Cardiac Muscle and Heart

Medical Cell and Tissue Biology

Dr. Cathleen Pettepher

January 19, 2001

Motor Innervation

4 Skeletal muscle fibers are richly innervated by motor neurons

– originate in the brain or spinal cord

– axons arborize on individual fibers-- motor end plate

– Each motor neuron and the fibers it controls -- motor unit

Myoneuronal Junction

4 Plasma membrane is thrown into junctional folds

– contains receptors

4 Synaptic vesicles of nerve terminal release acetylcholine

– bind receptors & initiate contraction

Stimulus transmission across the Synaptic Cleft

4 Depolarization of axon terminal

– opens voltage-gated Ca++ channels

4 Influx of Ca++

– fusion of synaptic vesicles with axon membrane

4 Release of ACh into synaptic cleft

4 ACh diffuses across synaptic cleft & binds to receptors on fiber plasma membrane

4 Ion influx leads to depolarization of sacrolemma

4 Action potential is generated thru T-tubule system

Muscle Spindles

4 specialized receptor unit:

– 2 types of modified muscle fibers

• nuclear bag fiber

• nuclear chain fiber

– neuron terminals

• annulospiral - rate

• flowerspray - duration

– surrounded by a fluid-filled space and a capsule

Cardiac Muscle

4 Like skeletal muscle:

– striated

– contractions are strong & utilize a lot of energy

4 Like smooth or visceral muscle

– contractions are continuous and initiated by inherent mechanisms

• modulated by external autonomic and hormonal stimuli

4 found in heart and pulmonary veins

4 Consists of long, branching fibers

4 Individual mono-nucleated muscle cells form the fibers

4 Delicate collagenous tissue which supports rich capillary network surrounding the cells

4 Central nuclei

4 juxtanuclear region

– rich in mitochondria

– Golgi apparatus

– glycogen and lipofuscin granules

4 Very large mitochondria densely packed between the myofibrils

Intercalated Disks

4 Transverse component

– fasciae adherentes

• attach to thin myofilaments

• analogous to Z disks

– maculae adherentes

4 Lateral component

– gap junctions

– permit rapid flow of contractile stimuli

Myofilament banding pattern

4 Arrangement of contractile proteins are identical to skeletal muscle

4 Mode & mechanism of contraction are identical except for:

– Membranous t-tubule system

– Ca++ supply

Sacroplasmic Reticulum

4 Does not form terminal cisternae

4 Diad formation

– T-tubule

– small terminals of sarcoplasmic reticulum

4 Located in vicinity of Z line

4 Slower contraction rate

Injury and Repair

4 Mature Cardiac Muscle cells DO NOT divide!

– formation of fibrous CT and loss of cardiac function -- myocardial infarction

4 What about Skeletal and Smooth muscle cells?

– Satellite cells >> myoblasts fuse within basal lamina to form myotubes >> fiber

– Smooth muscle cells are capable of mitosis

Heart

4 Pump with 4 chambers separated by an interatrial and interventricular septi

4 Contains valves that maintain a one-way flow of blood

4 Heart wall contains:

– cardiac muscle for contraction

– fibrous skeleton for attachment of valves

– internal conduction system

Heart Wall -- Epicardium

4 layer of mesothelial cells on outer surface of heart

– visceral pericardium

– overlies thin layer of CT

4 Subepicardium

– loose CT & white fat

– blood vessels and nerves

Heart Wall -- Myocardium

4 Cardiac muscle

4 Surrounded by thin layers of CT

4 Contains a dense capillary network

4 Thickest in left ventricle

Heart Wall -- Endocardium

4 inner layer

– endothelium resting on a basal lamina

4 middle layer

– “subendothelial”

– CT and smooth muscle cells

4 subendocardial layer -

– contains impulse-conducting system

Atrial Wall

4 Endocardial layer is thicker

4 Myocardium is thinner

– more elastic fibers

– absence of chordae tendinea and papillary muscles

4 Conducting fibers in nodal areas only

4 Myocytes are smaller

– Poorly developed T-tubule system

– Myosin chains differ

4 Atrial granules present

– juxtanuclear position

– contain ANF & BNF

• affects urinary excretion of Na+ and vascular smooth muscle contraction

Cardiac Fibrous Skeleton

4 Annuli Fibrosi

– fibrous rings around valves

4 Trigona Fibrosa

– dense collagenous CT

– some organized fibers

4 Membranous septum

– regularly arranged collagen fibers

Valves

4 Bilayered flaps of endocardium

4 Cover CT plates

4 Contain collagenous fibers which are continuous with rings of fibrous skeleton

4 Some smooth muscle fibers but no bv.

4 Semilunar valves

– thicker ventricular endocardium

– more elastic tissue

4 AV Valves

– endocardium thicker and more elastic on leaflets facing atrium

– prominent CT plates

– chordae tendinae attach to ventricular leaflet

Conducting System -- SA Node

4 located near SVC in subepicardium

4 pacemaker

4 modified muscle fibers

– thin, fusiform and branching in shape

– NO intercalated disks

– cross striations

4 surrounding a coronary vessel

Conducting System -- AV Node

4 Located in interatrial septum in sub-endocardium

4 Nodal fibers similar to SA Node

4 Impulses from SA to AV nodes follows “preferential pathway”

– formed from atrial myocytes

AV Bundle of His

4 Emanates from AV node and pierces fibrous skeleton

4 IV septum > branches enter into papillary muscles > ventricular apex > outer walls

4 Two types of Nodal fibers

Purkinje Fibers

4 Located in ventricular subendocardium

4 Large fibers

– 20-30um in diameter

– Binucleated cells

– Scanty myofibrils

– Sarcoplasm rich in glycogen

– intercalated disks

4 NO t-tubule system