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.

Smooth muscle comes in various types including: multi-unit smooth muscle, unitary (single unit) smooth muscle, and vascular smooth muscle. 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. Thanks to electric coupling this muscle subtype is able to have coordinated contraction.

Multi unit smooth muscle is present in the iris, ciliary muscle of the lens, and vas deferens. It has little to no electric coupling between cells thus acts as separate motor units. Because of this it is controlled by neural innervation, and is thus densely innervated. Vascular smooth muscle shares properties of both multi-unit and single unit smooth muscle.

The sympathetic nervous system signals the activation of the fight-or-flight response in the body (including increased heart, blood vessel and eye pupil dilation, and increased stress hormone release). The sympathetic nervous system signal is transmitted via two chains of neurons — the preganglionic neuron and the postganglionic neuron. The preganglionic neuron uses the neurotransmitter acetylcholine. The postganglionic neuron uses the neurotransmitter norepinephrine.