is auto-rhythmicity important for cardiac muscle?
Because it continuously needs to provide the pumping action to maintain a constant blood flow
Cardiac muscle contains a diad structure; what is it composed of?
- T-tubule
- Sarcoplasmic reticulum
Which cells set the rate of the heart?
Pace-maker cells in the SA node
Does cardiac muscle taper?
No, but skeletal muscle does
What do cardiac muscle fibers form? Why?
Continous, contractile band around the heart chambers that conducts a single impulse across a virtually continuous sarcolemma
What allows for a longer contraction in cardiac muscle tissue?
Longer retention of calcium in the SR
Why do you want longer contraction in cardiac muscle?
Because you want complete filling and emptying of the heart
How do T-tubules in cardiac muscle differ from skeletal muscle?
They are longer
What are three primary differences between cardiac muscle and skeletal muscle?
- Cardiac muscle sustains each impulse longer
- Cardiac muscle does not run low on ATP and does not experience fatigue
- Cardiac muscle is self-stimulating
Which component is present in skeletal and cardiac muscle, but not in smooth muscle?
No T-tubules are present
How are the sarcomeres of smooth muscle organized?
There are NO sarcomeres in smooth muscle (no striations)
What is the role of calcium in smooth muscle? Where does it come from?
- Ca2+ comes from outside of the cell
- Binds to calmodulin (NOT troponin) to trigger a contraction
What occurs during the contraction of smooth muscle?
- Sliding of the myofilaments (pulling on the plasma membrane) causes the fiber to shorten by “balling up”
- The fiber exhibits “dimples”
What are the “dimples” of smooth muscle called? What are they anchored to?
- Dense bodies
- Anchored to the cell membrane
What are the two types of smooth muscle tissue?
- Single-unit (visceral)
- Multi-unit
What allows for single-unit smooth muscle?
Gap junctions join smooth muscle fibers into large, continuous sheets
What does single-unit smooth muscle exhibit?
- Auto-rhythmicity
- Produces peristalsis
What is multi-unit smooth muscle?
- Does not act as a single unit
- Composed of many independent cell units
What does multi-unit smooth muscle respond to?
- ONLY to nervous input (independently responds to nerve stimulation)
- There is NO electrical coupling through gap junctions
What triggers impulses in single-unit smooth muscle?
Neurotransmitters released from the nerve fiber, whose actions are transmitted to adjacent fibers through gap junctions
Which muscle type has the longest contraction period? Why?
- Smooth muscle
- For maintenance of homeostasis
- Involved in the movement of material through the lumen of an organ
Does skeletal or cardiac muscle have a longer contraction period? Why?
Cardiac muscle does, as it must allow for complete filling and emptying
Which of these statements about smooth muscle contraction is FALSE?
A) It uses actin and myosin cross-bridges to create force
B) It can occur without a change in membrane potential
C) Ca2+ initiates contraction
D) Troponin plays the same role as in skeletal muscle
D) Troponin plays the same role as in skeletal muscle
If you want to study smooth muscle, it doesn’t matter what organ it comes from since all smooth muscles are alike.
A) True
B) False
B) False
As an axon enters a muscle, it branches into a number of axon terminals, each of which makes contact with a single muscle fiber. The portion of the sarcolemma in contact with the axon terminals is called the \_\_\_\_\_\_\_\_\_. A) Synaptic cleft B) Motor end plate C) Neuromuscular junction D) Synaptic knob E) Motor unit
B) Motor end plate
At the neuromuscular junction, calcium ions act to _______.
A) Increase the conduction speed of action potentials transmitted along the sarcolemma
B) Release the inhibition on Z-discs
C) Remove the blocking action of tropomyosin
D) Cause ATP binding to actin
E) Release synaptic vesicles from the axon terminal
E) Release synaptic vesicles from the axon terminal
** at the NEUROMUSCULAR JUNCTION (tropomyosin is not found at the neuromuscular junction)
The first thing that occurs when the axon terminal releases Ach is _______.
A) Calcium ions return to terminal cisternae of the SR
B) The troponin blocks the tropomyosin
C) Calcium diffuses into the axon terminal of the motor neuron
D) Diffusion across the synaptic cleft
E) The tropomyosin blocks the myosin
D) Diffusion across the synaptic cleft/
What type of ion channel opens in response to an action potential arriving at the axon terminal? A) Ligand-gated B) Voltage-gated sodium C) Voltage-gated D) Ligand-gated calcium E) Voltage-gated calcium
E) Voltage-gated calcium
What is the name of the enzyme that degrades ACh? A) Lipase B) ATPase C) Acetylcholinesterase D) Serine hydroxylase E) N-methyl transferase
C) Acetylcholinesterase
Acetylcholine receptor-channels allow the passage of \_\_\_\_\_\_ when open. A) Na+ B) K+ C) Both Na+ and K+ D) Cl- E) Ca2+
C) Both Na+ and K+
Drug “X” inhibits acetylcholinesterase action within the neuromuscular junction. What would drug “X” do to the membrane potential of the muscle fiber?
A) It would remain depolarized
B) It would repolarize more quickly than normal
C) It would hyperpolarize
D) Drug “X” would have no effect on the membrane potential
A) It would remain depolarized
In skeletal muscle, the Ca2+ from contraction comes from: A) Mitochondria B) Extracellular fluid C) Sarcoplasmic reticulum D) b and c E) a, b, and c
C) Sarcoplasmic reticulum
In smooth muscle, the Ca2+ from contraction comes from: A) Mitochondria B) Extracellular fluid C) Sarcoplasmic reticulum D) a and c E) b and c
E) b and c
If K+ concentration increases in the extracellular fluid surrounding a cell but does not change significantly in the cell’s cytoplasm, the cell membrane (depolarizes/hyperpolarizes) and becomes (more/less) negative
depolarizes
less
What ion is responsible for the repolarization phase of the muscle action potential, and in which direction does this ion move across the muscle fiber membrane? How might this be linked to hyper KPP?
- K+ leaves the muscle fiber cell
- Repeated contractions (in which K+ leaves) could contribute to elevated extracellular K+ (hyperkalemia)
Differentiate tonic and phasic smooth muscles.
- Tonic: continuously contracted (ex: esophageal sphincter)
- Phasic: alternates between contraction and relaxation (ex: intestine)
How can contraction be initiated in smooth muscle? What about skeletal?
- Smooth: electrical or chemical signals or both
- Skeletal: always begins with an action potential
What is the “motor end plate” region for smooth muscle cells?
- Smooth muscle does NOT have a specialized receptor region
- Neurotransmitters are released and simply diffuse across a cell surface until they find a receptor
What is the difference in how contraction force is varied in multiunit and single-unit smooth muscle?
- Multi-unit smooth muscle increases force by recruiting additional muscle fibers
- Single-unit smooth muscle increases force by increase Ca2+ entry
Is tropomyosin, t-tutuble and/or troponin present in smooth muscle?
- Tropomyosin is
- Troponin is NOT
- T-tubules are NOT
The dense bodies that anchor smooth muscle actin are analogous to what structure in a sarcomere?
Z-dics
What controls contraction and relaxation in smooth muscle?
Phosphorylation and dephosphorylation of the myosin light chain
What ratio determines the contraction state of smooth muscle?
Ratio of MLCK to MLCP
How may calcium sensitivity be altered in smooth muscle?
- Chemical signals (ex: neurotransmitters), hormones and paracrine molecules alter the Ca2+ sensitivity by modulating MLCP activity
- Calcium desensitization (calcium is less effective at causing a contraction
In smooth muscle, what is SR calcium release mediated by?
- RyR calcium release channel
- IP3-receptor channel
What is myogenic contraction?
Stretch on the cell depolarizes it an opens membrane Ca2+ channels (smooth muscle)
Compare the signal for crossbridge activation between smooth and skeletal muscle.
- Smooth: myosin is phosphorylated
- Skeletal: Ca2+ binds to troponin
Compare the signal that releases Ca2+ from the SR between smooth and skeletal muscle.
- Smooth: IP3 signal
- Skeletal: depolarization signal
Describe how changes in phosphatase activity alters myosin’s response to Ca2+.
- Low phosphatase activity sensitizes myosin (low calcium)
- High phosphatase activity desensitizes myosin (high calcium)
What are pacemaker potentials?
Always reach threshold and create regular rhythms of contraction
When tetrodotoxin, a poison that blocks Na+ channels, is applied to certain types of smooth muscle, it does not alter the spontaneous generation of action potentials. What conclusion can you draw about the action potentials of these types of smooth muscle?
- The depolarization phase of the action potentials must not be due to Na+ entry
- In these muscles, depolarization is due to Ca2+ entry
How can a neuron alter the amount of neurotransmitter it releases?
Increased frequency of action potentials in the neuron increases neurotransmitter release
Explain how hyperpolarization decreases the likelihood of contraction in smooth muscle?
- Many Ca2+ channels open with depolarization
- Hyperpolarization decreases the likelihood that these channels open
- The presence of Ca2+ is necessary for contraction
What causes relaxation in skeletal muscle?
Relaxation in skeletal muscle occurs when troponin releases Ca2+ and tropomyosin moves back to block actin’s binding site for myosin
The graded depolarization in skeletal muscle fiber that is elicited in response to one action potential is called _______.
EPP (end-plate potential)
Which of the following statements comparing smooth muscles to skeletal muscles is FALSE?
A) Skeletal muscles have more sarcoplasmic reticulum than smooth muscles.
B) Smooth muscles have more actin filaments than skeletal muscles.
C) Smooth muscles have less myosin than skeletal muscle.
D) Smooth muscles have longer myosin filaments than skeletal muscles.
A) Skeletal muscles have more sarcoplasmic reticulum than smooth muscles.
Smooth muscles just have less ORGANIZED SRs than skeletal muscle
Which protein creates the power stroke?
Myosin
How can smooth muscle contract when it has less organized sarcoplasmic reticulum?
It also utilizes calcium from the ECF
Which of the following does NOT shorten during muscle contraction?
A) The H zone
B) The sarcomere
C) The thin filament
B) The sarcomere
Individual skeletal muscle cells are known as A) myotomes B) myofilaments C) muscle fibers D) myofibrils
C) muscle fibers
All of the following are functions of skeletal muscle tissue except A) cooling of the body B) body movement C) maintenance of posture D) storage and movement of materials
A) cooling of the body
Which term describes the expanded tip of an axon at a neuromuscular junction? A) synaptic vesicle B) motor end plate C) synaptic cleft D) synaptic knob
D) synaptic knob
In skeletal muscle fibers, where do calcium ions occur at varying concentrations?
A) in the terminal cisternae of the sarcoplasmic reticulum
B) throughout the sarcoplasm
C) in regions where the thin and thick filaments overlap
D) all of the above
D) all of the above
Which list of muscle structures proceeds from smaller to larger in diameter?
A) myofilament, fascicle, muscle fiber, myofibril
B) fascicle, myofibril, muscle fiber, myofilament
C) myofilament, myofibril, muscle fiber, fascicle
D) myofibril, fascicle, muscle fiber, myofilament
C) myofilament, myofibril, muscle fiber, fascicle
Synaptic knobs are to axons as \_\_\_\_\_\_\_\_\_\_\_\_ are to sarcolemmae, in that both are structural modifications involved in transmitting electrochemical signals across the synaptic cleft. A) synaptic vesicles B) motor end plates C) sarcoplasmic reticula D) transverse tubules
B) motor end plates
Interactions between thick and thin filaments during muscle contraction occur in which sequence? A) attach, detach, pivot, return B) attach, pivot, detach, return C) detach, pivot, return, attach D) pivot, attach, detach, return
B) attach, pivot, detach, return
How many motor end plates does a typical skeletal muscle fiber contain?
A) only one
B) one at each end
C) dozens scattered along the sarcolemma
D) too variable to determine
A) only one
An overdose of cholinesterase inhibitors could produce effects most similar to those of A) flaccid paralysis B) tetanus C) Botox treatments D) botulism
B) tetanus
The unit of muscle structure that is composed of bundles of myofibrils, enclosed within a sarcolemma, and surrounded by a connective tissue covering called endomysium is a A) myofibril. B) fascicle. C) myofilament. D) muscle fiber.
D) muscle fiber
As actin and myosin filaments slide past each other during muscle contraction,
A) actin filaments shorten, while myosin filaments do not. B) myosin filaments shorten, while actin filaments do not. C) either actin or myosin filaments shorten, but not both at the same time. D) both actin and myosin filaments shorten. E) neither actin nor myosin filaments shorten.
E) neither actin nor myosin filaments shorten.
