Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 5Excitable Tissue: Muscle 109

CORRELATION BETWEEN MUSCLE

FIBER LENGTH & TENSION

The relation between initial fiber length and total tension in
cardiac muscle is similar to that in skeletal muscle; there is a
resting length at which the tension developed on stimulation
is maximal. In the body, the initial length of the fibers is deter-
mined by the degree of diastolic filling of the heart, and the
pressure developed in the ventricle is proportionate to the vol-
ume of the ventricle at the end of the filling phase (Starling’s
law of the heart). The developed tension (Figure 5–18) in-
creases as the diastolic volume increases until it reaches a max-
imum, then tends to decrease. However, unlike skeletal
muscle, the decrease in developed tension at high degrees of
stretch is not due to a decrease in the number of cross-bridges
between actin and myosin, because even severely dilated
hearts are not stretched to this degree. The decrease is due in-
stead to beginning disruption of the myocardial fibers.
The force of contraction of cardiac muscle can be also
increased by catecholamines, and this increase occurs without
a change in muscle length. This positive ionotropic effect of
catecholamines is mediated via innervated β 1 -adrenergic
receptors, cyclic AMP, and their effects on Ca2+ homeostasis.
The heart also contains noninnervated β 2 -adrenergic recep-
tors, which also act via cyclic AMP, but their ionotropic effect
is smaller and is maximal in the atria. Cyclic AMP activates
protein kinase A, and this leads to phosphorylation of the
voltage-dependent Ca2+ channels, causing them to spend
more time in the open state. Cyclic AMP also increases the
active transport of Ca2+ to the sarcoplasmic reticulum, thus
accelerating relaxation and consequently shortening systole.
This is important when the cardiac rate is increased because it
permits adequate diastolic filling (see Chapter 31).

METABOLISM

Mammalian hearts have an abundant blood supply, numerous

mitochondria, and a high content of myoglobin, a muscle pig-

ment that can function as an O 2 storage mechanism. Normally,

less than 1% of the total energy liberated is provided by anaero-

bic metabolism. During hypoxia, this figure may increase to

nearly 10%; but under totally anaerobic conditions, the energy

liberated is inadequate to sustain ventricular contractions. Un-

der basal conditions, 35% of the caloric needs of the human

heart are provided by carbohydrate, 5% by ketones and amino

acids, and 60% by fat. However, the proportions of substrates

utilized vary greatly with the nutritional state. After ingestion of

large amounts of glucose, more lactate and pyruvate are used;

during prolonged starvation, more fat is used. Circulating free

fatty acids normally account for almost 50% of the lipid utilized.

In untreated diabetics, the carbohydrate utilization of cardiac

muscle is reduced and that of fat is increased.

SMOOTH MUSCLE MORPHOLOGY

Smooth muscle is distinguished anatomically from skeletal

and cardiac muscle because it lacks visible cross-striations.

Actin and myosin-II are present, and they slide on each other

to produce contraction. However, they are not arranged in

regular arrays, as in skeletal and cardiac muscle, and so the

striations are absent. Instead of Z lines, there are dense bodies

in the cytoplasm and attached to the cell membrane, and these

are bound by α-actinin to actin filaments. Smooth muscle also

contains tropomyosin, but troponin appears to be absent. The

isoforms of actin and myosin differ from those in skeletal

muscle. A sarcoplasmic reticulum is present, but it is less ex-

tensive than those observed in skeletal or cardiac muscle. In

general, smooth muscles contain few mitochondria and de-

pend, to a large extent, on glycolysis for their metabolic needs.

TYP ES

There is considerable variation in the structure and function

of smooth muscle in different parts of the body. In general,

smooth muscle can be divided into unitary (or visceral)

smooth muscle and multiunit smooth muscle. Unitary

smooth muscle occurs in large sheets, has many low-resis-

tance gap junctional connections between individual muscle

cells, and functions in a syncytial fashion. Unitary smooth

muscle is found primarily in the walls of hollow viscera. The

musculature of the intestine, the uterus, and the ureters are ex-

amples. Multiunit smooth muscle is made up of individual

units with few (or no) gap junctional bridges. It is found in

structures such as the iris of the eye, in which fine, graded con-

tractions occur. It is not under voluntary control, but it has

many functional similarities to skeletal muscle. Each multi-

unit smooth muscle cell has en passant endings of nerve fibers,

but in unitary smooth muscle there are en passant junctions

on fewer cells, with excitation spreading to other cells by gap

FIGURE 5–18 Length–tension relationship for cardiac
muscle. Comparison of the systolic intraventricular pressure (top trace)
and diastolic intraventricular pressure (bottom trace) display the devel-
oped tension in the cardiomyocyte. Values shown are for canine heart.

Systolic

intraventricular

pressure

Diastolic

intraventricular

pressure

Diastolic volume (mL)

Developed

tension

Pressure (mm Hg)

270

240

210

180

150

120

90

60

30

0

10 20 30 40 50 60 70