Page 729 - Fundamentals of anatomy physiology

716 Unit 4 Fluids and Transport Intercalated disc
Figure 20–5 Cardiac Muscle Cells. Gap junction

Cardiac muscle cell Z-lines bound to
Mitochondria opposing plasma
Intercalated
disc (sectioned) membranes

Desmosomes

Nucleus b Structure of an intercalated disc
Cardiac muscle Intercalated discs
cell (sectioned)
Bundles of

myo brils

a Cardiac muscle cells

Cardiac muscle tissue LM × 575
c Cardiac muscle tissue
20

Table 20–1 Structural and Functional Differences between Cardiac Muscle Cells and Skeletal Muscle Fibers

Feature Cardiac Muscle Cells Skeletal Muscle Fibers

Size 10–20 m 3 50–100 m 100 m 3 up to 40 cm
Nuclei Typically 1 (rarely 2–5) Multiple (hundreds)

Contractile proteins Sarcomeres along myofibrils Sarcomeres along myofibrils

Internal membranes Short T tubules; no triads formed with sarcoplasmic reticulum Long T tubules form triads with cisternae of the sarcoplasmic
reticulum

Mitochondria Abundant (25% of cell volume) Much less abundant

Inclusions Myoglobin, lipids, glycogen Little myoglobin, few lipids, but extensive glycogen reserves

Blood supply Very extensive More extensive than in most connective tissues, but sparse
compared with supply to cardiac muscle cells
Metabolism (resting) Not applicable Aerobic, primarily lipid-based
Metabolism (active) Aerobic, primarily using lipids and carbohydrates Anaerobic, through breakdown of glycogen reserves
Contractions Twitches with brief relaxation periods; long refractory period Usually sustained contractions
prevents tetanic contractions
Stimulus for contraction Autorhythmicity of pacemaker cells generates action potentials Activity of somatic motor neuron generates action potentials
in sarcolemma

Trigger for contraction Calcium entry from the ECF and calcium release from the Calcium release from the sarcoplasmic reticulum
sarcoplasmic reticulum

Intercellular connections Branching network with plasma membranes locked together at Adjacent fibers tied together by connective tissue fibers
intercalated discs; connective tissue fibers tie adjacent layers
together