Lecture 2 and 3 Unit 1 Flashcards
what does cardiac muscle consist of?
branched, striated fibers with one or two centrally located nuclei
what are actin and myosin packaged in and how are they arranged?
they are packaged in myofibrils and arranged in sarcomeres (just like skeletal muscle)
what does the branching pattern in cardiac muscle form?
a network that can facilitate the transmission of electrical impulses in all directions
what are gap junctions?
small channels which allow electrical impulses to pass quickly from one cell to the next
where are gap junctions located?
in intercalated disks which lie between adjacent muscle fibers
what are the two structures that intercalated disks have?
desmosomes and gap junctions
what is the function of desmosomes?
they hold adjacent cells together
what is the pacemaker in the heart?
the SA node
what is the SA node?
a mass of cells in the right atrial wall
what do pacemaker cells do?
spontaneously discharge action potentials at a rate of 100-120 per minute or more
what do autonomic nerves do in the intact animal?
modify the rate of discharge so that the resting heart rate is ~70 beats per minute
how must the atria and ventricles contract?
in a coordinated fashion
what is the first event in cardiac muscle excitation?
depolarization of the SA node
what is the second event in cardiac muscle excitation
the impulses travel down and across both atria causing atrial muscle fiber contraction
what is located at the AV border?
poorly conducting tissue (small fibers, few gap junctions)
what happens in the presence of poorly conducting tissue?
the impulse is slowed by about 0.1 seconds
why is it beneficial for the impulse to be slowed by the poorly conducting tissue?
it gives the atria time to fully empty before the ventricles begin to contract
what are right and left AV bundles connected by?
the short Bundle of His
where does the bundle of his transmit action potential?
bundle branches, purkinje fibers, and the muscle of both ventricles
What happens in the sequence of excitation? (what part of the heart contracts first)
the lower portions of the ventricles contract first, pushing the blood upwards
what are the steps of action potentials of contractile fibers?
- rapid depolarization due to Na+ inflow when voltage-gated fast Na+ channels open
- plateau due to Ca2+ inflow when voltage gated slow Ca2+ channels open and K+ outflow when some K+ channels open
- depolarization due to closure of Ca2+ channels and K+ outflow when additional voltage gated K+ channels open
what does refractory mean
unresponsive or stubborn
what does refractory mean in physiology?
refers to a period of time when the muscle or nerve cell is unresponsive to stimulation
what is the absolute refractory period?
it refers to the time when the cell will not respond regardless of the strength of the stimulus
