Smooth Muscle Physiology
Structure of Smooth Muscle
- Spindle-shaped and small
- One nucleus per cell that is centrally located
- Capable of division throughout life, unlike skeletal muscle, which doesn’t divide
- Filaments
o Myosin, actin, and tropomyosin are present
o NO TROPONIN
o Filaments arranged diagonally
o Not in sarcomeres à no striations
- Dense bodies: anchor thin filaments to the plasma membrane or cytoplasmic
structures
- Location
o Walls of hollow organs: GI tract, blood vessels. Bladder, ureters, airways
o Stroma of organs like the prostate
Length-Tension relationship
- Cross-bridge activation
o Requires phosphorylation of myosin by myosin light chain kinase (MLCK)
o Ca2+ binds to Calmodulin à activates MLCK
o Phosphorylated myosin can bind to actin and cycle
o Relaxation: dephosphorylation by myosin light chain phosphatase (MLCP)
Sources of Ca2+
- Sarcoplasmic reticulum
- Extracellular fluid via voltage-gated Ca2+ channels
Removal of Ca2+
- Active transport: Ca2+ is pumped back into SR or out to ECF
- Slower than in skeletal muscle à longer single twitch
, Graded tension
- Tension depends on how much Ca2+ is present
- Not all or nothing
- More Ca2+ à more cross bridge activation à graded contraction
Smooth muscle tone
- Even at rest, low level Ca2+ keeps some cross bridges active
- Maintains baseline tension
Inputs influencing contraction
- Spontaneous electical activity
o Pacemaker cells: generate APs on their own
- Neurotransmitters
o Autonomic neurons via varicosities
o No motor end plate
o Single cell may receive input from multiple neurons
- Hormones
o AXect smooth muscle by binding to receptors
o Excitatory or inhibitory
- Local factors
o Paracrine signals, pH, O2/CO2, osmolarity, nitric oxide
o NO induces relaxation
- Stretch
o Activated mechanically gated ion channels à depolarization à contraction
Structure of Smooth Muscle
- Spindle-shaped and small
- One nucleus per cell that is centrally located
- Capable of division throughout life, unlike skeletal muscle, which doesn’t divide
- Filaments
o Myosin, actin, and tropomyosin are present
o NO TROPONIN
o Filaments arranged diagonally
o Not in sarcomeres à no striations
- Dense bodies: anchor thin filaments to the plasma membrane or cytoplasmic
structures
- Location
o Walls of hollow organs: GI tract, blood vessels. Bladder, ureters, airways
o Stroma of organs like the prostate
Length-Tension relationship
- Cross-bridge activation
o Requires phosphorylation of myosin by myosin light chain kinase (MLCK)
o Ca2+ binds to Calmodulin à activates MLCK
o Phosphorylated myosin can bind to actin and cycle
o Relaxation: dephosphorylation by myosin light chain phosphatase (MLCP)
Sources of Ca2+
- Sarcoplasmic reticulum
- Extracellular fluid via voltage-gated Ca2+ channels
Removal of Ca2+
- Active transport: Ca2+ is pumped back into SR or out to ECF
- Slower than in skeletal muscle à longer single twitch
, Graded tension
- Tension depends on how much Ca2+ is present
- Not all or nothing
- More Ca2+ à more cross bridge activation à graded contraction
Smooth muscle tone
- Even at rest, low level Ca2+ keeps some cross bridges active
- Maintains baseline tension
Inputs influencing contraction
- Spontaneous electical activity
o Pacemaker cells: generate APs on their own
- Neurotransmitters
o Autonomic neurons via varicosities
o No motor end plate
o Single cell may receive input from multiple neurons
- Hormones
o AXect smooth muscle by binding to receptors
o Excitatory or inhibitory
- Local factors
o Paracrine signals, pH, O2/CO2, osmolarity, nitric oxide
o NO induces relaxation
- Stretch
o Activated mechanically gated ion channels à depolarization à contraction
