Page 116 - Headlines Histology2024_Neat
P. 116
Impulse conducting system
• It is a network of modified cardiac muscle cells serve to initiate and coordinate
contraction of the myocardium. It consists of sinoatrial node, atrioventricular
node, atrioventricular bundles or bundles of His and Purkinje fibers.
• The Sinoatrial node (pacemaker region) is the place at which impulses for
cardiac contraction are generated.It is located at the boundary between the right
atrium and the anterior vena cava. It is composed of a network of thin,
branching nodal muscle cells that contain numerous ribosomes, mitochondria
and few myofibrils.
• The nodal cells are embedded in large amount of highly vascular connective
tissue containing many sympathetic and parasympathetic nerve fibers.
• From the SA node, nerve impulses are generated spontaneously at regular
intervals. However, the rate of the nerve impulses can be modulated by the
autonomic nervous system.
• The parasympathetic decreases the heart rate and the sympathetic increases the
heart rate. The nerve impulse spreads throughout the atria causing them to
contract forcing blood into the ventricles.
• The atrioventricular node is located in the lower part of the interatrial septum.
It has the same structure as the SA node. It receives stimuli from the SA node
and passes them rapidly to the entire ventricular myocardium via the
atrioventricular bundle.
• The Bundle of His divides within the interventricular septum to give rise to
smaller branches called Purkinje fibers.
The Purkinje fibers are similar to cardiac muscle cells except:
o They are larger.
o Sometimes binucleated.
o Pale cytoplasm with few peripherally situated myofibrils.
o Cytoplasm is rich in glycogen and mitochondria.
o Have no T-tubules.
116
