Skeletal Muscle Tissue
Skeletal Muscle Tissue
Arrangement
• Myofibrils – contractile elements of
muscle tissue
Skeletal Muscle Cont.
• Muscle fiber – Muscle cell; composed of
several myofibrils
Skeletal Muscle Cont.
• Each muscle fiber is surrounded by a thin
sheath of areolar connective tissue called
endomysium
Muscle Tissue Cont.
• Fascicles – A bundle of muscle fibers.
There are usually between 10 to 100
muscle fibers in a fascicle.
Muscle Tissue Cont.
• Each fascicle is surrounded by a layer of
dense irregular connective tissue called
perimysium
Muscle Tissue Cont.
• Whole muscle – made up of several
fascicles
Muscle Tissue Cont.
• The whole muscle is surrounded by a
dense irregular connective tissue called
epimysium
Muscle Tissue
• All three connective tissues (endomysium,
perimysium, epimysium) extend beyond
the muscle fiber to form a tendon.
Muscle Tissue
• Tendon – Composed of dense regular
connective tissue that attaches muscle to
the periosteum of the bone
General Features of Skeletal
Muscle
• Striated
General Features of Skeletal
Muscle
• Voluntary
General Features of Skeletal
Muscle
• Multinucleated
General Features of Skeletal
Muscle
• Controlled by the somatic (voluntary)
division of the nervous system
Microscopic Anatomy of Muscle
Fibers
• Muscle Fiber = Muscle Cell
Microscopic Anatomy cont.
• Sarcolema – plasma membrane of muscle
cells or muscle fibers
Microscopic Anatomy cont.
• The multiple nuclei of each muscle fiber is
located beneath the sarcolema
Microscopic Anatomy cont.
• T (tranverse tubules) – Invagination of the
sarcolema that tunnel in from the surface
to the center of each muscle fiber
Microscopic Anatomy cont.
• Sarcoplasm – cytoplasm of a muscle fiber
Microscopic Anatomy cont.
• Sarcoplasmic reticulum – fluid filled
system of membranous sacs. Calcium is
stored here.
Microscopic Anatomy cont.
• Dilated ends of SR are called terminal
cisterns
Microscopic Anatomy cont.
• Myofibrils are composed of functional units
called sarcomeres responsible for the
striations
Microscopic Anatomy cont.
• Each sarcomere is separated from the
next by z discs
Microscopic Anatomy cont.
• Sarcomeres are composed of thick
(myosin) and thin (actin) filaments
Microscopic anatomy cont.
• A band is the part of the sarcomere
composed of thick (myosin) and thin
(actin) filaments
Microscopic anatomy cont.
• The A band is the dark striation seen
under the microscope
Microscopic Anatomy cont.
• I Band is the part of the sarcomere that
contains only thin (actin) filaments
Microscopic Anatomy cont.
• I Band is the light striation seen
underneath the microscope
Microscopic Anatomy
• The H zone is the part of the A band that
contains only thick filaments (myosin)
Microscopic Anatomy
• M line is the middle of the sarcomere and
is composed of supporting proteins that
hold the thick filaments together
How does a nerve initiate
contraction?
• Neuromuscular junction – the region of
contact between a motor neuron and a
skeletal muscle fiber
Initiation of Contraction
• Synaptic cleft – the region between the
neuron and muscle fiber
Initiation of Contraction
• The tips of axon terminals are called
synaptic end bulbs
Initiation of Contraction
• Synaptic vessicles – membrane –
enclosed sacs that contain the
neurotransmitter acetylcholine (Ach)
located in the synaptic end bulb
Initiation of Contraction
• Motor end plate – the region of the
sarcolema opposite of the synaptic end
bulb
Initiation of Contraction
• Each motor end plate contains between 30
to 40 million Ach receptors
Initiation of Contraction / 4 Steps
1. Once the nerve impulse arrives at the
synaptic end bulb, the synaptic vesicles
release Ach via exocytosis.
Initiation of Contraction / 4 Steps
2. When two ACh molecules bind to the
ACh receptors at the motor end plate it
opens the cation channel and Na+ can
flow across the membrane.
Initiation of Contraction / 4 Steps
3. Once the inside of the muscle fiber is
more positively charged, a muscle action
potential is triggered, which propogates
along the sarcolema and into the T tubule
system.
Initiation of Contraction / 4 Steps
4. ACh is broken down by
acetylcholinesterase in the extracellular
matrix of the synaptic cleft.
Calcium’s Role
• Once the action potential propagates
along the sarcolema and into the T tubules
Ca2+ release channels in the SR
membrane open causing Ca2+ to flow out
of the SR into the cytosol.
Calcium’s Role
• Calcium binds to troponin on the actin
filaments causing the troponintropomyosin complexes to move away
from the myosin binding sites on actin.
Contraction / 4 Steps
1. ATP hydrolysis – ATP is hydrolyzed into
ADP and a phospate by ATPase on a
myosin head
Contraction / 4 Steps
2. Attachment of myosin to actin to form
crossbridges – myosin binds to actin on
the myosin binding site and the phosphate
is released.
Contraction / 4 Steps
3. Power stroke – The myosin pushes the
thin filament past the thick filament toward
the M line releasing ADP.
Contraction / 4 Steps
4. Detachment of myosin from actin – When
ATP binds to the myosin head, the myosin
head detaches from actin.
Contraction
• As the muscle contracts the I band and H
zone decreases
Relaxtion
Once nerve impulses stop;
1. Acetylcholinesterase breaks down the
remaining acetylcholine
2. Muscle action potentials stop
3. Calcium levels in cytosol decreases
4. Contraction stops
How do calcium levels decrease?
• Ca2+ release channels close
• Ca2+ active transport pumps move Ca2+
back into the SR
• In the SR calsequestrin binds to Ca2+
enabling more Ca2+ to be sequestered
within the SR
Rigor Mortis
• Calcium leaks out of the SR therefore
allowing myosin heads to bind to actin.
• ATP production ceases so myosin cannot
detach form actin.
• Muscles therefore become rigid (cannot
contract or stretch)
Atrophy
• Muscle fibers decrease in size due to loss
of myofibrils
Hypertrophy
• Muscle fibers increase in diameter due to
the production of more myofibrils.
ATP and Muscle
• Muscle fibers need ATP for powering the
contraction cycle and to pump Ca2+ into
the SR.
ATP and Muscle
•
1.
2.
3.
ATP is made by;
Creatine phosphate
Anaerobic cellular respiration
Aerobic cellular respiration
Creatine Phosphate
• When the muscle is relaxed creatine
kinase (CK) transfers a phosphate from
ATP to creatine forming creatine
phosphate and ADP.
Creatine Phosphate
ATP + Creatine → ADP + Creatine
Phosphate
This reaction is catalyzed by creatine kinase
Creatine Phosphate
• When a muscle contracts CK tranfers a
phosphate from creatine phosphate to
ADP forming ATP and creatine.
Creatine Phosphate
• Creatine Phosphate + ADP → Creatine
and ATP
This reaction is catalyzed by CK
Anaerobic Cellular Respiration
• Does not require oxygen
• ATP is formed by a process called
glycolysis
• A glucose is converted into two pyruvic
acid molecules
Anaerobic Respiration
• Glycolysis uses two ATP but forms 4 ATP
for a net gain of two
• Pyruvic acid is converted into lactic acid
Anerobic Respiration
• Muscle fibers attain their glucose via
diffusion from the blood and glycogen
stored within muscle fibers
Aerobic Respiration
• Requires oxygen
• Takes place in mitochondria
• The two molecules of pyruvic acid
produced in glycolysis enter the kreb
cycle.
• Aerobic respiration results in a net gain of
36 ATP.
Aerobic Respiration
• In aerobic respiration oxygen is attained
via the diffusion of oxygen from blood and
oxygen released by myoglobin
Aerobic Respiration
• Myoglobin is a protein found in muscle
cells that binds oxygen
Motor Units
• There is only one neuromuscular junction
per fiber.
Motor Units
• A somatic motor neuron branches out and
forms neuromuscular junctions with many
muscle fibers.
Motor Units
• A motor unit consists of a somatic motor
neuron plus all the skeletal muscle fibers it
stimulates
Motor Units
• All muscle fibers in a motor unit contract in
unison
Motor Unit
• Muscles that produce precise movements
are made up of small motor units.
Red Muscle Fibers
• Have a high myoglobin content
White Muscle Fibers
• Have a low myoglobin content
3 Main Types of Skeletal Muscle
Fibers
1. Slow Oxidative Fibers
2. Fast Oxidative-Glycolytic Fibers
3. Fast Glycolytic Fibers
Slow Oxidative Fibers
• Smallest in diameter
• Contain large amounts of myoglobin
• Generate ATP by aerobic cellular
respiration
• Large amounts of mitochondrial and blood
capillaries
• ATPase in the myosin head hydrolyzes
ATP slowly
Fast Oxidative-Glycolytic Fibers
• Intermediate in diameter
• High myoglobin content
• Generates ATP by aerobic and anaerobic
respiration
• High content of mitochondria and blood
capillaries
• ATPase hydrolyzes ATP quickly
Fast Glycolytic Fibers
•
•
•
•
•
Largest in diameter
Low myoglobin content
Few blood capillaries and mitochondria
Generate ATP by anaerobic respiration
ATPase hydrolyzes ATP quickly
Motor Unit
• Muscle fibers of a single motor unit are of
the same type
Origin and Insertion
• Most muscles cross at least one joint and
are attached to the articulating bones that
form the joint.
Origin and Insertion
• When a muscle contracts, it draws one
articulating bone toward the other.
Origin and Insertion
• The attachment of the stationary bone is
the origin.
Origin and Insertion
• The attachment of the movable bone is the
insertion
Twitch contraction
• The contraction of all the muscle fibers in
a motor unit in response to a single action
potential
Myogram
• A record of a muscle contraction
Myogram of a Twitch Contraction
1. Latent period
2. Contraction period
3. Relaxation period
Myogram of a Twitch Contraction
1. Latent period –
Lasts two milliseconds
Calcium ions are released from SR
Myogram of a Twitch Contraction
2. Contraction period –
10 – 100 msec
Myogram of a Twitch Contraction
3. Relaxation Period –
10 – 100 msec
Active transport of calcium into SR
Frequency of Stimulation
Wave summation –
When a second stimulus occurs before the
muscle has relaxed, the second
contraction is stronger than the first.
Frequency of Stimulation
Unfused tetanus –
When a skeletal muscle is stimulated at a
rate of 20 to 30 times per second, it can
only partially relax between stimuli
resulting in a sustained but wavering
contraction.
Frequency of Stimulation
• Fused tetanus –
When a skeletal muscle is stimulated at a
rate of 80 to 100 stimuli per second, a
sustained contraction results in which
individual twitches cannot be discerned.
Motor Unit Recruitment
• Not all motor units in a muscle are not
stimulated at once to prevent fatigue.
Concenteric Isotonic Contraction
• A muscle shortens and pulls on a tendon,
which produces movement and reduces
the angle at a joint.
Eccenteric Isotonic Contraction
• The length of a muscle increases during
contraction.
Isometeric Contractions
• The muscle doesn’t shorten because the
force of the load equals muscle tension.