Cardiac Mucle Flashcards
(9 cards)
Characteristics of cardiac muscle
Similar to skeletal in terms of structure, but operates similar to smooth muscle ( involuntary)
They have interacted discs that is where different fibres join together and striations which follow each other in one direction
The gap junctions allow an electrical signal to originate from a specialized part of
the heart and propagate from fibre to fibre in a structured and organized pattern
Excitation contraction in cardiac vs skeletal muscle
The contraction of cardiac muscle is dependent on extra cellular Ca2+, this calcium is what interacts with receptors of the sarcoplasmic reticulum to release more Ca2+.
Unlike skeletal where all Ca2+ released from sarcoplasmic reticulum
Has long action potentials and so a tetanic contraction cannot occur.
Role of calcium in cardiac muscle
1 The membrane is depolarized by Nat entry as an action
potential begins.
2. Depolarization opens L-type Ca?* channels in the T-tubules.
3. A small amount of “trigger” Ca?* enters the cytosol,
contributing to cell depolarization. That trigger Ca?* binds to,
and opens, ryanodine receptor Ca2+ channels in the
sarcoplasmic reticulum membrane.
4.Ca?t flows into the cytosol, increasing the Ca?t concentration.
5.Binding of Ca?* to troponin exposes cross-bridge binding
sites on thin filaments.
6 Cross-bridge cycling causes force generation and sliding
of thick and thin filaments.
7.Cazt-ATPase pumps return Ca?* to the sarcoplasmic
reticulum.
8.Ca?-ATPase pumps and Na/Ca?+ exchangers
remove Cat from the cell.
9.The membrane is repolarized when K* exits to end the action
potential.
Sinoatrial node role
The SA node has an intrinsic rate of approximately 100 beats per minute and can initiate cardiac contraction in the absence of any external control (hormonal or nervous)
SA node cells initiate an action potential due to an unstable
membrane potential that is continuously drifting towards threshold.
Sympathetic stimulation to the SA-node leads to an increase in the slope of the drift due to an increase of the permeability of the Na. + current
Parasympathetic stimulation to the SA-node leads to a decrease in the slope of the drift due to an overall
decrease of the inward current, and to hyperpolarisation of the membrane due to increased K+ permeability
Stimulation in atria spreads fibre through the gap junctions
.Ventricular stimulation spreads initially via a network of conduits which begins at the atrioventricular (AV) node and terminates at cardiac muscle fibres via the Purkinje fibres.
Tricuspid valve
Separates R atrium and R ventricle
Open when R atrial pressure > R ventricular pressure
Closed when R atrial pressure < R ventricular pressure
Pulmonary valve
Separates R ventricle and pulmonary artery (PA)
Open when R ventricular pressure > PA pressure
Closed when R ventricular pressure < PA pressure
Aortic valve
Separates L ventricle and aorta
Open when L ventricular pressure > Aortic pressure
Closed when L ventricular pressure < Aortic pressure
Mitral valve
Separates L atrium and L ventricle
Open when L atrial pressure > L ventricular pressure
Closed when L atrial pressure < L ventricular pressure
How stimulus spread through the heart
Stimulus spreads through the muscle fibres and stimulates both atria almost simultaneously.
• The AV-node and the bundle of His are the only pathway of the electric stimulus to travel from the
atria to the ventricles and the signal is delayed at the AV-node in order to allow the atria to empty
before ventricular contraction begins.
• The signal travels down the interventricular septum via the two bundle branches.
• The ventricles begin to contract as the stimulus spreads upwards depolarising muscle fibres via the purkinje fibres.
