Logan Steele
In biology, retractor muscles are those that enable the retraction of organs. For example, in male bullfrogs, pond snails, and dogs, retractor muscles are involved in the eversion and retraction of the penis. In snails, lesion experiments have shown that retractor muscle bands serve their designated function, while protractor muscle bands do not. In addition, retractor muscles are also found in the heart and vascular system of Neomeniomorpha. In the field of medicine, retractors are instruments used to hold away organs and tissues during surgery.
Characteristics and Values of Retractor Muscles
| Characteristics | Values |
|---|---|
| Definition | A muscle that enables the retraction of an organ or tissue. |
| Function | Retraction and eversion of organs or tissues, such as the penis in some animals, the male copulatory organ in pond snails, and the foot in mollusks. |
| Types | Proximal and distal retractors (suggested terminology for penis retractor and protractor muscles). Anterior and posterior pedal retractors in mollusks. |
| Location | Found in various animals, including mammals (marsupials and carnivorans), snails, dogs, and mollusks. |
| Human Application | Absent in humans regarding penis retractor muscles, but may have applications in other areas, such as cardiovascular anatomy. |
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What You'll Learn
The retractor muscle of the penis
A retractor muscle is a muscle that retracts a specific body part. For example, the retractor muscle in the penis is called the bull retractor penis muscle. This muscle is involved in the peripheral autonomic mechanism of erection. The nervous mechanism of relaxation of penis muscles has been a subject of research for a long time, and it was demonstrated over a century ago that electrical stimulation of the pelvic nerve could relax these muscles.
The penis is divided into three parts: the root, the body, and the glans. The root is the most proximal and fixed part of the penis, located in the superficial perineal pouch of the pelvic floor and containing erectile tissues and muscles. The body is the free part of the penis, located between the root and the glans, and is composed of three cylinders of erectile tissue. The glans is the most distal part of the penis, conical in shape, and formed by the distal expansion of the corpus spongiosum.
The root of the penis contains two muscles: the ischiocavernosus and the bulbospongiosus. The ischiocavernosus muscle surrounds the left and right crura of the penis and helps maintain an erection by forcing blood from the cavernous spaces in the crura into the corpora cavernosa. The bulbospongiosus muscle is associated with the bulb of the penis and contracts to empty the spongy urethra of any residual semen and urine. It also aids in maintaining an erection by increasing the pressure in the bulb of the penis.
In addition to the bull retractor penis muscle, there are other examples of retractor muscles in nature. For instance, the pond snail Lymnaea stagnalis has protractor and retractor muscles in its male copulatory organ, which is relatively large and eversible. The distinction between retractor and protractor muscles in the snail's male organ has been questioned, as lesion experiments showed that snails could still evert their preputium with several lesioned muscle bands, suggesting that all muscle bands could be considered retractors.
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Protraction and eversion
Protraction
Protraction refers to protruding or sticking out. In practice, this is the movement of 'reaching out' to something. With the shoulder at a 90-degree angle of abduction, the arms are brought forward as if to hug someone. The scapulae slide laterally and then anteriorly along the thorax, which is protraction. Protraction also applies to the movement of the jaw, allowing for anterior movement, as if moving the jaw out from the rest of the head.
Eversion
Eversion involves the movement of the sole of the foot away from the median plane, so that it faces in a lateral direction. It is a pair of movements that are limited to humans and some great apes. Eversion is the opposite of inversion, which is the movement of the sole of the foot towards the median plane, such as when an ankle is twisted.
The ability to protrude or evert an organ generally requires protractor and retractor muscles. For example, male copulatory behaviour in certain species of pond snails involves eversion (protraction) and retraction of the penis-carrying organ.
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Pedal retractor muscles
In general, retractor muscles are those that enable the retraction of organs or body parts. In the context of "pedal retractor muscles," this specifically relates to the movement of the foot in certain organisms.
The size of the anterior pedal retractors is directly related to the size of the anterior adductor muscle. When the anterior adductor muscle is reduced in size, the anterior pedal retractors also decrease in size. This relationship highlights the interconnectedness of muscle groups in the organism's anatomy.
While the term "retractor muscle" is commonly used, some research suggests that the distinction between retractor and protractor muscles may not have a clear physiological basis. For example, in the male copulatory organ of a freshwater snail, both retractor and protractor muscle bands have been described. However, experiments have shown that snails can still evert their preputium with lesions in these muscle bands, questioning the traditional terminology.
In summary, pedal retractor muscles are an integral part of mollusk anatomy, enabling the retraction and movement of the foot. Their function is supported by intricate muscle fibre arrangements that prevent overinflation and guide pedal extension. While the terminology of retractor and protractor muscles is commonly used, ongoing research continues to refine our understanding of muscle function and the most appropriate language to describe it.
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Protractor and retractor muscles in dogs
A retractor muscle is involved in the retraction of an organ or body part. In the case of the pond snail, Lymnaea stagnalis, the retractor muscle is involved in the retraction of the penis-carrying organ.
Now, moving on to the topic at hand, protractor and retractor muscles in dogs are associated with the locomotor function of their forelimbs and exhibit strut-like behaviour at the shoulder. This means that the forelimbs of dogs function as inverted struts during constant-speed running, resulting in minimal or no movement at the proximal joints. This has significant implications for the energetics of running and the synchronisation of lung ventilation with the locomotor cycle.
To investigate the role of these muscles, researchers modified locomotor forces by adding mass to the trunk, inclining the treadmill, adding mass to the wrists, or applying horizontal force to the trunk through a leash. Results indicated that the primary protractor muscles of the forelimb were active during the final part of the ipsilateral support phase and the beginning of the swing phase, which is consistent with the initiation of the swing phase. Similarly, the major retractor muscles of the forelimb also displayed activity during the swing phase.
The m. pectoralis profundus and m. latissimus dorsi muscles were inactive during the support phase of the ipsilateral limb when dogs ran without any load. When the dogs carried additional weight, these muscles showed minimal increase in activity, indicating that the forelimb acts as a compliant strut at the shoulder during constant-speed running on level surfaces.
In conclusion, the protractor and retractor muscles in dogs play a crucial role in their forelimb locomotor function, contributing to the swing phase and active braking during deceleration.
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Retractor muscles of the gills
A retractor muscle is a muscle that draws a body part backward. In the case of retractor muscles of the gills, they are involved in adjusting the position of the head and gills in certain organisms.
Cephalopods, for example, have funnel retractor muscles that are located on the sides of the roof of the anterior mantle cavity. These funnel retractor muscles are important for propulsion and respiration. Contractions of circular muscles in the mantle force jets of water out of the mantle cavity through the funnel. This mechanism is essential for the cephalopod's movement and breathing.
Additionally, cephalopods possess cephalic retractor muscles that originate on the pen and skirt cartilages and insert on the posterior surface of the cranium. These muscles, along with others, play a role in adjusting the position of the head.
The gills in cephalopods are located within the mantle cavity, along with the outlets of the digestive, excretory, and reproductive systems. The respiratory current enters the mantle cavity, passes over the gills, and then exits, ensuring the flow of oxygenated water across the gills for respiration.
In mollusks, the term "pedal retractor muscles" refers to the anterior and posterior muscles that control the retraction and back-and-forth movements of the foot. These muscles work in conjunction with the concentric rings of circular, oblique, and longitudinal muscle fibers to prevent overinflation of the foot as blood is pumped into it.
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Frequently asked questions
A retractor muscle is a muscle that enables the retraction of an organ. In some animals, the male penis possesses a retractor muscle that enables retraction into the prepuce.
An example of a retractor muscle is the retractor penis muscle, which occurs in marsupials and carnivorans but is absent in humans.
The function of a retractor muscle is to enable the retraction or eversion of an organ. For example, the retractor penis muscle contracts to retract the penis into its sheath and relaxes to allow the penis to extend from the sheath.
