Muscle tissue allows for the movements of the body and its parts. Muscle cells are specialized for contractility and electrical conductivity.
- Muscle cells are elongated. This shape functions well for contraction.
- The cytoplasm of muscle cells is called the sarcoplasm.
- Myofibrils are thread-like contractile structures in these cells.
- Myofibrils are composed of myofilaments of the contractile proteins actin and myosin.
i. found in hollow visceral organs such as the gut, uterus and blood vessels
ii. associated with various exocrine glands.
b. Striated Involuntary Muscle - found in the heart (cardiac muscle)
c. Striated Voluntary Muscle - makes up the skeletal muscles of the body
I. Skeletal Muscle - is found attached to the vertebrate skeleton. It produces most of the bodys movements.
1. The cells are
called skeletal muscle fibers (SMF). They are long and cylindrical in shape.
fiber is covered with a delicate connective tissue called the endomysium. Collagenou
fibers of the
endomysium interconnect each skeletal muscle fiber with adjacent cells in a bundle called the fascicle. The
collagen fibers from each cell become incorporated into the tendon joining the cells with the periosteum of the
5. The cell membrane of the SMF is called the sarcolemma.
6. Within the SMF
across a narrow space called the synapse. The sarcolemma across the synapse is specialized to response to
the nerve message and is called the motor end plate.
11. Blood vessels follow
the connective tissue wrappings into skeletal muscle tissue. A rich network of
capillaries surrounds each SMF within the endomysium.
channels are found in the epimysium and perimysium only.
II. Smooth Muscle - is often called involuntary. It is present in the walls of many visceral organs. Its activity is controlled by the autonomic nervous system. Peristalsis and the contractions of birth are examples of its activity.
A. Histological Characteristics:
1. Cells or fibers are elongated with tapered ends.
2. The fibers range in width from 3 to 8 um and in length from 18 to 200 um. In the pregnant uterine wall they enlarge from 70 to 500 um.
3. Each fiber is surrounded by a basal lamina and fine reticular fibers.
4. Elastic fibers also form close associations between adjacent smooth muscle cells.
5. Gap junctions are commonly seen
between adjacent fibers. These junctions, similar to those seen in the
intercalated discs between cardiac muscle fibers, are responsible for the coordinated action of sheets or
bundles of smooth muscle cells.
6. Within the smooth muscle fiber:
7. Smooth muscle fibers are grouped in
different numbers and orientations depending on their location and
a. Solitary fibers are found around the small arterioles.
b. The cells can be grouped in small isolated bundle forming a fascicle, for example, the arrector pili of a hair follicle. The cells in the bundle are closely packed so that the wide midsections are adjacent to the tapered ends of nearby cells.
c. In sheets, the bundles may show the same orientation, as in the wall of the intestine or vas
deferens. They may also be arranged in different orientations as seen in the stomach wall, urinary
bladder or uterus.
8. Smooth muscle tissue has only a
moderate blood supply. Capillaries run through the connective tissue
around bundles but do not enter the bundles.
9. Autonomic nerve end
branches terminate on some smooth muscle fibers. The excitation of fibers lacking
innervation can be due to
diffusion of neurotransmitters through the interstitial spaces or propagation of
excitation via gap junctions.
1. Cardiac muscle fibers (CMF) range from 9 to 22um in diameter.
2. They are arranged in columns with cells in adjacent columns connected by thin slanting cellular bridges.
3. The cells are discrete but the "bridges" give the impression of a syncitium.
4. Connective tissue (endomysium) is found between columns of cells.
5. The cells are uninucleated and show some striation.
6. Within the CMFs:
- the nuclei are ovoid and centrally located.
- Sarcoplasm is abundant and extends above and below the nucleus forming a spindle-shaped arrangement.
- Myofibrils are finer and seem less distinctly banded than in SMF. In some regions of the same cell, the myofibrils fuse giving a more striated appearance.
- Mitochondria are abundant and arranged in rows. These rows are separated from one another by myofibrils.
- Lipid droplets can be seen in the sarcoplasm. With age, lipofuchsin, a brown pigment, will accumulate in CMFs. This may occur to such an extent that the myocardium will have a brown color. This condition is referred to as "brown atrophy of the heart".
- The sarcoplasmic reticulum is less developed than in SMFs.
7. The intercalated discs are specialized cell junctions between CMFs. They are located At Z lines and carry out important functions for the cell:
- The disc contains gap junctions which permit the rapid conduction of chemical and electrical activities between CMFs.
- The disc also represents the location of firm adhesions between CMFs. This allows these cells to "pull together" during a contraction.
- Myofibrils anchor themselves via their Z lines on the discs. This allows the myofibrils to transmit the tension of their contraction through the entire CMF.
8. Purkinje fibers are CMFs specialized to function primarily for conduction of the electro-chemical waves of depolarization and repolarization through the myocardium.
a. These cells form a network close to the endocardium. They are particularly evident near the
b. Purkinje fibers are larger and thicker, about 50um in diameter.
c. They are paler than the surrounding CMFs.
d. They contain abundant sarcoplasm and relatively few myofibrils.
e. Purkinje fibers are seen to contain large numbers of glycogen granules.