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Anatomy and Cell Biology
Anatomy and Cell Biology 3309
Kem Rogers

o Around each muscle fiber is the endomysium This is a loose CT o Around each fascicle is the perimysium This is also a loose CT o Around each muscle is the epimysium This is a dense irregular CT The loose CTs are reticular CTs o Therefore, they have type III collagen The CTs align the contraction of the muscle o They all fuse to form the tendon There are 2 types of skeletal muscle fibers: o Red (type I) These are aerobic They contain myoglobin They are slow to fatigue o White (type II) These are anaerobic They have no myoglobin They are used for short bursts of power or energy, after which they fatigue Depending on the activity, the relative balance of these muscle fiber types can change As mentioned, sarcomeres are the individual units responsible for the contraction of skeletal muscle o They have a distinct structure The Z-lines delineate each sarcomere The A-band represents the thick myofilaments The I band represents the thin myofilaments; there are a lot of mitochondria here The H-zone is the distance between the lateral actin filaments (this distance can decrease during contraction) At the NMJ, each individual muscle cell is innervated o This is where we find a neuronal to muscle synapse o The neurotransmitter released here is Ach It leads to the released of Ca++ from the SR within the muscle cells Ca++ is required for the powerstroke, which causes muscle contraction The SR is found all over muscle fibers o It is a Ca++ store o When stimulated by APs, it releases Ca++ right onto the sarcomere o At each end, there is a terminal cistern as well as transverse T-tubule In skeletal muscle, a triad is formed by a T-tubule with terminal cisternae on each side o By contrast, in cardiac muscle, there is a diad instead Troponin hold tropomyosin over the myosin binding sites on actin o The release of Ca++ and its binding to troponin allows for tropomyosin to be pulled off the binding sites such that myosin can bind the actin Contraction mechanism o We start with myosin bound to actin o ATP binding to myosin causes it to release actin o ATP hydrolysis to ADP+Pi causes a the myosin to become energized This causes myosin to bend 5 nm parallel in the direction of the actin filament o The release of Pi causes attachment to the actin filament o The release of ADP causes the powerstroke o A new ATP molecule comes in to bind the myosin, such that the actin filament is released Therefore, rigor mortis occurs when there is no ATP in the dead body and as a result, the myosin stays bound to actin The main pathology associated with muscle is muscular dystrophy o It is a genetic disease that has no known cure o There are 9 different types Dystrophin is a protein found around each muscle fiber o In DMD, there is a loss of dystrophin o As a result, the muscle eventually starts to fall apart o Along with skeletal muscle, this also affects cardiac muscle, but not as fast The contractile mechanism of smooth muscle is different from that of cardiac or skeletal muscle o Recall that both cardiac and skeletal muscle are striated Cardiac muscle has very obvious structure to it o It is also striated o We dont see myofibrils o The cells are joined by intercalated disks, which 3 components Adherens junctions Desmosomes Gap junctions o The nucleus is central o There is very high blood supply o Cells are very often binucleated o There branches (in longitudinal section) o The control of cardiac muscle contraction is external Cardiomyocytes are post-mitotic cells o It is thought that they dont undergo cytokinesis, even though they may undergo the rest of mitosis This is thought to be the explanation for their binucleated-ness o Organelles are found at the poles of the nucleus o They contain abundant glycogen o They contain myosin HC In cardiac tissue, we also find cardiac fibroblasts, which are helper cells o These are responsible for turning over the matrix TEM reveals that cardiac muscle has the same contractile machinery as skeletal muscle Cardiac muscle vs. Skeletal muscle o Cardiac muscle fibers have more sarcoplasm This is to allow more access to Ca++ o The mitochondria in cardiac muscle are larger and better developed This is due to a higher energy requirement Essentially, every sarcomere has an associated mitochondrion o All the fibers in cardiac muscle are type I (i.e. red fibers with abundant myoglobin that are aerobic) o Glycogen is more common in cardiac muscle It is found in granule form between myofibrils o Cardiac cells dont express glucose-6-phosphatase Normally, cells express this As a result, there is no glucos
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