What is the hierarchy of muscle cell organization?
myofilament > myofibril > muscle fiber/cell > muscle fascicle
What are myofibrils?
series of sarcomeres linked together like "beads on a string", align w/ bands above and below
What is another name for muscle fiber?
- muscle cell
- multinuc w/ eccentric nuc
What is skeletal muscle derived from?
paraxial mesoderm (forms somites) and somitomeres in developing head of embryoblast
ends of nerve endings interact w/ muscle fibers at motor end plates
What are characteristics of skeletal muscle?
- multinuc, eccentric nuc
- long fibers (but not as long as muscle itself)
- voluntary control (innervated by cholinergic nerve endings)
- contraction results from shortening sarcomeres
What are the ends of muscle fibers attached to?
CT w/in muscle or CT tendons at each end of muscle
What is the isotropic band?
- (I band) "monorefringent"- plane of polarized light is not bent
- light in color
- bisected by Z disc
- made of only thin actin filaments
What is the anisotropic band?
- (A band) birefringent: plane of polarized light is bent
- made of thick and thin filaments (depends on your XS as to what the composition is)
What is the Hensen band?
- (H band); thick filaments only
- btw thin filaments of A band; at center of A band
What is the M line?
made of only thick myosin filaments w/ cross connections of myomesin
What is the Z disk?
bisects I band
What is a sarcomere?
area btw 2 Z disks; basic contractile unit of striated muscle
What composes actin filaments?
- G actin monomers that comprise filamentous F actin
- two F actins intertwine to form a helix
What is the purpose of tropomyosin?
occupies shallow grooves btw actin helices; masks myosin-binding sites on G actin
What is the purpose of troponin?
- located on tropomyosin molecules; 3 subunits:
- troponin T: binds troponin to tropomyosin
- troponin C: binds Ca2+ which causes conformational change in tropomyosin and exposure of myosin binding site on G actin
- troponin I: binds to actin to prevent actin-myosin interaction
What is the structure of myosin?
- two globular heads on two helices that intertwine
- heads have ATPase activity
What is myomesin?
holds myosins together
What is titin?
connects ends of myosin molecules to Z line
What is the sarcoplasmic reticulum?
sER in muscle cells; stores intracellular Ca2+ and release
What is sarcolemma?
cell membrane of striated muscle cells
What are transverse tubules?
invaginations of sarcolemma into interior of muscle fiber; occur at A-I junction in skeletal muscle and Z-intercalated disk in cardiac muscle
aka TM prots
congenital muscular dystrophies
gene mut or deletion in lamins of dystroglycans
oculopharyngeal muscular dystrophy
due to mut in Type 1 red muscle fibers
due to mut in type 2 white fibers
autosomal limb girdle muscular dystrophies
gene mut or deletion in sarcoglyans
What are terminal cisternae?
- cisternae of sarcoplasmic reticulum which parallel T tubule
- two, one for each side of T tubule; forms a triad in skeletal muscle and diad in cardiac muscle
How do terminal cisternae function?
have Ca2+ stored in it, released after deplolarization from AP; Ca2+ causes troponin to relieve tropomyosin inhibition
Where are terminal cisternae located?
at A-I junction; v closely assoc w/ myofibrils for best contraction
How are sER and calcium related?
release calcium once receives signal and re-uptakes calcium right after releasing it so that contractions do not continue to happen
When does rigor occur?
if levels of Ca2+ become too high or ATP levels become depleted (actin and myosin become cross-linked)
How are actin filaments anchored?
via dystrophin protein to TM proteins that are linked to basal lamina; maintains muscle integrity during contraction
dystrophin is lg prot so lots of chances for mutations (duschennes and beckers)
Red fibers (type 1)
- lots of myoglobin and oxidative enz
- -"slow twitch", sustained force, weight-bearing, enduring
- -sm diameter
- -NADH dk staining (bc of myoglobin)
- -abundant lipids
- -minimal glycogen
- -one motor neuron innervates many muscle fibers (less precise control)
- -located in limb muscles and long back muscles
White fibers (type 2)
- -"fast twitch", sudden movements; purposeful motion; easily fatigued
- -less myoglobin
- -NADH light staining
- -minimal lipids
- -abundant glycogen
- -few mit
- -one motor neuron innervates few muscle cells (precise control)
- -located in extraocular muscles, digits
What does endomysium surround?
What does perimysium surround?
bundles of muscle cells (fascicles)
What does epimysium surround?
entire muscle; many fascicles
What is the nt for skeletal muscle?
Ach; uses cholinergic (nicotinic) receptors
How does the motor unit work for skeletal muscle?
multipolar neuron from ventral horn of gray column synapses on ind fibers (motor end plate)
What are some characteristics of muscle fibers and nerves?
- bundle of skeletal muscle fiber innervated by single motor neuron
- every muscle fiber receives a nerve terminal
How do skeletal muscle cells regenerate?
stem cells--->myoblasts--->new muscle fibers
How can you tell an original muscle cell from new muscle cell?
new cell has a more central nuc (bc from myoblast) whereas an original cell has an eccentric nuc
Where are cardiac muscle nuclei located?
How do cardiac muscle cells contract?
same as skeletal muscle cells
What is an intercalated disk?
junctional complex that connects sequential cardiocytes; creates lots of attachment and communication sites for gap junctions
What are components of intercalated disks?
- maculae adherens (desmosomes)-attachment of intF
- fascia adherens-attachment of actin thin filaments
- gap junctions-ion transport to synchronize heart contraction
What is an intracellular bridge?
bridge between layers of cardiocytes; allows heart to contract at once
What is the function of mit in cardiocytes?
elongated around the myofibrils; lots bc need lots ATP for heart
What does the sarcoplasmic reticulum look like in cardiocytes?
less well-developed; diads; located at Z disks (but not intercalated disks)
What else is in cardiocytes?
- lots of cytoplasmic glycogen
- secretory granules in atria contain ANP
What is the function of ANP in cardiocytes?
- when the atria is stretched (bc of too much H20), cardiocytes in atria release ANP:
- osmotic balance
- distention atrial wall
- stimulates diuresis (counters ADH)
- relaxes cardio channels
What are purkinje fibers?
- modified cardiocytes; lgr than regular cardiocytes w/ central nuc and less myofibrils peripherally located
- binucleate; lots of glycogen granules
Where are purkinje fibers located?
How are purkinje fibers organized?
nodes and bundles
What is the function of purkinje fibers?
to facilitate conduction of contractile stimulus (not actually contract as skeletal...more of a transmitter to rest of heart)
How are cardiac muscle cells regenerated?
replaced w/ fibrous CT
What do smooth muscle cells look like?
- organized as bundles or sheets of cells
- spindle shaped
- involuntary control
- stains "frosty pink" color
- organelles (golgi and sec granules) around each end of nuc
What can smooth muscle cells secrete?
- elastin (tunica media cells of elastic arteries)
- type 4 and 3 collagen (tunica intima cells of blood vessels)
- lamin and proteoglycans (CTs)
- renin (juxtaglomerular cells in kidneys)
How do smooth muscles utilize gap junctions?
to act synchronously
How are smooth muscles innervated?
- nonmyelinated, ANS
- no specialized synapse
- only a few cells are innervated
- has gap junctions to allow concerted contractions
What are interstitial cells of Cajal?
(ICC): highly specialized pacemaker cells that modulate autonomic stimulation of smooth muscle cells and are important in frequency of rhythmic contraction (i.e. peristalsis)
How do smooth muscles contract?
- sliding filament mechanism w/ actin-myosin
- Ca2+ dep
What does Ca2+ do for smooth muscle contraction?
Ca2+ induces calmodulin dep act of myosin light chain kinase...initiates actin-myosin interaction
What are some characteristics of smooth muscle contraction?
- contractions are sustained (little ATP req)
How do smooth muscles regenerate?
What is the conducting pathway of the heart?
What is the parasympathetic innervation of the heart?
vagus nerve-slows down heart contraction
What is the sympathetic innervation of the heart?
- speeds up contraction at SA
- stim cardiac muscle cells to contract harder
- goes to each node and cell
What is variscosity?
bare nerve terminal of smooth muscle cells where NE is dumped onto