Motor Units, Muscle Contractions, and ATP 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 motor unit
1 motor neuron and all the muscle fibers it innervates
What types of movements are performed by small motor units?
Muscles that exert fine control
What types of movements are performed by large motor units?
Muscle that create large, less precise movements
How are the fibers of a particular large motor unit arranged within the muscle?
What is the functional significance of this?
The muscle fibers within a particular motor unit are spread throughout the
muscle – not clustered together
Stimulation of a single motor unit causes a 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
What is a 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 10-100ms; Ca2+ is being pumped back into the SR; number of active cross bridges is declining;
muscle tension declines to 0
All muscles ________ faster then they_________.
contratc, relax
Do all twitches occur at the same speed?
List examples of twitches that occur at different speeds.
Twitch contractions in some muscles are rapid and brief
Ex: extraocular muscles (eyes)
Twitch contractions in other muscles are slow and long lasting
Ex: gastrocnemius, soleus (calf)
Muscle responses are graded by __________ and/or ___________.
changing frequency of stimulation, changing strength of stimulation
Define wave/temporal summation.
Summation occurs in response to changes in
stimulus ____________. Physiologically, what
causes the increase in the strength of the
contraction?
What will happen if the frequency of stimuli continues to increase?
- Relaxation time between twitches becomes shorter and shorter
- Concentration of Ca2+ in the cytosol rises higher and higher
- Degree of summation becomes greater and greater
Define unfused and fused tetany.
Eventually, the muscle can reach a sustained, quivering contraction called unfused (incomplete) tetanus and then potentially fused (complete) tetanus
Define recruitment. Physiologically, what causes recruitment?
Recruitment - also called multiple motor unit summation; stimuli of increasing voltage are delivered, and more muscle fibers are called into play
- This controls the force of contraction more precisely
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
What is asynchronous activation? Why is it
important?
All motor units may be recruited simultaneously to produce an exceptionally
strong contraction, but typically, motor units are activated asynchronously
Asynchronous activation helps to prolong a strong contraction and delay/
prevent muscle fatigue
What is muscle tone? What creates it? Why is it important?
- the constant, slightly contracted state of all muscles
Muscle tone is due to spinal reflexes – groups of motor units are alternately
activated in response to input from stretch receptors within the muscles
Keeps muscles firm, healthy, and ready to respond
Define hypotonia and hypertonia.
Hyptonia reduced muscle tone | Floppy
Hypertonia, Excess muscle tone | too rigid
Fully define the following classifications of muscle contraction: isotonic, isometric, eccentric, and concentric. Think of examples of each type of contraction to help you
remember what they look like!
Isotonic Contractions - muscle changes in length and moves load
“iso” = same and “ton” = tension
Once enough tension is generated to move the load, tension remains
relatively constant
- Isotonic contractions can either be concentric or eccentric
Concentric Contractions - muscle shortens and does work
Ex: biceps brachii contraction to pick up a book
Eccentric Contractions - muscle lengthens and generates force – 50%
stronger contractions than concentric!
Ex: quadriceps contractions while walking downstairs
Many activities involve both types of isotonic contractions!
Isometric Contraction - muscle tension develops, but the load is not
moved – load is greater than the tension the muscle can develop
Ex: attempting to lift a piano with 1 hand
Why can eccentric contractions be stronger
than concentric?
Muscle Lengthens
Stretch Refelx
Muscle activates more muscle fibers
Some are optimized foeccentrics
Why do we need ATP to contract muscle? What
functions does it perform?
As a muscle contracts, 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
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
- Glycolysis/Anaerobic pathway: glycolysis and lactic acid formation
- Aerobic respiration - 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?
CP + ADP creatine + ATP
Creatine Phosphate ( CP ) - unique high energy molecule stored in muscle
fibers
CP donates a phosphate to ADP to instantly form ATP
Creatine Kinase - enzyme that carries out the transfer of phosphate
Muscle fibers have enough ATP + CP reserves to power the muscle cell for
15s
CP reserves are replenished during rest or inactivity
Define glycolysis. Write out the steps of glycolysis. Does this process require
Oxygen? How much ATP is produced? Define lactic acid. How long can we power a muscle cell using this method?
Glycolysis (“Sugar-Splitting”) = the 1st step in glucose breakdown
Process does not require Oxygen
Glucose is broken into 2 pyruvic acid molecules
2 ATPs are generated for each glucose broken down
Lactic acid defuses into blood stream
- Used as fuel by the liver, kidneys and heart
- It is converted back to pyruvic acid or glucose by the liver
Responsible for post-activity muscle soreness
Less efficient (5) but is 2.5x faster
Define aerobic respiration. Where does it
take place? What is the chemical equation
for the process? In this process, what are
the ingredients for ATP? What happens to
the CO2 produced? List the fuel sources for aerobic respiration in the order that they are used.
Aerobic respiration is the process by which cells convert oxygen and nutrients, such as glucose, into energy (in the form of ATP) in the presence of oxygen, producing carbon dioxide and water as byproducts.
Takes place in mitochondria
Glucose + O2 -> CO2 + H2O + ATP
Ingrediants for ATP is ADP and Pi (Inorganic Phosphate)
CO2 is breathed out
Fuel is Carbs, Fatty Acids and Amino Acids
When Oxygen is available, which ATP production pathway is used?
Aerobic Pathway
Define aerobic endurance. Define anaerobic
threshold.
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?
Exercises that require a surge of power but last only a few seconds rely entirely
on ATP and CP stores (Vault)
Exercises requiring slightly longer bursts of activity are fueled almost entirely by
anaerobic glycolysis
Prolonged endurance activities depend mainly on aerobic respiration – levels of
ATP and CP don’t change much – “pay as you go” (Triathlon)
What is fatigue? What might cause it? What RARELY causes it?
What must happen for a muscle to return to
its pre-exercise state?
- Oxygen reserves must be replenished
- Accumulated lactic acid must be reconverted to pyruvic acid
- Glycogen stores must be replaced
- ATP and creatine phosphate reserves must be resynthesized
