Download Muscles in the Human Body

Muscles in the Human Body
By: Student Name
There are 650 muscles in the human body and they play an important role in
movement, vital functions, heat regulation, among others. There are three
types of muscle: the skeletal muscle, the smooth muscle and the cardiac
muscle. While they all share some key features common to muscle cells such
as responsiveness, conductivity, contractility, extensibility and elasticity,
they also have major differences in other levels. The following essay will
compare and contrast the first two types, first in regards to location and
function, then overall appearance and major components of the muscle,
followed by the muscle cell anatomy and finally the method of contraction.
A) Skeletal Muscles
Skeletal muscle cells are found attached to tendons in bones and they are involved in
performing the body’s movements which includes walking or lifting weights. Smooth muscle cells,
on the other hand, are characteristic of the walls of hollow organs. This type of muscle can be
found in the gastro-intestinal track, where it assists the movement of food through peristalsis.
The same happens in the bladder, where muscles contract to expel the urine, or in the uterus,
where the contraction movements help during labor (Assefa et al., 2003).
One striking difference in the appearance of both muscles is the striations,
which are present in the skeletal muscle but not in the smooth (Hardin et
al, 2012). The former has a cylindrical, straight shape with several nuclei
in the periphery of each cell. This type of cell is multinucleated mainly to
facilitate the transport of proteins, considering that they would otherwise
have to cover large distances to travel along the cell, which can be up to
2-3cm long and (10-100 µm) in diameter (Alberts et al., 2008). Smooth
muscle cells are smaller, around 2-10 µm wide, and they have a fusiform
shape, sometimes referred to as spindle-shape, with tapered ends and a
single nucleus localized in the centre (Guyton and Hall, 2006).
The skeletal muscle is covered by a connective tissue called the
epimysium that protects it by reducing the friction between the
muscle and the surrounding bone and tissue (Hardin et al., 2012). The
whole muscle is divided into several sections called the fascicles, which
are surrounded by another connective tissue, the perimysium. Inside each
fasciculus there other internal structures, the muscle cells - also known as
muscle fibers because they are longer than they are wide (Guyton and
Hall, 2006). Each muscle cell is then wrapped around in another
connective tissue called the endomysium. The endomysium is the only
connective tissue present in smooth muscles (Anthony and Thibodeau,
1983). As shown in Figure 1, inside each muscle fiber there are specialized
intracellular structures called myofibrils , which are responsible for the
muscle contraction. Each myofibril is composed of sarcomeres, which are
the unit of repetition. In each sarcomere, there are thick and thin
filaments - the myosin and actin filaments, respectively. Myofibrils are
surrounded by the sarcoplasmic reticulum, which contains a fluid rich in
calcium ions, and also by the T-tubules, which connect to the outer
membrane, the sarcolemma, and contain ion channels for calcium (Guyton
and Hall, 2006).
2. Smooth Muscles
The structure of smooth muscle is significantly different. For a start, this type of cells does not
possess T-tubules and their sarcoplasmic reticulum is poorly developed. They also lack
sarcomeres (AnatomyGMC, 2011). “Actin filaments are found attached to dense bodies made
up of α-actinin, a protein which also appears in Z-lines in skeletal muscles; therefore, dense
bodies in smooth muscles act as Z-lines in skeletal muscles” (Assefa et al., 2003). These actin
filaments are either on the sarcolemma or on the cytoskeleton. Intertwined with them are the
thick filaments that have no regular alignment, which can explain the lack of striations. The
ratio of thin/thick filaments is significantly higher in smooth muscle when compared to skeletal
muscle (Shier et al., 2007).
One of the major differences between these two types of muscles lays in the regulation of
contraction. In skeletal muscles this happens voluntarily through the axon terminals of the
somatic motor neurons, and the contractions can vary from a simple twitch to a strong
contraction that enables the lift of heavy weights. Contrarily, in smooth muscles this is
involuntary (Assefa et al., 2003). Contraction is done instead through autonomic nerves,
hormones and local chemicals. Because of this discrepancy, the method of contraction changes
considerably. In the first case, the muscle only contracts if there is a stimulus from a somatic
neuron. On the one side, there is the axon terminal of the motor neuron with several vesicles
containing neurotransmitters known as Acetylcholine; on the other side, there is the
sarcolemma with T-tubules with the thin filaments nearby which need calcium in order to
contract. However, these calcium ions are in the terminal cisternae of the sarcoplasmic
reticulum, but they cannot move into the T-tubules because the ion channels are still blocked.
They only open when the neuron’s fibers connect to a muscle fiber, complex called the motor
unit (Guyton and Hall, 2006). The action potential arrives at the axon terminal, making the
vesicles release Acetylcholine. This neurotransmitter then binds to the receptors in the
sarcolemma, causing them to open. At this stage, sodium ions can enter the cell and induce the
opening of voltage gate channels, creating an action potential. The action potential then travels
across the sarcolemma and propagates down the T-tubules. Once this happens, the calcium ion
channels open due to the change in voltage and as a consequence the calcium can move out
from the cisternae into T-tubules and bind to the troponin that is attached to the thin
filaments. Myosin can start using ATP to slide along the actin filaments, which are then pulled
together causing the muscle to contract (Shier et al., 2007).
Unlike in skeletal muscle, smooth muscle contraction does not depend on troponin. The process
begins by an increase on the concentration of calcium. This is done through the sarcoplasmic
reticulum and also from the extracellular fluid. This increase in calcium increases the likelihood
of calcium binding to calmodulin (CaM), a calcium binding protein. When they bind, myosin light
chain kinase (MLCK) is activated and ready to phosphorylate the heads on the myosin chain due
to an increase of ATPase activity. This enables the myosin cross bridges to slide along actin
filaments, the same way that happens in skeletal muscle contraction, in order to create muscle
tensions (Michael, 1996).
In conclusion, from the previously outlined
differences between skeletal muscle cells and
smooth muscle cells, the most essential are the
cell appearance and the regulation of contraction.
While in skeletal muscles the contraction is
voluntary, in smooth muscles this occurs
involuntarily and in a slower pace. These
differences can essentially be reduced to the
different function/purpose of the muscle and where
it is located; all other differences arguably
follow from this most fundamental one.
Bibliography
Alberts, B. et al. (2008) Molecular Biology of the cell. 5th edition. New York: Garland Science.
AnatomyGMC Anatomy and Physiology Help: Chapter 10 Muscle Tissue. 2011 [video online] Available
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Anthony, C.P. and Thibodeau, G.A. Textbook of anatomy and physiology. 11th edition. St. Louis:
The C.V. Mosby Company (1983).
Assefa, N. et al. 2003. Human Anatomy and Physiology. Ethiopia: EPHTI
Guyton, A. C., Hall, J.E. (2006) Textbook of Medical Physiology. 11th edition. Philadelphia: Elsevier
Saunders.
Hardin, J. et al. Becker’s World of the cell. 8th edition. San Francisco: Pearson. 2012;
Michael, B. (1996) Biochemistry of Smooth Muscle Contraction. [e-book] Available at
<http://NCL.eblib.com/patron/FullRecord.aspx?p=316978> [Accessed 26 November 2013].
Shier, P. et al. (2007) Hole’s Human Anatomy and Physiology. 11th edition. New York: McGraw-Hill.
Figure 1 from page 1 taken from [online]
<Http://kurser.iha.dk/eit/bim1/Noter/BIOMECHANICS_OF_MUSCULOSKELETAL_TISSUES/CHP7.P
DF> [Accessed 26 November 2013].