Skeletal Muscle Unit
Chapter 6
Functions of skeletal muscles
Produce skeletal movement
Maintain posture and body position
Support soft tissues
Guard entrances and exits
Maintain body temperature
Store nutrient reserves
Organization
of muscle
 Epimysium: an exterior collagen layer that separates the muscle
from surrounding tissues.
 Perimysium: surrounds bundles of muscles fibers called fascicles.
 Perimysium holds the blood vessels and nerves that supply the
fascicles.
 Endomysium: surrounds individual muscle cells (the muscle
fibers), and contains the capillaries and nerve fibers that
directly contact the muscle cells.
 Endomysium also contains stem cell that repair damaged
muscles.
Tendon and aponeurosis
 At each end of the muscle collagen
fibers from the epimysium, perimysium,
and endomysium come together.
 These fibers attach the muscle to the
bone to allow for movement
Myoblasts
Myo: muscle
Blast: Build
Muscle Fiber
(Cell)
Structure
Sarcolemma: The cell membrane of a muscle cell,
which surrounds the sarcoplasm or cytoplasm of
the muscle fiber.
Transverse tubules: transmit action potentials
Myofibrils: Within each muscle fiber are hundreds
of lengthwise subdivisions called myofibrils.
Myofilaments: two types of myofibrils
Sarcomere
The 2 types of myofilaments are:
thin filaments: made of the protein
actin, and
thick filaments: made of the protein
myosin.
Sarcoplasmic
Reticulum: Surrounding
each myofibril is a
membranous structure
It is involved in
transmitting the action
potential to the
myofibril.
Ion pumps concentrate
calcium ions (Ca++)
that are released into
the contractile units of
the muscle
(sarcomeres) at the
beginning of a muscle
contraction.
A Band, I Band, Z Line
Sarcomeres: the
contractile units of muscle
 Structural units of myofibrils
 Skeletal muscles appear
striped or striated because
of the arrangement of
alternating dark, thick
filaments and light, thin
filaments
One sarcomere is
measured from one Z line
to another.
A Band
I band
 M line: The center of the A  Z lines: The centers of
band is the midline or M line the I bands are Z lines
 Zone of overlap: Where thick  Titin: Strands of protein
reach from the ends
filaments and thin filaments
of the thick filaments
overlap which is the densest,
to the Z line and
darkest area
stabilize the filaments
Thin Filament
 Thin filaments contain 4 proteins:
 F actin (2 twisted rows of globular G actin.
 Active sites on G actin strands bind to myosin.)
 nebulin (holds F actin strands together)
 tropomyosin (a double strand, prevents actin-myosin
interaction)
 troponin (a globular protein, binds tropomyosin to G
actin, controlled by Ca++)
Thin Filament
When a Ca++ ion binds to the receptor
on a troponin molecule, the troponintropomyosin complex changes, exposing
the active site of the F actin and initiating
contraction.
Thick Filament
 Thick Filaments contain twisted myosin subunits.
 The tail binds to other myosin molecules.
 The free head, made of 2 globular protein subunits,
reaches out to the nearest thin filament.
Thick Filament
 During a contraction, myosin heads interact
with actin filaments to form cross-bridges.
The myosin head pivots, producing motion.
 Thick filaments contain strands that recoil
after stretching.
Sliding Filament Theory
 In skeletal muscle contraction, the thin
filaments of the sarcomere slide toward the M
line, in between the thick filaments.
 This is called the sliding filament theory. The
width of the A zone stays the same, but the Z
lines move closer together.
Motor Unit
 Muscle fiber contraction is
initiated by neural stimulation
of a sarcolemma.
 Calcium ions trigger the
interaction of thick and thin
filaments, consuming ATP
and producing a pulling
force called tension.
 Neural stimulation occurs at
the neuromuscular junction.
 The electrical signal or
action potential travels
along the nerve axon and
ends at a synaptic terminal
which releases a chemical
neurotransmitter called
acetylcholine (ACh).
Action Potential
 ACh travels across a short gap and binds to
membrane receptors on the sarcolemma,
causing sodium ions to rush into the
sarcoplasm.
 The increase in sodium ions generates an
action potential in the sarcolemma which
travels along the T tubules.
 When the action potential reaches a triad,
calcium ions are released, triggering
contraction.
 This step requires the myosin heads to have
previously broken down ATP and stored the
potential energy in the “cocked” position.
ATP and CP Reserves
 At rest muscles produce more ATP than
needed, it is turned into creatine (a high
energy compound)
 Energy in creatine is used to assemble ADP and
Pi (phosphate) to recharge it to make ATP.
Contraction Cycle
 The
Contraction
Cycle has 5
steps:
 1. (Ca++
introduction
stimulated by
neurons)
Exposure of
active sites
 2. Formation
of crossbridges
3. Pivoting of myosin heads (uses ATP)
4. Detachment of cross-bridges
5. Reactivation of myosin
 Reformation of ATP by reassembling
ADP with Creatine Phosphate
Aerobic vs Anaerobic
Aerobic metabolism uses O2 and
provides 95% of ATP for a resting cell
 the primary energy source of resting
muscles
32 ATP molecules produced per
glucose molecule
Aerobic vs Anaerobic
 Anaerobic glycolysis: does
not use O2,
 provides 5% of the ATP for a
resting cell
 the breakdown of glucose
from glycogen
 primary energy source for
peak muscular activity
 2 ATP molecules
produced per molecule of
glucose
 skeletal muscles store
glycogen
 Glycolysis (breaks down
Bigglucose
Energyinto
Events
of Respiration
two molecules
of pyruvate)
 The citric acid cycle
(completes the breakdown
of glucose)
 Oxidative phosphorylation
(accounts for most of the
ATP synthesis)
Lactic Acid and Fatigue
 At peak levels of
exertion, muscles can’t
get enough oxygen to
support mitochondrial
activity. The muscle then
relies on glycolysis for
ATP.
 Glycolysis produces
pyruvic acid
 The pyruvic acid
produced by glycolysis,
which would normally be
used up by the
mitochondria, starts to
build up and is converted
to lactic acid.
Lactic Acid and Fatigue
 Fatigue is when the muscle uses reserves and
can no longer perform the required activity
 1. depletion of metabolic reserves
 2. damage to the sarcolemma/sarcoplasmic
reticulum
 3. low pH (lactic acid)
 4. muscle exhaustion and pain
Recovery Period
 After high levels of exertion, it can take hours or days for
muscles to return to their normal condition.
 During the recovery period, oxygen is once again
available and mitochondrial activity resumes.
 To process excess lactic acid and normalize metabolic
activities after exercise, the body uses more oxygen than
usual. This elevated need for oxygen, called the oxygen
debt, is responsible for heavy breathing after exercise.
 Cori cycle: Lactic acid is carried by the blood
stream to the liver, where it is converted back
into pyruvic acid, and glucose is released to
recharge the muscles’ glycogen reserves.
 Skeletal muscles at rest metabolize fatty acids
and store glycogen.
Players in Contraction
 Agonist (Batman):
Prime mover-does
the main portion of
the work
 Antagonist (Joker):
Resists the prime
mover-pulls against
 Synergist (Robin):
Assist the agonisthelper
Types of Contractions
 There are 2 basic patterns of
muscle tension:
 In isotonic contraction, the
muscle changes length, resulting
in motion.
 If muscle tension exceeds the
resistance, the skeletal muscle
shortens (concentric
contraction).
 If muscle tension is less than
the resistance, the muscle unshortens (eccentric
contraction).
 In isometric contraction, the
muscle is prevented from
changing length, even though
tension is developed.
 Heat Production and Loss: The more active
muscles are, the more heat they produce.
During strenuous exercise, up to 70 percent of
the energy produced can be lost as heat,
raising body temperature.
Levers
Origin, Action, Innervation
Building Muscle
 Hypertrophy: Extensive training can cause
muscles to grow by increasing the diameter of
the muscle fibers, which increases the number
of myofibrils, mitochondria and glycogen
reserves.
 Atrophy: Lack of muscle activity causes
reduction in muscle size, tone and power.
What you don’t use, you loose.
 Muscle tone is an indication of the
background level of activity.
 When inactive for days or weeks, muscles
become untoned. The muscle fibers break
down and become smaller and weaker.
 If inactive for long periods of time, muscle
fibers may be replaced by fibrous tissue.