n myofilaments of actin. A Z line bisects the I band, and serves as a point of attachment for actin filaments. The functional unit of skeletal muscle is the sarcomere, which extends from one Z line to the next. When a muscle contracts, all the sarcomeres in all the myofibrils shorten at the same time.
30.15 The unique structure of cardiac muscle cells allows action potentials to travel rapidly from one cell to another, and allows the cardiac muscle to contract as a whole unit. The unique structure in cardiac muscle is the gap junction. Gap junctions are intercellular channels, which permit free passage of ions and small molecules up to a molecular weight of about 1000 Daltons between cells (Junctions, 2004). They are constructed from between four and six copies of a group of membrane proteins appropriately known as connexins. Gap junctions allow changes in membrane potential to flow from cell to cell rapidly.
31.2 For the heart to pump productively, the contractions of the atria and the ventricles must be synchronized: both atria must contract at the same time, and so must both ventricles (Applegate, 2000, 249). This is achieved through the nodal system of specialized muscle fibers. The sinoatrial node (SA) is located in the posterior wall of the right atrium, close to the entrance to the superior vena cava. The SA node rhythmically initiates impulses from 70 to 80 times per minute, establishing the rhythm of the heart. It is known as the heart's pacemaker. Impulses from the SA node travel throughout the atrial myocardium and cause both atria to contract simultaneously.
At the same time, the impulses reach the second part of the conduction system, the atrioventricular (AV) node (Applegate, 2000, 250). This is located in the floor of
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