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Chapter 18 – The Heart

“Do you love me with all your heart?”

“My heart doesn’t love you at all. It’s a chunk of muscle that

pumps blood”

BI: Cardiac Muscle (Microscopic Anatomy)

Skeletal Muscle •  Striated •  Long, cylindrical •  Multinucleate •  Sliding Filament Model

Cardiac Muscle •  Striated •  Short, fat, branched out •  1-2 centrally located

nuclei •  Sliding Filament Model

Cardiac Muscle •  Sliding Filament Model –  Sarcomeres

•  A & I bands •  Z discs •  Myosin & actin •  T-tubules

Cardiac Muscle Skeletal Muscle •  Striated •  Long, cylindrical •  Multinucleate •  Sliding Filament Model

Cardiac Muscle •  Lots of mitochondria

(25-35% of cell volume)

•  Why?? – Need to be resistant

to fatigue

Cardiac Muscle Skeletal Muscle •  Striated •  Long, cylindrical •  Multinucleate •  Sliding Filament Model

Cardiac Muscle •  Endomysium –  Loose connective tissue

matrix –  Fills in intracellular

spaces –  Connected to fibrous

skeleton •  Allows it to be

tendon and insertion (pull or exert force)

Cardiac Muscle

Skeletal Muscle •  Fibers are structurally

and functionally separate

•  Move/contract independent of each other

•  You can delicately pick up something fragile, or crush something

Cardiac Muscle •  Fibers are physically and

electrically/functionally connected

•  Being linked allows the perfect timing needed for the heart to create and maintain pressure gradients so it can pump blood.

Remember, when we talk muscle fiber, we are talking about muscle cells

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Cardiac Muscle •  Physically connected – Intercalated discs – Desmosomes

Cardiac Muscle •  Physically connected – Intercalated discs – Desmosomes • Prevent physical

separation during contractions

Cardiac Muscle •  Electrically connected – Gap Junctions

Cardiac Muscle •  Electrically connected – Gap Junctions • Allows ions to pass from

cell to cell •  Ion =

current

Cardiac Muscle Ø “Functional Synctium” – Myocardium behaves as a

single coordinated unit

Ø WHY? – Reminder: Cardiac cells

need to be linked to have the perfect timing necessary to create and maintain high and low pressure gradients

BI: Contraction Skeletal Muscle •  Triggers: Neurons or

neighboring muscle cells

•  Resting potential (-70) •  Depolarization – Membrane potential

becomes less negative

•  Threshold potential (-55)

•  Triggers voltage gated channels

Cardiac Muscle •  Similar, but… •  It doesn’t need the

brain to tell it to contract!

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Autorhythmic/Pacemaker Cells •  Only in the heart •  Trigger their own depolarization – Allows heart to beat without the brain

telling it to – Keeps heart beating in correct rhythm

and coordination of chambers • Sets the rhythm •  Important for pressure!

Autorhythmic/Pacemaker Cells •  Skeletal muscle fibers have a steady/

stable resting membrane potential (-70mV) that they maintain via a Na+/K+ pump

•  A stimulus causes the Na+ channels to

open… Na+ passes in… membrane potential less negative… hits threshold (-55mV) and then voltage gated channels open

Skeltal Muscle Review

Autorhythymic/Pacemaker Cells

•  Cardiac pacemaker cells Have a changing/unstable resting potential…There are Na+ and K+ channels that allow Na+ to trickle in…The membrane potential slowly drifts toward the threshold (-40mV)

Cardiac Muscle Preview BI: Intrinsic Cardiac Conduction System

1.  Action potential initiation by autorhythmic (pacemaker) cells

2.  Sequence of excitation

Intrinsic Cardiac Conduction System 1.  Action potential initiation by autorhythmic

(pacemaker) cells

•  These cells have a changing/unstable resting potential

•  Na+ and K+ channels allow Na+ to trickle into the cell

•  Leak happens at a steady rate •  Membrane potential slowly drifts toward the

threshold (-55)

Intrinsic Cardiac Conduction System 1.  Action potential initiation by autorythmic

(pacemaker) cells

•  If an action potential comes along from another cell before leak hits threshold, that action potential “trumps” the leak

•  Therefore, the fastest leak controls the muscles

•  Autorythmic (pacemaker) cells at start of system have the leakiest membranes (fastest)

Don’t need to write this down…

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Sequence of Excitation 1.  Sinoatrial (SA) node –  R At wall, just below

superior vena cava entrance

–  Leakiest cells / reaches threshold first / sets pace for entire heart

–  Sends signal across atria <<Slight delay (0.1s) to allow atria to finish contraction; ensures all chambers don’t contract at same time>>

–  Atria contract, creates high atrial pressure

–  Blood moves into lower pressure ventricles

–  Sends signal across gap junctions to AV node

Sequence of Excitation 2.  Atrioventricular (AV)

node •  Interatrial septum

just above the tricuspid valve

3.  Atrioventricular bundle 4.  R/L bundle branches 5.  Purkinje Fibers –  Sequence of #3-5

causes ventricles to contract from the bottom up

–  Creates high pressure in ventricles

–  Blood moves into lower pressure pulmonary artery and aorta

Sequence of Excitation

We’re getting closer to the end.

Hang in there…

BI: Electrocardiography

Electrocardiogram (EKG or ECG): A graphic record of heart activity •  Detected by an electrocardiograph

Electrocardiography: ECG/EKG 3 distinguishable waves 1.  P Wave

–  Depolarization of SI Node through atria

–  0.08 sec, atria contract 0.1s after P begins

2.  QRS Complex –  Ventricular depolarization –  Precedes ventricular contraction

3.  T Wave –  Ventricular repolarization –  Atrial repolarization is masked by

QRS

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Take a deep breath…

WE ARE ALMOST DONE!

BI: Regulation of Heart Rate •  Autonomic Nervous System – Sympathetic Nervous System (SNS) •  Stimulation is activated by stress, anxiety,

excitement or exercise

– Parasympathetic Nervous System (PNS) •  Stimulation is mediated by acetylcholine and

opposes the SNS

– PNS dominates

•  Chemicals – Hormones –  Ions