Smooth Muscle

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Chapter: Anatomy and Physiology for Health Professionals: Support and Movement: Muscle Tissue

Smooth muscle lacks coarse connective tissue sheaths such as are found in the skeletal muscle.

Smooth Muscle

Smooth muscle lacks coarse connective tissue sheaths such as are found in the skeletal muscle. Most smooth muscle is made up of sheets of fibers that are closely apposed, such as in the walls of all except the smallest blood vessels and in the walls of hollow organs of var-ious systems. Usually, there are two sheets of smooth muscle, with their fibers situated at right angles to each other. In a smooth muscle’s longitudinal layer, fibers run parallel to the long axis of the organ. When these fibers contract, the organ shortens. In a circular layer, fibers are located around the organ’s circumference. When this layer contracts, the lumen of the organ con-stricts. Alterations of contraction and relaxation result in peristalsis. Examples of such smooth muscle action include the rectum, urinary bladder, uterus, lungs, and stomach.

The thin filaments of smooth muscle have no troponin complex. There are more thin filaments than thick filaments. However, the thick and thin fil-aments are arranged diagonally. Smooth muscles lack sarcomeres­ because there are no striations. There is an intermediate filament dense body network. These intermediate filaments resist tension. Dense bodies are also attached to the sarcolemma, anchoring thin fil-aments. These dense bodies correspond to the Z discs found in skeletal muscle. In smooth muscle sarco-plasm, calcium ions interact with calmodulin, which is a calcium-binding protein that activates the enzyme myosin kinase. This enables myosin heads to attach to actin. Stretched smooth muscles adapt to their new lengths and remain able to contract on demand, a con-dition known as plasticity.

In the digestive tract, pacesetter cells spon-taneously trigger contraction of entire sheets of muscle. They are also called pacemaker cells. When excited, they set the pace of contraction for the entire muscle sheet. The membrane potentials of these cells fluctuate and are self -excitatory. In the absence of external stimuli, they depolarize spontaneously. The rate and intensity of smooth muscle contraction can, however, be modified by both neural and chemical stimuli.

In smooth muscles, innervating nerve fibers exist, which are part of the autonomic (involuntary) nervous system. They have many bulb-like swellings called varicosities, which release neurotransmitters into a wide synaptic cleft near smooth muscle cells. These are known as diffuse junctions. The sarco-lemma of smooth muscles has many caveolae, which are pouch-like infoldings. The caveolae contain some extracellular fluid with a high concentration of cal-cium ions near the membrane. T-tubules are absent in smooth muscle. When calcium channels open in the caveolae, calcium ion influx occurs quickly. Most of these ions enter through calcium channels directly from the extracellular space. When cytoplasmic cal-cium is actively transported into the SR and out of the cell, contraction ends.

Myoblasts are the embryonic mesoderm cells from which most muscle tissue develops. Develop-ment occurs quickly, with the embryo experiencing skeletal muscle fiber contraction by week 7. The sur-faces of developing myoblasts are initially covered with ACh receptors. Spinal nerves eventually penetrate the muscle masses. Nerve endings seek out individual myoblasts and release agrin, a growth factor. Agrin activates MuSK, a muscle kinase. This stimulates clus-ters of ACh receptors at new neuromuscular junctions and maintains them in each muscle fiber. The nerve endings release another chemical to eliminate recep-tor sites that have not been innervated or stabilized by the released agrin.

Contraction of Smooth Muscle

Smooth muscle contraction is similar to that of skeletal muscle, using actin, myosin, calcium ions, and ATP; however, smooth muscle is also affected by another neurotransmitter—norepinephrine. Certain smooth muscles are stimulated by these neurotransmitters, whereas others are inhibited. A number of hormones also influence the actions of smooth muscles. Smooth muscle contracts and relaxes more slowly than skel-etal muscle. The whole muscular sheet responds to a stimulus in unison because there is electrical cou-pling of smooth muscle cells by gap junctions. This differs from skeletal muscle, in which the fibers are electrically isolated from each other. Skeletal muscle fibers are stimulated to contract by their own neu-romuscular junctions. The gap junctions of smooth muscle allow the transmission of action potentials from fiber to fiber. With the correct amount of ATP, it can maintain forceful contractions for a longer period. Smooth muscles can change length without changing how taut they are. A summary of smooth muscle contraction is

The sliding filament mechanism occurs for the interaction of actin and myosin.

A rise in the intracellular calcium ion level is the final trigger for contraction.

ATP energizes the sliding process.

Smooth muscle takes approximately 30 times lon-ger than skeletal muscle to contract and relax. How-ever, it uses much less energy. It can maintain the same amount of contractile tension for long periods, with less than 1% of the energy expended. In small arterioles and other visceral organs, the smooth mus-cle regularly maintains a small amount of contraction (smooth muscle tone) with fatigue. Because of its low energy requirements, the aerobic pathways of smooth muscle manufacture adequate amounts of needed ATP.

Most adjacent smooth muscles have slow, syn-chronized contractions, in which the whole muscu-lar sheet responds in unison to stimuli. As discussed earlier, this is related to gap junctions, which allow action potentials to be transmitted from fiber to fiber. The pacemaker cells control the pace of contractions. The speed of the ATPases of smooth muscle is much slower than in skeletal muscle. Myofilaments may con-nect during prolonged contractions, which help save energy. The contraction of smooth muscle is regulated by neural, hormonal, and local chemical factors.

Neural regulation: Neurotransmitter binding generates an action potential. This is coupled to increased calcium ions in the cytosol. Some types of smooth muscle only respond to ­neural stimulation­ with graded potentials, which are local electrical signals. Different autonomic nerves serve visceral smooth muscle, releasing different neurotransmitters. Each of these can excite or inhibit groups of smooth muscle cells. This is based on the type of receptor molecules on the sarcolemma of cells.

Hormonal regulation: Hormones are chemicals that have the ability to affect smooth muscle con-traction. The hormone gastrin, for example, sim-ulates the stomach to contract so that its churning actions of food are more efficient.

Local chemical regulation: Smooth muscles that lack a nerve supply depolarize sponta-neously or in response to chemicals that bind to protein-linked receptors. Other smooth muscle­ cells may respond to chemical as well as neural stimuli. Direct responses to chemical stimuli affects smooth muscle activity based on local tissue needs. Chemical factors can cause contraction or relaxation without action potentials­ by altering calcium ion entry into the sarcoplasm. It is important to understand that chemical factors may include some hormones as well as histamine, lack of oxygen, low pH, and excess carbon dioxide.

Unique factors of smooth muscle contraction include response to stretching and changes in length and tension. Stretching of smooth muscle causes contraction, resulting in the automatic movement of substances along internal tracts. The increased tension is only brief and the smooth muscle quickly adapts to stretching, and then relaxes. It is still able to contract as needed. Therefore, the stress relax-ation response is important for organs such as the intestines and stomach, which must store their con-tents for enough time so that digestion and nutrient absorption can occur. The urinary bladder also must store urine, which is continuously made, until it can be voided.

Types of Smooth Muscle

Smooth muscles vary in fiber arrangement, fiber orga-nization, innervation, and how they respond to stim-uli. There are two major types of smooth muscle:

Visceral smooth muscle: Also known as uni-tary smooth muscle, it exists in the walls of all hollow organs except for the heart. It is the most common type of smooth muscle. Its cells are arranged in opposing sheets, either circular or longitudinal. They are innervated by auto-nomic nerve fiber varicosities. The cells often show rhythmic spontaneous action potentials. They are electrically coupled by gap junctions. Therefore, the cells contract as a unit. Recruit-ment is not utilized in unitary smooth muscle and it is able to respond to various chemical stimuli. Visceral smooth muscle fibers can stim-ulate each other and adjacent fibers experience excitability. They display a pattern of repeated contractions known as rhythmicity, which is caused by self-exciting fibers. The wave-like motion of many tubular organs, known as peristalsis­, is caused by these features of vis-ceral smooth muscle. Peristalsis helps to move the contents of organs such as the intestines from the stomach to the outside of the body.

Multiunit smooth muscle: This type exists in the large lung airways, large arteries, arrector pili muscles of the hair follicles, and the internal eye muscles that adjust the pupils and allow the eyes to focus. Gap junctions and spontaneous depolarizations are rare. Multiunit smooth mus-cle has fibers that are structurally independent of each other. It has many nerve endings, with each forming a motor unit having a number of mus-cle fibers. Multiunit smooth muscle, has only one nucleus, responds to neural stimulation and has graded contractions involving recruitment. It is also innervated by the autonomic nervous system and responds to hormones.

1. Explain the actions of pacesetter cells.

2. Discuss the effects of gap junctions in smooth muscle.

3. Compare visceral smooth muscle and multiunit smooth muscle.

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