Chapter 10 muscle tissue Flashcards
6 functions of skeletal muscles
Produce skeletal movement Maintain posture and body position Support soft tissues Guard body entrances and exits Maintain body temperature Store nutrients
Organization of the skeletal muscle
Muscle fiber (cell) Muscle Fascicle ( bundle of fibers) Skeletal muscle (organ)
3 layers of connective tissue in muscles
Epimysium: dense layer of colligen fibers that surrounds the entire muscle
Perimysium:divides muscle into bundles of fibers called fascicles.
Endomysium: delicate, flexible and elastic connective tissue that surrounds each individual cell.
Each endomysium contains (3)
- capillary network that supply blood to the muscle
- myosatillite stem cells that help repair damaged muscle
- nerve fibers that control the muscle
Features of skeletal muscle fibers
Enormous
Multinucleated
Striated: contain thin (actin) and thick (myosin) filaments
A muscle cell is formed by myocytes that fuse together as a fetus.
Sarcolemma
Plasma membrane of a muscle fiber
Has membrane potential due to more negative cytoplasmic side and more positive extracellular side
Sarcoplasm
Cytoplasm of a muscle fiber
Transverse (T) tubules
Narrow tubes whose surfaces are continuous with the sacroplasm and extend deep into the sarcoplasm. Electrical impulses called action potentials travel along the T tubules and trigger the muscle fiber to contract
Myofibrils
Cylindrical fiber within the muscle fiber that contains actin, myocin and titin–elastic myofilaments associated with the thick filaments.
Sarcoplasmic reticulum (SR)
tubular network around each myofibril that is similar to a smooth ER.
Terminal cisternae
Expanded chambers of the SR that form on either side of the T tubules. Contains 40,000 times the Ca+ as cytosol. .
Muscle contraction begins when the Ca+ is released into the cytosol
Triad
A combination of 2 terminal cisternae with a t tubule in the center
Sarcomeres contain (4):
The smallest functional units of the muscle fiber and contain:
1. thick filaments
2. thin filaments
3. proteins that stabilize the position of thick and thin filaments
4. proteins that regulate the interactions between thick and thin filaments
Each sarcomeres have dark A bands and light I bands
A bands
Contain regions of overlapping thick (myosin) and thin (actin) filaments. Within the A band is:
- The M line-in the center of the A band, help stabilize the position of the thick filaments
- The H band- the light region on each side of the M line, contains thick filaments but no thin
- The zone of overlap- 3 thick surround each thin and 6 thin surround each thick. Triads are located in this zone
I bands
Region of the sarcomere that contains thin but no thick
Z lines bisect each I band and mark the boundary between each sarcomere
Titin
Extend from the tips of the thick filaments and attach to the Z line.
- Helps keep thick and thin filaments in proper allignment
- Aids restoring sarcomere length after contraction
Thin filaments
Contain 4 proteins: F-actin, nebulin, tropomyosin and troponin
F-actin in a double helix held together by a central nebulin
G- actin are the molecules that contain the active site for myocin to bind
Troponin-tropomyosin complex prevents myocin from binding
Tropomyocin
Covers the active sites on G-actin and prevents actin-myocin interaction
Troponin
Molecule that consists of 3 globular sub-units that bind to tropomyosin and G actin and a third binds to two Ca+ ions. When Ca+ binds to troponin it releases tropomyocin complex which exposed the active site for myocin.
5 levels of organization in skeletal muscle
Sarcomere Myofibril Muscle fiber Muscle fascicle Skeletal muscle
Thick filaments
Contains about 300 myocin molecules that are composed of a myosin tail and a double head on a hinge. The tails are twisted together and the head faces the nearest thin filament.
Cross- bridges
When the myosin heads interact with the thin filaments during contraction
What happens when the skeletal muscle contracts (4 steps)
- H bands and I bands of the sarcomere narrow
- The zones of overlap widen
- Z lines move closer together
- The width of the A bands remains constant
Events at the neuromuscular junction (5)
- ACh is released in the from the axon terminal into the synaptic cleft
- ACh stimulates an action potential that travels along the length of the axon
- Action potrntial reached the axon terminal and releases ACh by exocytosis
- ACh binds o the motor end plate of the muscle and alters the cells permeability to Na+
- Influx of Na+ creates action potential in sacrolemma then
Excitable membranes
Permit rapid communication between different parts of the cell
Cells on the inner surface are slightly negative compared to cells in the outer surface
This creates typical resting membrane potentials of -70 mV and -85 mV
Contain voltage gated channels that are activated and inactivated by changes in membrane potential.
Excitation contraction coupling (6 steps)
The generation of action potential in the sarcolemma and the start of a muscle contraction:
- Neural Control
- Excitation
- Release of Calcium ions
- Contraction Cycle Begins
- Sarcomere Shortening
- Generation of Muscle Tension
^ steps of the contraction cycle and cross bridge formation
- Contraction cycle begins
- Active site exposure
- Cross bridge formation
- Myosin head pivoting
- Cross bridge detachment
- Myosin reactivation
2 factors that affect muscle tension produced
- the fibers resting length at the time of stimulation, which determines the degree of overlap
- the frequency of stimulation, which affects the internal concentration of Ca+ and the amount of bound troponin
3 phases of a twitch
- latent:
- contraction phase
- relaxation phase
The amount of tension produced in a whole muscle is determined by (2 things)
- The tension produced by a single muscle fiber
2. The total number of muscle fibers stimulated
Motor unit
All the muscle fibers controlled by a single motor neuron
Isotonic Contractions
Tension increases and muscle length changes:
Concentric: The muscle tension exceeds the load and the muscle shortens
Eccentric contraction: the peak tension developed is less than the load and the muscle elongates due to contraction of another muscle or gravity
Isometric Contractions
Muscle as a whole does not change length and the tension produced never exceeds the load
Cardiac muscle characteristics
Relatively small
Single centrally placed nucleus
Typically branched
No triads
SR lack terminal cisternae
More sensitive to changes in extracellular Ca+
Each cardiac muscle cell contacts other cells by intercalated discs
Intercalated discs
Gap junctions and desmosomes
Cardiac muscle functional differences
- Automaticity
- Nervous system can alter the rate
- Contractions last 10x longer
- Cannot produce tetanic contractions
Smooth muscle characteristics
Forms sheets, bundles and sheaths around other tissues
Non striated
Involuntary
Long and slender
Spindle shaped
No T tubules
Thick filaments are scattered throughout
More myosin heads per thick filament
Thin filaments are attached to dense bodies
Smooth muscle functional differences
- Free calcium in cytoplasm triggers contraction
- Tension and resting length are not related (plasticity is the ability to function over a wide range)
- Multiunit smooth muscle cells and visceral smooth muscle which is not connected to a motor unit
- Smooth muscle tone can be controlled by both multiunit and visceral smooth muscle as well as hormonal or chemical factors
