Drg

Phrenic nucleus

FIGURE 2 Respiratory centers in the brain stem with functions described in text. Left, dorsal view of the pons and medulla with the cerebellum removed; right, transverse sections from three levels of the medulla; PRG, pontine respiratory group; preBot, pre-Botzinger complex, responsible for generating the normal respiratory rhythm; VRG, ventral respiratory group; DRG, dorsal respiratory group; IX cranial nerve (glossopharyngeal); X cranial nerve (vagus); AP, area postrema; NTS, nucleus tractus solitarius; CC, central canal.

Phrenic nucleus

FIGURE 2 Respiratory centers in the brain stem with functions described in text. Left, dorsal view of the pons and medulla with the cerebellum removed; right, transverse sections from three levels of the medulla; PRG, pontine respiratory group; preBot, pre-Botzinger complex, responsible for generating the normal respiratory rhythm; VRG, ventral respiratory group; DRG, dorsal respiratory group; IX cranial nerve (glossopharyngeal); X cranial nerve (vagus); AP, area postrema; NTS, nucleus tractus solitarius; CC, central canal.

respiratory centers by making discrete lesions in the pons and medulla in experimental animals. Recently, neuro-scientists targeted a specific class of neurons in the pre-Botzinger complex, which have neurokinin 1 (NK-1) receptors that are responsive to the neurotransmitter substance P. When these neurons are selectively killed by a toxin targeted to the NK-1 receptor in rats, the animals develop an abnormal and unstable ventilatory pattern over several days, resulting in severe arterial hypoxia and hypercapnia. However, the animals survive, suggesting that other parts of the brain can compensate for the loss of the central pattern generator in the pre-Botzinger, although the compensation is less than perfect.

Experiments can be done on the pre-Botzinger in vitro, using slices of the brain from experimental animals that continue to generate a basic respiratory rhythm. Such studies have shown that the major inhibitory neurotransmitters, GABA and glycine, are not necessary for rhythm generation. This rules out a reciprocal inhibition network model for rhythm generation. Current evidence supports a hybrid network model, which incorporates pacemaker neurons and synaptic interactions that fine-tune the duration of, and transition between, inspiration and expiration. Glutamate is the major excitatory neurotransmitter within the central pattern generator, as well as between the rhythm generating neurons and bulbospinal (premotor) neurons, and at spinal (e.g., phrenic) motor neurons.

other respiratory centers include the ventral and dorsal respiratory groups in the medulla. The ventral respiratory group (VRG) is a column of neurons that fire action potentials in phase with respiration. It includes neurons depolarizing during inspiration (inspiratory or I neurons) and expiration (expiratory or E neurons). The pre-Botzinger is in the rostral VRG. The dorsal respiratory group (DRG) contains mainly I neurons and is part of the nucleus of the solitary tract (or nucleus tractus solitarius, NTS). The afferent input from arterial chemoreceptors and lung mechanoreceptor synapses on neurons in the NTS near the DRG. The pons includes a group of neurons in the pontine respiratory group, which are involved in the phase transition between inspiration and expiration, and the reflex effects of lung mechanoreceptors on ventilation. Apneusis (abnormally long inspiration) can occur if the pons is lesioned in humans. All of these respiratory centers are bilaterally symmetrical on the right and left sides of the brain stem.

Efferent Pathways

Normal resting ventilation consists of the rhythmic contraction of the diaphragm during inspiration and

Volume

Internal intercostal T10

External intercostal T5

Phrenic

Internal intercostal T10

External intercostal T5

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