Smooth Muscle Figs and Endocrine Figs

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Smooth Muscle Figs and Endocrine Figs

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 14 Current Lecture Types of Smooth Muscle There are two main types of smooth muscles - single unit and multiunit. These terms refer to whether they act as a single unit or multiple units. In the single unit, the smooth muscle cells are coupled by extensive gap junctions. The excitation, whether by second messengers or depolarization, spreads through gap junctions so the whole thing behaves as one unit. You can see the autonomic postganglionic axon with synaptic boutons and varicosities releasing transmitters, and the types of smooth muscles that are of this sort occur throughout the body. But we will be focusing on the smooth muscle of blood vessels (constriction and dilation), in the intestine (responsible for motility), and in the full term uterus (powerful contractions during labor and delivery) Multi-unit: each cell works independently. Examples: 1. the ones in the eye, such as the one that controls the iris, and the biliary body as shown above (when they contract, they change the shape of the lens and are responsible for accommodation – how your eyes focus when they are looking at something close). 2. The non pregnant uterus, the smooth muscle is multiunit. At the end of pregnancy, in addition to proliferating, the smooth muscle cells become coupled through gap junctions. A few smooth muscles don’t receive any innervation. These tend to be intrinsically active, usually due to spontaneous changes in membrane potential. When we look at the heart, we can see the ionic mechanism by which the cardiac muscle cells and especially the pacemaker have rhythmic intrinsic changes in membrane potential that result in periodic action potentials. Well, a similar thing happens to smooth muscles - the smooth muscles that don’t receive innervation are all single unit. They are subject to hormonal and paracrine control, but are not necessarily innervated. Most smooth muscles receive innervation from autonomic neurons though. In general, when they get depolarized, they contract, and when hyperpolarized, they relax. In addition, hormones affect all smooth muscles whether they are innervated or not. 2 paracrine agents that have been well studied influence the activity of smooth muscle – histamine (important regulator of inflammation – in the immune and circulatory system and allergic response) and nitric oxide, the gas which is synthesized by endothelial cells (innermost cells lining blood vessels). NO diffuses out to the muscles of the blood vessels and causes them to relax (important in regulation of blood flow in tissues). There are also stretch sensitive tissue in some muscles – example would be bladder – when it gets full, and smooth muscle gets stretched, they respond with depolarization and contract, increasing pressure on the bladder contents. Blood vessels do the same thing, because they have has stretch sensitive channels. Excitation due to the stretching of blood vessels is important in maintaining blood pressure. Regulation of Ca2+ in Smooth Muscle Cells

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