Repetitive vocalizations evoked by electrical stimulation of avian brains. IV. Evoked and spontaneous activity in expiratory and inspiratory nerves and muscles of the chicken (Gallus gallus).

The activity in respiratory nerves and muscles in response to electrical stimulation of vocal substrates in the brain and to CO2 stimulation of the respiratory centers was studied in 28 adult chickens. It was found that the same nerves and muscles were active during both vocalization and respiration. Stimulation of vocal substrates resulted in short latency bursting in the expiratory nerves and muscles. As stimulation intensity increased, progressively longer duration bursts composed of numerous subbursts were produced. By relating muscle activity with sound production , such bursting was shown to underlie evoked vocalizations. Background activity in inspiratory nerves and muscles continued uninterruptedly past stimulus onset only stopping when expiratory activity began. Thereafter inspiratory bursting reciprocated with expiratory bursting and was shown to underlie the intervals between vocalizations. The pattern of activity which was evoked by stimulating vocal substrates was found to strongly interact with the pattern of activity evoked by CO2 stimulation of the respiratory system. Simultaneous records of respiratory and tracheal muscles demonstrated that the same information was sent to both groups of muscles during evoked vocalization. Activity in the respiratory muscles was recorded during spontaneous vocalization of a free-moving bird and was found to resemble that recorded from anesthetized birds. Finally the activity of single units in the obex region of the medulla was recorded during electrical stimulation of vocal substrates and during CO2 stimulation of the respiratory system. Rhythmically active units were found only in the medulla. Unit activity paralleled that found in the nerves and muscles. On the basis on the data accumulated, two models of the chicken vocal system are presented. The first is a model of the sound-producing structures of the chicken. The second is a model of the neural machinery which controls the sound-producing structures. The two models are used as a basis for an explanation of the production of voclizations by the chick of the same species.