Page 202 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Physiology of Muscle / 187

receptors. For example, stimulation of blood supply to cardiac muscle quickly
results in myocardial ischemia and the
β ‐adrenergic receptors causes smooth
VetBooks.ir muscle relaxation, while stimulation of α ‐ symptoms of a heart attack. Cardiac mus-
2
1
adrenergic receptors causes smooth mus-
cle may also undergo necrosis (cell death)
cle contraction. Some smooth muscle can if the loss of blood supply is prolonged or
also be stimulated to contract by stimula- extremely severe.
tion of muscarinic receptors by ACh.
Individual smooth muscle cells may have
multiple types of receptors in their cell Excitation and Contraction
membranes and respond to both auto-
nomic neurotransmitters. In this case, the Individual cardiac muscle cells do not
overall response of the smooth muscle also require nerve stimulation to contract, but
depends on the relative levels of the differ- action potentials must occur on the cell
ent neurotransmitters. membrane. Action potentials first occur
spontaneously within specialized myocar-
dial pacemaker cells within the heart, and
Cardiac Muscle these are propagated throughout the heart
by a specialized conduction system and
Cardiac muscle (sometimes known as from cell to cell via the gap junctions (at
involuntary striated muscle) has many intercalated disks). The impulse genera-
anatomic characteristics that are similar to tion and conduction system is described
those of striated skeletal muscle fibers, in detail in Chapter 18. Autonomic nerves
although the striations are fainter than in innervate the pacemaker cells, and these
skeletal muscle. Both types of muscle con- serve to modify the rate of spontaneous
sist largely of sarcoplasm, myofibrils, a sar- action potentials, which in turn determines
coplasmic reticulum, transverse tubules, contraction rate of the entire heart.
nuclei, and a sarcolemma. The most striking The cardiac action potential is much
difference is the tendency for cardiac slower than that of skeletal muscle. It
muscle fibers to branch and join, forming a lasts for hundreds of milliseconds
network. The heart is made up of cells that (1 ms = 1/1000 s), as opposed to 5 to 10 ms
are separate entities; however, unique in skeletal muscle. Also, the contraction
structures, found where cardiac muscle time in cardiac muscle lasts as long as the
cells meet end to end, are the intercalated action potential does. Instead of a sharp
disks. These disks can be seen with the spike potential, the cardiac action potential
light microscope (Fig. 1‐10) and are inter- has a long plateau, which extends the time
posed between muscle cells. The disks of both the action potential and the muscle
represent apposed cell membranes and contraction.
gap junctions. The gap junctions provide a As is true for the other two types of
mechanical attachment between cells and muscle, an increase in intracellular Ca
2+
permit electrical transmission from one must occur to bring about cardiac cell
cardiac muscle cell to the next. Action contraction. In cardiac cells the Ca
2+
potentials can readily spread from cell to enters the cell via electrically gated cell
cell, causing cardiac muscle to act electri- membrane channels and is also released
cally and mechanically as a functional from an extensive sarcoplasmic reticulum
syncytium, as if it were a single cell mass. network. Thus, cardiac muscle has some
Blood vessels and lymphatic vessels are similarities to both smooth and skeletal
2+
both plentiful in cardiac muscle. A gener- muscle. The Ca binds to regulatory pro-
ous blood supply is essential, because teins on the actin filaments, and contrac-
most ATP production depends on aerobic tion occurs in a manner similar to that in
metabolism. In humans disruption of the skeletal muscle.