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
