Smooth muscle lacks coarse connective tissue sheaths such as are found in the skeletal muscle.
Smooth muscle lacks coarseconnective tissue sheaths such as are found in the skeletal muscle. Most smoothmuscle is made up of sheets of fibers that are closely apposed, such as in thewalls of all except the smallest blood vessels and in the walls of holloworgans 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 parallelto 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. Whenthis layer contracts, the lumen of the organ con-stricts. Alterations ofcontraction and relaxation result in peristalsis. Examples of such smoothmuscle action include the rectum, urinary bladder, uterus, lungs, and stomach.
The thin filaments of smoothmuscle have no troponin complex. There are more thin filaments than thickfilaments. However, the thick and thin fil-aments are arranged diagonally.Smooth muscles lack sarcomeres because there are no striations. There is anintermediate filament dense body network. These intermediate filaments resisttension. Dense bodies are also attached to the sarcolemma, anchoring thin fil-aments. Thesedense bodies correspond to the Z discs found in skeletal muscle. In smoothmuscle sarco-plasm, calcium ions interact with calmodulin, which is a calcium-binding protein that activates the enzyme myosin kinase. This enables myosin heads to attachto actin. Stretched smooth musclesadapt to their new lengths and remain able to contract on demand, a con-ditionknown as plasticity.
In the digestive tract, pacesetter cellsspon-taneously trigger contraction of entire sheets of muscle. They are alsocalled pacemaker cells. When excited,they set the pace of contraction for the entire muscle sheet. The membranepotentials of these cells fluctuate and are self -excitatory. In the absence ofexternal stimuli, they depolarize spontaneously. The rate and intensity ofsmooth muscle contraction can, however, be modified by both neural and chemicalstimuli.
In smooth muscles, innervatingnerve fibers exist, which are part of the autonomic (involuntary) nervoussystem. They have many bulb-like swellings called varicosities, which release neurotransmitters into a wide synaptic cleft near smoothmuscle cells. These are known as diffusejunctions. The sarco-lemma of smoothmuscles has many caveolae, which are pouch-like infoldings. The caveolae contain someextracellular fluid with a high concentration of cal-cium ions near themembrane. T-tubules are absent in smooth muscle. When calcium channels open inthe caveolae, calcium ion influx occurs quickly. Most of these ions enterthrough calcium channels directly from the extracellular space. Whencytoplasmic cal-cium is actively transported into the SR and out of the cell,contraction ends.
Myoblasts are the embryonic mesoderm cellsfrom which most muscle tissue develops.Develop-ment occurs quickly, with the embryo experiencing skeletal muscle fibercontraction by week 7. The sur-faces of developing myoblasts are initiallycovered with ACh receptors. Spinal nerves eventually penetrate the musclemasses. Nerve endings seek out individual myoblasts and release agrin, a growth factor. Agrin activates MuSK, a muscle kinase. This stimulatesclus-ters of ACh receptors at new neuromuscular junctions and maintains them ineach muscle fiber. The nerve endings release another chemical to eliminaterecep-tor sites that have not been innervated or stabilized by the releasedagrin.
Contractionof Smooth Muscle
Smooth muscle contraction issimilar to that of skeletal muscle, using actin, myosin, calcium ions, and ATP;however, smooth muscle is also affected by anotherneurotransmitter—norepinephrine. Certain smooth muscles are stimulated by theseneurotransmitters, whereas others are inhibited. A number of hormones alsoinfluence the actions of smooth muscles. Smooth muscle contracts and relaxesmore slowly than skel-etal muscle. The whole muscular sheet responds to astimulus in unison because there is electrical cou-pling of smooth muscle cellsby gap junctions. This differs fromskeletal muscle, in which the fibers are electrically isolated from each other.Skeletal muscle fibers are stimulated to contract by their own neu-romuscularjunctions. The gap junctions of smooth muscle allow the transmission of actionpotentials from fiber to fiber. With the correct amount of ATP, it can maintainforceful contractions for a longer period. Smooth muscles can change lengthwithout changing how taut they are. A summary of smooth muscle contraction is
■■ The sliding filament mechanism occurs for the interaction of actin andmyosin.
■■ A rise in the intracellular calcium ion level is the final trigger forcontraction.
■■ ATP energizes the sliding process.
Smooth muscle takes approximately30 times lon-ger than skeletal muscle to contract and relax. How-ever, it usesmuch less energy. It can maintain the same amount of contractile tension forlong periods, with less than 1% of the energy expended. In small arterioles andother visceral organs, the smooth mus-cle regularly maintains a small amount ofcontraction (smooth muscle tone) withfatigue. Because of its low energy requirements, the aerobic pathways of smoothmuscle manufacture adequate amounts of needed ATP.
Most adjacent smooth muscles haveslow, syn-chronized contractions, in which the whole muscu-lar sheet respondsin unison to stimuli. As discussed earlier, this is related to gap junctions,which allow action potentials to be transmitted from fiber to fiber. Thepacemaker cells control the pace of contractions. The speed of the ATPases ofsmooth muscle is much slower than in skeletal muscle. Myofilaments may con-nectduring prolonged contractions, which help save energy. The contraction ofsmooth muscle is regulated by neural, hormonal, and local chemical factors.
■■ Neural regulation:Neurotransmitter binding generates anaction potential. This is coupled to increased calcium ions in the cytosol.Some types of smooth muscle only respond to neural stimulation with gradedpotentials, which are local electrical signals. Different autonomic nervesserve visceral smooth muscle, releasing different neurotransmitters. Each ofthese can excite or inhibit groups of smooth muscle cells. This is based on thetype of receptor molecules on the sarcolemma of cells.
■■ Hormonal regulation: Hormonesare chemicals that have the abilityto affect smooth muscle con-traction. The hormone gastrin, for example,sim-ulates the stomach to contract so that its churning actions of food aremore efficient.
■■ Local chemical regulation:Smooth muscles that lack a nervesupply depolarize sponta-neously or in response to chemicals that bind toprotein-linked receptors. Other smooth muscle cells may respond to chemical aswell as neural stimuli. Direct responses to chemical stimuli affects smoothmuscle activity based on local tissue needs. Chemical factors can causecontraction or relaxation without action potentials by altering calcium ionentry into the sarcoplasm. It is important to understand that chemical factorsmay include some hormones as well as histamine, lack of oxygen, low pH, andexcess carbon dioxide.
Unique factors of smooth musclecontraction include response to stretching and changes in length and tension.Stretching of smooth muscle causes contraction, resulting in the automaticmovement of substances along internal tracts. The increased tension is onlybrief and the smooth muscle quickly adapts to stretching, and then relaxes. Itis still able to contract as needed. Therefore, the stress relax-ation responseis important for organs such as the intestines and stomach, which must storetheir con-tents for enough time so that digestion and nutrient absorption canoccur. The urinary bladder also must store urine, which is continuously made,until it can be voided.
Types ofSmooth Muscle
Smooth muscles vary in fiberarrangement, fiber orga-nization, innervation, and how they respond tostim-uli. There are two major types of smooth muscle:
■■ Visceralsmooth muscle: Also known as uni-tary smoothmuscle, it exists in thewalls of all hollow organs except forthe heart. It is the most common type of smooth muscle. Its cells are arrangedin opposing sheets, either circular or longitudinal. They are innervated byauto-nomic nerve fiber varicosities. The cells often show rhythmic spontaneous action potentials. They are electricallycoupled by gap junctions. Therefore, the cells contract as a unit. Recruit-mentis not utilized in unitary smooth muscle and it is able to respond to variouschemical stimuli. Visceral smooth muscle fibers can stim-ulate each other andadjacent fibers experience excitability. They display a pattern of repeated contractions known asrhythmicity, which is caused byself-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 fromthe stomach to the outside of the body.
■■ Multiunitsmooth muscle: This type exists in the large lung airways, large arteries, arrector pilimuscles of the hair follicles, and the internal eye muscles that adjust thepupils and allow the eyes to focus. Gap junctions and spontaneous depolarizationsare rare. Multiunit smooth mus-cle has fibers that are structurally independentof each other. It has many nerve endings, with each forming a motor unit havinga number of mus-cle fibers. Multiunit smooth muscle, has only one nucleus,responds to neural stimulation and has graded contractions involvingrecruitment. It is also innervated by the autonomic nervous system and respondsto hormones.
1. Explainthe actions of pacesetter cells.
2. Discussthe effects of gap junctions in smooth muscle.
3. Comparevisceral smooth muscle and multiunit smooth muscle.