Muscle fibres contain many myofibrils which are long contractile units composed of repeating bands of thick and thin filaments. The thick filaments are composed primarily of the myosin protein and the thin filaments are composed of actin, tropomyosin, and troponin proteins. The sarcomere, the basic unit of muscle contraction, is the region between two Z lines and contains overlapping thick and thin filaments that slide past each during muscle contraction and relaxation. The resting membrane potential of cardiac muscle cells is maintained around -90mV by selective permeability to potassium ions, and cardiac muscle cells have an intrinsic ability to generate slow spontaneous action potentials through calcium channels in the sinoatrial node, leading to the prolonged

2. Electron Microscopic Structures of Muscle Fibres
Muscle fibres contain several hundred to several thousand myofibrils
which constitute more than 80% volume of skeletal muscle cell. The length
of skeletal muscle fibre ranges from 10 to 30cm.

3. Myofibrils
 The myofibrils are arranged in parallel to the long axis of the
muscle cell/Myo fibre.
 Each myofibril - 1 to 2 µm in diameter
 Extend entire length of the muscle fibre
 Special function - contraction
 Myofibrils are contractile units within the cell which consist of a
regular array of protein myofilaments

4. Myofilaments
Each myofilament runs longitudinally with respect to the muscle fiber.The
myofibril is formed by two types of myofilaments
 Thick (or) Myosin myofilaments
 Thin (or) Actin myofilaments

5. Thin filaments (1.0 μm in length) primarily contain
 Actin
 Tropomyosin
 Troponin proteins
Thick filaments (1.6 μm in length )primarily contain
 Myosin protein
The thin filaments are arranged hexagonally around the thick filaments.
Each thin filament, in turn, is surrounded by three thick filaments.

6. The myofilaments are arranged in an
orderly manner which results in regular
repetition of dense (dark) cross-bands
and less dense (light) bands. This
arrangement produces cross-striations
that are seen microscopically in the
skeletal and cardiac muscles.

7. The dark bands are called A bands (contain myosin filaments and the ends of
actin filaments)which are anisotropic i.e. they polarise visible light
Light bands are called I bands (contain only the actin filaments )which are
isotropic i.e. they do not polarise visible light.
Myosin filaments are linked to the Z lines by the gigantic, elastic protein titin
(also known as connectin)

8. Structure of Sarcomere
 Z lines/Z disk - hold the myofilaments in place. The myofibril between
two Z lines/Z disc is called a sarcomere.
 The centre of the sarcomere appears darker due to the overlap of both
actin and myosin filaments (A band)
 The dark A band may also contain a slightly lighter central region where
only the myosin is present (H zone)
 Centre of H zone have M line which contains creatine phosphokinase
enzyme
 The peripheries of the sarcomere appear lighter as only actin is present
in this region (I band)

9. Myosin - Protein
 A myosin consisting of two identical
subunits, each shaped like a golf club with
two heads
 The tail ends are coiled around each
other(double helix).
 These heads form the cross bridges.
between the thick and thin
filaments
 Each head has two important sites
 Actin binding site
 ATPase (ATP-splitting) site
 Hinges – Flexible part of arm
 Protruding arms + Head = Cross Bridges

10. Myosin Filament
 Many myosin molecules form the myosin filament. About 500 myosin
heads on each thick filament
 The myosin molecule is made up of six polypeptide chains:
o Two heavy meromysin (heavy chain)
o Four light meromysin. (light chain)
 Light Meromysin (LMM) makeup the major part of the tail
 Heavy Meromysin (HMM) makeup the globular head and neck region.

11. Actin Filament
Mainly contains 3 proteins
1. Actin
2. Tropomyosin
3. Troponin proteins
Actin
 G actin - globular
 F actin - filamentous.

12. Ionic distribution of the Cell

13. Membrane Potential
Difference in the electrical potential between the interior and the exterior
of the cell is called “membrane potential”. This potential difference across
the cell membrane makes the plasma membrane a polarized membrane
(electrically charged).
The unit of this membrane potential is milivolts (mV).
Typical membrane potential ranges from -40 mV to -100 mV

14. a) When the positive and negative charges are equally balanced on each side of the
membrane, no membrane potential exists.
b) When opposite charges are separated across the membrane, membrane potential
exists.

15. Membrane potential is due to Difference in the Permeability of
the Membrane for different ions
⇒ K+ is more soluble in internal water than
is Na+ and that this leads to K+
preferentially entering a cell.
⇒ The negative charges of proteins attract
K+ more strongly than Na+
⇒ The plasma membrane is virtually
impermeable to A– , these large, negatively
charged proteins are found only inside the
cell.
⇒ Plasma membrane has many more K+
leaky channels than it has Na+ leak
channels

16. Equilibrium Potential Vs Resting Membrane Potential
 Equilibrium potential is the membrane potential required to produce
electrochemical(balance the concentration and electrical difference)
equilibrium.
 Resting membrane potential is the voltage across a given cell membrane
during the resting stage.
Skeletal muscle = -75mV to -95mV
Cardiac muscle = -85mV to -95mV
Smooth muscle = -50mV to – 60mV

17. Equilibrium Potential

18. Action potential
 Action potential is the rapid change in the membrane potential which
shift the membrane potential from its normal negativity to positivity
 It last for few milliseconds.
 Channels present in the cell membrane helps to create action
potential.Channel's like
 Leaky channel – establish the membrane potential
 Na K ATPase pump
 Voltage gated channels

19. Stages of Action Potential

20. Events of Action potential

22. Sodium/Potassium ATPase Pump
The Na+, K+ pump which pumps 3 Na+ ions out
of the cell and 2 K+ ions into the cell against
their concentration gradient (active transport).
This pump generates some membrane
potential, because it pumps 3 Na+ ions out of
the cell for every 2 K+ ions pumped into the
cell, thus concentration of positively charged
ions outside cell become high compare to
inside the cell.
Hence, this pump is called as “electrogenic
pump”

23. Resting membrane potential in cardiac muscle
 The resting membrane potential in cardiac muscle is -90mV.
 It has long action potential (250msec).
Cardiac muscle = -85 to -95 mV
Conduction system = -90 to -100 mV
Ventricle muscle = -100 to -105 mV
S.A. node = -50 to -55 mV

24. Sinoatrial (SA) Node are the primary pacemaker site within the heart.
Not having true resting potential, but instead generate regular,
spontaneous action potentials.
Unlike non-pacemaker action potentials in the heart, and most other cells
that elicit action potentials (e.g., nerve cells, muscle cells), the
depolarizing current is carried into the cell primarily by relatively slow
Ca++ currents instead of by fast Na+ currents. There are, in fact, no fast
Na+ channels and currents operating in SA nodal cells. This results in
slower action potentials in terms of how rapidly they depolarize.
Therefore, these pacemaker action potentials are sometimes referred to
as "slow response" action potentials.

25. Action Potential in Cardiac Muscle
 Cardiac muscle has an inherent or intrinsic property of
generating its own action potentials rhythmically, independent of
nerve stimulation.
 This occurs in the peacemaker cells of the S.A. node which
depolarises faster than any other parts of the heart.
 The cardiac muscle has slower but prolonged action
potential than skeletal muscle that lasts for 150msec in atria and
300 msec in ventricle.

26. The causes for the prolonged action potential in cardiac
muscle cells are
Cardiac muscle has two separate channel systems.
 Voltage activated Na channel (fast channel).
 Voltage activated Ca channel (slow channel).
The slow channels are slow to open and remain in the open state for a few tenths
of a second. The slow channels are activated at a membrane potential of -30 to -40
mV
Activation of the fast Na channels causes the spike potential of the action
potential, whereas the slow channel prolongs the passage of Ca 2+ ions into the
interior of the cell, thus establishes the plateau in the action potential.
The inflow of Ca2+ ions into the cardiac muscle cells decreases K+ efflux through
voltage gated K channels. This delays the K+ ion permeability to outside which in
turn delays the repolarisation process of the action potential in cardiac muscle.
The prolonged action potential makes the cardiac muscle cells to have longer
contraction period than skeletal muscles.

28. • PHASE 0: DEPOLARIZATION
• Sodium rapidly into cell (calcium slowly into cell)
• PHASE 1
• Sodium channels close
• PHASE 2: PLATEAU PHASE
• Potassium rapidly out of cell
Calcium slowly into cell
Calcium from extracellular space and sarcoplasmic reticulum = plateau
• PHASE 3: RAPID REPOLARIZATION
• Calcium channels close
Potassium rapidly out of cell
Potassium and sodium ion positions reversed
• PHASE 4: RESTING POTENTIAL
• Leaky potassium channels
Sarcolemma impermeable to sodium
***Long absolute refractory period in cardiac muscle cells: phase 0 to phase 3
Second action potential cannot be initiated
Protective mechanism against tetanus (state of maximal contraction)