From http://www.cartage.org.lb/en/themes/Sciences/Zoology/AnimalPhysiology/Muscles/Muscles.htm
Skeletal muscle is the type normally thought of when one thinks of the
word muscle, but do not be mislead, this is only one of three
different types of muscle. The three different muscles, classified by
their general microscopic appearance, are skeletal, cardiac and
smooth. Let's start first with skeletal.
Skeletal muscle is characterized by elongated, multi-nucleated fibers,
or cells, with striations due to light, I band, and dark, A band,
alternations. The nuclei of the fibers are not centered, but rather
located towards the outer edge of the cells. These muscles are under
somatic or voluntary control. This means they are under immediate
conscious control by the nervous system. Skeletal muscle has a name
related to its function since it is usually associated with attachment
to and movement of bones and cartilage. Skeletal muscle requires
either extrinsic nerve or hormonal stimulation to create action
potentials, which trigger movement.
Cardiac, like skeletal muscle, is also striated. Though unlike
skeletal, cardiac muscle cells are short and branched with a single,
centered nucleus. They are also involuntary or not under immediate
conscious control. Rather than Z-disks, which join skeletal muscle
cells, intercalated disks join cardiac muscle fibers. Cardiac muscles
are located only in the heart. Unlike skeletal, cardiac muscle can
contract without extrinsic nerve or hormonal stimulation. It contracts
via its own specialized conducting network within the heart, with
nerve stimulation causing only an increase or decrease in rate of
conducting discharge. The heart also has some very beneficial features
such as an increased number and larger mitochondria, which allow it to
produce more ATP. This is very important since the heart is constantly
contracting and relaxing. Cardiac muscle can also convert lactic acid
produced by skeletal muscle to ATP. This is quite ingenious since
lactic acid is a by-product of muscle when in a deoxygenated state, a
state that would be detrimental to cardiac muscle. This muscle also
remains contracted 10 to 15 times longer than skeletal muscle due to a
prolonged delivery of calcium (see discussion of cardiac action
potential in Circulation section). Likewise, it also has a relatively
long refractory period, lasting several tenths of a second, allowing
heart to relax between beats. This also allows heart rate to increase
significantly without causing it to go into tetanus, which would be
fatal since it would cause blood flow to cease.
Lastly there is smooth muscle in which ellipsoidal cells have a single
centered nucleus. Like cardiac, smooth muscle is also involuntary.
Smooth muscle is not striated like cardiac and skeletal due to a
non-orderly arrangement of myosin and actin filaments. Its muscle also
lacks Z-disks, instead microfilaments are attached to each other by
dense bodies. Sliding of the filaments generates tension that is
transmitted to intermediate filaments, in turn they pull on dense
bodies attached to sarcolemma causing lengthwise shortening of muscle
fibers. Contractions are corkscrew-like, start more slowly and last
much longer in comparison to striated muscle. This is because smooth
muscle lacks T-tubules, therefore it takes longer for calcium to reach
the filaments as well as be reabsorbed by the terminal cisternae.
Smooth muscle can also stretch to a greater extent than striated. The
bladder, for example, contains calmodulin instead of troponin, which
striated muscle contains. In smooth muscle, the myosin binding sites
on actin are blocked by caldesmon. When the calcium-calmodulin complex
binds to the caldesmon, the caldesmon moves away from the myosin
binding sites on the actin molecule and cross-bridges can form. Smooth
muscles also is sensitive to mechanical stimulation, and may contract
when stretched. Depoalariztion of smooth muscle often involves the
inward flow of calcium across the cell membrane, rather than sodium.
There are two different types of smooth muscle, single-unit and
multi-unit. Single-unit cells, usually small and spindle shaped, are
connected to one another by gap junctions which allows excitation to
spread throughout the muscle without neuronal input. Internal
pacemaker cells usually regulate contraction in this type of muscle.
Single unit cells form the walls of vertebrate viceral, hollow organs
such as the urinary bladder, ureters, uterus, stomach, intestines,
small arteries and veins. Multi-unit cells each act independently and
only when they receive neuronal input. This type of muscle forms the
iris of the eye, large arteries, bronchioles and arrector pili muscle
(for elevating hairs - the "goose bump" muscles).
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