Ch. 12 Cardiac and Smooth Muscles

Question 1

Are cardiac and smooth muscles voluntary or involuntary?

Answer 1

Involuntary

Question 2

Which nervous systems regulates cardiac and smooth muscles?

Answer 2

Autonomic nervous system

Question 3

Contraction of cardiac and smooth muscles is due to?

Answer 3

Myosin/actin cross bridges stimulated by Ca2+

Question 4

Cardiac Muscles

Answer 4

Striated, branched sarcomeres, contraction via sliding filament

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)
–also generates action potentials on its own

Question 5

Myocardium

Answer 5

Mass of cardiac muscle cells connected to each other via gap junctions

• -action potentials (A.P.s) that occur at any once cell can stimulate all the cells in the myocardium
• -it behaves as a single functional unit
• -produces A.P.s automatically (i.e. w/o innervation)
• -Ca2+ channels involved in excitation-contraction coupling different from those in skeletal muscle
• -E.C. coupling slower
• -single nucleus usually

Question 6

Smooth Muscle

Answer 6

Found in blood vessel walls, bronchioles, digestive organs, urinary and 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 and can form cross bridges w/ actin its entire length
–connected by dense bodies

Arrangement allows contraction even when greatly stretched

Question 7

Smooth Muscle: Single-Unit

Answer 7

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 ACh stimulation

Question 8

Smooth Muscle: Multi-Unit

Answer 8

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 9

Autonomic Innervation of Smooth Muscle

Answer 9

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 passent (“in passing”)

Question 10

Excitation-Contraction Coupling in Smooth Muscle

Answer 10

Source for most Ca2+ is extracellular space via voltage-gated calcium channels

Ca2+ binds to calmodulin (no troponin in smooth muscle); activates myosin-light chain kinase (MLCK)

MLCK phosphorylates myosin light chains, allowing cross bridge formation

Stimulation is graded - increased stimulation –> increased Ca2+ entry –> stronger contractions

Contractions are slow and sustained