Chapter 12 Flashcards
what are skeletal muscles connected to?
two or more bones by tendons
What tissue surrounds muscle (epimysium) and tendon connective tissue?
continuous
what divides muscle into bundles (fascicles) of muscle cells?
Perimysium
What surrounds muscles fibers?
endomysium
Sarcolemma
plasma membrane
Are muscle fibers multinucleated?
yes
Sarcoplasm
cytoplasm
label a muscle diagram
components of a muscle fiber
- myofibrils
- mitochondria
- sarcoplasmic reticulum (Ca storage)
- T tubules
Lateral sacs
- terminal cisternae
- store calcium (increasing the capacity of the sarcoplasmic reticulum to release calcium) and release it when an action potential courses down the transverse tubules, eliciting muscle contraction
triad
- T tubule + two lateral sacs
- responsible for the regulation of excitation-contraction coupling
myofibrils
- Give skeletal and cardiac muscle striated appearance
- Orderly arrangement of thick and thin filaments
- Actin (thin)
- Myosin (thick)
Filaments form ?
sarcomeres (like car that are linked together)
A band
Dark band
Thick filaments
myosin
H zone
Thick filaments
No overlap
M line
Links thick filaments
I band
Light band
Thin filament
actin
No overlapping
Z line
Links thin filaments
Sarcomere
Functional unit
Z line to Z line
label sarcomere diagram
Actin
- Contractile protein
- Each G (globular monomer proteins) actin has a binding site for myosin
Tropomyosin
- Regulatory protein
- Overlaps binding sites on actin for myosin
- Blocks myosin binding
Troponin
- Regulatory protein
- Ca2+ binding to troponin regulates skeletal muscle contraction
Name the three protein complexes of troponin
Attaches to actin
Attaches to tropomyosin
Binds Ca2+ reversibly
Thick myofilament
- Myosin tail is toward the M line
- Myosin head is toward the I band
Myosin head binding sites
- Actin binding site
- Nucleotide-binding site for ATP and ATPase
Titin
- 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
Sliding filament
- Muscle contraction
- Shortening of muscle
- Thick and thin filaments overlap
- Neither thick nor thin filaments shorten
- Filaments slide past each other
What happens within a sarcomere during contraction?
- A band stays the same length
- I band shortens
- H zone shortens (basically disappears)
- Sarcomere shortens (Z move closer together)
Sliding filament is due to
cyclical formation and breaking of cross bridges = crossbridge cycle
Look at figure 12.7!
What sequence of events whereby an action potential in the sarcolemma causes contraction must occur?
- Dependent on neural input from the motor neuron
- Requires Ca2+ release from the sarcoplasmic reticulum
Each motor neuron innervates?
several muscle cells
Each muscle fiber receives input from?
a single motor neuron
Acetylcholine released and bind to receptors on?
- Motor end plate
- High density of acetylcholine receptors
- Highly folded
- End-plate potential
Motor neuron AP always creates a?
muscle cell AP
If there’s no Ca2+ what happens?
- troponin holds tropomyosin over myosin binding sites on actin
If there’s no Ca2+ are there any crossbridges?
- No crossbridges form between actin and myosin
- Muscle relaxed
If Ca2+ present what occurs
- it binds to troponin, causing movement of troponin, causing movement of tropomyosin, exposing binding sites for myosin on actin
If Ca2+ is present are there any crossbridges?
- Yes, crossbridges form between actin and myosin
- Cycle occurs; muscle contracts
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
Memorize figure 12.8
what type of gating are on the sarcoplasmic reticulum Ca2+ channels
- Voltage-gated opening
- Coupled to T tubules by ryanodine and DHP receptors
Ca2+ -induced opening
Ca2+ -induced closing
what must occur for a muscle contraction to end?
Ca2+ must leave troponin, allowing tropomyosin to cover myosin binding sites on actin
What must occur to remove Ca2+ from cytosol
- Ca2+ -ATPase in the sarcoplasmic reticulum
- Transports Ca2+ from cytosol into the sarcoplasmic reticulum
Twitch
- Contraction produced in a muscle fiber in response to a single action potential
- An all-or-nothing event for a muscle fiber at rest
Phases of a twitch
- Latent period
- Contraction phase
- Relaxation phase
Latent period
- Time from action potential in muscle cell to onset of contraction (few milliseconds)
- Excitation-contraction coupling
Contraction phase
- Time tension is increasing (10–100 msec)
- Crossbridge cycling (as long as theres still Ca2+)
Relaxation phase
- Time tension is decreasing back to zero (longer than contraction phase)
- Ca2+ reuptake to SR
- Fewer crossbridges less force
Isometric twitch contraction
- Length constant
- Contractile elements contract, generating tension
- When load > tension
- Muscle does not shorten, load not lifted
Isotonic twitch contraction
- Constant tension
- When tension > load
- Load is lifted as muscle shortens
during normal muscle contractions which contractions are rare and which ones mostly occur?
- Purely isotonic contractions are rare
- purely isometric contractions occur
Even if the load is constant, isometric precedes?
isotonic phase of contraction
Isometric continues (tension increases) until?
- tension exceeds load
- Then isotonic contraction begins
Tension remains constant as
muscle shortens
Graded muscle contractions depend on what factors:
- Tension produced by each fiber (Number of active crossbridges that bind to actin)
- Number of fibers contracting
- Frequency of stimulation
if more crossbridges that bind?
more force is generated
Frequency of stimulation
- increases in the frequency of action potentials in muscle fibers increase tension in two ways
- Treppe (step-wise)
- Summation
summation
Action potential
2 msec
Contraction
10–200 msec
Contractions can overlap and sum
Factors that affect the force generated by individual muscle fibers
- Fiber diameter
- Number of thick and thin filaments/area = constant
- Fiber length
- Optimal length
Force-generating capacity =
inherent ability of muscle to generate force
Force-generating capacity depends on
the number of crossbridges in each sarcomere and the geometric arrangement of sarcomeres
More crossbridges/sarcomere ->
more force
Larger diameter-> _____ -> ________
more filaments-> more force
At the onset of contraction what affects force generated?
Length of fiber
Optimal length
Resting length of muscle at which the fiber can develop the greatest amount of tension
situ
most muscles are at optimal length
More muscle fibers contracting =
greater force
Recruitment
- Stimulating more muscle fibers to contract
- occurs at the level of the motor unit
Motor unit recruitment
- 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
Muscles for delicate movements
Small motor units
Muscles for strength
Large motor units
Small motor units
- small fibers
- Small motor neuron cell bodies
- Small axon diameters
Large motor units
- Large fibers
- Large motor neuron cell bodies
- Large axon diameters
Order of motor unit recruitment is related to?
size of motor units
Small units recruit? Large units recruit?
first ; last
Larger neurons are more difficult to?
- depolarize to threshold
- Requires greater synaptic input
- Small neurons excited first (low input), then large neurons (high input)
What is required for muscles to generate work?
ATP, drives crossbridge cycling
Sources of ATP
- Phosphorylation of ADP by creatine phosphate
- Oxidative phosphorylation of ADP in mitochondria
- Anaerobic glycolysis
Role of the creatine/phosphate system
- At rest, small store of ATP
- Must quickly increase ATP synthesis
- Use of ATP drives the reaction to the right
creatine/phosphate reaction
Creatine + ATP-> creatine + ATP
creatine/phosphate can supply up to _____the quantity of resting ATP
5 times
What energy system is initially used during exercise?
- Oxidative phosphorylation
- Initially, glycogen stores supply glucose
- Up to 30 min, glucose and fatty acids in blood
why must O2 supply be kept adequate?
- Increases ventilation
- Increases heart rate and contraction
- Dilates vessels to muscle
- Transient increase in GLUT4
Exercise of heavy intensity involves what?
- Anaerobic glycolysis
- Lactate
- Only 2 ATP molecules per glucose molecule
Basis for skeletal muscle classification
*Velocity of contraction
- Fast versus slow
*Primary energy source
- Oxidative versus glycolytic
Differences in speed of contraction are dependent on what?
- Dependent on rate of myosin ATPase activity
- Higher rate = faster crossbridge cycling
ATP hydrolysis =
rate-limiting step of cycle
Fast fibers
Myosin with fast ATPase activity
Slow fibers
Myosin with slow ATPase activity
Fast fibers contract ___times more rapidly than slow fibers
2-3
Fast fibers relax ___ rapidly compared to slow fibers
more
Rate of Ca2+ -ATPase is ___ in fast twitch fibers
faster
Slow fiber contractions last approximately ___ times longer than fast fiber contractions
10
Glycolytic fibers
Anaerobic glycolysis
Fewer mitochondria
Many glycolytic enzymes
High glycogen stores
Use little oxygen—anaerobic
Large diameter
Quick to fatigue
Oxidative fibers
Oxidative phosphorylation
Many mitochondria
Myoglobin (red)
Small diameter
Resistant to fatigue
Many capillaries
Three skeletal muscle fiber types
Slow oxidative
Fast oxidative
Fast glycolytic
Properties of slow oxidative fibers
- Low myosin ATPase
- High oxidative capacity—aerobic
- Small diameter
- Fatigue slowly
Properties of fast glycolytic fibers
- High myosin ATPase activity
- High glycolytic capacity
- No myoglobin (so they appear white)
- Large diameter
- Fatigue rapidly
Properties of fast oxidative fibers
- Intermediate myosin ATPase activity
- High oxidative capacity—aerobic
- Myoglobin
- Slow to fatigue, but more rapid than slow oxidative fibers
- Intermediate diameter
Recruitment order:
- Slow oxidative fibers
- Fast oxidative fibers
- Fast glycolytic fibers (sprinting)
Resistance to fatigue: High-intensity exercises
- Glycolytic fibers
- Buildup of lactate
- Altered enzyme activity
- Recovery within minutes to hours
Strong contractions cause
compression of blood vessels
Resistance to fatigue: Low-intensity exercises
- Depletion of energy reserves (glycogen)
- Long time to recover
- Psychological fatigue
- Will to win
Muscle Receptors for Coordinated Activity
Muscle spindle
Golgi tendon organ
Smooth muscle
- 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 steps of excitation-contraction coupling
- Most Ca2+ comes from outside the cell
- Voltage-gated Ca2+ channels in plasma membrane
- Ca2+ triggers release of Ca2+ from sarcoplasmic reticulum
- Ca2+ binds to calmodulin
- Ca2+ -calmodulin activates myosin light-chain kinase
- MLCK phosphorylates myosin
- Crossbridge cycling
Relaxation of smooth muscle
- Phosphatase removes phosphate from myosin
- Ca 2+ is removed from cytoplasm
Myosin ATPase contraction is ____times ____ in smooth muscle than in skeletal muscle
10–100 ; slower
Neural regulation of smooth muscle contraction
- Innervated by autonomic nervous system
- Sympathetic and/or parasympathetic
- May be excitatory or inhibitory
- Precise response depends on the receptor type
- Neurotransmitter is released from varicosities
- Diffuse binding of neurotransmitter to receptors
Classification of smooth muscle
Single-unit smooth muscle
Multi-unit smooth muscle
Single-unit smooth muscle
- Most common type
- Intestinal tract
- uterus
- Muscle fibers activated synchronously
- Fibers connected by gap junctions
- Contract together as a single unit
Multi-unit smooth muscle
- Located in large airways and arteries, eye (ciliary muscle, iris)
- Few, if any, gap junctions
- Each fiber acts individually
Receives own innervation
Smooth Muscle: Pacemaker
Single-unit smooth muscle
Slow-wave potentials
Cycles in resting Vm
Pacemaker potentials
Cardiac Muscle
- striated with same sarcomeres
- Troponin-tropomyosin regulation
- Gap junctions (within intercalated disks)
- Pacemaker cells
- Innervated by autonomic nervous system
Ca2+ in cardiac muscles comes from
extracellular fluid and sarcoplasmic reticulum
