Cardiac Muscle

Question 1

Answer 1

•smooth muscle

The figure represents a blood vessel wall with endothelial cells and smooth muscle. One mechanism for smooth muscle cell contraction is shown in the figure. Following a stimulus, calcium enters the cell via a calcium channel and binds to calmodulin (rather than troponin as in skeletal and cardiac muscle cells). The calcium-calmodulin complex activates myosin light chain kinase (MLCK). Activated MLCK phosphorylates myosin in the cell, which activates myosin. (Remember in skeletal and cardiac, myosin is always active but the binding site on actin is blocked.) Once myosin is active, it interacts with actin and enters the cross-bridge cycle. As shown in the figure, contraction can terminate and relaxation occur through activation of myosin light chain phosphatase (MLCP), which dephosphorylates and inactivates myosin. In the figure, activation of MLCP is shown via nitric oxide (NO) signaling via guanylate cyclase and cGMP. Activation of MLCP can also occur by beta-2-adrenergic receptor signaling via adenylate cyclase and cAMP (not shown in the figure).

Question 2

Answer 2

cardiac muscle

Question 3

Answer 3

skeletal muscle

Question 4

differences between skeletal muscle and cardiac muscle

Answer 4

•cardiac muscle has a higher concentration of mitochondria

• high O2 demand
• doesn’t fatigue

•in cardiac muscle, calcium induced calcium release

• L-type Ca++ channels open allowing calcium to enter from the extracellular space, which then stimulates the release of calcium from sarcoplasmic reticulum stores
• cardiac muscles have short fibers
• cardiac cells are branched
• cardiac muscles have only 1-2 nuclei, skeletal muscle cells are multi nucleated
• intercalated disks that have gap junctions - increase electrical conductivity, cells act as one

-cardiac muscles are interconnected, muscle cells are not

Question 5

smooth muscle cells

Answer 5

• walls of blood vessels and airways
• approx same size as cardiac cells
• have only one nucleus
• not branched
• no sarcomeres
• form layers that are usually arranged so that one runs parallel to an organ and the other wraps around it
• no T-tubules and not an extensive sarcoplasmic reticulum
• actin and myosin organized into dense bodies attached to sarcolemna
• contraction produces a twisting or corkscrew motion
• no troponin - calmodulin binds calcium and activates myosin cross-bridge formation
• steady supply of ATP for sustained contractions, not powerful contractions
• can maintain contractions through a latch state during which actin and myosin remain locked together in the absence of Ca++ ions

-no ATP required - sustained contractions without using energy

•two types

-single unit smooth muscle cells and multiunit

•not under voluntary control

• spontaneous
• pacesetter cells
• ANS or hormones

•neuromuscular junctions not present, but varicosities, enlargements along autonomic nerves release neurotransmitters into synaptic clefts

Question 6

single unit smooth muscle cells

Answer 6

• gap junctions - electrically coupled
• hollow organs

*small blood vessels

•stimulated spontaneously or by stretch

Question 7

multi unit smooth muscle cells

Answer 7

•rarely possess gap junctions

*autonomic nerves or hormones

• large blood vessels
• respiratory airways