Cardiac Muscle Forms a Functional Syncytium
Although the molecular basis of contraction is the same for cardiac and skeletal muscle, the two muscle types differ in regard to electrical linkages between neighboring cells, and this difference has important consequences.
Individual skeletal muscle cells are electrically isolated (insulated) from one another, so action potentials cannot “jump” from one skeletal muscle cell to another. As described in Chapter 5, an action potential in a skeletal muscle cell is initiated only in response to an action potential in the somatic motor neuron that innervates the skeletal muscle cell. Each neural action potential causes release of the neurotransmitter acetylcholine, which activates nicotinic cholinergic receptors on the skeletal muscle cell, which in turn depolarizes the muscle cell to threshold for the formation of an action potential. Once formed, the action potential propagates along the length of that particular muscle cell and then stops. The muscle action potential causes the cell to contract. Neighboring cells may or may not contract at the same time, depending on whether or not action potentials are initiated in the neighboring cells by their motor neurons.In contrast, cardiac muscle cells are electrically linked to one another. When an action potential is started in a single cardiac muscle cell, it propagates along the length of that cell. At specialized points of contact with neighboring cells, ionic currents then “jump the gap” and initiate action potentials in the neighboring cells. Because cardiac action potentials propagate from cell to cell through cardiac tissue, neighboring cardiac muscle cells all contract in synchrony, as a unit, and then they all relax. In this regard, cardiac muscle tissue behaves as if it were a single cell. Cardiac muscle is therefore said to form a functional syncytium (literally, “acts like same cell”).
The specialized cellular structures that allow cardiac action potentials to propagate from cell to cell are evident under the light microscope (see Figure 19-1). Cardiac muscle appears as an array of fibers (individual cardiac muscle cells) that are arranged approximately in parallel but with some branching. Adjacent cells are joined together by dark-appearing structures called intercalated disks. Electron microscopy has revealed that within these disks are tiny open channels between neighboring cells. These nexi, or gflp junctions, provide points of contact between the intracellular fluid of adjacent cells. When an action potential depolarizes the cell on one side of an intercalated disk, positive ions flow through the gap junctions and into the neighboring cell. This local, ionic current depolarizes the neighboring cell to threshold for the formation of an action potential. In effect, an action potential propagates from cell to cell through the gap junctions that are located within the intercalated disks. Skeletal muscle does not have intercalated disks or nexi (gap junctions).