Muscle Tissue

food we eat along gastrointestinal tract
Expels waste we produce
Changes amount

muscle
Composes 40-50% of weight of the adult

700 skeletal muscles in

Muscle
Body movement
Maintenance of posture
Protection and support
Storage and movement of

of posture, protection and support, storage and movement of material,

What are the five major functions of skeletal muscle?

system stimulation
neurons secreting neurotransmitters that bind to muscle cells
Conductivity
electrical change

protein fibers acting like compressed coils
when contraction ended, tension in

muscle cells
Typically as long as the entire muscle
Often referred to

of connective tissue:
epimysium, perimysium, endomysium
Provide
protection
sites for blood vessel and

dense irregular connective tissue
surrounds whole skeletal muscle
Perimysium
dense irregular tissue surrounding

composed of dense regular connective tissue
formed by the three connective

sheet of dense irregular connective tissue
external to the epimysium
separates individual

to deep fascia
composed of areolar and adipose connective tissue
separates muscles

vascularized by extensive blood vessels
Deliver oxygen and nutrients, removing waste

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Epimysium
Tendon
Deep fascia
Skeletal muscle
Artery
Vein
Nerve
Perimysium
Fascicle
Endomysium
Muscle fiber

of connective tissue surrounding the whole skeletal muscle and providing

What are the locations of the endomysium, perimysium, and epimysium?

The perimysium surrounds the muscle fascicles and contains extensive blood vessels and nerves.

The endomysium is the innermost layer surrounding and electrically insulating muscle fibers.

comprising muscle)
Contains typical cellular structures
e.g., Golgi apparatus, ribosomes, vesicles
Has specialized

of muscle
Myoblasts
embryonic cells which fuse
form single skeletal muscle fibers during

in adult skeletal tissue
may be stimulated to differentiate if tissue

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Muscle fiber

Myoblasts

Satellite cell

Satellite cell

Nuclei

Muscle fiber

Myoblasts fuse
to form a skeletal
muscle fiber.

a skeletal muscle fiber
sarcolemma
Invaginations of the sarcolemma
T-tubules, or transverse tubules

pumps along sarcolemma and T-tubules
create concentration gradients for Na+ and

channels and voltage-gated K+ channels
also present
necessary for propagation of electrical

to smooth endoplasmic reticulum
Surround bundles of contractile proteins
Terminal cisternae
blind sacs

T-Tubules (Figure 10.3 b)
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–

Interstitial fluid

Sarcoplasm

Sarcolemma

(b) Sarcolemma and T-tubules

T-tubule

3 Na+ out

Na+/K+
pump

Voltage-gated
Na+ channel

–

2K+ in

Voltage-gated
K+ channel

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Na+

K+

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sarcoplasmic reticulum
stored bound to specialized proteins, calmodulin and calsequestrin
Voltage-gated Ca2+

(Figure 10.3c)

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Ca2+ pump

Calsequestrin

Calmodulin

(c) Sarcoplasmic reticulum

Terminal cisterna

Sarcoplasm

SR membrane

Ca2+

Voltage-gated
Ca2+ channel

structures
extend length of muscle fiber
compose 80% of volume of

a)
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Muscle

Fascicle

Muscle fiber

Sarcoplasmic
reticulum

Triad

Terminal
cisternae

T-tubule

Sarcolemma

Myofibrils

Sarcomere

Myofilaments

Nucleus

Openings into
T-tubules

(a) Skeletal muscle fiber

Nucleus

Sarcoplasm

Mitochondrion

Nucleus

filaments
Assembled from bundles of protein molecules, myosin
each myosin protein

filaments
Primarily composed of two strands of protein, actin
Two strands twisted

filaments (continued)
Tropomyosin
twisted “stringlike” protein
cover small bands of the actin strands
covers

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Myosin molecule
G-actin
Muscle

Myofibril

Myofilaments

Heads

Actin binding site

ATP and ATPase binding site

Myosin heads

(a) Thick filament

(b) Thin filament

Tropomyosin

F-actin

Myosin binding site

Troponin

Ca2+ binding site

Tail

in repeating units, sarcomeres
Number varies with length of myofibril
Composed of

(continued)
Form alternating patterns of light and dark regions
Appears striated under

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Muscle fiber
(a)
Sarcomeres
Myofilaments
Myofibril
I band
A

I band

Z disc

H zone

Z disc

M line

Sarcomere

filaments
I bands
region containing only thin filaments
extend from both directions of

(continued)
A band
central region of sarcomere
contains entire thick filament
contains partially overlapping

(continued)
H zone
central portion of A band
thick filaments only present; no

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Connectin
Z disc
Thin filament
Thick

Sarcomere

Z disc

Thin filament

(b)

I band

A band

H zone

I band

M line

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(c)
Z disc
Thin filaments
Connectin
and

I band
Thin filaments
Connectin

A band
Thick filaments
Thin filaments

H zone
Thick filaments

M line
Thick filaments
and accessory
proteins

Transverse
sectional plane

and functional proteins
Connectin
protein extending from Z discs to M line
extends

and functional proteins (continued)
Nebulin
actin-binding protein
part of I band of the

with energy production
Muscle with high ATP requirement
Abundant mitochondria for aerobic

with energy production (continued)
Myoglobin
molecule unique to muscle tissue
reddish globular protein

myosin protein.

What are the primary components of thick and thin

Thin filaments are composed primarily of actin protein. Tropomyosin and tropin are associated regulatory proteins.

there thick filaments only, with no thin filament overlap?

neuron nerve cells
transmit nerve signals from brain or spinal

(continued)
Varied number of fibers a neuron innervates
small motor units less

(continued)
Fibers dispersed throughout most of a muscle
Stimulation producing weak contraction

where motor neuron innervates muscle
Usually mid-region of muscle fiber
Has synaptic

(continued)
Synaptic knob
The expanded tip of the axon
Axon enlarged and flattened

terminal
Secondary synaptic cleft (junctional folds)
Mitochondria of synaptic terminal

knob (continued)
Has voltage-gated Ca2+ channels in membrane
Ca2+ flowing down concentration

end plate
Specialized region of sarcolemma
Has numerous folds
increase surface area

(continued)
Synaptic cleft
Narrow fluid-filled space
Separates synaptic knob and motor end

end
plate
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channels
Sarcolemma
Synaptic knob
Sarcoplasm
Ach receptor
Junction fold
Motor end plate
–Na+
ACh
Vesicle
with ACh
Ca2+
Synaptic
cleft
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unit is a single motor neuron and the muscle fibers

What is a motor unit, and why does it vary in size?

There is an inverse relationship between size and degree of control. Muscles needing greater power but less control have bigger motor units.

interact
Sarcomeres shorten
Tension is exerted on portion of skeleton where muscle

EXCITATION OF A SKELETAL MUSCLE FIBER
Release of neurotransmitter acetycholine (ACh)

SARCOLEMMA, T-TUBULES, AND SARCOPLASMIC
RETICULUM: EXCITATION-CONTRACTION COUPLING

ACh binding triggers propagation of an action potential
along the sarcolemma and T-tubules to the sarcoplasmic
reticulum, which is stimulated to release Ca2+.

SARCOMERE: CROSSBRIDGE CYCLING

Ca2+ binding to troponin triggers sliding of thin
filaments past thick filaments of sarcomeres;
sarcomeres shorten, causing muscle contraction.

Ca2+

Sarcomere

Sarcolemma

Muscle
fiber

Neuromuscular
junction

Synaptic vesicle (contains ACh)

Action potential

T-tubule

ACh

Ach receptor

Sarcoplasmic
reticulum

Terminal
cisterna
of SR

Thick filament

Thin filament

Ca2+

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1

2

3

Ca2+

First physiological event
Muscular fiber excitation by motor neuron
Occurs at neuromuscular

Calcium entry at synaptic knob
Nerve signal propagated down motor axon
Triggers

Release of ACh from synaptic knob
Merging of synaptic vesicles with

Binding of ACh at motor end plate
Diffusion of ACh across

end plate
1
NEUROMUSCULAR JUNCTION: EXCITATION OF A SKELETAL MUSCLE FIBER
Ca2+ entry

Nerve signal

1a

1a

ACh receptor

Synaptic cleft

Interstitial
fluid

Synaptic vesicles
(contain ACh)

A nerve signal is propagated down a motor axon and triggers
the entry of Ca2+ into the synaptic knob.

Ca2+ binds to proteins in synaptic vesicle membrane.

Release of ACh from synaptic knob

Binding of ACh to ACh receptor at motor end plate

ACh diffuses across the fluid-filled synaptic cleft in the
motor end plate to bind with ACh receptors.

Ca2+

Ca2+

1b

ACh

1c

1b

Synaptic
vesicle

ACh

1c

Voltage-gated
Ca2+ channel

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Synaptic knob

Calcium binding triggers synaptic vesicles to merge
with the synaptic knob plasma membrane and ACh
is exocytosed into the synaptic cleft.

Nerve signal triggers the entry of calcium into the synaptic

What triggers the binding of synaptic vesicles to the synaptic knob membrane to cause exocytosis of ACh?

Clinical View: Myasthenia Gravis
Autoimmune disease, primarily in women
Antibodies binding

coupling
Links skeletal muscle stimulation to events of contraction
Consists of three

end-plate potential at the motor end plate
Binding of ACh to

end-plate potential at the motor end plate (continued)
Net gain of

of action potential along the sarcolemma and T-tubules
Action potential triggered

of action potential along the sarcolemma and T-tubules (continued)
Action potential

from the sarcoplasmic reticulum
Opening of voltage-gated Ca2+ channels
found in terminal

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2
b
2
Synaptic
cleft
Voltage-gated
Na+ channel
2a
Voltage-gated
K+ channel
Interstitial

Sarcolemma

Voltage-gated
Na+ channel

Voltage-gated
K+ channel

2b

EPP

2a

2c

2c

Sarcolemma

Terminal cisterna
of sarcoplasmic
reticulum

T-tubule

SARCOLEMMA, T-TUBULES, AND SARCOPLASMIC RETICULUM:
EXCITATION-CONTRACTION COUPLING

Development of an end-plate potential (EPP) at the motor end plate

Binding of ACh to ACh receptors in the motor end plate triggers the opening
of these chemically gated ion channels. Na+ rapidly diffuses into and K+
slowly diffuses out of the muscle fiber.

The result is a reversal in the electrical charge difference across the
membrane of a muscle fiber at the motor end plate, which is called
an end-plate potential (EPP). (The inside which was negative is now
positive.)

Initiation and propagation of an action potential
along sarcolemma and T-tubules

An action potential is propagated along the sarcolemma
and T-tubules.

First, voltage-gated Na+ channels open, and Na+ moves
in to cause depolarization.

Second, voltage-gated K+ channels open, and K+ moves
out to cause repolarization.

Release of Ca2+ from the
sarcoplasmic reticulum

When the action potential reaches
the sarcoplasmic reticulum, it
triggers the opening of
voltage-gated Ca2+ channels
located in the terminal cisternae
of the sarcoplasmic reticulum

Ca2+ diffuses out of the cisternae
sarcoplasmic reticulum into the
sarcoplasm.

Ca2+

Ca2+

Ca2+

Ca2+

Terminal cisterna

Voltage-gated
Ca2+ channels

ACh
receptor

ACh

Na+

Na+

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K+

K+

Sarcoplasm

Motor end plate

skeletal muscle stimulation at the neuromuscular junction are coupled to

What two events are linked in the physiologic process call excitation-contraction coupling?