Human Physiology, 14th edition (2016)

Muscle 397

Skeletal Muscle Cardiac Muscle Smooth Muscle

Striated; actin and myosin

arranged in sarcomeres

Striated; actin and myosin arranged in

sarcomeres

Not striated; more actin than myosin; actin inserts into

dense bodies and cell membrane

Well-developed sarcoplasmic

reticulum and transverse tubules

Moderately developed sarcoplasmic reticulum

and transverse tubules

Poorly developed sarcoplasmic reticulum; no

transverse tubules

Contains troponin in the thin

filaments

Contains troponin in the thin filaments Contains calmodulin, a protein that, when bound to Ca^2 1 ,

activates the enzyme myosin light-chain kinase

Ca^2 1 released into cytoplasm from

sarcoplasmic reticulum

Ca^2 1 enters cytoplasm from sarcoplasmic

reticulum and extracellular fluid

Ca^2 1 enters cytoplasm from extracellular fluid,

sarcoplasmic reticulum, and perhaps mitochondria

Cannot contract without nerve

stimulation; denervation results

in muscle atrophy

Can contract without nerve stimulation; action

potentials originate in pacemaker cells of

heart

Maintains tone in absence of nerve stimulation; visceral

smooth muscle produces pacemaker potentials;

denervation results in hypersensitivity to stimulation

Muscle fibers stimulated

independently; no gap junctions

Gap junctions present as intercalated discs Gap junctions present in most smooth muscles

Table 12.8 | Comparison of Skeletal, Cardiac, and Smooth Muscle

Clinical Investigation CLUES

Mia had both palpitations and hypertension, for which

the cardiologist prescribed a calcium-channel blocker.

• What are the calcium channels blocked by this
  drug, and how does this action relieve Mia’s
  palpitations and hypertension?


• Which particular calcium-channel blocking drug was
  likely prescribed?

| CHECKPOINT

16. Explain how cardiac muscle differs from skeletal
    muscle in its structure and regulation of contraction.
    17a. Contrast the structure of a smooth muscle cell
    with that of a skeletal muscle fiber and discuss the
    advantages of each type of structure.
    17b. Distinguish between single-unit and multiunit smooth
    muscles.
    17c. Describe the events by which depolarization of a smooth
    muscle cell results in contraction and explain why
    smooth muscle contractions are slow and sustained.

CLINICAL APPLICATION

Calcium-channel blockers are drugs that block the voltage-

gated Ca^2 1 channels involved in the contraction of all types of

muscles and in the pacemaker activity of the heart. One subcat-

egory of these drugs, called the dihydropyridines, are relatively

specific for blocking the calcium channels in vascular smooth

muscles. This leads to vasodilation, which lowers the vascular

resistance and blood pressure as a treatment for hypertension.

Verapamil is more specific for the heart and is used to treat

angina (pain) and arrhythmias, and dilitiazen ( Cardizem ) pro-

motes both vasodilation and a slowing of the heartbeat.

Mia went for Botox injections to cause paralysis of facial

muscles that produce wrinkles. This works because

Botox causes destruction of a protein required for the

exocytosis of ACh at the motor end plates. She suffered

from nocturnal leg cramps, likely because low blood Ca^2 1

makes nerves and their muscles more excitable, even

though Ca^2 1 within the myofibers stimulates muscle con-

traction by binding to troponin. The physician advised Mia

to stretch her leg muscles slowly, because this activates

the secondary endings more than it does the primary end-

ings, leading to a less active monosynaptic stretch reflex

than would a rapid stretching of the muscles that might

provoke a muscle spasm. Her chest pain was not a result

of a myocardial infarction, as revealed by normal blood

tests for CK-MB and troponin T. She had palpitations and

hypertension, for which the cardiologist prescribed a cal-

cium channel blocker. A calcium-channel blocker such as

Cardizem could address both issues by slowing the heart

and promoting relaxation of vascular smooth muscle,

producing vasodilation that lowers blood pressure.

See the additional chapter 10 Clinical Investigation on

Multifocal Motor Neuropathy in the Connect site for

this text.

Clinical Investigation SUMMARY