Factors influencing contractile activity of smooth muscle

Many factors influence the contractile activity of smooth muscle. The strength of contraction of multiunit smooth muscle may be enhanced by stimulation of a greater number of cells, or contractile units. This mechanism is directly comparable to motor-unit recruitment employed by skeletal muscle. As the number of contracting muscle cells increases, so does the strength of contraction. However, this mechanism is of no value in single-unit smooth muscle. Due to the presence of gap junctions, all of the muscle cells in the tissue are activated at once.

Other factors that influence contractile activity include:

• Autonomic nervous system

• Hormones and blood-borne substances

• Locally produced substances

• Intracellular calcium concentration

The autonomic nervous system (ANS) modifies contractile activity of both types of smooth muscle. As discussed in Chapter 9, the ANS innervates the smooth muscle layer in a very diffuse manner, so neurotransmitter is released over a wide area of muscle. Typically, the effects of sympathetic and parasympathetic stimulation in a given tissue oppose each other; one system enhances contractile activity while the other inhibits it. The specific effects (excitatory or inhibitory) that the two divisions of the ANS have on a given smooth muscle depend upon its location.

Many hormones and other blood-borne substances (including drugs) also alter contractile activity of smooth muscle. Some of the more important substances include: epinephrine; norepinephrine; angiotensin II; vaso-pressin; oxytocin; and histamine. Locally produced substances that may alter contraction in the tissue in which they are synthesized include: nitric oxide; prostaglandins; leukotrienes; carbon dioxide; and hydrogen ion.

All of these factors (ANS stimulation, blood-borne and locally produced substances) alter smooth muscle contractile activity by altering the intracellular concentration of calcium. An increase in cytosolic calcium leads to an increase in crossbridge cycling and therefore an increase in tension development. The concentration of calcium within the cytoplasm of the smooth muscle cell may be increased by several mechanisms, including:

• Voltage-gated Ca++ channels

• Ligand-gated Ca++ channels

• Stretch-activated Ca++ channels

Voltage-gated Ca++ channels open when the smooth muscle cell is depolarized. Calcium then enters the cell down its electrochemical gradient. Ligand-gated Ca++ channels are associated with various hormone or neurotransmitter receptors. Binding of a given substance to its receptor causes the ligand-gated Ca++ channel to open and, once again, Ca++ ions enter the cell. This process, which occurs without a significant change in membrane potential (due to a simultaneous increase in Na+ ion removal from the cell), is referred to as pharmacomechanical coupling.

Inositol triphosphate (IP3)-gated channels are also associated with membrane-bound receptors for hormones and neurotransmitters. In this case, binding of a given substance to its receptor causes activation of another membrane-bound protein, phospholipase C. This enzyme promotes hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) to IP3. The IP3 then diffuses to the sarcoplasmic reticulum and opens its calcium channels to release Ca++ ions from this intracellular storage site.

Finally, an increase in volume or pressure within a tube or hollow organ causes stretch or distortion of the smooth muscle in the organ wall. This may cause activation of stretch-activated Ca++ channels. The subsequent influx of calcium initiates contraction of the smooth muscle. This process is referred to as myogenic contraction and is common in blood vessels.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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