Sleep, 1(2): 199-204
© 1978 Raven Press, New York
Short Report
Membrane Potential of Spinal Motoneurons
During Natural Sleep in Cats
Loyd L. Glenn, Arthur S. Foutz, and William C. Dement
Sleep Research Center, Department of Psychiatry, Stanford
University School of Medicine, Stanford, California
Summary: The membrane potential of spinal motoneurons was recorded during
wakefulness, NREM sleep, and REM sleep in minimally restrained, behaving
cats. At the onset of sleep, the membrane potential generally increased in
polarization in rough proportion to time spent asleep, During the postural atonia
of REM sleep, the membrane potential of all motoneurons was tonically hyperpolarized, Antecedents of NREM sleep electromyographic suppressions, and
REM sleep myoclonic twitches were seen as transient hyperpolarizations and
depolarizations, respectively, Key Words: Motoneuron-Sleep-Spinal cordMembrane potential-Postural tone.
The postural tonus of mammals is similar in relaxed wakefulness and nonrapid eye movement (NREM) sleep. In contrast, during rapid eye movement
(REM) sleep, electromyographic activity ceases in virtually all postural muscles
(Jouvet, 1962). This flaccidity has been attributed to a tonic motoneuronal inhibition arising from the caudal brainstem region of Magoun and Rhines (Magoun and
Rhines, 1946; Pompeiano, 1976). Reflex and electro myographic investigations have
led to the hypothesis that increased postsynaptic inhibition causes a hyperpolarization in alpha motoneurons during REM sleep (Giaquinto et aI., 1964; Gassel et aI.,
1965). Recently, such a hyperpolarization has been directly measured in trigeminal
motoneurons (Nakamura et aI., 1978). The present experiment was undertaken to
determine whether spinal motoneurons become hyperpolarized during REM sleep.
METHODS
The materials and methods were described in preliminary reports (Glenn et aI.,
1978a,b; Glenn et aI., in press). In brief, nine cats were chronically implanted for
Accepted for publication November 1978.
Address reprint requests to Dr. Glenn, Sleep Research Center, TD-114, Stanford University School
of Medicine, Stanford, California 94305.
199
L. L. GLENN ET AL.
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the bipolar recording of cortical and lateral geniculate body potentials, eye movements, and dorsal neck muscle electromyogram. In addition, the tibial nerve was
attached to chronic subcutaneous stimulating electrodes and the lumbosacral cord
prepared for later microelectrode explorations in the restrained animal. After
adaptation to the restraint, glass microelectrodes (4-10 megaohm broken-tip 2 M
potassium acetate) were passed through a small vertebral opening into the ventral
gray of segments L6-S I, while the cats cycled through sleep and wakefulness.
Stimulation of hindlimb nerves for the purpose of antidromic ally identifying
motoneurons often dislodged the intracellular microelectrode. Therefore, a novel
identification criterion, based only on the action potential characteristics of a
neuron, was applied to identify the present alpha motoneuron population (Glenn
et aI., in press). Neurons were included if (1) the action potential height and
membrane potential were over 40 m V, (2) the action potential had an inflexion on
the rising phase and a potent, single-phased afterhyperpolarization over 50 msec in
duration, (3) the neuron was histologically located in the ventral gray as indicated
by dye marks, and finally (4) the intracellular potential of the cell was recorded
during more than one of the following states: wakefulness, NREM sleep, and
REM sleep. The motoneuron in Fig. 1 not only had a pronounced inflexion on the
rising phase, but was fractionated into two components, as many were. Discrimination and rejection of injured or deteriorating cells was no more difficult in this
preparation than under acute conditions. Recorded motoneurons had afterhyperpolarization durations from 50 to 105 msec. Since the afterhyperpolarization duration is inversely proportional to the size of these cells (Eccles et aI., 1958), our
population is comprised of small as well as large motoneurons.
Only a few large motoneurons were held through all states of sleep and wakefulness; however, a composite was formed of the sleep-related membrane potential changes on the basis of different impalements maintained through different
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FIG. 1. Action potential of a spinal motoneuron elicited by a 15 nA, 0.5 msec microelectrode current pulse (six superimposed traces). By straddling the threshold of
the larger spike, the two components could be fractionated. Later electrographic
recordings of this motoneuron are shown in the middle trace of Fig. 2B. Calibration:
20 mY, I msec.
Sleep. Vol. I. No.2. 1978
SPINAL MOTONEURONS IN SLEEP
201
sets of states. Since anesthetizing, curarizing, or tranquilizing agents were not
used in these recording sessions, it should be appreciated that the recorded intracellular potentials are those which occur in naturally perceiving, behaving, and
sleeping animals.
RESULTS AND DISCUSSION
The majority of motoneurons increased in polarization during the transition
from wakefulness to NREM sleep. As predicted from reflex studies of hindlimb
muscles (Gassel et aI., 1965), this hyperpolarization was not sudden, but very
gradual. Some cells hyperpolarized only slightly (top and middle records of Fig.
2A), while others descended more distinctly in association with the onset of cortical slow waves (bottom record of Fig. 2A). The intracellular potential did not
fluctuate in relation to spindles or interspindle lulls. The median difference between the membrane potential in quiet wakefulness and NREM sleep was -3.6
mY, with a range of 0 to -10.4 mV (N = 5, x = O,p < 0.05, sign test). A more
depolarized condition recurred if the cat was aroused. Otherwise, the gradual
hyperpolarization usually continued on a roughly linear course, steadily increasing in polarity as the sleep episode deepened and as ponto-geniculo-occipital
(PGO) waves began to occur in the lateral geniculate bodies. Phasic NREM sleep
was defined and distinguished from NREM sleep by the occurrence of three or
more PGO waves in 10 sec. The median membrane potential difference between
NREM sleep and phasic NREM sleep was -3.0 mY, ranging from +0.6 to -10.2
mV (N = 9, x = 1, p < 0.05, sign test). At the onset of REM sleep, the
motoneurons hyperpolarized more abruptly (Fig. 2B). This difference varied from
2 or 3 mV in some cells (upper records) to about 20 mV in others (lower record).
The median difference between phasic NREM sleep and REM sleep was -5.0 mV
with a range of -1.5 to -23.8 mV (N = 18, x = 0, p < 0.001, sign test). These
differences in potential are consistent with the values reported for trigeminal
motoneurons (Nakamura et aI., 1978).
During phasic NREM sleep, brief outstanding hyperpolarizations (20-500
msec) 0.5 to 10.5 mV in amplitude were often seen on the usual subthreshold
background potential variations (Fig. 2C). These fleeting, negative deflections
were sometimes coincident with an isolated eye movement or an electromyographic suppression. The time course and amplitude of the deflections suggest that
they are antecedents of the brief electromyographic suppressions reported to
occur in a variety of muscles during NREM sleep (Pivik and Metz, 1975). The
transient inhibition of motoneurons in NREM sleep was unexpected, since lumbosacral monosynaptic reflexes were not suppressed during neck electromyographic suppressions in cats.
Both flexor and extensor muscles of the hindlimb twitch in a disorganized
manner during the background atonia of REM sleep (Gassel et aI., 1964). In
lumbosacral motoneurons, high amplitude depolarizations of short duration were
commonly observed; sometimes these reached threshold and resulted in single or
multiple action potentials. Figure 2D exemplifies a transient potential that occurred during the repetitive phasic clusters of REM sleep. A cluster ofPGO waves,
Sleep, Vol. I. No.2, 1978
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FIG. 2. Membrane potential of spinal motoneurons during sleep state transitions and phasic events. A: Membrane potentials of three motoneurons in a
transition from wakefulness to NREM sleep. The record begins in unequivocal wakefulness and ends in unequivocal NREM sleep. Cell trace filtered at 0- 75
Hz. Figure 2C is derived from the segment of tracings above line "C". B: Same as in Fig. 2A, except that the three records begin in unequivocal phasic
NREM sleep and end in REM sleep (PGO wave tracings were deleted from the figure for clarity). Figure 2D is derived from the segment above line "D". c:
Phasic inhibition of motoneurons during NREM sleep. Notice the dorsal neck muscle phasic suppression. Cell trace filtered at 0.15-150 Hz. D: Phasic
depolarization and hyperpolarization during a cluster of phasic events. Notice the increase in background activity (over a wide range offrequencies) during
the PGO wave train in the LGN trace. Cell trace filtered at 0.15-150 Hz. The labels of the upper tracings in A apply to all tracings of both A and B. The time
calibration marks of all six tracings of A and B apply only to the cell trace and are \0 mV in amplitude. ECoG, electrocorticogram; EMG, neck
electromyogram; LGN, lateral geniculate nucleus recording; Cell, lumbar alpha motoneuron membrane potential.
SPINAL MOTONEURONS IN SLEEP
203
eye movements, and myoclonic twitches coincided with a biphasic excitatoryinhibitory wave in the motoneuron. All intermittently occurring transient neuronal
potentials in NREM sleep were hyperpolarizing. During REM sleep, both excitatory and inhibitory transients occurred, with the latter predominating. This is in
partial agreement with Nakamura et al. (1978), who reported hyperpolarizations
during the phasic episodes of REM sleep. Isolated, well-defined hyperpolarizing
potentials like those of NREM sleep were observed in few cells during REM
sleep. The 6-10 Hz periodicity in eye movements and PGO waves characteristic
of REM sleep (Mergner et al., 1976) was often reflected in the motoneuron membrane potential (Fig. 2D). In both NREM and REM sleep, transient membrane
fluctuations were only loosely associated in time with PGO waves or other indications of phasic activity. The motoneuronal membrane potential consistently increased in variability during the clustered phasic episodes of REM sleep (Fig. 2D).
The following summary is drawn for sleep-related changes of the membrane
potential in lumbosacral alpha motoneurons during sleep: (1) During the transition
from wakefulness to NREM sleep and from NREM to phasic NREM sleep, there
is a gradual increase in membrane polarity (0 to 10 mY) that is roughly proportional to the time spent asleep. (2) Transient hyperpolarizations (50-200 msec)
occur during phasic NREM sleep; these are evidently the intracellular antecedent
of the brief intermittent electromyographic suppressions seen in this state. (3)
During the transition from NREM to REM sleep, essentially all motoneurons
hyperpolarize (1.5 to 23.8 mY). (4) Antecedents to twitches commonly appear
during REM sleep in the form of both subthreshold and suprathreshold phasic
depolarizations. (5) The clustered phasic episodes of REM sleep are not consistently associated with either an increase or decrease in membrane polarity, but do
consistently result in increased membrane potential variability.
ACKNOWLEDGMENT
This research was supported in part by National Institute of Neurological and
Communicative Disorders and Stroke grant NS 10727, and by NIH career investigator award MH 05804 to Dr. Dement.
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