MUSCLE.

I. Types of muscle: A. Striated muscle Skeletal muscle - voluntary muscle 2. Cardiac muscle - heart muscle

II. Basic components of muscle cell organization A. sarcolemma - plasmalemma of muscle cells - well developed basement membrane. Sarcolemmal tubular invaginations called transverse tubules - associated with the sarcoplasmic reticulum. B. sarcoplasm - cytoplasm of muscle cells excluding the myofibrils. C. sarcoplasmic reticulum - smooth endoplasmic reticulum of muscle cells. D. sarcomere - basic contractile unit of a myofilament/myofibril (skeletal and cardiac muscle only) F. myofibril or myofilament - a string of sarcomeres (skeletal and cardiac muscle only) G. myofiber - a single muscle cell E. sarcosome - specialized long mitochondrion found (skeletal and cardiac muscle only).

H. epimysium - thick layer of collagenous connective tissue that surrounds large bundles of muscle. II. Basic components of muscle cell organization J. fascicle - bundle of muscle cells (myofibers) bounded by perimysium. I. perimysium - collagenous connective tissue continuous with epimysium that separates fascicles of muscle cells. K. endomysium - very thin layer of connective tissue that separates individual muscle cells. fascicle perimysium

III. Skeletal - voluntary muscle B. Skeletal muscle is connected to bone a tendon. A. As just described, epimysium, perimysium, and endomysium surrounds the components of skeletal muscle as. C. Individual muscle cells are syncytial (multinucleate). Nuclei at periphery of cell. D. Each individual muscle cell is called a muscle fiber/myofiber. Within the sarcoplasm of these cells are numerous myofilaments/myofibrils.

E. The myofilaments/myofibrils are linear arrays of structures known as sarcomeres - connected in end to end repeating pattern. The sarcomeres contain filaments of actin and myosin that interact to cause contraction of the muscle cells. Skeletal/voluntary muscle III. Skeletal - voluntary muscle

III. Skeletal - voluntary muscle

F. Axons from CNS form multiple synapses on sarcolemma. Called myoneural junctions, neuromuscular junctions, or motor endplates. Contain synaptic vesicles filled with acetylcholine. Nervous impulses (action potentials) cause acetylcholine in vesicles to be released into the gap between the axon and the muscle cell This initiates an electric action potential in the sarcolemma that causes the muscle to contract. III. Skeletal - voluntary muscle

A = Artefact; Ec = Euchromatin; Hc = Heterochromatin; H = Herzmuskelzelle; Ko = Kollagenfasern (Querschnitt); Ma = Macula adherens; Mi = Mitochondrium (Crista-Typ); Nf = Neurofilamente; Nt = Neurotubuli; P = Plasmalemm; Po = postsynaptische Membran; Pr = praesynaptische Membran; S = synaptischer Spalt; SR = sarcoplasmatisches Reticulum; SV = synaptische Vesikel; TB = terminaler Bouton (= motorische Endplatte, enthält Axolemma). Electron micrograph of motor endplate synapse

G. Sarcoplasmic reticulum - stores calcium that is released to cause contraction. 1. Organization - extensions of the sarcolemma called Transverse tubules (T- tubules) extend between parallel cisternae of the sarcoplasmic reticulum that encircle individual myofilaments (myofibrils). 2. Allows very rapid transmission of surface action potentials to the interior of the cell ----> releasing calcium ions from the sarcoplasmic reticulum and allowing the sarcomeres of the myofilaments to all contract at essentially the same time.

V. Cardiac muscle A. Unlike striated muscle, cardiac muscle cells are not a syncytium for most part, though some cells may have two nuclei. B. The structure is similar to that of striated skeletal muscle - myofilaments (myofibrils) composed of sarcomeres are present. Contraction is mediated by release of Ca+2 from sarcoplasmic reticulum. Fewer T-tubules.

C. Cardiac muscle differs from skeletal muscle in four major ways, V. Cardiac muscle 1. cardiac muscle cells are branched and 4. muscle cells held together by intercalated disks. 2. have 1-2 nuclei per cell 3. nucleus located in center of cell in cross-sections a. Interdigitating regions of the sarcolemma of adjacent cardiac muscle cells that b. Hold the cells together c. Act to conduct ionic changes between cells. d. The intercalated disks form the irregular, jagged, dark lines that are characteristic of stained cardiac muscle. Intercalated disks:

2. The intercalated disk is composed of a number of structures that are organized along adjacent muscle cell sarcolemmas in a repeating array, a. Macula adherens ( desmosomes) - previously described structures that hold cells together. Located between adjacent myofibers. b. zonula adherens - Located where myofilaments end at muscle cell sarcolemma. c. gap junctions - allow transfer of ions between muscle cells. Allows cells to coordinate their activities. Action potentials can spread quickly between the sarcoplasmic reticula of adjacent cardiac muscle cells via gap junctions. V. Cardiac muscle

D. The sarcoplasmic reticulum of cardiac muscle cells is not as regular as that of skeletal/voluntary muscle. V. Cardiac muscle E. Purkinje fibers - modified cardiac muscle cells found in the ventricle that are specialized for conduction of electrical impulses. Conduct the contraction impulses that originate in the sinoatrial node (then passed via atrioventricular node) to the cardiac muscle fibers.

VI. Smooth muscle A. This sort of muscle consists of long, overlapping, spindle shaped cells that look similar to fibroblasts. B. Smooth muscle cells similarity to fibroblasts is evident in that they are able to synthesize collagen, elastin, and proteoglycans. Scanning EM of vascular smooth muscle

C. There are no sarcomere structures, but filaments of actin and a type of myosin are present. Thus, contraction is much less organized and occurs more slowly than it does in skeletal or cardiac muscle. Another reason for this slower contraction is that smooth muscle cells do not contain a transverse tubule system. D. The actin and myosin filaments are not constrained by a sarcomere/myofilament arrangement. Thus, the actin and myosin filaments are able to achieve a greater degree of overlap when contraction occurs resulting in a greater degree of contraction. VI. Smooth muscle

E. Smooth muscle cells have the ability to remain contracted for long periods of time. F. Bundles of smooth muscle are organized as fascicles similar to what is seen in striated and cardiac muscle, Thus, a perimysium with endomysium between cells and epimysium deliniating bundles of fascicles can be identified.

G. The contraction of smooth muscle cells is involuntary and the neuromuscular junctions controlling contractile rhythms may be on the surrounding epimysium rather than directly on muscle cells. As a result, neurotransmitters have to diffuse across this connective tissue layer and onto the plasmalemma of the smooth muscle cells in order to have an effect on contraction. This is another reason for the slower contraction of smooth muscle cells. H. Smooth muscles exhibit spontaneous contractile activity (doesn't require nervous stimulation). Thus, the innervation that is present acts to modify the contractile activity rather than initiate it. VI. Smooth muscle