4/25/16 1 Chapter 18 – The Heart “Do you love me wiayour heart?” “My heart doesn’t love you at a. It’s a chunk of muscle at 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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Chapter 18 – The Heart (Microscopic Anatomy) 18 – The Heart “Do you love me wi" a# your heart?” “My heart doesn’t love you at a#. ... • Leak happens at a steady rate
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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
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>>