PHYSL 212 Lecture 9: Cardiac and Smooth Muscle (Terms)

Cardiac muscle is (striated/unstriated), and (larger/smaller) than skeletal muscle.

Unstriated, smaller

What are the elements within each intercalated disk? What are their respective roles in myocyte-to-mycoyte communication?

Desmosomes: mechanically/physically couple myocytes, holding them together

Gap junctions: electrically couple myocytes, allowing ion flow

Describe the microscopic filamentous structure of cardiac muscle (bands) in order.

In a sarcomere: z line, I band, A band (H zone and M line), I band, z line

Compare thin filaments in cardiac muscle to thin filaments in skeletal muscle.

Similar: F actin double helix, tropomysoin, troponin (TnC, TnT, TnI), nebulin

TnC has one calcium binding site in cardiac muscle, while TnC has 4 calcium binding sites in skeletal muscle

Compare thick filaments in cardiac muscle to thick filaments in skeletal muscle.

All have heavy and light myosin chains, but different isoforms

Titin (accessory protein) is different isoform, providing better elastic recoil in cardiac myocytes

Compare the T-tubule/sarcoplasmic reticulum complex in cardiac and skeletal muscle.

SR in cardiac muscles is not as well developed but equally important

T-tubules are 10x larger in cardiac myocytes

Cardiac muscle (the heart) is innervated by ___.

Autonomic nervous system: both divisions (sympathetic and parasympathetic)

Do ANS neurons trigger/turn on cardiac muscle contraction? If not, what does?

Neurons modulate cardiac muscle contraction (its always happening, they determine how fast)

Pacemaker triggers cardiac muscle contraction

Cardiac muscle expresses specific receptors for ___, which then control ___.

Circulating transmitters (epinepherine and norepinepherine), heart rate

Explain how the sarcoplasmic reticulum and t-tubule complex differs in cardiac and skeletal muscle.

Cardiac:

• L-type channels (voltage gated)

• Ryanodine receptors not physically coupled to channel (Ca²⁺ release channel)

• Dyad: t-tubule only attached to SR on one side

• TnC only has one low affinity binding site

Skeletal:

• DHP receptors (voltage gated)

• Ryanodine receptors physically coupled to channel

• Triad: t-tubule attached to SR on both sides

• TnC has two low affinity, two high affinity binding sites

Explain the role of extracellular Ca²⁺ in cardiac muscle contraction. How does this compare to skeletal muscle?

Extracellular Ca²⁺ is completely necessary for cardiac myocytes to contract, as the influx of calcium through L-type channels from the outside of the cell needs to happen in order for ryanodine receptor channels to open (SR releases calcium for muscle contraction)

In skeletal muscle contraction is not dependent on extracellular Ca²⁺, action potential arrival stimulates DHP receptor that physically opens ryanodine receptors regardless of whether or not Ca²⁺ flowed through L-type channels

How does contraction reaction speed of cardiac and skeletal muscle compare, and how does this explain the structure of their t-tubule SR complexes?

Skeletal muscle: quick, instant contraction upon receiving stimulus. Triad with SR on both sides of t-tubule so that intracellular Ca²⁺ increases very fast

Cardiac muscle: modulation of the heart is gradual. Dyad with SR on one side, intracellular Ca²⁺ increases slower

In CICR, where does the calcium come from? How does their differing location change their roles, and how much does each contribute?

Ca²⁺ from the outside of the cell (extracellular) via L-type channel: 10% of the Ca needed for contraction, initial calcium influx that causes more calcium to be released.

Ca²⁺ from the SR (intracellular) via ryanodine receptor: 90% of the Ca needed for contraction, amplifies the small amount of initial calcium to make up what is needed

In cardiac muscle, which pumps contribute the most to the removal of calcium from the cytoplasm to reset cross bridge cycles?

NCX and PMCA: ~10% low contribution

SERCA2A and calsequestrin: 90% high contribution

Explain the complete excitation-contraction coupling mechanism in cardiac muscle, from AP arrival to muscle contraction.

1. Muscle AP arrives and is propagated into T-tubule

2. Depolarization voltage change activates/opens L-type receptors on t-tubule membrane

3. Extracellular calcium enters the muscle cell via the L-type receptors

4. Ca²⁺ binds to the ryanodine receptors on the SR cisternae within the cell

5. Upon Ca²⁺ binding, calcium ions are released from storage in the SR

6. Conformational change in troponin, moving tropomyosin out of the way of myosin binding site

7. Actin binds to the myosin head, causing it to undergo cross-bridging contraction (P_i released, power stroke)

8. Ca²⁺ is removed from the cytoplasm via SERCA 2A until approximately < 10^-7M to keep low enough for next cycle

9. Troponin resumes its original conformation and tropomyosin slips back to block the binding site

Smooth muscle is (striated/unstriated), meaning actin and myosin are (more/less) organized.

Unstriated, less

Describe the microscopic filamentous structure of smooth muscle.

Thick and thin filaments are interspersed, no organized bands

Compare the main participants of cross bridging in skeletal/cardiac muscle and smooth muscle.

Skeletal/cardiac: actin, tropomyosin, troponin, myosin

Smooth: actin, tropomyosin, myosin, calmodulin (no troponin)

No t-tubules in smooth muscle!

Smooth muscle is innervated by the __.

Autonomic nervous system

Does specialization of the membrane occur on multiunit or unitary smooth muscle? Explain.

Unitary

Each individual myocyte is innervated in multiunit → varicosities completely cover cell membrane, unable to specialize. Can’t contract together due to not being coupled

Only some myocytes innervated in unitary → areas of the muscle membrane are left open for function, not covered by varicosities. Able to contract together to perform specific functions due to gap junctions

What are the general locations that multiunit and unitary smooth muscle are found, based on their functions?

Multiunit: fine motor control and small precise movement (iris, cilliary muscle, piloerector muscle)

Unitary: specialized organ tissue (GI tract, uterus, bladder)

Smooth muscle myosin heads can only interact with actin (form cross bridges) when ___. Why?

Light chain is phosphorylated. Phosphorylation increases ATPase activity of the head, allowing it to bind to actin

Describe the slow-wave electrical activity that occurs in smooth muscle, including all contributing ions and channels.

1. Voltage gated calcium channels open, allowing the membrane to depolarize

2. An action potential spike occurs once membrane threshold is reached

3. Upon action potential, calcium influx into the cell due to opening of more V-gated calcium channels

4. Calcium dependent K+ channels open, which begins to repolarize the membrane as positive charge flows out

5. Because K+ channels close slowly, slow hyperpolarization occurs as more charge flows out

6. V gated calcium channels close, decreasing internal calcium concentration, adding to hyperpolarization

7. Due to decreased calcium concentration, Ca dependent K+ channels close, allowing the membrane to be repolarized

Explain the process by which MLCK is activated to phosphorylate the myosin heads in smooth muscle.

1. 4 calcium ions bind to calmodulin, forming a Ca²⁺-calmodulin complex

2. The complex binds to MLCK, causing a conformational change that exposes an ATPase on the enzyme

3. ATP is hydrolyzed by the MLCK and the phosphate is added to the light chain of smooth muscle myosin heads

4. ATPase activity on the myosin head increases, allowing actin binding

Explain the process by which the myosin head light chain is dephosphorylated in smooth muscle. Why does this need to happen?

Myosin light chain phosphatase removes the phosphate from the myosin head light chain, which inhibits actin binding. Required for relaxation of smooth muscle

Myosin light chain phosphatase (is/is not) Ca²⁺ dependent. Explain.

Is not, must be able to dephosphorylate myosin heads in order to terminate contraction, which occurs in low concentration of calcium

Describe the elements of smooth muscle that allow calcium to enter upon excitation.

Voltage dependent L-type channels → allows extracellular Ca2+ to enter the cell upon action potential depolarization

Ligand-gated channels → extracellular ligands allow flow of calcium into cell

Ryanodine receptors RyR3, IP3 → release Ca2+ from the SR

Explain the complete excitation-contraction coupling mechanism in smooth muscle, from AP arrival to calcium removal and contraction termination.

1. Muscle AP arrives, depolarizing membrane and activates/opens V gated L-type calcium channels

2. Extracellular calcium enters the muscle cell via the L-type receptors

3. 3 contributors to SR calcium store release:

   1. G-protein receptor allows IP3 to move to SR membrane and activate Ca2+ release channels, causing calcium SR stores to be released

   2. Ca²⁺ binds to the ryanodine receptors on the SR cisternae within the cell, causing SR calcium stores to be released

4. Calcium binding to calmodulin, forming complex

5. Calcium calmodulin complex activates MLCK, phosphorylating myosin light chain

6. Actin binds to the myosin head, causing it to undergo cross-bridging contraction (P_i released, power stroke)

7. Ca²⁺ is removed from the cytoplasm via SERCA until approximately < 10^-7M to keep low enough for next cycle

8. MLCP dephosphorylates myosin light chain so that muscle can relax

In smooth muscle, which pumps contribute the most to the removal of calcium from the cytoplasm to reset cross bridge cycles?

Low: NCX, PMCA

High: SERCA, calreticulin

How does the latch-bridge mechanism work? Why does it happen?

Once calcium concentration drops, MLCP removes the phosphate group while the myosin head is still attached to actin, maintaining the cross-bridge as the detachment happens very slowly. The contraction is sustained without expending energy to create a new cross bridge for a new contraction

If an organ needs to be in a consistent state of contraction (ex. bladder), expending ATP to create a new bridge as soon as the old one dissipates would burn through too much energy

Compare skeletal, cardiac, and smooth muscle in terms of: method/location of excitation, primary calcium sensor, main contributors to E-C coupling, main contributors to contraction termination.

Skeletal: NMJ synapse (AP), troponin (TnC), DHP receptor physically coupled to ryanodine receptor RyRs, SERCA and calsequestrin

Cardiac: Gap junction at intercalated disc or pacemaker, troponin (TnC), L-type calcium channel and CICR of ryanodine receptor RyR2, SERCA and calsequestrin

Smooth: autonomic synaptic transmission or hormones or gap junctions between myocytes, calmodulin, V-gated calcium channels and Ca release channels modulated by IP3 and ryanodine receptor RyR3, SERCA and calreticulin and MLCP