BIOL 208 Lecture Notes - Lecture 7: Adrenal Medulla, Cardiac Muscle, Thyroid Hormones
● Chemical control (extrinsic control of heart beat)
● A) hormones
● epinephrine is the hormone that will accelerate the HR, norepinephrine is the NT of this.
sympathetic release of epinephrine will accelerate HR and rate of contraction
● *** as it turns out, the adrenal medulla, which releases epinephrine, will also release about
20% norepinephrine. its a mix of them both, but is mostly epinephrine, but end up doing thr
same thing, wether its nervous control or hormonal control. they accelerate the heart rate
and the strength of contraction
● thyroxin is another. when this is released in large quantities, it increases HR, not as rapidly, it
will take a longer period of time for receptors to respond to this higher level of thyroxin. it
brings about a longer and sustained increased HR level. it takes longer to kick in, but when it
kicks in it lasts longer
● B) various ions
● calcium - need certain level of this circulating and is involved in a lot of things. have normal
levels, important to keep cardiac muscles strong
● calcium is needed for proper muscle contraction, cell division, blood clotting
● essential for proper muscle contraction
● elderly will sometimes take calcium pills to strengthen their heart. this is to ensure if you have
a high level of calcium, there will be more calciums binding the TT complex, allowing more
cross bridges to attach, power stroke and so forth
● hypercalcemia - if you have too much calcium. a higher than normal amount of calcium in the
blood. this brings about an increased heart rate to the point where the heart muscle becomes
spastic
● hypocalcemia - too little calcium. very poor strength of heart contraction. the heart beats very
feebly, very weak
Ectopic-focus
**is an abnormal pacemaker that may appear elsewhere in the heart and assume control of the HR -
that is, other cells of the heart can assume this function should the SA node falter**
● in many instances the pacemaker just doesn’t work anymore or might have a heart attack,
but all of a sudden other atrial cells take over the role of a pacemaker
● or sometimes have renegades, anything can go wrong with your heart, and times when
pacemaker is running slower than normal other cells just will take over
● you have a pacemaker established in a place where it shouldn’t be
● can be quite detrimental, mess up overall cardiac rhythm
Arrythmias/dysrhythmias
● less than normal heart rates, heart rates that are not normal. usually faster
● abnormal heart beat rhythm
● conditions of irregular heart beat
● 1) flutter
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