Download Prelab 2 Muscle

Prelab Exercise 2 – MUSCLE
MUSCLE TISSUE
The structure of muscle cells (fibers) reflects their specialized capacity for contraction.
The cells are much longer than they are wide and are usually arranged in parallel sheets
or bundles. Contraction invariably involves shortening of the long axes of the cells.
Common to all muscle cells are cytoplasmic myofilaments, actin and myosin, which are
involved in the dynamics of contraction.
Different types of muscle cells vary markedly in form, size, structure, innervation, and
the force with which they contract. Considerable variation occurs in muscle cells of a
given type depending on their location and function.
SKELETAL MUSCLE (striated, voluntary)
Morphology.
Individual skeletal muscle fibers (i.e., muscle cells) are cylindrical and do not branch.
They are multinucleated, with peripheral nuclei. The striations are due to the arrangement
of myofibrils. The sarcomere (the portion of a myofibril located between two Z bands) is
the unit of organization. Each sarcomere can be subdivided into a darker central A band
(which is divided by the M band) and two lighter I bands.
Compare the appearance of skeletal muscle as seen with the light microscope with that
seen with the electron microscope (refer to illustrations in your textbooks and figs. 6-9 of
the “EM of muscle” from the virtual histology site). What elements comprise the bands
within the sarcomere? The transverse tubule (T-tubule)-system passes through the
muscle fiber at the level of the junction of the A and I bands. It is bracketed by two
tubular elements of the sarcoplasmic reticulum. This mechanism is important for
extending membrane excitation into the center of the muscle fiber and for release of
stored calcium from the sarcoplasmic reticulum. This, in turn, is the necessary element in
excitation/contraction coupling.
Connective tissue components.
The connective tissue sheath system of skeletal muscle consists of endomysium,
perimysium, and epimysium). It is via these connective tissue investments that blood
vessels get to the
Muscle-tendon attachments.
The connective tissue of the muscle blends with tendon at the musculotendinous junction.
Additionally, there are interdigitations of the collagenous fibers with the end of muscle
fibers with apparent anchor points between actin inside the muscle fibers with
specializations that attach to collagen extracellularly.
Motor nerve terminals/sensory nerve receptors in skeletal muscle.
Each skeletal muscle fiber has one neuromuscular junction (motor end plate). However,
a single motor nerve fibers innervates many muscle fibers (this collection of muscle
fibers innervated by one motor nerve fiber is called a motor unit.
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Prelab Exercise 2 – MUSCLE
Muscle proprioception.
Neuromuscular spindles and Golgi tendon organs are highly specialized sensory receptors
that are a part of the proprioceptive component of the somatosensory system. Muscle
spindles are comprised of a bundle
of modified skeletal muscle fibers
(“intrafusal” fibers) that are
organized in parallel with the
major muscle fibers of the muscle
("extrafusal"muscle fibers). The
intrafusal fibers have a motor
innervation (gamma motor nerve
fibers) and a sensory innervation
(the most important is the
annulospiral ending). This sensory
receptor axon detects stretch of the
muscle. The sensory nerve fiber
relays this information to the
spinal cord, where reflexes of
varying complexity are activated
to maintain posture or to regulate
the activity of opposing muscle
groups involved in motor activities
such as walking. Neurotendinous
spindles (Golgi tendon organs) are
located near the junction of
tendons with muscles. They
provide the CNS with information concerning tension within a tendon. In concert with
the muscle spindle, they provide information that allows for regulation of muscle tone.
One of these structures, the muscle spindles, may be found in your slides of skeletal
muscle tissue.
SMOOTH MUSCLE (non-striated, involuntary)
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Prelab Exercise 2 – MUSCLE
Smooth muscle fibers are spindle shaped and may appear as sheets, or as individuals
scattered in connective tissue. They do not have sarcomers, even though they do have
actin and myosin. They contain a single nucleus that is more euchromatic than most
fibroblasts. The nucleus is cigar to cucumber shaped and occasionally takes on a
“corkscrew” appearance when the muscle is contracted.
Smooth muscle cells may be arranged singly or in small groups (“multiunit
organization”). They can also be packed densely in organs such as the GI tract
(“visceral/unitary organization”). In that location they appear in distinct layers at right
angles to each other, with intervening connective tissue.
Examine the ultrastructure of smooth muscle cells in figure 1 of the “EM of Muscle”
module on the virtual histology site.
CARDIAC MUSCLE (striated, involuntary)
Cardiac muscle is found only in the wall of the heart (the myocardium). These fibers have
a single nucleus and appear in branched chains. Individual cells are connected at
intercalated discs, which have an irregular organization (they don’t appear in a straight
line). This not only anchors one cardiac muscle cell to the next but also facilitates the
spread of excitation. The structure of the cardiac sarcomere is similar to skeletal muscle
fibers except that there is a dyad (instead of a triad, as in skeletal muscle), with one T
tubule in relation to one sarcoplasmic reticulum element. Also, this dyad is located at the
Z line.
What is the structure of the intercalated disc as revealed by the electron microscope (figs.
3-5, “EM of muscle” module of virtual histology site)? Study the cross-striations of the
muscle fibers and note that the sarcomere and its subunits are not as clearly delineated as
in skeletal muscle.
Cardiac Muscle
1. Central nucleus
2. Branching
3. Intercalated Discs
4. Striations of muscle fibrils
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Prelab Exercise 2 – MUSCLE
CHECK LIST
Identify the three types of muscle in light microscope and electron microscope
images.
-skeletal (voluntary, striated)
-smooth (involuntary, non-striated)
-cardiac (involuntary, striated)
Identify the connective tissue sheath system of skeletal muscle
-epimysium
-perimysium
-endomysium
Be able to define and to identify the following terms:
-myofibrils
-sarcoplasm
-sarcolemma
-sarcoplasmic reticulum
-sarcomere
-transverse tubule system
-neuromuscular junction
-motor unit
-muscle proprioception including muscle spindles
-neurotendinous spindles (Golgi tendon organs)
-intercalated disc
Understand the fibrillar and banding patterns of skeletal and cardiac muscle shown
in both light and electron microscopic images. Identify I, M, A, Z, H bands. Know
the location of the contractile and regulatory proteins associated with each band.
Recognize the diversity, arrangement and location of smooth muscle.
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