Muscles and Muscles Tissue Part B Flashcards
Whole Muscle Contraction
Same principles apply to contraction of both single fibers and whole muscles
muscle tension
Contraction produces muscle tension, the force exerted on load or object to be moved
Contraction may
may/ may not shorten muscle
Isometric contraction
no shortening; muscle tension increases but does not exceed load numbers
Isotonic contraction
muscle shortens because muscle tension exceeds load
Force and duration of contraction
contraction vary in response to stimuli of different frequencies and intensities.
Each muscle is served
by at least one motor nerve
Motor nerve contains
axons of up to hundreds of motor neurons
Axons branch
into terminals, each of which forms NMJ with single muscle fiber
Motor unit define
is the nerve-muscle functional unit
Motor unit
unit consists of the motor neuron and all muscle fibers (four to several hundred) it supplies
Smaller the fiber number, the greater the fine control
The smaller fiber number is
the greater the fine control
Muscle fibers from a motor unit
are spread throughout the whole muscle, so stimulation of a single motor unit causes only weak contraction of entire muscle
Muscle twitch
simplest contraction resulting from a muscle fiber’s response to a single action potential from motor neuron
In muscles twitch the muscles fiber
Muscle fiber contracts quickly, then relaxes
Myogram
is when twitch can be observed and recorded
Tracing
line recording contraction activity
Three phases of muscle twitch
Latent period
Period of contraction
Period of relaxation
Latent period
events of excitation-contraction coupling.
No muscle tension seen
Period of contraction
cross bridge formation
Tension increases
Period of relaxation
Ca2+ reentry into SR
Tension declines to zero
Muscle contracts
faster than it relaxes
Differences in strength and duration of twitches causes
are due to variations in metabolic properties and enzymes between muscles.
Example twitches
eye muscles contraction are rapid and brief, whereas larger, fleshy muscles (calf muscles) contract more slowly and hold it longer
Normal muscle contraction
is relatively smooth, and strength varies with needs
A muscle twitch is seen
only in lab setting or with neuromuscular problems, but not in normal muscle
Graded muscle responses
vary strength of contraction for different demands
Required for proper control of skeletal movement
Responses are graded by:
Changing frequency of stimulation
Changing strength of stimulation
Muscle response
to changes in stimulus frequency
Single stimulus
results in single contractile response (i.e., muscle twitch)
Wave Temporal summation
results if two stimuli are received by a muscle in rapid succession.
Muscle fibers do not have time
completely relax between stimuli, so twitches increase in force with each stimulus
Additional Ca2+ that
is released with second stimulus stimulates more shortening
If stimuli frequency increases
muscle tension reaches near maximum
Produces smooth
continuous contractions that add up (summation)
Further increase in stimulus frequency
frequency causes muscle to progress to sustained, quivering contraction referred to as unfused (incomplete) tetanus
fused (complete) tetanus
because contractions “fuse” into one smooth sustained contraction plateau
Prolonged muscle contractions
lead to muscle fatigue
Recruitment (or multiple motor unit summation):
stimulus is sent to more muscle fibers, leading to more precise control
Types of stimulus involved in recruitment:
Subthreshold stimulus
Threshold stimulus
Maximal stimulus
Subthreshold stimulus
stimulus not strong enough, so no contractions seen
Threshold stimulus
stimulus is strong enough to cause first observable contraction
Maximal stimulus
strongest stimulus that increases maximum contractile force
All motor unit
have been recruited
Recruitment works
on size principle
Motor units with smallest
muscle fibers are recruited first
Motor units with larger
larger fibers are recruited as stimulus intensity increases
Largest motor units
activated only for most powerful contractions
Motor units in muscle
usually contract asynchronously
Some fibers contract
while others rest,
Helps prevent fatigue
Muscle Tone
Constant, slightly contracted state of all muscles. Due to spinal reflexes
Groups of motor units
alternately activated in response to input from stretch receptors in muscles
Muscle Tone
Keeps muscles firm healthy, and ready to respond
Isotonic contractions
muscle changes in length and moves load.
Isotonic contractions can be either concentric or eccentric:
Concentric contractions and examples
muscle shortens and does work
Example: biceps contract to pick up a book
Eccentric contractions and examples
muscle lengthens and generates force.
Example: laying a book down causes biceps to lengthen while generating a force
Isometric contractions
Load is greater than the maximum tension muscle can generate, so muscle neither shortens nor lengthens
Electrochemical and mechanical events
same in isotonic or isometric contractions, but results are different
In isotonic contractions
actin filaments shorten and cause movement
In isometric contractions
cross bridges generate force, but actin filaments do not shorten.
Myosin heads
“spin their wheels” on same actin- binding site
ATP supplies the energy needed for the muscle fiber
first to:
Move and detach cross bridges
ATP supplies the energy needed for the muscle fiber second to:
Pump calcium back into SR
ATP supplies the energy needed for the muscle fiber third to:
Pump Na+ out of and K+ back into cell after excitation-contraction coupling
Available stores of ATP
depleted in 4–6 seconds
ATP is the only source of energy
for contractile activities; therefore it must be regenerated quickly
ATP is regenerated quickly by three mechanisms:
Direct phosphorylation of ADP by creatine phosphate (CP)
Anaerobic pathway: glycolysis and lactic acid formation
Aerobic pathway
Creatine phosphate
is a unique molecule located in muscle fibers that donates a phosphate to ADP to instantly form ATP
Creatine kinase
kinase is enzyme that carries out transfer of phosphate
Muscle fibers have enough ATP
and CP reserves to power cell for about 15 seconds
Muscle fibers have enough ATP
CP reserves to power cell for about 15 seconds
Creatine phosphate + ADP →
creatine + ATP
ATP can also be generated
by breaking down and using energy stored in glucose
Glycolysis
first step in glucose breakdown.
Does not require oxygen.
Glucose is broken into 2 pyruvic acid molecules.
2 ATPs are generated for each glucose broken down
Low oxygen levels prevent pyruvic acid
from entering aerobic respiration phase
Normally, pyruvic acid enters
mitochondria to start aerobic respiration phase; however, at high intensity activity, oxygen is not available
Bulging muscles compress
blood vessels, impairing oxygen delivery
In the absence of oxygen
referred to as anaerobic glycolysis, pyruvic acid is converted to lactic acid.
Lactic acid
Diffuses into bloodstream.
Used as fuel by liver, kidneys, and heart.
Converted back into pyruvic acid or glucose by liver
Anaerobic respiration yields only
5% as much ATP as aerobic respiration, but produces ATP 2½ times faster
Aerobic Respiration
Produces 95% of ATP during rest and light-to-moderate exercise.
Slower than anaerobic pathway
Aerobic Respiration consist of
of series of chemical reactions that occur in mitochondria and require oxygen
Aerobic Respiration breaks
glucose into CO2, H2O, and large amount ATP (32 can be produced)
Fuels used include (Aerobic Respiration)
glucose from glycogen stored in muscle fiber, then bloodborne glucose, and free fatty acids.
Fatty acids(Aerobic Respiration)
are main fuel after 30 minutes of exercise
Energy systems used during sports
Aerobic endurance
Anaerobic threshold
Aerobic endurance
Length of time muscle contracts using aerobic pathways
Light-to-moderate activity, which can continue for hours
Anaerobic threshold
Point at which muscle metabolism converts to anaerobic pathway
Muscle Fatigue
Fatigue is the physiological inability to contract despite continued stimulation
Possible causes of muscles fatigue include:
Ionic imbalances can cause fatigue
Levels of K+, Na+ and Ca2+ can change disrupting membrane potential of muscle cell
Increased inorganic phosphage
(Pi) from CP and ATP breakdown may interfere with calcium release from SR or hamper power
Decreased ATP and increases magnesium.
As ATP levels drop magnesium levels increase and this can interfere with voltage sensitive T tubule proteins
glycogen
decreases in muscles fatigue
Lack of ATP
rarely a reason for fatigue, except in severely stressed muscles
Excess Postexercise Oxygen Consumption
All replenishing steps require extra oxygen.
Formerly referred to as “oxygen debt”
muscles pre-exercise state:
Oxygen reserves are replenished.
muscles pre-exercise state:
Lactic acid is reconverted to pyruvic acid
muscles pre-exercise state:
Glycogen stores are replaced
muscles pre-exercise state:
ATP and creatine phosphate reserves are resynthesized
