Muscle Histology Flashcards
WHat are the two functions of muscle tissue?
contractility and conductivity
WHat are the 3 types of muscle tissue?
smooth (unstriated, involuntary, visceral)
skeletal (striated - voluntary)
Cardiac (striated - involuntary)
What is the embryological origin of muscle tissue?
mesoderm
What is the muscle cytoplasm called?
sarcoplasm
What is the muscle cell plasma membrane called?
sarcolemma
What is the SER called in muscle?
the sarcoplasmic reticulum
What is the term for a linear unit of muscle?
What is an individual muscle cell called?
a linear unit is a sarcomere
an individual muscle cell is a myofiber
What are 5 characteristics of skeletal muscle?
- straited
- t-tubule system
- well developed sarcoplasmic reticulum
- multinucleated
- nuclei located at cell periphery
What is responsible for the cross-striations in muscle cells?
the myofibilrs, which are tiny cylindrical rods within the sarcoplasm of the muscle cell
the banding is due to registry of alternating light and dark portions of myofibrils adjacent to each other
Name the bands/lines/units
The length of a sarcomere is between what?
2 Z lines
What is the A-band?
the anisotropic band
it’s the borad, dark-staining band
What is the I-band? Whats inside?
THe I band is the isotropic band, which is narrower than the A band and light-staining
It’s bisected by the Z-disc
What is the light-staining area that bissects the A band?
the H band - it’s the middle of the sarcomere wher ethe myosin doesn’t have any heads
WHat bisects the H band?
the M line
What comprises the myofibrils?
myofilaments: actin and myosin
where are the actin filaments located within the sarcomere?
from the Z-disc strething to the edge of the H band
Where are myosin filaments located in the sarcomere?
from one end of the A-band to the other end of the A-band.
there will be cross-bridges estending from each myosin filament to its neghborin gactin filaments in the A band (the area where they overlap)
What proteins are located in the M-line?
It’s at the center of the H band
contains myomesin and C protein that interconnect the thick myosin filaments to maintain teir specific lattice arrangement
What protein in the Z disc is used to anchor the actin filaments?
alpha actinin
In skeletal muscle, where is the sarcoplasmic reticulum located?
It forms a network of cisterns or membranous tubules that run between and around the myofibrils
they tend to form “collars” at each A-I junction-one on each side which are spearated from each other by T-tubules
What does the sarcoplasmic reticulum do?
It binds and releases Ca2+
What are the invaginations of the sarcolemma called? Where are they typically seen in seletal muscle?
They are called T-tubules and they’re located at the A-I junctions between the two cisternae “col.ars” of the sarcoplasmic reticulum
The lumen of the T-tubule is continuous with what?
the extracellular space
What is a triad?
the combination of a t-tubule and 2 lateral cisternae collars
What do the T-tubules faciliate?
transmission of electrical impulses from sarcolemma to interior depths of cell
HOw many “triads” are there per sarcomere in skeletal muscle?
2 (one at each A-I junction)
What two forms of actin are located in myofibrils?
G-actin (globular actin) which is the globular monomer that polymerizes to form….
F-actin (fibrous actin), which is two helically wound strands of polymerized G-actin
Myosin is composed of 4 __ and 2 __, which are combined for form ___ meromyosin and ___ meromyosin.
4 light chains and 2 heavy chains
make up heavy meromyosin and light meromyosin.
The light meromyosin is the end that doesn’t have the heads (made of the intertwining 2 heavy chains)
the heavy meromyosin is the rod like portion tha tlies parallel to the backgone and then the heads.
WHat are the two components of heavy meromyosin?
- rod-like portion that lies parallele to the backbone of the filament
- a globular head which etends laterally as a cross bridge to connect to actin filament - it has to have actin binding sites
What type of enzymatic acitvity can be found near the myosin globular heads?
ATPase
What does it mean to say that myosin heads are polarized?
they are directed away from the midpoint of myosin filament (which is why there’s a bare sonze making the M line)
What are the 3 regulatory molecules involved in skeletal muscle contraction?
tropomyosin
troponin complex
calcium ion
What is tropomyosin? What is troponin?
Tropomyosin is the fibrous protein that is helically wound along the groove sof the F-actin helix.
Troponin is the complex that binds the tropomyosin to actin in such a way that the tropomyosin blocks the actin-myosin interactions
What are the three component sof the troponin complex?
TnT - binds entire trponin molecule to propomyosin
TnC - binds Ca++
TNI - binds to actin preventing actin myosin interactions
What does calcium ion do?
It binds to TnC and causes a conformation shift in the complex so that tropomyosin shifts over exposing previously blocked active sites on the actin filament
What does the protein titin do?
it’s a large, linear, elastic protein that helps position the myosin filaments precisely within the sarcomere
It extends from the z-disc to half of the myosin filament - so there are two per sarcomere
What does the protein nebulin do?
It’s a long non-elastic protein - two molecules of which wrap around each actin filament to help anchor it to the z-disc (in accition to alpha actinin)
it probably also regulates the length of the actin - otherwise it might unwrap and become too long
What hypothesis is generally accepted about the contraction mechanism of skeletal muscle?
the sliding filament hypothesis
What happens to the following during contraction?
a band
I-band
Z-disc
H-band
The A band remains constant
The I band becomes smaller and may even disapear (because the actin filaments are now overlapping with the myosin filaments completely)
The Z discs will get closer together as the sarcomere shortens
The H band narrows and is eventually obliterated
What are the 3 connective tissue coats around muscle tissue?
endomysium - surrounds each individual muscle fiber (made of fine reticular fibers)
Perimysium - connective tissue sheath around groups of muscle fibers (fasicles)
Epimysium - the outermost sheath of CT that surroudns the entire muscle
The connective tissue sheaths extend from the end of the muscles to form what?
the muscle tendons
True or false, the length of indvidual muscle cells within a muscle fascicle can vary.
True - they can extend from one end oall the way to the other, from one end to the middle, or just located in the middle
What are the 3 general types of skeletal muscle fibers?
red
white
intermediate
Describe how blood reaches the muscle fibers.
There is a very rich blood supply to muscles for obvious reasons
the larger arteries follow the perimysium
artieroles penetrate the fasicles
capillaries run parallele to individual myofibers
In what connective tissue layer is the lymphatics system located?
In the perimysium
Describe the motor end plate.
It’s the myo-neural junction
- the myelina sheath of the axon is lost as the nerve fibers approach the surface of muscle cell
- the axon bbranches close to the surface of the muscle cells so branches occupy the recesses in the surface of the muscle fiber (the primary synaptic clefts)
- subneural apparatu (seconday synaptic clefts formed by infolding of sarcolemma) contain acetylcholinersterases
- Synaptic vesicles are located in the axon terminals and contain acetylcholine
What does a motor nunit consist of?
a motor neurn and the muscle fibers innervated by it
there can be lots of fibers innervated by one motor neuron
The closer the radio o motor neurons to the number of muscle fibers innervated, the ____ the control
the greater (or finer) the control
What are the two muscle sensory structures we discussed?
. muscle spindles
golgi tendon organs
What are muscle spindles comprised of?
a group of small specialized muscle fibers called intrafusal fibers enclosed in a CT capsule
The central region of the intrafusal fibers will have many nuclei in what two different arragnemets?q
- nuclear bag fibers
- nuclear chain fibers
How many CT capsules are there in a muscle spindle?
2
an internal capsule immediately surrounding the intrafusal fibers and an external capusule surrounding the internal capsule
Describe the sensory innervation of the muscle spindles.
Describe the motor innervation of muscle spindles.
sensory:
- large diameter fibers forming annulospiral or primary endings around the nuclear bag or chain fibers
- smaller diameter fibers forming flower spray endings on the intrafusal fibers outside the mid-region
Motor:
efferent fibers form motor end plates on either side of the mid-region of intrafusal fibers
What are golgi tendon organs comprised of?
spindle-shaped bodies comprised of collagen and enclosed by a thin capsule
What are golgi tendon organs sensitive to?
stretching forces on the tendon
True or false: hypertrophy of muscle tissue is due to the increase in the number of muscle fibers.
False - it is due to the increase in size of the fibers already present
How does cardiac muscle differ from skeletal muscle. What is the main diagnostic feature?
- the sarcoplasmic reticulum is less well developed than in skeletal muscle
- just one nucleus per cell
- The nucleus is located at the center of the cell (not periphery)
- Intercalated disks are the diagnostic feature (although sometimes hard to see under staining)
- The cells are elongated and branching (unlike skeletal which are just tubes)
True or false: skeletal muscle myofibrils are more distinct than in cardiac muscle.
True - in cardiac muscle the myofibrils branch and blend with adjacent myofibrils so they are less distinct.
Where is sarcoplasm typically seen in cardiac muscle cells?
at each pole of the nucleus
Where are intercalated disks located in cardiac muscle? What do they ook like?
they occur wher eyou’d expect to see a Z disc - it marks the spot of a cell-cell attachment
they look like dark step-like cross bands
How does the T-tubule system differ from skeletal muscle to cardial muscle?
in cardiac muscle the t-tubule system is larger (wider) and located at the Z lines instead of at the A-I junction
How does the sarcoplasmic reticulum differn from skeletal muscle to cardiac muscle?
In cardiac muscle it ia less developed and only forms a simple plexiform arrangement of the tubular elements without ternimal disternae
Skeletal muscle has triads, cardiac muscle has ____.
diads - a T-tubule and a terminal portion of adjacent sarcoplasmic reticulum
What are the three cell-to-cell junction types found in an intercalated disc of cardiac muscle?
- gap junctions = nexuses
- desmosome
- fascia adherens (anchoring points for actin filaments)
what are atrial granules? Where are they located? WHat do they contain?
They are unique to atrial cardiac muscle cells
they contain atrial natriuretic peptide
this serves to lower blood pressure by decreasing the renal tubules capability to resorb sodium and water
What types of connective tissue fibers occur around the cardiac muscle cells? How about in between bundles of cardiac muscle cells?
reticular and collagenous fibers around each cardiac muscle cell
collagenous and elastic fibers between bundles of cells
the vascular supply for the cardiac muscle is derived from what?
the coronary arteries
Branches from what nervous system terminate on cardiac muscle?
autonomic - sympathetic and parasympathetic
The conduction system of cardiac muscle is comprised of what fibers?
purkinje fibers - specialized muscle cells
How are purkinje fibers different from other cardiac muscle cells?
the myofibrils are reduced in number
they contain more sarcoplasm
the nucleia re more rounded
they are larger in idamter
they lack T-tubules
they have more glycogen
How does cardiac muscle differ from skeletal muscle in terms of emryological origin?
they are both from mesoderm, but the myoblasts fuse in skeletal muscle and not in cardiac muscle
Does smooth muscle have striations? T-tubules?
no for both
In addition to contractility and conductivity, what is another function of smooth muscle?
it can produce extreacellular products like colagen, elastin, glycosaminoglycans, proteoglycans and growth factors
(not always)
What does smoothe muscle look lik eunder a microscope?
They are fusiformly shaped (ends tapered, oval mucleus in center, corkscrew shape when contracted, cluster in sheeths)
cytoplasm is eosinophilic
each smooth muscle cells is surrounded by what type of connective tissue?
reticular fibers
Mitochondria and other organelles tend to cluster where in smooth muscle cells?
at the poles of the nuclei
FOr the actin in smooth muscle cells, is there also tropomyosin? Troponin?
there is tropomyosin, but not troponin
How are the actin and myosin filaments arranged in smooth muscle?
not in a precise manner like in skeletal and cardiac
more like bundles that run obliquely through cytoplasm
What are the dense bodies of smooth muscle comparable to in skeletal muscle?
the z-discs, because the dense bodies contain alpha actinin to achnor actin myosin filament bundles
Where are the dense bodies located? (2)
- located along the inner aspects of the sarcolemma
- scattered throughout the cytoplasm
What are caveolae?
they are pinocytotic-like invaginations of the sarcoplasmic reticulum
may function as the equivalent of the T-tubules
may also work in concert with the sarcoplasmic reticulum to modulate Ca+ availability
What are the steps in the mechanism for smooth muscle contraction?
- Ca++ binds to calmodulin
- complex activates myosin light chain kinase
- myosin light chain kinase phosphorylates one of the two light chains of the myosin molecule head
- this exposes an actin binding site on myosin head
- myosin head binds to actin and contraction occurs
Why does smooth muscle contraction take longer (and is more prolonged) than skeletal muscle?
the phosphorylation process is slower, so it takes longer for the myosin heads to be able to bind to actin and longer to let go
Why doe smooth muscle contraction require less energy than skeletal muscle?
ATP hydrolysis takes longer than in skeletal muscle
How are impulses transmitted from muscle cell to muscle cell in smooth muscle?
through gap junctions
Do smooth muscle cells retain mitotic capability?
yes - think about the pregnant uterus
