Muscle complete lecture

1. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The Muscular System

2. Introduction
 All activities that involve movement depend on muscles
 650 muscles in the human body
 Various purposes for muscles for:
 Locomotion
 Upright posture
 Balancing on two legs
 Support of internal organs
 Controlling valves and body openings
 Production of heat
 Movement of materials along internal tubes
 Three types of muscles in the human body
 Skeletal
 Cardiac
 Smooth

3. The Muscular System
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
 Muscles are responsible for all types of
body movement – they contract or
shorten.
 Three basic muscle types are found in
the body
Skeletal muscle
Cardiac muscle
Smooth muscle

4. Smooth Muscle Characteristics
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
 Has no striations
 Spindle-shaped cells
 Single nucleus
 Involuntary – no
conscious control
 Found mainly in the
walls of hollow
organs, wall of
vessels.
 Responsible for
peristalsis and
movement of blood Figure 6.2a

5. Cardiac Muscle Characteristics
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
 Has striations
 cylindrical but branched ( y
shaped)
 Cardiac muscle fibers are
attached to adjacent fibers
through by thick plasma
membrane called intercalated
discs
 Usually has a single nucleus
 Joined to another muscle cell
at an intercalated disc
 Involuntary
 Found only in the heart
Figure 6.2b

6. Skeletal Muscle Characteristics
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
 Skeletal muscle cell is also called a
muscle fibre
 Cells are multinucleate
 Striated – have visible banding
 Voluntary – subject to conscious control
 Cells are surrounded and bundled by
connective tissue

7. Connective Tissue Wrappings of
Skeletal Muscle
Slide 6.4b
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Fascia: on the
outside of the
epimysium
• Epimysium:
covers the entire
skeletal muscle
Figure 6.1

8. Connective Tissue Wrappings of
Skeletal Muscle
Slide 6.4a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
 Endomysium –
around single
muscle fiber
 Perimysium –
around a
fascicle
(bundle) of
fibers Figure 6.1

9. Muscle.
Muscle Fibers
Myofibrils
Microfilaments
(Divided into Sarcomere units)
Actin myosin

10. Muscle Fibers
 Each muscle has many muscle bundles.
 Each is muscle bundle is composed of muscle
fibers or cells.
 Muscle fibers are long, cylindrical and
multinucleated.
 The cytoplasm in these cells is called
sarcoplasm
 The plasma membrane is called sarcolemma
 Endoplasmic reticulum is known as sacroplasmic
reticulum which is specialized for storing calcium.

11. Myofibrils.
 Each muscle fiber contains large number of myofibrils.
 They have a diameter of 1-2 micrometers
 They extend throughout the length of the cell.
 Myofibrils are striated having dark and light bands.
Dark Bands: A-band {Central Light band called H zone}
Light Bands: I-bands
Line intersecting A-band: M-line
Line intersecting I-band: Z-line/disc.

12. Sarcomere.
 The contractile unit of myofibrils.
 The region between two sucessive Z-lines is called
the sarcomere
 The two Microfilaments are:
1. Actin: These are thin filaments having Diameter
of only 6-7 nanometers.
2. myosin: Thicker filaments with diameter of 16
nanometers. (almost double compared to actin)

13. Arrangement of Actin and myosin
 Actin and myosin are arranged in the myofibrils in such a
way that they overlap at some points.
 These overlapped areas are the darker regions in the A-
bands where as the central bright band in the A band is
called the H zone where only myosin is present.
 I-band covers the area where only actin is present

17. ACTIN MOLECULE
• Actin molecules are arranged in two chains which twist
around each other like a pearl necklace.
• Another protein called tropomyosin also twists around
the actin chain.
•Troponin is also a major protein which is important in
muscle contraction

18. MYOSIN MOLECULE
 myosin molecule consists of a tail and two heads
 They consist of two polypeptide chains coiled together
 The globular heads are also called cross bridges

19. TITIN MOLECULE
 One end of the titin molecule is elastic and is attached
to the Z disk, acting as a spring and changing length as
the sarcomere contracts and relaxes. The other part of
the titin molecule tethers it to the myosin thick
filament

20. SARCOPLASMIC RETICULUM
 sarcoplasm surrounding the myofibrils of each muscle
 fiber is an extensive reticulum (Figure 6-4), called the
sarcoplasmic reticulum.
 This reticulum has a special organization that is
extremely important in controlling muscle
contraction,

21. MECHANISM OF MUSCLE
CONTRACTION
The initiation and execution of muscle contraction occur
in the following sequential steps.
1. An action potential travels along a motor nerve to its
endings on muscle fibers.

22. MECHANISM OF MUSCLE
CONTRACTION
2. At each ending, the nerve secretes a small amount of
the neurotransmitter substance acetylcholine.
3. The acetylcholine acts on a local area of the muscle
fiber membrane to open multiple “acetylcholine-gated”
cation channels through protein molecules floating in
the membrane.

23. MECHANISM OF MUSCLE
CONTRACTION
4. Opening of the acetylcholine-gated channels allows
large quantities of sodium ions to diffuse to the interior
of the muscle fiber membrane. This causes a local
depolarization that in turn leads to opening of voltage-
gated sodium channels. This initiates an action potential
at the membrane.

24. MECHANISM OF MUSCLE
CONTRACTION
5. The action potential travels along the muscle fiber
membrane in the same way that action potentials travel
along nerve fiber membranes.
6. The action potential depolarizes the muscle
membrane, and much of the action potential electricity
flows through the center of the muscle fiber. Here it
causes the sarcoplasmic reticulum to release large
quantities of calcium ions that have been stored within
this reticulum.

25. MECHANISM OF MUSCLE
CONTRACTION
7. The calcium ions initiate attractive forces between the
actin and myosin filaments, causing them to slide
alongside each other, which is the contractile process.

26. MECHANISM OF MUSCLE
CONTRACTION
8. After a fraction of a second, the calcium ions are
pumped back into the sarcoplasmic reticulum by a
Ca++ membrane pump and remain stored in the
reticulum until a new muscle action potential comes
along;
this removal of calcium ions from the myofibrils causes
the muscle contraction to cease.

27. MOLECULAR BASIS OF MUSCLE
CONTRACTION
 Recall structure of myosin and actin..
 MYOSIN head has ATPase activity,which cleaves ATP
to ADP and phosphate that change configuration of
the mysoin head and cause it to bend at the arm.
 Actin filament is composed of F-actin, tropomyosin
and troponin molecules, the latter two cover the active
sites on actin preventing it from reacting with myosin

28. MOLECULAR BASIS OF MUSCLE
CONTRACTION
 When SR releases Ca 2+, it binds with troponin C and
changes its configuration such that active sites on actin
are uncovered and react with the myosin head.

29. WALK ALONG THEORY
It is postulated that when a head attaches to an active site, this attachment
simultaneously causes profound changes in the intramolecular forces between the
head and arm of its cross-bridge. The new alignment of forces causes the head to
tilt toward the arm and to drag the actin filament along with it. This tilt of the
head is called the power stroke. Then, immediately after tilting, the head
automatically breaks away from the active site. Next, the head returns to its
extended direction.
In this position, it combines with a new active site farther down along the actin
filament; then the head tilts again to cause a new power stroke, and the actin
filament moves another step. Thus, the heads of the cross-bridges bend back and
forth and step by step walk along the actin filament, pulling the ends of two
successive actin filaments toward the center of the myosin filament.

31. SMOOTH MUSCLES
 There are two types of smooth muscles
 Multi-unit smooth muscle
 Unitary or syncytial smooth muscle

32. Multi-unit smooth muscle
 This type of smooth muscle is composed of discrete,
separate smooth muscle fibers, often is innervated by a
single nerve ending
 The outer surfaces of these fibers, like those of skeletal
muscle fibers, are covered by a thin layer of basement
membrane–like substance, a mixture of fine collagen and
glycoprotein that helps insulate the separate fibers from
one another.
e.g, iris, ciliary body muscles, piloerector muscles

33. Unitary/syncytial smooth muscle
 This type of smooth muscle is usually arranged in
bundles or sheets
 They carry abundant gap junctions that allows free
flow of ions and action potential from one cell to
another
 It is also visceral smooth muscle because it lines the
walls of most bodily viscera

35. CHEMICAL BASIS OF SMOOTH
MUSCLE CONTRACTION
 Chemically, the interaction between actin and myosin
is similar as seen in skeletal muscle contraction.
 The actin in smooth muscle lacks the tropomyosin
complex
 The contraction is activated by calcium and ATP is
degraded to ATP to fuel the process

36. PHYSICAL BASIS FOR SMOOTH
MUSCLE CONTRACTION
 Most actin filaments are attached to dense bodies,
which in turn are either attached to the cell membrane
or dispersed around the cell.
 Each dense body has a cluster of actin filaments
attached to it, in the middle of the cluster lies a myosin
filament
 Myosin filaments in smooth muscles have sidepolar
bridges

37. COMPARISON
 Smooth muscles in comparison with skeletal muscle have a
slow and tonic contraction
 Following are the differences that allow for this
i. Slow cycling of myosin bridges
ii. Low energy requirement
iii. Slowness of onset and relaxation
iv. Maximum force of contraction is greater
v. Latch mechanism
vi. Stress relaxation

38. REGULATION OF CONTRACTION
 Calcium ions bind with a regulatory protein called
calmodulin found in smooth muscles
 calcium-calmodulin complex activates myosin light chain
kinase (MLCK)
 When myosin light chain is phosphorylated, it repetitively
interacts with actin to create intermittent pulls resulting in
contraction.

40. CESSATION OF CONTRACTION
 An enzyme, myosin phosphatase, is needed to de-
phosphorylate the myosin light chain, and cease
contraction.
 It is found in the cytosol.
 Therefore, the time required to relax greatly depends
on the amount of myosin phosphatase available in the
cell.