Muscle tissue Flashcards
Four types of contracting cells
- Myoepithelial cells: expels secretion from Gland
- Pericytes: around small blood vessels
- Myofibroblasts: secrete Collagen
- Myoid cells: outside basal lamina of seminiferous tubules in testes
Types of muscles
> Skeletal muscle
Cardiac muscle
Smooth muscle
Skeletal muscle
- cross striations
- contraction is quick and vigorous
- contraction is voluntary
- LOCOMOTION; MASTICATION; PHONATION
structure of skeletal muscle
- muscle composed of fibers
- each Fiber: multinucleate structure–> symplast
- cells: myoblasts (somites)
layers
- Epimysium (dense irregular with blood vessels and nerves)
- Perimysium (surrounds fascicles)
- Endomysium (surrounds fibers)
- Sacrolemma (surrounds fibers)
Skeletal muscle fiber
- nuclei are in the periphery
- MYOFIBRIL fill 80% of the Fiber
- myofibrils surrounded by sacroplasm with mitochondria, sER, glycogen
Cross striation of skeletal muscle
- Alternation of I and A bands –> Arrangement of myofilaments
- I bands: light, thin, bisected by Z lines (Actin)
- A bands: dark, thick, bisected by H lines and further bisected by M lines (actin and Myosin)
- H bands: thick Filaments only (Myosin)
Z lines divide each myofibril in sacromeres
Thin (actin) filament
three proteins:
- Actin (Kügelchen)
- Tropomyosin (Bänder)
- Troponin
Actin
- globular proteins
- globules are called G-ACTIN
- Many g-actin globules from a strand of F-ACTIN
- two f-actin make a double helix
Tropomyesin
- coil protein
- 7 globules of g-actin Long
- in regulating interactions between actin and Myosin –> “hides” the active Myosin binding sites on actin filaments
Troponin
three regulatory proteins:
1- Troponin C: binds to Calcium
2- Troponin T: binds to tropomyosin
3- Troponin I: binds to actin –> inhibits Myosin-actin binding
Thick (Myosin) filament
- composed of Myosin
- each Myosin molecule has tail –> fixed to M line
- has two heads
- heads hydrolyze ATP, bind to active sites on actin and form cross bridges
Structural proteins in a sacromere apart from Myosin and actin
Titin (Myosin associated)
Nebulin (actin associated)
Myomesin (inside M line)
Alpha actinin (inside Z line)
The sliding Filament theory
- thick and thin Filaments slide over each other
- all Filaments stay constant in length
- amout of overlap: contraction and Relaxation
- A bands: constant in width
- I & H bands: narrow during contraction
- Z lines move Closer togehter –> contraction
Conduction System for contractile Stimuli
- T tubules of sacrolemma
2. Terminal cisternae
T (transverse) tubules
- extensions of the Plasma membrane (sacrolemma)
- run perpendicular to myofibrils
Terminal cisternae
- flattened cisternal arranement in sER around each myofibril
Triad
- one tubules and two cisternae
function: Propagation of Signal to sER to release Ca
Motor unit
- motor Neuron and all muscle fibers it supplies
- Action potentials from axons are transmitted across NEUROMUSCULAR JUNCTIONS
Neuromuscular Junction /motor end plate
- Presynaptic part: has vesicles with acetylcholine
- Synaptic cleft
- Postsynaptic part (sacrolemma): contains receptors
depolarization is passed on by T tubules to the sacroplasmic reticulum
–> release of Ca initiates contraction
Contraction cycle
- Ca binds to Troponin
- -> Change in tropomyosin
- -> active sites on actin are revealed
- -> Myosin binds to actin
- -> Myosin pulls actin to M line
- -> ATP binds to Myosin head
- -> Myosin detaches from actin
Types of skeletal muscle
> Type I: (Aerobic/ red): mitochondria, Myoglobin
—> “slow-twitch” fibers: can contract continuously
> Type II: (anaerobic/ White): few mitoch. and Myoglobin
—> “fast-twitch” fibers: can contract intensely and sporadic
Cardiac muscle
- in myocardium in heart
- in proximal portions of great blood vessels
- striated but involuntary
Cardiac muscle Fiber/ cardiomyocyte
- Long cylindrical cells
- split longitudinally in branches
- interconnected by intercalated Discs
- 1 or 2 centrally located nuclei
- contains: myofibrils, mitochondria, glycogen, sER
T-tubules in cardiac muscle
2x larger in Diameter than in skeletal muscle
Sacroplasmic reticulum in cardiac muscle
One t tubule and one terminal cisterna of sER –> DIAD
Intercalated Disc and the three types of contact
- Zonulae adherentes: actin Filaments insert for MECHANICHAL COUPLING
- Maculae adherentes (desmosomes): intermediate Filaments insert for MECHANICAL COUPLING
- Gap Junction: in longitudinal Portion, ELECTRICAL COUPLING
–> FOR SIMULTANEOUS CONTRACTION
Smooth muscle
- in blood vessels and inner organ walls
- involuntary
- mediated by autonomic nervous System and Hormones
- in inner Organs: inner circular layer and outer longitudinal –> for peristaltic contractions
- in blood vessels: circular arrangements
Smooth muscle cell/ myocyte
- fusiform
- non-striated
- centrally located, elongated nucleus
- intercellular spaces filled with loose CT
- Interconnected by gap junctions
Ultrastructure of smooth muscle
- MYOFILAMENTS criss cross the cytoplasm
- INTERMEDIATE FILAMENTS(Desmin)
- DENSE BODIES: spots when fibers cross
- Thin (actin) Filaments: only associated with tropomyosin
- CAVEOLAE: invaginations in Plasma membrane, analogue to T tubules, in contact with sER
Smooth muscle contraction
- contraction: shortening of cells and Twist of nucleus
- -> globular shape of myocyte
MECHANISM
- -> depolarization vie calveolae and sER
- -> release of Ca
- -> Ca binds to calmodulin
- ->Activates Myosin
- -> Myosin binds actin
Important difference between smooth muscle cells and cardiac muscle cells
Smooth muscle cells can divide, while cardiac myocytes cannot
