Smooth muscle and cardiac muscle

A gap junction is a set of channels joining two cells' cytoplasm, so ions, and with them electrical changes, pass straight from cell to cell. Desmosomes rivet cells; tight junctions seal them.

• Correct: Lets ions pass directly from one cell's cytoplasm to the next:
• Rivets the cells together against pulling:
• Seals the space between them so nothing leaks through:

Calcium released from the SR binds troponin, which moves tropomyosin off the binding sites on actin so cross-bridges can form.

• Myosin heads:
• Correct: Troponin:
• Actin directly:

Cardiac muscle is striated like skeletal muscle, but its short, branched fibers are joined end to end by intercalated discs.

• Smooth muscle:
• Skeletal muscle:
• Correct: Cardiac muscle:

Calcium–calmodulin binds and activates myosin light-chain kinase, the enzyme that phosphorylates the myosin heads.

• Tropomyosin moves off actin's binding sites: Tropomyosin shifting off actin is the skeletal muscle switch, triggered by troponin. In smooth muscle the switch is on myosin.
• Troponin changes shape: Smooth muscle has no troponin.
• Correct: Myosin light-chain kinase is activated: Correct. MLCK is the enzyme calcium–calmodulin turns on.
• Myosin phosphatase is activated: Myosin phosphatase removes phosphate and causes relaxation; it is the opposite step.

Calcium still rises and still binds calmodulin, but calcium–calmodulin cannot switch on the blocked kinase. Myosin heads stay unphosphorylated and do not cycle, so the strip barely contracts.

• Calcium entering the fibers: no change. The drug acts after calcium entry; the calcium channels still open when the fibers depolarize.
• Calcium bound to calmodulin: no change. Calmodulin still binds the calcium that enters; the block is on the kinase that calcium–calmodulin activates.
• Phosphorylated myosin heads: down. With the kinase blocked, no phosphate is moved onto the myosin heads.
• Force of contraction: down. Unphosphorylated myosin heads cannot bind actin and cycle, so little force develops despite the calcium.

When myosin heads are dephosphorylated while still attached to actin, they detach and reattach very slowly. These latch-bridges hold tension for hours at a small fraction of the ATP cost skeletal muscle would pay.

• Its cross-bridges never detach, as in rigor mortis: Rigor happens when there is no ATP at all. Latch-bridges do detach, only slowly, and the vessel can still relax.
• It makes its ATP without using mitochondria at all: Smooth muscle does have mitochondria; the saving comes from spending less ATP, not from making it differently.
• It stays in complete tetanus from constant nerve firing: Tetanus is a skeletal muscle response to rapid firing and costs a lot of ATP. Smooth muscle holds tension through the latch state.
• Correct: Myosin heads held on as slowly cycling latch-bridges: Correct. Slowly cycling latch-bridges hold tension cheaply.

Each increase in volume stretches the wall and raises tension briefly, then the smooth muscle slowly lets its tension fall back while staying at the new length. This stress-relaxation response keeps pressure low as the bladder fills.

• Tetanus of the bladder wall's smooth muscle: Tetanus is a sustained skeletal muscle contraction; a contracting wall would raise pressure, not keep it low.
• Correct: Stress-relaxation of the wall: Correct. Stress-relaxation lets the wall accommodate each new volume.
• Autorhythmicity of pacesetter cells: Autorhythmicity describes cells that fire on their own; it would cause contractions, not a quiet fill.
• Rigor of the bladder's cross-bridges: Rigor means locked cross-bridges from a lack of ATP; it would stiffen the wall.

Intestinal smooth muscle is single-unit. Pacesetter cells produce slow, spontaneous waves of depolarization, and gap junctions spread the resulting action potentials through the sheet, so it contracts as one unit without any input from the brain or spinal cord.

• Motor neurons inside the fluid stimulate each fiber separately: The loop has been cut off from the brain and spinal cord, and the fluid contains no motor neurons.
• Each fiber contracts on its own, at its own random rhythm: Random, independent contractions would not give regular, coordinated waves; the fibers are coupled.
• Stored acetylcholine is released at neuromuscular junctions: Smooth muscle has no neuromuscular junctions, and the outside nerves have been cut.
• Correct: Pacesetter waves spread by gap junctions: Correct. Pacesetter rhythm plus gap junctions coordinates the sheet.

An action potential in a ventricular muscle fiber lasts about 250 to 300 ms, and the fiber cannot fire again until it has nearly relaxed. New action potentials cannot pile up during a contraction, so twitches cannot sum, and the heart relaxes and fills between beats.

• Cardiac muscle has no troponin on its thin filaments: Cardiac muscle does use troponin, as skeletal muscle does.
• Cardiac fibers are not linked by gap junctions: Cardiac fibers are linked by gap junctions in their intercalated discs; that spreads the beat, and it has nothing to do with preventing tetanus.
• Correct: Its action potential lasts almost as long as its twitch: Correct. The long action potential blocks summation.
• Cardiac muscle has no sarcoplasmic reticulum to store calcium: Cardiac muscle has an SR; it is smaller than skeletal muscle's but supplies most of the calcium.

Every cardiac fiber contracts with every beat, because gap junctions spread each action potential through the whole heart. Force is graded instead by how much calcium each beat releases (and by how far filling stretches the fibers), which sets how many cross-bridges form.

• By recruiting fibers that were resting during the last beat: There are no resting fibers to recruit: all of them contract every beat.
• By summing beats into tetanus: The heart cannot sum beats into tetanus, because each action potential lasts nearly as long as the contraction.
• Correct: More calcium released per beat: Correct. More calcium per beat gives a stronger contraction.
• By switching its fibers over to anaerobic glycolysis: Cardiac muscle is almost entirely aerobic and has little anaerobic capacity; switching fuel pathways does not make beats stronger.