Motor neurons supplying which type of muscle fiber have the lowest activation threshold?
slow oxidative (followed by fast oxidative glycolytic, with fast glycolytic fibers having the highest activation threshold)
Why are stronger contractions produced when the brain sends stronger signals down the motor neurons?
greater activity from the brain = more motor neurons activated = stronger contraction
tension
the force produced by a contracting muscle fiber/whole muscle when it is stimulated
twitch
the mechanical response (contraction) of a muscle fiber/whole muscle to a single stimulus (measured as tension)
latent period
the time between application of the stimulus and development of tension
How can a muscle produce different amounts of tension even though muscle cells respond to stimuli in an all-or-none fashion?
Each individual cell responds in an all-or-none way, but different stimuli recruit different amounts of cells to produce a twitch. (The recorded output is the sum of the twitches of all of the stimulated cells.)
Time to Peak Tension (TPT)
- the time from the first increase in tension until the peak tension is developed
- a good indicator of the speed with which the signal for cross-bridge cycling is delivered
Half Relaxation Time (1/2 RT)
- the time from the peak tension until half of the active tension has disappeared
- measured in this way due to some lag in elastic elements during the second half of relaxation
- an indicator of the speed with which the calcium pumps operate to remove calcium from the cell cytoplasm
recruitment
increasing the intensity of the stimulus and/or the duration will result in more of the fibers being stimulated
summation
If the frequence of stimulation is high enough, a second twitch adds to the previous one
Why is summation possible?
the excitation process is of a much shorter duration than the contraction phase
tetanus
- the situation where twitches are fused to a steady tension development
- can result in a very small amount of tension or a maximal development of tension
fatigue
failure to maintain maximal tension
passive tension
- tension due to the external stretching of the elastic elements associated with the muscle
- leads to the muscle “resisting” lengthening due to its spring quality
At what length can optimal contraction and tension output occur?
- very slightly stretched past the resting length
- less tension is developed when shorter or stretched
fatigue index calculation (%)
fatigue index = (force at end of stimulation/maximum force) x 100
What do we know about calcium re-uptake when tetanus is achieved?
At tetanus, the concentration of calcium in the cytoplasm becomes steady. This is because the sarcoplasmic reticulum cannot reuptake calcium enough to counteract its release into the cytoplasm.
Why is the tension of a muscle in tetanus greater than the tension of a single muscle twitch?
In tetanus, all elastic elements are stretched = greater force on load
sinoatrial (SA) node
the pacemaker of the heart, located in the right atrium
myogenic
able to produce action potentials without stimulation from the nervous system (for example, cells in the SA node)
atrioventricular (AV) node
first destination of action potentials produced by the SA node (slow transmission from SA –> AV ensures that ventricles contract after the atria)
Why is it important that the ventricles contract after the atria?
this allows ventricles to fill with blood from the atria before they contract
Can the AV node generate its own impulses?
Yes. Without input from the SA node, the AV node can generate its own rhythm, but it will be slower
How many chambers are in a frog’s heart?
Three (two atria and one ventricle)
