Control of heart rate: Role of SAN & AVN in the cardiac cycle, Parasympathetic & sympathetic nerves

Summary

In this educational video, Ms. Estrich explores the control of heart rate, detailing the role of the sinoatrial (SA) node, atrioventricular (AV) node, and conductive tissues in the cardiac cycle. She explains how the autonomic nervous system, particularly the sympathetic and parasympathetic branches, regulates heart rate in response to stimuli like blood pressure and pH changes. The video emphasizes the importance of these mechanisms for maintaining homeostasis and the significance of the slight delay between atrial and ventricular contractions for efficient blood pumping.

Takeaways

• 💓 The heart's rate is controlled by the nervous system, specifically the autonomic nervous system, which is responsible for involuntary actions.
• 🫀 Cardiac muscle is myogenic, meaning it can contract and relax without external stimulation, but the rate is modulated by the nervous system.
• 🔌 The sinoatrial node (SAN), also known as the pacemaker, initiates the cardiac cycle by releasing a wave of depolarization that causes the heart to contract.
• 🚦 The atrioventricular node (AVN) is situated between the atria and ventricles and is crucial for the transmission of electrical signals from the atria to the ventricles.
• 🛤️ The bundle of His and Purkinje fibers are conductive tissues that ensure the electrical signal travels efficiently through the heart, coordinating the contraction of the ventricles.
• 🕒 There is a slight delay between atrial and ventricular contractions, which allows the atria to fully contract and fill the ventricles with blood before the ventricles contract.
• 🧠 The medulla oblongata in the brain is the coordinator center for heart rate, receiving signals from various receptors and sending impulses through the sympathetic or parasympathetic nervous system.
• 🏃‍♂️ The sympathetic nervous system increases heart rate in response to stimuli such as stress or exercise, while the parasympathetic nervous system decreases it, promoting relaxation.
• 🩸 Changes in blood pressure and pH levels are detected by receptors and can trigger responses that adjust the heart rate to maintain homeostasis.
• ♻️ The heart's response to pH changes is crucial for removing acidic byproducts like carbon dioxide and lactic acid, ensuring the body's enzymes and proteins function properly.

Q & A

What is the primary function of the sinoatrial (SA) node, also known as the pacemaker?

The SA node's primary function is to generate electrical impulses in the form of depolarization waves, which initiate the contraction of cardiac muscles, thus controlling the heart rate.

How does the atrioventricular (AV) node contribute to the cardiac cycle?

The AV node receives the electrical impulse from the SA node and, after a slight delay, transmits it to the ventricles, ensuring that the atria contract and empty their blood into the ventricles before the ventricles contract.

What is the role of the bundle of His and the Purkinje fibers in the heart's conduction system?

The bundle of His and the Purkinje fibers are conductive tissues that transmit the electrical impulses from the AV node to the ventricles, ensuring a coordinated contraction of the heart's chambers.

Why is there a delay between atrial and ventricular contractions?

The delay allows the atria to fully contract and empty their blood into the ventricles before the ventricles contract, ensuring that the ventricles are filled to capacity before pumping blood out of the heart.

How does the autonomic nervous system regulate the heart rate?

The autonomic nervous system, through the sympathetic and parasympathetic branches, modulates the frequency of the depolarization waves released by the SA node, thereby increasing or decreasing the heart rate.

What is the role of the medulla oblongata in controlling the heart rate?

The medulla oblongata in the brain serves as the coordinator center for heart rate control, receiving input from various receptors and sending signals to the SA node via the autonomic nervous system.

How do changes in blood pH affect the heart rate?

A decrease in blood pH, indicating increased acidity, triggers chemoreceptors to send signals to the medulla oblongata, which in turn increases the heart rate via the sympathetic nervous system to facilitate the removal of acidic molecules.

What is the significance of the non-conductive tissue between the atria and ventricles?

The non-conductive tissue ensures that the electrical impulses from the SA node do not directly reach the ventricles, allowing for the necessary delay and coordination of atrial and ventricular contractions.

How does the heart respond to increased blood pressure?

In response to increased blood pressure, baroreceptors in the aorta and carotid artery send signals to the medulla oblongata, which then decreases the heart rate through the parasympathetic nervous system to lower blood pressure.

What is the purpose of the slight delay in the cardiac cycle between atrial and ventricular contractions?

The slight delay ensures that the atria have enough time to contract fully and transfer blood to the ventricles, allowing the ventricles to contract with maximum efficiency and pump blood effectively.

Outlines

00:00

🫀 Control of Heart Rate

In this segment, Ms. Estrich introduces the topic of heart rate control, emphasizing the cardiac muscle's myogenic nature, which allows it to contract and relax without external stimuli. The focus is on how the nervous system regulates the rate of these contractions and relaxations. Key structures in the heart, including the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His, and Purkinje fibers, are highlighted for their roles in the cardiac cycle. The SA node, or pacemaker, initiates the depolarization wave that causes the heart to contract. The AV node, bundle of His, and Purkinje fibers are crucial for the propagation of this depolarization wave through the heart, ensuring efficient blood pumping. The video also links to a previous lesson on the cardiac cycle for further understanding.

05:02

🔬 How the Heart Contracts

This paragraph delves into the mechanics of how the heart contracts, starting with the SA node's release of depolarization that triggers atrial systole. The AV node's subsequent depolarization wave is directed through the bundle of His and Purkinje fibers, causing the ventricles to contract from the apex upwards for optimal blood ejection. The paragraph also discusses the strategic delay caused by non-conductive tissue between the atria and ventricles, which allows the atria to fully contract and empty into the ventricles before the ventricles contract. The autonomic nervous system, particularly the medulla oblongata, is introduced as the regulator of the heart rate through the sympathetic and parasympathetic pathways, which can either increase or decrease the heart rate.

10:03

🌡️ Response to Blood Pressure and pH Changes

The third paragraph explores how the heart responds to changes in blood pressure and pH levels. It explains that increased blood pressure stretches pressure receptors, leading to a decrease in heart rate via the parasympathetic nervous system. Conversely, decreased blood pressure results in an increased heart rate through the sympathetic nervous system. The importance of maintaining proper pH levels is also discussed, with the heart rate increasing during high respiration to facilitate the removal of carbon dioxide and lactic acid, preventing enzyme and protein denaturation. The paragraph emphasizes the role of the autonomic nervous system in these responses, with the medulla oblongata as the central coordinator.

15:05

🏃‍♂️ The Role of Nervous System in Heart Rate Regulation

The final paragraph provides a detailed look at how the nervous system, specifically the sympathetic and parasympathetic branches, regulates the heart rate in response to stimuli such as blood pressure and pH changes. It outlines the process from the detection of stimuli by receptors, the coordination by the medulla oblongata, to the effector response in the cardiac muscle. The paragraph clarifies the importance of understanding the increase in electrical impulses and the specific nervous system pathways involved in these responses. It also addresses common pitfalls in understanding this process, such as the need to specify the increase in impulses and the involvement of either the sympathetic or parasympathetic nervous system.

Keywords

💡Cardiac Cycle

The cardiac cycle refers to the sequence of mechanical and electrical changes that occur in the heart as it contracts and relaxes. In the video, it is mentioned that the control of the heart rate is linked to the stages of the cardiac cycle, which includes atrial systole, ventricular systole, and diastole. The video explains how different structures within the heart control the timing of these stages.

💡Myogenic

Myogenic is a term used to describe the ability of cardiac muscle to contract and relax without the need for external stimulation. The video emphasizes that while the heart can initiate contractions autonomously, the rate of these contractions is regulated by the nervous system, which is a key point in understanding how the heart rate is controlled.

💡Pacemaker

The pacemaker, also known as the sinoatrial (SA) node, is a group of specialized cells located in the right atrium of the heart. As explained in the video, it is responsible for generating electrical impulses that spread across the atria, triggering atrial systole. The pacemaker's role is crucial in initiating the cardiac cycle.

💡Atrial Systole

Atrial systole is the phase of the cardiac cycle where the atria contract to push blood into the ventricles. The video script describes how the SA node's depolarization wave causes both atria to contract, illustrating the process of atrial systole and its significance in the overall cardiac function.

💡Ventricular Systole

Ventricular systole is the contraction of the ventricles, which pumps blood out of the heart to the lungs and the rest of the body. The video explains that after atrial systole, the atrioventricular (AV) node releases a depolarization wave that travels through the bundle of His and Purkinje fibers, leading to the contraction of the ventricles.

💡Diastole

Diastole is the period of relaxation between cardiac contractions, during which the ventricles and atria fill with blood. The video script mentions diastole as a critical phase in the cardiac cycle, ensuring that the heart can replenish its blood supply before the next contraction.

💡Bundle of His

The bundle of His is a bundle of specialized conductive tissue that carries the electrical impulses from the AV node down the septum of the heart and into the ventricles. The video highlights the importance of the bundle of His in ensuring that the electrical signals reach the ventricles, which is essential for the proper coordination of the cardiac cycle.

💡Purkinje Fibers

Purkinje fibers are conductive tissues that spread throughout the walls of the ventricles, ensuring that the depolarization wave reaches all parts of the ventricles for a coordinated contraction. The video script uses the analogy of squeezing toothpaste from a tube to explain how the contraction of the ventricles, starting from the apex and moving upwards, helps to efficiently pump blood out of the heart.

💡Autonomic Nervous System

The autonomic nervous system is a part of the nervous system that controls involuntary actions, such as the regulation of the heart rate. The video discusses how the autonomic nervous system, through the sympathetic and parasympathetic branches, controls the rate at which the SA node releases depolarization waves, thereby controlling the heart rate.

💡Medulla Oblongata

The medulla oblongata is a part of the brainstem that serves as the coordinator center for the autonomic nervous system's control of the heart rate. The video script explains that the medulla oblongata receives signals from various receptors and sends impulses along the sympathetic or parasympathetic nervous system to the heart, thereby regulating the heart rate in response to changes in blood pressure or pH.

Highlights

Introduction to the control of heart rate and its relation to the cardiac cycle.

Explanation of cardiac muscle's myogenic nature and its regulation by the nervous system.

Role of the sinoatrial (SA) node as the heart's natural pacemaker.

Function of the atrioventricular (AV) node in the cardiac conduction system.

Description of the Bundle of His and Purkinje fibers in electrical conduction within the heart.

Process of atrial systole triggered by the SA node's depolarization wave.

Sequence of ventricular systole following atrial systole due to the AV node's depolarization.

Importance of the non-conductive tissue between atria and ventricles for proper cardiac function.

Mechanism of the heart's squeezing action for efficient blood expulsion.

Influence of the autonomic nervous system on the heart rate via the SA node.

Role of the medulla oblongata in coordinating heart rate control.

Sympathetic nervous system's effect on increasing heart rate.

Parasympathetic nervous system's role in decreasing heart rate.

Link between heart rate control and homeostasis, particularly in response to blood pH changes.

Response of the heart to blood pressure changes as a mechanism for maintaining homeostasis.

Importance of the timing delay between atrial and ventricular contraction for efficient blood pumping.

Detailed explanation of how changes in blood pressure and pH are detected and responded to by the heart.

Summary of the key components involved in the heart rate control process, including receptors, coordinator, and effectors.

Transcripts

hi everyone and welcome to learn a level

biology for free with ms estrich

in this video i'm going to go through

the control of the heart rate

so make sure you are subscribed if you

aren't already to keep up to date on all

the videos

so control of the heart first of all

links to

the control of the stages in the cardiac

cycle which you learnt

in year 12. so you would have already

known that cardiac muscle is myogenic

meaning it can contract and relax on its

own accord it doesn't require a stimulus

however the rate at which it contracts

and relaxes

is controlled by the nervous system so

we'll be coming on to that

and what we're going to be looking at is

how

we have the different stages

that the different chains of the heart

contract

in this cycle is controlled

so what is it that triggers atrial

systole

so the contracting of the atria

ventricular systole or systole

which is the contracting of the

ventricles and then we have a diastole

or distally

and this is when ventricles and atria

both

relax and if you can't quite remember

this i'll link

up here my video to the cardiac cycle

so you can revise that first so it's all

to do with

key structures in the heart that you

haven't actually learned about yet

so the first one is what we call the

cyanoatrio

node and this is located just here

in the right atrium and it's also known

as the

pacemaker and it's a group of cells or

in other words a tissue in the right

atrium

which can release a wave of electricity

which we call

depolarization and when electricity or

depolarization

hits cardiac muscle it causes it to

contract

so that's why it's called the pacemaker

because the san

is the cell that can release this wave

of depolarization

and start off the muscle contractions

there's also the avn which is the

atrioventricular

node and this is located in between the

atria

and the ventricles as the name suggests

and it's within the border of the left

and the right hand side

so although i've drawn it here it might

actually be slightly further over

within this diagram

you then have your bundle of hiss and

these

are conductive tissues

which run down the septum of the heart

and up the walls of the two

ventricles and lastly the perkin

fibers which you might see written as

purkinje

fibers in some textbooks aqhas tend to

use the phrase pertinent so perkline

fibers

and these are conductive tissues that go

all the way through the walls of the

ventricles

so those are the key tissues involved

but how they actually control

the cardiac cycle is what we'll have a

look at so step one

the san releases a wave of

depolarization

across the two atria and that is what

causes atrial systole

both atria will contract

the next step is the avn releases

another wave of depolarization

now this doesn't go directly down into

the ventricles

because there's actually a layer of

non-conductive tissue that separates

the atria and the ventricles and it's

not one that you can

see it's just the layer of tissues that

separates those

top and bottom chambers is insulating

and that's why the bundle of hiss is

important

because as the avm releases this second

wave

of depolarization the depolarization

wave cannot go directly downwards

so it has to go through this conductive

tissue

in the septum which is what the bundle

of hiss is

so it travels down ception and the

bundle of hiss then moves

up the outer walls of the ventricle

and that can then pass the wave

depolarization

finally through the perkline fibers

which are then going to branch into all

of the walls of the ventricle

so as a result what that then means is

you'll actually have in your ventricles

it's the apex of the heart

of the ventricles which will contract

first and apex just means the tip

because it has to travel down the bundle

of hiss

so you'll have some contraction in the

septum but

then the apex and these outer walls

contract first

and then the purple fibers called the

wrap cause the rest of the walls

to contract and that's really useful

because if you think about getting

toothpaste

out of a toothpaste tube

to get the maximum amount of toothpaste

out you should squeeze from the bottom

and push all the way up if you squeeze

in the middle or at the top

you're not going to get as much

toothpaste and that's exactly the same

with the heart

squeezing at the bottom and then moving

that contraction all the way up

forces out the maximum amount of blood

from the heart

now you could also be asked why is it an

advantage

that we have this non-conductive tissue

which actually causes a very slight

delay

between the atria contracting

and the time it takes for the

depolarization to get to the ventricles

and therefore the ventricles contracting

and the reason for that is it allows

enough

time for the atria to fully contract

pump all of the blood in the ventricles

so they are then

full before they contract

so it's only a very slight delay it's

milliseconds but that is all it takes

to give enough time for the right atria

to fully empty

so that is what controls the cardiac

cycle

but how quickly that san

releases that wave of depolarization

is controlled by the nervous system and

it's a part of the nervous system called

the autonomic

nervous system and what that means is

it's automatic

it's subconscious you do not think about

making it happen it will automatically

happen

and the coordinator center is the

medulla oblongata

in the brain so that is the part of the

brain which controls the heart rate

and we can see here in the picture where

the medulla oblongata is

and there are nerves

connecting to directly to the san

in the heart and what that means

is it's able to control how quickly the

wave of depolarization is released from

the san and there's two different

routes that can be taken depending on

whether you want the san to be releasing

ways of depolarization more rapidly to

increase the heart rate

or more slowly to decrease the heart

rate

so the sympathetic nervous system

or the sympathetic nerve we can see here

is shown

in purple and any impulses sent down the

sympathetic nervous system

will actually have the effect of

increasing

the heart rate you also have the

parasympathetic nervous system

and that has the opposite effect any

impulses sent down their parasympathetic

nervous system

will trigger the san to release the wave

of depolarization

more slowly and therefore it decreases

the heart rate

so we'll have a look at a few examples

of how this links to homeostasis

and the two key examples you need to

know about are how the heart responds to

changes in ph

of the blood and your blood pressure so

those are the two stimuli

which will then trigger whether the

impulse goes down the parasympathetic

or the sympathetic nerve system so this

is to do with nervous response so you

still need to think about

how you have your stimulus which we have

here

stimuli are detected by receptors and

if it's to do the ph of the blood it's a

chemical so it's a chemical receptor

or chemoreceptor if it's to do with the

blood pressure it's a pressure receptor

also known as a barrow receptor the

location of these two receptors is the

same

they're found in the aorta but also the

carotid artery

and that is the artery which branches

out of the aorta

to the rest of the body so just in

general what might

cause these changes or these stimuli

and why we have to respond so changes in

pressure

that could be due to stress anxiety diet

genetics

if your blood pressure is too high it

can cause damage to the linings of the

walls of the arteries

that can lead to blood clots and then

potentially heart attack or stroke

so it's very important that these

mechanisms

are put in place to reduce the blood

pressure

if the blood pressure is too low that

might mean that this insufficient supply

of oxygenated blood to aspiring cells

but also insufficient removal of waste

and that could then build up toxins

so response to ph this links to the idea

of

enzymes so the ph of your blood will

decrease

during times of high respiration so

for example exercise and that's because

carbon dioxide

is a product of aerobic respiration

but also lactic acid is a product of

anaerobic

respiration so these two

acids can build up in the blood

quite rapidly if the heart rate doesn't

increase

to get the blood to the lungs for the

carbon dioxide to be removed

or to get the blood to the liver to get

the lactic acid broken down

and if those acids aren't removed

rapidly

enzymes could denature and proteins

within the blood can do nature like

hemoglobin

so it's very important that the response

is to increase the heart rate

to be able to remove those acidic

molecules

so again this is um much like the

synapses topic i did it's another

long answer quite text heavy bit of

theory

where you could be asked for a four or

five mark question

to go through the whole process

so i've split it into the flow diagram

you might be more familiar with the idea

of a stimulus

detected by a receptor what is the

coordinator

that coordinates the response the

effector

that actually will implement the

response

and what is the response so you can then

see

where the marks are broken down so the

first thing i'm looking at first example

is an increase in pressure

so the receptors you'd get a mark for

pointing out would be the pressure

receptors or

barrow receptors and for pointing out

there in the walls of the arteries of

the

aorta and the carotid artery

and what happens is if the blood

pressure is too

high that actually stretches the blood

vessels

and that in turn stretches the pressure

receptors

and that is what then triggers the

action potential along the sensory

neuron

so the coordinate sensor center is in

the brain

that would be your medulla oblongata in

the brain

and what happens is more impulses

are sent to the medulla oblongata and

then you'll have more impulses sent

along the

parasympathetic nervous system to the

san

which will then decrease the frequency

of electrical impulses

and that then means that we'd have

the effector is the cardiac muscle the

san tissues as well

releasing fewer ways to depolarization

and the response is the reduced

heart rate so i've just noticed

throughout there are a few typos here so

sorry about that but hopefully listening

at the same time

you've spotted what the correct version

should be

um what i did want to point out is the

most common reason

people miss marks is in this section

here

you need to be pointing out that it's

more electrical impulses go to the

medulla oblongata

more impulses are going along the

parasympathetic nervous system

so you get more impulses at the sam

because you will continually have

impulses

but you'll only get a change in response

if you have

more impulses so you would have to have

them more in your answer

now decrease pressure same idea but

opposite so still the same receptors but

this time if the blood pressure is lower

they're not being stretched

you still get more electrical impulses

since the medulla oblongata

but this time it will trigger more

impulses along

the sympathetic nervous system so more

impulses reach the san

and this is where our effector is it's

the cardiac muscle

in particular the san tissues within

that cardiac muscle

and because they're now receiving more

impulses from the sympathetic nervous

system

the response is an increase in heart

rate

so lastly we'll have a look at one

example of ph

so decrease in ph so that would mean

it's becoming more acidic

so there must be more carbon dioxide or

lactic acid

this is detected by chemoreceptors

in the wall of the aorta and the carotid

artery

so what will then happen is more

electrical impulses

are sent to the medulla oblongata again

you have to have that

more and then more impulses are

sent via the sympathetic nervous system

to the san

and that will increase the frequency of

electrical impulses

so the effector is still the cardiac

muscle in particular the san tissues

because they're receiving more impulses

from the sympathetic nervous system

it's going to fire that wave of

depolarization more frequently

the response then is increase heart rate

to deliver the blood to the lungs

to remove the carbon dioxide more

rapidly

so the key here is more electrical

impulses

and making sure you're stating whether

it's the sympathetic

or parasympathetic nervous system

so that is it for controlling the heart

rate

[Music]

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