MUSCLE TISSUE
- Capable of Contraction
- Composition = Muscle cells + CT (carries blood vessels and nerves, each muscle cell
is supplied with capillaries
and nerve fiber) - Muscle cells are elongate (therefore they are termed fibers) and lie
in parallel arrays (with the longitudinal axis of the muscle).

THREE HISTOLOGICAL TYPES OF MUSCLE FIBERS

1) Skeletal (Striated) = striated (striped in appearance under microscope), voluntary
(under conscious control)
2) Cardiac = striated, involuntary
3) Smooth = non-striated, involuntary

SKELETAL MUSCLE
I. CELLS (FIBERS)

1) Very long compared with most other cells, up to several cm long, 10-100
micrometers in diameter
2) Multinucleate, nuclei are located peripherally
3) Development:
Mesenchymal cell ---> Myoblast (proliferative) ---> Myotubule ---> Muscle Cell

II. ARRANGEMENT OF FIBERS - similar to tendon arrangement
- Blood vessels, lymph vessels, and nerves penetrate muscle
with perimysium
- Endomysium contains capillaries and nerve fibers
III. STRIATION ULTRASTRUCTURE (Fibers ---> Myofibrils --->
Myofilaments)
- Proteins are actin (thin filaments) and myosin (thick
filaments), also tropomyosin and troponin are associated
with thin filaments



Sarcomere = the smallest contractile unit of skeletal
muscle, bounded by Z-lines, 2-3 micrometers long.
Z-line = disc-like structures to which actin filaments
attach on both sides; composed of alpha-actinin and a dense amorphous matrix
A-band = "anisotropic" band: birefringent in polarized light (as light source
rotated 360 degrees becomes light-dark-light-dark); signifies greater than 1 molecular
species present (in this case, myosin & actin filaments)
I-band = "isotropic" band: maintains darkness in polarized light; signifies
a singular molecular species is present (actin filaments)
H-zone = pale central region in A-band; due to absence of thin filaments;
outer portions of A-band with both filaments
M-line = thick filaments interconnected by cross-linking fine radial filaments,
acts to maintain regular spacing and arrangement of thick filaments

ELECTRON MICROSCOPY ULTRASTRUCTURE


- During contraction, A-band width remains unchanged, I-band width decreases,
H-zone also decreases, Sarcomere
shortens (SEE HANDOUT)

MYOFILAMENTS
1) Thin = composed mainly of F-actin (polymer of globularG-actin subunits) in
two-stranded double helix (1 micrometers in length); associated with actin double
helix is a long slender filament of tropomyosin that lies in the groove between the
2 F-actin strands; Troponin (globular protein) is attached to tropomyosin at regular
intervals.
2) Thick = composed of myosin (1.5 micrometers long) arranged in bundle;
Structurally has smooth central region with projections at each end; each myosin
molecule is shaped like a golf club with a shaft and head (SEE HANDOUT)

To Lecture 10