Cardiac Muscle & Cycle

4 Characteristics of Cardiac Muscle Cell Fibers

• Striated

• Involuntary

• Have Branches

• Interconnected by intercalated discs

What are the 2 cell junctions in Cardiac Muscle Fibers?

• Desmosomes- anchoring junctions

• Gap Junctions- communicating junctions

__ Junctions allow the heart to act as a __

Gap junctions allow heart to act like a syncytium=coordinated unit

Contractile Fibers

• % of all cardiac muscle tissue

• shorten or not

• membrane potential

• depolarization

Contractile fibers-

• 99% of cardiac muscle tissue

• can shorten

• stable membrane potential →

• cannot self depolarize or start own ap signals

Autorhythmic Fibers

• % of all cardiac muscle tissue

• shorten or not

• membrane potential

• depolarization

Autorhythmic fibers-

• 1% of cardiac muscle tissue

• cannot shorten

• unstable membrane potential →

• can self depolarize & starts own ap signals

The Cardiac Conduction System is made up of what fibers?

Autorhythmic Fibers

What are the 2 functions of the Cardiac Conduction System

• to initiate & transmit electrical impulses in heart

• cause rhythmic contractions

What 5 structures does the Cardiac Conduction Sys consist of?

1. Sinoatrial node

2. Atrioventricular Node

3. Atrioventricular Bundle

4. Left & Right Bundle Branch

5. Purkinje Fibers

What is a sinus rhythm & what induces it?

• normal heartbeat rhythm

• induced by the Sinotrial node (pacemaker)

Definition of Atrioventricular/AV Bundle

• only electrical connection between the atria & ventricles

What do the Purkinje fibers do?

• release AP to 2 ventricles through electrical impulses

What are the 3 major steps of Contractile Cardiac Muscle Cells?

1. Autorhythmic fibers start and conduct AP through heart 

2. Contractile fibers respond to those autorhythmic fiber APs

3. Contractile fibers then reach threshold and generate APs

3 Contractile Cardiac Muscle Cell Phases

1. Depolarization

2. Plateau phase

3. Repolarization

Depolarization

• AP + or -

• Ion

• Muscle action

• AP positive

• Na+ in

• Muscle contracts

Plateau Phase

• Ion

• Ap + or -

• Muscle action

• Ca+ in

• AP Stays + & Depolarized

• Muscles stay contracted

Repolarization

• AP + or -

• ion

• muscle action

• AP turns negative

• K+ out cell

• Muscle relaxes

Electrocardiogram (EKG/ECG)

tracor used to record electrical activity (all APs)

What are the 3 waves the ECG traces & what happens in each?

• P wave- 2 atria depolarize

• QRS wave- 2 ventricles depolarize

• T wave- 2 ventricles repolarize

What 3 segments (time span intervals) do ECGs trace

• PQ interval- start of P wave to start of QRS complex

• QT interval- start of QRS to end of T wave

• ST interval- end of QRS to start fo T wave

Which interval stays depolarized

The ST segment interval

Systole vs Diastole

• Systole- contraction caused by depolarization

• Diastole- relaxation caused by repolarization

What defines a Cardiac Cycle?

It is a 1 heartbeat event

• What is the total length of the cardiac cycle &

• how many periods does it have in one heartbeat?

• .8 seconds

• 3 periods in a heartbeat

Atrial Systole

• time length

• atria

• ventricles

• 0.1 sec

• atria- systole

• ventricles- diastole

Ventricular Systole

• time length

• atria

• ventricles

• 0.3 sec

• atria- diastole

• ventricles-systole

Relaxation Systole

• time length

• atria

• ventricles

• 0.4 sec

• atria- diastole

• ventricles- diastole

4 steps in Atrial Systole

• starts when both atria contract after P wave

• AV valves open,

• SL valves close

• Ventricles are filled with blood

5 steps in Ventricular Systole

• starts when both atria contract after QRS wave

• AV valves close (1st heart sound!)

• Isometric Contraction

• SL valves open

• Blood pumped out of ventricles into aorta & pulmonary arteries

5 steps in Relaxation:

• both ventricles relax after T wave

• SL valves close (2nd heart sound!)

• Isometric Relaxation

• AV valves open in middle of period

• Blood fills ventricles

Isometric Contraction:

• when does it occur

• describe

• result

• occurs in Ventricular Systole

• it is the mini period where all 4 valves are closed

• result- unchanging blood volume =contraction

Define Stroke Volume

\amount of blood pumped out of each ventricle per heartbeat

Define Cardiac Output

Cardiac Output- amount of blood pumped out of each ventricle per minute

What is the equation for cardiac output & ex

• CO= heart rate x stroke volume

  • 75 bpm x 70 ml= 5250 ml or 5l of blood pumped per min

What are the 2 parts of the Autonomic System’s effects on heart rate?

• Parasympathetic- lowers HR to resting

• Sympathetic-

  • increases HR to stimulate adrenal medulla

    to release epinephrine & norepinephrine

What is the value for resting cardiac output & what increases it?

• 5L per min

• exercise (increases bpm)

3 factors that affect stroke volume

• Afterload

• Preload

• Contractility

Preload:

• meaning

• if increase, what results

• degree of stretch of heart muscle

• ↑ Preload= ↑ SV ↑ CO 

Contractility

• meaning

• if increases what results

• ability of heart to contract (independent of preload)

• ↑ Contractility= ↑ SV ↑ CO 

Afterload

• meaning

• if increases what results

• back pressure exerted on SL valves by arterial blood

• ↑ Afterload= ↓SV ↓CO

Where is the Cardiovascular Center?

The Medulla Oblongata