LECTURE OUTLINE & REVIEW QUESTIONS: MUSCLE TISSUE
SKELETAL MUSCLE
Connective Tissue Components
Epimysium (dense irregular c.t.)
Envelops entire muscle
Continuous with tendon (dense regular c.t.)
Perimysium
Continuous with epimysium
Surrounds fascicles (bundles of muscle cells)
Endomysium
Continuous with perimysium
Surrounds individual muscle cells
C.T. Functions
Paths for blood vessels, nerves, lymphatics
Transmit contractile forces:
muscle cell > endomysium > perimysium > epimysium >
tendon > periosteum > bone
Allow muscles, fascicles, and individual muscle cells to act as
units.
Skeletal Muscle Cells (Myofibers)
General Morphology and Terminology:
Elongate cylinders bluntly tapered at ends. Lengths vary.
Each cell is a syncytium, formed by the fusion of myoblasts.
Each cell is multinucleated (~35 nuclei/mm).
Nuclei are located peripherally, i.e., just beneath the cell membrane.
Special terminology:
Sarcolemma = muscle cell membrane
Surrounded externally by a basal lamina.
Sarcoplasm = muscle cell cytoplasm
Skeletal muscle cells are striated.
A-bands and I-bands alternate with each other.
A-bands (LM: dark staining; EM: dense staining)
I-bands (LM: lighter staining; EM: less dense staining)
Z-lines: dark lines dividing I-bands in half.
Sarcomere: section of muscle cell between two Z-lines.
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ANATOMY 25
Why are myofibers striated ?
Sarcoplasm is packed with cylindric myofibrils.
Myofibrils have A-bands, I-bands, Z-lines.
Myofibrils held in register by desmin, so all their bands align.
Why are myofibrils striated ? Answer required the electron microscope
(EM).
Myofibrils are cylindric arrays of thick and thin
myofilaments.
The organization of the myofilaments explains the striations.
I-bands contain only thin myofilaments, so more light
(LM) or more electrons (EM) pass through and the band is
lighter or less dense, except for the Z-line.
A-bands contain thick myofilaments, so less light (LM) or
fewer electrons (EM) pass through and the band is darker
or less dense.
The EM also revealed additional bands: a less dense H-zone
in the middle of the A-band and a dense M-line in the middle
of the H-zone.
The outer ends of the A-bands contain both thin and thick
myofilaments. The H-zone contains only thick
myofilaments. So, although the entire A-band is darker
than the I-band, the ends of the A-band are darker than the
central H-zone, except for the M-line.
Structure of Myofilaments
Myofilaments are contractile proteins.
Thin myofilaments are composed of three proteins:
Actin:
G-actin monomers  F-actin chain
Two F-actin chains  Actin molecule
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LECTURE OUTLINE & REVIEW QUESTIONS: MUSCLE TISSUE
Each G-actin has a myosin-binding site.
Tropomyosin:
Two polypeptide chains  tropomyosin molecule
Troponin: Three subunits.
TnC: binds calcium ions
TnT: links troponin to tropomyosin
TnI: inhibits actin-myosin binding (see below)
Thick myofilaments are composed of myosin molecules.
Myosin molecule: tail + hinge area + double head
Two “heavy” chains + four “light chains”
Head area has: actin-binding site + ATP-binding site
Thick myofilament = bundled myosin molecules.
Heads project in a spiral arrangement.
Accessory proteins keep the myofilaments in position.
Actinin and titin: link ends of thick and thin myofilaments to
Z-line.
Myomesin: links thick myofilaments at M-line.
Histophysiology of Contraction
EM Observations of sarcomeres during muscle contraction.
A-band width does not change.
I-band width decreases and Z-lines approach ends of A-band
H-zone width decreases.
Conclusion: during contraction thin myofilaments slide into
A-band between thick myofilaments.
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ANATOMY 25
Molecular mechanisms of contraction.
Background data.
Myosin head ATP site has ATPase activity.
ATP  ADP + Pi + energy release
Myosin head can “swivel” at hinge area.
Myosin head actin-binding site will link with myosinbinding site on G-actin, if tropomyosin does not block the
G-actin molecules.
Mechanisms: A simplified explanation.
Calcium ions bind to TnC site of troponin in thin
myofilament.
Troponin changes shape  tropomyosin molecule moves
 myosin-binding sites on G-actin molecules are exposed.
ATP locks into ATP site on myosin head  ATP broken
down  released energy forms link between actin-binding
site of myosin head and myosin-binding site of G-actin.
Myosin head swivels and moves the linked G-actin.
ADP + Pi released from myosin head. New ATP binds to
myosin head  actin-myosin bond broken  myosin
head swivels back to original position
Process repeated with next G-actin in chain. Repeat,
repeat, repeat…..
Some interesting facts.
There are ~ 500 myosin heads / thick myofilament.
Each head undergoes ~ 5 reaction cycles / second.
Each cycle moves a thin myofilament a distance equal to ~ 1%
of its length into the A-band.
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LECTURE OUTLINE & REVIEW QUESTIONS: MUSCLE TISSUE
From myofilaments to myofibers to muscle.
Thin filaments slide into A-bands  sarcomeres shorten 
Myofibrils shorten  linking proteins (dystrophins) transmit
forces to muscle cell sarcolemma and basal lamina  muscle
cells gets shorter and fatter  muscle gets fatter and shorter
(30 to 50% of its resting length)  forces transmitted to
endomysium  perimysium  epimysium  tendon 
periosteum  bone  bone moves at joint.
Stopping contraction.
ATP  myosin ATPase site  actin-myosin link broken.
Calcium ions released from TnC of troponin  troponin
changes shape  tropomyosin moves  myosin-binding sites
on G-actin molecules blocked.
(CONTINUED IN LECTURE PART B.)
Where do the calcium ions (Ca++) come from ?
Sarcoplasmic reticulum (SR):
Smooth endoplasmic reticulum
Surrounds myofibrils
Stores and releases calcium ions
ANATOMY 25
Action potential (electrochemical wave) travels along
sarcolemma  T-tubules  terminal cisternae  receptors
 SR calcium channels open  Ca++ released……
What causes an action potential (AP) in the sarcolemma ?
Skeletal muscle cells are controlled by somatic motor
neurons located in the CNS (central nervous system).
The axon of a motor neuron branches to end in motor endplates (myoneural junctions) on muscle cells.
A single neuron + the muscle cells it controls is called a motor
unit.
Motor unit sizes vary. In muscles performing fine movements,
units are small. In muscles performing larger scale movements,
units are large.
Anatomy of a motor end-plate:
Axon branch loses its myelin sheath and ends in several
axon terminals (terminal bulbs).
At start of muscle contraction:
Ca++ channels in SR open  Ca++ into sarcoplasm  Ca++
bind to TnC of troponin    contraction
Each axon terminal sit in an indentation of the
sarcolemma, the sole plate. Sarcolemma in sole plate is
highly folded to increase surface area. End-plate is roofed
over by Schwann cells.
At the start of muscle relaxation:
Ca++ channels in SR close  Ca++ /Mg/ATPase pumps in SR
activated  Ca++ back into SR  troponin changes shape 
tropomyosin moves and blocks myosin sites on actin
Axon terminal contains unit membrane vesicles of the
neurotransmitter, acetylcholine (Ach). Sarcolemma in sole
plate contains acetylcholine receptors. Axon terminal and
sole plate are separated by a narrow gap (synaptic cleft).
What opens the SR calcium channels ?
Terminal cisternae of SR are in contact with transverse
tubules (T-tubules), tubular invaginations of the sarcolemma.
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How it works…
Neuron generates AP (nerve impulse)  AP travels along
axon to axon terminal  opens calcium channels in axon
terminal  calcium ions enter axon terminal  ACh
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LECTURE OUTLINE & REVIEW QUESTIONS: MUSCLE TISSUE
released (merocrine secretion) into synaptic cleft  Ach
diffuses across cleft  binds to ACh receptors in sole plate
sarcolemma  AP initiated in sarcolemma  travels
along sarcolemma to T-tubules  triggers opening of
calcium channels in sarcoplasmic reticulum
Summation of Events in Skeletal Muscle Contraction
Motor neuron AP  axon  axon terminals  calcium influx
 Ach release  synaptic cleft  ACh binds to ACh receptors
in sole plate  AP in T-tubules  calcium release from SR 
calcium binds to TnC  troponin changes shape 
tropomyosin moves  actin myosin-binding sites exposed 
ATP binds to myosin  actin-myosin links formed  ATP
breakdown  myosin heads swivel  thin myofilaments moved
into A-band  myofibrils shorten  linking proteins
(dystrophins) transfer forces to sarcolemma and basal lamina 
muscle cell shortens  force transmitted to endomysium 
perimysium  epimysium  tendon  periosteum  Sharpey’s
fibers  bone  BONE MOVES.
Skeletal muscle cells vary in their morphology and physiology.
Type 1: slow, oxidative, or red fibers
slow ATPase rate; high oxidative rate; moderate glycolytic activity;
moderate SR calcium pumping capacity; moderate cell diameter
Type 2: fast, glycolytic, or white fibers
fast ATPase rate; low oxidative rate; high glycolytic activity; high
SR calcium pumping capacity; large cell diameter
Depending on its function, any specific muscle may contain
mostly Type 1 fibers; mostly Type 2 fibers; or a variable mixture
of both
CARDIAC (HEART) MUSCLE
Cardiac Muscle Cells
Cylindric cells with one or two nuclei. May branch at their ends.
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ANATOMY 25
Have the same striations as skeletal muscle cells (A-bands, Ibands, Z-lines, etc.)
Have the same arrangement of myofilaments as skeletal
muscle cells, but no myofibrils.
Have a sarcoplasmic reticulum, but it differs in details from
that of skeletal muscle.
Are joined end-to-end by intercalated discs containing:
Desmosomes and extensive tight junctions (fasciae
adherentes) that bind the cells together.
Gap junctions that allow intercellular communication.
The contractile mechanisms in cardiac muscle are the same as in
skeletal muscle, but the nerve supply differs.
Unlike skeletal muscle cells, cardiac cells will contract without
a nerve supply.
However, their contractions are regulated by visceral motor
(autonomic) neurons, not somatic motor neurons as in
skeletal muscle.
The autonomic neurons release either acetylcholine
(cholinergic fibers) or norepinephrine (noradrenergic fibers).
In the heart:
Noradrenergic fibers make contact with some cardiac
muscle cells in the ventricles.
Cardiac myoneural junctions differ from the motor endplates of skeletal muscle. They are called en passant – in
passing- terminations.
The nerve branches “pass” from cell to cell. Each
branch has a series of bubble-like expansions that
contact the cardiac cell membrane. The “bubbles”
contain vesicles of norepinephrine.
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LECTURE OUTLINE & REVIEW QUESTIONS: MUSCLE TISSUE
The action potentials are transmitted from the cells with
en passant contacts to other cells via the gap junctions in
the intercalated discs.
Both noradrenergic and cholinergic fibers end on
specialized conduction system cells (pacemaker tissue).
These cells transmit the action potential to surrounding
cardiac muscle cells.
SMOOTH MUSCLE
Smooth Muscle Cell Morphology
Elongated spindles; a single, centrally located nucleus; no
striations.
Myofilament Organization
Bundles of thin myofilaments (actin and tropomyosin) and thick
myofilaments (a form of myosin different from that in skeletal or
cardiac muscle) criss-cross through the cytoplasm.
The myofilaments attach to:
Dense areas (plaques) in the plasmalemma
Dense bodies (the protein actinin) in the cytoplasm
Thin myofilaments do not contain troponin.
Its calcium binding functions are served by other proteins.
Sarcoplasmic Reticulum
Smooth muscle cells have some ser, but it is not organized into a
sarcoplasmic reticulum.
To initiate contraction, calcium ions are transported into the
cell from the extracellular fluid by calcium pumps in the cell
membrane.
Smooth muscle cells have abundant caveolae and these may
be the sites of at least some of the calcium transport.
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ANATOMY 25
Contractile Mechanisms
Basically the same molecular interactions as in skeletal and cardiac
muscle.
Actin-myosin interactions move the thin myofilaments.
Forces are transmitted to the dense plaques in the
plasmalemma.
Smooth muscle can:
Maintain contraction much longer than skeletal or cardiac
muscle.
Adapt to maintain the same tension when stretched.
General Classes of Smooth Muscle
Visceral or Unitary: sheets of cells; cells connected by gap
junctions; cells act as a syncytium.
Multiunit: no gap junctions; cells act individually.
Control of Smooth Muscle
Various smooth muscle cells contract in response to:
Stretching
Hormones
Nerve stimulation
Visceral smooth muscle
En passant endings on some cells. Action potential
relayed to other cells via gap junctions.
Multiunit smooth muscle
En passant endings on all cells.
REVIEW QUESTIONS
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LECTURE OUTLINE & REVIEW QUESTIONS: MUSCLE TISSUE
SKELETAL MUSCLE
Matching
1.
2.
3.
4.
Connective tissue around a single skeletal muscle cell.
Connective tissue around a bundle of skeletal muscle cells.
Connective tissue around an entire skeletal muscle.
A bundle of muscle cells surrounded by perimysium.
Answers: (a) fasciclce (b) perimysium (c) endomysium
(d) epimysium (e) none of these
5.
6.
7.
8.
17. The connective tissue elements of a skeletal muscle _?_. (a)
transmit contractile forces (b) are pathways for blood vessels and
nerves supplying muscle cells (c) allow muscle cells, fascicles,
and whole muscles to act as individual units (d) all of these (e)
none of these
18. A single skeletal muscle cell is a multinucleated syncytium
formed by the fusion of _?_. (a) fibroblasts (b) myoblasts (c)
lipoblasts (d) mesenchymal cells (e) none of these
19. Which is structure is largest ? (a) myofiber (b) myofibril (c)
thick myofilament (d) thin myofilament.
Muscle cell cytoplasm
Muscle cell membrane
Muscle celll smooth endoplasmic reticulum
Segment of a myofiber or myofibril between two Z-lines
20. Skeletal muscle cells are striated because their sarcoplasm is
packed with striated myofibrils held in register by _?_. (a) Zlines (b) desmin (c) M-lines (d) keratin (e) none of these
Answers: (a) sarcomere (b) sarcolemma (c) sarcoplasm
(d) sarcoplasmic reticulum (e) none of these
21. Myobibrils are striated because they are cylindrical arrays of
thick and thin myofilaments held in register _?_. (a) titin (b)
actinin (c) myomesin (d) all of these (e) none of these
9. Thick myofilaments
10. Thin myofilaments
11. An array of myofilaments
Answers: (a) myofibril (b) troponin + tropomyosin + actin
(c) myosin (d) none of these
12.
13.
14.
15.
16.
ANATOMY 25
I – Band
A - Band
H - Zone
M - line
Z - line
Answers: (a) thick myofilaments only (b) thin myofilaments
only (c) proteins connecting thin myofilaments (d)
proteins connecting thick myofilaments (e) thick and
thin myofilaments
MULTIPLE CHOICE
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22. A sarcomere includes _?_. (a) two dark Z-lines (b) halves of two
light I-bands (c) an entire dark A-band (d) all of these (e) none
of these
23. If you observed a single sarcomere during muscle contraction,
which of the following events would not occur ? (a) I-bands
shorten (b) H - zones shorten (c) A-bands shorten (d) Zlines approach the ends of the A-band (e) sarcomere shortens
24. Which muscle cell molecule contains an actin-binding and an
ATP-binding site ? (a) troponin (b) myosin head (c) myosin
tail (d) actin (e) tropomyosin
25. Which muscle cell molecule changes shape when it binds with
calcium ions ? (a) troponin (b) myosin head (c) myosin tail (d)
actin (e) tropomyosin
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LECTURE OUTLINE & REVIEW QUESTIONS: MUSCLE TISSUE
26. Which molecule covers the myosin binding sites of actin in
resting muscle ? (a) troponin (b) myosin head (c) myosin tail
(d) actin (e) tropomyosin
27. During muscle contraction, which is the second event to occur
in the following list ? (a) troponin changes shape (b) calcium
binds to troponin TnC (c) tropomyosin moves (d) myosinbinding sites on G-actin molecules are exposed (e) calcium ions
released from sarcoplasmic reticulum
28. During muscle contraction, which is the third event to occur in
the following list ? (a) myosin head swivels (b) actin-myosin
linkage forms (c) ATP binds to myosin head and ATP  ADP
+ P + energy (d) thin myofilament moves into A-band (e) ADP
+ P released from myosin head
29. After a myosin head has swiveled and moved its attached Gactin molecule, the actin-myosin bridge is broken and the
myosin head returns to its original position when _?_. (a) ADP
+ P is released from the myosin head (b) a new ATP locks into
the myosin head (c) calcium is released from troponin (d)
tropomyosin moves (e) none of these
30. A contracting muscle cell gets shorter and fatter because _?_. (a)
sarcomeres get fatter during contraction (b) myofibrils get fatter
during contraction (c) myofilaments get fatter during
contraction (d) dystrophin proteins transmit motions of the thin
myofilaments to the sarcolemma and basal lamina (e) all of
these
31. The shortening and fattening of contracted muscle cells results
in tensile (pulling) forces in the muscle’s connective tissue
elements. Which sequence best describes the order in which
these forces are exerted ? (a) endomysium  perimysium 
epimysium  tendon  periosteum and Sharpey’s fibers 
bone (b) epimysium  perimysium  endomysium  tendon
 periosteum and Sharpey’s fibers  bone (c) endomysium 
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ANATOMY 25
epimysium  perimysium  tendon  periosteum and
Sharpey’s fibers  bone
32. During muscle relaxation, which is the third event to occur in
the following list ? (a) Ca/Mg/ATPase pumps in sarcoplasmic
reticulum are activated (b) calcium is released from troponin (c)
troponin changes shape (d) tropomyosin moves to block
myosin-binding sites on actin (e) thin myofilaments slide out of
the A-band
33. A motor neuron plus the skeletal muscle cells it controls is
called a _?_. (a) myoneural junction (b) motor end-plate (c)
motor unit (d) fascicle (e) none of these
34. Skeletal muscle cells are controlled by _?_. (a) visceral motor
neurons (b) autonomic neurons (c) somatic motor neurons (d)
all of these (e) none of these
35. The contact area between the axon terminals of a motor neuron
and the sarcolemma of a skeletal muscle cell is called a __?__.
(a) T-tubule (b) motor end-plate (c) motor unit (d) muscle
spindle (e) sarcoplasmic reticulum
36. Which is the third even to occur in the following list ? (a)
calcium ions enter axon terminal (b) nerve impulse travels down
axon (c) acetylcholine is released from axon terminal (d)
acetylcholine binds to receptors in sole plate sarcolemma (e)
acetylcholine diffuses across synaptic cleft
37. Which is the second even to occur in the following list ? (a)
action potential travels along T-tubules (b) calcium gates in
terminal cisternae open (c) action potential travels along
sarcolemma (d) calcium enters sarcoplasm (e) calcium combines
with troponin
38. All skeletal muscle cells are histologically and physiologically
identical. (a) true (b) false
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LECTURE OUTLINE & REVIEW QUESTIONS: MUSCLE TISSUE
39. Type 2 skeletal muscle cells are also called _?_. (a) slow (b)
oxidative (c) red (d) all of these (e) none of these
40. Type 1 skeletal muscle cells are also called _?_. (a) white (b) fast
(c) glycolytic (d) all of these (e) none of these
41. Cardiac muscle cells _?_. (a) may branch (b) contain one or two
centrally located nuclei (c) are linked by intercalated discs (d)
are regulated by autonomic nerve cells (e) all of these.
42. Cardiac muscle cells have _?_. (a) the same striations as skeletal
muscle cells (b) the same arrangement of thick and thin
myofilaments as skeletal muscle cells (c) a sarcoplasmic
reticulum (d) all of these (e) none of these
ANATOMY 25
48. At the beginning of contraction in smooth muscle, calcium ions
are _?_. (a) released from the sarcoplasmic reticulum (b)
transported into the cell from the extracellular fluid (c) released
from T-tubules (d) all of these (e) none of these
49. Various smooth muscle cells may contract in response to _?_. (a)
hormones (b) a nerve action potential (c) stretching (d) all of
these (e) none of these
50. In _?_ smooth muscle, the cells are connected by gap junctions
and only some of the cells have en passant autonomic nerve
endings. (a) visceral or unitary (b) multiunit
43. Intercalated discs contain _?_. (a) desmosomes (b) gap junctions
(c) fasciae adherentes (d) all of these (e) none of these
44. Which component of the intercalated disc allows an action
potential to be transmitted from one cardiac muscle cell to
another ? (a) desmosomes (b) gap junctions (c) fasciae
adherentes (d) all of these (e) none of these
45. Noradrenergic nerve axons _?_. (a) make en passant contacts
with all cardiac muscle cells (b) contain norepinephrine (c)
contain acetylocholine (d) end in motor end-plates similar to
those in skeletal muscle (e) none of these
46. Smooth muscle cells _?_. (a) are spindle-shaped (b) contain one
centrally located nucleus (c) are non-striated (d) all of these (e)
none of these
47. Select the incorrect statement. In smooth muscle _?_. (a) the
myofilaments are arranged in myofibrils (b) thin myofilaments
do not contain troponin (c) myofilaments are anchored to
cytoplasmic dense bodies (d) myofilaments are anchored to
dense areas in the plasmalemma (e) there is no organized
sarcoplasmic reticulum
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