NPB 101L: Frog Cardio and Human Relfexes Quiz

Frog heart - differences

3-chambered heart (2 atria and 1 trabeculated ventricle)

Primary Pacemaker = sinus venosus

Atrioventricular myocardium: a collection of cells that form a funnel between the atria and ventricle

Cornus Arteriosum: artery that comes out of the ventricle

No Purkinje fibers; Bundle of His

Less developed SR and no coronary circulation

Cardiac Muscle

striated; cells shorter than skeletal muscle; cells are connected via intercalated discs

intercalated discs contain

desmosomes - mechanically bind the cells together with adherin proteins

gap junctions - electrically couple cells together

Where do action potentials originate from?

the sinoatrial (SA) node -> spontaneously fires AP's at regular intervals of 60-100 times/min

Where do the AP from the SA node travel?

AP from SA node travel from right atrium to left, then to atrioventricular (AV) node

Where do the APs travel to from the AV node?

AP travel from the AV node to His-Purkinje fiber system and to ventricles

What are Pacemaker Cells?

Cells that send electrical signals through the heart that keep it beating at a steady rythm (SA node, AV node, and Bundle of His are capable of pacemaker activity)

What is the primary pacemaker?

SA node -> pace is set by the fastest pacemaker cells

Overdrive suppression

when faster pacemaker cells suppress the activity of slower pacemaker cells

Effective Refractory Period

equivalent to the absolute refractory period in nerves, and the ventricles cannot be activated (to keep the heart beating in a rhythmic pattern)

Relative Refractory Period

an action potential can occur but takes longer/greater stimulation

Cardiac Action Potentials are characterized by

rapid depolarization, prolonged plateau, and extended refractory period. This is crucial for coordinated contractions and effective blood pumping by the heart.

Cardiac Action Potential (aka Ventricular Action Potential) Phases

phase 0 - rapid depolarization - sodium moves into cell through voltage-gated sodium channels. The cell becomes positive.

phase 1- initial repolarization. sodium channels close, potassium channels open, potassium moves out of cell.

phase 2 - the plateau phase of repolarization. Characterized by a prolonged influx of calcium ions (Ca²⁺) through voltage-gated calcium channels, counterbalanced by potassium moving out.

phase 3 - rapid repolarization. Fast potassium moves out of cell. the T wave on the ecg.

phase 4- returning to resting state (maintained by sodium-potassium pump)

SA Node Action Potential Phases

Phase 0: Depolarization is caused by Ca2+ influx

No plateau phase

Phase 3: Repolarization from K+ efflux

Phase 4: Unstable resting potential due to opening of a non-specific cation channel

Calcium Induced Calcium Release (CICR)

calcium moves into the cell and stimulates release of calcium from the sarcoplasmic reticulum.

Calcium Induced Calcium Release Steps

1. Ca2+ influx (an action potential activates L-type voltage-dependent Ca2+ channels in the T tubules)

2. Ca2+ release (Ca2+ that enters the cell triggers the release of more calcium from the sarcoplasmic reticulum (SR) through ryanodine receptors

3. Thin filament interaction (the calcium released from the SR binds to troponin C on the thin filament, which moves tropomyosin out of the way; thin filament can now interact with the think filament, which generates force and actomyosin ATPase activity)

Sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA)

primarily responsible for reuptake of calcium into the intracellular stores; high calcium affinity

Sodium-calcium exchanger (NCX)

exchanges calcium for sodium ions across the plasma membrane, allowing for rapid calcium removal; High transport capacity but lower calcium affinity, making it more efficient for large calcium influxes

Plasma membrane Ca2+ ATPase

Primarily responsible for extruding calcium from the cell across the plasma membrane; Plays a "housekeeping" role in maintaining basal calcium levels

Cardiac Cycle

the sequence of mechanical and electrical events that repeat with every heartbeat

Cardiac Cycle Order

1. ventricular filling

2. isovolumetric contraction

3. ventricular ejection

4. isovolumetric relaxation

Ventricular FIlling

AV valves open, semilunar valves closed

Isovolumetric contraction

all valves closed, ventricular contraction causes an increase in pressure but no change in volume

Ventricular ejection

semilunar valves open, ejection of blood causes an increase in pressure and a decrease in volume

Isovolumetric relaxation

all valves closed, ventricles relax causing a decrease in pressure but no change in volume

Systole

contraction and emptying (Isovolumetric contraction and ventricular ejection)

Diastole

relaxation and filling (isovolumetric relaxation and ventricular filling)

Frank-Starling Law

heart will contract with more force during systole if filled to a greater extent during diastole; more filling = increased end diastolic volume = increased stroke volume

Extrasystolic contractions

premature ventricular contraction often caused by depolarization in the ventricle rather than at the SA Node; can be either larger or smaller than the beat immediately before it because of reduced filling time (smaller -> Frank-Starling Law) and increased Calcium buildup (larger)

Compensatory Pause

A skipped beat is sometimes caused by extrasystole to resume the proper timing of the SA Node; beats following a compensatory pause are usually much larger in force than the previous beat (Frank-Starling Law applies here)

The Vagus Nerve contains?

Parasympathetic efferents to the hear

Neurotransmitter: ACh, Receptor: mAChR

What happens when the vagal nerve is stimulated?

Vagal stimulation decreases HR (bradycardia) and prolonged stimulation causes cardiac arrest; Vagal stimulation slows/halts spontaneous AP generation of the SA node, but has less effect on the non-SA pacemakers (other pacemakers will take over in generation of heart rate at the next fastest pace)

Vagal Escape

occurs when the heart has been stimulated so much by the vagus nerve that it either slows to an extremely slow rate or stops. In response, the sympathetic system will release NE to start the heart again, and release it from the "overpowering vagal control".

What is stroke volume?

volume of blood pumped each beat

What is cardiac output? How is it calculated?

Cardiac output is the amount of blood pumped out by each ventricle in one minute. It can be calculated by the following equation: cardiac output = heart rate x stroke volume.

What is a reflex?

A reflex is an involuntary, stereotyped response that occurs in response to an appropriate stimulus. They are graded so the strength of the response is proportional to the strength of the stimulus

monosynaptic

reflex arc in which a single synapse separates the afferent and efferent limbs (ex: knee jerk)

polysynaptic reflex

involves more than one synapse within the reflex arc between the afferent and efferent neurons (advantage: the reflex arc can be modulated by other neuronal inputs)