Skeletal Muscle Contraction
8th ed 50.5
7th ed 49.6

In order to move all animals require muscle
activity in response to nervous system input.

Skeletal muscles responsible for voluntary
movement.
Biceps
contracts

Skeletal muscles are
attached to bones by
tendons and are
responsible for their
movement.
Forearm
flexes
Triceps
relaxes
Tendons
Biceps
relaxes
Forearm
extends
Triceps
contracts

Organization of
skeletal muscles:
Have a hierarchy of
smaller and smaller
units


Bundles of long fibers
running parallel to the
length of the muscle
Each fiber is made of
longitudinally arranged
myofibrils
Muscle
Bundle of
muscle fibers
Single muscle fiber
(cell)
Nuclei
Plasma membrane
Myofibril
Light
Z line
band Dark band
Sarcomere
Muscle


Myofibrils are composed
of thin and thick
filaments
Thin filaments are made
of two strands of
regulatory proteins and
two stands of actin
Bundle of
muscle fibers
Single muscle fiber
(cell)
Nuclei
Plasma membrane
Myofibril
Light
Z line
band Dark band
Sarcomere

Thick filaments are made
of staggered myosin
molecules
TEM
I band
A band
M line
0.5 µm
I band
Thick filaments
(myosin)
Thin filaments
(actin)
Z line
H zone
Sarcomere
Z line

Skeletal muscles are striated – arrangement of
filaments create dark and light bands
0.5 µm
Z
Skeletal muscle tissue under a light
microscope
H
A
Sarcomere
I
Skeletal muscle tissue under a electron
microscope

Sarcomere – repeating contractile unit of a muscle








Thin filaments are attached at the Z line and proceed to the center of the
sarcomere
Thick filaments are attached at the M line in the center of the sarcomere
Area near edge of sarcomere with only thin filament is the I band
Region corresponding to the length of the thick filament is the A band
Center of A band containing only thick filaments is called the H zone
At a relaxed state thin and thick filaments partially overlap
This overlapping arrangement is key to the contraction mechanism
This regular arrangement produces dark and light bands and hence make
the fibers look striated
Sarcomere
TEM
I band
A band
M line
0.5 µm
I band
Thick filaments
(myosin)
Thin filaments
(actin)
Z line
H zone
Sarcomere
Z line
LE 49-29

Sliding-Filament
model of muscle
contraction (focus
on a single
sarcomere)

During contractions
the thin and thick
filaments do not
change in length but
increase the overlap.
0.5 µm
Z
H
A
Sarcomere
Relaxed muscle fiber
Contracting muscle fiber

This shortens the
length of the
sarcomere
Fully contracted muscle fiber
I


Myosin molecule (thick filament) has a globular
“head” and a long “tail”. Tail adheres to other tails.
Thin filaments are actin molecules along with
regulatory proteins
Thick filament
Thin filaments
Thin filament
Myosin head
Thick
filament
Steps in muscle contraction:
 Myosin head is bound to ATP in a low-energy
configuration
Thick filament
Thin filaments
Thin filament
Myosin head (low-energy
configuration)
Thick
filament

ATP is hydrolyzed to ADP and Pi and the
head is now in high-energy configuration
Thick filament
Thin filaments
Thin filament
Myosin head (low-energy
configuration)
Thick
filament
Actin
Cross-bridge
binding site
Myosin head (highenergy configuration)
Thick filament
Thin filaments
Thin filament

Head binds with
actin filament at
the myosin binding
sites; forms cross
bridge
Myosin head (low-energy
configuration)
Thick
filament
Actin
Cross-bridge
binding site
Myosin head (highenergy configuration)
Cross-bridge

Thick filament
Thin filaments
Head releases the ADP and
Pi and returns to low-energy
configuration; Actin (thin)
filament moves towards the
center of the sarcomere
Thin filament
Myosin head (low-energy
configuration)
Thick
filament
Thin filament moves
toward center of sacomere.
Actin
Myosin head (lowenergy configuration)
Cross-bridge
binding site
Myosin head (highenergy configuration)
Cross-bridge

Myosin binds to new ATP molecule and
resumes the cycle
Thick filament
Thin filaments
Thin filament
Myosin head (low-energy
configuration)
Thick
filament

Nerves conduct signals by changing the
voltage on the membranes (action potential)

Sensory neuron: nerve cell that receives
information from the internal or external
environments and transmits the signal to the
central nervous system (brain and spinal
cord)

Motor neuron: transmits signals from brain
or spinal cord to muscles or glands.

Synapse: junction where one
neuron communicates with
another neuron or with
muscle/gland etc.

Synaptic terminal: A bulb at
the end of the axon in which
neurotransmitter molecules
are stored and released.

Synaptic cleft: narrow gap
separating synaptic knob of a
transmitting neuron or its
effector cell.
Synaptic terminal
of motor neuron
Synaptic cleft
Spinal cord


Motor
unit 1
Motor unit: A
single neuron and
all the muscle
fibers it controls
When motor
neuron produces
action potential all
the muscle fibers
in its motor unit
contract as a
group.
Motor
unit 2
Synaptic terminals
Nerve
Motor neuron
cell body
Motor neuron
axon
Muscle
Muscle fibers
Tendon
Regulation of skeletal muscle contraction:



Synaptic terminal receives action potential and
releases neurotransmitter Acetylcholine (ACh)
ACh binds to receptors in the muscle and triggers
action potential in the muscle fiber.
Action potential is propagated along the plasma
membrane and down the T-tubule.
Synaptic terminal
of motor neuron
Synaptic cleft
ACh
T TUBULE
PLASMA
MEMBRANE
Synaptic terminal
of motor neuron
Synaptic cleft
T TUBULE
SR
ACh

Action potential
triggers Ca2+
release from
sarcoplasmic
reticulum
PLASMA
MEMBRANE
Ca2+
CYTOSOL
Ca2+



Myosin cross-bridges attach and detach, powered
by ATP pulling the actin filament towards center of
the sarcomere
When action potential ends Ca2+ is absorbed back
into the sarcoplasmic reticulum
Muscle contraction ends, muscle fibers relax
CYTOSOL
Ca2+
Synaptic terminal
of motor neuron
Synaptic cleft
PLASMA
MEMBRANE
T TUBULE
SR
ACh
Ca2+
CYTOSOL
Ca2+
Motor
neuron axon
Mitochondrion
Synaptic
terminal
T tubule
Ca2+ released
from sarcoplasmic
reticulum
Sarcoplasmic
reticulum
Myofibril
Plasma membrane
of muscle fiber
Sarcomere

Ca+ and regulatory proteins and their role
in muscle fiber contraction



Actin filaments have regulatory proteins on them.
Tropomyosin, trponin complex and Ca2+ regulate
muscle contraction
At rest tropomyosin covers the actin binding sites
preventing actin and myosin from interacting
Tropomyosin
Ca2+-binding sites
Actin
Myosin-binding sites blocked.
Troponin complex




When Ca2+ is released into the cytosol from the
sarcoplasmic reticulum it binds to troponin complex.
This changes the alignment of the troponin
That in turn shifts the position of the tropomyosin, exposing
the myosin binding sites on the actin filament
When Ca2+ concentration drops the binding sites are
covered and contraction stops.
Ca2+
Myosinbinding site
Myosin-binding sites exposed.
Tropomyosin
Ca2+-binding sites
Actin
Troponin complex
Myosin-binding sites blocked.
Ca2+
Myosinbinding site
Myosin-binding sites exposed.