Download Cardiac Muscle Cells

CARDIAC MUSCLE CELLS
Erin Mears
AP BIO 5/13/14
Muscle cells, also known as myocytes, are divided into three distinct groups:
Skeletal (or striated), smooth, and cardiac muscle. The three types of muscle cells
are similar in look and function. Each type will extend and contract to fulfil their
purpose of motion, conduct an action potential, quickly react to stimulation, and
retain its elasticity after constant expansion and contraction. These long, tubular
cells band together to create the fibrous structure of muscle tissue that binds with
nerves, blood vessels, and connective tissue. Their movement is controlled by the
nerve signals to expand and contract, whether it is voluntary or involuntary. Each
type, however, has very different specific tasks in accordance to its own muscle
group. Skeletal muscles comprise of all voluntary muscle coordination which
allows us to move, while smooth muscles pertain to the involuntary muscle
interaction that occur within the internal organs, such as the bronchi of the lungs or
the stomach. These muscles are not controlled directly with voluntary nerve
impulses like skeletal muscle is.
Cardiac muscle have unique qualities in comparison to its smooth and
skeletal myocyte counterparts. These cells are found only in the heart, and serve
the specific function to pump blood throughout the body. They are designed in a
way to allow all of the cells to oscillate at once, contracting together strong enough
to allow blood flow throughout the body. The tubular cells can branch out from
each other unlike skeletal or smooth muscle tissues. These branches of cells
connect to one another at adhere junctions. This creates a stronger junction that
enables the heart muscles to contract forcefully without tearing any of the muscle
fibers. These cells also may have between one or two nuclei per cell, usually
centrally located, a unique adaption to muscle cells.
Cardiac myocytes form long tubular structures that branch out into several
connecting cells, attaching to one another at intercalated disks. Inside the disks
contain gap junctions and desmosomes. Gap junctions are a protein-lined tunnel
region that allow two cells to communicate to one another by letting ions float
through the gap to a neighboring cell in a depolarization wave. These junctions
allow for mass cellular communication. This action potential is what triggers the
heart to beat within itself, to cause the cells from atria to ventricle to coordinate
their contraction in unison in order to induce proper blood flow. The desmosomes
act like cellular staples, holding the cells together during these stressful
contractions. These molecular complexes of cell-adhesion proteins and linking
proteins connect cells together and provide a great resistance to mechanical stress
due to their high adhesion.
Inside the myocyte walls several mitochondria can be found, in a much
higher quantity than in a normal cell. This is due to the high amount of energy
produced and consumed that is necessary to keep the heart pumping. This high
mitochondrial density produces adenosine triphosphate quickly in order to resist
fatigue while maintaining a steady rhythm of action potential. The sheer amount of
mitochondria per cell also reflects the greater dependence for cellular respiration
for the production of ATP in these cells. Due to the unique properties of muscle
cells, a few standard organelles have been given specialized terminology to better
describe the organelles. For example, the typical cytoplasm is renamed sarcoplasm,
similar to cytoplasm, but it contains large amounts of glycosomes and myoglobin,
an oxygen binding protein, as well as myofibrils within the sarcoplasm, all held
together by the sarcolemma (plasma membrane). The sarcoplasmic reticulum hold
common traits with its counterpart, the smooth endoplasmic reticulum. The
sarcoplasmic reticulum forms a network around each myofibril of the muscle fiber.
This network is composed of groupings of two dilated end-sacs called terminal
cisternae, and a single transverse tubule, or T tubule, which travels through the cell
from one side to the other. Together these three components form the triads that
exist within the network of the sarcoplasmic reticulum. The sarcoplasmic reticulum
serves as reservoir for calcium ions, in which the T tubule signals the sarcoplasmic
reticulum to release calcium from the gated membrane channels to stimulate a
cardiac muscle contraction
The muscle cells communicate with the rest of the body by connecting
directly with nerves through neuromuscular junctions, similar to how a neuron
transmits signals with the use of synapses. In a normal muscle cell, a nerve impulse
will transmit through the nerve to the axon terminal, where a neurotransmitter is
released from the vesicles, depolarizing and spreading widely across the cells to
directly send a signal and move the muscle. The cardiac muscle cells may either
being in an active state or a rest phase, a polarized state. During the resting
potential, ions such as sodium, calcium and potassium are separated, causing the
electrical cell to generate an impulse. This impulse will force the ions to cross the
cell membrane which will create contractions of the heart muscles. This combined
movement of contractions and depolarization evokes a wave of movement
throughout the heart. After the heart has contracted, the ions will scatter back to
their regular resting area, allowing the heart muscle to relax in a state of
repolarization. This involuntary muscle will cycle throughout the organism’s life to
maintain proper blood flow and cellular respiration.
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