BPK 105 Study Guide - Final Guide: Intercalated Disc, Cardiac Muscle, Resting Potential
27 Apr 2018
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Modules 7 & 8 - Objectives - Part 3
● Describe the flow of blood through the heart.
Refer to figure 12.10 in textbook.
● Describe the structure and function of cardiac muscle cells.
Cardiac muscle cells are elongated, branching cells that contain one, or occasionally two,
centrally located nuclei (figure 12.13). Cardiac muscle cells contain actin and myosin
myofilaments organized to form sarcomeres, which are joined end-to-end to form myofibrils (see
chapter 7). The actin and myosin myofilaments are responsible for muscle contraction, and their
organization gives cardiac muscle a striated (banded) appearance much like that of skeletal
muscle. However, the striations are less regularly arranged and less numerous than in skeletal
muscle.
Like skeletal muscle, cardiac muscle relies on Ca2+ and ATP for contraction. Calcium ions enter
cardiac muscle cells in response to action potentials and activate the process of contraction
much as they do in skeletal muscle. ATP production depends on O2 availability. Cardiac muscle
cells are rich in mitochondria, which produce ATP at a rate rapid enough to sustain the normal
energy requirements of cardiac muscle. An extensive capillary network provides adequate O2 to
the cardiac muscle cells. Unlike skeletal muscle, cardiac muscle cannot develop a significant
oxygen deficit. Development of a large oxygen deficit could result in muscular fatigue and
cessation of cardiac muscle contraction.
Cardiac muscle cells are organized into spiral bundles or sheets (see figure 12.9). When cardiac
muscle fibers contract, not only do the muscle fibers shorten but the spiral bundles twist to
compress the contents of the heart chambers. Cardiac muscle cells are bound end-to-end and
laterally to adjacent cells by specialized cell-to-cell contacts called intercalated disks (figure
12.13). The membranes of the intercalated disks are highly folded, and the adjacent cells fit
together, greatly increasing contact between them and preventing cells from pulling apart.
Specialized cell membrane structures in the intercalated disks called gap junctions (see
chapter 4) allow cytoplasm to flow freely between cells. This enables action potentials to pass
quickly and easily from one cell to the next. The cardiac muscle cells of the atria or ventricles,
therefore, contract at nearly the same time. The heart’s highly coordinated pumping action
depends on this characteristic.
● Describe action potentials in cardiac muscle.
Like action potentials in skeletal muscle and neurons, those in cardiac muscle exhibit
depolarization followed by repolarization. In cardiac muscle, however, a period of slow
repolarization greatly prolongs the action potential (figure 12.14). In contrast to action potentials
in skeletal muscle, which take less than 2 milliseconds (ms) to complete, action potentials in
cardiac muscle take approximately 200 to 500 ms to complete. In addition, unlike in skeletal
muscle, action potentials in cardiac muscle are conducted from cell to cell.
Document Summary
Modules 7 & 8 - objectives - part 3. Describe the flow of blood through the heart. Describe the structure and function of cardiac muscle cells. Cardiac muscle cells are elongated, branching cells that contain one, or occasionally two, centrally located nuclei (figure 12. 13). Cardiac muscle cells contain actin and myosin myofilaments organized to form sarcomeres, which are joined end-to-end to form myofibrils (see chapter 7). The actin and myosin myofilaments are responsible for muscle contraction, and their organization gives cardiac muscle a striated (banded) appearance much like that of skeletal muscle. However, the striations are less regularly arranged and less numerous than in skeletal muscle. Like skeletal muscle, cardiac muscle relies on ca2+ and atp for contraction. Calcium ions enter cardiac muscle cells in response to action potentials and activate the process of contraction much as they do in skeletal muscle.
