5F · Module 4 Review: Smooth and Cardiac Muscle

Practice this module: MCQ · Flashcards · Problems, Module 4

Smooth muscle structure and types¶

Spindle-shaped, no striations, dense bodies, caveolae, no troponin, but it does have an SR (shared with all muscle).
Found in vessels, airways, GI, bladder, uterus, eye. Not the heart.
Phasic and tonic both exist. Most smooth muscle is single-unit (gap junctions); the eye is multi-unit.
Smooth and cardiac cells are coupled; skeletal cells are not.

APs depolarize on Ca²⁺ and vary widely; they are not like skeletal APs.
Ca²⁺ from outside and from the SR: CICR through the RyR, or GPCR → PLC → IP₃ → IP₃ receptor. Both SR channels are ligand-gated.
Calmodulin → MLCK phosphorylates myosin → contraction. MLCP dephosphorylates → relaxation. MLCP is Ca²⁺-independent and does not touch MLCK.
SERCA does most Ca²⁺ removal; RyR does not remove Ca²⁺.
Force-velocity curves differ by degree of myosin phosphorylation.

Messenger	Ligand + receptor	Effect	Smooth muscle result
IP₃ ↑	ACh + M₃; NE + alpha-1 (Gq)	Ca²⁺ release; rho kinase inhibits MLCP	Contraction
cAMP ↓	NE + alpha-2 (Gi)	Less MLCK inhibition	Contraction
cAMP ↑	NE + beta-2 (Gs)	MLCK inhibited	Relaxation
cGMP ↑	Nitric oxide, no receptor	MLCP ↑, curve shifts right	Relaxation

Pacemakers start the beat; AP spreads through gap junctions. Autonomic nerves modulate.
Phase 2 Ca²⁺ influx (20%) triggers CICR (80%). Ca²⁺ binds troponin C.
Removal: SERCA 70%, NCX 28%, PMCA 2%. Only cardiac (versus skeletal) uses all three.
Frank-Starling: stretch → filament proximity → Ca²⁺ sensitivity → more force.
Sympathetic: beta-1 → cAMP → PKA → enhanced contraction and enhanced relaxation. Parasympathetic: M₂, weak, reversed.
Most modulation of smooth and cardiac muscle runs through GPCRs.