MMED1005 Lecture Notes - Lecture 21: Vagus Nerve, Medulla Oblongata, Depolarization

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7 Jun 2018

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Cardiovascular system lecture 2: how does the heart work?

Learning objectives:

- Describe the structural and functional properties of cardiac muscle

- Explain the conduction system of the heart

- Describe the movement of blood during the cardiac cycle

- Define cardiac output

- Explain the intrinsic and extrinsic factors involved in the regulation of heart rate and

stroke volume

Main points:

- Cardiac muscle cells contain sarcomeres that are responsible for muscle contraction

- Blood flow through heart is determined by pressures in chambers and presence of

unidirectional valves

- The frequency of pacemaker action potentials drives heart rate which can be

increased by sympathetic stimulation and decreased by parasympathetic stimulation

- An increase in venous return will increase stroke volume. Stroke volume can also be

increased by stimulation of the SNS which increases force of contraction

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Muscle cells: also known as myocytes

- Found in 3 places:

o Attached to the skeleton (skeletal)

o Hollow organs (smooth)

▪ Stomach, bronchi, blood vessels

o Heart (cardiac)

Skeletal muscle:

- Voluntary muscles (innervated by the somatic nervous system)

- Striated (due to overlapping thick and thin filament)

- Multinucleated

- Non-branched

- Primary role: movement of the body

Smooth muscle:

- Involuntary muscle (innervated by the ANS autonomic nervous system)

- Non-striated

- Single nucleus

- Tapered

- Primary role: move contents within a hollow organ (blood vessel, intestine)

Cardiac muscle:

- Involuntary muscle (ANS)

- Striated

- Single nucleus

- Branched

- Intercalated disks – makes these cells unique from other muscle cells

- Primary role: pump blood out of the heart

- ***remember from the previous lecture that the myocardium (thickest, middle layer

of the heart) is comprised of muscle fibres

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Inside the muscle cell – definitions: (sarco = flesh)

- sarcolemma: muscle cell membrane

- sarcoplasm: cytoplasm of a muscle cell

- sarcoplasmic reticulum: specialised endoplasmic reticulum of muscle cells

- sarcomere: structural unit of a myofibril

- transverse (T-) tubule: plasma membrane extends deep into the cell

Inside the muscle cell: myofibrils

- made up of myofibrils (intracellular structure)

- each myofibril contains repeated sections of sarcomere

- each sarcomere contains actin and myosin (proteins) that

slide past each other when the muscles contract and relax

Cardiac muscle:

- connected by intercalated discs

- intercalated discs have two components that form a

syncytium

o desmosomes

▪ strong mechanical connections that tie

adjacent cells together

o gap junctions provide ionic and electrical

continuity between adjacent cells

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Document Summary

Describe the structural and functional properties of cardiac muscle. Explain the conduction system of the heart. Describe the movement of blood during the cardiac cycle. Explain the intrinsic and extrinsic factors involved in the regulation of heart rate and stroke volume. Cardiac muscle cells contain sarcomeres that are responsible for muscle contraction. Blood flow through heart is determined by pressures in chambers and presence of unidirectional valves. The frequency of pacemaker action potentials drives heart rate which can be increased by sympathetic stimulation and decreased by parasympathetic stimulation. An increase in venous return will increase stroke volume. Stroke volume can also be increased by stimulation of the sns which increases force of contraction. Found in 3 places: attached to the skeleton (skeletal, hollow organs (smooth, stomach, bronchi, blood vessels, heart (cardiac) Voluntary muscles (innervated by the somatic nervous system) Striated (due to overlapping thick and thin filament) Involuntary muscle (innervated by the ans autonomic nervous system)

Related Questions

Question 1 (1 point)Save Question 1Save
Which of the following will increase blood flow to a given regionof the body?

A smaller pressure gradient between the arteries and thatregion.

Vasoconstriction of the arterioles feeding that region.

An increase in blood viscosity via dehydration.

An increase in diameter of the arterioles feeding the region.
Question 2 (1 point)Save Question 2Save
Which of the following statements pertaining to the innervation ofthe heart is TRUE?

The heart is directly activated by the somatic nervoussystem.

The heart muscle is directly activated by the autonomic nervoussystem.

NE released by sympathetic neurons binds to beta-adrenergicreceptors and increases heart rate.

ACh released by parasympathetic neurons binds to nicotinicreceptors and decreases heart rate.
Question 3 (1 point)Save Question 3Save
The prolonged depolarization in cardiac myocytes is due primarilyto which channel type?

Voltage-gated sodium channels

Voltage-gated potassium channels

Funny (F-type) sodium channels

Long-lasting (L-type) calcium channels
Question 4 (1 point)Save Question 4Save
Which of the following statements about the pacemaker potentialgenerated by the SA node is FALSE?

Its intrinsic rate is about 70 depolarizations per minute.

The rate of depolarization can be affected by autonomicinnervation.

Funny (F-type) sodium channels play a role in the gradualdepolarization.

In the complete absence of extrinsic input (hormones or neurons),the SA node will continue to depolarize
Question 5 (1 point)Save Question 5Save
Match the description in the left hand column with the terms in theright hand column that BEST correspond (one match per term, allselections are used).

The period of the cardiac cycle when the heart is contracting

The period during diastole when the pressure in the atria exceedsthe pressure in the ventricles.

The period during systole when all heart valves are closed.

The difference between end-diastolic volume and end-systolicvolume.

The period of the cardiac cycle when the heart is relaxing.

1.
Systole
2.
Diastole
3.
Stroke volume
4.
Isovolumetric contraction
5.
Ventricular filling

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c) How going from laying to standing affects arterial blood pressure in the head region?

When we are laying down the Mean Arterial Pressure (MAP) is at the same level of the heart which is about 100 mm Hg. When we try to stand up from the lying position, our head is placed above our heart and gravity starts to pull the blood back downwards to our heart and feet. Return of venous blood is reduced and this leads to a decrease in cardiac stroke volume and decline in mean arterial pressure which is reduced to 60-70 mm Hg. This decreases the flow of blood to head. In response to this reduction in arterial blood pressure in the brain, parasympathetic nervous system is withdrawn and sympathetic nervous system is activated to pump more blood to head region via increased heart rates.

d) How the baroreceptors reflex explains the higher HRs during standing

When a person stands up, baroreceptor reflexes are rapidly activated to restore arterial pressure so that mean arterial pressure normally is not reduced by more than a few mmHg when a person is standing compared to lying down. However, in order to maintain this normal mean arterial pressure, the person who is standing upright has increased systemic vascular resistance (sympathetic mediated), decreased venous compliance (due to sympathetic activation of veins), decreased stroke volume (due to decreased preload), and increased heart rate (baroreceptor-mediated tachycardia).

1. Fill in the words or phrases that best complete each sentence. Be as specific as possible.

a. Erythrocytes contain the enzyme -------- , which catalyzes the conversion of metabolically produced CO₂ and water into --------- .

b. Most old erythrocytes are removed from circulation and destroyed by cells called ------------- , as they rupture passing through the narrow capillaries of the organ called the --------- .

c. Undifferentiated cells called------------ reside in the bone marrow, where they continuously divide and-------------- to give rise to each of the types of blood cells.

d. The process of leukocytes squeezing through the capillary ----------- to exit the vasculature is known as ----------- . Once they leave the bloodstream to fight a pathological condition, they never return.

e. The genetically-determined glycoprotein and glycolipid antigens found on the surface of an erythrocyte are called-------------- , and a person with agglutinins that react with type A and blood type B has type------------- blood.

f. During embryonic development, ninety-nine percent of the cardiac fibers are specialized for the function called----------- , whereas the remainder is specialized for-------------- .

g. The action potential delay at the-------------------- ensures that atrial excitation and contraction are complete before------------------ excitation and contraction commence.

h. The--------------------- is the volume of blood in the ventricle after ejection has been completed. An increase of this volume occurs when the stroke volume is .

i. The three cations, K⁺, Ca²⁺, and Na⁺ have an important effect of heart function. Increased blood levels of Na⁺ blocks----------------- inflow and results in a decrease in the force of contraction, while an excess of---------------- blocks the generation of action potentials.

j. An increase in parasympathetic activity ---------------- has the following effect on stroke volume: ---------------- . An increase in parasympathetic activity --------------- atrial contractility.

MMED1005 Lecture Notes - Lecture 21: Vagus Nerve, Medulla Oblongata, Depolarization

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