cardiac muscle: defining features
shorter fibers than striated that are fused together for added length; intercoalated discs at the point of attachment; branched; centrally located nucleus
cardiac muscle: contraction
needs no stimulation; all fibers beat on their own at different frequencies, but because neighboring contractions pass it down, the fasted fiber dictates speed of all others; nervous system can override this natural beating
cardiac muscle: purkinje fibers
specialized fibers to control the rate of contraction among the 4 cardiac chambers
same force; muscle fibers shorten against the resistance of a movable force/object
same length; muscle fibers do not shorten against the resistance of an immovable force/object
recording contractions: time 2
electrical activity occurs at the nerve electrode, and then it dissipates
recording contractions: time 3
electrical activity at the muscle electrode along the sarcolemma and t-tubules, then dissipates
recording contractions: what accounts for the delay before step 4?
the time it takes to change the permeability of the SER to ca ions
recording contractions: what accounts for the delay before step 3?
the time it takes the electrical signal to travel from one cell to another
recording contractions: what accounts for the delay before step 2?
the time it takes for the signal to go from the brain to the distal end of the neuron
the minimum amount of strength required of the stimulus to generate a response from the motor unit
organ level contraction
variable contraction of whole muscles based on recruitment of motor units taking place in the CNS
how is the contraction strength controlled?
the CNS fires more units at once for a stronger response; fewer at once for a weaker response
applying a stimulus and then a second (and third, fourth, etc.) before the first twitch has completely ended
applying stimuli evenly spaced so that the amplitude remains the same at each jump up; never fully relaxes
what is the drawback of twitch fusion?
it stops after the last stimulus ends, or muscles fatigue - which is after only 20 seconds
staggering the stimuli to different motor units so that some can be contracted while others are resting, allowing for a longer contraction
energetics of contraction
contraction requires ATP to be synthesized as quickly as it is being used (aka very fast), so while ATP is going to the relaxed muscles to contract them, ADP is going to getting a high energy P from phosphocreatine (ultimate energy source in the background is still glucose)
what happens if the PCr process doesn't provide enough energy/quickly enough?
the process finally goes anaerobic
fast and powerful twitches with fewer mitochondria, low consumption of oxygen (glycolytic)