Victor Forge
The presence of sphincter muscles in monkeys is a topic that has been studied by scientists, particularly in relation to the internal anal sphincter and rectum of the vervet monkey. Investigations have also been conducted on the histomorphology of the sphincteric musculature of the lower urinary tract in rhesus monkeys, comparing it to humans. These studies contribute to our understanding of the functional basics of the complex human urinary system.
| Characteristics | Values |
|---|---|
| Topic | Characteristics of the internal anal sphincter and the rectum of the vervet monkey |
| Study | Observations on the functional internal anal sphincter of the vervet monkey |
| Methodology | Strips of sphincter in vitro contracted to noradrenaline and adrenaline |
| Result | Strips of rectal circular muscle relaxed to noradrenaline |
| All strips contracted to acetylcholine | |
| All strips relaxed to field electrical stimulation | |
| 5-Hydroxytryptamine relaxed all strips |
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What You'll Learn
- The internal anal sphincter of the vervet monkey
- The response of the internal anal sphincter to presacral nerve stimulation
- The effect of dopamine on the esophageal smooth muscle
- The urethral sphincter muscle in male and female rhesus monkeys
- The evolution of the sphincteric musculature of the lower urinary tract
The internal anal sphincter of the vervet monkey
In one study, strips of the internal anal sphincter in vitro contracted when exposed to noradrenaline and adrenaline, indicating the presence of alpha-excitatory adrenoceptors. Rectal circular muscle, on the other hand, relaxed in response to noradrenaline and exhibited both inhibitory alpha and beta-adrenoceptors. When treated with hyoscine or atropine, high doses of acetylcholine caused relaxation in the strips by stimulating intramural inhibitory neurons. These relaxations were blocked by tetrodotoxin and hexamethonium.
Further investigations revealed that the internal anal sphincter responded differently to certain substances compared to sympathetically denervated or reserpinized animals. For example, relaxations caused by acetylcholine, nicotine, and DMPP were not blocked by propranolol and guanethidine, indicating that these relaxations were unrelated to their actions as adrenergic blocking drugs. Additionally, all strips relaxed when subjected to field electrical stimulation, which stimulated intramural inhibitory neurons.
In vivo studies also provided insights into the vervet monkey's internal anal sphincter. Close arterial injections of adrenaline or noradrenaline inhibited the spontaneous contraction waves of the rectum but contracted the sphincter. These responses were blocked by phentolamine. Furthermore, hypogastric nerve stimulation relaxed the rectum but contracted the sphincter, while sacral nerve root stimulation caused after-contractions in both the rectum and sphincter.
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The response of the internal anal sphincter to presacral nerve stimulation
Another study investigated the physiology of the internal anal sphincter of the vervet monkey. Strips of the sphincter contracted in response to noradrenaline and adrenaline, with adrenoceptors that were predominantly alpha-excitatory. Rectal circular muscle strips relaxed when exposed to noradrenaline and contained both inhibitory alpha- and beta-adrenoceptors. All strips contracted with acetylcholine, and high doses caused relaxation by stimulating intramural inhibitory neurons. Similar relaxations were observed with nicotine and DMPP.
The internal anal sphincter is crucial for maintaining urinary and faecal control. Nerve damage due to surgery, injuries, or health conditions like diabetes can impact the sphincter's function. Age-related changes in muscle strength and nerve function can also affect the sphincter's ability to maintain continence.
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The effect of dopamine on the esophageal smooth muscle
Dopamine is a neurotransmitter in the autonomic nervous system that has been observed to have an inhibitory effect on the cholinergic transmission in the guinea-pig ileum. Dopamine is also known to have smooth muscle relaxant effects in vascular tissue, which extend to the smooth muscle of the gastrointestinal tract.
These studies suggest the presence of selective dopamine receptors, which mediate contraction of the esophageal body and relaxation of the sphincter muscle. The relaxation of the lower esophageal sphincter by dopamine has been observed in opossums, with evidence suggesting the presence of dopamine receptors in the esophageal smooth muscle of this species.
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The urethral sphincter muscle in male and female rhesus monkeys
In male rhesus monkeys, the urethral sphincter muscle is part of the lower urinary tract musculature. It consists of a striated and a smooth muscle part. The prostate gland is attached dorsolaterally and does not encircle the urethra as it does in humans. The external urethral sphincter covers the ventral surface of the prostate in a crescent shape above and below the verumontanum and along the proximal bulbar urethra. The levator ani muscles form an open circle around the external sphincter.
In female rhesus monkeys, the external urethral sphincter covers the ventral surface of the urethra in a horseshoe shape. This horseshoe shape envelops the distal vagina caudally. The levator ani muscles do not support the proximal urethra in females. The urethral sphincter complex, composed of smooth and striated muscle components, is inseparable in both male and female rhesus monkeys.
The urinary continence mechanism in rhesus monkeys, as in humans, is formed by a combination of detrusor, trigone, and urethral sphincter muscles. However, the internal sphincter (musculus sphincter vesicae) found in humans is absent in rhesus monkeys. Additionally, a vesical sphincter muscle, present in the human bladder outlet, is also not found in rhesus monkeys.
While the urethral sphincter muscle exists in both male and female rhesus monkeys, the differences in the lower urinary tract anatomy between this species and humans limit the suitability of the rhesus monkey as an animal model for studying the longitudinal muscle systems of the human bladder neck.
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The evolution of the sphincteric musculature of the lower urinary tract
The physiology of the muscle systems of the human lower urinary tract is still not known in detail. To better understand the functional basics of this complex organ system, experiments are often performed on animal models, including rhesus monkeys. However, detailed comparative studies of the lower urinary tract of rhesus monkeys and humans are limited.
To address this gap, a detailed study was conducted on the sphincteric musculature of the lower urinary tract of rhesus monkeys, involving 18 male and female subjects. The study aimed to establish anatomical correspondences and differences between the two species. Three-dimensional reconstructions were employed to illustrate the topographical anatomy of the different muscle systems.
The results revealed that both male rhesus monkeys and humans possess a urethral sphincter muscle that operates independently of the pelvic floor musculature. This urethral sphincter muscle consists of a smooth and a striated muscular part. Interestingly, a urinary diaphragm was found to be absent in both rhesus monkeys and humans. In contrast, female rhesus monkeys exhibit a unique feature: a striated muscle encircling the urethra and vagina simultaneously. Another distinction is the absence of a vesical sphincter muscle, found in the human bladder outlet, in rhesus monkeys.
Understanding the evolutionary aspects of the sphincteric musculature of the lower urinary tract is crucial for clinical applications. By comprehending the anatomical and physiological similarities and differences between species, we can better interpret the results of animal model studies and translate them into human contexts. This knowledge can inform the development of effective treatments and interventions for urinary disorders, such as incontinence, which can arise from malfunctions of the urethral sphincters.
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Frequently asked questions
Yes, monkeys have sphincter muscles. Studies have been conducted on the vervet monkey and the rhesus monkey (Macaca mulatta) to investigate the characteristics of their internal anal sphincter and rectum.
The internal anal sphincter in monkeys, such as the vervet monkey, plays a crucial role in their digestive system, similar to its function in humans.
Yes, there are some differences in the sphincteric musculature of the lower urinary tract between monkeys and humans. For example, in female rhesus monkeys, a striated muscle encircles the urethra and vagina together, while this is not observed in women.
Scientists employ various techniques, including immunohistochemical staining, three-dimensional reconstructions, and serial sections, to study the sphincter muscles in monkeys, specifically in rhesus monkeys.
Research on the internal anal sphincter of vervet monkeys has revealed its response to different substances. For instance, strips of the sphincter muscle contracted with noradrenaline and adrenaline, and relaxed with acetylcholine, nicotine, and DMPP.
