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KIN 426 > Cardiac Autonomic Regulation & Reflexes > Flashcards

Cardiac Autonomic Regulation & Reflexes Flashcards

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1
Q

What is the average adult resting HR?

A

about 70 bpm
significantly higher in children

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2
Q

How does HR change during sleep?

A

decreases by 10-20 beats
decreased HR - increased EDV + SV -> increased blood flow (vagal tone)

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3
Q

When does HR exceed 100 bmp?

A

during emotional excitement or muscular activity
parasympathetic

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4
Q

What is a well trained athletes resting HR?

A

50 bpm

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5
Q

How is the SA node usually under the tonic influence of both divisions of the autonomic NS?

A

sympathetic enhances automaticity, and parasympathetic inhibits it

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6
Q

How does HR change cause reciprocal action in both divisions?

A

HR increases - less parasympathetic and more sympathetic activity
HR decreases - more parasympathetic and less sympathetic activity

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7
Q

What division typically predominates during rest?

A

parasympathetic

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8
Q

What occurs when there is a blockade of parasympathetic tone?

A

increases HR significantly

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9
Q

What drug blocks parasympathetic tone?

A

atropine

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10
Q

What occurs when there is a blockade of sympathetic tone?

A

decreases HR significantly

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11
Q

What drug blocks sympathetic tone?

A

propanolol

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12
Q

What occurs when there is a blockade of sympathetic and parasympathetic tone?

A

HR will stabilize to about 100 bmp
intrinsic HR

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13
Q

Where do cardiac parasympathetic fibers originate?

A

medulla oblongata
in cells that lie in the dorsal motor nucleus of the vagus or in the nucleus ambiguus

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14
Q

What is the pathway of cardiac parasympathetic fibers?

A

medulla oblongata -> inferior through the neck (close to common carotid arteries) -> mediastinum to synapse on postganglionic cells on the epicardial surface or within the heart

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15
Q

Where are most cardiac ganglion cells found?

A

in plexuses near the SA and AV conduction tissues

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16
Q

What does the right vagus nerve predominately affect?

A

SA node
stimulation slows SA firing or may stop it for several seconds

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17
Q

What does the left vagus nerve predominately affect?

A

mainly inhibits AV conduction
varying degrees

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18
Q

How does the distribution of efferent vagal fibres overlap?

A

left vagal stimulation also depresses the SA node, and right vagal stimulation impedes AV conduction

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19
Q

What are the SA and AV nodes rich in?

A

cholinesterase
activated with vagus nerve stimulation

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20
Q

What is the function of cholinesterase?

A

causes the effects of any vagal impulse to be brief because it rapidly hydrolyzes (breaks down) neurally released Ach

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21
Q

What is the speed of effects of vagal activity on SA and AV nodes?

A

display a very short latency
initial rises in HR are fast (vagal control)

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22
Q

What happens to HR when vagus nerves are stimulated at a constant frequency for several seconds?

A

HR abruptly decreases and attains a steady state value within 1 or 2 cardiac cycles

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23
Q

What happens to HR when vagus nerve stimulation is discontinued?

A

HR returns to its basal level very quickly

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24
Q

What do the brief latency and rapid decay of response in vagal nerves allow?

A

vagus nerves to exert beat-by-beat control for SA and AV nodal function
- because of the abundance of cholinesterase that rapidly hydrolyzes Ach = increased HR

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25
How is HR regulated due to parasympathetic influences predominating over sympathetic effects at the SA node?
regulation of HR is mediated by suppression of the release of norepinephrine from sympathetic nerve endings and ACH released from neighbouring vagus nerve endings
26
What is the role of Ach in parasympathetic predominance of SA node?
inhibits the release of norepinephrine
27
Where do cardiac sympathetic fibres originate?
in the intermediate columns of the upper five or six thoracic and lower one or two cervical sections of the spinal cord
28
What is the pathway of cardiac sympathetic fibers?
intermediate columns -> emerge through white communicating branches and enter the paravertebral chains of ganglia -> pre and postganglionic neurons synapse mainly in the stellate and middle cervical ganglia
29
Where do middle cervical ganglia lie close to?
the vagus nerves in the superior portion of the mediastiunum
30
What do sympathetic and parasympathetic nerves join to form?
a (cardiac) plexus of mixed efferent nerves to the heart
31
How do postganglionic sympathetic fibers approach the heart?
approach the base of the heart along the adventitial surface of the great vessels; they are distributed to various chambers as an extensive epicardial plexus, they then penetrate the myocardium, usually oolong the coronary vessels
32
What type of adrenergic receptors are usually in the nodal regions and what activates and inhibits them?
beta type activated by beta-adrenergic antagonists, such as isoproterenol, and are inhibited by beta-adrenergic blocking agents, such as propranolol
33
How are the left and right sympathetic nerves distributed?
Like vagus nerves, they are distributed differentially left fibres - myocardial contractility right fibres - heart rate * dog experiment, likely in humans
34
What are the two factors responsible for the more gradual onset of HR response to sympathetic activity than to vagal activity?
1. the response to sympathetic activity depends mainly on the intracellular production of second messengers, mainly cAMP, in the automatic cells in the SA node 2. the postganglionic nerve endings of each of the two divisions release neurotransmitters at different rates
35
Explain why the response to sympathetic activity depends mainly on the intracellular production of second messengers, mainly cAMP, in the automatic cells in the SA node
this is a slower process than vagal the muscarinic receptors that respond to the Ach released from the vagal terminals are coupled directly to the Ach-regulated K+ channels, allowing a prompt response
36
Explain why the postganglionic nerve endings of each of the two divisions release neurotransmitters at different rates
intense vagal activity releases enough Ach during a brief period to arrest the heartbeat intense sympathetic activity, enough norepinephrine is released during each cardiac cycle to change cardiac behaviour by only small increments thus, vagal nerves can exert beat-by-beat control, whereas sympathetic nerves can't alter cardiac function very much within the one cardiac cycle
37
What does stimulation of cardiac sympathetic nerves result in?
the overflow of substantial amounts of norepinephrine into the coronary sinus blood amount of overflow parallels the amount of norepinephrine released
38
What does simultaneous vagal stimulation with stimulation of cardiac sympathetic nerves result in?
decrease the norepinephrine overflow by about 30%
39
What are the receptors on the sympathetic adrenergic terminal membrane activated by, and what does that activation cause?
muscarinic receptors are activated by Ach, which causes inhibition of norepinephrine
40
What causes, in part, vagal activity to decrease HR and contractility?
antagonizing the facility effects of any concomitant sympathetic activity neuropeptide Y can transmit with norepinephrine in sympathetic adrenergic nerves, can inhibit Ach from neighbouring vagal fibres
41
Explain sympathovagal balance
primarily sympathetic control at peak exercise measurable sympathetic activity at rest as exercise intensity increases, increased sympathetic control and decreased vagal control
42
What can be inferred when microneurography is used to measure MSNA in trained and untrained heart failure patients
sympathetic nerve activity decreased at rest with exercise training (HR would as well) heart failure patients are more sympathetically active at rest
43
What is circulatory occlusion?
isometric constriction of blood flow builds up metabolic by-products
44
Why does BP decrease then increase again during the onset of circulatory occlusion?
decreases due to the deactivation of mechanoreceptors increases again due to the isolation of metaboreceptors (still sensing metabolic activity)
45
What happens to MSNA during circulatory occlusion?
stays the same even though the muscle is no longer contracting
46
Why does MSNA stay the same during circulatory occlusion?
muscle metaboreflex contributor to exercise presser response - the reflex that will increase breathing, HR, and BP during exercise
47
What are metaboreceptors?
type III and IV afferent chemically sensitive (H+ ions -> lactic acid) nerve to brainstem
48
What is the mechanoreflex?
mechanic receptors in the muscle that stimulate the nervous system when deforms (afferent, sensory) signal an increase in HR (efferent)
49
What is the baroreceptor reflex?
acute changes in arterial blood pressure reflexly elicit changes in HR via the baroreceptors (stretch) located in the aortic arch and carotid sinuses (more sensitive)
50
What range of BP is the baroreceptor reflex relationship the most pronounced?
intermediate range about 70 to 160 mm Hg
51
What occurs if BP goes below the intermediate range of the baroreceptor reflex?
HR maintain a constant high value
52
What occurs if BP goes above the intermediate range of the baroreceptor reflex?
HR maintain a constant low value
53
What results when pressure is delivered to the carotid sinuses?
they are compressed, and transmural pressure decreases, leading to stimulating hypotensive stimulus and increased HR, MSNA, and BP
54
What results when suction is delivered to the carotid sinuses?
carotid sinuses are stretched, and transmural pressure increases, leading to a stimulated hypertensive stimulus and decreased HR, MSNA, and BP
55
How are alterations in HR achieved?
by reciprocal changes in vagal and sympathetic neural activity over an intermediate range of arterial processes (about 100 to 200 mmHg) - above range: increased HR by the intense sympathetic and virtual absence of vagal activity - below range: decreased HR by the intense vagal and constant low level of sympathetic activity
56
What is the Bainbridge reflex?
infusions of blood or saline (volume) accelerated HR - occurred independent of arterial BP
57
When did tachycardia occur during the Bainbridge reflex?
when central venous pressure rose sufficiently to distend the right side of the heart, abolished by the bilateral transection of vagi both atria have receptors that influence HR
58
Where are the receptors in the atria located that influence HR?
in venoatrial junctions right - with vena cava left - with pulmonary veins
59
What occurs with distension of atrial receptors?
sends impulse centrally to vagi efferent impulses are carried by fibres from both autonomic division to the SA node
60
What other reflexes does an increase in blood volume activate?
baroreflex also tends to change HR in opposite direction
61
What is the actual change in HR evoked by?
changes in blood volume, which is the result of antagonistic reflex effects
62
What mechanisms cause a increase in HR?
intravenous infusion -> increased right atrial pressure -> stimulate atrial receptors -> bainbridge reflex -> increase HR
63
What mechanisms cause a decrease in HR?
increased right atrial pressure -> increased cardiac output -> increased atrial pressure -> baroreceptor reflex -> decrease HR
64
What do infusions of blood cause?
Increase in HR and CO proportionally, SV remains the same
65
What does a reduction of blood volume cause?
decreased CO but increased HR
66
When does the Bainbridge reflex prevail?
when blood volume is raised causing an increase in HR and CO, but no change in SV
67
When does the baroreceptor reflex prevail?
when blood volume is diminished causing an increase in HR but decrease in CO and SV
68
explain the baroreflex loop
cardiac and systemic pressure sensors (baroreceptors) -> medullar CV control center (MCCC) -> sympathetic NS -> adrenal medulla -> epinephrine -> norepinephrine -> increase HR and SV -> increase CO -> parasympathetic NS -> Ach -> decrease HR -> decrease CO CO -> arterial pressure -> cardiac and systemic pressure sensors (baroreceptors)

KIN 426 (12 decks)

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