W4-L4: Heart Rate

Question 1

What is the normal heart rhythm?

Answer 1

• The inherent rhythm of the heart is 100b/min

Question 2

What is the role of the SA node and AV node?
(purkinje fibers)

Answer 2

The SA (sinoatrial) node generates an electrical signal that causes the upper heart chambers (atria) to contract.

The signal then passes through the AV (atrioventricular) node to the lower heart chambers (ventricles), causing them to contract, or pump.

The SA node is considered the pacemaker of the heart.

Question 3

Order of Cardiac Conduction (8)

Answer 3

Question 4

ECG Waves

What is the P wave?

Answer 4

depolarization of atria before atria contract

Question 5

What is the QRS complex?

Answer 5

Signals electrical changes from ventricular depolarization before
ventricles contract

• Atrial repolarization follows P wave, but produces a wave so small that QRS complex usually obscures it

Question 6

What is the T wave?

Answer 6

Represents ventricular repolarization that occurs during ventricular diastole

Question 7

Different Phases of the Normal ECG from Atrial Depolarization to Ventricular Repolarization

Answer 7

Question 8

explain

Answer 8

Parasympathetic
innervates atrium, SA and AV node

Sympathetic
innervates atrium and ventricle muscles
SA and AV nodes

Question 9

Stimulation of sympathetic cardioaccelerator nerves releases what?

What are chronotropic and inotropic

Answer 9

epinephrine and norepinephrine

• Cause chronotropic (heart beats faster) and inotropic (increase myocardial contractility) effects on the heart

Question 10

Sympathetic stimulation also produces generalized vasoconstriction everywhere, BUT where does it not?

Answer 10

Sympathetic stimulation also produces generalized vasoconstriction except in coronary arteries

• Norepinephrine, released by adrenergic fibers, acts as a vasoconstrictor
• Dilation of blood vessels under adrenergic influence occurs from decreased adrenergic activity

Question 11

Question 12

How does Parasympathetic Influence blood flow?

Answer 12

• Parasympathetic neurons release acetylcholine, which delays rate of sinus discharge to slow HR
• Bradycardia results from stimulation of vagus nerve from medullaʼs cardioinhibitory center
• Parasympathetic stimulation excites some tissues and inhibits others
• At start and during low/moderate intensity exercise, HR increases largely by inhibition of parasympathetic stimulation
• HR in strenuous exercise increases by additional parasympathetic inhibition and direct activation of sympathetic cardioaccelerator nerves

Question 13

What is Bradycardia?

Answer 13

Bradycardia results from stimulation of vagus nerve from medullaʼs cardioinhibitory center

Bradycardia is a condition characterized by a slower than normal heart rate, typically defined as fewer than 60 beats per minute in adults.

Question 14

Central Command: Input from Higher Centers

Answer 14

• Central command provides greatest control over HR during exercise
• Heart rapidly “turns on” during exercise by decreasing parasympathetic inhibitory input and increasing stimulating input from central command
• Central command in cardiovascular regulation explains how emotional state can affect cardiovascular response (thus, creating difficulty obtaining “true” resting values for HR and BP)

Question 15

Three mechanisms continually assess the nature and intensity
of exercise and muscle mass activated? What are they?

Answer 15

1. Reflex neural input from mechanical deformation of type III afferents within active muscles
2. Chemical stimulation of type IV afferents within active muscles
3. Feed-forward outflow from motor areas of central command

Type III and Type IV afferents are sensory nerve fibers that transmit information about muscle activity to the central nervous system:
- Type III Afferents: These are myelinated fibers that respond to mechanical changes such as muscle stretch and tension.
- Type IV Afferents: These are unmyelinated fibers that respond to chemical changes within the muscle, such as the presence of metabolic byproducts like lactic acid.

Question 16

What are baroreceptors?
What is the baroreceptor Reflex?
What are Cardiopulmonary receptors?

Answer 16

• Aortic arch and carotid sinus contain pressure-sensitive baroreceptors
• Negative feedback
• Cardiopulmonary receptors assess mechanical activity in the left ventricle, right atrium, and large veins

Question 17

What Physical Factors Affect Blood Flow?

Answer 17

• Friction between blood and internal vascular wall creates resistance (force) that impedes blood flow

Three factors determine resistance:
1. Blood thickness (viscosity)
2. Length of conducting tube
3. Blood vessel radius

Question 18

Answer 18

Key Points:
- Type IV Afferents: These sensory nerves are sensitive to chemical changes (e.g., increased metabolites) within muscles.
- Free-flow vs. Ischemia:
- In free-flow conditions, the BP increase is moderate and returns to baseline during recovery.
- Under ischemia, the accumulation of metabolic byproducts stimulates type IV afferents more intensely, causing a greater increase in BP.
- Clinical Relevance: The study highlights how sensory feedback from type IV afferents can significantly affect cardiovascular responses during exercise, especially under conditions of reduced blood flow.

Summary:
This classic study demonstrates that type IV afferents, when chemically stimulated by metabolic byproducts during ischemia, significantly augment the blood pressure response during exercise. This highlights the important role of these afferents in modulating cardiovascular responses to changes in muscle metabolism.