Introduction to Muscle
• Movement is a fundamental characteristic of
all living things.
• Muscles cells are capable of shortening and
converting the chemical energy of ATP into
mechanical energy.
• Types of muscle
– Skeletal
– Cardiac
– Smooth
Skeletal Muscle
– Has obvious stripes called striations and multiple nucleuses
– Under voluntary control
• Attached to the skeletal system
• Responsible for movement.
Cardiac Muscle
• Striated muscle but is involuntary
• Intercalated discs connect adjacent cells together
Smooth Muscle
• Found in the walls of hollow visceral organs, such as the
stomach, intestines ,bladder.
– It is not striated and is involuntary
Characteristics of Muscle Tissue
• Excitability, or responsiveness
– the ability to respond to stimuli
• Conductivity
– the ability to produce and conduct an action
potential along the cell membrane
• Contractility
– the ability to shorten creates movement by:
• Skeletal: pulling on bones
• Visceral: Movement created by visceral organs.
• Extensibility
– the ability to be stretched
• Elasticity
– the ability to recoil after being stretched
Connective Tissues of a Muscle
Tendon
Deep fascia
Epimysium
Perimysium
Endomysium
Skeletal Muscle
• Each muscle is composed of various tissue types. They
include muscle tissue, blood vessels, nerve fibers, and
connective tissue.
• The connective tissue located is important for supplying
a framework for the blood vessels and nerves. They also
contribute to the elastic qualities of muscle aiding in
force production.
• The three connective tissue sheaths are:
– Endomysium – fine sheath of connective tissue
surrounding each muscle fiber
– Perimysium –connective tissue that surrounds groups
of muscle fibers called fascicles
– Epimysium – an overcoat of dense regular connective
tissue that surrounds the entire muscle
The Muscle Fiber
Anatomy of a Skeletal Muscle Fiber
• Each fiber is surrounded by a sarcolemma
• ( cell membrane around the muscle)
– the sarcolemma contains voltage-gated channels
able of generating an action potential
– The action potential travels along the sarcolemma
and dips into the center of the muscle via
transverse-tubules. (T-tubules)
• Sarcoplasmic reticulum (SR) (bathes the muscle
fibers)
– Extensions of the T-tubules which store intracellular
calcium (Ca+2)
• Once an action potential reaches the T-tubules Ca+2
from the terminal cysterna of SR is released into the
sarcoplasm triggering a muscle contraction.
Myofilaments: Banding Pattern
Myofilaments: Banding Pattern
The thin and thick filaments overlapping forming
sarcomeres.
• Z disc
– anchors the thin filaments and connects myofibrils
to one another
– Z-disc to Z disc = one sarcomere
• A band
– the length of the thick filaments
• I band
– the length of thin filaments within a sarcomere that
is not overlapping with the thick filaments
• H zone
– the length of thick filaments within in a sarcomere
that do not overlapping with the thin filaments
Resting Muscle
• When a muscle is in a relaxed state the
sarcomere is at its normal length.
• The H-zone is visible.
Contracted Sarcomeres
• Muscle cells shorten because their individual
sarcomeres shorten as myosin pulls actin toward the
center of the sarcomere.
– pulling Z discs closer together
• Notice neither thick nor thin filaments change length they
overlap as sarcomeres shorten
• H-zone disappears
Resting muscle/Contracted muscles
Thick Filaments
• Arranged in a bundle
with heads directed
outward in a spiral array
• Myosin heads:
– Form cross bridges
with actin.
– Myosin contains the
enzyme ATPase
which hydrolyzes
ATP to create
movement.
Thin Filaments
• Actin: Two intertwined strands fibrous protein containing the
active site for myosin heads.
• Tropomyosin : Prevents actin and myosin cross bridge
formation
• Troponin: a protein attached to tropomyosin that calcium
binds to allowing cross bridge formation and muscle
contraction.
The Neuromuscular Junction
Neuromuscular Junctions (Synapse)
• Functional connection between nerve fiber and
muscle cell
• Neurotransmitter (acetylcholine/ACh) released from
nerve fiber stimulates muscle cell
• Components of synapse (NMJ)
– synaptic knob is swollen end of nerve fiber (contains ACh)
– junctional folds region of sarcolemma
• increases surface area for ACh receptors
• contains acetylcholinesterase that breaks down ACh and causes
relaxation
– synaptic cleft = tiny gap between nerve and muscle cells
Electrically Excitable Cells
• Plasma membrane is polarized or charged
– resting membrane potential due to Na+ outside of
cell and K+ and other anions inside of cell
– difference in charge across the membrane =
resting membrane potential (-90 mV cell)
• Stimulation opens ion gates in membrane
– ion gates open (Na+ rushes into cell and K+
rushes out of cell)
• quick up-and-down voltage shift = action potential
– spreads over cell surface as nerve signal
Excitation (Steps 1 and 2)
• AP opens voltage-gated calcium channels. Calcium
stimulates exocytosis of synaptic vesicles containing ACh =
ACh release into synaptic cleft.
Excitation (steps 3 and 4)
Binding of ACh to receptor proteins opens Na+ and K+
channels resulting in jump in RMP from -90mV to +75mV
forming an end-plate potential (EPP).
Excitation (step 5)
Voltage change in end-plate region (EPP) opens nearby
voltage-gated channels producing an action potential
Excitation-Contraction Coupling (steps 6 and 7)
Action potential spreading over sarcolemma enters T
tubules -- voltage-gated channels open in T tubules
causing calcium gates to open in SR
Excitation-Contraction Coupling (steps 8 and 9)
• Calcium released by SR binds to troponin
• Troponin-tropomyosin complex changes shape
and exposes active sites on actin
Contraction (steps 10 and 11)
• Myosin ATPase in myosin head hydrolyzes an ATP molecule,
activating the head and “cocking” it in an extended position
• It binds to actin active site forming a cross-bridge
Contraction (steps 12 and 13)
• Power stroke = creates
muscle contraction
– myosin head pulls the
actin over it.
– With the binding of more
ATP, the myosin head
will detach and break the
cross bridge.
Relaxation (steps 14 and 15)
Nerve stimulation ceases and acetylcholinesterase
removes ACh from receptors. Stimulation of the muscle
cell ceases.
Relaxation (step 16)
• Active transport needed to pump calcium back into SR to
bind to calsequestrin
• ATP is needed for muscle relaxation as well as muscle
contraction
Relaxation (steps 17 and 18)
• Loss of calcium from sarcoplasm moves
troponin-tropomyosin complex over active sites
– Muscle fiber returns to its resting length
Motor Units
• A motor neuron and the
muscle fibers it innervates.
• Fine control
– small motor units contain as few
as
20 muscle fibers per nerve fiber
– eye muscles
• Allow for greater dexterity
because of a lower neuron to
muscle fiber ratio.
• Strength control
– gastrocnemius muscle has 1000
fibers per nerve fiber
• One motor neuron controls many
muscle fibers which allows for
strength production.
Recruitment and Stimulus Intensity
• Strength of muscle
contraction is dependant of
# of motor units recruited
• Multiple motor unit
summation
– The harder the activity is the
more motor units will be
recruited.
• Lifting 1 lb vs. 100 lbs
– How do you explain the rapid
strength gains when you first
start training?
Muscle Twitch
• A single stimulus results in a single muscle twitch
• Each twitch has time to recover but develops more
tension than the one before (treppe phenomenon)
Muscle Response: Stimulation Frequency
• Higher frequency stimulation generates gradually more strength
– each stimuli arrives before last one recovers
• temporal summation or wave summation
– incomplete tetanus = sustained fluttering contractions
• Maximum frequency stimulation
– muscle has no time to relax at all
– twitches fuse into smooth, prolonged contraction called complete tetanus
Isometric and Isotonic Contractions
• Isometric muscle contraction
– develops tension without changing length
– important in postural muscle function and antagonistic muscle joint
stabilization
• Isotonic muscle contraction
– tension while shortening = concentric
– tension while lengthening = eccentric
Metabolism and Skeletal Muscle Fibers Types
• There are 3 different types skeletal muscle
fibers based histological differences, duration of
a twitch and the method of ATP production
– slow oxidative fibers
– fast oxidative fibers
– fast glycolytic fibers
• Proportions genetically determined
Fast Glycolytic, Fast-Twitch Fibers
• Fast glycolytic, fast-twitch fibers:
– rich in enzymes for phosphagen and glycogen-lactic acid
systems
– Limited # of mitochondria and high concentration of
glycogen stores makes it adapted for anaerobic
metabolism
• a lack of myoglobin in glycolytic fibers results in a white color
– sarcoplasmic reticulum releases calcium quickly so
contractions are quicker which are required for
movements that produce speed and power.
• extraocular eye muscles, gastrocnemius and biceps
brachii
Slow- Twitch Fibers
• Slow oxidative, slow-twitch fibers
– Oxidative fibers contain greater amounts of
mitochondria and myoglobin which binds
oxygen.
– Rich blood supply and high concentration of
myoglobin these fibers appear red in color.
• adapted for endurance (resistant to fatigue)
– Soleus and postural muscles of the back are
predominantly this type.
Fast Oxidative Fibers
• Fast oxidative fibers:
– characteristics of both fast and slow fibers.
– have a fast twitch (use ATP quickly)
– Increased mitochondria make it moderately
resistant to fatigue
– Usually make up 10% of fibers.
• Training will make these fibers adapt to
become functionally more fast or slow.
Cellular Adaptations to Physical
Demands
• Strength training: high intensity training
stresses anaerobic pathways.
– Increased # and size of glycolytic associated
enzymes and substrates
• ATP, creatine phosphate and glycogen
• Endurance training: Enhance the aerobic
pathways.
– increased # and size of mitochondrial
membranes and associated enzymes.
– This will increase O2 uptake ( VO2 max) which will
delay the formation of lactic acid.