2. Functional histology of the
• The heart is composed of:
1- Cells full of contractile muscle proteins (e.g. ventricular muscle)
2- Cells with few contractile elements: (e.g. sinoatrial nodal "SAN")
• The cardiac muscle fibers:
• Similar to skeletal muscle in striated structure
• Similar to smooth muscle in being involuntary
3. The intercalated
• connect the ends of adjacent muscle
fibers at Z lines
• 1) Provide a strong union between
fibers to act as one unit
• 2) Contain gap junctions ~ electrical
continuity between cardiac cells
4. •Properties of the cardiac muscle
• Defined as :
it is the ability of the heart to beat regularly & initiate its own regular
repetitive beats independent on nerve supply.
• Due to the presence of automatic cells in the heart:
• SAN Sinoatrial node (initiates cardiac impulses at a rate of 90- 105 I min .)
• 2 other automatic cells have the same property
• • AVN; Atrioventricular node (rate 40- 60 I min)
• • Purkinje fibers (rate 15-40 I min)
11. • Nodal action potential
compared to ventricular action
• 1- Nodal A.P. has unstable membrane potential.
• 2- Nodal A.P. has spontaneous diastolic
• 3- Nodal A.P. has slower upstroke (absence of
fast Na current)
• 4- Nodal A.P. reaches peak at only +10 mV.
• 5- Nodal A.P. has no plateau.