the heart Flashcards
- Compare and contrast the microscopic anatomy of cardiac muscle vs. skeletal muscle fibers.
Both
- Striated and contract by the sliding filament mechanism
Cardiac muscle
- Cells are short, fat, branched, and interconnected
- Each fiber contains one or two large, pale, centrally located nuclei
- Intercellular spaces are filled with a loose connective tissue matrix (endomysium) containing numerous capillaries - this matrix connects to the fibrous cardiac skeleton, which acts as a tendon and an insertion - gives cardiac cells something to pull or exert their force against
Skeletal muscle
- Cells are long cylindrical, multinucleate - nuclei are just underneath the plasma membrane
- Cells are all independent of each other both structurally and functionally
- Name and describe the special intercellular junctions present in cardiac muscle tissue.
Intercalated discs: contain desmosomes (prevent adjacent cells from separating during contraction) and gap junctions (allow ions to pass from cell to cell, transmitting current across the entire heart)
- What is the functional significance of these junctions (#16)?
- The gap junctions allow ions to pass from, transmitting current across the entire heart
- Desmosomes prevent adjacent cells from separating during contraction
- The gap junctions electrically couple cardiac cells, therefore the myocardium behaves as a single coordinated unit/functional syncytium
- What is the significance of the large numbers of mitochondria and the rich blood supply to cardiac muscle tissue?
The large numbers of mitochondria make the cardiac cells highly resistant to fatigue
The rich blood supply to cardiac muscle tissue probably delivers O2 to the mitochondria to keep the cardiac muscle functioning - the heart relies almost exclusively on aerobic respiration, so cardiac muscle cannot operate effectively for long without oxygen → damage to myocardium would be from a lack of oxygen (therefore, need a rich blood supply)
- What is a functional syncytium? Identify the functional syncytia of the heart.
Functional syncytium: myocardium having as a single coordinated unit
Atrial syncytia and ventricular syncytia
Gap junctions tie cardiac muscle cells together to form a functional syncytium. This allows the wave of depolarization to travel from cell to cell across the heart. As a result, either all of the fibers in the heart contract as one unit or the heart does not contract at all
- Describe the three main functional differences between skeletal and cardiac muscle.
Means of stimulation
- Certain cardiac muscle fibers exhibit autorhythmicity or automaticity. Some cardiac muscle cells are self excitable. The heart contains 2 kinds of myocytes, one being the contractile cardiac muscle cell responsible for the heart’s pumping activity. However, certain locations in the heart contain special noncontractile cells (pacemaker cells) that spontaneously depolarize. ← automaticity or autorhythmicity
- Because heart cells are electrically joined together by gap junctions, these cells can initiate both their own depolarization and the rest of the heart. No neural input is required
- Skeletal muscle fibers must be stimulated by a nerve ending to contract
Syncytium vs motor unit
- All the fibers in the heart contract as a unit or the heart doesn’t contract at all
- In skeletal muscle, impulses do not spread from cell to cell. Only skeletal muscle fibers that are individually stimulated by nerve fibers contract. The strength of contraction increases as more motor units are recruited
- Such recruitment doesn’t happen in the heart because it acts as a single huge motor unit
Length of absolute refractory period
- Absolute refractory period is the period of time during an action potential when another action potential cannot be triggered - ion channels are not prepared to open and thus cells cannot be excited
- In skeletal muscle, the absolute refractory period is much shorter than the contraction, allowing multiple contractions to summate (tetanic contractions)
- If the heart were to contract tetanically, it would be unable to relax and fill, therefore useless as a pump. To prevent this, the absolute refractory period in the heart is nearly as long as the contraction itself
- Identify (in order) the components of the heart’s conduction system.
The sinoatrial (SA) node generates the impulses → the impulses pause (0.1 s) at the atrioventricular (AV) node → the atrioventricular (AV) bundle connects the atria to the ventricles → the right and left bundle branches conduct the impulses through the interventricular septum → the subendocardial conducting network/Purkinje fibers depolarize the contractile cells of both ventricles
- The Sinoatrial (SA) node (in the right atrial wall) generates impulses and sets the pace for the heart as a whole (bc no other region of the conduction system or myocardium has a faster depolarization rate). This makes the SA node the heart’s pacemaker, and its characteristic rhythm (sinus rhythm) determines heart rate
- From the SA node, the depolarization wave spreads via gap junctions throughout both atria and via the internodal pathway to the atrioventricular (AV) node (in inferior portion of the interatrial septum). At the AV node, the impulse is delayed for about 0.1 seconds (allows the atria to respond and complete their contraction before the ventricles contract). Once through the AV node, the signaling impulse passes rapidly through the rest of the system
- From the AV node, the impulse sweeps to the atrioventricular (AV) bundle/bundle of His (in superior part of the interventricular septum). The AV bundle is the only electrical connection between the atria and ventricles (they are not connected by gap junctions).
- The AV bundle persists briefly, then splits into two pathways - the right bundle branches and the left bundle branches (course along the interventricular septum toward the heart apex)
- The subendocardial conducting network/Purkinje fibers (long strands of barrel-shaped cells with few myofibrils) completes the pathway through the interventricular septum, penetrates into the heart apex, and then turns superiorly into the ventricular walls. The right and left bundle branches excite the septal cells, but the bulk of ventricular depolarization depends on the large fibers of the conducting network and cell-to-cell transmission of the impulse via gap junctions between the ventricular muscle cells. Since the left ventricle is much larger than the right, the subendocardial conducting network is more elaborate in left ventricle than the right ventricle
- What are autorhythmic cells?
Autorhythmic cells are cardiac pacemaker cells - have the ability to depolarize spontaneously (their unstable resting potential that continuously depolarizes, drifting slowly toward threshold) and thus pace the heart
- What is a pacemaker potential? Why are pacemaker potentials important?
Pacemaker potentials are the spontaneously changing membrane potentials generated by cardiac pacemaker cells. Cardiac pacemaker cells have the special ability to depolarize spontaneously and thus pace the heart. Pacemaker cells have an unstable resting potential that continuously depolarizes, slowly drifting toward threshold ← pacemaker potentials
Pacemaker potentials initiate the action potentials that spread throughout the heart to trigger its rhythmic contractions
- How is a pacemaker potential generated (i.e., what ion channels are involved and at what point do the channels open/close)?
- In the sarcolemma, hyperpolarization at the end of an action potential both closes K+ channels and opens Na+ channels. The Na+ influx alters the balance between K+ loss and Na+, and the membrane interior becomes more positive (e.g. -60 mV to -40 mV)
- At threshold (about -40 mV), Ca2+ channels open, allowing entrance of Ca2+ from extracellular space. This influx of Ca2+ (rather than Na+) produces the rising phase of the action potential and reverses the membrane potential in pacemaker cells ← depolarization
- During the falling phase of action potential, Ca2+ channels inactivate. The falling phase of the action potential and repolarization reflect opening of K+ channels and K+ efflux from the cell, bringing the membrane potential back to its most negative voltage ← repolarization
- Once repolarization is complete, K+ channels close, K+ efflux declines, and the slow depolarization to threshold begins again
- What part of the myocardium normally has the fastest spontaneous depolarization rate?
SA node;
Bc it has the fastest spontaneous depolarization rate, it generates impulses and sets the pace for the heart as a whole
- What is the functional significance of the 0.1 second delay in impulse transmission at the AV node?
The 0.1 second delayed impulse allows the atria to respond and complete their contraction before the ventricles contract
This delay reflects the smaller diameter of the fibers here and the fact that they have fewer gap junctions for current flow → the AV node conducts impulses more slowly than other parts of the system (think cars forced to merge from 4 lanes to 2)
When the atria contract, blood gets pushed into the ventricles
- What is the function of the Purkinje fibers?
Purkinje fibers/subendocardial conducting network does the bulk of ventricular depolarization (and kind of ventricular contraction, b/c ventricular contraction happens almost immediately after the ventricular depolarization wave)
- What part of the heart is excited by the left and right bundle branches?
The septal cells of the interventricular septum
- What part of the conduction system serves as the sole electrical connection between the atria and the ventricles?
The AV bundle
- What is the normal pacemaker of the heart?
The SA node
- Compare and contrast the action potential of skeletal and cardiac muscle cells.
The action potential and contractile phase lasts much longer in cardiac muscle than in skeletal muscle. In skeletal muscle, the action potential typically lasts 1-2 ms, and the contraction for a single stimulus 15-100 ms. In cardiac muscle, the action potential lasts 200 ms or more because of the plateau, and tension development persists for 200 ms or more. This long plateau in cardiac muscle has 2 consequences…
- Ensures that the contraction is sustained so that blood is ejected efficiently from the heart
- Ensures that there is a long refractory period so that tetanic contractions can’t occur and the heart can fill again for the next beat
- In cardiac muscle, what are the sources of Ca2+ which trigger contraction?
10-20% of Ca2+ required for contraction come through the plasma membrane from the extracellular space. Once in the plasma membrane, this triggers Ca2+ influx → stimulates opening of Ca2+ ion release channels of the sarcoplasmic reticulum - SR is where the other 80-90% of Ca2+ comes from (and maybe mitochondria too)
