Download Skeletal Muscle

NAME: EFE-ODENEMA KOBIRUO
MATRIC NO: 14/MHS02/022
DEPT: NURSING SCIENCE
COURSE CODE: ANA 203
HISTOLOGY OF MUSCLE AS A TISSUE
MUSCLE
Contractility is a fundamental property of cells and the majority of them contain essentially the
same contractile machinery as that found in muscle cells. In muscle cells, however, a larger
proportion of the cells' resources are given over to this function than in other cell types.
Muscle function:
1. contraction for locomotion and skeletal movement
2. contraction for propulsion
3. contraction for pressure regulation
Muscle classification: muscle tissue may be classified according to a morphological
classification or a functional classification.
Morphological classification (based on structure)
There are two types of muscle based on the morphological classification system
1. Striated
2. Non striated or smooth.
Functional classification
There are two types of muscle based on a functional classification system
1. Voluntary
2. Involuntary.
Types of muscle: there are generally considered to be three types of muscle in the human body
Skeletal muscle: which is striated and voluntary
Cardiac muscle: which is striated and involuntary
Smooth muscle: which is non striated and involuntary
These are the three types of muscle:
Skeletal Muscle
Contractions move part of the skeleton. Also called 'voluntary' because usually its contractions
are under your control. Skeletal muscle cells are elongated or tubular. They have multiple nuclei
and these nuclei are located on the periphery of the cell. Skeletal muscle is striated. That is, it has
an alternating pattern of light and darks. Skeletal muscle: moves joints by strong and rapid
contractions
It has a stripy appearance, because of the repeating structure of the muscle: there are many
myofibrils (fibers), each one of which is made up of repeating units called muscle sarcomeres.
Each sarcomere is 2.5 m long. Can you work out how many sarcomeres are there (placed end to
end) in your biceps muscle, which approximately 25cm long? Skeletal muscle is designed as a
bundle within a bundle arrangement
Cardiac Muscle
Cardiac muscle cells are not as long as skeletal muscles cells and often are branched cells.
Cardiac muscle cells may be mononucleotide or binucleated. In either case the nuclei are located
centrally in the cell. Cardiac muscle is also striated. In addition, cardiac muscle contains
intercalated discs.
Cardiac muscle makes up the muscular walls of the heart (myocardium). It is 'involuntary'
because its contractions are not under your control. However, it has a similar ultrastructural
organisation to skeletal muscle. So, it too has a stripy appearance because of the repeating units
called muscle sarcomeres.
(click here to find out more)
Smooth Muscle
Smooth muscle cell is described as spindle shaped. That is, they are wide in the middle and
narrow to almost a point at both ends. Smooth muscle cells have a single centrally located
nucleus. Smooth muscle cells do not have visible striations although they do contain the same
contractile proteins as skeletal and cardiac muscle, these proteins are just laid out in a different
pattern.
Found in the walls of most blood vessels and tubular organs such as the intestine. It is also
'involuntary'. However, it does NOT have a stripy appearance, because it does not have repeating
sarcomeres. The contractile proteins, myosin and actin are much more randomly arranged than in
skeletal or cardiac muscle.
NOTE
Skeletal Muscle and Cardiac Muscle are also called 'striated muscle', because they have dark and
light bands running across the muscle width when they are looked at under the microscope.
Confusingly the prefixes myo- and sarco- (respectively from the Latin and Greek, both meaning
muscle) are often used when naming structures and organelles associated with muscle.
Thus the plasma membrane of muscle cells is sometimes called the sarcolemma and their
cytoplasm sarcoplasm.
Their endoplasmic reticulum is called sarcoplasmic reticulum and their mitochondria are
sometimes called sarcosomes.
The contractile fibres that lie in the sarcoplasm are known as myofibrils and the embryonic
precursors of skeletal muscle cells are called myoblasts.
The sarcolemma is the name given to the plasma membrane of the muscle cell. There are
specialized invaginations of the sarcolemma that run transversely across the cell. These
invaginations are known as T tubules (short for transverse tubules). The T tubules are essential
for carrying the depolarization brought to the cell by a motor nerve impulse down into the
muscle cell where it can have an affect on the terminal cisternae. We will cover more about this
in the unit on the physiology of muscle contraction.
The cytosol is the cytoplasm of the muscle cell.
The sarcoplasmic reticulum is the endoplasmic reticulum of the muscle cell. There are sac-like
regions of the sarcoplasmic reticulum known as terminal cisternae. The terminal cisternae act as
calcium storage sites. The calcium ions stored in the terminal cisternae are essential in muscle
contraction. We will cover more about this in the unit on the physiology of muscle contraction.
NOTE: this is not calcium storage for use in general body physiology as we would see with bone
tissue, but rather is calcium storage for muscle contraction.
Muscle: Cell Junctions
There are several kinds of cell-cell junctions. A description of these can be found here.
Cardiac cells are special, amongst the muscle types, because they are connected to each other by
intercalated discs - structures that are only found in cardiac muscle cells. These can be seen in
this diagram, as darkly staining irregular lines, at 90 degrees to the striped sarcomeric pattern.
Intercalated discs contain three different types of cell-cell junctions:
1. Fascia adherens junctions (anchoring junctions)
o where actin filaments attach thin filaments in the muscle sarcomeres to the cell
membrane.
2. Expanded desmosomes
o sites of strong adhesion, that help to keep the muscle cells connected when they
contract.
3. Gap junctions
o large and small, which provide direct contact between the cardiac cells, facilitating
electrical communication, so that waves of depolarization spread rapidly over the entire
heart, by passing from cell to cell.
Skeletal muscle does not have any cell-cell junctions.
Smooth muscle contains gap junctions, to allow a rapid spread of depolarization, as in cardiac
muscle.
Muscle: Muscle regeneration
Skeletal muscle contains numerous 'satellite cells' underneath the basal lamina, as shown in the
photograph opposite. These are mononucleated quiescent cells. When the muscle is damaged,
these cells are stimulated to divide. After dividing, the cells fuse with existing muscle fibres, to
regenerate and repair the damaged fibres.
The skeletal muscle fibres themselves, cannot divide. However, muscle fibres can lay down new
protein and enlarge (hypertrophy).
Cardiac muscle can also hypertrophy. However, there are no equivalent to cells to the satellite
cells found in skeletal muscle. Thus when cardiac muscle cells die, they are not replaced.
Smooth cells have the greatest capacity to regenerate of all the muscle cell types. The smooth
muscle cells themselves retain the ability to divide, and can increase in number this way.
As well as this, new cells can be produced by the division of cells called pericytes that lie along
some small blood vessels.
Smooth muscle can also hypertrophy.
Muscle: Stimulation
This picture shows nerve fibres approaching skeletal muscle fibres, and forming a neuromuscular
junction. The structure of the neuromuscular junction is described in more detail here.
Skeletal muscle is stimulated via a nerve impulse, which depolarizes the muscle. However, not
all the muscle fibres in the muscle fibre will necessarily be activated at once. Sometimes, a
subset of muscle fibres is activated, depending on how much force is needed.
When the muscle is stimulated, calcium ions are released from its store inside the sarcoplasmic
reticulum, into the sarcoplasm (muscle).
Then the calcium ions bind to a protein called troponin, on the thin filaments, which in turn
allows myosin to bind to actin. This interaction makes the thick and thin filaments slide past each
other, to make the muscle shorten.
Invaginations of the plasma membrane (sarcolemma) of the muscle fibres are called T (or
transverse) tubules. The T-tubules lie over the junction between the A- and I-bands (see
diagram).
The two terminal cistemae of the SR together with their associated T tubule are known as a triad.
Inside the muscle fibre, the T-tubules lie next to the terminal cisternae of an internal membrane
system derived from the endoplasmic reticulum, called the sarcoplasmic reticulum (SR), which
is a store of calcium ions. Stimulation of the muscle fibre, causes a wave of depolarisation to
pass down the t-tubule, and the SR to release calcium ions into the sarcoplasm. Calcium is
pumped back up into the SR to lower calcium ion concentration in the sarcoplasm, to relax the
muscle (turn off contraction).
Cardiac Muscle also has T-tubules, and SR. However the T-tubules lie over the Z-line in
cardiac muscle, are less numerous and wider. The SR is smaller and less elaborate, and stores
less calcium ions. Cardiac muscle cells also depend on extracellular calcium ions, that enter
through the T-tubules and triggers release of calcium ions from the SR.
Cardiac muscle cells are electrically connected through gap junctions, so that waves of electrical
stimuli pass around the heart from cell to cell, and all the cells are stimulated to contract.
Smooth Muscle. The thick and thin filaments are attached to alpha-actinin in dense bodies
(equivalent to Z-lines in skeletal muscle), which are attached to the plasma membrane by
intermediate filaments. The thin filaments do not have troponin. This type of muscle responds to
a increase in calcium, following nerve stimulation through a protein called calmodulin. Binding
of calcium to calmodulin, results in the activation of an enzyme (myosin light chain kinase) that
phosphorylates myosin, which activates it, enabling it to interact with actin.
Muscle: Skeletal and Cardiac Muscle Ultrastructure
This is a high power, light micrograph of a muscle fibre showing the banding pattern. There are
light stripes - which are called the 'Z' lines, and darker wider stripes called the 'A' bands. (A - for
anisotropic - because in a polarizing light microscope, the dark bands are birefringent)
The Z-lines are midway inside a light band, called the 'I' band. (I for isotropic - because in a
polarising microscope, these bands are much less birefringent than the A bands).
The muscle sarcomere is the repeating unit (S) between two Z-lines.
N - is a nucleus on the outside of the fibre.
This is an electron micrograph (EM) of a skeletal muscle fibre. (The sarcomeres of cardiac
muscle have a very similar organisation).
Notice how the stripes appear less regular than in the light microscope. This is because the
repeating muscle sarcomeres are arranged in longitudinal structures called 'myofibrils' (these run
from top right to bottom left of the picture).
This is the same EM with labels to show the organisation of the muscle sarcomeres. Z-lines mark
the boundaries of the sarcomeres. The dark staining region in the centre of the sarcomere is
called the A (anisotropic) band. The lighter staining band, through which the Z-line passes is
called the I (isotropic) band.
A diagram of a muscle sarcomere is shown below.
Thin filaments, which consist mostly of the proteins actin, troponin and tropomyosin, insert
directly into the Z-lines.
Thick filaments consist mostly of the proteins myosin and titin. Titin connects the thick filaments
to the Z-line.
There are about 300 molecules of myosin in each thick filament, and at the end of each molecule
are globular heads that stick out and bind to actin in the thin filament. When the muscle is
stimulated, the heads bind to actin, and pull on actin to pull the thin filaments towards the Mline, increasing the overlap between thick and thin filaments, and making the muscle shorten.
This uses ATP for energy.
In relaxed muscle, troponin/tropomyosin stops myosin from binding to actin. When muscle is
stimulated, calcium is released, binds to troponin, and this then allows myosin to bind to actin.
SMOOTH MUSCLE
Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the
gastrointestinal tract, the bladder, or the uterus. Its structure differs greatly from that of skeletal
muscle, although it can develop isometric force per cross-sectional area that is equal to that of
skeletal muscle. However, the speed of smooth muscle contraction is only a small fraction of that
of skeletal muscle.
Structure: The most striking feature of smooth muscle is the lack of visible cross striations
(hence the name smooth). Smooth muscle fibers are much smaller (2skeletal muscle fibers (10-unit and
multi-unit smooth muscle (Fig. SM1). The fibers are assembled in different ways. The muscle
fibers making up the single-unit muscle are gathered into dense sheets or bands. Though the
fibers run roughly parallel, they are densely and irregularly packed together, most often so that
the narrower portion of one fiber lies against the wider portion of its neighbor. These fibers have
connections, the plasma membranes of two neighboring fibers form gap junctions that act as low
resistance pathway for the rapid spread of electrical signals throughout the tissue. The multi-unit
smooth muscle fibers have no interconnecting bridges. They are mingled with connective tissue
fibers.
Fig. SM1. Single-unit and multi-unit
smooth muscle.
Electron micrographs of smooth muscle reveal that the actin filaments are organized through
-actinin, a Z-band protein in skeletal muscle. Thus,
it is assumed that the dense bodies function as Z-lines. The ratio of thin to thick filaments is
much higher in smooth muscle (~15:1) than in skeletal muscle (~6:1). Smooth muscle is rich in
intermediate filaments that contain two specific proteins, desmin and vimentin.