MUSCLE TISSUE
MUSCLE TISSUE
 A primary tissue type, divided into:
 skeletal muscle
 cardiac muscle
 smooth muscle
Connective Tissue Organization
1. Epimysium
• Exterior collagen layer
• Surrounds entire muscle
• Separates muscle from surrounding tissues
2. Perimysium
 Surrounds muscle fiber bundles (fascicles)
 Contains blood vessel and nerve supply to fascicles
 3. Endomysium
 Surrounds individual muscle cells (muscle fibers)
 Contains capillaries and nerve fibers contacting
muscle cells
 Contains satellite cells (stem cells) that repair damage
 Endomysium, perimysium, and epimysium come together
at ends of muscles to form connective tissue attachment
to bone matrix i.e., tendon.
Structure of Skeletal Muscle
Structure of Skeletal Muscle
SKELETAL MUSCLE CELL
- Striated/striped appearance of skeletal muscle cell is
due to the orderly arrangement of the thin and thick
filaments .
that makeup the majority of the contractile proteins.
- The contractile proteins are made of 3 types of
filaments;
1. THIN FILAMENT
2. THICK FILAMENT
3. ELASTIC FILAMENT
CONTRACTILE PROTEINS
1. THIN FILAMENT
 Thin filament is placed hexagonally around myosin
 Make and break contacts with myosin during
contraction
 Has 3 parts;
i) ACTIN PROTEIN
(i.e. the main molecule of this filament).
FUNCTION: Binds to myosin head.
CONTRACTILE PROTEINS
ii) TROPONIN
FUNCTION: Regulatory function by binding to Ca 2+
iii) TROPOMYOSIN
FUNCTION: Has a regulatory function by
blocking/unblocking
the binding site of actin to the myosin head
CONTRACTILE PROTEINS
2.
THICK FILAMENT
 Thick filament: composed of structural protein,
myosin.
- has 2 main parts
i) Myosin head - possesses actin binding site and
ATPase activity.
ii) Myosin tail– forms the shaft of thick bands.
CONTRACTILE PROTEINS
3. ELASTIC FILAMENT
– made of titin molecule
FUNCTION: – Fixes the thick filament to a z disc.
Muscle Sarcomere
 Sarcomere - functional unit of muscle cell
 Consist of thick and thin filaments – myofilaments/
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myofibril
Myofilaments that lies between two ‘z’ linessarcomere
- Dark & light bands alternate
- Light I band is Isotropic
- Dark A band is anisotropic
- Z line bisects I band
- H zone: No overlap of actin
Myofibrils
 Each myofibril consists of two types of protein
filaments -"thick filaments", and "thin filaments".
 Thick filaments and the thin filaments within
myofibrils overlap in a structured way, forming units
called sarcomeres.
 Sarcomeres are sections of myofibril that are
separated from each other by areas of dense material
called "Z discs".
The sarcomeres are also described in terms of the
bands/zones within which one or both of the two
filaments occur.
•
The "A band" is a relatively darker area within the
sarcomere that extends along the total length of the thick
filaments.
• The "H zone" is at the centre of the A band of each
sarcomere. As shown below, this is the region in which
there are only thick filaments, and no thin filaments.
The "I band" is the region between adjacent A bands, in
which there are only thin filaments, and no thick filaments.
( Each I band extends across two adjacent sarcomeres.)
Arrangement of Filaments
Cross Bridge
Muscle contraction
[SLIDING FILAMENT THEORY]
 Involves the sliding of the actin over the myosin
which causes the muscle to shorten and
therefore develop tension
 The process of muscular contraction can be
explained by the sliding filament theory
Muscle contraction
[SLIDING FILAMENT THEORY]
Excitation: Step 1
 Action potential (AP) travels to nerve ending
 ACh release at the neuromuscular junction
 Activation of receptors by ACh
 Voltage gated Na channels open
 AP is generated in the muscle fiber
Muscle contraction
 Depolarization of sarcolemma
 Action potential transmitted down T-tubule
 Sarcoplasmic reticulum releases Ca++
 Ca++ binds to troponin C
 Active actin site is exposed
Muscle contraction
Contraction: Step 2
1. Myosin heads are activated
Myosin head hydrolyses (breakdown) ATP to
release energy
Myosin head gains energy so it becomes
activated.
Activated myosin head is ready to bind with actin
filament (cross bridge)
Muscle contraction
2. Activated myosin head binds rapidly to myosinbinding sites on actin
3. Myosin head still bonded to actin;
i) Changes its shape
ii) Moves towards the center of sarcomere
Above steps, i) and ii) called power stroke
4. Myosin head is still bonded to actin
Muscle contraction
5. ATP binds to the myosin in the myosin-actin complex
6. Myosin head detaches from actin
7. Myosin head hydrolyses ATP to release energy.
8. Steps 2 – steps 8 is repeated
• During muscle contraction the thin actin filaments
slide over the thick myosin filament.
• This results in a reduction in the distance from Z line
to Z line
• H zone increases while while I band decreases.
The Sliding Filament Model of Muscle
Contraction
Muscle contraction
Relaxation: Step III
• Ca++ is pumped back to sarcoplasmic reticulum
• Active pump utilizing ATP
• Actin sites are covered by troponin
• Ca++ remains stored in the reticulum
NEUROMUSCULAR JUNCTION
 Each muscle fibre is connected to a nerve fibre
branch coming from a nerve cell.
 These nerve cells are called motor neurons, and
extend outward from the spinal cord
 The motor neuron and all the muscle fibres it
innervates are called a motor unit.
 Stimulation from motor neurons initiates the
contraction process.
 The site at which motor neuron attaches on the
muscle cell is known as the neuromuscular
junction
NEUROMUSCULAR JUNCTION
 At this junction, the sarcolemma forms a pocket
known as the motor end plate
 The end of the motor neuron is not in direct
contact with the muscle fibre but is separated by a
short gap known as the neuromuscular cleft
 A nerve impulse reaching the end of the motor
nerve
stimulates
the
release
of
the
neurotransmitter acetylcholine which diffuses
across the synaptic cleft and binds to the receptor
sites on the motor end plate
NEUROMUSCULAR JUNCTION
 This causes an increase in permeability of
sarcolemma to sodium and sodium diffuses into
muscle fibre resulting in a depolarisation called the
end-plate potential (EPP)
 This EPP is usually large enough to exceed the
threshold that is the signal to start the contractile
process.