Exam #1: Skeletal Muscle Flashcards
What is the difference between the three main types of muscle?
- Skeletal & Cardiac are striated
- Smooth is non-striated
What type of muscle contains intercalated discs?
Cardiac
What is the functional unit of skeletal muscle?
Sarcomere
What is the definition of a sarcomere?
Z-disc to z-disc
What is the function of intercalated disc found in cardiac muscle?
- Mechanical connection
- Electrical connection
What is contained in intercalated discs that allows for electrical connection?
Gap junctions
What are the two types of smooth muscle?
Multiunit & Unitary
What is unitary smooth muscle?
Group of fibers that contract together as a single unit:
- Found in visceral smooth muscle
- Pacemaker cells make it spontaneously active
- Contains gap junctions that work as a functional syncytium
How is multiunit smooth muscle different from unitary smooth muscle?
Discrete smooth muscle fibers that contract independently
- Not spontaneously active
- Stimulated to contract by motor nerves
- Each cell has a synaptic connection with a varicosity
*Found in the eyes e.g. ciliary muscle & in piloerector muscle
What is a myofibril?
Myofibril= cylindrical structure made up of end-to-end chains of sarcomeres
What connective tissue element surrounds a bundle of myofibrils?
Endomysium
Define muscle cell.
Muscle fiber, a bundle of myofibrils surrounded in endomysium
What is a fascicle?
A bundle of muscle fibers
What connective tissue element en-sheaths fasciculi?
Perimysium
What connective tissue element en-sheaths bundles of fasciculi?
Epimysium i.e. the covering around the entire muscle
Where are the nuclei & mitochondria located in a muscle cell?
Sarcoplasm
Are skeletal muscle cells uninucelate or multinucleate?
Multinucleate
What is a T-Tubule?
Transverse Tubule, extensions of cell membrane into the muscle cell;
- Because these structures are an extension of the plasma membrane, they contain extracellular fluid
What is the function of the T-Tuble? What does the ionic composition of a T-tubule resemble?
Function= electromechanical coupling- conveys the action potential to the muscle
- ECF (high Na+)
What are thick filaments composed of?
Myosin filaments
What are thin filaments composed of?
Actin filaments
When muscle cells contract, which filaments slide relative to which?
Thick filaments slide relative to stationary thin filaments & z-discs approximate
What is Titin?
Titin= “spring-like” protein that attaches the thick filaments to the z-disc .
- Note that Titin continues into the thick filament & is encased; it connects on the M-line on the opposite side of the z-disc.
What is the function of Titin?
- Titin acts as a spring that changes length as the sarcomere contracts & relaxes.
- Helps to give muscle it’s elasticity
What diseases are associated with Titin mutations?
- Hereditary myopathy with early respiratory failure
- Familial dilated cardiomyopathy
What are the three units of a triad?
1) Sarcoplasmic reticulum cisterna
2) Transverse-Tubule (ECF)
3) Sarcoplasmic Reticulum Cisterna
What is the function of the triad? Describe the sequence of events that occurs as an action potential is propagated into a muscle cell.
- Transverse tubules function to conduct action potentials into the cell
- Sarcoplasmic reticulum contains high concentrations of Ca++
- Ca++ release from the SR in response an action potential traveling down the T-Tubule is the basis of the electromechanical coupling
Specifically, what are the functions of the Sarcoplasmic Reticulum?
- Store Ca++
- Releases Ca++ in muscle cytosol
- Re-sequesters Ca++
What is dystrophin? What is the function of dystrophin?
- Large protein that forms a rod that attaches the thin actin filament to a transmembrane protein
- Gives muscles strength by connecting fibrils to the ECM
What family of diseases is associated with dystrophin mutations?
Muscular Dystrophy
What is the difference between Duchenne, Becker’s & Limb-girdle dystrophy?
Duchenne=
- Severe reduction in dystrophin in skeletal & cardiac m.
- X-linked recessive (males)
- Fatal prior to 30 y/o
Becker’s=
- Less severe with dystrophin present but reduced or altered
Limb-Girdle= NOT caused by dystrophin pathology itself; rather, other components of the dystrophin-glycoprotein complex
What is tropomyosin?
A protein attached to thin filaments that covers the active site on the actin filament
What are the three subunits of the Troponin complex? What do they bind to?
- TnC= Ca++
- TnT= Tropomyosin
- TnI= Actin
What binds to actin in actin-myosin cross-bridge cycling?
Myosin heads of thick filaments bind to active sites on actin filaments (thin filaments)
What prevents myosin binding in the “no-Ca++” state?
Troponin complex & tropomyosin
Describe the events that follow Ca++ influx into the sarcolemma.
- Ca++ binds TnC (Troponin complex) & causes a conformational change that shifts tropomyosin & troponin out of the way of
- Actin binding sites are exposed
List the steps of the actin-myosin cross-bridge cycle starting with myosin attached to actin.
1) Myosin is attached to actin
2) ATP binds to the myosin head causing dissociation from actin
3) Myosin head is now in the relaxed state
4) ATP is hydrolyzed to ADP & Pi, which causes the myosin head to become “cocked”
5) An actin-myosin (w/ ADP+Pi) cross-bridge forms b/c of the action of Ca++ exposing actin binding sites
6) Power stroke occurs with the release of Pi, myosin pulls along actin
7) ADP is released & the actin-myosin complex is stuck in a rigid state
How does muscle relax?
- ATP binding to the actin-myosin complex causes dissociation of myosin from actin
- ATP also pumps Ca++ back into the SR
What is rigor mortis?
After death ATP is depleted & actin-myosin complexes are unable to detach, causing muscle to become stiff
List the steps of excitation contraction coupling.
1) Action potential travels down a motor neuron
2) Voltage-gated Ca++ channels open causing Ca++ influx in the pre-synaptic terminal
3) Acetylcholine is released into the synaptic clef & binds Nicotinic AChR on post-synaptic membrane
4) NAChR opens–Na+ flows in leading to depolarization
5) Voltage gated Na+ channels open & reach threshold leading to an action potential
6) Action potential spreads through the T-Tubule
7) L-Type Ca++ channels in the T-Tubule act as voltage sensors
8) L-Type Ca++ channels are mechanically coupled to Ca++ release channels in the SR
9) Ca++ is released from the SR & spreads into the myofibrils
10) Excitatory pulse of Ca++ causes muscle contraction
11) Ca++ is then pumped back into the SR
12) Ca++ binding proteins bind Ca++ in the SR
What is a dihydropyridine (DHP) receptor?
Synonym for L-Type voltage-gated Ca++ channel in T-Tubule
What is a Ryanodine receptor?
Synonym for Ca++ release channel in SR
What is the function of the Ca++ binding proteins in the SR?
Maintenance of Ca++ gradient so the Ca++ pump can function more efficiently
What are Calsequestrin?
Ca++ binding protein in the SR
Which is longer, the muscle action potential or the muscle contraction?
Muscle contraction
What is phosphocreatine? What is its function?
- Phosphorylated creatine that serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle
- Donates phosphate to ADP when ATP reserves are depleted
What is creatinine?
Breakdown product of phosphocreatine in skeletal muscle that is usually produced at a constant rate
What is the motor unit?
A motor neuron and the skeletal muscle fibers innervated by that motor neuron’s axonal terminals.
- Smallest unit that the nervous system can “turn on”
- Any 1 motor neuron innervates 1,000 muscle cells
What is a motor neuron pool?
All the motor neurons that innervate a single muscle
What is the difference between a small motor unit & a large motor unit?
Small motor unit= muscles with fine control
Large motor unit= muscles with large power & less fine contro
How do we produce graded muscle contractions?
- Contract small motor units first, then recruit large motor units
- Make motor units work harder by increasing the frequency of action potentials
What is a single muscle twitch?
Response of single muscle fiber to a single maximal stimulus
What is the difference between isometric & isotonic contraction?
Isometric= not changing length BUT can develop tension Isotonic= constant tension but change in length
*Can be done experimentally but does not really happen in life
