Excitable Tissues II Cardiac and Smooth Muscle PDF

Excitable Tissues II: Cardiac and Smooth Muscle Kulliyyah of Medicne & Health Sciences (KMHS) Universiti Islam Antarabangsa Sultan Abdul Halim Mu'adzam Shah (UniSHAMS)  Learning outcomes.. At the end of the lecture ,the students should be able to :  describe the physiological properties of car...

Excitable Tissues II: Cardiac and Smooth Muscle Kulliyyah of Medicne & Health Sciences (KMHS) Universiti Islam Antarabangsa Sultan Abdul Halim Mu'adzam Shah (UniSHAMS)  Learning outcomes.. At the end of the lecture ,the students should be able to :  describe the physiological properties of cardiac and smooth muscles.  describe the concept of excitation-contraction coupling.  describe the molecular basis of smooth muscle contraction. Classifications  properties of muscle tissue All muscle tissues have 4 characteristics in common: 1. Contractility: is the ability of muscle cells to forcefully shorten. Contractility allows muscle tissue to pull on its attachment points and shorten with force. ( muscles can only pull, never push.) 2. Excitability: ability to generate an action potential in response to a stimulus. 3. Extensibility: is the ability of a muscle to be stretched or extended. 4. Elasticity: is the ability to a muscle to return to its original length when relaxed i.e the ability to return to its original shape.  Myocardial cells have several different specific electro-physiologic properties: 1. Automaticity, 2. Excitability, 3. Conductivity, 4. Contractility, 5. Rhythmicity, 6. Refractoriness. Characteristics of cardiac muscle cell The heart is well supplied by Autonomic nervous network. These nerves affect cardiac pumping in 2 ways : By changing the rate of contraction: (Chronotropism). By changing the strength of contraction: (Inotropism).. Increasing rate of electrical impulses conduction = dromotropic agent is often inotropic & chronotropic (but not always).. Myocardial relaxation Cardiac Muscle (ventricle) action potential  Phase 0 (Depolarization Phase) Rapid depolarization occurs after threshold potential is reached owing to fast Na‫‏‬+ influx. The gradient of this line should be almost vertical.  Phase 1 (Early rapid Repolarization Phase) Repolarization begins to occur as Na‫‏‬+ channels close and K‫‏‬+channels open (K‫‏‬+ efflux). Phase 1 is short in duration and does not cause repolarization below 0mV.  Phase 2 (Plateau Phase) A plateau occurs owing to the opening of slow long lasting L-type Ca2+‫‏‬channels,which offset the action of K‫‏‬+ channels by Ca2+ influx, no further depolarization is possible now so Tetany is not possible in cardiac muscle. This time period is the absolute refractory period (ARP). The plateau should not be drawn completely horizontal as repolarization is slowed by Ca2‫‏‬+ channels but not stop it totally.   Phase 3 (Repolarization Phase) - The L-type Ca2+‫‏‬channels close and K+‫‏‬ efflux now causes repolarization as seen before. The relative refractory period (RRP) occurs during phases 3 and 4.  Phase 4 - The Na‫‏‬/K‫‏‬pump restores the ionic At first repolarization is rapid and then slows down, gradients by pumping 3Na‫‏‬out of the resulting in a plateau during which the cell in exchange for 2K‫‏‬. transmembrane potential remains around the 0 mV - The overall effect is, therefore, the slow level and declines but slowly (phase 2). In phase 3, loss of positive ionic charge from repolarization proceeds rapidly until the resting potential is reached. within the cell. Why repolarization is slower than depolarization in cardiac muscle? 1. Slow opening K‫‏‬+channels which is responsible for K‫‏‬+ efflux. 2. Plateau occurs owing to the opening of slow long lasting L-type Ca2+‫‏‬channels. 3. Depolarization involves the high-speed Purkinje system to rapidly conduct action potentials throughout the ventricles, whereas the propagation of repolarization does not involve these pathways, and therefore, it is limited primarily to slower cell-to-cell conduction outside of the Purkinje system. REMEMBER excitation–contraction coupling in skeletal muscle Cardiac muscle contraction Inotropism,Ca+2 is an inotropic agent  SMOOTH MUSCLE  Smooth muscle is described to have the character of the Plasticity (stress relaxation).  is defined as the ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it.  As in urinary bladder :the pressure increase is slight until the organ is relatively full.  Is known to be Fatigue resistance muscle as it uses less ATP ~ 10-100 times less than skeletal muscles. SMOOTH MUSCLE.. Summary… REFERENCES  Ganong WF, 2009. Review of Medical Physiology. 23rd Ed. Appleton & Lange: USA.  Lauralee Sherwood. 2015. Human Physiogy. 9th Ed. Thomson, Brooks/Cole: USA.  Eric P. Widmaier. 2007. Vander’s Human Physiology –The mechanism of body function. 11th. McGraw-Hill: USA.  Robert M. Berne. 2004. Physiology. 5th Ed. Mosby,Elsevier Sc.: USA.  John T. Hansen. 2003. Netter’s Atlas of Human Physiology. 1st Ed. MediMedia: USA.