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4/25/16
1
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