Virtual Medical Centre

Symptoms

Nausea and Vomiting

• Nausea
• Retching
• Vomiting
• Symptoms of emesis
• Causes of nausea and vomiting
• Mechanisms of emesis
• Central nervous system control
  • Chemoreceptor trigger zone (CTZ)
  • Integrative trigger zone
• Afferent pathways
• Efferent pathways

Vomiting can also be referred to as emesis, and consists of the following stages:

Nausea

Nausea is an unpleasant sensation of wanting to vomit, and is often associated with cold sweat, pallor, salivation, loss of gastric tone, duodenal contraction, and the reflux of intestinal contents into the stomach. Nausea generally precedes vomiting, but can occur by itself. The system that brings about the loss of gastric tone, of gastric relaxation, is the efferent part of the long loop intestinal reflex that relaxes the gut during food intake.

Retching

Retching is a strong involuntary effort to vomit, and usually follows nausea. During retching, the abdominal muscles, chest wall and diaphragm all contract without any expulsion of gastric contents.

Vomiting

Vomiting is the forceful expulsion of the contents of the gastrointestinal system out through the mouth. From an evolutionary perspective, it is thought to have evolved as a defense mechanism of the body, serving a protective function to rid the body of noxious substances that have been ingested, rather than allowing them to be retained and absorbed by the intestine. Contrary to popular belief, the stomach itself does not actively expel its contents during vomiting. The stomach, oesophagus, and their relevant sphincters are all in fact relaxed during vomiting. Most of the force that expels the contents arises from the contraction of the diaphragm, which is the major respiratory muscle, and the abdominal muscles, which are the muscles involved in active expiration.

Symptoms of emesis

The symptoms of emesis include:

1. Profuse salivation
2. Sweating
3. Elevated heart rate
4. Pallor
5. Nausea
6. Retching movements

Causes of nausea and vomiting

The following table highlights the major and most common causes of nausea and vomiting:

Mechanisms of emesis

The mechanisms of emesis can be divided into three components:

1. Afferent inputs go to the central nervous system (CNS), relaying the signals of emetic stimuli
2. These signals are received, recognised, and centrally processed. They then form integrated emetic efferent signals coming from the CNS
3. These motor and chemical efferent pathways relay signals that lead to the co-ordinated respiratory gastrointestinal and abdominal muscle expulsive actions of vomiting

Central nervous system control

There are two medullary centres of vomiting in the brain known as the sensory "chemoreceptor trigger zone (CTZ)" and the integrative centre.

Chemoreceptor trigger zone (CTZ)

The CTZ is located in the medulla of the brain. It has a defensive blood-brain barrier for detecting circulating toxins in the blood and cerebrospinal fluid (CSF), and is sensitive to a number of circulating emetic agents including morphine, intravenous copper sulphate, and certain circulating metabolic emetic agents associated with uremia, infections and radiation. When activated, the CTZ does not initiate vomiting itself, but relays stimuli to the integrative vomiting centre which produces the actual act of emesis.

Integrative vomiting centre

The integrative vomiting centre coordinates activities of the neural structures near it to produce a complex patterned response, resulting in the processing and action of the vomiting reflex. The centre is located in the medullar. The motor component of the vomiting centre is controlled by both somatic and autonomic systems, meaning that both voluntary and involuntary systems are involved in the process. Their inputs are coordinated by the vomiting centre. Somatic efferent pathways control respiratory and abdominal musculature, and visceral efferent components mediating changes in gastric tone and motility, while salivation, pallor and sweating are autonomic epiphenomena. The autonomic nervous system is not essential for the mechanical act of vomiting, but the activation of efferent nerves of the abdominal organs in the emetic process is proportional to the duration and intensity of the nausea that accompanies the process.

Afferent pathways

The vomiting centre is predominantly activated by three different mechanisms:

1. By nervous impulses from the stomach, intestinal tract, and other portions of the body, resulting in a reflexive activation
2. By stimulation from the higher brain centres
3. By the chemoreceptor trigger zone (CTZ) sending impulses

Afferent impulses may also arise from other sites, such as unpleasant sights and odours, as well as severe parietal pain. The most common afferent pathways are in the viscera, or abdominal organs. Vomiting can be provoked by occlusion of the coronary vessels, distension of the intestine, and irritation of the gastrointestinal mucosa. In the gastrointestinal tract, mechanoreceptors in the intestinal wall are activated by abnormal contractions, distension or physical damage. Potentially harmful chemical stimuli can also activate chemoreceptors located in the intestinal wall. These receptors then release information to the vomiting centre.

Efferent pathways

The neural pathways involved in the motor act of vomiting are associated mainly with the phrenic nerve to the diaphragm, the spinal nerves to the abdominal and intercostal muscles, efferent visceral autonomic fibres to the gut, and the viscera efferent fibres to parts of the voluntary muscles of the pharynx and larynx. The vomiting reflex is mediated by both the autonomic and somatic systems, and consists of two phases:

1. Prodomal phase (pre-ejection) - relaxation of gastric muscles followed by small intestinal retrograde peristalsis
2. Ejection phase - comprises of retching and vomiting including expulsion of gastric contents