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We left our study of penguin histology with a close look at bone. Bone forms the fixating points for skeletal muscle (or most skeletal muscle anyway). To generate the powerful movements require to whip through the water, penguins recruit muscle fibers to propel themselves. Here's a closer look... MuscleSkeletal muscle is invested by a thin connective tissue blanket composed of collagen and elastic fibers. Remember, collagen was the same fiber used to form bone. Collagen is particularly strong and is well suited to be part of the musculature. Muscle is composed of the following components:
Don't let these terms scare you off, for they are simple concepts once you look at a histological section of muscle.
There are many muscles in penguins that are involved in swimming, and you will learn more about them in the anatomy section. The basic operation for all skeletal muscles is the same. After a synapse occurs, a signal is given to each muscle fiber to contract. Within each muscle fiber is a bundle of filaments. These filaments are made of proteins called actin and myosin. The two of these muscle fibers make up the sarcomere. The sarcomere is the functional unit of the muscle.. The sarcomere shortens each time the muscle contracts. When the actin and myosin pull on each other, the muscle shortens. When the muscle shortens, this is called contraction. |
On the right, you can see a cross-section of penguin muscle. The light pink and tan colored bodies
you see distributed throughout the image are actual muscle cells, also known as muscle fibers. If you look
closely, you can visualize the eccentric nucleus of each muscle fiber. Look for the small purple dots on the sides
of each muscle fiber. Running between these tan and pink bodies is the endomysium, a thin layer of
collagen tissue that separates each cell from one another. If you look in the upper right, just off the center of the image,
you can see a large empty space of collagen tissue. This is part of the perimysium. Perimysium is thicker
collagen tissue that creates fascicles, or bundles of muscle fibers. Several fascicles rolled up together create
the muscle that you can see and feel. These bundles of fascicles are surrounded by what? If you guessed
epimysium, you are correct. From the outside-in, it goes epimysium to perimysium to
endomysium. Click 
Penguin skeletal muscles are under voluntary control. That means that they won't function unless
the penguin wants them to function. In order to get muscles contracting, the brain sends signals through motor
nerves. Nerves consist of many, many neurons, or single nervous cells. As a neuron descends from the central
nervous system, it branches (i.e., ramifies). Each branching of a neuron connects with each muscle fiber. One motor
neuron services one motor unit, a collection of muscle fibers. When stronger contractions are needed, e.g. when
a penguin needs to escape a predator, more motor units are recruited. As more motor units are recruited, more muscle
fibers contract and create stronger muscle contractions, propelling penguins. The image on the left was performed
with a scanning electron microscope. You can see a "thick" muscle fiber descending from the upper left down to the
lower right. A thin wisp of a neuron (N) can be seen winding down from the upper left and synapsing with
the muscle fiber at the myoneural junction (MJ).
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