2. INTRODUCTION
Muscle Tissue : 3 types
Skeletal Muscle
Cardiac / Heart Muscle
Smooth Muscle/ Muscles of the internal organ (bowel).
Skeletal muscle along with bones/skeleton forms musculoskeletal
system.
3. SKELETAL MUSCLE
Human body contains over 600 skeletal muscles
• constituents - 40-50% of total body weight.
Characteristics: They are -
under voluntary control
have origin – insertion
most are attached by tendons to bones
have nerve supply, blood supply.
4. They are arranged in 3 interconnecting broad groups:
Neck and Trunk muscle
Upper limb muscle
Lower limb muscle
5. NECK MUSCLE: FROM FRONT
From front:
1. SCM
2. Trapezius
3. Scalene: ant., mid.,
post
4. Omohyoid
5. Platysma
6. NECK MUSCLE: FROM BACK
From back:
1. Trapezius
2. Splenius
3. Rhomboides Major,
minor
4. Levator scapulae
13. SKELETAL MUSCLE CELL/FIBRE
Long, cylindrical - referred as muscle
fiber.
Multinucleated, Nuclei located at
periphery
Numbers remain constant.
Striated – have visible banding.
Voluntary – subject to conscious
control.
Cells/Fibres are surrounded and
bundled by connective tissue = great
force, but tires easily
14. SKELETAL
MUSCLE FIBRE
Size: Skeletal muscle
fiber can be quite
large for human cells,
with diameters up to
100 μm and lengths
up to 30 cm (in the
Sartorius of the
upper leg).
15. DEVELOPMENT
Skeletal muscle cells originate from the paraxial mesoderm,
forming somites, then dermamyotome and finally the myotome.
During early development, embryonic myoblasts, each
with its own nucleus, undergo frequent divisions and
coalesce to form the multinucleated skeletal muscle fibers.
The nuclei of the myotube are still located centrally in the
muscle fibre.
In the course of the synthesis of the myofilaments/myofibrils, the
nuclei are gradually displaced to the periphery of the cell.
17. EMBRYONIC ORIGIN
Multiple nuclei mean
multiple copies of genes,
permitting the production
of the large amounts of
proteins and enzymes
needed for muscle
contraction.
18. HISTOLOGY
• Striated - due to the
arrangement of Actin and
Myosin in sequential order
from one end of the fiber
to the other.
• Multinucleated.
• Peripherally located nuclei.
19. IMPORTANT TERMINOLOGY
Some terminology associated with muscle fiber is rooted in the
Greek Sarco, which means “Flesh.”
The cell membrane of muscle fiber is the Sarcolemma.
The cytoplasm is referred to as Sarcoplasm.
The specialized smooth endoplasmic reticulum, which stores,
releases, and retrieves calcium ions (Ca++) is called the
Sarcoplasmic Reticulum (SR).
The functional unit of a skeletal muscle fiber is the Sarcomere, a
highly organized arrangement of the contractile myofilaments
Actin (thin filament) and Myosin (thick filament), along with
other support proteins.
20. THE SARCOMERE
Each packet of
microfilaments
(Actin & Myosin)
and their
regulatory
proteins, troponin
and tropomyosin
(along with other
proteins) is called
a sarcomere.
21. A bands: a dark band; full length of thick (myosin) filament
M line - protein to which myosins attach
H zone - thick but NO thin filaments
I bands: a light band; from Z disks to ends of thick filaments
Thin but NO thick filaments
Extends from A band of one sarcomere to A band of the
next sarcomere
Z disk: filamentous network of protein. Serves as attachment
for actin myofilaments
22. SARCOMERE: Z DISK TO Z DISK
About 10,000 sarcomeres per
myofibril
Each is about 2 µm long
Differences in size, density, and
distribution of thick and thin
filaments gives the muscle fiber a
banded or striated appearance.
Titin filaments: elastic chains of
amino acids; keep thick and thin
filaments in proper alignment
23. SARCOLEMMA
Cell membrane of muscle cell:
Surrounds the sarcoplasm (cytoplasm of fiber)
Contains many of the same organelles seen in other cells
- mitochondria, SR, Nucleus.
An abundance of the oxygen-binding protein,
Myoglobin and Myofibrils.
They are punctuated by openings called the transverse
tubules (T-tubules).
23
24. T TUBULES
Narrow tubes
that extend
into the
sarcoplasm at
right angles
to the surface.
Filled with
extracellular
fluid.
25. T tubules play an important role in muscle contraction:
Muscle action potential, which is the movement of electrical
charge, travelling along T tubules triggers the release of
calcium(2+) ions from the sarcoplasmic reticulum.
This allows the calcium (2+) ions to flood into the
sarcoplasm,
... which causes actions and movements of proteins
(including actin, myosin, troponin, and tropomyosin) within
the myofibrils that eventually result in muscle contraction.
26. SARCOPLASMIC RETICULUM (SR)
An elaborate, smooth endoplasmic reticulum –
• runs longitudinally and surrounds each myofibril
• form chambers called terminal cisternae on either side of the T-
tubules
A single T-tubule and the 2 terminal cisternae form a triad.
SR stores Ca++ when muscle not contracting.
When stimulated, calcium released into sarcoplasm.
SR membrane has Ca++ pumps that function to pump Ca++ out of
the sarcoplasm back into the SR after contraction.
27. MYOGLOBIN
A low molecular weight oxygen binding heme protein that is found
exclusively in heart and skeletal muscle cells.
It constitutes up to 5–10% of all the cytoplasmic proteins found in the
muscle cells.
In blood, myoglobin is bound primarily to plasma globulins, a complex
which is filtered by the kidneys.
If the plasma concentration exceeds the plasma binding capacity
(1.5 mg/dl in humans), myoglobin begins to appear in the urine.
High concentrations of myoglobin can change the color of the urine to
a dark red-brown color.
28. MYOFIBRIL
Cylindrical structures within the muscle fiber
They are bundles of protein filaments (=myofilaments)
Two types of myofilaments
1. Actin filaments (thin filaments)
2. Myosin filaments (thick filaments)
At each end of the fiber, myofibrils are anchored to the
inner surface of the sarcolemma
When myofibril shortens, muscle shortens (contracts)
32. ACTIN (THIN) MYOFILAMENT
Composed of 3 major proteins
1. F (fibrous) actin
2. Tropomyosin
3. Troponin
Two strands of fibrous (F) actin
form a double helix extending
the length of the myofilament;
attached at either end at
sarcomere.
Composed of G actin monomers
each of which has a myosin-
binding site.
Actin site can bind myosin during
muscle contraction.
33. Tropomyosin:
An elongated protein winds along the groove of the F actin
double helix.
Troponin: is composed of 3 subunits:
Tn-A : binds to actin
Tn-T :binds to tropomyosin,
Tn-C :binds to calcium ions.
34. MYOSIN(THICK) MYOFILAMENT
Single filament contains
roughly 300 myosin molecules.
Myosin molecules shaped like
golf clubs.
Each molecule consists of two
heavy myosin molecules
wound together to form a rod
portion lying parallel to the
myosin myofilament and two
heads that extend laterally.
35. Myosin heads:
1. Can bind to active sites on the actin molecules to form
cross-bridges. (Actin binding site)
2. Attached to the rod portion by a hinge region that can
bend and straighten during contraction.
3. Have ATPase activity: activity that breaks down
adenosine triphosphate (ATP), releasing energy. Part of
the energy is used to bend the hinge region of the
myosin molecule during contraction
36. TYPES OF MUSCLE FIBRE
Skeletal muscles consist of 3 main fibre types.
1. Slow twitch fibres (Type I):
They contract slowly but can contract repeatedly over long periods.
They have a good blood supply, hence they are ‘red fibres’, and are
suited to endurance activity using the aerobic energy system which
relies on oxygen from the blood for the supply of energy.
They are smaller and develop less force than fast twitch fibres.
Walking or cycling for 30 minutes at low intensity would use mostly
slow twitch fibres.
37. 2. Fast twitch fibres (Type IIa):
They have a fast contraction speed and can use aerobic (oxygen
dependant) energy sources as well as anaerobic (no oxygen used)
energy sources.
They are ‘white fibres’ as they are less reliant on oxygen supplied by
the blood for energy and therefore fatigue faster than slow twitch
fibres.
They are suited to speed, strength and power type activities, such as
moderately heavy weight training (8-12 reps) and fast running events
such as the 400metres.
38. 3. Fast twitch fibres (Type IIb):
They contract extremely rapidly, create very forceful muscle
contractions and fatigue quickly.
They are also ‘white fibers’ but unlike IIa fibres they can only use
anaerobic energy sources.
Like type IIa fibres the fast twitch type IIb fibres are also suited to
speed, strength and power type activities.
Heavy weight training (1-3 reps), power lifting, and 100metre sprints
are examples of activities that predominantly require IIb fibres.
39. HOW IT VARIES FROM INDIVIDUAL TO
INDIVIDUAL?
An individual’s muscle fibre type is determined by 3 factors.
1. Genetics:
You are genetically programmed to having a certain percentage of
each muscle fibre based on your parents’ genes.
It is thought that the average person is born with around 60% fast
twitch and 40% slow twitch fibres, however some individuals can be
born with larger amounts of fast twitch or slow twitch fibres and may
therefore be more suited to either high force or long duration
activities.
40. 2. Hormone levels within the blood:
The amount of hormone in the blood will affect the fibre type of an
individual and how big the fibres are.
Hormone levels naturally fluctuate throughout a person’s lifetime, so
some change in fibre type and distribution can occur as we grow and
mature.
Males and females also have different levels of certain hormones
produced and the type of exercise (i.e. light vs heavy weights) we do
will affect the level of hormones released as well.
41. Some of these hormones are ‘catabolic’, that is they stimulate muscle
breakdown, while others are ‘anabolic’, that is they stimulate the
growth and repair of muscle tissue.
So depending on our age, gender and the type of training we do, we
can cause an increase or decrease in the production of certain
hormones.
The result of this is either an increase or decrease in the slow or fast
twitch muscle fibres.
42. 3. Training undertaken:
Fibre type and the ability to change fibre type is a common area of
debate amongst exercise physiologists.
There is no evidence yet to show that fibre type can be changed,
however there is evidence to show that fibres adapt to the type of
training they are exposed to.
For example, if a person with predominantly slow twitch ‘endurance’
fibres trained predominantly with heavy weights their slow twitch
fibres would overtime begin to behave more like fast twitch fibre.
43. THE NEUROMUSCULAR JUNCTION
Another specialization of the skeletal muscle is the site where a motor
neuron’s terminal meets the muscle fiber—called the neuromuscular
junction (NMJ).
This is where the muscle fiber first responds to signaling by the motor
neuron.
Every skeletal muscle fiber in every skeletal muscle is innervated by a
motor neuron at the NMJ.
Excitation signals from the neuron are the only way to functionally
activate the fiber to contract.
44. NEUROMUSCULAR JUNCTION
Nerve impulse reaches myoneural junction:
• Acetylcholine is released from motor neuron
• Ach binds with receptors in the muscle membrane to allow sodium to
enter
• Sodium influx will generate an action potential in the sarcolemma
Action potential travels down T tubule
Sarcoplamic reticulum releases calcium
Calcium binds with troponin to move the troponin - tropomyosin
complex
Binding sites in the actin filament are exposed.
46. Myosin head attach to binding sites and create a power stroke
• ATP detaches myosin heads and energizes them for another
contaction
• When action potentials cease, the muscle stop contracting.
47. MUSCULAR CONTRACTION
The sliding filament model:
Muscle shortening occurs due to the movement of the
actin filament over the myosin filament
Formation of cross-bridges between actin and myosin
filaments
Reduction in the distance between Z-lines of the
sarcomere
51. TYPES OF CONTRACTION:
Isotonic - Muscle changes its length while contraction. 2 types:
Eccentric - Muscle elongates as it contracts. Example - Landing from jump.
Concentric - Muscle shortens as it contracts.
Isometric - Muscle length is constant. Static contraction.
Isokinetic - Muscle contracts at constant velocity; best for muscle
strengthening. Example – cycling on stationary bike.
52. ENERGY SOURCES FOR CONTRACTION
ATP provides immediate energy for muscle contractions from 3
sources:
• Creatine phosphate - During resting conditions stores energy to
synthesize ATP.
• Anaerobic respiration - Occurs in absence of oxygen and
results in breakdown of glucose to yield ATP and lactic acid.
• Aerobic respiration -
Requires oxygen and breaks down glucose to produce ATP,
carbon dioxide and water
More efficient than anaerobic
53. FUNCTION OF SKELETAL MUSCLE
1. Locomotion: The best-known feature of skeletal muscle is its
ability to contract and cause movement.
2. Maintain posture: Skeletal muscles act not only to produce
movement but also to stop movement, such as resisting gravity to
maintain posture.
Small, constant adjustments of the skeletal muscles are needed to hold
a body upright or balanced in any position.
3. Stabilize bone and joint: Muscles also prevent excess
movement of the bones and joints, maintaining skeletal stability and
preventing skeletal structure damage or deformation.
54. Joints can become misaligned or dislocated entirely by pulling on the
associated bones; muscles work to keep joints stable.
4. Control internal movement of substances: Skeletal muscles
are located throughout the body at the openings of internal tracts to
control the movement of various substances. These muscles allow
functions, such as swallowing, urination, and defecation, to be under
voluntary control.
5. Protect internal organs: Skeletal muscles also protect
internal organs (particularly abdominal and pelvic organs) by
acting as an external barrier or shield to external trauma and by
supporting the weight of the organs.
55. 6. Heat generation: Skeletal muscles contribute to the maintenance
of homeostasis in the body by generating heat.
Muscle contraction requires energy, and when ATP is broken down,
heat is produced. This heat is very noticeable during exercise, when
sustained muscle movement causes body temperature to rise, and in
cases of extreme cold, when shivering produces random skeletal
muscle contractions to generate heat.
In short,
skeletal muscles maintain posture, stabilize bones and joints, control
internal movement, and generate heat.
56. REVIEW: SKELETAL MUSCLE STRUCTURE
Skeletal muscle:
• Under voluntary control
• Has an origin and insertion
Types:
• Type 1 - “slow twitch” are aerobic
• Type 2 - “fast twitch” are anaerobic
57. Connnective tissue covering:
Muscle – Composed of multiple fascicles (bundles) surrounded by
epimysium.
Muscle Fascile (bundle) - Composed of multiple muscle fibers (cells)
surrounded by perimysium.
Muscle Fibre (Cell) - Elongated muscle cell composed of multiple
myofibrils surrounded by endomysium.
Myofibril – Composed of multiple myofilaments arranged end to end
without a surrounding tissue
Sarcomere – Composed of interdigitated thick (myosin) and thin (actin)
filaments organized into bands, Z line to Z line defines the length of
the sarcomere.
58. A band: length of the
thick filament/myosin,
which does not
change with
contraction
I band (actin only) –
present both side of Z
disc
H band (myosin only)
– In the mid portion of
myosin
I band, H band,
Sarcomere length all
change with
contraction.
59. Composition: Skeletal muscles contain connective tissue, blood
vessels, and nerves.
Muscles attach to bones directly or through tendons or
aponeuroses.
60. Skeletal muscle fibers: long, multinucleated cells.
The membrane of the cell is the sarcolemma; the cytoplasm of
the cell is the sarcoplasm.
The sarcoplasmic reticulum (SR) is a form of endoplasmic
reticulum.
Muscle fibers are composed of myofibrils.
The striations are created by the organization of actin and
myosin resulting in the banding pattern of myofibrils.
61. Myosin - Thick filament; has “head” that binds ATP and attaches
to thin filaments (actin)
Actin - Thin filament; fixed to Z bands, associated with troponin
and tropomyosin
Troponin - Associated with actin and tropomyosin, binds Ca++
ions
Tropomyosin - Long molecule lies in helical groove of actin and
blocks myosin from binding to the actin
62. FUNCTIONS OF SKELETAL MUSCLE
Body movement (Locomotion)
Maintenance of posture
Respiration
Diaphragm and intercostal contractions
Communication (Verbal and Facial)
Constriction of organs and vessels
Peristalsis of intestinal tract
Vasoconstriction of b.v. and other structures (pupils)
Production of body heat (Thermogenesis)
