Iiismooth Muscle

A. Smooth muscle fibers (cells) have the following characteristics:

1. They have cytoplasmic dense bodies (equivalent to Z disks), which contain «-actinin

2. Smooth muscle fibers have subplasmalemmal dense plaques, which contain vin-culin and talin

3. Smooth muscle fibers contain actin and myosin filaments, as well as desmin and vimentin intermediate filaments (Vimentin is found predominately in vascular smooth muscle.)

4. They have numerous invaginations of the cell membrane called caveolae (equivalent to T tubules)

5. They use the protein calmodulin to bind calcium

6. Smooth muscle fibers synthesize many extracellular components such as laminin, collagen, and elastin

B. Contraction mechanisms

1. Intrinsic. Smooth muscle fibers of the gastrointestinal tract contract through intrinsically generated excitations that are passed to neighboring cells by gap junctions. Postganglionic autonomic neurons only modify the intrinsic activity.

2. Neural. Smooth muscle fibers of the large arteries, ciliary muscle and iris of the eye, and ductus deferens contract through neural excitations generated by postganglionic autonomic neurons that pass over rhe muscle surface and synapsi en passant. No neuromuscular junctions are present as in skeletal muscle.

3. Hormonal. Myoepithelial cells of the mammary gland and smooth muscle fibers of the myometrium contract through hormonal excitations generated by oxytocin secreted from the neurohypophysis. Epinephrine and norepinephrine also stimulate smooth muscle fiber contraction.

C. Smooth muscle repair (regeneration) is very high. IV. COMPARISONS AND CONTRASTS (Table 6-3)

Table 6-3

Comparison of Muscle Types

Table 6-3

Comparison of Muscle Types

Muscle Type

Characteristic

Skeletal

Cardiac

Smooth

Nucleus

Multinucleated; peripheral location

Single nucleus; central location

Single nucleus; central location

Bands

A and 1 bands

A and 1 bands

Absent

Z disks

Present

Present

Absent; dense bodies are present

T tubules

Present

Present

Absent; caveolae are present

Cell junctions

Absent

Absent

Gap junctions

Neuromuscular junction

Present

Absent; contraction is intrinsic

Absent; contraction is intrinsic, neural, or hormonal

Stretch receptors

Present

Absent

Absent

Calcium ion binding

Troponin

Troponin

Calmodulin

Regeneration

Limited

None

High

V. SELECTED PHOTOMICROGRAPHS

A. Skeletal muscle (Figure 6-3; see I B)

Figure 6-3. (A) Light micrograph of skeletal muscle cut longitudinally. Note the A hand (dark) and I band (ligfit). The I hand is bisected by the Z disk (arrows). A sarcomere (Z disk to Z disk) is indicated by the double-headed arrow. A peripherally located nucleus (N) is shown. (B) Electron micrograph of skeletal muscle cut longitudinally. Note the A band (dark) and I band (light) and H band. The I band is bisected by the Z disk (single-headed arrows). A sarcomere is indicated. A peripherally located nucleus (N) is shown. (Reprinted with permission from Fawcett DW: A Textbook of Histology, 12th cd. New York, Chapman Hall, 1994, p 280. Courtesy of Don W. Fawcett, M.D.) (C-E). Electron micrographs of skeletal muscle cut in cross-section showing the characteristic arrangement of myofilaments in the H band, A band, and 1 band, respectively. The H band shows only thick myofilaments (large black dots). The A band shows both thick myofilaments (large black dots) surrounded by six thin myofilaments (small black dots). The I band shows only thin myofilaments (small black dots). (C and E reprinted with permission from Weiss L: Histology: Cell and Tissue Biology, 5th ed. New York, Elsevier, 1983, p 264. D reprinted with permission from Fawcett DW: A Textbook of Histology, 12th ed. New York, Chapman Hall, 1994, p 280. Courtesy of Don W. Fawcett, M.D.)

Figure 6-3. (A) Light micrograph of skeletal muscle cut longitudinally. Note the A hand (dark) and I band (ligfit). The I hand is bisected by the Z disk (arrows). A sarcomere (Z disk to Z disk) is indicated by the double-headed arrow. A peripherally located nucleus (N) is shown. (B) Electron micrograph of skeletal muscle cut longitudinally. Note the A band (dark) and I band (light) and H band. The I band is bisected by the Z disk (single-headed arrows). A sarcomere is indicated. A peripherally located nucleus (N) is shown. (Reprinted with permission from Fawcett DW: A Textbook of Histology, 12th cd. New York, Chapman Hall, 1994, p 280. Courtesy of Don W. Fawcett, M.D.) (C-E). Electron micrographs of skeletal muscle cut in cross-section showing the characteristic arrangement of myofilaments in the H band, A band, and 1 band, respectively. The H band shows only thick myofilaments (large black dots). The A band shows both thick myofilaments (large black dots) surrounded by six thin myofilaments (small black dots). The I band shows only thin myofilaments (small black dots). (C and E reprinted with permission from Weiss L: Histology: Cell and Tissue Biology, 5th ed. New York, Elsevier, 1983, p 264. D reprinted with permission from Fawcett DW: A Textbook of Histology, 12th ed. New York, Chapman Hall, 1994, p 280. Courtesy of Don W. Fawcett, M.D.)

B. Neuromuscular junction (Figure 6-4; see I G)

Figure 6-4. (A) Electron micrograph of a neuromuscular junction. A myelinated axon (N) loses its myelin sheath (at the arrow) and ends in a synaptic terminal on the surface of a skeletal muscle fiber (muscle). At the junction of the nerve and muscle fiber, the cell membrane of the muscle fiber is thrown into junctional folds (jf; brackets). The boxed area is shown at high magnification in B. (Reprinted with permission from Ellisman M: J Cell Biol 68:752, 1976.) (B) High magnification of the neuromuscular junction (boxed area in A). A collection of synaptic vesicles (st>) that contain acetylcholine (ACh) is indicated along with the presynaptic membrane (single arrow) where ACh is released. The postsynaptic membrane (double arrows) that contain ACh receptors (AChR) is shown. The bracket indicates the postsynaptic membrane of the skeletal muscle fiber thrown into junctional folds (jf). The synaptic cleft (large arrow) containing the electron-dense basal lamina and acetylcholinesterase (AChE) is shown. (Reprinted with permission from Lindgren CA, Emery DG, Haydon PG: Intracellular acidification reversibly reduces endocytosis at the neuromuscular junction. J Neurosci 17:3074,1997.)

Figure 6-4. (A) Electron micrograph of a neuromuscular junction. A myelinated axon (N) loses its myelin sheath (at the arrow) and ends in a synaptic terminal on the surface of a skeletal muscle fiber (muscle). At the junction of the nerve and muscle fiber, the cell membrane of the muscle fiber is thrown into junctional folds (jf; brackets). The boxed area is shown at high magnification in B. (Reprinted with permission from Ellisman M: J Cell Biol 68:752, 1976.) (B) High magnification of the neuromuscular junction (boxed area in A). A collection of synaptic vesicles (st>) that contain acetylcholine (ACh) is indicated along with the presynaptic membrane (single arrow) where ACh is released. The postsynaptic membrane (double arrows) that contain ACh receptors (AChR) is shown. The bracket indicates the postsynaptic membrane of the skeletal muscle fiber thrown into junctional folds (jf). The synaptic cleft (large arrow) containing the electron-dense basal lamina and acetylcholinesterase (AChE) is shown. (Reprinted with permission from Lindgren CA, Emery DG, Haydon PG: Intracellular acidification reversibly reduces endocytosis at the neuromuscular junction. J Neurosci 17:3074,1997.)

C. Intercalated disk (Figure 6-5; see 11 A 3)

Figure 6-5. Electron micrograph of an intercalated disk in cardiac muscle. An intercalated disk is found at the junction of two cardiac myocytes and is typically arranged in a stair-srep pattern. The intercalated disk consists of a fascia adherens (fa), desmosomes (des), and gap junction (gap). The gap junction is always oriented parallel to the myofilaments. (Reprinted with permission from Wheater PR: Functional Histology. New York, Churchill Livingstone, 1987, p 94.)

Figure 6-5. Electron micrograph of an intercalated disk in cardiac muscle. An intercalated disk is found at the junction of two cardiac myocytes and is typically arranged in a stair-srep pattern. The intercalated disk consists of a fascia adherens (fa), desmosomes (des), and gap junction (gap). The gap junction is always oriented parallel to the myofilaments. (Reprinted with permission from Wheater PR: Functional Histology. New York, Churchill Livingstone, 1987, p 94.)

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