What is a Sphincter Muscle?
To define sphincters, let us understand that these are special circular muscles able to open and close certain body parts. One of the vital sphincters muscle is the sphincter pylori, a thick middle layer of the stomach muscle opening to pylorus caving into the small intestine. Mostly, the sphincters muscle acts as a valve to regulate fluid such as bile, urine, or faecal substance. The mechanism of the sphincter may be involuntary depending on the autonomic nervous system or voluntary operated by the somatic nervous system. The sphincter ani externus control the anal opening by contracting.
The sphincter urethrae are the most paramount voluntary mechanism of urination. Sphincter papillae are located in the iris, making it contract in the presence of bright luminosity.
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The sphincter meaning is a ring-shaped muscle that relaxes or contracts to open or close a body passage or opening. To define sphincter, these muscles are smooth muscles that remain in a contraction state. These are specialized latch systems in the contractile filaments permitting sphincters to maintain contracting position with minimum effort.
The internal anal sphincter is an exception that blocks the backward flow of intraluminal contents. The lower oesophagal sphincter thwarts gastric acid into the oesophagus. Dysfunction of the oesophagal sphincter causes continuous exposure of oesophagal mucosa to gastric acid, which can cause dysplastic heartburn that may become cancerous. The sphincters muscle function of external anal sphincters is to control defecation; it is a short tube of skeletal muscle surrounding the inferior portion of the anal canal.
Sphincters Muscle Stomach
There are six different sphincter muscles in the abdomen. The upper oesophagal sphincter (UES) is located at the end of the pharynx, where it guards the opening of the oesophagus. UES prevents the induction of air in the oesophagus while breathing and prevents food from getting inside the respiratory system. Because of the location in the pharynx, it also controls vomiting and burping.
The lower oesophagal sphincter (LES), also called cardiac sphincter, is located at the end of the oesophagus, where it joins the stomach. The principal function of LES is to permit food to pass from the oesophagus to the stomach and to enable air to pass from the stomach while burping. It also prevents the backward flow of gastric acid into the oesophagus.
Pyloric Sphincter
The pyloric sphincter is situated between the stomach and duodenum. This sphincter allows chyme (partially digested food) to transmit from the stomach to the duodenum for further digestion and absorption of nutrients through mucus into the blood.
Sphincter of Oddi
The sphincter of Oddi (SO) is located at the influx of the bile duct and pancreatic duct at the duodenum. SO opens to allow bile from the gallbladder and enzymes from the pancreas to mix to break down food particles for assimilation.
Ileocecal Sphincter
At the junction of the small and large intestine, the Ileocecal sphincter is situated. The functionality of this sphincter is not clear, except it allows chyme from the small intestine to the large intestine.
Anal Sphincter
This sphincter is located at the end of the gastrointestinal tract at the last part of the rectum. This muscle has both inner and outer components to allow the passage of the stool. The automatic nervous system controls the inner sphincter while the outer is managed by the somatic nervous system. There are other sphincters in the body as well.
What is the Function of Sphincter Muscles?
The urethral sphincter controls the holding and pouring of urine. Similar to the anal sphincter, it has inner and outer muscles controlled by the involuntary and voluntary nervous system.
Iris Sphincter
This sphincter, also known as sphincter papilla, is located at the iris, the coloured part of the eye. It surrounds the pupil of the iris and contracts the pupil in bright light through the pupillary light reflex or during accommodation. The diameter of the pupil controls the amount of light that enters the iris reaching at the back of the eye at the retina.
Sphincter muscles examples are iris sphincter, anal sphincter, ileocecal sphincter, pyloric sphincter; there are about fifty to sixty different types of sphincter muscles in the body. Some are microscopic, such as millions of precapillary sphincters in the circulatory system. Some are controlled by the involuntary nervous system; some respond to specific stimuli; others are controlled by the voluntary nervous system. Sphincter muscles can degenerate or be damaged, leading to several health issues. The dysfunction of the lower oesophagal sphincter (LES) permits gastro acid to enter the oesophagus resulting in acid reflux.
FAQs on Sphincters
1. What is a sphincter muscle and what is its primary function in the body?
A sphincter is a circular, ring-like muscle that surrounds a body opening or a tube. Its primary function is to constrict (close) or relax (open) the passage it guards. This allows it to control the movement of substances like food, fluids, and waste. For example, sphincters in the digestive system regulate the one-way flow of food and prevent backflow, ensuring processes like digestion occur in a coordinated manner.
2. Where are the major sphincters located in the human body?
Sphincters are found throughout the human body, playing crucial roles in various systems. Key examples include:
- Digestive System: The lower esophageal sphincter (LES), pyloric sphincter, ileocecal sphincter, sphincter of Oddi, and internal and external anal sphincters.
- Urinary System: The internal and external urethral sphincters control the flow of urine from the bladder.
- Cardiovascular System: Precapillary sphincters regulate blood flow into capillary beds.
- Vision System: The iris sphincter (pupillary sphincter) in the eye controls the size of the pupil in response to light.
3. What is the role of the pyloric sphincter in digestion?
The pyloric sphincter is located at the junction between the stomach and the small intestine (duodenum). Its main role is to act as a gatekeeper, controlling the passage of partially digested food, known as chyme, from the stomach into the intestine. It opens periodically to release small amounts of chyme, ensuring that the small intestine is not overwhelmed and has enough time to properly neutralise stomach acid and continue the process of digestion and nutrient absorption.
4. How does the sphincter of Oddi regulate the flow of bile and pancreatic juice?
The Sphincter of Oddi is a muscular valve that guards the opening of the hepatopancreatic duct where it enters the duodenum (the first part of the small intestine). Its function is crucial for Digestion and Absorption. It remains closed between meals, allowing bile to be stored and concentrated in the gallbladder. When fatty foods enter the duodenum, hormonal signals cause the Sphincter of Oddi to relax and open, releasing bile and pancreatic enzymes to aid in the digestion of fats and proteins.
5. What happens if the lower esophageal sphincter (LES) does not function correctly?
If the lower esophageal sphincter (LES), which is located between the esophagus and the stomach, weakens or relaxes inappropriately, it can fail to close properly. This allows acidic stomach contents to flow back up into the esophagus, a condition known as acid reflux. Chronic acid reflux leads to Gastroesophageal Reflux Disease (GERD), causing symptoms like heartburn, chest pain, and potential damage to the esophageal lining over time.
6. What is the main difference between a valve and a sphincter?
While both control fluid flow, their structure and mechanism differ. A valve is typically a flap-like structure made of tissue (like the valves in the heart or veins) that allows one-way flow and is passively pushed open or closed by pressure differences. In contrast, a sphincter is a ring of circular muscle that actively contracts or relaxes to open or close a passageway. Sphincters provide more regulated control, often under the influence of nerve or hormonal signals, rather than just passive pressure.
7. How are sphincter muscles controlled? Are they voluntary or involuntary?
The control of sphincters can be either voluntary or involuntary, depending on the type of muscle they are made of.
- Involuntary Sphincters: Most sphincters in the body, such as the lower esophageal sphincter and the internal anal sphincter, are made of smooth muscle. They are controlled by the autonomic nervous system and function without our conscious thought.
- Voluntary Sphincters: Some sphincters, like the external anal sphincter and the external urethral sphincter, are made of skeletal muscle. These are under conscious, or voluntary, control, allowing us to decide when to defecate or urinate.
8. Is the mouth technically considered a sphincter?
This is a great question that touches on anatomical definitions. The muscle surrounding the mouth is the orbicularis oris. Functionally, it acts exactly like a sphincter: it's a circular muscle that contracts to close an opening (the mouth). However, in classical anatomical terminology, it is often simply classified as a facial muscle or a 'circular muscle' rather than being formally named a 'sphincter'. So, while it functions as one, it isn't always labelled as such in textbooks alongside internal body sphincters.
