Stimulation of Smooth Muscle

Like cardiac muscle, smooth muscle is involuntary and capable of contracting without nervous stimulation. Some smooth muscle contracts in response to chemical stimuli such as hormones, carbon dioxide, low pH, and oxygen deficiency and in response to stretch (as in a full stomach

Mucosa

Epithelium -

Lamina propria Muscularis mucosae

Submucosa

Muscularis externa

Circular layer-

Mucosa

Epithelium -

Submucosa

Muscularis externa

Circular layer-

Longitudinal layer

Esophagus Cross Sections

Figure 11.22 Layers of Visceral (single-unit) Smooth Muscle in a Cross Section of the Esophagus. Many hollow organs have alternating circular and longitudinal layers of smooth muscle.

Longitudinal layer

Figure 11.22 Layers of Visceral (single-unit) Smooth Muscle in a Cross Section of the Esophagus. Many hollow organs have alternating circular and longitudinal layers of smooth muscle.

or bladder). Some single-unit smooth muscle, especially in the stomach and intestines, has pacemaker cells that spontaneously depolarize and set off waves of contraction throughout an entire layer of muscle. Such smooth muscle is autorhythmic, like cardiac muscle, although with a much slower rhythm.

But like cardiac muscle, smooth muscle is innervated by autonomic nerve fibers that can trigger or modify its contractions. Autonomic nerve fibers stimulate smooth muscle with either acetylcholine or norepinephrine. The nerve fibers have contrasting effects on smooth muscle in different locations. They relax the smooth muscle of arteries while contracting the smooth muscle in the bronchioles of the lungs, for example.

In single-unit smooth muscle, each autonomic nerve fiber has up to 20,000 beadlike swellings called varicosities along its length (figs. 11.21 and 11.23). Each varicosity contains synaptic vesicles and a few mitochondria. Instead of closely approaching any one myocyte, the nerve fiber passes amid several myocytes and stimulates all of them at once when it releases its neurotransmitter. The muscle cells do not have motor end plates or any other specialized area of sarcolemma to bind the neurotransmit-ter; rather, they have receptor sites scattered throughout the surface. Such nerve-muscle relationships are called diffuse junctions because there is no one-to-one relationship between a nerve fiber and a myocyte.

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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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