Flashcards in Lecture 3: Histology of the Heart Deck (53):
What are the specialized needs of cardiac muscle?
1. Must pump blood continuously throughout life
2. Entire muscle must contract nearly SYNCHRONOUSLY with each beat
3. Entire muscle must relax with each beat (NO TETANIC contractions)
4. Highest oxygen consumption and energy requirements of any organ in the body
5. Must adapt appropriately to changes in circulatory demand (preload, rate, adrenergic stimulation)
What are the three layers of the heart wall?
1. Endocardium (inner)
3. Epicardium (outer)
What is the structure of the epicardium?
Composed of a two layer sac (mesothelium and connective tissue)
Inner layer = visceral layer of epicardial sac
Outer layer = parietal layer of epicardial sac
Serous fluid is present between visceral and parietal layer
What is the visceral layer of the epicardium?
Inner layer of epicardium
What is the parietal layer of the epicardium?
Outerlayer of the epicardium
Where do coronary arteries lie?
What is the difference between a leaflet and semilunar valve?
Leaflet valve has chordae tendinae attached to it (mitral and tricuspid)
Similunar doesn’t have chordae tendinae attached to it (aortic and pulmonary)
What are the key characteristics of the endocardium?
Is contiguous with the blood vessels
Has anti-thrombotic properties that may be reduced by injury/inflammation
Inflammation = endocarditis
What is the difference between atrial and ventricular endocardium?
Atrial endocardium + subendocardium is THICK
Ventricular endocardium are THIN
What are the annuli fibrosi?
The four fibrous rings at the base of the heart to which the myocardium is attached, one around each A-V orifice and one each around the aorta and the pulmonary artery
What is the aortic annulus?
The fibrous rings of the aorta
What is the fibrous skeleton of the heart? Significance?
An attachment for the myocardium
ELECTRICALLY separates atria from ventricles
What makes up the fibrous skeleton of the heart?
The annuli fibrosis
Triangular regions between the aortic annulus and the AV valves
Membranous ventricular septum
Composed of dense connective forming an aponeurosis with thick COLLAGEN fibers
What is the difference between left and right ventricle?
Inflow and outflow are close in LV
Inflow and outflow are farther apart in RV (because pulmonary artery is at a more obtuse angle)
What are the specialized conduction tissues?
1. SA node
2. internodal pathway in atrium
3. A-V node in interventricular septum
4. Bundle of His (right and left bundle branches)
5. Purkinje fibers
What is the left anterior fascicle and posterior fascicle?
Reference to the two ultimate ends of the left bundle branch
Covers most of the endocardium of the left ventricle
What are purkinje cells?
Specialized cardiac myocytes
Extensively connected to each other due to lateral gap junctions and intercalated discs that promote rapid impulse conduction
Have relatively few myofibrils (so very nerve-like)
What are the histological features of the purkinje cells?
They have a large diameter
What are intercalated disks?
Provide MECHANICAL and ELECTRICAL junctions between cells arranged end-to-end
What are gap junctions?
Provide electrical connections between adjacent cardiac myocytes
What are the histological features of the myocardium?
Composed of muscle cell bundles
Separated by connective tissue containing blood vessels
Have CENTRALLY placed nuclei
What are the features of the ventricular myocardium? Significance of layers?
Thick and comprised of several DIFFERENT muscle layers oriented in different directions
Different layers = longitudinal shortening and circumferential shortening with a twisting motion
The “wringing” of cardiac muscle ensures most blood gets ejected
What is the orientation of the superficial, middle and deep strands of myocardium?
Superficial = oblique
Middle = circumferential
Deep = longitudinal
ALL LINKED TOGETHER
What is a syncytium?
A multinucleated mass of cytoplasm that is NOT separated into individual cells
Myocytes are considered a “functional” syncytium
What is the significance of the endomysium?
A fine network of fibrocollagenous connective tissue that surrounds each myocyte
Provides myocyte with supportive framework
Endomysial weave coordinate the transmission of force and prevent slippage BETWEEN cells
What is the perimysium? Significance?
A network of connective tissue thicker than the endomysium
Bears SHEARING forces between groups of cardiomyocytes
Prevents malalighnment between bundles
Abnormal accumulation and/or change in the quality of the connective tissue increases myocardial stiffness
How do myocytes fit together in heart tissue?
Connected via intercalated discs
Each cell connects IN SERIES with 2 or 3 others, forming complex branching pattern
Groups of cells from one bundle will branch out at an angle to connect to cells of adjacent bundles
There is also VERTICAL contact among myocytes
What are the key characteristics of skeletal muscle?
Large, striated, elongated MULTInucleated fibers
What are the key characteristics of cardiac muscle?
Irregular branched cells with one or two CENTRALLY-located nuclei
What are the key characteristics of smooth muscle?
Fusiform (tapered) cells with one central nucleus
What are the differences between adhering and gap junctions?
Adhering junctions permit transmission of TENSION
Gap junctions allow flow of small SOLUTES from one cell to another
-can be blocked by increased calcium levels to limit damage
What is the ADAPTATION that allows heart to pump blood continuously throughout life?
What is the ADAPTATION that allows the entire muscle to contract with each beat?
1. specialized conduction system
2. gap junctions: cell-cell conduction
3. Adhering junctions and extracellular matrix (ECM) unify contraction within each chamber
What is the ADAPTATION that allows the blood flow to be unidirectional?
What is the adaptation that ensures increased venous return is met with increased contractility?
ECM and titin produce sufficient passive stiffness to assure an upslopping L-T relation (length and tension)
How is the myocardial cell organized?
Into sarcomeric units
What are the key characteristics of thin filaments?
Composed of actin, troponin and tropomyosin
Troponin binds to Ca and open up tropomyosin binding site
What are the key characteristics of thick filaments?
ATP binding and ATPases
What connects thick filaments to Z lines?
What is Titin?
The substance responsible for the connection of thick filaments to Z lines
Largest molecule in our body
What is the Z line? I band? A band? M line? H zone?
Z line = end of sarcomere (German for Zwischen which means “between”)
I band = the thin filaments (isotropic through actin)
A band = thin And thick filaments (anisotropic through myosin)
M line = the middle of the sarcomere
H zone = myosin only
What does isotropic mean?
Uniformity in all orientations
What happens in diastole/relaxation?
As sarcomere is passively stretched, titin first extends
At longer sarcomere lengths, the elastic PEVK (proline, glutamate, valine, lysine) region changes conformation producing an increase in PASSIVE tension
How do cardiac and skeletal titin compare?
Cardiac muscle is very hard to OVERSTRETCH
Skeletal muscles, however, have a point that is optimal in providing strength so you can under or over stretch it
Cardiac muscles however do not generally have a Lmax (max length at which contractility will be highest) because it is hard to overstretch the titin
Cardiac sarcomere is STIFFER than skeletal sarcomere and contributes to the mechanical force
What is significant about the cardiac sarcomere?
You don’t have to stretch these guys out AS FAR in order to generate the same amount of force as skeletal sarcomere
Same stretch distance = more force when compared to skeletal sarcomere
What is the relationship between sarcomere L and T? Does it change in disease states?
A positive relationship between L and T in all human hearts
Passive stiffness is increased in diseased hearts, we do not observe consistent defects in length-dependent contractile reserve in failing vs. non-failing hearts
What are the other specialized structures of the heart?
1. Sarcoplasmic Reticulum
3. Ryanodine Receptor
4. Lots of mitochondria
Where are SR and T-tubules located?
Close to the mitochondria
What is the function of the atrial myocytes? Significance?
Atrial myocytes as an endocrine organ
Secretory granules are located in Golgi region
Secretes NATRIURETIC peptides like ANP and BNP
What does ANP/BNP do? Secreted by?
Secreted by atrial myocytes (and sometimes by ventricular myocytes)
Regulate body fluid homeostasis through
iii. suppression of the renin-angiotensin-aldosterone system
Under pathological states ventricular myocytes produce more of the natriuretic peptides
How are myocytes organized?
Cellular components integrate to form larger working units
-contractile protein myofilament myofibril myocyte
What are the macro cardiac scaffolds?