ANAT 335 Exam 3

Myocardium

cardiac muscle cells

Cellular structure of cardiac muscle

• striations from regular arrangements of myofibrils (sacromeres)

• branched at ends usually

• intercalated disks: provide mechanical and electrical connections between cardiac muscle cells

• desmosomes

• gap junctions

• cardiac muscle is a functional syncytium (many cells acting as one)

Cardiac desmosomes

at intercalated disks and provide structural support (spot welds)

Cardiac gap junctions

at intercalated disk and have protein channels linking the cytosols of adjacent cells; small molecules like ions can quickly pass from cell to cell

Excitation-Contraction (EC) coupling in cardiac muscle

1. Excitation: electrical signal (action potentials)

2. EC coupling: chemical signals (intracellular Ca2+ release or Ca2+ transient)

3. Contraction: mechanical signal (contraction)

Excitation= action potential in a ventricular cardiac cell

• very negative resting membrane potential (-89mV) due to lots of K+ leak

• current enters from neighboring cell through GAP junctions (these replace need for graded potentials)

• rapid opening of voltage gated Na+ channels is responsible for the rapid depolarization phase

• prolonged plateau of depolarization is due to slow, prolonged opening of voltage-gated Ca2+ channels (L-TYPE CHANNELS)

• slower opening of voltage-gated K+ channels is responsible for the repolarization phase

• long duration of action potential due to L-type channels

L-type channels

• L=long lasting

• slow prolonged opening of voltage gated Ca2+ channels and same thing as DHPRs in t-tubule

• cause plateau of depolarization

Steps of excitation

1. The membrane is depolarized by Na+ entry as an action potential begins= excitation, no neuronal input is required

2. Depolarization opens L-type Ca2+ channels in T-tubules

3. small amount of “trigger” Ca2+ enter the cytosol, contributing to cell depolarization. That trigger Ca2+ binds to and opens ryanodine receptor Ca2+ channels in sarcoplasmic reticulum membrane

4. Ca2+ flows into the cytosol, increasing Ca2+ concentration

5. Binding of Ca2+ to troponin exposes cross bridge binding sites on thin filaments

6. cross bridge cycling causes force generation and sliding of thick and thin filaments (contraction)

7. Ca2+-ATPase pumps return Ca2+ to the sarcoplssmic reticulum

8. CA ATPase pumps and Na/Ca exchangers remove Ca from the cell

9. The membrane is repolarized when K+ exits to end the action potential

Timing of action potentials in cardiac muscle

• prolonged refractory period prevents tetanus of cardiac muscle

• can not summate force in cardiac muscle, this allows time for ventricles to relax and fill with blood prior to the next heartbeat

Summary of muscle types table

Cardiovascular system attributes

• role in homeostasis is the main transport system

• we need circulatory system because diffusion of solutes 100 micrometers or more would be too slow and inefficient

• has three parts: heart, blood vessels, and blood

Heart

the biological pump; generates force to move blood; 2 parts (electrical and mechanical)

Blood

the fluid connective tissue through which O2/CO2/wastes/ nutrients and messengers like hormones are transported

Blood vessels

the “tubing” through which blood flows; they play and active role in the movement of blood

Parts of blood

• total blood volume ~5.5L

• Plasma is 55% (~3.0L) part of ECF, least dense “lightest”

• “Buffy coat”: has WBC and platelets; insignificant volume, middle of centrifudge

• RBC/hematocrit is ~45% (2.5L). Mainly involved in gas transport. Heaviest layer

Erythrocytes (RBC)

• biconcave disks

• large surface area for smaller volume

• 7 micrometer in diameter

• carries hemoglobin

• organelles are extruded (no DNA, DNA in buffy coat)

Blood differentiation

1. Reticulocyte differentiates into RBC for oxygen transport

2. Megakaryocyte differentiates into platelets for clotting

3. Promyelocyte differentiates into WBC for immunity defense

4. Bone marrow lymphocyte precursor differentiates into B and T lymphocytes for immunity defense

Components of the circulatory system

• 2 pumps and 2 circulatory systems (heart is a dual pump)

• systemic circulation is out in the body

• pulmonary circulation is the blood going to and coming from lungs

• artery: carries blood away from the heart

• vein: carries blood towards the heart

• left side of heart is thicker than right because left pumps to systemic circulation

• perfusion: passage of blood through a vascular bed; blood moves by bulk flow from high to low pressures

• most vascular beds are in parallel, but pulmonary circulation is in series

Capillary beds in series vs parallel

• most beds are in parallel because this means equal amounts of O2, nutrients and waste go through the capillaries

• in series the capillary at the end would have barely any nutrients and be mostly wastes

Endothelium

inside part of myocardium lining the chambers

Pericardium

• outside part of heart

• the pericardial space/fluid is between pericardium and myocardium

Types of cardiac muscle cells

• pacemaker cells

• conducting cells

• contractile cells

pacemaker cells

• small fraction of cardiac muscle cells that have automaticity; SA node normally determines the heart rate

  • SA node: 100-120 APs/min

  • AV node: 60-80 APs/min

  • Conducting cells 30-50 APs/min

Conducting cells

• techinally all cardiac cells are conducting cells because they all conduct potentials

• small fraction are specialized to rapidly spread the electrical stimulus throughout the chambers

• bundle of HIS, right and left bundle branches, and Purkinje fibers

Contractile cells

99% of cardiac muscle cells whose activity allows blood to be pumped out of the heart