Morphological and physiological properties of caudal medullary expiratory neurons of the cat

doi:10.1016/0006-8993(87)91410-7

Brain Research

Volume 401, Issue 2, 20 January 1987, Pages 258-266

https://doi.org/10.1016/0006-8993(87)91410-7 Get rights and content

Abstract

The activity of respiratory neurons in the caudal part of the nucleus retroambigualis (NRA) was recorded intracellularlyin decerebrated, spontaneously breathing cats. Spinal projections of these neurons were determined by antidromic stimulation at the C₃ segment. A high concentration of bulbospinal expiratory (BS-E) neurons was identified in the caudal NRA, whereas the inspiratory (I) neurons, not activated antidromically, were also found to be intermingled in the same region. The BS-E neurons had ramp-like depolarizing potentials during expiration, and repolarized rapidly at the onset of phrenic nerve discharge. The I neurons depolarized abruptly in the early I phase, and repolarized gradually thereafter, namely, they were early-I neurons. Intracellular current injections revealed postsynaptic inhibition of the BS-E neurons during inspiration, as evidenced by inhibitory postsynaptic potential reversal. Using the technique of intracellular labeling with horseradish peroxidase, seven well-stained expiratory cells located in the caudal NRA revealed detailed information about axonal morphology: the axon projected rostrally and dorsomedially for the first 2 mm after emerging from the soma, then turned caudally and ventrally along two different courses, and crossed the midline of the medulla almost at the same rostrocaudal level as the soma. No axon collaterals were observed along the length of the stained portion, indicating that the BS-E neurons cannot influence other respiratory neurons in the brainstem. It has been concluded that the NRA expiratory neurons are involved only in spinal action, and that they receive a postsynaptic inhibition during inspiration. The neighboring early-I neurons are likely to be candidates for the inhibitory inputs to the BS-E neurons.

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However, it cannot be at present completely excluded that other respiration-related regions, such as the raphe nuclei, the cVRG or the BötC, may contribute to the cough-gating mechanism. Another important site of action of neuroactive substances affecting the cough reflex is the cVRG where bulbospinal expiratory neurons are located intermingled with other types of respiratory and non-respiratory neurons (e.g. Arita et al., 1987; Iscoe 1998). Reasonably, this could be a neural structure very important in the control of the cough reflex since the expulsive expiratory phase represents the main purpose of this defensive reflex.
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Classically, the respiratory drive of spinal expiratory motor neurons is attributed to the activity of excitatory premotor neurons that are located in the caudal VRG (Iscoe, 1998). These premotor neurons have an expiratory incrementing discharge pattern attributed to the summation of a tonic excitatory drive of still uncertain origin (speculatively some “locomotor center” and/or central chemoreceptors)(Iwamoto et al., 1996) and inhibitory volleys that occur during the inspiratory and post-inspiratory phases (Bainton and Kirkwood, 1979; Ballantyne and Richter, 1986; Arita et al., 1987; Anders et al., 1991). The post-inspiratory input could conceivably originate from neurons located in the immediate vicinity of the expiratory premotor neurons (Ballantyne and Richter, 1986) although inhibitory post-inspiratory interneurons are commonly found throughout the ventral respiratory column, most particularly in the Bötzinger region (Rybak et al., 2007).
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Identifying the major ionotropic neurotransmitter in a respiratory neuron is of critical importance in determining how the neuron fits into the respiratory system, whether in producing or modifying respiratory drive and rhythm. There are now several groups of respiratory neurons whose major neurotransmitters have been identified and in some of these cases, more than one transmitter has been identified in particular neurons. This review will describe the physiologically identified neurons in major respiratory areas that have been phenotyped for major ionotropic transmitters as well as those where more than one transmitter has been identified. Although the purpose of the additional transmitter has not been elucidated for any of the respiratory neurons, some examples from other systems will be discussed.
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Morphological and physiological properties of caudal medullary expiratory neurons of the cat

Abstract

Neurosci. Lett.

Brain Research

Exp. Neurol.

Brain Research

J. Neurosci. Meth.

Respir. Physiol.

Brain Research

Post-synaptic inhibition of bulbar inspiratory neurones in the cat

J. Physiol. (London)

The non-uniform character of expiratory synaptic activity in expiratory bulbospinal neurones of the cat

J. Physiol. (London)

Morphology of inspiratory neurons located in the ventrolateral nucleus of the tractus solitarius of the cat

J. Comp. Neurol.

The Brain Stem of the Cat. A Cytoarchitectonic Atlas with Stereotaxic Coordinates