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Muscle tissue
There are four characteristics associated
with muscle tissue:
Excitability
- Tissue can receive & respond to stimulation
Contractility
- Tissue can shorten & thicken
Extensibility
- Tissue can lengthen
Elasticity
- After contracting or lengthening, tissue
always wants to return to its resting state
Types of muscle tissue:
Skeletal muscle tissue
• Associated with & attached to the skeleton
• Under our conscious (voluntary) control
• Microscopically the tissue appears striated
• Cells are long, cylindrical & multinucleated
Connective tissue component
Dense
irregular CT
Dense
irregular CT
Surrounded by
perimysium
10-100 muscle
fibers
Reticular fiber
Surrounded by
epimysium
Surrounded by
endomysium
Microanatomy of a Muscle Fiber (Cell)
Transverse (T) tubules: (invaginations
of the sarcolemma), spread of action
potential
Sarcoplasm: contains
myoglobin (oxygen
binding and release)
Sarcoplasmic
sarcolemma (plasma membrane)
reticulum: (Ca+2
Terminal
stores)
cisternae:
Dilated sacs
mitochondria
thick myofilament
thin
myofilament
Myofibril:
arranged in
compartments called
sarcomeres
nuclei
triad
Sacromere
A band: The darker middle part of
the sarcomere, which extends the
entire length of thick filaments.
I band: is a lighter, less dense
area that contains thin filaments
but no thick filaments
H zone: narrow, in the center of
each A band contains thick
filaments but no thin filaments.
M line: proteins that hold the thick
filaments together at the center of
the H zone form middle of the
sarcomere.
Z disc: Narrow,plate-shaped
regions of dense protein material
Structural Proteins
Titin, half sarcomere, from a Z disc to an M line (Elastic)
Myomesin: M line. holds the thick filaments
Nebulin: anchor thin filaments to Z discs
Dystrophin: links thin filaments integral membrane proteins of the sarcolemma.
α-actin: Z disc
Proteins
Structural: Titin, Myomesin, Nebulin,
Dystrophin and α-actin
Contractile: Actin (thin) and Mysin
(thick)
Regulatory: Tropomyosin and
Troponin
*
*
In relaxed muscle, myosin is blocked from
binding to actin because strands of
tropomyosin cover the myosin binding site
on actin. The tropomyosin strand is held in
place by troponin.
When Ca+2 bind to troponin, it undergoes
a change in shape; this change moves
tropomyosin away from myosin-binding sites
on actin, allowing myosin to bind to actin
and muscle contraction to begin.
Sliding Filament Theory
• Myosin heads attach to actin molecules (at binding (active) site)
• Myosin “pulls” on actin, causing thin myofilaments to slide across
thick myofilaments, towards the center of the sarcomere
• Sarcomere shortens, I bands get smaller, H zone gets smaller, &
zone of overlap increases
• As sarcomeres shorten, myofibril shortens. As myofibrils shorten,
so does muscle fiber (filament itself do not shorten)
• Once a muscle fiber begins to contract, it will contract maximally
• This is known as the “all or none” principle
Cardiac muscle tissue
Striated like skeletal muscle
Shorter in length and less circular (squarish) in transverse section
exhibit branching; Y shape appearance.
Centrally located nucleus
Makes up myocardium of heart
Unconsciously (involuntarily) controlled
Cardiac muscle tissue
intercalated discs: thickenings of the sarcolemma
1) Desmosomes
2) Gap junctions
Have endomysium, but no epimysium
same arrangement of actin and myosin, and the same bands
(T) tubules of cardiac muscle are wider but less abundant than those
of skeletal muscle. one T tubule per sarcomere, located at the Z
disc.
Smooth muscle tissue
• Makes up walls of organs & blood vessels
• Tissue is non-striated & involuntary
• Cells are short, spindle-shaped & have a single
nucleus
• Tissue is extremely extensible, while still retaining
ability to contract
Smooth muscle tissue
smaller than skeletal
muscle fibers
The sarcoplasm contains
both thick filaments and
thin filaments, not arranged
in sarcomeres as in striated
muscle.
Contain intermediate
filaments (desmin),
structural protein
lack transverse tubules
Have endomysium, but no
perimysium or epimysium
dense bodies, attached to
intermediate filaments,
which are functionally
similar to Z discs
Smooth muscle tissue
The tension generated by
the thick and thin filaments
during contraction is
transmitted to intermediate
filaments. pulling the
dense bodies. (shortening
of the muscle fiber)
Calcium ions flow into
smooth muscle
sarcoplasm from both the
interstitial fluid and
sarcoplasmic reticulum
smooth muscle contraction
starts more slowly and
lasts much longer