MD137 Cardiac Muscle PDF 2024

Physiology School of Medicine 2024 Cardiac Muscle MD137: Principles of Lecturer: Dr K.McCullagh Physiology CLASSIFICATION OF THE TYPES OF MUSCLES Involuntary Reflexes Chapter 13 Cardiac Muscle In Rhodes Textbook  Cardiac...

Physiology School of Medicine 2024 Cardiac Muscle MD137: Principles of Lecturer: Dr K.McCullagh Physiology CLASSIFICATION OF THE TYPES OF MUSCLES Involuntary Reflexes Chapter 13 Cardiac Muscle In Rhodes Textbook  Cardiac Muscle Cardiac muscle share characteristics of both skeletal and smooth muscle. Striated Myosin and actin filaments form sarcomeres. Contraction occurs by means of sliding thin filaments. Unlike skeletal muscle fibers, these fibers are short, branched, and connected via gap junctions called intercalated discs.  Cardiac Muscle Cells Contract without Innervation Autorhythmic cells (pacemakers) – Signal for contraction – Smaller and fewer contractile fibers compared to contractile cells – Do not have organized sarcomeres Contractile cells – Striated fibers organized into sarcomeres Cardiac muscle vs. skeletal muscle 1. Smaller and have single nucleus per fiber 2. Branch and join neighboring cells through intercalated disks 3. Gap junctions 4. T-tubules are larger and branch 5. Sarcoplasmic reticulum is smaller 6. Mitochondria occupy one-third of cell volume Cellular Structure of Cardiac Muscle Cardiac muscle  Myocardium A myocardium is a mass of cardiac muscle cells connected to each other via gap junctions. Action potentials that occur at any cell in a myocardium can stimulate all the cells in the myocardium. It behaves as a single functional unit (similar to single- unit smooth muscle). The atria of the heart compose one myocardium, and the ventricles of the heart compose another myocardium.  Pacemaker Potential Cardiac muscle can produce action potentials automatically (without innervation). – Begin in a region called the pacemaker Heart rate is influenced by autonomic innervation and hormones. Excitation-contraction coupling in cardiac muscle * * DHPR * * * DHPR * *Mechanism Different or non-existent in skeletal muscle EC-coupling  Calcium Entry is a Feature of Cardiac EC Coupling Action potential starts with the heart pacemaker cells Ca2+ -induced Ca2+ release (different to sk. muscle) - Voltage-gated L-type Ca2+ channels (DHPR) in the cell membrane open - Ryanodine receptors (RyR) open in the sarcoplasmic reticulum (SR) Open in response to inflow of Ca2+ (different to sk. muscle) Called Ca2+ spark - Summed sparks create a Ca2+ signal Calcium binds to troponin Crossbridge cycle as in skeletal muscle Relaxation calcium removed from cytoplasm - Into the SR with Ca2+ -ATPase pump - Out of cell through the Na+-Ca2+ exchanger (NCX)  Action Potentials: Skeletal vs Cardiac Muscle For cardiac muscle APs the absolute refractory period is ~250 ms. This prevents tetanic contractions which would interfere with the heart’s ability to pump. Sk.muscle tetanic contractions Duration of Muscle Contraction in the Three Types of Muscle Skeletal Cardiac Smooth Tension 0 1 2 3 4 5 Time (sec)  Comparison of the Three Muscle Types Blank Skeletal Smooth Cardiac Appearance under Light Striated Smooth Striated Microscope Fiber Arrangement Sarcomeres No Sarcomeres Sarcomeres Location Attached to bones; a few sphincters Forms the walls of hollow organs Heart muscle close off hollow organs and tubes; some sphincters Tissue Morphology Multinucleate; large, cylindrical Uninucleate; small spindle- Uninucleate; shorter branching fibers shaped fibers fibers Internal Structure T-tubule and sarcoplasmic reticulum No t-tubules; sarcoplasmic T-tubule and sarcoplasmic (SR) reticulum reticulum Fiber Proteins Actin, myosin; troponin and Actin, myosin; tropomyosin Actin, myosin; troponin and tropomyosin tropomyosin Continued…  Comparison of the Three Muscle Types Blank Skeletal Smooth Cardiac Control Ca2+ and troponin Ca2+ and calmodulin Ca2+ and troponin Fibers independent of one Some fibers electrically linked Fibers electrically linked via gap another via gap junctions; others junctions independent Ca2+ from S R Ca2+ from E CF and SR Ca2+ from E CF and SR Contraction Speed Fastest Slowest Intermediate Contraction Force of Single Not graded Graded Graded Fiber Twitch Initiation of Contraction Requires ACh from motor neuron Stretch, chemical signals. Can Autorhythmic be autorhythmic Neural Control of Somatic motor neuron Autonomic neurons Autonomic neurons Contraction Hormonal Influence on None Multiple hormones Epinephrine Contraction