This document provides an introduction to skeletal muscle structure and function. It discusses the following key points in 3 sentences or less: Skeletal muscle forms about 50% of body weight, is voluntary and striated, and its main functions are tension development and shortening under nervous system control. Skeletal muscle is composed of bundles of muscle fibers which contain myofibrils made up of repeating structural units called sarcomeres, the basic contractile units of muscle. Each sarcomere contains thin actin filaments and thick myosin filaments arranged in a repeating pattern that gives skeletal muscle its striated appearance visible under electron microscopy.

1. Introduction to skeletal muscle
Dr AnuPriya J

2. Muscle physiology
• Muscle , another excitable tissue like the
nervous tissue, forms about 50% of the total
body weight
• 40% - skeletal muscle – voluntary & striated
• 10% - cardiac muscle – involuntary & striated
& smooth muscle – involuntary & nonstriated

3. Muscle - Types

4. Skeletal muscle
• Forms the great mass of somatic musculature
• Main function is tension development &
shortening
• Under the control of nervous system
• Coordinated activity of different muscles –
provide useful movement & maintainance of
posture

5. Skeletal muscle
• Voluntary & striated
• Attached to bones at both ends by tendons
• Fusiform in shape with tapering ends
• Has a belly & tendons on either side

6. Structure of skeletal muscle

7. • Skeletal muscle is made up of bundles of
muscle fibers (muscle cells) .
• The muscle fibers are arranged longitudinally
and parallel to one another
• Muscle fibers/cells are the building blocks of
the muscular system, like the neurons in
nervous system.
Structure of skeletal muscle

8. Structure of skeletal muscle

9. Structure of skeletal muscle

10. • Muscle cell/ muscle fiber is long, cylindrical &
multinucleated.
• The nuclei are peripherally located
• Diameter varies from 10 to 100 µm
• Length varies – often extends the entire
length of the muscle
Structure of skeletal muscle

11. • The cell membrane of the muscle fiber/cell is
known as sarcolemma
• Muscle fibers are composed of myofibrils of
1µm diameter, arranged parallel along the
long axis of the muscle fiber.
• They are separated by cytoplasm/sarcoplasm
• Smooth ER – Sarcoplasmic reticulum
Muscle cell

12. Myofilaments
• Each myofibril is formed of myofilaments
• 2 types – thick and thin filaments
• THICK FILAMENTS – 1500 in number, 1.6µ
long, 10-14 nm wide - MYOSIN
• THIN FILAMENTS – 3000 in number, 1 µ long,
7 nm wide – ACTIN, TROPOMYOSIN &
TROPONIN ratio of 7:1:1

13. Myofilaments

14. Myofilaments
• Each thick filament consists of 500 myosin
molecules & each thin filament consists of
300-400 actin , 40-60 tropomyosin, 40-60
troponin molecules ratio 7:1:1

15. Myofilaments
• The thick and thin filaments practically
interdigitate
• The arrangement of thick and thin filaments
in the myofibrils results in the striated
appearance of muscle fibers

16. Structure of a myofibril –
Electron microscopy

18. Myofibril - myofilaments
• A cross section of myofibril shows that each
thick filament is surrounded by 6 thin
filaments & each thin filament is in turn
surrounded by 3 thick filaments

19. Transverse / cross section through part
of a myofibril

20. Structure of a myofibril –
Electron microscopy
• Electron microscopy shows cross striations &
are characteristic of skeletal muscle
• These cross striations are not visible in
unstained preparations & under ordinary
microscope
• The cross striations are of alternate dark and
light bands.

21. Sarcomere
• The portion of the myofibril between two Z
lines is known as sarcomere
• Sarcomere is the structural and functional
unit of the myofibril
• The width of the sarcomere is 2.5 μ.
• It consists of an A band 1.6 μ & half of I band
0.5 μ on either side (1.6+0.5+0.5 = 2.6 μ)

22. Sarcomere

23. Sarcomere

25. Sarcomere

26. • The dark bands are called A band/ Anisotropc
band because they are anisotropic to
polarised light (i.e., they can rotate the plane
of polarised light and are bifringent)
• The light bands are called I band/ Isotropic
band (they do not rotate the plane of
polarised light & hence not bifringent)
Sarcomere

27. • A band contains thick filaments & are made
up of the protein myosin molecule and are
arranged in a parallel fashion.
• The I bands contain thin filaments made of 3
proteins – actin, tropomyosin and troponin
Sarcomere

28. Sarcomere
• The thick and thin filaments of the myofibrils
are arranged in such a way that they ( the A
& I bands) coincide with the A & I bands of
all myofibrils, thus giving a striated
appearance to the skeletal muscle.

30. Sarcomere

31. • In the middle of the dark A band, there is a
lighter zone(band/area) called as H zone. It is
the area where the thin filament do not
overlap with the thick filament.
• In the middle of the H zone, there is a darker
line called M line, formed of a protein called
myomesin. This binds to fibrils and connects
adjacent thick filaments to one another.
Sarcomere

32. • In the middle of the I band, there is a dark line
called the Z line/ Z disk.
• The thin filaments are attached to the Z line and
they extend to either side of the Z line to
interdigitate with the thick filament.
• The Z line passes from myofibril to myofibril,
attaching them all the way across the muscle
fiber.
• It is composed of filamentous proteins –
α actinin, desmin, vimentin
Sarcomere

33. Muscle proteins
• Myosin & actin are contractile proteins. They
are directly involved in tension generation and
shortening
• Troponin and tropomyosin are regulatory
proteins. They regulate the actin-myosin
interaction. Hence called so.

34. Muscle proteins
in
Desmin
Vimentin

35. Myosin
• 2 heavy chains, 4 light chains

36. Myosin

37. Myosin
• The part of the helix projecting out is called
the arm
• The protruding arm and head together called
as CROSS BRIDGE

38. Myosin
• There are 2 hinges; one present between the
arm and the body, the other between the
head and arm
• So movements are possible in these places on
either directions

39. Myosin
Myosin head has
1. Actin binding site
2. Site of ATPase activity – catalytic site that
hydrolyses ATP
Types of myosin
Myosin I – seen in association with cell membrane
with one head for myosin end
Myosin II – present in skeletal muscle – has 2 heads

40. Myosin
• Direction of cross bridge opposite in 2 halves
of sarcomere – so no cross bridge in centre of
sarcomere
• Tail directed towards center
• Head away from center
• Hence no heads but only tails in centre

41. Myosin

42. Actin
• Double stranded protein
• Made of F actin which is formed by
polymerisation of the globular protein G actin
• Mol wt 43000
• Myosin binding site during muscle contraction

43. Tropomyosin
• Double stranded protein
• Mol wt 70000
• Long filaments located on the groove between
the 2 actin strands
• Resting state – loosely attached to F actin &
physically covers active sites of actin strands
• So no interaction between actin & myosin in
resting state
• Each tropomyosin covers about 7 active sites on
the actin molecule

44. Troponin

45. Thin filament proteins

46. • Calcium binds to troponin C – exposure of
active sites of actin strands i.e., myosin
binding sites

48. Muscle Proteins

49. Structural proteins
• α Actinin – binds actin to Z line
• Titin – connects Z line to M line. Provides
muscle with its elasticity
• Nebulin – helps align actin molecules in the
actin filament
• Desmin, Vimentin – associated with Z line
• Myomesin - M line, formed of a protein called
myomesin. This binds to fibrils and connects
adjacent thick filaments to one another.

50. Structural proteins
• Dystrophin glycoprotein complex – structural
support and strength to myofibrils, transmits
the force generated by the contraction to
cytoskeleton
• Congenital defect – dystrophin gene defect –
different muscular dystrophies – largest gene
also present in cardiac & smooth muscle &
brain

51. Structural proteins

52. Motor point
• The area on the skin which corresponds to the
point of entry of nerve on the muscle.
• This area, when stimulated, gives the
maximum contraction.

53. Motor point
• In a long nerve, there are several motor
points.
• When the nerve supply to the muscle is intact
& when the muscle is stimulated over its
motor points, it is the nerve that is stimulated.
• Clinically, the muscle is stimulated electrically
at motor points, to prevent atrophy of the
muscle in certain muscular & neurological
disorders.

54. Motor unit
• The motor neuron, its axon & branches, and
the muscle fibers supplied by it, constitute a
motor unit.
• The number of muscle fibers supplied by a
motor unit varies (the size of the unit varies
inversely with the precision of the movement
performed by the part)

56. Motor unit
• In muscles which are concerned with fine,
precise, skilled movements, there are only few
muscle fibers (3-6) per motor unit ex: muscles of
hand, extraocular muscles
• Whereas, motor units supply 160-200 muscle
fibers - back muscles ; 2000 – gastrocnemius
muscle
• One motor unit supplies only one type of muscle
fiber.
But, in a muscle, there may be more than one
motor unit.

57. Motor endplate
• Site where the axon & muscle fiber meet

58. Classification of skeletal muscle fiber types
TYPE I TYPE II A TYPE II B
OTHER NAMES Slow oxidative (SO) Fast oxidative
glycolytic (FOG)
Fast glycolytic (FG)
COLOUR Red Red White
MYOSIN ATPASE
ACTIVITY
Slow Fast Fast
CALCIUM PUMPING
CAPACITY OF
SARCOPLASMIC
RETICULUM
Moderate High High
DIAMETER Small Large Large
GLYCOLYTIC
CAPACITY
Moderate High High
OXIDATIVE
CAPACITY
High Moderate Low
ASSOCIATED
MOTOR UNIT TYPE
Slow Fast. Resistant to
fatigue (FR)
Fast, fatiguable (FF)
MEMBRANE
POTENTIAL
-90 mV -90mV -90mV

59. Classification of skeletal muscle fiber types
TYPE I TYPE II A TYPE II B
OTHER NAMES Slow oxidative (SO) Fast oxidative
glycolytic (FOG)
Fast glycolytic (FG)
COLOUR Red Red White
MYOSIN ATPASE
ACTIVITY
Slow Fast Fast
CALCIUM PUMPING
CAPACITY OF
SARCOPLASMIC
RETICULUM
Moderate High High
DIAMETER Small Large Large
GLYCOLYTIC
CAPACITY
Moderate High High
OXIDATIVE
CAPACITY
High Moderate Low
ASSOCIATED
MOTOR UNIT TYPE
Slow Fast, Resistant to
fatigue (FR)
Fast, fatiguable (FF)
MEMBRANE
POTENTIAL
-90 mV -90mV -90mV

60. Types of motor units
On the basis of the type of muscle fiber they
innervate, and thus on the basis of the
duration of their twitch contraction, motor
units are divided into:
• Slow (S)
• Fast, resistant to fatigue (FR)
• Fast, fatiguable (FF)
motor units

61. Motor units – Size principle
• The recruitment of motor units during muscle contraction
follows a general scheme, the size principle
• A specific muscle action is developed first by the
recruitment of S muscle units that contract relatively
slowly to produce controlled contraction.
• Next, there is recruitment of FR muscle units resulting in a
more powerful response over a short period of time.
• Lastly, FF muscle units recruited for the most demanding
tasks.
• Example :
Leg muscle – S for standing foll by FR for walking foll by FS for
running

62. Give reason - Striated appearance of
sarcomere on electron microscopy

63. Note :
• Neuromuscular junction – Axon terminal & a
single muscle fiber

65. Sarcotubular system

66. 3 component model of a skeletal
muscle
• Contractile component – thick and thin filaments –
myosin,actin
• Series elastic component – elastic tissue of the muscle
that is present in series with the contractile component
of the muscle – i.e., the elastic tendon of the muscle
• Parallel elastic comp – elastic tissue of the muscle that
is attached parallel to the contractile component – i.e.,
the structural elastic tissue of the muscle such as
connective tissue sheaths of the muscle, sarcolemma &
gap filaments. Presence of this component explains
why the muscle regains its original length after it is
passively stretched i.e., property of elasticity

67. Thank you

68. Excitation contraction coupling