Medical Physiology, 3rd Edition

Neurons That Control Ventilation

The neurons that generate the respiratory rhythm are located in the medulla

A classical method for determining which parts of the CNS imageN32-2 are responsible for controlling respiratory output is to transect the neuraxis at different levels and to observe changes in breathing. Using this approach in the second century, Galen was the first to determine the location of the respiratory controller. As a physician for gladiators in the Greek city of Pergamon, he observed that breathing stopped after a sword blow to the high cervical spine. A similar blow to the lower cervical spine paralyzed the arms and legs, but allowed respiration to continue. He reproduced these lesions in live animals and correctly concluded that the brain sends information via the midcervical spinal cord to the diaphragm.

N32-2

Experimental Preparations for Studying the Neural Control of Ventilation

Contributed by George Richerson

One of the most difficult challenges in neurobiology today is to understand how neurons function within neural networks to generate normal behaviors. Although it is one of the most primitive in the mammalian brain, the neural network controlling respiration is still highly complex. The experimental preparations and techniques used to study respiration are also shared by neuroscientists studying other neural networks and include the following:

Intact, awake, behaving animals and humans are used to permit direct correlation of results with behavior. Investigators primarily use this approach to measure body movements, lung volume changes, or electrical activity in muscles or peripheral nerves.

Anesthetized, paralyzed, mechanically ventilated animals are used to permit better control of experimental variables and more intensive surgical techniques. Paralysis reduces movement of the brain so that individual neurons can be studied using extracellular recordings of single neurons.

The whole brain or large portions of the brain can be isolated in vitro by perfusing the brain via the arterial system, or by removing the spinal cord and lower brainstem of a neonatal rat from the body and keeping them alive submerged in artificial CSF. Because the brainstem circuitry is intact, a respiratory rhythm may still be produced. Elimination of the lungs and heart results in reduction of movement, which makes intracellular recording easier.

Brain slices are prepared by cutting the medulla into thin slices and are kept alive in artificial CSF. These slices can be used to study individual neurons in relative isolation; for example, with intracellular microelectrodes or patch-clamp recordings. Simple synaptic connections, limited to a restricted subset of those present in vivo, can also be studied.

Dissociated neurons can be studied immediately or after days or weeks in tissue culture. These approaches isolate neurons from synaptic input and permit highly stable recordings so that their biophysical properties can be studied with a high degree of precision and control.

These approaches to studying neural networks are at the same time very powerful and very limited. Results can vary depending on the species studied, on age, and on whether the subject is awake or anesthetized. Each experimental preparation has advantages for answering specific questions, as well as disadvantages. For example, it is not possible to define the properties of individual neurons while they are still part of a complex neural network. On the other hand, the respiratory rhythm is usually no longer present in “reduced” preparations, which makes it difficult to know if a given neuron is actually involved in respiratory control. All of these approaches suffer, more or less, from effects due to measurement of the responses—the biological equivalent of the Heisenberg uncertainty principle. There is ischemia in the center of en-bloc tissue, traumatic damage of neurons in brain slices, dedifferentiation of some neuronal properties in culture, and effects of anesthetics and prolonged surgery in vivo. For this reason, it is important that each result be verified and each theory be tested using many of these approaches, instead of relying on only one.

Eighteen centuries later, Lumsden used a similar approach in cats. He found that transection of the CNS between the medulla and spinal cord (Fig. 32-2, spinomedullary transection) causes ventilation to cease as a result of loss of the descending input to phrenic and intercostal motor neurons in the spinal cord. However, even after a spinomedullary transection, respiratory activity continues in muscles innervated by motor neurons whose cell bodies reside in the brainstem. During the period that would have been an inspiration, the nostrils continue to flare, and the muscles of the tongue, pharynx, and larynx continue to maximize airway caliber—although this respiratory activity cannot sustain life. Thus, spinomedullary transection blocks ventilation by interrupting output to the diaphragm, not by eliminating the respiratory rhythm. We can conclude that the neural machinery driving ventilation lies above the spinal cord.

image

FIGURE 32-2 Effect of brainstem transections. A dorsal view of the brainstem and spinal cord, with the cerebellum removed, and records of integrated nerve activity during one respiratory cycle following the indicated transection. During inspiration, integrated nerve activity (a moving average of the amplitude and frequency of action potentials) increases in the nerves to the tongue (e.g., CN XII) and the diaphragm (phrenic nerve).

When Lumsden, in the 1940s, made a transection between the pons and the medulla (see Fig. 32-2, pontomedullary transection), he noticed that breathing continued, but with an abnormal “gasping” pattern. Others have since observed relatively normal breathing after a transection at this level, and concluded that the gasping seen by Lumsden was due to surgical damage to the respiratory CPG in the rostral medulla. Today, most respiratory neurophysiologists believe that the respiratory CPG is located in the medulla and that other sites, including the pons, only shape the respiratory output to produce the normal pattern. imageN32-3

N32-3

Evolution of the Respiratory Central Pattern Generator

Contributed by George Richerson

The medulla actually has two identical respiratory CPGs—one on each side. All of the elements of the respiratory controller—drive inputs, sensory feedback, RRNs, and motor neurons—are bilateral. Thus, after a midsagittal transection, each side of the medulla generates an independent respiratory rhythm. As noted in the textbook, the location of the respiratory CPG is not universally agreed upon.

The respiratory CPG probably evolved in fish, where blood-gas exchange across the gills requires pumping by branchial structures; this necessity explains why the CPG is located in the medulla. CPGs generate all repetitive motor activities (see pp. 396–397). In invertebrates, these include swimming in sea slugs and movements of the stomach in lobsters. The mechanisms of rhythm generation discovered in these systems have led to establishment of general principles that have been valuable in promoting understanding of CPGs in mammals.

The pons modulates—but is not essential for—respiratory output

Although the medulla alone can generate a basic respiratory rhythm, both higher CNS centers and sensory inputs fine-tune this rhythm. For example, the pons contains neurons that affect respiratory output.

Lumsden found that a midpons transection has only a modest effect: an increase in tidal volume and a slight decrease in respiratory rate. A bilateral vagotomy—interrupting the two vagus nerves, which carry sensory information from pulmonary stretch receptors—has a similar but smaller effect. However, combining a midpons transection with a bilateral vagotomy causes the animal to make a prolonged inspiratory effort—inspiratory apneusis—that is terminated by a brief expiration. A brainstem transection above the pons does not alter the basic respiratory pattern of eupnea. These observations led Lumsden to propose that (1) the caudal pons contains an apneustic center (i.e., it can cause apneusis), and (2) the rostral pons contains a pneumotaxic center that prevents apneusis (i.e., it promotes coordinated respirations). He believed that these two regions and the medulla are required for normal breathing. imageN32-4 Although this viewpoint is common in current literature, it is held by only a minority of respiratory physiologists.

N32-4

Role of the Pons in the Control of Ventilation

Contributed by George Richerson

See the special issue of Respiratory Physiology and Neurobiology dealing with pontine influences in breathing. The following is the lead/introductory article:

McCrimmon DR, Milsom WK, Alheid GF: The rhombencephalon and breathing: A view from the pons. Respir Physiol Neurobiol 143:103–104, 2004.

What is the modern view? We now appreciate that the apneustic center is not a specific nucleus but is distributed diffusely throughout the caudal pons. The pneumotaxic center is located in the nucleus parabrachialis medialis and adjacent Kölliker-Fuse nucleus in the rostral pons. However, the pneumotaxic center is not unique in preventing apneusis because simply increasing the temperature of the animal can reverse apneusis induced by lesions in the pneumotaxic center. Moreover, lesions in many locations outside the pneumotaxic center can also induce apneusis. Today we still do not understand the role of the apneustic center, and the consensus is that the pneumotaxic center plays a general role in a variety of brainstem functions—including breathing—but is not required for eupnea (Box 32-2). Thus, the terms apneustic center and pneumotaxic center are generally of only historical significance.

Box 32-2

Normal and Abnormal Respiratory Patterns

Respiratory output can change for a variety of reasons. Many patterns, both normal and abnormal, have recognizable characteristics summarized below. Figure 32-3 illustrates some of these.

image

FIGURE 32-3 Respiratory patterns. These records are typical of either phrenic nerve activity or lung volume. Those on the left come from experimental animals, and those on the right, from humans. All of the patterns theoretically could occur in experimental animals or humans, and many occur in clinical settings.

Eupnea

Normal breathing.

Sighs

Larger than normal breaths (see Box 32-4) that occur automatically at regular intervals in normal subjects, possibly to counteract collapse of alveoli (atelectasis).

Yawn

An exaggerated sigh (see Box 32-4).

Tachypnea

An increase in respiratory rate.

Hyperventilation

An increase in alveolar ventilation (see pp. 675–676)—caused by an increase in respiratory frequency or an increase in tidal volume—that decreases arterial image. Seen in pregnancy and liver cirrhosis (due to increased progesterone), in panic attacks, and as a compensation to metabolic acidosis (see p. 642).

Kussmaul Breathing

Extremely deep, rapid breathing seen with metabolic acidosis, such as in diabetic ketoacidosis (see p. 1185).

Central Neurogenic Hyperventilation

Rapid, deep breathing causing a decrease in arterial image. Although first described in a small number of patients with focal brain lesions, it is now believed that this pattern reflected coexisting lung disease or other systemic illness in the majority of these patients.

Cheyne-Stokes Respiration

A benign respiratory pattern. Cycles of a gradual increase in tidal volume, followed by a gradual decrease in tidal volume, and then a period of apnea. Seen with bilateral cortical disease or congestive heart failure, or in healthy people during sleep at high altitude.

Gasping

Maximal, brief inspiratory efforts separated by long periods of expiration. Seen in severe anoxia, as well as in terminal, agonal breathing exhibited by patients with brainstem lesions or cardiac arrest.

Apneusis (Inspiratory)

Prolonged inspirations separated by brief expirations, typically seen in animals with lesions of the rostral pons plus bilateral vagotomy. Rarely seen in humans.

Apnea

Cessation of respiration.

Vagal Breathing

Slow, deep inspirations caused by interruption of vagus nerve input to the brainstem. Rarely seen in humans.

Ataxic Breathing

Highly irregular inspirations, often separated by long periods of apnea. Seen mainly with medullary lesions.

Cluster Breathing

Similar to ataxic breathing, with groups of breaths, often of differing amplitude, separated by long periods of apnea. Seen with medullary or pontine lesions.

Biot Breathing

First described in patients with meningitis by Biot (in 1876), with breaths of nearly equal volume separated by periods of apnea. Biot breathing is also considered to be a variant of ataxic or cluster breathing.

The dorsal and ventral respiratory groups contain many neurons that fire in phase with respiratory motor output

In the 1930s, Gesell imageN32-5 and colleagues used extracellular microelectrode recordings to monitor single neurons, finding that many neurons within the medulla increase their firing rate during one of the phases of the respiratory cycle. Some of these neurons fire more frequently during inspiration—inspiratory neurons—whereas others fire more often during expiration—expiratory neurons.

N32-5

Robert Gesell

See the following reference:

Gesell R: Respiration and its adjustments. Annu Rev Physiol 1:185–216, 1939.

Not all neurons that fire in phase with the respiratory cycle are involved in control of breathing. For example, because they are located within the chest cavity, aortic baroreceptors (see p. 534) produce an output that varies with lung inflation, but they are primarily involved in the control of cardiovascular—not respiratory—function. Conversely, some neurons whose firing does not correlate with the respiratory cycle may be essential for respiratory control. For example, central chemoreceptors may fire tonically (i.e., they do not burst during inspiration) and yet are critical for maintaining respiratory output by providing tonic drive (see pp. 700–701). Thus, not all RRNs (e.g., those stimulated by the aortic baroreceptors) play a direct role in respiration, and respiratory control involves more than just RRNs (e.g., chemoreceptor neurons).

Although electrical recordings from RRNs cannot identify all neurons necessary for producing respiratory output, this mapping has proved very useful in defining neurons that are candidates for controlling ventilation. On each side of the medulla, two large concentrations of RRNs—the dorsal and ventral respiratory groups—are grossly organized into sausage-shaped columns, oriented along the long axis of the medulla (Fig. 32-4). Many neurons of these two regions tend to fire exclusively during either inspiration or expiration.

image

FIGURE 32-4 Dorsal and ventral respiratory groups and their motor output. A, Inspiratory output. B, Expiratory output. These are dorsal views of the brainstem and spinal cord, with the cerebellum removed. The dorsal respiratory group (DRG) includes the nucleus tractus solitarii (NTS). The ventral respiratory group (VRG) includes the Bötzinger complex (BötC), pre-Bötzinger complex (preBötC), nucleus ambiguus (NA), nucleus para-ambigualis (NPA), and nucleus retroambigualis (NRA). The color coding indicates whether the neurons are primarily inspiratory (red) or primarily expiratory (green).

The pons also contains RRNs. imageN32-4 Although, as discussed above, pontine neurons may not be needed to produce a normal respiratory rhythm, they can influence respiratory output.

The dorsal respiratory group processes sensory input and contains primarily inspiratory neurons

The dorsal respiratory group (DRG) primarily contains inspiratory neurons (Table 32-2). It extends for about one third of the length of the medulla and is located bilaterally in and around the nucleus tractus solitarii (NTS), which receives sensory input from all viscera of the thorax and abdomen and plays an important role in control of the autonomic nervous system (see p. 348). The NTS is viscerotopically organized, with the respiratory portion of the NTS ventrolateral to the tractus solitarius, just beneath the floor of the caudal end of the fourth ventricle (see Fig. 32-4). These NTS neurons, as well as some immediately adjacent neurons in the dorsal medulla, make up the DRG.

TABLE 32-2

Properties of the DRG and VRG imageN32-27

 

VRG

PROPERTY

DRG

ROSTRAL

INTERMEDIATE

CAUDAL

Location

Dorsal medulla

Midway between dorsal and ventral surfaces of medulla

Major component

Nucleus tractus solitarii (NTS)

Nucleus retrofacialis (NRF) or Bötzinger complex (BötC) imageN32-8

Pre-Bötzinger complex (preBötC), nucleus ambiguus (NA), and nucleus para-ambigualis (NPA)

Nucleus retroambigualis (NRA)

Dominant activity

Inspiratory

Expiratory

Inspiratory

Expiratory

N32-27

Properties of the Dorsal and Ventral Respiratory Groups

Contributed by George Richerson, Walter Boron

eTable 32-2 is an expansion of Table 32-2.

eTABLE 32-2

Properties of the DRG and VRG

 

VRG

PROPERTY

DRG

ROSTRAL

INTERMEDIATE

CAUDAL

Location

Dorsal medulla

Midway between dorsal and ventral surfaces of medulla

Major component

Nucleus tractus solitarii (NTS)

Nucleus retrofacialis (NRF) or Bötzinger complex (BötC)

Pre-Bötzinger complex (preBötC), nucleus ambiguus (NA), and nucleus para-ambigualis (NPA)

Nucleus retroambigualis (NRA)

Dominant activity

Inspiratory

Expiratory

Inspiratory

Expiratory

Major input

Sensory via CN IX and X

Rostral VRG

Major output

(a) Premotor neurons → spinal cord → primary muscles of inspiration

Interneurons → DRG and caudal VRG

(a) Motor neurons (via CN IX and X) → accessory muscles of inspiration

Premotor neurons→ spinal cord → accessory muscles of expiration

 

(b) Interneurons → VRG

 

(b) Premotor neurons → spinal cord → primary and accessory muscles of inspiration

 

As might be surmised from the sensory role of the NTS, one of the major functions of the DRG is the integration of sensory information from the respiratory system. Indeed, some of the DRG neurons receive sensory input—via the glossopharyngeal (CN IX) and vagus (CN X) nerves—from peripheral chemoreceptors, as well as from receptors in the lungs and airways (see above). Some of the RRNs in the DRG are local interneurons. Others are premotor neurons, projecting directly to various pools of motor neurons—primarily inspiratory—in the spinal cord and ventral respiratory group (see Fig. 32-4). imageN32-6

N32-6

Dorsal Respiratory Group Neurons

Contributed by George Richerson

For example, an interneuron in the DRG may synapse on a DRG premotor neuron, which, in turn, may descend into the spinal cord and reach one of the paired phrenic motor nuclei in the ventral horn of the spinal cord. There, the premotor neuron synapses on the cell bodies of phrenic motor neurons, whose axons follow the phrenic nerve to the diaphragm.

The ventral respiratory group is primarily motor and contains both inspiratory and expiratory neurons

The ventral respiratory group (VRG)imageN32-7 contains both inspiratory and expiratory neurons (see Table 32-2). It is ventral to the DRG, about midway between the dorsal and ventral surfaces of the medulla. The VRG lies within and around a series of nuclei that form a column of neurons extending from the pons nearly to the spinal cord and is thus considerably longer than the DRG (see Fig. 32-4). Like the DRG, the VRG contains local interneurons and premotor neurons. In contrast to the DRG, the VRG also contains motor neurons that innervate muscles of the pharynx and larynx, as well as viscera of the thorax and abdomen. Sensory information related to pulmonary function comes indirectly via the DRG. Thus, the VRG plays more of an efferent role, whereas the DRG primarily plays an afferent role.

N32-7

Ventral Respiratory Group

Contributed by George Richerson

Aside from input from muscle spindle fibers (see p. 388), which provide feedback to pharyngeal and laryngeal motor neurons, the VRG does not receive any monosynaptic sensory input.

Rostral VRG

Within the nucleus retrofacialis is a region known as the Bötzinger complex. This region is the rostral VRG, which contains primarily expiratory interneurons projecting to other respiratory nuclei, including the caudal VRG.

Intermediate VRG

Within and near the nucleus ambiguus and the nucleus para-ambigualis is the intermediate VRG. The majority of RRNs in the intermediate VRG are inspiratory. The nucleus ambiguus contains somatic motor neurons that leave the medulla via CN IX and X to supply the larynx and pharynx and prevent collapse of the upper airways during inspiration. In addition, this nucleus contains parasympathetic preganglionic neurons that supply airways in the lung and other structures via CN X. Finally, the nucleus ambiguus contains some motor neurons with respiration-related activity that send axons out the trigeminal nerve (CN V), presumably for opening the mouth; the facial nerve (CN VII), for flaring the nostrils; and the accessory nerve (CN XI), for activating other accessory muscles of respiration during strong inspirations.

The nucleus para-ambigualis surrounds the nucleus ambiguus and contains premotor neurons that project to inspiratory motor neurons in the spinal cord and interneurons that have local connections to other neurons of the medulla. Like some neurons in the DRG, the inspiratory premotor neurons in the nucleus para-ambigualis drive primary muscles of inspiration. However, unlike neurons in the DRG, the inspiratory premotor neurons in the nucleus para-ambigualis also drive accessory muscles.

The pre-Bötzinger complex (see p. 706) is in the rostral portion of the intermediate VRG. It is not yet well defined anatomically, but instead is defined by the electrophysiological properties of its component neurons, which continue to produce bursts of respiratory activity in brain slices. The anatomical location roughly corresponds to a region slightly ventral and lateral to the nucleus ambiguus just caudal to the Bötzinger complex. The pre-Bötzinger complex contains inspiratory neurons that include local interneurons and premotor neurons. There is evidence that this region may be the respiratory CPG.

Caudal VRG

Within the nucleus retroambigualis is the caudal portion of the VRG, which contains mostly expiratory premotor neurons. Many of these travel down the spinal cord to synapse on motor neurons that innervate muscles of expiration, such as the internal intercostal and abdominal muscles. Because a quiet expiration is a passive event, these are accessory muscles of expiration. Thus, the premotor neurons in the caudal VRG are normally silent and become active only during forced expirations. These premotor neurons appear to receive their stimulatory drive from neurons in the rostral VRG.

The VRG consists of three regions that perform specific functions. (1) The rostral VRG or Bötzinger complex (BötC)imageN32-8 contains interneurons that drive the expiratory activity of the caudal region. (2) The intermediate VRGimageN32-7 contains somatic motor neurons whose axons leave the medulla via CN IX and CN X. These fibers supply the pharynx, larynx, and other structures, thus maximizing the caliber of the upper airways during inspiration. The intermediate VRG also contains premotor neurons that project to inspiratory motor neurons in the spinal cord and medulla. Within the rostral pole of the intermediate VRG is a group of inspiratory neurons defined as the pre-Bötzinger complex (preBötC), which, as we will see below, may be part of the respiratory CPG, contributing to generation of the respiratory rhythm. (3) The caudal VRG contains expiratory premotor neurons that travel down the spinal cord to synapse on motor neurons that innervate accessory muscles of expiration, such as abdominal and certain intercostal muscles (see p. 608).

N32-8

Origin of the Terms Bötzinger Complex and Pre-Bötzinger Complex

Contributed by Emile Boulpaep, Walter Boron

The Bötzinger complex—the most rostral portion of the VRG—is not named after a scientist or even a grateful patient. Rather, the Bötzinger complex received its name from Professor Jack Feldman (currently of the University of California, Los Angeles) while he was at a banquet for a scientific meeting in Hirschhorn, Germany, in 1978.

Feldman had discovered this region and reported on it at a meeting in Stockholm the previous year, but had not published the work. At the Hirschhorn meeting, a colleague who had been at the Stockholm meeting presented some new data based on Feldman's previous presentation. Worrying that this forgotten corner of the medulla would go unnamed—or, worse yet, be named after the wrong person—Feldman proposed a toast in which he suggested that this anatomical area be named after some aspect of the Hirschhorn meeting. So Professor Feldman picked up the bottle of wine that happened to be at the table—Bötzinger (a rather unremarkable white wine)—and the rest is neurophysiological history … the Bötzinger complex.

Twelve years later, Feldman and his colleagues identified an area slightly caudal to the Bötzinger complex. This area appeared to be the source of the respiratory rhythm in the preparation with which they were working. The group thought that the logical name—the post-Bötzinger complex—would connote less importance than the authors thought the region deserved. So they chose the anatomically incorrect pre-Bötzinger complex. The reviewers of the subsequent paper seemed not to notice, and the name stuck.

Reference

Smith JC, Ellenberger HH, Ballanyiy K, et al. Pre-Bötzinger complex: A brainstem region that may generate respiratory rhythm in mammals. Science. 1991;254:726–729.