Muscle Tissue Flashcards
What is the primary function of muscle tissue?
Contraction
What does the muscular contraction contribute to?
movement/locomotion circulatory control respiratory control peristalsis glandular secretion body heat production
What are muscle tissue components?
1) myofibers -contractile cells that move skeletal elements
2) CT wrappings-harness the pull of the contraction, contains neurovasculature
What is a muscle fiber or myofiber?
elongated cell
What is a nerve fiber?
Elongated cell process
What is a CT fiber
Extracellular fiber
What is striated muscle?
1) displays a uniform light/dark banding pattern (microscopically)
2) this highly organized intracellular machinery facilitates an increased rate of contraction
What is skeletal muscle?
1) voluntarily controlled contractions (PNS)
2) comprises most of the body muscles
What is cardiac muscle?
1) involuntarily controlled contractions (ANS, hormones)
2) localized to the heart
What is smooth muscle?
1) lacks a regular banding pattern
2) involuntarily controlleed.contractions (ANS, hormones)
optimized for slow continuous contractions
located in many places throughout the body (in walls of most tubes of body)
What are myoblasts?
Derived from mesenchyme
Single nucleus
What are myotube?
Fused myoblasts
Multinucleated with centrally located nuclei
What are myofibers?
Contains contractile elements and peripheral nuclei
What are satellite cells?
Functions as a stem cell (provides limited ability to regenerate)
What kind of growth does skeletal muscle experience?
Skeletal muscle growth only has hypertrophic growth and very limited hyperplasia
What is hypertrophy?
Growth by increasing in size
What is hyperplasia?
Growth by adding number of cells
What provides growth when a muscle is broken or torn?
Satellite cells stimulate to divide and bridge broken gaps
What does connective tissue in skeletal muscle do?
1) harnesses the pull of the contraction and conveys the neurovasculature
2) relays the contraction to the desired place and also allows for places for neurovasculature
What is the endomysium?
loose CT surrounding individual myofibers
What is perimysium?
dense irregular CT surrounding fasicles (bundles of myofibers)
What is epimysium?
dense irregular CT surrounding the entire muscle
What is myotendinous junction?
convergence of CT wrappings with tendon/ aponeurosis
What is Dense regular connective tissue?
Located in tendons, aponeuroses, ligaments, Sharpey’s fibers ·
Comprised of thick bundles of Type I collagen all running parallel
- fibroblasts are the main (and usually only) cell type present
- not highly vascularized or innervated (doesn’t repair itself well after injury)
What is the function of Dense Regular Connective Tissue?
Functions to limit range and direction of movement in one direction
What are sharpey’s fibers?
Fibers found between tendinous and bone insertions
What are the different skeletal muscle fiber types?
Type I (slow oxidative or red fibers) Type IIa (fast oxidative glycolytic or intermediate fibers Type IIB (fast glycolytic or white fibers)
What are Type I (slow oxidative or red fibers) skeletal muscles?
1) relatively high content of myoglobin and mitochondria
2) able to maintain contractions for relatively long periods of time
- slow-twitch fatigue-resistant cells
3) principal fibers in erector spinae muscles
IE: postural muscles
What are Type IIa (fast oxidative glycolytic or intermediate fibers) skeletal muscles?
1) relatively high content of myoglobin and mitochondria and lots of glycogen
2) capable of anaerobic glycolysis
• fast-twitch fatigue resistant
3) generate high peak muscle tension over prolonged periods
IE: most skeletal muscles
What are Type IIB (fast glycolytic or white fibers) skeletal muscles?
1) relatively low content of myoglobin and mitochondria but lots of glycogen
2) adapted for short bursts of rapid contractions but fatigue easily
• fast-twitch fatigue-prone
3)generate high peak muscle tension over short periods
IE: occular musculature (allows for short burst of movement to move the eye
Does training change the ratio of fiber types?
No, Individual muscles are comprised of multiple fiber types but Training enhances desirable fiber types but does not change ratio of fiber types
What are sarcomeres?
Contractile unit of striated muscle cell
What is a myofibril?
Series of sarcomeres linked together
What do skeletal muscle fibers appear like when cut longitudinally?
1) show a series of alternating light and dark bands
2) seen microscopically when skeletal muscle is cut longitudinally
What makes up muscle fascicle?
Made up of muscle (myofibers) fibers
where are the nuclei found in skeletal muscle?
Nuclei are always at periphery
What are myofibrils made up of ?
Myofilaments
When are sarcomere contracts what areas do the contracting?
The Z-lines pull together
What do thin filaments mostly consist of?
actin
What part of the sarcomere do thin filaments occupy?
1) occupy I band (with some overlap into A band to H band)
2) extend both ways from Z line
(essentially start at Z line and extend into the A band up until the H band, on contraction the Z lines are pulled together and the H band is overlapped)
What is the structure of thin filaments?
G actin (not a strand, but forms the F actin) F actin (2 strands present) Tropomyosin and troponin (together form the 3rd strand in an actin filament)
What is G actin?
globular form which polymerizes to form F actin
What is F actin?
filamentous form (2 strands are present in each actin filament)
What is tropomyosin?
Filamentous part that is a part of the 3rd strand in the actin filament
What is troponin?
Globular part that is part of the together form the 3rd strand in the actin filament
What do thick filaments consist of mostly?
Myosin
What part of the sarcomere do thick filaments occupy?
Occupy A band
What is the structure of a thick filaments?
1) myosin shaft anchors myosin in the thick filament
2) myosin globular head is flexible and has ATPase activity
What are the accessory proteins of the sarcomere?
alpha actinin
Myomesin
Titin
Desmin
What is alpha actinin?
links actin thin filaments to Z-line
What is myomesin?
holds myosin thick filaments in register at M-line (spacing protein)
What is titin?
links ends of thick filaments to Z -line (parallel to filament array)
What is desmin?
link adjacent myofibrils together (at Z-line) and myofibrils to cell membrane (holding everything in register)
What is sarcolemma?
Muscle cell plasma membrane
What are Transverse (T) tubules?
1) invaginations of the sarcolemma located at A-I junctions
2) lumen is continuous with the extracellular space
What is the function of Transverse (T) tubules?
carry the membrane depolarization into the muscle cell
What is the sarcoplasmic reticulum?
1) cylindrical sheets of SER surrounding each myofibril
2) expanded into terminal cisternae adjacent to each T-tubule
What is the function of Sarcolasmic reticulum?
Store calcium intracellularly
What is the triad of the skeletal muscle cell membrane systems?
Triad = 1 T tubule + 2 terminal cisternae
How does the sliding filament model describe contraction?
1) thin filaments slide past the thick filaments (toward M line)
- this process obliterates H band (but A band is unchanged)
2) Z-lines are pulled toward thick filaments (narrowing I bands)
3) M-line doesn’t change, H band gets narrower (represents the gap of the thin filaments on both sides, the I band gets narrower because the z-line is moving. The A band width doesn’t change at all because it is where the thick filaments are.)
In a non-contracting condition what prevents myosin and actin interaction?
1) prior to contraction, actin and myosin are not physically in contact
2) troponin/tropomyosin complex (in the way) prevents myosin/actin interaction
What initiates the process of muscle contraction?
1) sarcolemma receives a nervous signal to contract at a neuromuscular junction
- acetylcholine receptor is a ligand gated ion channel
- this local depolarization leads to the opening of voltage gated channels
What occurs due to the local depolarization due to the opening of voltage gated channels?
the depolarization spreads across sarcolemma and into muscle cell vja T tubule system
• mediated by more voltage gated ion channels throughout the membrane
What occurs when the depolariazation reaches the A-I junction?
1)signal in T tubule system causes release of Ca 2+
from terminal cisternae of sarcoplasmic reticulum into cytoplasm
2) ryanodine receptor-primary voltage gated Ca 2+
release channel
What is the troponin complex?
1) troponin T - binds complex to tropomyosin near myosin binding site
2) troponin I -inhibitory portion that blocks myosin interaction with actin
3) troponin C - Ca 2+ binding portion; causes conformational change allowing myosin access to actin filament
What causes the troponin-tropomyosin complex to expose the myosin binding site?
When Ca 2+ binds with troponin C the troponin-tropomyosin complex moves deeper into the thin filament and exposes the myosin binding site
What does the myosin head act as?
An ATPase (breaks ATP down to ADP and phosphate)
At rest what is bound to myosin?
ADP and Pi
What is the energy state of at rest myosin?
this is the high energy state where myosin is in the “cocked” position
When Ca 2+ binds to troponin what happens?
when Ca 2+ binds to troponin, myosin is then able to bind to actin since the myosin binding site is free
Once the myosin attaches to the troponin what happens?
ADP and Pi are released and myosin undergoes a conformational change causing actin to slide
What is the sliding of the actin known as?
The power stroke where force generation occurs and puts the myosin into a low energy state (“uncocked”)
What happens in the “uncocked or low energy” state?
Myosin binds to another ATP molecule which causes the myosin to release from actin
How does myosin return to the resting conformation when ATP is bound?
myosin cleaves ATP to ADP and P; and resumes the resting conformation (returning myosin to the high energy state, “cocked” position)
What is required for continuous cycles of muscle contractions to occur?
ATP and Ca 2+ needs to be available
True or False, each contraction is a result of one cycle that occurs?
False, each contraction requires many cycles to result in the myosin head walking along the actin
How do contraction stop?
1) when signal to contract ceases, Ca 2+ is quickly returned to SR and
troponin-tropomyosin complex returns to peripheral position
2) myosin no longer has access to actin and contraction stops
What causes calcium to constantly pump back into the sarcoplasmic reticulum?
Calcium - ATPase pumps
What are the 3 stages of rigor mortis?
Primary flaccidity
Rigor
Secondary flaccidity
What occurs at primary flaccidity?
1) muscles exhibit no resistance to movement
2) Ca 2+ is still sequestered away in the SR
3) approximately 0-3 hours post mortem
What occurs in the rigor stage?
1) Approximately 3-12 hours post mortem
2) nonuniform contractions start to occur in the skeletal musculature
3) membrane degradation is allowing Ca 2+ to leak into cytosol
4) residual ATP is being used to sustain the contractions
5) the muscle locks in rigor when the last ATP molecule is depleted
What occurs in secondary flaccidity?
1) After 12 hours post mortem
2) pH is dropping due to lactic acid production
lysosomal enzymes are being released due to membrane degradation
3) myosin heads are gradually being digested by these enzymes
4) rigor dissipates as the myosin heads are digested
What is the motor end plate of skeletal muscle?
1) the neuromuscular junction between a motor neuro and its associated muscle fiber
What is acetylcholinesterase?
Enzyme found on sarcolemma breaks down acetylcholine which is returned and recycled in axon terminal
What is the function of the motor end plate?
1) electrical stimulus in neuron causes release of Ach across synaptic cleft
2) Ach binds to receptors on the sarcolemma directly across the synaptic cleft
3) depolarization spreads across sarcolemma and into T tubules
True or false: there is only one motor end plate on each muscle fiber?
false there are multiple motor end plates on every muscle fiber
What is a motor unit?
a lower motor neuron and the group of muscle fibers it innervates
What is each motor unit’s function?
1) each lower motor neuron supplies muscle fibers throughout muscle
2) ensures muscle will contract as whole rather than just locally
What does fewer numbers of muscle fibers/lower motor neuron signify?
More precise contractile control
ie: motor units for ocular muscles have only a few muscle fibers
motor units for erector spinae muscles have hundreds of muscle fibers
What do graded contractions depend on?
~ the number of motor units involved
~ the size of motor units involved
~ the length and rate of stimulation of the involved motor units
What does a loss of motor innervation result in?
Disuse atrophy (breakdown) - reversible if motor innervation is restored
What does afferent innervation of skeleton muscles consist of?
Muscle spindle
Intrafusal fibers
extrafusal fibers
What is a muscle spindle?
contains modified muscle fibers (spindle cells) and sensory neuron endings
What are nuclear bag fibers?
spindle cells with several nuclei clustered in mid region
What are nuclear chain fibers?
spindle cells with several nuclei in a chain configuration
Where is the location of sensory neurons in regards to the spindle cells?
Sensory neuron spirals around the spindle cells
What surrounds the spindle cells and neuron in a muscle spindle?
a connective tissue capsule surrounds the spindle cells and neuron
What is the function of muscle spindles?
1)functions to monitor the degree of stretch in a muscle
• similar encapsulated receptors (tendon organs) are found in tendons
What are intrafusal fibers
alternative name for the two types of spindle cell
Whatare extrafusal fibers?
alternative name for the typical contractile skeletal myofiber
What is a cardiac muscle fiber?
chains of cardiac muscle cells joined on end
How many nuclei and where is the nuclei located in a cardiac muscle cell?
a single central nucleus (sometimes 2 nuclei)
Are there satellite cells in cardiac muscle? What is the significance of this?
no satellite cells which means no capacity for regeneration
What is an intercalated disc?
1) apposed cell borders of contiguous cardiac muscle cells
* may appear as straight line or stair-step
What is the transverse component of an intercalated disc?
is perpendicular to myofibrils occurs at Z lines
What is the lateral component of an intercalated disc?
Parallel to myofibrils
What are the 3 kinds of cell junctions present in intercalated disc?
Fascia adherins
Desmosomes
Gap junctions
What are fascia adherins?
1) broad adherent junction- major constituent of transverse component
2) electron dense, patch-shaped adhering junction
What is the function of fascia adherins?
holds actin thin filaments to sarcolemma
bind actin together to allow simultaneous contractions
What are desmosomes?
1) located in both transverse and lateral components of disc
2) site of strong adhesion (intermediate filaments)
What is the function of desmosomes?
minimize separation of contiguous muscle cells during contraction
What is a gap junction?
1) major constituent of lateral component
2) provide direct ionic continuity between contiguous cells
3) made of connexons (tubes) that allow for diffusion waves of nutrient and/or ions
What is the function of gap junctions?
1) facilitates synchronized muscle contraction
2) solely used for communication and not a structural junction
What is the primary differences between cardiac and skeletal muscle contractions?
1) T tubules located at Z lines(not A-1 junction as in skeletal muscle)
2) sarcoplasmic reticulum is smaller and stores less Ca 2+ intracellularly
3) cardiac muscle contraction very dependent on extracellular calcium
4) force of contraction is calcium dependent and can be modulated
Do cardiac muscles use the sliding filament mechanism as described for skeletal muscles?
Yes, it is the same mechanism
What does it mean that cardiac muscle contraction is myogenic?
The muscle contracts intrinsically and at a spontaneous (no external stimulation required) rhythm and it does not require neural stimulation for each contraction, instead the SA node controls the contractions
What are purkinje fibers?
1) specialized conducting fibers designed to distribute contractile impulse in a precise sequence throughout the cardiac muscle
What do the ANS and hormoes regulate in cardiac muscle contraction?
Rate and strength of contraction (does not initiate the contraction)
