A&P 2 - Lesson 8

contractile cells

-Purkinje fibers distribute the stimulus to the contractile cells, which make up most of the muscle cells in the heart -resting potential --of a ventricular cell about -90 mV --of an atrial cell about -80mV

heart rate

-SA node generates 80-100 action potentials per minute -parasympathetic stimulation slows heart rate --normal HR less than 80-100 beats/min because of parasympathetic innervation -AV node generates 40-60 action potentials per minute --pacemaker cells in AV node can take over role of SA node if SA node pacemaker cells are damaged/non-functional

the electrocardiogram (ECG or EKC)

-a recording of electrical events in the heart -obtained by electrodes at specific body locations -abnormal patterns diagnose damage -features of an ECG --P wave- atria depolarize --QRS complex- ventricles depolarize --T wave- ventricles repolarize

heartbeat

-a single contraction of the heart -the entire heart contracts in series --first the atria --then the ventricles -two types of cardiac muscle cells 1. conducting system -controls and coordinates heartbeat 2. contractile cells -produce contractions that propel blood

the conducting system

-a system of specialized cardiac muscle cells --initiates and distributes electrical impulses that stimulate contraction -automaticity --cardiac muscle tissue contracts automatically --does not require nervous or endocrine system input

refractory period

-absolute refractory period --long --cardiac muscle cells cannot respond -relative refractory period --short --response depends on degree of stimulus

prepotential

-aka pacemaker potential -resting potential of conducting cells --gradually depolarizes toward threshold --after depolarization the cells depolarize to resting potential and start the depolarizing cycle again ---cycle is rhythmic -SA node depolarizes first, establishing heart rate

the cardiac cycle

-begins with action potential at sinoatrial (SA) node --transmitted through conducting system --produces action potentials in cardiac muscle cells (contractile cells) -electrocardiogram (ECG or EKG) --electrical events in the cardiac cycle can be recorded on an electrocardiogram

abnormal pacemaker function

-bradycardia-abnormally slow heartbeat -tachycardia- abnormally fast heartbeat -ectopic pacemaker --abnormal cells that generate high rate of action potentials than theSA or AV node --can bypass conducting system and disrupt ventricular contractions

the role of calcium ions in cardiac contractions

-contraction of a cardiac muscle cell --is produced by an increase in calcium ion concentration around myofibrils

Purkinje fibers

-distribute impulse through ventricles -atrial contraction is completed -ventricular contraction begins

the atrioventricular (AV) node

-in floor of right atrium -receives impulse from SA node (step 2) -delays impulse (step 3) by approx. 100 msec before it can travel towards the ventricles -atrial contraction begins -damage to conducting pathways that disrupt normal rhythm results in conduction deficits

The sinoatrial (SA) node

-in posterior wall of right atrium -contains pacemaker cells -connected to AV node by internal pathways -begins atrial activation

the AV bundle

-in the septum -carries impulse to left an right bundle branches --which conduct to Purkinje fibers(step 4) -and to the papillary muscles

conducting cells

-interconnect SA and AV nodes -distribute stimulus through myocardium -in the stria, conducting cells in the internal pathways distribute stimuli to atrial muscle cells -in the ventricles, AV bundle and the bundle branches distribute stimuli to ventricular myocardium through the Purkinje fibers

timing of refractory periods

-length of cardiac action potential in ventricular cell --250-300 msec ---30 times longer than skeletal muscle fiber ---long refractory period prevents summation and tetany

cardiac arrhythmias

-premature atrial contractions- momentary interruption of atrial rhythm (can be caused by stress, caffeine, drugs) -paroxysmal atrial tachycardia- premature atrial contraction triggering a flurry of atrial contractions - ventricles can keep up -atril fibrillation - impulses generating up to 500 beats/min cause atrial walls to quiver instead of producing organized contractions (ventricular rate remains normal, so hard to detect in older people) -premature ventricular contractions- Purkinje cell or ventricular myocardial cell cause premature contraction - single PVC not dangerous (caused by stimulants, epinephrine, ion changes) -ventricular tachycardia (VT, V-Tach) - 4 more PVCs without and intervening normal beats (can indicate serious problems) -ventricular fibrillation (VF) - responsible for cardiac arrest, rapidly fatal since ventricles quiver and stop pumping blood

structures of the conducting system

-sinoatrial (SA) node - wall of right atrium. pacemaker cells located here -atrioventricular (AV) node - junction between atria and ventricles. another set of pacemaker cells located here -conducting cells - found throughout myocardium

time intervals between ECG waves

P-R interval --from start of atrial depolarization -- to start of QRS complex Q-T interval --from ventricular depolarization --to ventricular repolarization