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In a healthy heart, the SA node continuously produces action potentials, setting the rhythm of the heart (sinus rhythm), and so is known as the heart's natural pacemaker. The rate of action potentials produced (and therefore the heart rate) is influenced by the nerves that supply it. [2]
Dogs have ear mobility that allows them to rapidly pinpoint the exact location of a sound. Eighteen or more muscles can tilt, rotate, raise, or lower a dog's ear. A dog can identify a sound's location much faster than a human can, as well as hear sounds at four times the distance. [41] Dogs can lose their hearing from age or an ear infection. [42]
These afferents synapse with the visceral motor nucleus of the vagus nerve, located in the reticular formation of the brain stem. The efferent portion is carried by the vagus nerve from the cardiovascular center of the medulla to the heart, of which increased stimulation leads to decreased output of the sinoatrial node . [ 3 ]
The canine space (also termed the infra-orbital space) [1] is a fascial space of the head and neck (sometimes also termed fascial spaces or tissue spaces). It is a thin potential space on the face, and is paired on either side. It is located between the levator anguli oris muscle inferiorly and the levator labii superioris muscle superiorly.
The reflex was originally demonstrated by Bainbridge in 1915 who observed an increase is heart rate following infusion of blood or saline into the jugular vein of anaesthetized dogs. [6] The response was reduced by cutting the cardiac sympathetic nerves and abolished by cutting the vagus nerve and he therefore concluded that it was a neural reflex.
Bainbridge is best remembered for showing that an increase in pressure on the venous side of the heart resulted in an increased heart rate due to denervation of vagal influences to the heart. The eponymous "Bainbridge reflex" is named after him, being explained as an increased heart rate due to an increase of right atrial pressure.
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In a normal heart beat rhythm, the SA node usually suppresses the ectopic pacemaker activity due to the higher impulse rate of the SA node. However, in the instance of either a malfunctioning SA node or an ectopic focus bearing an intrinsic rate superior to SA node rate, ectopic pacemaker activity may take over the natural heart rhythm. [3]