Factors affecting the tension on the external tympaniform membranes of the syrinx during respiration and vocalization were studied in 35 adult chickens. These membranes are controlled by the interaction of the sternotrachealis and the tracheolateralis muscles, tracheal air flow, and interclavicular air sac pressures to produce sound. The muscles are innervated by the hypoglossal nerves, with the left nerve playing the dominant role in innervating both the left and the right sternotrachealis muscles. Contraction of the tracheolateralis muscles applies tension on the external tympaniform membranes, holding them out of the tracheal air flow. The sternotrachealis muscles relax the membranes by pulling the trachea caudally. This allows interclavicular air sac pressure to push the membranes into the tracheal air flow which causes them to vibrate and produce sound. Section of the sternotrachealis muscles eliminates low level vocalizations and alters the character of calls at any intensity. Inactivating both pairs of muscles has less effect on vocalization because, without the tension caused by the tracheolateralis muscles the external tympaniform membranes are already relaxed, eliminating the need for the sternotrachealis muscles. Loss of pressure in the interclavicular air sac will stop vocalizations in chickens, even when the tracheal air flow is maintained at a rate adequate for calling. Tracheal air flow and interclavicular air sac pressure changes both result from the direct action of the respiratory muscles.
The brains of 22 anesthetized chickens were systematically explored with stimulating electrodes for electrically-evoked vocalizations. Both active and inactive areas for evoked vocalizations as determined from 821 electrode penetrations were plotted. An apparently continuous system, active for vocalization, extends from the preoptic region rostrally to the most caudal portion of the medulla posteriorly. All portions of the striatum and spinal cord sampled were negative for vocalizations at the stimulus intensity used. Thresholds were lowest; calling was the most natural sounding and was elicited in the absence of other behavior in the area ventro-medial to the inferior colliculus. Most of the brain yielded only repetitive vocalizations ranging from brief ''cuk'' sounds to longer duration squawk sounds similar to those given by normal chickens. In the medulla both repetitive sounds in response to continuous stimulation and single protracted sounds per stimulation were evoked. Both types of vocalization could be elicited from the same site by varying the stimulus intensity. In portions of the midbrain, pons and medulla vocalization substrates corresponded to areas reported to yield respiratory responses in this and other species. Results are in agreement with those reported for chronically-implanted awake birds and are interpreted to suggest a single set of neural mechanisms controlling the production of vocalizations.
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