Histology of Muscle Flashcards
General Features of Skeletal Muscle
- striated (voluntary)
- T-tubule system
- well-developed sarcoplasmic reticulum
- multinucleated
- nuclei located at cell periphery (just inside sarcolemma)
A band
broad, dark-staining band
anisotropic
I band
narrower, light-staining band
isotropic bond
Z-disk
dense-staining, narrow
bisects I band
distance between two Z-disks = sarcomere
H band
pale-staining, narrow
bisects A-band
Located at the middle of the sarcomere
M-line
thin
bisects H band
Functional unit of the myofibril
sarcomere
Sarcomere length varies as a function of degree of ________
contraction of muscle cell
Actin filaments
thin
extend from Z-disks to edge of H-band
myosin filaments
thick
extend from one end of A-band to other end of A-band
thin cross bridges extend from each myosin filament towards neighboring actin filaments
M line
at center of H-band
contains myomesin, C protein and other proteins that interconnect thick myosin filaments to maintain their specific lattice arrangement
Z disk
(a) region where attachment of the ends of the actin filaments in adjacent sarcomeres occurs
(b) appears filamentous and somewhat electron dense
(c) contains alpha-actinin
network of cisterns or membranous tubules running between and around myofibrils
form collars at AI junctions
Skeletal muscle - sarcoplasmic reticulum
________ binds and releases Ca (membranes contain voltage-gated Ca-release channels)
Sarcoplasmic reticulum
Forms T-tubules
Sarcolemma
T-tubules typically seen at ___________ between _________ (skeletal muscle)
AI junction between two collars of SR
Triads
T-tubule + 2 lateral cisternae of SR (skeletal muscle)
2 triads/sarcomere
Actin is made up of ____
G-actin (globular) and F-actin - composed of 2 helically wound G-actin polymers
Myosin = _______ and _________
4 light chains and 2 heavy chains
Light chain Myosin
Meromyosin
Long molecular subunits -rigid
packed together longitudinally to form backbone of myosin filament
Heavy chain myosin
Heavy meromyosin
Long molecular subunits, more flexible
2 components - rod-like portion lies parallel to backbone of filament, globular head = crossbridge from thick myosin filament to actin filament
Globular head of heavy chain meromyosin
Cross bridge from thick myosin filament to actin filament
ATPase activity w/ actin-binding sites
No cross bridges at ________ line
M- line (globular heads directed away from midpoint of myosin filament)
Tropomyosin
Fibrous, arranged head to tail in linear series
helically wound along grooves of F-actin helix (blocks actin-myosin binding)
Troponin complex
TnT: binds entire troponin molecule to tropomyosin
TnC: great affinity for Ca
Tnl: binds to actin, preventing actin-myosin interactions
Ca
binds to TnC
Binding to TnC induces a conformational shift in tropomyosin, exposing previously blocked active sites on the actin filament (myosin binding sites)
alpha-actinin
component of Z-disk
holds actin filaments in register by binding them in parallel array
Titin
Large, linear elastic protein
Helps position myosin filaments precisely within the sarcomere
extends from each half of a myosin filament to the Z-disc (like a spring)
Nebulin
long, non-elastic protein
two molecules of nebulin wrap around each thin filaments
help anchor thin filament to Z-disk
Changes in banding patterns during contraction
A-band remains constant
I-band becomes smaller
Z-disc come closer together (sarcomere shortens)
H-band narrows and is eventually obliterated
sheath of fine connective tissue surrounding individual muscle
fibers
endomysium
perimysium
(1) connective tissue sheath around groups of muscle fibers (fascicles)
(2) continues as a major component of a tendon
outermost sheath of connective tissue; surrounds entire muscle
epimysium
Vascular supply to skeletal muscle
a. larger arteries follow perimysium
b. arterioles penetrate fascicles
c. capillaries parallel individual myofibers
subneural apparatus
(1) secondary synaptic clefts formed by infolding of sarcolemma
(2) acetylcholinesterase located at surface of sarcolemma
Motor units
consist of a motor neuron and the muscle fibers innervated by it
Muscle spindles
Small specialized muscle fibers (inrafusal fibers) enclosed in connective tissue capsule
Central - nuclear bag fibers, nuclear chain fibers
capsules - internal immediately surrounds intrafusal, external capsule surrounds internal capsule
Sensory innervation of muscle spindle
[1] large diameter fibers form annulospiral or primary endings around mid-portions of nuclear bag and nuclear chain fibers
[2] smaller diameter afferent fibers form flower spray or secondary endings on the intrafusal fibers some distance from the mid-region
Motor innervation of muscle spindle
efferent fibers form motor end plates on either side of the mid-region of the intrafusal fibers
Golgi tendon organs
(1) spindle-shaped bodies comprised of collagen and enclosed by a thin capsule
(2) afferent (sensory) nerve fibers penetrate between the collagen fibers
(3) sensitive to stretching forces on the tendon
Embryologic origins of skeletal muscle
- loose mesenchyme –> myoblasts
- myoblasts fuse –> multinucleate myotubes
a. myofilaments appear
(1) irregularly arranged at first
(2) gradually become aligned into myofibrils
Hypertrophy of muscle d/t ____________
increase in size of fibers, not number
General featurs of Cardiac Muscle
- striated (involuntary)
- T-tubule system
- sarcoplasmic reticulum less well developed than in skeletal muscle
- single nucleus/cell
- nucleus located at center of cell
- intercalated disks are a diagnostic feature of this tissue
Cardiac muscles cells have _________ nucleus/nuclei located ________
1 nucleus
centrally located
myofibrils of cardiac muscle cells appearance
branch and sometimes blend with adjacent myofibrils, sarcomeres/a-bands/i-bands/z-discs all present
Intercalated disks
a. dark cross-bands; frequently appear step-like
b. occur where a Z-disk should be observed
c. mark spots of cell-to-cell attachments
represent a complex of several types of cell-to-cell junctions
[1] gap junctions (nexuses)
[2] desmosomes
[3] fascia adherens (serve as anchoring points for actin filaments)
T-tubules in cardiac muscle
[1] larger (wider) than those in skeletal muscle
[2] located at Z-lines instead of at A-I junction
Sarcoplasmic reticulum in cardiac muscle
[1] less developed than that of skeletal muscle
[2] simple plexiform arrangement of tubular elements
[a] no terminal cisternae
diads
cardiac muscle
T-tubule + terminal portion of adjacent sarcoplasmic reticulum
Fat droplets abundant in ________ muscle
cardiac
Atrial granules
a. unique to atrial cardiac muscle cells
b. contain atrial natriuretic peptide which functions to lower blood pressure
[1] decreased renal tubules capability to resorb (conserve) sodium and water
Connective tissue of Cardiac muscle
- each cardiac muscle cell is surrounded by a fine net of reticular and collagenous fibers
- collagenous and elastic fibers occur between bundles of cells
Vascular supply to cardiac cells
- excellent: vessels are derived from coronary arteries
- lymphatic drainage present
Nerve supply to cardiac muscle
- branches of sympathetic and parasympathetic terminate on muscle cells
Purkinje fibers
Specialized muscle fibers - conduction system of cardiac muscle
a. myofibrils reduced in number
b. contain more sarcoplasm
c. nuclei more rounded
d. large diameter
e. lack T-tubules
f. more glycogen
Embryological origins of cardiac muscle
- mesoderm –> myoblasts
- no fusion of myoblasts
General features of smooth muscle
No striations
No t-tubules
involuntary
Locations of smooth muscle
walls of hollow viscera (most of the gastrointestinal tract; portions of reproductive and urinary tracts, walls of blood vessels, larger ducts of some compound glands, respiratory passages, and small bundles in the skin)
functions of smooth muscle
contractility, conductivity, production of extracellular products
Connective tissue contributions of smooth muscle cells
a. every cell is surrounded by a thin external lamina that separates the plasma membrane of adjacent smooth muscle cells
b. reticular fibers are prevalent in the external lamina: help harness the force of contraction
Dense bodies
comparable to Z-disks of skeletal and cardiac muscle
(1) contain a-actinin
(2) serve as anchor sites for actin-myosin filament bundles as
well as intermediate filaments
Located along inner aspects of the sarcolemma, scattered throughout cytoplasm
Caveolae
pinocytotic-like
(1) may function as the equivalent of the T-tubule in skeletal and cardiac muscle
(2) may also work in concert with the sarcoplasmic reticulum to modulate Ca++ availability
Contractile mechanism for smooth muscle
- Ca calmodulin form a complex
- Ca++-calmodulin complex activates myosin light chain kinase
- myosin light chain kinase phosphorylates one of the two light chains associated with the myosin molecule head
- phosphorylation of the light chain exposes an ‘actin-binding’ site on the myosin molecule
Innervation of smooth muscle
- sympathetic innervations: synaptic vesicles contain norepinephrine
- parasympathetic innervation: synaptic vesicles contain acetylcholine
- variations in numbers of cells in a group that are innervated
Regneration of smooth muscle
- smooth muscle cells retain mitotic capability (e.g., pregnant uterus)
- smooth muscle cells can also be formed from pre-existing pericytes
