Respiratory activity in glossopharyngeal, vagus and accessory nerves and pharyngeal constrictors in newborn rat in vitro

Abstract

1. Previously, in a brainstem-spinal cord-rib preparation from neonatal rats we demonstrated that a decrement in extracellular pH (from about 7.4 to 7.1) caused expiratory activity in an internal intercostal muscle (IIM) during the first half of the expiratory phase (Ea). As the initial step in finding nerves or muscles firing during the second half of the expiratory phase (Eb), the patterns of activity in the glossopharyngeal, vagus and accessory nerves were examined in the present study. 2. Since the emerging motor rootlets of these three nerves (> 20; collected into about 10 bundles before the jugular foramen) are distributed in a continuous fashion from rostral to caudal levels of the brainstem, visual identification was impossible. Therefore, antidromic compound action potentials evoked by stimulation of the glossopharyngeal nerve (IX), the pharyngeal branch of the vagus nerve (PhX), the superior laryngeal nerve (SLN), the cervical vagus nerve (CX) and the accessory nerve (XI) were recorded from the peripheral stumps of the various rootlets. Nerve rootlets could be categorised into rostral, intermediate and caudal groups (rostIX-XI, intIX-XI, caudIX-XI). The rostIX-XI rootlets showed their largest potential on IX stimulation, while the intIX-XI and caudIX-XI rootlets showed their largest potentials on CX stimulation. The intIX-XI rootlets showed larger potentials on PhX and SLN stimulation than the caudIX-XI rootlets. 3. Activity was recorded simultaneously from the central stumps of the rootlets in the above three groups. Most rootlets showed inspiratory bursts. Under low pH conditions, all representatives of group rostIX-XI, most of intIX-XI and about half of caudIX-XI showed additional bursts during the Ea phase. Groups intIX-XI and caudIX-XI but not rostIX-XI also showed discrete bursts during the Eb phase in some preparations. In general, expiratory activity was prominent in intIX-XI. The spinal branch of XI showed no consistent respiratory activity. 4. Since the intIX-XI rootlets showed Eb bursts and large antidromic potentials on stimulation of PhX and SLN (which innervate the inferior pharyngeal constrictor muscle (IPC)), electromyograms were recorded from the rostral and caudal parts of IPC (rIPC and cIPC). Under low pH conditions, cIPC showed bursts during the Ea and Eb phases, while rIPC showed bursts predominantly during the Eb phase. 5. These results indicate that recording from rIPC would be a useful way of examining the neuronal mechanisms responsible for Eb phase activity.

Figure 1. Experimental arrangement

A, low-power magnification photograph of the brainstem-spinal cord preparation in the experimental chamber. Five suction electrodes for electrical stimulation are already set up. A piece of the occipital bone remained in this particular preparation (*). B, high-power magnification photograph of the boxed region in A. To make discrimination among motor rootlets easier, the sensory rootlets of the glossopharyngeal and vagus nerves were cut (filled and open circle, respectively). At least seven relatively large nerve bundles (a-g) can be seen in this preparation. After these seven rootlets had been cut carefully near the brainstem, their peripheral stumps were incorporated into suction electrodes for recording. Antidromic action potentials evoked by stimulation of the glossopharyngeal nerve (IX), the pharyngeal branch of the vagus nerve (PhX), the superior laryngeal nerve (SLN), the cervical vagus nerve (CX) and the accessory nerve (XI) were recorded from the seven rootlets (Fig. 2). SpXI, spinal branch of the accessory nerve.

Figure 7. Photograph of pharyngeal constrictor muscles

Electromyograms were recorded from the two enclosed regions, which correspond to the rostral and caudal parts of the inferior pharyngeal constrictor muscle (rIPC, cIPC). IX, glossopharyngeal nerve: PhX, pharyngeal branch of the vagus nerve: SLN, superior laryngeal nerve: CX, cervical vagus nerve.

Figure 2. Antidromic compound action potentials evoked by peripheral nerve stimulation

Mean electrical potentials (n= 20 potentials in each case) in each nerve rootlet evoked by stimulation of the glossopharyngeal nerve (IX), the pharyngeal branch of the vagus nerve (PhX), the superior laryngeal nerve (SLN), the cervical vagus nerve (CX) and the accessory nerve (XI). Data are from the preparation shown in Fig. 1. Records in a-g were obtained from the nerve rootlets shown as a-g, respectively, in Fig. 1B. Records in h and i were obtained from the spinal branch of the accessory nerve (SpXI) when the ascending bundle representing this nerve was incorporated into the suction electrode up to the C1 or C3 level, respectively (the same electrode being used in each case). The number at the right end of each trace indicates the factor by which the value shown against the ordinate scale bar (see ‘standard’ trace in same vertical column (a-i)) should be divided (e.g. a bar of the length representing 0.4 mV for IX in a would represent 0.067 mV for PhX in a).

Figure 3. Relation between the position of the rootlet and the amplitude of the compound action potential

Ordinates in plots A-E (each obtained from a different preparation) show the relative amplitude of the potential in each nerve rootlet evoked by IX, PhX, SLN, CX and XI stimulation. The peak amplitude of the largest evoked potential in a given rootlet was used as the standard (i.e. 1.0) for the other evoked potentials in the same rootlet. On two occasions in A and B, a rootlet, when cut, divided into two strands that could not be incorporated into a single suction electrode. Recordings were therefore made separately from the two strands. The data obtained from the two strands making up a single rootlet are plotted close together. Recordings obtained from SpXI are plotted on the right side of B-E. C1, C2, C4: rootlets of SpXI emerging from C1, C2 and C4 segments, respectively. a(C1), a(C3): ascending bundle representing SpXI nerve incorporated into the electrode up to segmental level C1 or C3, respectively.

Figure 4. Respiratory-related discharges under normal and low pH conditions

A and B, records of discharges in the C4 ventral root (C4), internal intercostal muscle (IIM), the most rostral group of rootlets (rostIX-XI) and the intermediate and caudal groups of rootlets (intIX-XI and caudIX-XI). A and B were obtained from two different preparations. The left, middle and right panels show records obtained under normal pH conditions, low pH conditions and high potassium conditions, respectively. Vertical broken lines in B indicate boundaries of the respiratory phases. I, E, Ea and Eb indicate the following phases: inspiratory, expiratory, first half of expiratory and second half of expiratory, respectively. The intIX-XI rootlet in B showed a discernible burst during the Ea phase in some respiratory cycles (*).

Figure 5. Summary of the pattern of activity

The histograms show the percentage of nerve rootlets that showed the specified type of activity in rostIX-XI, intIX-XI and caudIX-XI during E (at normal pH), and Ea and Eb (at low pH) phases. The patterns of activity were categorised into the following five types. Burst, bursts always occurred; Tonic, tonic activity without any clear peak and with an amplitude more than two times the noise level; WeakT, tonic activity with an amplitude less than two times the noise level; Spike, large spike-like potentials less than 10 times per respiratory cycle; None, no discernible activity. Data were obtained from 10 preparations.

Figure 6. Motor activity in the spinal branch of the accessory nerve under normal and low pH conditions

A and B, records of discharges in the C4 ventral root (C4), an internal intercostal muscle (IIM) and the spinal branch of the accessory nerve (SpXI) under normal (A) and low (B) pH conditions. Arrow in B indicates the time electrical stimulation was applied to the contralateral C4 lateral funiculus (c C4LF).

Figure 8. Electromyograms recorded from the inferior pharyngeal constrictor muscle

A and B, records obtained from C4, IIM, rIPC and cIPC under normal and low pH conditions, respectively.