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July 1999
A. Mammals
The neural substrate responsible for respiratory
rhythm generation and mediation of respiratory reflexes
lies within the brain stem of mammals. Groups of respiratory premotoneurons and neurons innervating upper
airway muscles are found in the caudal medulla near the
nA and the Bötzinger complexes. In addition, at least one
site of respiratory rhythmogenesis has been identified, in
neonatal mammals, in the “pre-Bötzinger” complex which
is situated in the reticular formation of the rostral medulla, at the level of the hypoglossal nuclei (585). These
outflows probably derive, in an evolutionary sense, from
the branchial motoneurons of more primitive, gill-breathing vertebrates that retain their primary roles in respiratory rhythm generation in present-day fish and larval amphibians. Accordingly, the reticular formation is thought
to be the site both of the primary respiratory rhythm
generator in fish and amphibians and of the respiratory
and suckling rhythms in neonatal mammals.
Because the detailed organization of central respiratory control in mammals has been exceedingly well reviewed recently (67, 200, 529), a brief synopsis, for comparison with nonmammalian vertebrates, will be
sufficient here. Two models have been proposed to try to
explain respiratory rhythmogenesis in mammals. One proposes that the central respiratory rhythm generator consists of burster or pacemaker neurons, which show spontaneous rhythmic oscillations in membrane potential in
the absence of synaptic inputs or alternatively require a
tonic excitatory input before they exhibit rhythmic oscillatory activity. The second postulates that respiratory
rhythm is produced by neural networks that exhibit oscillatory behavior due to synaptic interactions alone. Indeed, although pacemaker-like neurons have been identified in the pre-Bötzinger region in neonatal mammals in
vitro, when sensory input was removed (585), most recently Richter (529) has argued for a hybrid of these two
in vivo, whereby under normal conditions of sensory
input, the synaptic interactions between respiratory neurons override the effects of pacemaker inputs. In their
recent review of the literature on the central control of
breathing in mammals, Bianchi et al. (67) have proposed
that respiratory rhythm is not generated by a single conditional pacemaker process. Their argument was based
on the assumption that brain stem respiratory activity
results from the sequential activation of many populations of neurons to produce a three-phase motor act
(breathing) in which each process is conditioned by the
previous one and initiates the next. An alternate view
would be that the coordination of the groups of respiratory neurons would be performed by a different entity.
This entity would be responsible for processing the relevant sensory signals and would ensure precise spatial and
temporal pattern of muscle activation during each breath
so that the respiratory system meets the demand of the
organism. It is to help understand the relationship between respiratory rhythm and pattern that the concept of
a central respiratory pattern generator has emerged (203,
439). Because the mechanisms underlying the generation
of central respiratory rhythms are not the prime subject of
this review, central pattern generation will be referred to
nonspecifically and the generator designated as the CPG.
B. Cyclostomes
This group of vertebrates is composed of the myxinoids (e.g., Myxine, the hagfish) and the petromyzontes
(e.g., Lampetra, the lamprey). They are jawless fishes,
possibly related to the primitive, extinct agnathans, but
with highly specialized life-styles. In the hagfishes, water
is drawn in through the nostrils by the action of a muscular membrane known as the velum and exits from a
series of gill pouches via a single external opening. The
ammocoete larva of the lamprey has a series of finely
divided gill slits which it ventilates unidirectionally by
means of the velum. Water flow is utilized both for gas
exchange and filter-feeding. Adult lampreys are ectoparasites and have powerful suckers around the mouth with
which they attach themselves to their fish hosts. The gills
are enclosed in a series of pouches that are ventilated
with tidal flow of water in and out of the small external
openings of each pouch. Expiration is the active phase
with muscles in the walls of the pouches contracting
against the elastic recoil of the branchial basket.
Spontaneous bursts of respiration-related activity
have been recorded from the isolated brain stem of the
lamprey. Recording sites included respiratory motor nuclei in the caudal half of the medulla, innervating the
VIIth, IXth and Xth cranial nerves and sites near the
trigeminal (Vth) motor nuclei, in the rostral half of the
medulla (538, 541, 622). Periodic bursts of small spikes
recorded from the rostral medulla, at the levels of the V
motor nuclei, continued after isolation of the isthmictrigeminal region by transections and occurred before
bursts recorded from the IX and X cranial nerve roots.
Electrical stimulation of this area excited respiratory motoneurons monosynaptically and could entrain or reset
the respiratory rhythm. These observations suggest that
the motor pattern for respiration is at least partly generated and coordinated in the rostral half of the medulla in
the lamprey and is transmitted to respiratory motoneurons through descending pathways (539).
C. Fish
Water contains less oxygen per unit volume than air
and yet is considerably more dense and viscous. Consequently, fish have to work relatively hard to extract suf-
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