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The respiratory pattern generator of mammals is anatomically organized in lateral respiratory columns (LRCs) within the brainstem. LRC compartments serve specific functions in respiratory pattern and rhythm generation. While the caudal medullary reticular formation (cMRF) has respiratory functions reportedly related to the mediation of expulsive respiratory reflexes, it remains unclear whether neurons of the cMRF functionally belong to the LRC. In the present study we specifically investigated the respiratory functions of the cMRF. Tract tracing shows that the cMRF has substantial connectivity with key compartments of the LRC, particularly the parafacial respiratory group and the Kölliker-Fuse nuclei. These neurons have a loose topography and are located in the ventral and dorsal cMRF. Systematic mapping of the cMRF with glutamate stimulation revealed potent respiratory modulation of the respiratory motor pattern from both dorsal and ventral injection sites. Pharmacological inhibition of the cMRF with the GABA-receptor agonist isoguvacine produced significant and robust changes to the baseline respiratory motor pattern (decreased laryngeal post-inspiratory and abdominal expiratory motor activity, delayed inspiratory off-switch and increased respiratory frequency) after dorsal cMRF injection, while ventral injections had no effect. The present data indicate that the ventral cMRF is not an integral part of the respiratory pattern generator and merely serves as a relay for sensory and/or higher command-related modulation of respiration. On the contrary, the dorsal aspect of the cMRF clearly has a functional role in respiratory pattern formation. These findings revive the largely abandoned concept of a dorsal respiratory group that contributes to the generation of the respiratory motor pattern.

Summary Background There is limited knowledge of the breathing strategy and impact on the patency of the upper respiratory tract (URT) in swimming horses. Objectives To describe the respiratory responses and endoscopic appearance of the URT during tethered swimming in horses. Study design Prospective descriptive study. Methods Ten race‐fit horses, with no history of URT obstruction, were examined during tethered swimming. Endoscopic examination, heart rate, sound recordings and above and below water video recordings were obtained. Plasma lactate concentration was measured before and 5 min after swimming and tracheal endoscopy was performed 30 min after exercise to assess for presence of blood or mucus. Four horses also underwent endoscopy during exercise on the track. Results Mean (±s.d.) breathing frequency was 28 ± 5 breaths/min during swimming, with a brief inspiration (mean ± s.d. TI = 0.51 ± 0.08 s), followed by a period of apnoea (1.59 ± 0.53 s) and then a short, forced expiration (TE = 0.42 ± 0.5 s). During apnoea all horses exhibited complete collapse of the URT including closure of the external nares, nasopharynx and rima glottidis (with bilateral adduction of the arytenoid cartilages and vocal folds) and, in two horses, epiglottic retroversion. No horses had URT collapse during overground exercise. Locomotor‐respiratory coupling was not observed during swimming. Median (IQR) plasma lactate post swim was 4.71 mmol/L (2.08–8.09 mmol/L) vs 0.68 mmol/L (0.65–0.71 mmol/L) preswim. Post swim endoscopy revealed grade 1 exercise‐induced pulmonary haemorrhage (EIPH) in 2 horses. Median mucus grade was 1 (range 0–3). Main limitations Overground endoscopy was not performed in all horses. Conclusions Horses experienced complete URT collapse associated with post inspiratory apnoea when swimming. The reason for this is unknown but may be to aid buoyancy or associated with the mammalian dive response – a survival reflex to preserve oxygen stores and prevent water entering the lungs.