Muscle Tissue

Question 1

skeletal muscle

Answer 1

Striated (voluntary)
T-tubule system
Well-developed sarcoplasmic reticulum
Multinucleated
Nuclei at cell periphery

Cell shape:
-Cylindrical (10 m –100 m in diameter)
-Great variations in length (1 mm -- 4  cm)
-Bulky muscles: larger cells
-Delicate muscles: smaller cells
Nuclei
-Several hundred/cell
-Flattened, oval, and elongated
-Located at cell periphery, beneath sarcolemma

Origins:
Loose mesenchyme -> myoblasts
Myoblasts fuse -> multinucleate myotubes
Myofilaments appear
-Irregular at first
-Gradually become aligned into myofibrils

Question 2

smooth muscle

Answer 2

General features

• No striations
• No T-tubules
• Involuntary

Sympathetic: synaptic vesicles contain norepinephrine

Parasympathetic: synaptic vesicles contain acetylcholine

Iris (eye): every muscle fiber is innervated

Other areas: only small percentage of muscle cells innervated

Cells retain mitotic capability
Example: pregnant uterus
Can be formed from pre-existing pericytes

Question 3

cardiac muscle

Answer 3

Striated (involuntary)
T-tubule system
Sarcoplasmic reticulum less well developed than in skeletal muscle
Single nucleus/cell; centrally located
Intercalated disks are diagnostic features of cardiac muscle tissue
numerous mitochondria
No fusion of myoblasts

One cell/nucleus
Centrally located
oval and fairly large

Myofibrils
-Branch and blend with adjacent myofibrils
~Thus less distinct than in skeletal muscle
-Sarcomeres, A-bands, I-bands, Z-disks, etc. are all present
Sarcoplasm most apparent at poles of nucleus

Atrial granules

• Unique to atrial cardiac muscle cells
• Contain atrial natriuretic peptide
• Lowers blood pressure by decreasing renal tubules ability to resorb (conserve) sodium and water

Question 4

myofiber

Answer 4

muscle cells

Question 5

myofibril

Answer 5

Tiny cylindrical rods within sarcoplasm

Responsible for appearance of cross-striations
Due to registry of adjacent myofibrils to one another

composed of actin and myosin filaments

Question 6

myofilament

Answer 6

actin and myosin filaments

Question 7

sarcolemma

Answer 7

plasma membrane

Numerous invaginations along cell surface
Form the T-tubules
Lumen of T-tubule continuous with extracellular space
T-tubules wind between myofibrils
Typically seen at A-I junction between the two ‘collars’ of the sarcoplasmic reticulum

Question 8

sarcoplasm

Answer 8

cytoplasm

Between myofibrils
Beneath sarcolemma
Around nuclei
Mitochondria: near nuclei, beneath sarcolemma, between myofibrils
Sarcoplasmic reticulum

Question 9

sarcoplasmic reticulum

Answer 9

smooth er

Network of cisterns and/or membranous tubules running between and around myofibrils
Tends to form ‘collars’ at A-I junctions
‘Collars’ connected by longitudinal components
Two ‘collars’ at each A-I junction
‘Collars’ separated from each other by T-tubules
Binds and releases Ca ++ (membranes contain voltage-gated Ca ++-release channels)

Question 10

fascicle

Answer 10

a

Question 11

actin

Answer 11

Thin
Extend from Z-disks to edge of H-band

G-actin (globular actin)

• Globular monomer
• Polymerizes to form F-actin

F-actin (fibrous actin)

• Composed of G-actin monomers
• Two helically-wound strands of polymerized G-actin

Question 12

myosin

Answer 12

Thick
Extend from one side of A-band to other side of A-band
Thin cross-bridges extend from each myosin filament towards neighboring actin filaments

Question 13

tropomyosin

Answer 13

Fibrous; arranged head to tail in a linear series

Helically wound along grooves of F-actin helix

Ca++

• Binds to TnC
• Binding to TnC induces a conformational shift in tropomyosin, exposing previously blocked active sites on the actin filament

Question 14

actinin

Answer 14

Component of the Z-disk

Holds actin filaments in register by binding them in parallel array

Question 15

troponin

Answer 15

Troponin complex (3 subunits)

TnT: binds entire troponin complex to tropomyosin

TnC: great affinity for Ca++

TnI: binds to actin, preventing actin-myosin interactions

Question 16

T-tubule

Answer 16

Lumen of T-tubule continuous with extracellular space
T-tubules wind between myofibrils
Typically seen at A-I junction between the two ‘collars’ of the sarcoplasmic reticulum

Question 17

triad

Answer 17

T-tubule + 2 lateral cisternae of sarcoplasmic reticulum
Membranes of T-tubules appear to be closely ‘coupled’ to membranes of sarcoplasmic reticulum
Facilitates transmission of electrical impulse from sarcolemma to interior depths of cell
Two ‘triads’/sarcomere (at the A-I junctions)

Question 18

diad

Answer 18

T-tubule + a terminal portion of adjacent sarcoplasmic reticulum

Question 19

nebulin

Answer 19

Long, non-elastic protein

Two molecules of nebulin wrap around each thin filament

Help anchor thin filament to Z-disk

Question 20

titin

Answer 20

Helps position myosin filaments precisely within the sarcomere
Extends from each half of a myosin filament to the Z-disk

Question 21

caveolae

Answer 21

smooth muscle
Pinocytotic-like invaginations of sarcolemma
Probably play the role of the T-tubules found in skeletal and cardiac muscle
May work in concert with sarcoplasmic reticulum to modulate Ca++ availability

Question 22

dense bodies

Answer 22

smooth muscle
Two locations
-Inner aspects of sarcolemma
-Throughout cytoplasm
Comparable to Z-disks of skeletal & cardiac muscle
-Contain -actinin
-Serve as anchor sites for actin-myosin interactions as well as intermediate filaments (vimentin; desmin)

Question 23

Z-disk

Answer 23

Region where attachment of ends of actin filaments in adjacent sarcomeres occurs
Appears filamentous and electron dense
Contains -actinin

Question 24

sarcomere

Answer 24

linear unit

Question 25

intercalated disk

Answer 25

Dark cross-bands; frequently appear step-like Occur where a Z-disk should be observed Mark spots of cell-cell attachments (i.e, at ends of cells)

Question 26

Be able to describe and explain the predictable filament pattern that you would observe in cross sections through the I-band, A-band, H-band.

Answer 26

M-line At center of H-band Contains myomesin, C-protein, and other proteins interconnect the thick myosin filaments to maintain their specific lattice arrangement

Question 27

Be able to explain the changing banding patterns that you would see as a muscle (skeletal or cardiac) contracts.

Answer 27

a

Question 28

Be able to explain the function(s) of the sarcoplasmic reticulum serves in muscle tissue

Answer 28

a

Question 29

epimysium

Answer 29

Outermost sheath of c.t.; surrounds entire muscle

Question 30

perimysium

Answer 30

Connective tissue sheath around groups of muscle fibers (fasicles) Continuous with tendon fibers

Question 31

endomysium

Answer 31

Sheath of fine c.t. around individual muscle fibers | Fine reticular fibers and ground substance

Question 32

Be able to describe what Purkinje fibers are and do.

Answer 32

``` Composed of modified cardiac muscle cells called Purkinje fibers (cells) Myofibrils reduced in number Contain more sarcoplasm Nuclei more rounded Large diameter Lack T-tubules More glycogen ```

Question 33

neuromuscular spindles

Answer 33

Small specialized muscle fibers in a c.t. capsule -Small fibers = intrafusal fibers Two zones -Central (equatorial) region of intrafusal fibers ~Nuclei chain fibers ~Nuclear bag fibers -Polar regions are long and tapered Capsules - Internal capsule immediately surrounds intrafusal fibers; encloses inner axial space - External capsule surrounds internal capsule and creates periaxial space between inner and outer capsules Innervation Sensory -Large fibers form annulospiral or primary endings around nuclear bag & nuclear chain regions -Smaller diameter afferents form flower spray or secondary endings on intrafusal fibers on polar segments Motor -Efferent fibers form motor end plates on either side of mid-region of intrafusal fibers Act like miniature strain gauges

Question 34

Golgi tendon organs

Answer 34

Structure - Spindle-shaped bodies comprised of collagen and enclosed by a thin capsule - Afferent (sensory) fibers penetrate between the collagen fibers - Sensitive to stretching forces on tendons

Question 35

motor end plates

Answer 35

Myelin sheath lost as nerve fiber approaches surface of muscle cell Axon branches near surface of muscle cell -Branches occupy recesses in cell surface -Recesses called synaptic troughs or primary synaptic clefts Sub-neural apparatus Synaptic vesicles Sub-neural apparatus -Secondary synaptic clefts formed by folds in sarcolemma -Acetylcholinesterase located at surface of sarcolemma Synaptic vesicles -In axon terminals -Contain acetylcholine

Question 36

Contractile mechanisms

Answer 36

Ca++ + calmodulin form a complex Ca++ probably stored in caveolae 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 light chain Exposes an ‘actin-binding’ site on the myosin molecule Enables myosin molecule to ‘straighten out’ and form small bipolar filaments Slow process Smooth muscle contraction takes longer and is more prolonged than skeletal or cardiac muscle contraction