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Muscle Tissue
Topics:
1.Introduction.
2. Overview of muscle tissue.
3. Organization of muscle tissue
4. Skeletal muscle tissue
a. Properties of skeletal muscle
b. Microscopic anatomy
i. Sarcoplasm
ii. Myofibrils
iii. Sarcoplasmic reticulum
iv. Filaments & Sarcomere
v. Contractile proteins
1. Excitation-contraction coupling.
2. Sliding filament mechanism of muscle
contraction: The Contraction Cycle
3. Neuromuscular junction (NMJ).
4. Neuromuscular transmission.
5. Muscle metabolism.
6. Control of muscle tension.
Motor unit
Muscle tone
7. Cardiac muscle tissue.
8. Smooth muscle tissue.
9. Regeneration of muscle tissue.
10. Disorders: Homeostatic imbalance.
Organization of Muscle tissue
Big skeletal Muscle (Biceps brachii)
Composed of Fascicles
Each fascicle consists of Muscle fiber (Cell)
Each muscle fiber (or muscle cell) consists of
Myofibrils
Each myofibril consists of Filaments
Thick and thin
Thick filament consists of Myosin mol.
And
Thin filament consists of Actin, Troponin and
Tropomyosin
Muscle Tissue
11. Introduction:
A. Alternate contraction and relaxation of muscle
gives motion to living organisms.
B. Skeletal muscles, axial skeleton and appendicular
skeleton when functions together, give variety of
movements to organisms.
C. Myology: is the scientific study of muscle tissue.
12. Overview of muscle tissue:
a. Types of muscle tissue:
i. Skeletal muscle: also known as striated
muscle.
ii. Cardiac muscle: It is striated but involuntary.
iii. Smooth muscle: located in viscera. It is nonstriated and involuntary.
b. Functions of muscle fibers:
 Muscle performs five functions:
i. Body movements.
ii. Stabilize body position.
iii. Moving substances within the body.
iv. Generating heat.
13. Skeletal muscle tissue:
A. Each skeletal muscle is a separate tissue
composed of cells called muscle fiber.
B. Connective tissue components of skeletal
muscle.
i. Epimysium: outer most layer covering the
whole muscle.
ii.Perimysium: surrounds a group of muscle
fiber---Fascicle.
iii.Endomysium: cover individual muscle fiber in
a fasciculus.All these layers are extension of deep fascia.
C. Microscopic anatomy of a skeletal muscle
fiber:
i.
ii.
Skeletal muscle arises from Myoblast. Even
in mature skeletal muscle cell, they persist
as Satellite cell.
Sarcolemma: T-tubules and sarcoplasm
- each skeletal muscle fiber is covered by a
membrane called Sarcolemma.
- Each muscle fiber contains T (transverse)
tubule. They are tiny invaginations of the
Sarcolemma that quickly spreads the
action potential to all parts of the muscle
fiber.
iii. Myofibrils and Sarcoplasmic reticulum:
- Each muscle fiber contains myofibrils that
consists of thick and thin filaments (
(myosin & actin)
- The Sarcoplasmic reticulum encircles
each myofibril. It stores calcium ion in
relaxed state of the muscle.
iv.Filaments and the sarcomere:
a. Myofibrils are composed of filaments
(Thick and thin) and they are arranged in
units called Sarcomere.
b. Sarcomere is the basic functional unit of a
myofibril and show distinct dark and light
areas( A band & I band)
- Z line passes through the center of the I
band.
- H zone at the center of each A band,
contains thick filaments only.
v.Muscle proteins:
a. Contractile protein (myosin & actin) generate
force during contraction.
- Myosin: it is the main component of thick
filament.It functions as a motor protein.
- Actin: the main component of thin
filaments, connect to the myosin for
sliding on the myosin.
b. Regulatory protein help switch the contraction
on & off.
i. Tropomyosin & troponin are part of thin
filament.
ii. In relaxed state, tropomyosin, which is
held in place by troponin, block the
myosin binding site on actin, preventing
myosin from binding to actin.
14. Excitation-contraction coupling:
It means how an excitation (action potential) in the
skeletal muscle leads to contraction of that muscle.
At resting muscle, calcium is stored in the
sarcoplasmic reticulum(by calcium pump) and the
cytosol is free of calcium.
i. Action potential reaches the sarcoplasmic
reticulum(SR) via the T tubule. Action potential
stops the calcium pump and calcium comes out of
the SR in the cytosol by passive diffusion.
ii. Calcium ion now combine with troponin C &
forms a complex called Troponin-Tropomyosin
complex.This complex then changes the
orientation of tropomyosin (which moves
laterally) exposing the myosin binding site on
actin. Myosin head immediately attaches with the
binding site and forms a cross-bridge.
iii. There is immediate hydrolysis of myosin – ATP
ase and ATP is released. ATP then gives energy
for the myosin cross bridge to attach with the
next binding site—thereby sliding the actin on
myosin—leading to shortening of the muscle
fiber. Muscle contraction now occur.
The contraction cycle:
i.
ii.
‘The contraction cycle’ is a repeating sequence of
events that causes actin filaments to slide on the
myosin filaments.
It consists of 4 phases:
a. ATP hydrolysis occur at the myosin head and
this energizes the myosin head.
b. Energized myosin head now make attachment
to actin to form crossbridges.
c. The ‘power stroke’ which causes the myosin
head to generate force to rotate towards the
center of the sarcomere, sliding the thin
filament over the thick filament.
d. The detachment of myosin from actin.
At the end of ‘Power stroke’, myosin head remains
attached to actin to bind with another molecule of
ATP. Myosin head then detaches from actin. Myosin
ATPase again hydrolyses ATP at the myosin head and
a new cycle repeats. The cycle continues.
5. Neuro-muscular junction( NMJ):
i. It is the biggest synapse of the body.
ii. A synapse is a junction between the two neurons.
iii. Parts of NMJ: motor neuron ending in synaptic
end bulb, synaptic vesicle within the end-bulb
containing acetylcholine (Ach), synaptic cleft,
motor end-plate, receptor at motor end-plate for
Ach.
iv. Every NMJ has a motor end-plate which end in a
single skeletal muscle fiber.
6. Neuro-muscular transmission:
i.
Action potential in the motor neuron reaches the
synaptic end bulb or end feet where it releases a
neurotransmitter acetylcholine(Ach) from the
synatic vesicle into the synaptic cleft.
ii. Ach. then crosses the synaptic cleft and combine
with the receptors at the motor end-plate to form
a complex called Ach.-receptor complex.
iii. Ach-receptor complex then lead to the formation
of action potential in the motor end-plate.
iv. This action potential (AP) is propagated along
the sarcolemma and enters the skeletal muscle
through the T-tubules to reach the SR and causes
the muscle to contract.
7. Muscle metabolism: ATP production in muscle
cell.
i.Creatine phosphate & ATP: It can power maximal
muscle contraction for 15 sec.
Phosphorylcreatine +ADP- creatine + ATP
ATP + H20 - H3P04 + 7.3 Kcal
ii.Anerobic glycolytic pathway: Pyruvate in muscle &
glycogen in liver. Anerobic glycolysis _ 4 ATP
iii. Aerobic glycolysis - 40 ATP. It involves
complete oxidation of glucose via cytric acid cycle.
8. Control of muscle tension:
A. Motor unit: A motor neuron and the muscle fibers
it stimulates form a motor unit. (10.14) A single
motor unit may innervate 10 or as many as 2000
muscle fibers.
B. A twitch: It is a brief contraction of all the muscle
fibers in a motor unit in response to a single action
potential.
i. Myogram: it is the record of a muscle
contraction and include: latent period,
contraction and relaxation.
ii. The refractory period: It is the time when a
muscle will not respond to any stimuli of any
strength. Refractory period of cardiac muscle
is much longer than that of skeletal muscle.
C. Summation & Tetanus:
i. Summation: it is a phenomena in skeletal
muscle, where the force of contraction is
increased by the application of a second
stimulus
ii.Tetanus: can be complete and incomplete.
- Incomplete tetanus: is a sustained muscle
contraction that permits partial relaxation
between stimuli
- Complete tetanus: is a sustained muscle
contraction that lacks even partial
relaxation between stimuli
D.Muscle tone:
It is a state of partial contraction of muscle, which
results in a firmness –which is called muscle tone. At
any time even in a relaxed muscle, some muscle fibers
remain in a contracted state. Muscle tone is very
important for maintaining posture.
E. Isotonic and isometric contractions:
i. Isotonic contraction: It means when a muscle
contracts with shortening of the muscle fibers.
It performs work. Tension within the muscle
remain the same.(fig. 10.17 a)
ii. Isometric contraction: Here muscle
contraction occur without shortening of the
muscle fibers. Tension is increased within the
muscle during contraction (fig. 10.17 c).
9. Cardiac muscle tissue:
a. Cardiac muscle tissue is found in the heart.
The fibers are arranged similar to skelrtal muscle
fiber.
-they connect to the adjacent muscle fiber by
intercalated disc which contain desmosomes and
gap-junction.
b. Cardiac muscle contraction last longer than that of
the skeletal muscle due to prolonged duration of the
cardiac action potential
c. Cardiac muscle fiber has its inherent rhythmicity. It
can generate a spontaneous discharge to initiate a
contraction (pace-maker)
10. Smooth muscle tissue:
a. it is non-striated and involuntory and classified into
two types.
i. Visceral smooth muscle. Visceral smooth
muscle are found in the wall of the hollow
organ & blood vessels. Fibers are arranged in a
net work fashion.
ii. Multiunit smooth muscle.It is found in large
airways, erector pili muscle and in the muscle
of iris. of the eye.
b. Microscopic anatomy:
- The duration of contraction & relaxation
is longer than in skeletal muscle.
- Here regulator protein that binds calcium
ion in the cytosol is Calmodulin (troponin
C in skeketal muscle)
- Calmodulin activates the enzyme myosin
light chain kinase which fascilitates
myosin-actin binding & allows
contraction to occur at a relatively slower
rate.
11. Regeneration of muscle tissue:
A. skeletal muscle fibers cannot divide and have
limited power of regeneration
- Extensive repair results in fibrosis—scar
tissue.
B. Cardiac muscle fibers cannot divide or regenerate.
C. Smooth muscle fibers have limited capacity for
division and regeneration.
12. Disorders: Homeostatic Imbalance:
A. Neuro-muscular disease: involves somatic motor
neuron & NMJ or fibers itself. Example:
Myopathy is a disorder of the skeletal muscle
tissue itself.
B. Autoimmune disorders: Myesthenia Gravis. It is
characterized by severe muscular weakness
caused by antibodies directed against Ach.
receptor at NMJ.
C. Inherited muscle – destroying disease: Muscular
destrophy: It is characterized by degeneration of
individual muscle fibers leading to progressive
atrophy . The most common form is Duchenne
muscular destrophy. Gene therapy some day
might cure this condition.
D. Properties of muscle tissue:
a. Electrical excitability: ability to respond to stimuli,
producing action potential.
b. Contractility: ability to contract when stimulated by
action potential.
c. Extensibility: ability to stretch without being
damaged.
d. Elasticity: ability of the muscle tissue to return to
its original length and shape after contraction.
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