Chapter 12: Muscles Flashcards
are large, multinucleate cells that appear striped or striated under the microscope
skeletal muscle fibers
-striated but they are smaller, branched, and uninucleate -Cells are joined in series by junctions called intercalated disks
cardiac muscle
fibers are small and lack striations
smooth muscle
moves bones closer together
flexion
moves bones away from each other
extension
- The tissue surrounding muscle (epimysium) and tendon connective tissue are continuous
- perimysium
- fascicles
skeletal muscles
extends into the muscle body, dividing muscle into bundles (fascicles) of muscle cells
perimysium
contain 100s-1000s of muscle cells—muscle fibers, which extend the length of the muscle
fascicles
what are the components of a muscle fiber?
-many myofibrils
-sarcoplasmic reticulum
-many mitochondria ~ high energy
-transverse tubules (T tubules)
-lateral sacs (terminal cisternae)
~ Ca2+
-triad
T tubule + 2 lateral sacs
triad
-Give skeletal and cardiac muscle striated appearance
-Orderly arrangement of thick and thin filaments
~actin
~myosin
myofibrils
due to thick and thin filaments that run parallel to the long axis
striations
form sarcomeres
filaments
what is the structure of a sarcomere?
- A band
- H zone
- M line
- I band
- Z line
- dark band
- thick filaments
A band
- thick filaments
- no overlap
H zone
links thick filaments
M line
- light band
- thin filament
- no overlapping
I band
links thin filaments
Z line
has acces- sory proteins that link the thin filaments together, similar to the accessory proteins shown for the M line
Z disk
contractile protein
actin
has binding site for myosin
each G actin
- regulatory protein
- overlaps binding sites on actin for myosin
tropomyosin
-regulatory protein
-complex of 3 proteins
~Attaches to actin
~Attaches to tropomyosin
~Binds Ca2+ reversibly
troponin
-Myosin tail is toward the M line
-Myosin head is toward the I band
-Myosin head binding sites
~actin binding site
~ATPase activity
thick myofilament
- Is a very elastic protein
- Supports protein in muscle
- Anchors thick filaments between the M line and the Z line
- Provides structural support and elasticity
titin
- crossbridge cycle
- excitation-contraction coupling
- muscle cell metabolism
sliding filament model
how muscles generate force
crossbridge cycle
how muscle contractions are turned on and off
excitation contraction coupling
how muscle cells provide ATP to drive the crossbridge cycle
muscle cell metabolism
- shortening of muscle
- thick & thin filaments overlap
- neither thick nor thin filaments shorten
- filaments slide past each other
muscle contraction
what happens within a sarcomere during contraction?
- A band stays the same length
- I band shortens
- H zone shortens
- Sarcomere shortens
due to cyclical formation and breaking of cross bridges = crossbridge cycle
sliding
what happens to actin and myosin during contraction?
do not change length but instead slide past one another
do not change length but instead slide past one another
Cyclical formation of links between actin and myosin
what happens to myosin during the cyclical formation of links between actin and myosin (sliding)?
*myosin head undergoes conformation changes
-high energy form
~ADP and Pi bound to myosin
~High affinity for actin
-low energy
~ADP and Pi released from myosin
*relies on hydrolysis of ATP
myosin head moves, propelling thin filament toward center of muscle
power stroke
detach when ATP binds
thick and thin filaments
returns to the initial cocked position when ATP is hydrolyzed.
myosin head
- ATP binds to myosin; Myosin releases actin
- Myosin hydrolyzes ATP; Energy from ATP rotates the myosin head to the cocked position. Myosin binds weakly to actin.
- Power stroke begins when tropomyosin moves off the binding site.
- Myosin releases ADP at the end of the power stroke.
contraction cycle
- binding of myosin to actin
- power stroke
- rigor (myosin in low energy form)
- unbinding of actin and myosin
- cocking of the myosin head (myosin in high energy form)
crossbridge cycle
- Sequence of events whereby an action potential in the sarcolemma causes contraction
- Dependent on neural input from the motor neuron
- Requires Ca2+ release from the sarcoplasmic reticulum
excitation contraction coupling
what is the role of the neuromuscular juncton in excitation-contraction coupling?
-Each motor neuron innervates several muscle cells
-Each muscle fiber receives input from a single motor neuron
-Acetylcholine released
-Motor end plate
~High density of acetylcholine receptors
-End-plate potential
-Motor neuron AP always creates a muscle cell AP
what is the role of Ca2+ in excitation contraction coupling?
If no Ca2+ → troponin holds tropomyosin over myosin binding sites on actin
- no crossbridges form between actin & myosin - muscle relaxed
what happens in excitation contraction coupling if Ca2+ is present?
If Ca2+ present → binds to troponin, causing movement of troponin, causing movement of tropomyosin, exposing binding sites for myosin on actin
- Crossbridges form between actin and myosin - Cycle occurs; muscle contracts
what are the steps of excitation-contraction coupling?
- Action potential in sarcolemma
- Action potential down T tubules
- DHP receptors of T tubules open Ca2+ channels (ryanodine receptors) in lateral sacs of SR
- Ca2+ increases in cytosol
- Ca2+ binds to troponin, shifting tropomyosin
- Crossbridge cycling occurs
how is muscle action potential initiated?
- somatic motor neuron releases ACh at neuromuscular junctions
- Net entry of Na+ through ACh
receptor-channel initiates a
muscle action potential.
what are the 3 metabolic pathways to obtain energy?
- phosphocreatine breakdown
- anaerobic glycolysis
- aerobic respiration
short burst of energy
Phosphocreatine breakdown
- produces lactate and acid
- quick, no oxygen required, small amount of energy released
anaerobic glycolysis
- citric acid cycle and electron transport chain
- slow, requires oxygen, large amount of energy released
aerobic respiration
- psychological effects
- protective reflexes
central fatigue
- decrease in neurotransmitter release
- decrease in receptor activation
- at neuromuscular junction
peripheral fatigue
change in muscle membrane potential
excitation-contraction coupling
what is the basis for skeletal muscle classification?
-velocity of contraction
-primary energy source
~oxidative vs glycolytic
what are the differences in speed of contraction?
- fast twitch
- slow twitch
what are the differences in speed of contraction dependent on?
- dependent on rate of myosin ATPase activity
- ATP hydrolysis= rate limiting step of cycle
- Higher rate = faster crossbridge cycling
myosin with fast ATPase activity
fast fibers
myosin with slow ATPase activity
slow fibers
- contract 2-3 times more rapidly than slow fibers
- relax more rapidly
fast fibers
why do fast fibers relax more rapidly?
rate of Ca2+-ATPase is faster
contractions last 10 times longer than fast fiber contractions
slow fibers
what are the 3 skeletal muscle fiber types?
- slow oxidative (type 1)
- fast oxidative-glycolytic (type 2A)
- fast glycolytic (type 2X)
-slow myosin ATPase
-high oxidative capacity-aerobic
~mitochondria
~rich blood supply
~myoglobin (red)
-small diameter
~little tension
-fatigue slowly
slow oxidative fibers (type 1)
-fast myosin ATPase activity
-high glycolytic capacity
~high glycogen stores
~many glycolytic enzymes
-no myoglobin (white)
-large diameter
~greater tension
-fatigue rapidly
fast glycolytic fibers (2X)
- intermediate myosin ATPase activity
- high oxidative capacity-aerobic
- myoglobin
- slow to fatigue, but more rapid than slow oxidative fibers
- intermediate diameter
fast oxidative fibers (type 2A)
what are the long term responses to aerobic exercise?
- increased oxidative capacity
- some fast glycolytic fibers can be converted to fast oxidative fibers
- increase in size & # of mitochondria
- increase in # of capillaries surrounding muscle fibers
what are the long term responses to high intensity exercise?
- decreased oxidative capacity
- some fast oxidative fibers can be converted to fast glycolytic fibers
- decrease in size & # of mitochondria
- increase in fiber diameter
- reduced resistance to fatigue
the mechanical response of an individual muscle cell to a single action potential.
twitch
Graded muscle contractions depend on two factors, what are they?
-tension produced by each fiber
~# of active crossbridges that bind to actin
~more crossbridges that bind–>more force
-number of fibers contracting
what are the Factors affecting the number of active crossbridges and thus the force generated by the contraction of individual muscle fibers?
- frequency of stimulation
- fiber diameter
- changes in fiber length
Frequency of stimulation: increases in the frequency of action potentials in muscle fibers increase tension in two ways, what are they?
- treppe
- summation
-Amount of tension developed depends on amount of Ca2+ bound to troponin
-At high frequencies, release exceeds reuptake
~Ca2+ increases in cytosol
-eventually saturates system
~All troponin has Ca2+ bound to it
~Crossbridge cycling maxed out
~Maximum tetanic contraction
cause of summation and tetanus
how does fiber diameter affect force generating capacity?
-fiber diameter varies
-larger diameter–> more filaments –> more force
~more crossbridges –> more force
~more sarcomeres in parallel –> more force
Length of fiber at the onset of contraction affects force generated
fiber length
- Resting length of muscle at which the fiber can develop the greatest amount of tension
- Due to maximum overlap of thick filament crossbridges and thin filaments
optimal length
Decrease crossbridge overlap
nonoptimal lengths
what does more muscle fibers contracting result in/
greater force
-More fibers contracting → greater tension
-Recruit motor units
~Activation of the motor neuron activates all muscle fibers in the motor unit
~Increases in tension occur in steps proportional to the size of the motor unit
motor unit recruitment
what are motor unit sizes like?
-number of motor units varies in different muscles
~small: delicate movements (3-5 fibers)
~large: strength movements (100s-1000s)
-all muscle fibers in a single motor unit are of the same fiber type
~fast twitch
~slow twitch
-Order of motor unit recruitment is related to size of motor units ~small units recruited first ~large units recruited last -asynchronous recruitment ~avoids fatigue and maintains tension
size principle
-create force and moves a load ~concentric action ~eccentric action -the muscle contracts, shortens, and creates enough force to move the load.
isotonic contractions
is a shortening action
Concentric action
is a lengthening action
eccentric action
-create force without moving a load ~series elastic elements ~sarcomeres -the muscle contracts but does not shorten. The force created cannot move the load
isometric contractions
elastic components of muscles
Series elastic elements
what do sarcomeres and elastic elements do during isometric contractions?
Sarcomeres shorten while elastic elements stretch, resulting in little change in overall length
act as levers to enhance the speed or power of limb movements
long bones
any elongated, rigid object that rotates around a fixed point called a fulcrum
lever
occurs when an effort applied overcomes resistance (load) at some other point
rotation
what are the 2 advantages conferred by a lever?
-To exert more force against a resisting object than the force applied to the lever
~Human moving a heavy object with help of crowbar
-To move the resisting object farther or faster than the effort arm is moved
~movement of rowing a boat
- Has fulcrum in the middle between effort and resistance (EFR)
- Atlanto–occipital joint lies between the muscles on the back of the neck and the weight of the face
- Loss of muscle tone occurs when you nod off in class
first class lever
- Resistance between fulcrum and effort (FRE)
- Resistance from the muscle tone of the temporalis muscle lies between the jaw joint and the pull of the digastric muscle on the chin as it opens the mouth quickly
second class lever
- Effort between the resistance and the fulcrum (REF)
- Most joints of the body
- The effort applied by the biceps muscle is applied to the forearm between the elbow joint and the weight of the hand and the forearm
third class lever
- Lacks striations
- Found in internal organs and blood vessels
- Under involuntary control by the autonomic nervous system
- Spindle-shaped
- Small—approximately 1/10 the size of skeletal muscle
- Contains actin and myosin
- No sarcomeres
- Dense bodies
smooth muscle
located in Vascular, gastrointestinal, urinary, respiratory, reproductive, ocular systems
smooth muscle
what are the contraction patterns of smooth muscle?
- phasic (periodic)
- tonic (continuously)
how does smooth muscle communicate with neighboring cells?
- Single-unit smooth muscle, or visceral smooth muscle
- Multi-unit smooth muscle
example of a phasic smooth muscle that is usually relaxed
esophagus
example of a phasic smooth muscle that cycles between contraction and relaxation
intestine
example of a tonic smooth muscle that is usually contracted
a spinchter that relaxes to allow material to pass
a tonic smooth muscle whose contraction is varied as needed
vascular smooth muscle
are connected by
gap junctions, and the cells contract as a single unit
Single-unit smooth muscle cells
are not electrically linked,
and each cell must be stimulated independently.
Multi-unit smooth muscle cells
are the slowest to contract and relax
smooth muscles
-actin is more plentiful
-lacks troponin
-less myosin
~myosin filaments are longer
~entire surface of filament covered with myosin heads
-extensive cytoskeleton
~intermediate filaments and dense bodies
-amount of SR varies and is less organized
-no t-tubules but caveolae
smooth muscle
membrane invaginations
caveolae
-form a cytoskeleton
~Actin attaches to the dense bodies
~Each myosin molecule is surrounded by actin filaments
Intermediate filaments and protein dense bodies
has
hinged heads all along its length
smooth muscle myosin
what does smooth muscle contraction begin with?
Begins with increase in cytosolic Ca2+
what happens in smooth muscle contraction after cytosolic Ca2+ increases?
Ca2+ binds to calmodulin and begins a cascade that results in contraction
during smooth muscle contraction, what happens after the cascade started by Ca2+?
Dephosphorylated myosin may remain attached to actin for a period of time during a latch state
Contraction (of smooth muscle) caused by electrical signaling (change in membrane potential)
electrochemical coupling
Contraction (of smooth muscle) caused by chemical signaling
pharmacomechanical coupling
the signal for contraction in smooth muscle
increased cytosolic calcium
the first step of relaxation of smooth muscle
removal of Ca2+ from the cytosol
fire action potentials when they reach threshold
slow wave potentials
always depolarize to threshold
pacemaker potentials
