Motor Units, Muscle Contraction, and ATP Study Guide Flashcards
Define muscle tension
the force exerted on an object by a contracting muscle
define muscle load
the opposing force exerted on the muscle by the weight of the object to be moved
Define a motor unit.
1 motor neuron and all the muscle fibers it innervates
What types of movements are performed by small motor units?
Fine control exerted by muscles (like in fingers)
What types of movements are performed by large motor units?
Large, less precise movements (like the thigh muscle)
How are the fibers of a particular large motor unit arranged within the muscle? What is the functional significance of this?
Muscle fibers in a motor unit are spread throughout muscle – not clustered together
Stimulation of a motor unit causes weak but uniform contraction throughout the muscle
What is a muscle twitch
simplest form of contraction – a muscle fiber’s response to a single action potential
- Muscle fiber contracts quickly then relaxes
All muscles ____ faster than they _____.
- contract
- relax
What is a myogram
Myogram: graphical recording of muscle activity
Define each part of a muscle twitch and state what is occurring.
- Latent period: 1st few milliseconds following stimulation; excitation-contraction coupling is occurring; cross bridges begin to cycle, but muscle tension is not yet measurable
- Period of contraction: cross bridges are active; myogram tracing rises to a peak; period lasts 10-100ms
- Period of relaxation: final phase lasting 10-100ma; Ca2+ is being pumped back into the SR; number of active cross bridges is declining; muscle tension declines to 0
Do all twitches occur at the same speed? List examples of twitches that occur at different speeds.
Twitches do not always occur at the same speed
Twitch contractions in some muscles are rapid and brief (extraocular muscles)
Twitch contractions in other muscles are slow and long lasting (gastrocnemius, soleus)
Muscle responses are graded by ________ and/or ________.
- changing frequency of stimulation
- changing strength of stimulation
Define wave/temporal summation.
2+ stimuli are received in rapid succession before relaxation is complete
Summation occurs in response to changes in stimulus ________.
frequency
Physiologically, what causes the increase in the strength of the contraction?
Muscle fibers do not have time to completely relax between stimuli
Additional calcium was squeezed into the cytosol of a partially contracted muscle
Contractions are added together
What will happen if the frequency of stimuli continues to increase?
Relaxation time between twitches becomes shorter
Concentration of Ca2+ in the cytosol rises higher
Degree of summation becomes greater
Define unfused and fused tetany.
Fused (complete tetany): contractions fuse into 1 smooth, sustained contraction plateau
Unfused (incomplete tetany): sustained, quivering contraction
Define recruitment. Physiologically, what causes recruitment?
multiple motor unit summation; stimuli of increasing voltage are delivered, more muscle fibers are called into play
Controls the force of contraction more precisely
* Caused by changes in stimulus strength
Define subthreshold, threshold, and maximal stimuli.
- Subthreshold stimulus: stimulus is not strong enough – no contractions are seen
- Threshold stimulus: stimulus is strong enough to cause 1st observable contraction
- Maximal stimulus: strongest stimulus that increases contractile force – all motor units are recruited
Explain the size principle. Explain how the size principle helps us to do things like filling a pot with water and keeps us from looking like Elaine from Seinfeld.
- Motor units with the smallest muscle fibers are recruited first – they are controlled by the smallest, most excitable motor neurons
- Motor units with larger muscle fibers are recruited next and contractile strength increases
- The largest motor units, containing large, coarse muscle fibers, are controlled by the largest and least excitable (highest threshold) neurons – these units are only activated when the most powerful contraction is necessary
- typically motor units are activated asynchronously
What is asynchronous activation? Why is it important?
When motor units aren’t all activated at once – helps to prolong a strong contraction and delay/prevent muscle fatigue
What is muscle tone? What creates it? Why is it important?
- Muscle tone: the constant, slightly contracted state of all muscles
- Muscle tone is due to spinal reflexes – groups of motor units are alternately activates in response to input from stretch receptors within the muscles
- Muscle tone is important because it keeps muscles firm, healthy, and ready to respond
hypotonia
very low muscle tone
Hypertonia
very high muscle tone
isotonic muscle contraction
muscle changes in length and movies load, once enough tension is generated to move the load, tension remains relatively constant, contractions can be either concentric or eccentric
concentric muscle contractions
muscle shortens and does work
Biceps brachii contraction to pick up a book
eccentric muscle contractions
muscle lengthens and generates force – 50% stronger contractions than concentric
Ex. quadriceps contractions while walking downstairs
isometric muscle contractions
muscle tension develops, but the load is not moved – laid is greater than the tension the muscle can develop
Ex. attempting to lift a piano with one hand
Muscles contract isometrically when they act primarily to maintain upright posture or hold joints stationary while movement occurs elsewhere
Ex. holding a plank or wall sit
Cross bridges are formed and generate force, but they do not slide the thin filaments
Why do we need ATP to contract muscle? What functions does it perform?
Atp supplies the energy to:
Move and detach cross bridges
Operate the calcium pump in the SR
Operate the Na+-K+ pump in the plasma membrane
Atp is the only source of energy for contractile activities
Why can eccentric contractions be stronger than concentric?
Force is generated with the lengthening of the muscle
More cross bridges are formed
Energy and oxygen consumption is less
List the 3 methods we use of regenerate ATP. Which of these methods is the fastest? Which creates the most ATP?
Direct phosphorylation of ADP by creatine phosphate (CP) FASTEST
Anaerobic pathway: glycolysis and lactic acids formation
Aerobic respiration: CREATES THE MOST ATP
Write out the steps of direct phosphorylation of ADP. Know the roles of CP and CK. How long can we power a muscle cell using this method?
- Creatine Phosphate (CP): unique high energy molecule stored in muscle fibers
1. CP donates a phosphate to ADP to instantly form ATP - Creatine Kinase (CK): enzyme that carries out the transfer of phosphate
2. Muscle fibers have enough ATP + CP reserves to power the muscle cell for 15 seconds
3. CP reserves are replenished during rest of inactivity - CP + ADP → creatine + ATP
Define glycolysis. Write out the steps of glycolysis. Does this process require Oxygen? How much ATP is produced?
Glycolysis: “sugar splitting” – 1st step in glucose breakdown
- Doesn’t require oxygen
- Broken down into 2 pyruvic acid molecules
- 2 ATPs are generated for each glucose broken down
- Can provide most of the ATP needed for 30-40 seconds of strenuous muscle activity
Define lactic acid. How long can we power a muscle cell using this method?
Lactic acid is diffused into the bloodstream
Lactic acid:
- used by fuel by the liver, kidneys, heart
- Converted back to pyruvic acid or glucose by the liver
- Responsible for post-activity soreness
- Stored ATP + CP and glycolysis can support about a minute of strenuous activity
- In absence of oxygen, pyruvic acid is converted to lactic acid
Define aerobic respiration
During rest and light-moderate exercise, 95% of the ATP used for muscle activity comes from aerobic respiration. It is slower than anaerobic pathway, but creates more ATP – up to 32 molecules
Where does aerobic respiration take place
starts with glycolysis and then moves into multiple reactions that place within the mitochondria
chemical equation for aerobic respiration
Glucose(C6H12O6) + O2 → CO2 + H2O + ATP
In aerobic respiration, what are the ingredients for ATP
requires consistent O2 and glucose
What happens to the CO2 produced in aerobic respiration
diffuses out of the muscle and into the blood – it will be removed by the lungs
List the fuel sources for aerobic respiration in the order that they are used.
- As exercise begins, muscle glycogen provides most of the fuel
- Secondly, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids are the major fuel sources
- Lastly, after about 30 mins, fatty acids become the major energy fuels
When Oxygen is available, which ATP production pathway is used?
Aerobic pathway
Define aerobic endurance
the length of time a muscle can continue to contract using aerobic pathways
Anaerobic threshold
the point at which muscle metabolism converts to anaerobic glycolysis
Which pathway would power a gymnast performing a vault? How about a triathlete completing an Olympic distance triathlon?
- Gymnast performing a vault: ATP + CP stores – Direct phosphorylation of ADP by creatine phosphate (CP)
- Triathlete competing an olympic distance triathlon: aerobic respiration
- Sprinting, weight lifting: anaerobic (slightly longer bursts of energy)
What must happen for a muscle to return to its pre-exercise state?
- Oxygen reserves must be replenished
- Accumulated lactic acid must be reconverted back into pyruvic acid
- Glycogen stores must be replaced
- ATP and creatine phosphate reserves must be resynthesized
What is fatigue? What might cause it? What RARELY causes it?
- Fatigue: the physiological inability to contract despite continued stimulation, prevents total depletion of ATP
Possible causes:- Ionic imbalances: changes in levels of K+, Na+, Ca2+ can disrupt membrane potentials
- Increased inorganic phosphate (Pi) from CP and ATP breakdown may interfere with calcium release from the SR
- Decreased ATP and increased Mg2+ act on voltage-sensitive proteins in the T Tubule and decrease Ca2+ release from the SR
- Decreased glycogen is highly correlated with fatigue
- RARELY a cause: lack of ATP and rise of lactic acid
Define excess postexercise oxygen consumption (EPOC)
The extra amount of oxygen that the body must consume for these restorative processes - formerly called the oxygen debt
How does an Oxygen deficit occur?
when the body needs more oxygen than is currently available – the difference between the oxygen needed by muscles and the amount present
