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Cardiac Muscle Notes

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This is an extract of our Cardiac Muscle document, which we sell as part of our Physiology and Pharmacology Notes collection written by the top tier of Oxford students.

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Cardiac muscle -striated
-myogenic- initiate contractions itself, doesn't require nervous stimulation but its activity can be modulated by nervous impulses from the ANS Structure
-cells are uninucleate, centrally placed nuclei and are connected to each other by intercelated disks (seen as dark lines on a light microscope) arranged in layerslaminae which are branched and iterdigitate Intercalated zones:
-main type of membrane to membrane contact is fascia adherens- the actin filaments at the ends of the terminal sarcomeres insert into anchor proteins, catenins/vinculin/alpha actinin which also bind to transmembrane proteins cadherins- these spand the plasma membranes and bind to identical cadherins on adjacent cells. Light microscopy, zona adherens can be seen as a small electron dense plaque on the cytoplasmic side of the plasma membrane fasciae adherns and transmit contractile forces
-Desmosomes: provide anchorage for the intermediate filament
-The adhering junctions allows transmission of mechanical forces- cadherins are transmembrane proteins, overlapping segments of the cadherin molecules bind to anchoring proteins desmoplakin
-Z lines, ruffled cell membranes abut each other to form intercalated disks-intercalated discs contain gap junctions (Cx43)- allow for the spread of electrical excitation between neighbouring fibres gap junctions allow the heart to function as a 3D synctium
-T Tubule system is present
-straited muscle- sarcomeric arrangement
-Between muscle fibres are collagenous tissue with rich capillary network
-Heterogenous organ- myocytes adapated for different purposes in different regions of the heart, some cells involved in contraction, conduction, pacemaking- SA node, AV node, fast conduction system: HIS bundle, purkinje fibre, atrial and ventricular myocytes
-atrial and ventricular myocytes specialisied for contraction so are much bigger cells and are packed with myofibres. Function of the heart
-muscular pump that pumps blood around the body
-nervous system alters the force and rate of heart beats Excitation: contraction is not iniated by neurons but by electrical excitation originating the heart's pacemaker. Cardiac muscle receives synaptic input from the autonomic neurons which is used to modulate the heart rate

Spread of impulse through the heart
-SA node, a specialised knot of myocytes in the right atrium that generate an action potential rhythmically - SA node- pacemaker
-Due to the presence of Gap junctions, the impulse is propagated through the atria where it triggers atrial contraction
-the atria are separated from the ventricles by a non conducting fibrous ring of tissue- atrioventricular septum- excitation can only spread into the ventricle at the AVN
- conduction through the AV node is slow, ensures the atria contract fully before the ventricles contract
-after a 100ms delay, conduction is fast through the HIS and purkinje fibres. From the purkinje fibres action potential is conducted from the base to the top of the ventricles- allows for synchronous activation of contraction in the ventricles Action potentials
-As the heart is a heterogenous organ, the myocytes in specific regions of the heart have different functions which mean they are specialised by having a distint set of ion channels
-the ion channels found in different regions of the heart are regulated in different ways
-Due to this the the intiation time, duration and shape of an action potential that occurs in the different regions of the heart varies according to the cell type ACTION POTENTIAL IN THE Sino Atrial Node
-SAN - spontaneous depolarisations-generating action potentials- determines the rate of the heart beat- always draw the graph

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