This document provides information about muscle physiology and the structure of muscle fibers at the microscopic level. It discusses the basic units of muscles including myofibrils, myofilaments, sarcomeres, and the proteins actin and myosin that make up the thin and thick filaments. It describes the striated banding pattern visible in muscle fibers due to the organized arrangement of these contractile proteins. It also covers concepts related to muscle membrane potential including ion gradients and the role of the sodium-potassium pump in establishing the resting potential.

1. Muscle Physiology

2. Muscle Physiology-Syllabus
 Basic muscle unit characteristic
 Electrical phenomenon in muscle cell
 Muscle action potential, excitation and propagation of impulse
 Latent period, refractiveness, threshold level
 All and none law
 Contractile mechanism - excitation - contraction coupling
 Neuro-muscular transmission
 Types of muscle contraction
 Phenomenon of Fatigue, Rigor mortis

4. 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.

5. 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

6. 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

7. 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.

8. 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.

9. 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)

10. 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)

11. 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

12. 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.

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

14. 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
The interstitial fluid and the intracellular fluid are electrolytic solutions
containing 147 mmol/L positive ions (cations) outside the cell and almost
the same concentration of negative ions (anions) about 155 mmol/L inside
the cell.
Higher concentration of Na+ ions (142 mmol/L) and lower concentration of
K+ ions (5 mmol/L) characterise the interstitial fluid.
The intracellular fluid has more of K ions (140 mmol/L) and less of Na+ ions
(14 mmol/L).

15. 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.
The unbalanced charges responsible for the
potential accumulate in a thin layer along opposite
surfaces of the membrane.
The vast majority of the fluid in the ECF and ICF is
electrically neutral. The unbalanced charges
accumulate along the plasma membrane.

16. Three major factors contribute to the membrane potential.
• Differential permeability of the membrane to diffusion of ions
• The Na+, K+ pump
• Negatively charged anions are trapped inside the cell

17. Ionic distribution of the Cell

18. Membrane Permeability of the Cell
⇒ 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+ because K+ has a smaller hydration shell (a sphere
of water molecules attracted by the ion’s positive charge).
The ion must lose its hydration shell before it can bind to a
protein, and it is easier for K+ to do this than for Na+. This
effect also reinforces the accumulation of K+ over Na+ inside
the cell.
⇒ The plasma membrane is virtually impermeable to A– , these
large, negatively charged proteins are found only inside the
cell. After they have been synthesized from amino acids
transported into the cell, they remain trapped within the cell.
It attracts more K than Na.
⇒ Plasma membrane has many more K+ leak channels than it
has Na+ leak channels

19. Effect of the Movement of Potassium on Membrane
Potential/Differential Permeability Of The Membrane To Diffusion Of
Ions
The concentration gradient tending to move K+ out of the cell and the electrical conduction gradient tending to move the
ions into the cell.

20. Effect of Sodium/Potassium Pump on Membrane Potential
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”