The presence of gap junctions within the intercalated discs of contractile cardiac myocytes allows for the rapid passage of ions from one cell to another. Once pacemaker cells in the sinoatrial node of the heart spontaneously generate action potentials, this wave of depolarization spreads into neighboring contractile myocytes via gap junctions. These gap junction connections are crucial to the heart operating in a unified and coordinated fashion, and are responsible for the characteristic wavelike contraction of the heart from the apex to the base. 

The pericardial membrane is the tissue that surrounds the heart. The easiest way to determine the answer in this problem is to understand that "cardial" indicates pertinence heart and that "peri" is a prefix meaning "around."

Cardiac contraction begins in the sinoatrial node. The impulse travels through both atria then followed by arriving at the atrioventricular node, which slows the impulse to allow for complete atrial contraction and ventricular filling. Then the impulse travels through the bundle of His, which branches into the right and left bundle branches and through the Purkinje fibers in the walls of both ventricles generating a strong contraction.

Cardiac output (CO) is defined as:

Rearrange to solve for heart rate.

Of the three major muscle groups, smooth muscle is the only type not composed of sarcomeres. As a result, smooth muscle does not appear striated under a microscope. Smooth muscle is under involuntary control, innervated by the autonomic nervous system, and contains mononucleated cells.

Skeletal muscle is striated, multinucleated, and under voluntary control. Cardiac muscle is striated, mononucleated, and under involuntary control.

The physician has removed a sample of skeletal muscle (the extensor hallicus longus) as well as smooth muscle, which typically surrounds structures like sweat glands.

The smooth muscle is not under voluntary control and uses a unique mechanism of contraction, different from the striated pattern expected of sarcomeres in skeletal muscle. Instead of sarcomeres, smooth muscle contracts by using myosin light chain kinase to phosphorylate myosin, which can then interact with actin to cause a contraction. Due to the absence of striations, we can conclude the muscle type 3 corresponds to smooth muscle, and will be present in the sample.

Skeletal muscle is known for having distinct striations. Gap junctions, as seen in muscle type 2, would be characteristic only of cardiac muscle, and would not be found in skeletal muscle. We can conclude that muscle type 1 is skeletal muscle, and will be present in the sample.

There are three primary types of muscle tissue: smooth, skeletal, and cardiac. Skeletal muscle and cardiac muscle are striated, but smooth muscle is not.

Striations are formed by the organized units of actin and and myosin (sarcomeres) in a linear pattern. Smooth muscle still uses the mechanics of actin and myosin to generate contractile forces, but the filaments are not organized into sarcomeres. As a result, smooth muscle cells are not as linear as skeletal or cardiac muscle cells and contraction will not result in the standard "shortening" of the cell. Rather, contraction of a smooth muscle cell will cause the entirity of the cell to shrink inward. This is essential for smooth muscle functions that cause a reduction in surface area, such as the constriction of boold vessels.

One of the two types of smooth muscle cells is called single-unit, where the cells are connected by gap junctions and the cells contract as a single unit. The other type of smooth muscle is called multi-unit, where the cells are not linked and each cell has to be individually stimulated. Single-unit cells are often found in the GI tract. A good example of mulit-unit cells would be in vascular smooth muscle. 

Smooth muscle has thick and thin filaments that are not arranged in sarcomeres. Thus, smooth muscle does not appear striated, and appears homogenous. Smooth muscle comes in various types including: multi-unit smooth muscle, unitary (single unit) smooth muscle, and vascular smooth muscle.

Smooth muscle comes in various types including: multi-unit smooth muscle, unitary (single unit) smooth muscle, and vascular smooth muscle. Multi-unit smooth muscle is present in the iris, ciliary muscle of the lens, and vas deferens. It behaves as separate motor units and has little to no electric coupling between cells. It is densely innervated and contraction is controlled by neural innervation. Unitary smooth muscle is the most common type of smooth muscle and is present in the uterus, gastrointestinal tract, ureter, and bladder. It is spontaneously active (has slow waves) and has “pacemaker” activity that is modulated by hormones and neurotransmitters. There is a high degree of electric coupling and allows for coordinated contraction. Vascular smooth muscle has properties of both multi-unit and single unit smooth muscle. Smooth muscle is not striated.