Unit 4: Muscle Flashcards
What are the types of muscle tissue?
Skeletal
Cardiac
Smooth
What is the function of skeletal muscle?
moves the skeleton by pulling on tendons attached to bones
What is the function of cardiac muscle?
pumps blood through heart chambers and blood vessels
What is the function of smooth muscle?
regulates movement of “contents” through hollow organs and blood vessels
What are the key properties of muscles?
Excitability
Contractility
Extensibility
Elasticity
What is the excitability property of muscle?
Ability to respond to stimuli such as chemical, electrical or mechanical
What is the contractility property of muscle?
Ability to develop tension in response to stimulus.
Enables movement of the structures to which muscles are attached.
Contractility due to interaction of myofilaments (thick and thin filaments composed of myosin and actin)
What is the extensibility property of muscle?
Ability to stretch
What is the elasticity property of muscle?
Ability to return to normal resting length
What does a skeletal muscle consist of?
Contractile tissue composed of muscle cells (aka muscle fibers or myocytes)
Series of collagen-based connective tissue coverings (the “mysiums”)
What do the Series of collagen-based connective tissue coverings (the “mysiums”) of skeletal muscle do?
Binds muscle fibers together
Help links skeletal muscle to bone (usually), skin, or fascia
Help transduces force to attached structure(s)
Conveys blood vessels and nerves
What is the control of contraction & relaxation in skeletal muscle? How is it innervated?
Voluntary
Innervation by motor neurons provide contraction signa
How is skeletal muscle developed?
During embryological development, multiple mesodermal stem cells (myoblasts) fuse to form a single skeletal muscle cell
Each skeletal muscle cell (aka muscle fiber or myocyte) is a syncytium (multinucleated cell)
Can muscle fibers divide?
Once fusion occurs, muscle fibers cannot divide
How do muscle cells enlarge?
Muscles enlarge by cell hypertrophy, not hyperplasia
How does regeneration occur in skeletal muscle?
Some myoblasts (satellite cells) remain in muscle
Regenerative capacity is robust, but can be overwhelmed
What happens with myoblasts (satellite cells) that remain in skeletal muscle?
Satellite cells = stem/progenitor cell-like cells
Can divide and fuse to generate new muscle fibers
Activated when needed for healing/repair
What happens when the regenerative capacity of skeletal muscle is overwhelmed?
After severe muscle trauma or due to disease (e.g. muscular dystrophy)
Fibrosis (replacement of muscle with fibrous tissue) occurs when repair process fails and/or is overwhelmed
What is the shape of skeletal muscle?
Long cylindrical cells (look polygonal in cross section); length varies by location (few mm to >500 mm).
What do the nuclei look like in skeletal muscle?
Multinucleated, with nuclei pushed to periphery, just beneath plasma membrane (sarcolemma)
What does the cytoplasm look like in skeletal muscle?
Cytoplasm (sarcoplasm) filled with myofibrils, bundles of longitudinal arrays of myofilaments
How do the thick and think filaments appear in skeletal muscle?
Overlapping arrangement of thick and thin filaments in myofibrils gives striated appearance
What are striations?
alternating pattern of light and dark bands visible by light microscopy
What are found between myofibrils in skeletal muscle?
Mitochondria and glycogen deposits found between myofibrils
• Generate ATP for muscle contraction
What is the appearance of skeletal muscle fibers in longitudinal sections?
Portions of elongated, cylindrical cells
Portions of multiple fibers seen in parallel
Slivers of endomysium in “spaces” between fibers
Striations perpendicular to long axis of fiber are visible Slender, elongated nuclei at periphery of each fiber
What is the appearance of skeletal muscle fibers in cross sections?
Fibers often look polygonal rather than cylindrical
Myocyte nuclei, when visible, are at periphery
Each muscle fiber surrounded by endomysium
May see small vessels, fibroblast nuclei in endomysium
What are the layers of connective tissue that surround skeletal muscle?
Endomysium
Perimysium
Epimysium
What is endomysium?
delicate layer of reticular fibers that immediately surrounds individual muscle fibers. It carries small diameter blood vessels and fine neuronal branches
What is perimysium?
thicker CT layer that surrounds a group of fibers to for a bundle or fascicle. Fascicles are functional units of muscle fibers. Larger blood vessels and nerves travel here
What is epimysium?
sheath of dense CT that surrounds a collection of fascicles that constitutes the muscle (deep investing fascia). Major vascular and nerve supply of the muscle penetrates the epimysium. Continuous with tendon attaching to bone
What are the functional units of muscle?
Multiple muscle fibers grouped to form a bundle or fascicle
Each fascicle surrounded by thicker layer of connective tissue (perimysium)
Larger branches of blood vessels and nerves are found in perimysium
Each fascicle functions as a unit; muscle fiber in each fascicle work together
Multiple fascicles are grouped together to form a muscle
Each muscle encased in a sheath of dense connective tissue called the epimysium
What is the myotendinous junction?
Site of attachment of a muscle fiber and the contiguous connective tissue of the tendon
How are the myotendinous junctions attached?
Muscle fibers send numerous cytoplasmic projections into connective tissue of tendon
- Interdigitations increase surface area of contact and enhance adhesion
Collagen fibrils of tendon linked to basal lamina of muscle fiber by multi-adhesive matrix proteins
What do nerves, arteries, veins, and lymphatics do in skeletal muscle?
Run together in a neurovascular bundle through the connective tissue coverings of a skeletal muscle
What vasculature surrounds each muscle fiber?
Capillary beds surround each muscle fiber
• provides oxygen and nutrients
• carries away CO2 and waste
What do nerves of skeletal muscles contain?
axons of somatic motor neurons
Cell bodies of these neurons are in brain or spinal cord
How do axons travel to skeletal muscle?
travel in neurovascular bundle to skeletal muscle and branch to innervate each fiber to control contraction
How do axons control skeletal muscle movement?
Axons of motor neurons branch to innervate varying numbers of muscle fibers
- Few branches to innervate small number muscle fibers (fine movement)
- Many branches to innervate hundreds/thousands of muscle fibers (gross movement)
What does motor innervation do for skeletal muscle homeostasis?
Motor innervation of muscles maintains muscle function
Motor neuron damage causes denervation atrophy
-Decreased muscle fiber diameter and myofilament
expression
What is the structure of myofibrils in the sarcoplasm?
Long bundles of myofilaments (thick & thin filaments)
Extend the entire length of the fiber
Occupy the bulk of space in the sarcoplasm
What are myofibrils?
arrays of a repeating unit called a sarcomere
sarcomere contains thick and thin filaments organized in specific overlapping pattern
What are the types of skeletal muscle fibers?
Red fibers
Intermediate fibers
White fibers
What are red fibers? What is the rate of contractile speed and fatigue?
Slow twitch
Slowest speed of contraction, but fatigue resistant
Found in limb muscles, erector spinae muscle
What are intermediate fibers? What is the rate of contractile speed and fatigue?
fast oxidative-glycolytic fibers
Fast speed of contraction and relatively fatigue resistant
Major muscles of the legs
What are white fibers? What is the rate of contractile speed and fatigue?
Fast glycolytic/twitch
Fastest speed of contraction, but fatigue quickly
Found in muscles generating “explosive” force or controlling movement of eye & digits
What is the A band?
Dark bands that contains thick filaments and overlapping portion of thin filaments
What is the I band?
Light bands that contain the remaining portion of thin filaments
What is the Z line/disc?
Protein dense region separating one sarcomere from next. Contains proteins that anchors thin filaments
What is the H zone/band?
Central, lighter portion of A band containing only thick filaments (no thin segments)
No myosin head, contains M line
What is the M line?
Protein dense region at center of H zone. Contains proteins that anchor thick filaments
What are aligned in myofibrils and muscle fibers?
Adjacent sarcomeres in myofibrils are aligned (“in register”) so A bands, I bands, etc. are all aligned in the myofibril
Myofibrils in sarcoplasm of a single muscle fiber also aligned so they are in register within the muscle fiber
What is myosin?
hexamer composed of 2 heavy chains & 4 light chains
Molecular motor protein
Major component of thick filaments
How does myosin form thick filaments?
Tails of myosin heavy chains associate laterally in antiparallel fashion to form thick filament
What is the arrangement of thick filament with myosin?
Form thick filament with “bipolar” arrangement
-Central bare zone
-Myosin heads radiate around ends
What does the myosin head do?
Contains actin binding site and ATP binding site/ATPase activity
ATP hydrolysis/release of Pi + ADP causes movement of myosin head (molecular motor) to power muscle contraction
What is actin?
Major component of thin filaments
Filament formed from globular actin
Contain binding site for myosin head
What are regulatory proteins of the sarcomere?
Associate with actin on thin filament
Tropomyosin
Troponin
What does tropomyosin do?
covers myosin binding sites on actin
What is Troponin and what does it do?
Heterotrimer
Troponin T binds to tropomyosin
Troponin I binds to actin
Troponin C (TnC) binds to calcium ions
- Ca2+ binding to TnC causes conformational change, this moves tropomyosin & makes myosin binding site on actin accessible
What happens to the filaments when skeletal muscle contracts?
thin filaments slide over the thick filaments
How do thin filaments slide over the thick filaments when skeletal muscle contracts?
Result of sequential interaction between myosin head and actin
Myosin head pivots and bends, pulling actin along with it
Thick filaments do not move, thin filaments slide toward the M line
What drives thin filaments to slide over the thick filaments when skeletal muscle contracts?
Driven by Ca2+- and ATP-dependent interaction between myosin and actin
What is the appearance of skeletal muscle contraction?
Z discs (line) move closer together (sarcomere length reduced)
I bands narrow
H zone (band) narrows/disappears
A band remains the same length because thick filaments do not move
Thick/thin filaments do not change in length
How are sarcomeres oriented?
parallel to long axis of the skeletal muscle cell Thus…contraction of muscle fibers in a muscle occurs in a single direction
What is the structure of the sarcoplasmic reticulum?
Special type of sER
Composed of a system of membranous sacs and anastomosing channels encircling each myofibril
Arranged as a series of adjacent networks or segments
Dilated sacs at ends of each network are called terminal cisternae
What is the function of the sarcoplasmic reticulum?
Stores high concentration of stored Ca2+ in the terminal cisternae
Calsequestrin: Ca2+ binding protein in SR
Ca2+ released from the SR binds TnC, allows actin-myosin interaction, sliding filament mechanism
What are the mitochondria and glycogen deposits in the sarcoplasmic reticulum?
Between the myofibrils, adjacent to to SR
Function: Provide ATP for muscle contraction
What is the structure of transerve tubules (T-tubules)
Invaginations of sarcolemma into the cell
Form network of channels lined by specialized plasma membrane that wrap around each myofibril
Lumen of T-tubules is continuous with extracellular space/contains extracellular fluid
WhereT-Tubule wraps around myofibril, it associates with terminal cisternae of two sarcomeres (triad)
What is the function of transverse tubules (T-tubules)?
Allow action potential from surface of muscle fiber to travel rapidly to interior of cell
Permits simultaneous contraction of all sarcomeres in the cell
Action potential in T-tubule membrane signals to adjacent SR in triad, resulting in Ca2+ release that allows contraction
What is the structure of the neuromuscular junction?
axon forms a dilation called the axon terminus (aka synaptic end bulb, synaptic bouton, synaptic knob) that houses synaptic vesicles containing Acetylcholine (Ach)
Where do axon terminals lie for muscle fibers?
In the depression of the sarcolemma of the muscle fiber
Sarcolemmaunderlyingaxonterminusisdeeplyfolded
Folds contain Ach Receptors (AchR)
What is the function of the neuromuscular junction?
Signaling at NMJ initiates cascade of events leading to skeletal muscle contraction
How does an action potential cause muscle contraction in muscle cells?
Action potential reaches axon terminus
Ach released and binds AchR on sarcolemma
Ach binding to AchR leads to influx of Na+
Ultimately leads to muscle cell action potential/membrane
How many axon termini and what is the size of neuromuscular junctions?
NMJ are small but diffuse structures that contain multiple axon termini
Size may correlate with muscle fiber diameter
How do neuromuscular junction motor units discharge?
It is a graded response to contraction signal
Not all discharge at once
