physiology 1 lecture 25 cardiac muscle excit. - cont. - coupl. action potentials

PHYSIOLOGY 1PHYSIOLOGY 1

LECTURE 25LECTURE 25

CARDIAC MUSCLECARDIAC MUSCLE

EXCIT. - CONT. - COUPL.EXCIT. - CONT. - COUPL.

ACTION POTENTIALSACTION POTENTIALS

Cardiac MuscleCardiac Muscle

Cardiac muscle differs from both Cardiac muscle differs from both smooth muscle and skeletal muscle, smooth muscle and skeletal muscle, being somewhere in between the two. being somewhere in between the two. Therefore, cardiac muscle is considered Therefore, cardiac muscle is considered an intermediate form between the more an intermediate form between the more primitive smooth muscle and advanced primitive smooth muscle and advanced skeletal muscle. Thus, it has properties skeletal muscle. Thus, it has properties of both smooth and skeletal muscle as of both smooth and skeletal muscle as well as it’s own unique properties.well as it’s own unique properties.

Cardiac MuscleCardiac MuscleStructureStructure

SIZE -SIZE - Cardiac muscle Cardiac muscle

tends to be smaller tends to be smaller than skeletal muscle than skeletal muscle but larger than but larger than smooth muscle. smooth muscle. About 10 to 15 About 10 to 15 m m in diameter and in diameter and around 50 around 50 m in m in length.length.


Contractile Contractile ProteinsProteins--

In cardiac muscle In cardiac muscle the contractile the contractile proteins are proteins are isomers of the isomers of the skeletal muscle skeletal muscle sarcomeres and sarcomeres and operate in a operate in a similar manner.similar manner.


T- Tubules -T- Tubules - In cardiac muscle In cardiac muscle

only one T -tubule only one T -tubule occurs per occurs per sarcomere and Triad sarcomere and Triad formation is formation is somewhat more somewhat more rudimentary due to rudimentary due to the small size of the the small size of the sarcoplasmic sarcoplasmic reticulium. reticulium.


Organelles -Organelles - Cardiac muscle is Cardiac muscle is

normally normally mononucleated but mononucleated but a few may be a few may be multinucleated. multinucleated. Otherwise organelles Otherwise organelles are similar to most are similar to most other cells with the other cells with the exception of the SRexception of the SR


The SR in cardiac The SR in cardiac musclemuscle cells is similar cells is similar to smooth muscle and to smooth muscle and takes up less than 5% takes up less than 5% of cellular volume. of cellular volume. Therefore, it lacks Therefore, it lacks space for large space for large amounts of Ca++ and amounts of Ca++ and the cell is dependent the cell is dependent on external Ca++ for on external Ca++ for contraction.contraction.


Intercellular Intercellular Connections -Connections -

Cardiac muscle cells Cardiac muscle cells are connected by a are connected by a intercalated disk. In intercalated disk. In reality these are reality these are interdigitated bundles interdigitated bundles of tightly bound of tightly bound collagen and elastin collagen and elastin fibers containing many fibers containing many gap junctions (An gap junctions (An electrical syncytium)electrical syncytium)


Sarcomere SizeSarcomere Size - - Due to the very Due to the very

heavy quantity of heavy quantity of connective tissue connective tissue surrounding cardiac surrounding cardiac myocytes the myocytes the resting sarcomere resting sarcomere length is somewhat length is somewhat compressed at 1.6 compressed at 1.6 M.M.


There are four types of cardiac muscle There are four types of cardiac muscle histologically “P” cells or pacemaker cells, histologically “P” cells or pacemaker cells, transition cells, Purkinje cells, and normal transition cells, Purkinje cells, and normal cardiomyocytes. All of these are classified cardiomyocytes. All of these are classified as cardiac muscle cells. However the “P”, as cardiac muscle cells. However the “P”, transition, and Purkinje cells are all part of transition, and Purkinje cells are all part of what is known as the cardiac conduction what is known as the cardiac conduction system and do not participate in muscle system and do not participate in muscle contraction but generate and transmit the contraction but generate and transmit the cardiac action potential.cardiac action potential.

Cardiac MuscleCardiac MuscleExcitation - Contraction - Excitation - Contraction -

CouplingCoupling

In cardiac muscle external CaIn cardiac muscle external Ca++++ is is once again required due to the small once again required due to the small volume of the SR.volume of the SR.

1. Calcium induced calcium release1. Calcium induced calcium release 2. Mechanism of calcium influx2. Mechanism of calcium influx 3. Role of calcium in tension 3. Role of calcium in tension

developmentdevelopment 4. Relaxation and calcium efflux4. Relaxation and calcium efflux


CouplingCoupling

Calcium Induced Calcium Induced Calcium Release -Calcium Release -

Passage of the Passage of the cardiac action cardiac action potential opens the potential opens the slow calcium/sodium slow calcium/sodium voltage gated voltage gated channels. Cachannels. Ca++++ influx influx opens Caopens Ca++++ sensitive sensitive calcium channels in calcium channels in the SR.the SR.


CouplingCoupling

Mechanism of Calcium Mechanism of Calcium Influx -Influx -

Calcium will influx through Calcium will influx through the slow voltage gated the slow voltage gated channels. But channels. But catacholamines also catacholamines also activate cAMP which in turn activate cAMP which in turn activates a cAMP activates a cAMP dependent Protein Kinase dependent Protein Kinase which phosphoralates Cawhich phosphoralates Ca++ ++

channels causing them to channels causing them to open increasing [Caopen increasing [Ca++]++]..


CouplingCoupling

Role of Ca++ in Role of Ca++ in Tension Tension development -development -

Increased cytosolic Increased cytosolic CaCa++++ concentrations concentrations increase crossbridge increase crossbridge cycling rates thereby, cycling rates thereby, increasing both the increasing both the force and speed of force and speed of myocyte contraction.myocyte contraction.


CouplingCoupling

Calcium release Calcium release from the SR and from the SR and extracellular Ca+extracellular Ca++ bind to Troponin + bind to Troponin C and excitation C and excitation contraction contraction coupling proceeds coupling proceeds as in skeletal as in skeletal muscle.muscle.


CouplingCoupling

Relaxation and Relaxation and Calcium efflux -Calcium efflux -

Calcium is removed Calcium is removed from the cardiac from the cardiac muscle cytosol by our muscle cytosol by our standard Castandard Ca++++ATPase ATPase into the SR and into into the SR and into the interstitial space, the interstitial space, but it is joined by a but it is joined by a NaNa++CaCa++++ exchanger 1 exchanger 1 CaCa++++ out for 3 Na out for 3 Na++ in. in.

Cardiac MuscleCardiac MuscleFast Cardiac Action Fast Cardiac Action

PotentialPotential

There are two cardiac action There are two cardiac action potentials produced by coronary potentials produced by coronary myocytes, the fast AP and the slow myocytes, the fast AP and the slow AP. The fast cardiac action potential AP. The fast cardiac action potential is characterized by containing a is characterized by containing a plateau phase. We will take up the plateau phase. We will take up the slow cardiac AP when we discuss the slow cardiac AP when we discuss the conduction system of the heart.conduction system of the heart.


PotentialPotential

The Fast Cardiac The Fast Cardiac Action Potential -Action Potential -

Four phasesFour phases Phase 4 or resting Phase 4 or resting

phase (Not really a phase (Not really a steady resting phase steady resting phase but rises slightly but rises slightly toward threshold)toward threshold)

Inwardly rectifying Inwardly rectifying potassium current.potassium current.


PotentialPotential

Phase 0 -Phase 0 - Phase 0 of the fast Phase 0 of the fast

cardiac action cardiac action potential is due to potential is due to the opening of the the opening of the fast Na+ voltage fast Na+ voltage gated channel gated channel similar to the ones similar to the ones encountered in nerve encountered in nerve and skeletal muscle and skeletal muscle action potentials.action potentials.


PotentialPotential

Phase 1 -Phase 1 - Phase 1 of the Phase 1 of the

fast cardiac action fast cardiac action potential is due to potential is due to a transient a transient outward outward potassium potassium current.current.


PotentialPotential

Phase 2 - PlateauPhase 2 - Plateau Phase 2 of the fast Phase 2 of the fast

cardiac action cardiac action potential is due to a potential is due to a slow inward slow inward calcium, sodium calcium, sodium channel balanced channel balanced by three separate by three separate outward potassium outward potassium currents.currents.


PotentialPotential

Phase 3 - Phase 3 - RepolarizationRepolarization

Phase 3 of the fast Phase 3 of the fast cardiac action cardiac action potential is due to potential is due to the action of the the action of the voltage gated voltage gated potassium channels potassium channels and a decline in and a decline in calcium/sodium calcium/sodium conductance.conductance.


PotentialPotential

Refractoriness -Refractoriness - Due to the long Due to the long

plateau phase plateau phase ( phase 2) of the fast ( phase 2) of the fast cardiac action cardiac action potential the cardiac potential the cardiac cells have time to cells have time to contract before the contract before the action potential is action potential is finished. finished. Can not Can not generate tetanygenerate tetany..

physiology 1 lecture 25 cardiac muscle excit. - cont. - coupl. action potentials

Documents

types of cardiac muscle

cardiac muscle structure

muscle contraction

cardiac muscle external

cardiac myocytes

primitive smooth muscle

skeletal muscle sarcomeres

advanced skeletal muscle