The types of muscles of the human being can be grouped into smooth muscle and striated muscle. Skeletal muscle, in turn, is classified into two groups known as skeletal skeletal muscle and cardiac skeletal muscle..
A muscle is a tissue made up of cells called "muscle fibers" that have the ability to contract in the face of electrical stimuli, that is, to reduce their length, generating mechanical forces.
Muscle tissue allows joint displacement, body movement, and ambulation. It also participates in the fulfillment of specific functions in specialized tissues such as digestive tissue, blood vessels, the bronchial tree, the heart, among others..
The muscles also make up the sphincters, which are muscular structures that surround a tube, allowing it to open or close, promoting the emptying of the content inside..
Skeletal muscle is structurally related, as its name implies, with bones and joints, while smooth muscle is associated with visceral functions and cardiac striated muscle is typical of the heart's function as a pump..
A fundamental difference between the different types of muscles is that one group is under the voluntary control of the nervous system (the skeletal muscles), others are involuntary muscles (the visceral muscles, which are smooth muscles), and others have automatic functions (such as muscle cardiac).
Just as neurons do, muscle fibers can be excited by mechanical, chemical, or electrical stimuli, generating an action potential that is transmitted along their plasma membrane. However, these cells have a contractile mechanism that is activated by this action potential..
The contraction of muscle fibers is possible due to the presence of contractile proteins called actin and myosin, whose union represents one of the molecular “motors” that converts the chemical energy from the hydrolysis of ATP into movement.
To facilitate understanding and analysis, we will separate the characteristics of the three main types of muscles: skeletal, cardiac, and smooth muscles..
This type of muscle is characterized by the fact that each of its cells (muscle fibers) is surrounded by connective tissue, which electrically isolates them from others. For this reason, each muscle fiber must be innervated by a nerve fiber that is under voluntary control of the nervous system..
The set of muscle fibers innervated by a single nerve fiber is called a "motor unit" and this unit responds in unison to the stimulation of its nerve fiber.
Larger motor units are generally used for “gross” movements, but small motor units are used for fine and delicate movements that require a high degree of control..
The functional unit of a skeletal muscle is known as a "sarcomere." Each sarcomere is delimited by two “Z lines” and is composed of actin and myosin filaments (contractile proteins) interdigitated with each other..
The areas within successive sarcomeres that contain only fine active filaments make up the so-called "clear areas" or "clear striations" that are observed in the light microscope. The areas of the sarcomeres that contain thick myosin filaments give rise to the “dark striations” of the skeletal muscles.
Skeletal muscle contraction involves the sliding of actin and myosin fibers (one over the other) and not the shortening of these protein fibers.
The heart is made up of a special class of striated muscle that, unlike skeletal muscle, has tight connections between its fibers that allow it to function as a syncytium..
It is an automatic muscle, that is, it is a muscle capable of producing its own stimulation (contraction), without the need for the function of the nervous system. The cardiac innervation of the nervous system only provides a control mechanism of the contractile function, but does not originate it..
The heart's contractile apparatus, which allows it to function as a pump, is also made up of sarcomeres delimited by two Z lines. Its fibers or muscle cells (cardiac myocytes) are branched and linked together through structures called "intercalary discs. "And" slit joints ".
Intercalary discs are low resistance structures through which electrical excitation can be conducted from one cell to another..
Cardiac “automatism” is in charge of specialized muscle cells that generate spontaneous and rhythmic electrical activity that is transmitted to the atria so that they contract in unison and, with a certain delay, pass to the ventricular system, which contracts sequentially after these.
Smooth muscle differs from skeletal muscle in that it does not have transverse striations visible under the microscope. It also has actin and myosin as a sliding contractile apparatus, but these proteins are not arranged in a regular and orderly way as is the case in skeletal muscle..
Instead of Z lines, smooth muscle muscle fibers have dense bodies in their cytosol that are attached to the plasma membrane and which in turn are attached to actin filaments. In general, these muscles have few mitochondria and their mechanical activity depends on glucose metabolism..
They are involuntary muscles, that is, they are innervated by nerve fibers that are not under the control of the will (as much as you want, you cannot induce the movement of your intestines voluntarily).
There are several types of smooth muscles, some with automatic activity (such as the fibers of the heart muscle) and some not.
Muscles are essential tissues for the functioning of most of the organic systems that make up us. Not only do they allow us joint movement and displacement that differentiates us from sessile organisms such as plants, but they also allow us to relate to the environment and to all the entities that surround us..
From a visceral point of view, muscles fulfill essential functions for life. For example, the heart pumps blood throughout the body, without which we could not live.
Smooth muscle, which is found in the hollow viscera, is essential for the function of the gastrointestinal, genitourinary and respiratory tracts, among others..
This type of muscle also makes up the walls of blood vessels, working to control blood pressure. In the eye there are muscles that control the opening and closing of the pupil, regulating the entry of light and facilitating vision..
They are also part of the sphincters in general, so they participate in functions such as defecation, the emission of urine, etc..
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