Abstract
The diving response is characterized by bradycardia, apnea, and increased peripheral resistance. This reflex response is initiated by immersing the nose in water. Because the anterior ethmoidal nerve (AEN) innervates the nose, our hypothesis was that intact AENs are essential for initiating the diving response in voluntarily diving rats. Heart rate (HR) and arterial blood pressure (BPa) were monitored using implanted biotransmitters. Sprague-Dawley rats were trained to voluntarily swim 5 m underwater. During diving, HR decreased from 480 ± 15 to 99 ± 5 bpm and BPa increased from 136 ± 2 to 187 ± 3 mmHg. Experimental rats (N = 9) then received bilateral AEN sectioning, while Sham rats (N = 8) did not. During diving in Experimental rats 7 days after AEN surgery, HR decreased from 478 ± 13 to 76 ± 4 bpm and BPa increased from 134 ± 3 to 186 ± 4 mmHg. Responses were similar in Sham rats. Then, during nasal stimulation with ammonia vapors in urethane-anesthetized Experimental rats, HR decreased from 368 ± 7 to 83 ± 4 bpm, and BPa increased from 126 ± 7 to 175 ± 4 mmHg. Responses were similar in Sham rats. Thus, 1 week after being sectioned the AENs are not essential for initiating a full cardiorespiratory response during both voluntary diving and nasal stimulation. We conclude that other nerve(s) innervating the nose are able to provide an afferent signal sufficient to initiate the diving response, although neuronal plasticity within the medullary dorsal horn may be necessary for this to occur.
Highlights
The diving response is triggered when animals dive underwater (Butler and Jones 1997)
It is currently thought that the neural circuitry of the diving response starts with stimulation of the nose and upper respiratory tract (Butler and Jones 1997; McCulloch 2012), through activation of the anterior ethmoidal nerve (AEN) that innervates the external nares and nasal passages (Greene 1963)
The central axon terminals of the AEN project to the spinal trigeminal nucleus caudalis (Panneton et al 2006; Hollandsworth et al 2009), which is known as the medullary dorsal horn (MDH)
Summary
The diving response is triggered when animals dive underwater (Butler and Jones 1997) This reflex response includes apnea, an immediate and intense decrease in heart rate (HR), and an increase in arterial blood pressures (BPa) caused by peripheral vasoconstriction (McCulloch et al 2010). These dynamic cardiovascular adjustments allow an animal to efficiently use on-board oxygen stores while under water and unable to breathe (Butler and Jones 1997). Secondary neurons within the MDH appear to be an important part of the afferent pathway involved in integrating the signal that produces the cardiorespiratory changes observed during diving (McCulloch and Panneton 1997; McCulloch 2005; Panneton et al 2006)
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