Guttural pouches are large, auditory-tube diverticula that contain between 300 and 600 ml of air. They are present in odd-toed mammals, some bats, hyraxes, and the American forest mouse. They are paired bilaterally just below the ears, behind the skull and connect to the nasopharynx. Due to the general inaccessibility of the pouches in horses, they can be an area of infection by fungi and bacteria, and these infections can be extremely severe and hard to treat. The condition guttural pouch tympany affects several breeds, including the Arabian horse. The condition predisposes young horses to infection, often including severe swelling and often requires surgery to correct. The guttural pouch is also the site of infection in equine strangles.
Structure The guttural pouches are located behind the cranial cavity, caudally the skull and below the wings of the atlas (C1). They are enclosed by the parotid and mandibular salivary glands, and the pterygoid muscles. The ventral portion lies on the pharynx and beginning of the esophagus, with the retropharyngeal lymph nodes located between the ventral wall and pharynx. The left and right pouches are separated by the longus capitis and rectus capitis ventralis muscles dorsomedially. Below these muscles, the two pouches fuse to form a median septum. The guttural pouches connect the middle ear to the pharynx. The opening into the pharynx is called the nasopharyngeal ostium, which is composed of the pharyngeal wall laterally and a fibrocartilaginous fold medially. This opening leads to a short soft tissue passageway into the respective guttural pouch. The openings are located rostrally to enable drainage of mucus when the head is lowered and prevent fluid build-up. The plica salpingopharyngea, a mucosal fold at the caudal portion of the Eustachian tube, forms an uninterrupted channel between the medial lamina of the Eustachian tube and the lateral wall of the pharynx. The plica salpingopharyngea can sometimes act as a one-way valve trapping air in the pouch, causing guttural pouch tympany. Each pouch is moulded around the stylohyoid bone which divides the medial and lateral compartments. The medial compartment is much larger, and protrudes more caudally and ventrally. The epithelium is pseudostratified and ciliated containing mucus-secreting goblet cells; lymph nodules are also present. The compartments of each guttural pouch contain many important structures including several cranial nerves and arteries that lie directly against the pouch as they pass into and out of foramina in the caudal aspect of the skull. The glossopharyngeal, vagus, accessory and hypoglossal nerves; the sympathetic trunk leaving from the cranial cervical ganglion; and the internal carotid all cause a mucosal fold indent within the medial compartment, visible when viewed endoscopically. The facial nerve is in contact with the dorsal part of the pouch. The external carotid artery passes ventral to the medial compartment before crossing to the lateral wall of the lateral compartment. The pouch also covers the temporohyoid joint.
Function The function of the guttural pouches has been shown with experimental data to participate in the rapid cooling of arterial blood destined for the brain and surrounding structures. In other words, the horse's guttural pouches are 'brain-cooling devices', cooling blood within the internal carotid arteries during hyperthermia that occurs during heavy exercise. However, this proven function has been called into question by another study that neither examined the guttural pouches nor the internal carotid arteries; and others have argued that a cooling function would require an unattainable high rate of inspiratory air flow partly diverted into the guttural pouches. The issue of necessary guttural pouch air flow rates, to provide rapid cooling of the internal carotid arteries, has been solved by further supporting evidence from microvascular studies of the guttural pouch mucosa. Many of the guttural pouch mucosal superficial arterioles and capillaries extend outwards, forming two types of vascular plexuses surrounding the internal carotid arteries: one with capillary bundles parallel to the internal carotid arteries in the outer layer of the tunica adventitia (outer peri-arterial plexus) and the other with vein-artery-vein triads within the inner layer of the tunica adventitia (inner peri-arterial plexus). These guttural pouch microvascular plexuses, engulfing the internal carotid arteries, are typical of countercurrent heat exchangers recognised in other animal species, supporting the data that guttural pouches participate in selective brain cooling, even at lower air flow rates. This completes the triad of internal carotid artery cooling that protects the horse's brain from hyperthermia. The triad including the function of guttural pouches achieving arterial cooling, via both by utilizing inspiratory air cooling as well as microvascular countercurrent heat exchangers surrounding internal carotid arteries, and finally afterwards where the internal carotid arteries project upwards passing through the intracranial cavernous venous sinuses accepting cooled venous blood from the nasal sinuses.
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