A&P 2 Chapter 18: Heart (week 2)

chordae tendineae =

"heart strings"

when ventricles *contract* to eject blood, AV valves are

*forced to close*

heart depolarized and contracts without

*nervous system stimulation*

When ventricles *contract* to eject blood SL valves are forced

*to open*

Pacemaker cells

-1% of myocardium -noncontractile cells -spontaneously depolarize

Contractile Cells

-99% of myocardium -responsible for contraction

Ventricles contract:

-AV valves close ("Lub"), then -SL valves open Ventricles eject blood

Skeletal Muscle Different Properties (6):

-Abundant SR; -Has terminal cisterns -Ca+2 is stored in the SR -Cells are independent -No gap junctions; APs do not spread from cell to cell -Contractions range from fast twitches to slow sustained contractions (tetanus)

2. Atrioventricular (AV) node

-Impulses from SA node travel through atria to AV node -0.1 sec delay lets atria contract before ventricles -AV node as an inherent rate of 50 impulses/minute if input from SA node is absent

Intrinsic cardiac conduction system

-Network of noncontractile (pacemaker) cells -Initiate and distribute impulses to coordinate depolarization and contraction of heart

summary of SL valves opening and closing

-Open when ventricles contract -Close when ventricles relax

summary of AV valves opening and closing

-Open when ventricles relax -Close when ventricles contract

1. Sinoatrial (SA) node

-Pacemaker of the heart- depolarizes faster than rest of myocardium -Generates impulses about 75 times per minute (called sinus rhythm)

Ventricles relax:

-Semilunar (SL) valves close ("Dup") then -AV valves open Ventricles begin to fill again

Cardiac Muscle Different Properties:

-Sparse SR; No terminal cisterns -Requires Ca+2 stored in SR and extracellular Ca+2 -Cells interconnect at intercalated discs -Gap junctions allow APs to spread quickly from cell to cell -Slow rhythmic twitches (no tetanus)

ECG Step 3:

-Ventricular depolarization begins: causes *QRS complex* -ventricles contract -Atria repolarize and relax -no wave seen for atrial repolarization

Gap junction function?

-let ions pass from cell to cell -electrically couple cells -*let heart cells contract in unison*

excitation-contraction coupling is the connection between...

-the connection between a muscle cell firing an AP and contraction of the muscle cell -similar in skeletal and cardiac muscle

Takes about ______ sec from initiation at SA node to complete contraction

0.22 sec

Summary: Intrinsic Conduction System

1. *The sinoatrial (SA) node* (pacemaker) sets off APs -APs spread to both atria -Atria contract in unison 2. APs spread to *atrioventricular (AV) node*; slow down to let atria relax 3. APs spread to ventricles; Ventricles contract then relax Atria and ventricles are electrically insulated from each other; APs can only travel this path

Similarities of Skeletal and Cardiac Muscle (4)

1. Both are striated muscle due to the presence of sarcomeres 2. Muscle *action potential* sets off muscle *contraction* 3. Wave of depolarization travels down *T tubules*- triggers release of Ca+2 from sarcoplasmic reticulum (SR) 4. *Excitation-contraction coupling occurs*: Ca+2 binds to troponin: thin filaments slide and sarcomeres shorten

2 Function of Pacemaker cells:

1. Fire APs that spread through heart via gap junctions 2. Initiate depolarization of entire heart

Structures at intercalated discs?

1. Gap Junctions 2. Desmosomes

3 waves of ECG:

1. P wave 2. QRS complex 3. T wave

Coordinated heartbeat is a function of:

1. Presence of gap junctions 2. Intrinsic cardiac conduction system

Cardiac intrinsic conduction system:

1. SA node 2. AV node 3. AV bundle 4. R and L bundle branches 5. Subendocardial conducting network

Skeletal Muscle properties:

1. Striated 2. Voluntary 3. Long cylindrical fibers 4. Each fiber is independent 5. No gap junctions

Cell junctions list:

1. Tight Junctions 2. Desmosomes 3. Gap Junctions

2 kinds of myocytes:

1. contractile cells 2. pacemaker cells

Cardiac Muscle Properties (5):

1. striated 2. involuntary 3. short, fat branched cells 4. cells are highly interdependent 5. gap junctions present at intercalated discs

mitral or bicuspid valve has how many cusps?

2

Valve *closure* is associated with

2 heart sounds

Tricuspid valves has how many cusps?

3

SL valves have how many cusps? are they anchored?

3 cusps; do not need to be anchored

AV bundle and subendocardial conducting network depolarize ______ times/min in absence of AV node input

30 times/min

electrocardiogram (ECG) measures

APs sweeping across the whole heart

SL valves are under much less tension than

AV valves and do not need to be anchored

size of AV valves? why?

AV valves are large; allow ventricles to fill easily -slide 41

The heart valves are controlled by ventricular contraction and relaxation. Review which ventricular action allows the atrioventricular valves to open? Which ventricular action makes them close? Do the same for the semilunar valves.

AV valves opens when ventricles are relaxed and passively filling; they close when the ventricle contracts SL opens when the ventricles contract; SL close when the ventricles relax and pressure drops

ventricular contraction *forces*

AV valves to close

Ventricular Systole begins:

Atria have already contracted (atrial systole) to complete filling of the ventricles, then relax (atrial diastole)

ECG Step 1:

Atrial depolarization, initiated at SA node, causes *P wave*. Sets off atrial contraction (atrial systole)

Excitation-contraction coupling occurs:

Ca+2 binds to troponin

Tight Junction:

Impermeable junctions prevent molecules from passing through the intercellular space.

The complete cardiac cycle is from

P wave to the beginning of another P wave

When does atrial repolarization occur?

QRS, we don't see it happening on the graph

SL valves size compared to AV valves?

SL valves are smaller than AV valves

ventricular contraction forces

SL valves to open

What if SA node is damaged? Why might the ventricles continue to beat?

The atria will not contract but the ventricles will contract at a slower pace set by the AV node

ECG Step 4:

Ventricles remain depolarized and contracted(plateau phase of AP)

ECG Step 5:

Ventricular repolarization begins; causes *T wave*; ventricles relax

ECG Step 6:

Ventricular repolarization is complete Ventricles are relaxed

the heart contracts as

a unit called *functional syncytium*

Gap Junctions:

allow ions and small molecules to pass from one cell to the next for intercellular communication

each AV valve separates

an atrium from its ventricle

as the ventricles *relax* and pressure falls, the SL valves

are *allowed to close*

when ventricles are *relaxed* and filling, AV valves...

are allowed to *open*

P wave

atrial depolarization

the rhythm of the heart can be altered by the

autonomic nervous system

cardiac pacemakers cells are called __________ and spontaneously fire _________

autorhythmic, APs

AV valves prevent...

backflow of blood into the atria when the ventricles contract

Desmosomes:

bind adjacent cells together

Cardiac cycle is all the events associated with

blood flow through the heart that occur during a single heartbeat

cusps are anchored by what? why?

chordae tendineae and papillary muscles to withstand the force of contraction and prevent *eversion* of the valves

Lub (1st sound) =

closure of AV valves

Dup (2nd sound) =

closure of SL valves

4. Right and left bundle branches

conduct the impulses along *interventricular septum to apex*

pacemaker cells do not

contract

depolarization of contractile cells *sets off*

contraction

intrinsic conduction system pathway coordinates

contraction of heart chambers

ventricular relaxation and ____________ allows valves to _________

decrease in pressure, close

5. . Subendocardial conducting network (Purkinje fibers)

depolarizes contractile cells of both ventricles. Contraction proceeds from apex toward atria

relaxation of a heart chamber is called

diastole

Atriventricular valves separate what?

each atrium from its ventricle

semilunar (SL) valves separate

each ventricle from the artery into which it ejects

the heart contracts as a unit to ensure

effective pumping action

closure of SL valve prevents

ejected blood from re-entering the heart

SL prevents

ejected blood from returning to its ventricle

pacemaker cells spontaneously

fire action potentials

complete cardiac cycle

from p wave to p wave

contractile cells are connected to each other and to autorhythmic cells by

gap junctions

systole =

heart chamber contracts

diastole =

heart chamber relaxes

cardiac cycle:

heart sounds and valve closures

Contractile cardiac cells are the most of

heart, bulk of the heart

ECG Step 2:

impulse slows at AV node PR interval

SL valves are smaller because it

increases velocity of ejection of the blood

skeletal muscle fibers contract

independent of each other, contrary to cardiac muscle

Cardiac muscle cells interdigitate at

intercalated discs

pacemaker cells are localized along a pathway called the

intrinsic conduction system

when do the heart sounds "lub" and "dup" occur?

lub = QRS complex; atrial repolarization and ventricle depolarization dup = t wave; atrial repolarization and ventricle repolarization

It is called the mitral valve because it resembles a

miter?

self excitable cardiac muscles are called

myocytes

Pacemaker cells do not require

nerves firing, in contrast to skeletal muscle fibers that require stimulation by motor neuron

the heart valves ensure what?

one way blood flow

3. Atrioventricular (AV) bundle (bundle of His)

only electrical connection between atria and ventricles.

ventricular relaxed *allows* AV valves to

open

chordae tendineae are attached to what?

papillary muscles projecting from the walls of the ventricles

systole

period of contraction

diastole

period of relaxation

Heart valves open or close due to

pressure differences as *ventricles* contract or relax

Valves open or close due to

pressure differences as *ventricles* contract or relax

Function of desmosomes?

prevent the cardiac cells from separating as the heart beats

repolarization of contractile cells *allows*

relaxation

Some cardiac muscle cells are

self-excitable -(not seen in skeletal muscle)

Contractile cardiac cells are similar to what? why?

skeletal muscle cells, do not spontaneously fire APs

Review ECG summary

slide 90

what you should know

slide 91

Cardiac Cycle

slide 96-103

skeletal muscle fiber contract only when it is

stimulated by the nerve fiber that innervates it

ECG measures APs

sweeping across the whole heart

contraction of a heart chamber is called

systole

Both the atria and the ventricles go through

systole and diastole

both the ventricles and the atria go through

systole and diastole

Cusps (flaps) of the AV valves are anchored by

tendinous cords- chordae tendineae

These are *mechanical* events that are set off by

the *electrical* events (depolarization and repolarization) that immediatelyprecede them

why are they called semilunar?

they resemble half moons

what happens when Ca binds to troponin?

thin filaments slide and sarcomeres shorten

pacemaker cells spread thru the heart

to other pacemaker cells and contractile cells via *gap junctions*

all cardimycoytes contract in

unison or none contract

both atria contract in ________ then __________ as both ventricles contract

unison; relax

Each semilunar (SL) valve separates its

ventricle from the blood vessel (artery) into which it ejects

QRS complex

ventricular depolarization

T wave

ventricular repolarization