MUSCULAR SYSTEM
Structure and Function
Skeletal Muscle Properties
1.
Excitability = ability to receive and respond to a stimulus

2.
3.
4.
Also called irritability or responsiveness
Contractility = ability to shorten when stimulated
Extensibility = ability to be stretched
Elasticity = ability to recoil and resume resting length
Organization of Skeletal Muscle



muscle cell = muscle fiber
A “muscle” is composed of many muscle fibers arranged
in bundles called fascicles
Fascia = connective tissue that separates muscles; also
forms tendons
Organization of Skeletal Muscle

Three layers of connective tissue:
Muscle fiber
endomysium
perimysium
epimysium
Organization of Skeletal Muscle

Three layers of connective tissue:
1.
2.
3.

Epimysium outermost layer; surrounds the entire muscle
Perimysium surrounds a bundle of muscle fibers (fascicle)
Endomysium surrounds individual muscle fibers
All three layers of connective tissue come together to
form a tendon which attaches the muscle to bone
Organization of Skeletal Muscle
epimysium
perimysium
endomysium
Organization of Skeletal Muscle

Hierarchy of organization:
Muscle
fascicle 
fiber 
myofibrils 
myofilaments (actin/myosin)
Skeletal Muscle Anatomy

Sarcolemma = plasma membrane of muscle cell

Sarcoplasm = cytoplasm of muscle cell

Sarcoplasmic reticulum = stores calcium for contraction


Transverse tubules = network of narrow tubes that form
passageways through a muscle fiber
Myofibrils = protein filaments (myofilaments)

Actin and myosin
Skeletal Muscle Anatomy
nucleus
sarcolemma
mitochondria
sarcoplasm
myofibril
Skeletal Muscle Anatomy

Sarcomere = functional unit of skeletal muscle
 There
may be thousands of sarcomeres in one fiber aligned
end to end
Myofilaments


Form dark and light bands which gives skeletal muscle
a striped appearance
Interaction between these myofilaments within each
sarcomere causes the sarcomere to shorten during
contraction
Actin (thin filament)




The most abundant protein in eukaryotic cells
Forms the light band of the sarcomere
Each actin molecule has an active site capable of
interacting with myosin
At rest, active sites are blocked
Myosin (thick filament)




Motor protein
Forms the dark band of the sarcomere
Each myosin molecule has a globular head and a long
fibrous tail
Myosin head interacts with actin active site during
contraction
LABEL
THIS
DIAGRAM
IN YOUR
NOTES!!!
Sarcomere Structure
 DArk
band = A band = Myosin (thick)
 LIght band = I band = actin (thin)
Sarcomere structure


Z line = marks the boundaries of each sarcomere
M line = middle of the sarcomere
Sarcomere structure


H zone = region of sarcomere where thick filaments do
not overlap with thin filaments
Zone of overlap = region of sarcomere where thick
filaments overlap with thin filaments
Sarcomere Structure
sarcomere
Thin
filament
Z line
I band
Animation
H zone
A band
Thick
filament
Z line
I band
M line
Microscopes…Skeletal Muscle

In your lab notebook, create a new Lab titled “Skeletal
Muscle”
 Part

I. Microscopic observations
Draw a picture of skeletal muscle under HIGH POWER
 Identify
patterns
and label multiple nuclei, muscle fibers, banding
Label…A, I, M, Z, H
animation
Label…A, I, M, Z, H

Changes to the sarcomere
I
band gets smaller
 Z lines move closer together
 H zone decrease
 Zones of overlap get larger
 Width of A bands doesn’t change
animation
Sliding Filament Theory (model)



The theory of how muscle contracts
Contraction occurs as the thin filament “slides” past the
thick filament
Involves 5 different molecules and calcium
 Myosin
 Actin
 Tropomyosin
 Troponin
 ATP
Role of Nervous System in Muscle
Contraction

Skeletal muscles contract only under nervous system control
 Neuromuscular
junction = association of nerve fiber with muscle
fiber
 Acetylcholine = the neurotransmitter that acts on skeletal muscle
by triggering calcium release

Calcium is the “key” to allowing actin and myosin to
interact during contraction
Before contraction begins…

Muscle at rest
 Troponin
is bound to actin filaments
 Tropomyosin binds to the troponin
 Blocks
 Myosin

active site preventing contraction from occuring
heads (cross bridges) are “primed” for contraction
Signal from nervous system (impulse) triggers the release
of calcium
animation
What do you know about a muscle at rest?
The contraction cycle
1.
Calcium binds to
troponin, shifting
tropomysoin,
exposing actin
active site
The contraction cycle
2.
Myosin head binds
to actin active site
(cross-bridge forms)
The contraction cycle
Attached myosin
head pivots toward
center of sarcomere
3.

This step requires energy that
was stored in myosin head
prior to contraction
The contraction cycle
4.
Cross-bridges
detach; myosin
binds another ATP
The contraction cycle
5.
Myosin is
reactivated.
The entire
cycle can now
be repeated
The contraction cycle

Ends when calcium ion concentration returns to normal
 Accomplished
reticulum
Contraction
animation
by active transport of calcium into sarcoplasmic
Sketches in your lab book
1. Skeletal muscle

If you do not, get a
microscope and do this
first!
2. Actin Filament

Label actin molecules,
troponin, tropomyosin
and an active site
3. Myosin Filament


Should contain multiple
myosin molecules
Label head and tail
4. Sarcomere (at rest vs.
contracted)



Without detail of
individual filaments
Label A band, I band,
M line, Z line, H zone
Describe changes to
these structures during
contraction
Sketches
The Contraction Cycle
BIG PICTURE
 Skeletal muscle fibers shorten as thin filaments interact with
thick filaments and sliding occurs


The trigger for contraction is calcium
Calcium ions are released when the muscle fiber is stimulated
by an impulse from a motor neuron

Contraction is an active process

Relaxation and return to normal are passive processes
Rigor Mortis






Upon death, circulation stops and skeletal muscles are
deprived of nutrients
Muscle fibers run out of ATP
Calcium ions cannot be cleared
Sustained contraction occurs
All body muscles are involved
Individual becomes “stiff”