Each skeletal muscle fiber has many bundles of myofilaments. Each bundle is called a myofibril. The myofilaments of a myofibril are arranged in a regular fashion so that their ends are all lined up. This is what gives the muscle its striated appearance. The contractile units of the cells are called sarcomeres. The sarcoplasmic reticulum is a specialized endoplasmic reticulum that stores calcium ions needed for muscle contraction.

1. SKELETAL MUSCLE

A. Fiber Types.
Skeletal muscle fiber can be classified into red, white, and intermediate types.
B. Cross~striations
1. The A band contains both thin and thick myofilaments. It appears as a dark band by electron microscope.
2. The I band contains only thin myofilaments. It appears as a light band by electron microscope.
3. The H band bisects the A band and contains only thick myofilaments.
4. The Z disk bisects the I band. The distance between two Z disks delimits a sarcomere. The Z disk contains aactinin, which anchors thin filaments to the Z disk.

C. Thin myofilaments
1. F-actin has an active site that interacts with two globular heads of myosin.
2. Tropomyosin blocks the active site on F-actin during relaxation.
3. Troponin C has four binding sites for calcium.

Table. Characteristics of Muscle Fiber Types                                                              
                                                                                  Fiber Type                                         
               Characteristic                                     Red                     White                      Intermediate     
Speed of contraction                                          Slow                    Fast                         Fast
Myoglobin content                                              High                    Low                         High
Amount of mitochondria                                     Many                   Few                         Many
Generation of ATP                                             Aerobic                Anaerobic               Aerobic
Glycogen content                                               Low                      High                       Intermediate
Succinate, clehydrogenase level                        High                     Low                        Medium to high
NADH clehydrogenase level                               High                     Low                        Medium to high 
NADH = reduced nicotinamide adenine dinucleotide.

D. Thick myofilaments
I. Myosin can be cleaved by trypsin into light meromyosin and heavy meromyosm. Heavy meromyosin has two globular heads. The globular heads have actin-binding sites and have ATPase activity.
2. Titin anchors myosin to the Z disks and helps the muscle to accomodate extreme stretching.
E. Changes in contracted and stretched muscle. The cross - striational pattern of skeletal muscle changes when it is contracted or stretched. These changes are caused by the degree of interdigitation of the thin and thick myofilaments.

Table. Conformational Changes in Contracted and Stretched Muscle                                       
                                                 Change (Compared to Relaxed Muscle)                                    
         Band                    Contracted Muscle                       Stretched Muscle                              
      A band                    No change                                   No change
       I band                    Shortens                                       Lengthens
      H band                    Shortens                                       Lengthens 
      Z disks                     Move closer together                    Move farther apart                             

F. A triad consists of a transverse tubule (T tubule) flanked by two terminal cisternae of the sarcoplasmic reticulum. It is located at the A-1 junction.
1. A T tubule is an invagination of the cell membrane. It transmits a depolarization signal from the neuromuscular junction to the depths of a muscle cell.
2. Terminal cisternae are dilated sacs of sarcoplasmic reticulum that store, release, and reaccumulate calcium ions.
3. In response to a nerve depolarization, calcium ions are released from the terminal cisternae, initiating a muscle contraction.

G. Innervation. A single axon of an alpha motoneuron may innervate a single muscle cell, or an axon may branch and innervate many muscle cells (160 or more). An axon together with all the muscle cells it innervates is called a motor unit.
H. Neuromuscular junction
1. Synaptic terminals are found in depressions of the skeletal muscle fiber called pri. mary synaptic clefts. The sarcolemma in this area is thrown into numerous folds that are called secondary synaptic clefts.
2. The basal laminae of both the nerve cell and muscle cell fuse and line the primary and secondary synaptic clefts.
3. Synaptic terminals contain a few mitochondria and many synaptic vesicles, which store acetylcholine.
4. The membrane of the synaptic terminal is called the presynaptic membrane. This is where exocytotic release of acetylcholine occurs. The sarcolemma of the synaptic cleft is called the postsynaptic membrane. It contains acetylcholine receptors that function as Na' ion channels. The synaptic cleft contains acety1cholinesterase.
I. Skeletal muscle regeneration is limited. Following injury or extensive excercise, satellite cells proliferate and fuse to form new skeletal muscle cells. Triad Terminal Terminal cisterna T tubule cisterna Figure 5-2. Ultrastructure of the triad. Ion fluxes are depicted.

2. CARDIAC MUSCLE

A. Cardiac myocytes
1. Cardiac myocytes contract through intrinsically generated excitations that are passed to neighboring cells by gap junctions; that is, the heart beat is myogenic.
2. Cardiac myocytes contain a diad, which consists of a T tubule flanked by one cisterna of sarcoplasmic reticulum. It is located at the Z disk.
3. The ends of neighboring cardiac myocytes have interdigitating cell surfaces that are attached by intercalated disks. Intercalated disks consist of a fascia adherens, desmosomes, and gap junctions.
B. Purkinje myocytes are modified cardiac myocytes.
C. Cardiac muscle regeneration is virtually nonexistent. Injured cardiac myocytes are generally replaced by connective tissue.

3. SMOOTH MUSCLE

A. Structural characteristics of smooth muscle cells include the following:
1. Cytoplasmic dense bodies (equivalent to Z disks) that contain a-actinin
2. Subplasmalemmal dense plaques that contain vinculin and talin
3. Actin and myosin filaments, as well as desmin and vimentin intermediate filaments (vimentin is found predominately in vascular smooth muscle)
4. Numerous invaginations of the cell membrane called caveolae (equivalent to T tubules)
B. Functional characteristics of smooth muscle cells include the following:
1. They use the protein calmodulin to bind calcium.
2. They synthesize many components of the extracellular matrix, such as laminin, Collagen, and elastin.
C. Contraction mechanisms
1. Smooth muscle cells 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. Smooth muscle cells 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 the muscle surface and synapse en passant.
3. Myoepithelial cells of the mammary gland contract through hormonal excitations generated by oxytocin secreted from the neurohypophysis.
D. Smooth muscle regeneration is very high. 

•  skeletal, cardiac and smooth muscle
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