Ch 12- cardiac/smooth muscles

Question 1

Cardiac Muscle

Answer 1

• striated, sarcomeres (branched), contraction via sliding filaments
• ->Unlike skeletal muscle fibers, these fibers are short, branched, and connected via gap junctions called intercalated discs (ELECTRICAL synapses that permit impulses to be conducted cell to cell) –> could have just ONE neuron signaling all fibers
• ->also, generates action potentials on it’s own

Question 2

Myocardium

Answer 2

mass of cardiac muscle cells connected to each other via gap junctions.

• -A.P.s that occur at any one cell can stimulate all the cells in the myocardium
• -It behaves as a single functional unit.
• -Produces action potentials automatically (i.e. without innervation)
• -Ca2+ channels involved in excitation-contraction coupling different from those in skeletal muscle (Ca comes from SR and extracellular space)
• -E.C. coupling slower
• single nucleus usually

Question 3

Smooth Muscle

Answer 3

• found in: blood vessel walls, bronchioles, digestive organs, urinary & reproductive tracts–produce peristaltic waves to propel contents of these organs
• NO sarcomeres, but still contain large amounts of actin and myosin
• Long actin filaments attached to dense bodies
• Myosin filaments are stacked vertically/horizontally and can form cross bridges with actin its entire length (connected by dense bodies)
• Arrangement allows contraction even when greatly stretched

Question 4

Smooth Muscle: Single-Unit

Answer 4

Multiple gap junctions that make neighboring cells behave as a unit

• Most smooth muscles are single-unit.
• They display pacemaker activity moderated by stretch or autonomic innervation.
• Only a few cells in a single-unit receive acetylcholine stimulation.

Question 5

Smooth Muscle: Multiunit

Answer 5

Require individual nerve innervation (no pacemaker activity)

• Few or no gap junctions
• Arrector pili muscles in skin and ciliary muscles in eyes are multi-unit

Question 6

Autonomic Innervation of Smooth Muscle

Answer 6

• Neurotransmitter is released along the length of an autonomic neuron from varicosities, receptor proteins along the entire surface of smooth muscle cells
• A number of smooth muscle cells are stimulated at once
• Form synapses “en passant” (“in passing”)

Question 7

Excitation-Contraction Coupling in Smooth Muscle

Answer 7

• ->Source for most Ca2+ is extracellular space via voltage-gated calcium channels
• Ca2+ binds to calmodulin (b/c NO troponin in smooth muscle) –> activates myosin light-chain kinase (MLCK)
• MLCK phosphorylates myosin light chains, allowing cross bridge formation and contraction
• Stimulation is graded–↑ stimulation –> ↑ Ca2+ entry  –> stronger contractions
• Contractions are slow and sustained