Smooth muscle


Smooth muscle tissue is also known as visceral muscle tissue. It is layered in a distinctive pattern of circular layers. This smooth muscle can be found surrounding the walls of the blood vessels, the bronchioles in the lungs, and the sphincter muscles used in the GI tract. The GI tract, which is tubular by design, also houses longitudinal muscles in addition to the smooth muscles of the circular pattern. These same longitudinal muscles can also be found in the ureters of the urinary system, the ductus differentia of the male reproductive system, and the uterine tubes of the female reproductive system. This systematic layering of smooth longitudinal muscles and circular smooth muscles contribute to the body’s ability to push fluids through its tubular organs. Through the process of alternate contraction, fluids pass through an undulating encouragement of these particular systems.


Striations found in the skeletal muscles and the cardiac muscles are created by sarcomeres. These sarcomeres are not located in the smooth muscle tissue of the autonomic nervous system. The smooth muscle fibers are in fact equipped with large volumes of actin and smaller volumes of myosin. This combination of elements creates a thin to thick myofiliment ratio of 16:1 as opposed to the striated ratio of 2:1. The smooth muscle gains operative ability due to the length of the myosin myofiliments as well as the lack of organized sarcomere fibers. This functional status is directly impacted upon by this arrangement. The smooth muscle fibers have to be reliable, even when stretched or tested, to operate autonomously. A perfect visual example of this is the urinary bladder, which can stretch as much as two and a half times their original resting state. For these muscles, the resting state is a constant tension state. Skeletal muscles have to rely upon the elasticity of their structure and can no longer be functional if they are stretched beyond the point of overlapping myosin fibers and sarcomeres.


Smooth Muscles
Image: Smooth Muscles


Single unit and multi unit smooth muscle groups create the two distinct muscular groups of the functional categories. Single unit smooth muscles are held together via electric adjacent cells by which are responsible for synonymous function. The electrical synapse of these muscles creates numerous gap junctions. This means the muscle fibers act as a single unit.

In contrast, the multi unit smooth muscle fibers have limited, if any, gap junctions. The cells rely on individual stimulation and respond to the individual stimulation on an independent basis. They rely on neurons for their action potentials. This is very similar to the state of skeletal muscles, as both groups numerous motor units must be activated to create a unified response. The stimulation given off by single unit smooth muscles is that of pacemaker activity, and often their stimuli can create new stimuli for other mass units of cells. A single unit smooth muscle unit will also react differently than other muscle groups when considering electrical activity and their response to muscular contraction. Single unit muscles will react with myogenic energy and intrinsic contraction when stretched to and beyond capacity.

A perfect example of this is within the GI tract. When the luminal contents of the small artery are stretched the GI tract is able to stimulate myogenic contraction. This is in direct contrast to skeletal muscles and is actually devoid of necessary nerve stimulation.


Smooth muscles and skeletal muscles vary greatly when considering neural control. A skeletal muscle is created with one conjunction of a single somatic nerve fiber. The membrane of the skeletal muscle fiber then houses a localized neurotransmitter at this neuromuscular junction point. The receptors for this neurotransmitter are housed at the same junction point. In contrast, the smooth muscle fiber is equipped with neurotransmitter receptor proteins. An autonomic nerve fiber is positioned away from the actual smooth muscle fibers. Along this autonomic nerve, stimulation releases neurotransmitter molecules despite the location distance. This structure of autonomic neurotransmitter molecules can be detected along the nerve fiber, appearing as bulges in the locations responsible for the release of the molecules. These bulges, also known as varicosities, are responsible for the release of molecules that affect numerous smooth muscle fibers.
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