Hemilingual spasm: Pathophysiology

In the present offering, the authors provide evidence for the role of the hypoglossal motonucleus in causing a cranial nerve hyperactivity syndrome, namely hemilingual spasm. During a microvascular decompression operation to treat hemilingual spasm, transcranial stimulation elicited a delayed electromyographic (EMG) response from the tongue. This late volley of EMG activity occurred with a latency of approximately 40 ms, lasted approximately 50 ms, and disappeared when the offending vessel was displaced away from the exit zone of the hypoglossal nerve root along medulla oblongata. This late tongue EMG response resembles those found in facial muscles of the patients with hemifacial spasm (HFS). In HFS, electrical stimulation of a branch of facial nerve may elicit an EMG response with a latency of approximately 10 ms in muscles innervated by another branch of the nerve, followed by a variable volley of EMG activity that may last 100 ms or longer. This abnormal response, known as the lateral spread response, is a characteristic sign for hemifacial spasm that disappears after the offending vessel is moved off the facial nerve root. The results of the present study indicate that the EMG signs of hemilingual spasm are similar to those of HFS and that the tongue spasms are most likely caused by hyperactivity of the hypoglossal motonucleus. Based on the authors’ knowledge, the above detailed electrophysiological findings related to hemilingual spasm have not been previously reported in the literature.     

Microelectrode recordings from the facial nerve in man

Microneurographic recordings have for the first time been obtained from the human facial nerve trunk, close to its exit from the stylomastoid foramen. The aim was to search for evidence of an afferent or sympathetic component of the facial nerve at this level and to study the fascicular organization of motor fibres. Single unit discharges of motor axons were occasionally discerned, and all recordings showed multiunit motor impulses preceding the EMG activity of the appropriate facial muscles by about 5 ms during both blink reflexes and voluntary contractions. No evidence of low-threshold mechanoreceptive afferents was found. Electron microscopic studies at the level of recording showed unmyelinated axons but attempts to record nociceptive and sympathetic activity failed. However, deep facial pain evoked by intraneural stimulation suggested the presence of nociceptive afferents of non-cutaneous origin. Intrafascicular recording and stimulation showed that most fascicles were composed of motor axons innervating muscles within the whole ipsilateral half of the face.     

Suppression of visual cortical activity following tactile periorbital stimulation; its role during eye blinks

Summary Single units were recorded in area 17 of anesthetized and paralyzed cats. The discharges of cortical units, either spontaneous or driven by two dimensional visual drifting noise, were analyzed during unilateral tactile stimulation (air puffs or taps) of the skin around the eye. This stimulation evokes a blink response in the normal non-curarized animal. The activity in the branch of the facial nerve innervating the orbicularis oculi muscles responsible for the blink was also recorded on the stimulated side. Following the mechanical stimulation, the discharges of both simple and complex visual cells were strongly inhibited with a latency of 70–80 ms. Inhibition was sometimes preceded by a brief increase in firing rate. This typical response was present only when the cutaneous stimulus was effective in triggering a discharge in the motor nerve which drives the orbicularis oculi muscles. Moreover, when a visual response was evoked by a temporary masking of the visual stimulus, this response was suppressed by the association with a tap stimulation.Supported by the DGRST grant (DN 81.E.1489) to P.B.     

Strength-duration and activity-dependent excitability properties of frog afferent axons and their intraspinal projections.

1. Excitability properties of afferent axons and terminal regions in frog dorsal roots (DR) and spinal cords in vitro were investigated by antidromic activation from three sites--the root, the entry zone (dorsal white matter or DW), and deep within the dorsal horn (DH)--while recordings were made from the DR. 2. Two approaches were used to assess physiological differences between telodendria and trunk axons. Rheobases and strength-duration time constants (tau sd) of single DR fibers were measured by stimulation in the DH or in the DW. Conduction velocity was estimated on the basis of onset latencies of evoked spikes (the time from stimulation to action potential arrival at the recording electrodes). Population supernormality was evaluated on the basis of responses to conditioned and unconditioned submaximal stimuli delivered to the DH or to the proximal end of isolated DRs. 3. Single-fiber action potentials occurred at longer latencies after DH stimulation than after DW stimulation. Estimated intraspinal conduction velocity was congruent to 0.6 m/s. Extraspinal conduction velocity in these fibers averaged 22.2 m/s. Average tau sd was longer in the DH than in the DW (670 microseconds vs. 204 microseconds). 4. DH and DR test responses evoked 10-150 ms after a conditioning stimulus had increased areas relative to unconditioned test responses. Conditioning-associated changes in evoked responses were greater with the DH stimulation site than with the DR stimulation site, and these changes were not altered by treatment designed to block synaptic transmission. 5. We conclude that membrane properties determining tau sd differ between large afferent axons and fine terminal regions of those axons.(ABSTRACT TRUNCATED AT 250 WORDS)     

Anatomical and physiological study of respiratory motor innervation in lampreys

The innervation of gill muscles of lampreys was investigated in a semi-intact preparation in which the respiratory rhythm was maintained for more than 2 days. Lesion experiments showed that the muscles of gill 1 are innervated by nerves VII (facial) and IX (glossopharyngeal), and those of gill 2 by nerve IX and the first branchial branch of nerve X (vagal). The other gills are supplied by the other branchial branches of nerve X. Retrograde tracers, injected in peripheral respiratory nerves, showed that branchial muscles are innervated by VII, IX and X motoneurons. Within the X nucleus, the motoneuron pools were branchiotopically organized, but with considerable rostro-caudal overlap. Electrophysiological recordings were used to show that the onset of activation of the branchial muscles was increasingly delayed with the distance from the brainstem, but that motoneuronal activity recorded with surface electrodes began at approximately the same time in all pools. The conduction velocity of VII and caudal X motor axons was found to be the same. Differences in the length of motoneuron axons appear to account for the rostro-caudal delay in gill contraction. The data presented here provide a much needed anatomical and physiological basis for further studies on the neural network controlling respiration in lampreys.     

Localization of motoneurons innervating the muscle of the hard palate in the rat: A horseradish peroxidase study

Using the horseradish peroxidase-technique, the myotopical arrangement of motoneurons innervating the transverse palatine muscle in the rat was studied. It appears that this muscle is innervated by axons from cells located in the ipsilateral intermediate subnucleus of the facial motor nucleus. By nerve transection and electrophysiological experiments it is shown that the transverse palatine muscle is innervated by the inferior as well as the superior buccolabial branch of the facial nerve.     

Distribution of climbing fibres on cerebellar Purkinje cells in X-irradiated rats. An electrophysiological study.

1. The distribution of climbing fibres on cerebellar Purkinje cells has been studied with intracellular recordings in X-irradiated and normal rats. 2. In the treated rats, multiple steps in the post-synaptic potential were elicited in 57% of the Purkinje cells by graded stimulation of the climbing fibres, the response was all-or-none in character in the other cells and in all Purkinje cells recorded in normal animals. In the neurones exhibiting the former type of response, no collision was seen along the afferent fibres during interaction experiments between just-threshold juxtafastigial and maximal olivary stimulations, whereas a collision always occurred when all-or-none responses were recorded. 3. These results show that in X-irradiated rats, the majority of Purkinje cells have a multiple innervation by two to four climbing fibres, instead of the one-to-one relationship seen normally. 4. Input resistances and total electrotonic lengths of Purkinje cells were measured in normal and treated rats. Mean values for these two parameters were higher than normal in multiply innervated cells. 5. Mean time course and mean current for reversal of the post-synaptic potential elicited in Purkinje cells by stimulation of the climbing fibres were nearly the same in mono- and in multiply innervated neurones. In multiply innervated cells, time courses and currents for reversal were independent of the size of the response or varied slightly with it, suggesting that the climbing fibres involved innervated territories whose electrotonic distance from the recording site were either the same or slightly different. 6. Interactions between two all-or-none steps of the graded post-synaptic potential evoked in multiply innervated cells by juxtafastigial and olivary stimulations revealed either a very weak or a very marked shunting effect between synapses of the two climbing fibres involved. 7. These results indicate that the over-all distribution of climbing fibre synapses on multiply innervated Purkinje cells is not grossly abnormal and that two fibres contacting a given cell can be either intermingled on the same dendrites, or segregated on distinct dendritic branches. 8. In general, the present study does not suggest the existence of a strong competition among climbing fibres innervating each Purkinje cell during development at least when granule cells are absent.     

Axonal projection patterns of ventrolateral medullospinal sympathoexcitatory neurons

We studied the following properties of cat ventrolateral medullary (VLM) neurons that projected to the thoracic spinal cord: the relationship between their spontaneous activity and that in the inferior cardiac postganglionic sympathetic nerve, their responses to baroreceptor-reflex activation, their axonal conduction velocities, the funicular trajectories of their axons, the likely sites of termination of their axons, and their axonal branching patterns. Microstimulation in the second thoracic spinal segment (T2) antidromically activated 67 VLM neurons (as determined with time-controlled collision of spontaneous and evoked action potentials), whose activity was correlated to inferior cardiac sympathetic nerve discharge (as determined with spike-triggered averaging). We tested the effect of baroreceptor-reflex activation on the firing rate of 20 of these VLM-spinal neurons. Because the firing rate decreased in each instance, these neurons apparently subserved a sympathoexcitatory function. The axonal branching patterns of 51 VLM-spinal sympathoexcitatory neurons were studied. Thirty-four neurons were antidromically activated by stimulation in the T2 gray matter and in more caudal thoracic spinal segments (T11 and/or T6). In each case, the antidromic response evoked by stimulation in the T2 gray matter was due to activation of an axonal branch rather than the main axon (via current spread to the white matter). This was demonstrated with tests that included time-controlled collision of the action potentials initiated by stimulation in T2 and a more caudal thoracic spinal segment. Some VLM-spinal axons that projected to T11 branched in T6 as well as in T2. These data indicate that some VLM-spinal neurons exerted widespread excitatory influences on sympathetic outflow. Seventeen VLM sympathoexcitatory neurons that innervated the T2 gray matter could not be antidromically activated by stimulation in T5, T6, and T11 despite an extensive search at each level. Thus the axonal projections of some VLM-spinal neurons were restricted to upper thoracic segments. Antidromic mapping in T2 revealed that the axons of VLM sympathoexcitatory neurons coursed through the dorsolateral or ventrolateral funiculus to innervate the region of the intermediolateral nucleus. Mean axonal conduction velocity was 3.5 +/- 0.3 m/s. Those VLM-spinal axons restricted to upper thoracic segments generally were located dorsally and/or medially to those that innervated widely separated thoracic segments. The discharges of 35 other VLM neurons that were antidromically activated by T2 stimulation were not related to sympathetic nerve activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Action potentials in slow muscle fibres of the frog during regeneration of motor nerves.

1. Pyriformis muscles of Rana temporaria were denervated by crushing the sciatic nerve inside the pelvis. At different times during regeneration of the nerve, slow muscle fibres were examined for the presence of all-or-none responses. 2. During the early period of re-innervation (35-60 days), slow muscle fibres were found to be non-selectively re-innervated by (foreign) fast-conducting motor axons. All slow fibres during this period were able to produce full sized action potentials, with overshoot. 3. Action potentials markedly decreased in amplitude and eventually disappeared completely between day 61-110 following denervation; during this period, slow muscle fibres were re-innervated by slowly-conducting motor axons. 4. Functional re-innervation by slowly-conducting motor axons, in the late stages of re-innervation, was not a necessary condition for the suppression of the action potential. Slow fibres innervated by fast motor axons, as well as denervated slow fibres, lost the action potential simultaneously with slow fibres which were re-innervated by slowly-conducting motor axons. 5. It is suggested that (small) slowly-conducting motor axons can exert a 'trophic' influence on the slow fibre membrane, independent of their synaptic function.     

Long-term potentiation/depotentiation are accompanied by complex changes in spontaneous unit activity in the hippocampus

Typically, long-term potentiation (LTP) has been assessed as long-lasting changes in field potentials or intracellularly recorded postsynaptic potentials evoked by activation of a set of afferents. In the present experiment, we determined changes in spontaneous unit activity in the dentate gyrus (DG) following high-frequency (HFS) or low-frequency stimulation (LFS) of the medial perforant pathway. Experiments were performed in anesthetized rats. Field potentials and unit recordings were obtained alternatively from the same recording electrode. Of 39 single units isolated (from 25 independent sessions), the spontaneous discharges of 13 units (33%) increased, while 7 units (18%) decreased their discharges following HFS that induced significant LTP of the field potentials. Such opposing modulations of unit discharges following HFS were observed on simultaneously recorded units. LFS applied following HFS also induced bi-directional effects on unit discharges. Of 20 single units isolated from a subset of recordings (12 experiments) to which LFS was applied, 6 units increased and 4 units decreased their discharges. LFS produced a long-lasting (>20 min) depotentiation, to the baseline level, on field potentials in four recording cases. The autocorrelation functions indicated that the isolated unit discharges were comparable to those of the putative DG granule cells and interneurons, shown in previous studies. The results suggest that changes in synaptic efficacy following HFS or LFS produce rather dynamic changes in cell activity in the DG.     

Comparison of the effects of stimulating groups of static gamma axons with different conduction velocity ranges on cat spindles.

In cat peroneus tertius muscles, static gamma axons were prepared in groups of three to four according to the conduction velocity of their axons (fast, intermediate, or slow). Effects of stimulating these groups (at 20, 30, and 50 Hz) on spindle ensemble discharges during sinusoidal stretch (peak-to-peak amplitude, 0.5 mm; frequency linearly increasing from 0.5 to 8 Hz in 10 s) were compared. Ensemble discharges were obtained by digital treatment of the discharges in afferent fibers from all the spindles in peroneus tertius as recorded from the muscle nerve. Stimulation of each group prevented ensemble discharges from falling to very low levels during shortening phases. However, this effect was clearly larger when the group of fast-conducting axons was stimulated. In view of the known effects of the activation of bag(2) and chain fibers (either separately or together) on single primary ending discharges during comparable sinusoidal stretches, this stronger effect supports the view that static gamma axons with faster conduction velocities are more likely to supply more bag(2) fibers than slower ones. Possibly the proportions of bag(2) and chain fibers activated during motor activity are determined by a recruitment of static gamma motoneurons related to their size.     

Increases in reflex excitability of monkey masseter motoneurons before a jaw-bite reaction-time response.

Reflex excitability of the motoneurons innervating the masseter muscle of monkeys was tested before a phasic voluntary activation of the jaw-closing muscles (a RT bite response). Single test shocks were delivered to the Mes V which supplies a monosynaptic excitatory input to the jaw-muscle motoneurons. Changes in reflex excitability were assessed by measuring the amplitude of the synchronous muscle potential evoked by the test shock. Amplitudes of the muscle potentials evoked by shocks which occurred just before the beginning of the voluntary EMG response, as judged by the onset of EMG activity of the masseter muscle contralateral to the test shock were many times larger than potentials evoked immediately following the visual RT stimulus. Curves relating the average amplitude of the evoked response to its time before the beginning of the voluntary response suggest that the reflex excitability of the motoneuron pool begins to increase 25-45 ms before the first detectable EMG activity occurs. These results suggest that inputs arrive at the motoneurons of agonist muscles used in rapid RT tasks substantially before changes in the EMG of the muscle are noted. These results, in part, would account for the time interval noted between the beginning of neural activity in suprasegmental structures which presumably excites spinal motoneurons, and the first EMG activity of muscles which are innervated by these motoneurons.     

Cells of origin of the branches of the facial nerve: A retrograde HRP study in the rabbit

The origin of different branches of the facial nerve in the rabbit was determined by using retrograde transport of HRP. Either the proximal stump of specific nerves was exposed to HRP after transection, or an injection of the tracer was made into particular muscles innervated by a branch of the facial nerve. A clear somatotopic pattern was observed. Those branches which innervate the rostral facial musculature arise from cells located in the lateral and intermediate portions of the nuclear complex. Orbital musculature is supplied by neurons in the dorsal portion of the complex, with the more rostral orbital muscles receiving input from more laterally located cells while the caudal orbital region receives innervation from more medial regions of the dorsal facial nucleus. The rostral portion of the ear also receives innervation from cells located in the dorsomedial part of the nucleus, but the caudal aspect of the ear is supplied exclusively by cells located in medial regions. The cervical platysma, the platysma of the lower jaw, and the deep muscles (i.e., digastric and stylohyoid) receive input from cells topographically arranged in the middle and ventral portions of the nuclear complex. It is proposed that the topographic relationship between the facial nucleus and branches of the facial nerve reflects the embryological derivation of the facial muscles. Those muscles that develop from the embryonic sphincter colli profundus layer are innervated by lateral and dorsomedial portions of the nuclear complex. The muscles derived from the embryonic platysma layer, including the deep musculature, receive their input from mid to ventral regions of the nuclear complex.     

Mauthner axon networks mediating supraspinal components of the startle response in the goldfish

Evidence is presented for the role of a single goldfish Mauthner axon impulse in initiating a reflex characterized by a rapid tailflip. In restrained animals, the reflex was evoked by electrical stimulation of the spinal cord or of afferent nerves. Under these conditions the Mauthner axon impulse was both sufficient and necessary for the reflex. The primary excitatory drive for this reflex was from the 8th cranial nerve, though it could also be evoked by stimulating the optic tectum. On the output side, a bilateral electromyogram recorded in the mandibular muscles occurred simultaneously with a unilateral response of the tail musculature, following activation of either Mauthner axon. This reflex could also be activated by direct intracellular stimulation of the Mauthner axon.Cranial relay interneurons were found to be postsynaptic to both Mauthner axons; their single impulses evoked activity in the mandibular muscles of only one side. Either Mauthner axon impulse alone was sufficient to produce orthodromic activation of the bilateral interneuron. Hyperpolarization of an interneuron sufficient to block spike initiation generally did not influence the reflex, even though a single impulse in the interneuron was sufficient to generate a maximum mandibular electromyogram. Repetitive stimulation at increasingly higher frequencies revealed at least two components of the mandibular response, an indication of different motoneuron pools. A few interneurons were found which innervated either opercular or extraocular muscles. Thus, they probably innervated groups of cranial muscles in a segmental manner.Elements of the Mauthner reflex were labeled with horseradish peroxidase staining techniques. Retrograde transport of the enzyme was used to determine the location of motoneurons innervating the cranial and trunk muscles. The Mauthner axon and its postsynaptic interneurons were injected with horseradish peroxidase from intracellular micropipettes. The interneuron cell bodies are contralateral to the motoneurons they innervate. Their axons appear to make contact with the ipsilateral Mauthner axon. They then cross the midline to reach the opposite Mauthner axon, with which they also apparently establish contact. Terminals branching from this rostral process innervate cranial motor nuclei. Also, as dendritic processes issue from the interneuron soma, these cells are potential candidates for reflex pathways which are activated by inputs which are not relayed through the Mauthner axon.A Mauthner axon impulse initiates a reflex which has both unilateral and bilateral components. This paper has focused on the experimental demonstration of a group of cranial interneurons which are an important relay in this reflex. While the Mauthner axon impulse appears sufficient and necessary for the cranial and trunk components of the model reflex, any one interneuron is sufficient for unilateral activation of the cranial components of the reflex. However, there are presumably three or more such interneurons postsynaptic to the Mauthner axon at that segmental level.     

Modulation of spinal reflexes by pyramidal tract stimulation in an in vitro brainstem-spinal cord preparation from the hamster

Summary Electrophysiological evidence is presented showing that the pyramidal tract (PT) of the hamster modulates spinal reflexes in an in vitro brainstem-spinal cord preparation. Three spinal reflexes were studied. Stimulation of a dorsal root (DR) while recording from a ventral root (VR) of the same spinal segment evoked two reflexes: the monosynaptic reflex, and a long latency polysynaptic reflex. Stimulation of a DR while recording from a DR immediately rostral to it elicited a volley of antidromic discharges characteristic of the dorsal root reflex (DRR). The effect of PT stimulation on reflex transmission was tested by stimulating the PT at varying intervals prior to evoking a reflex. The results show that the amplitude of the monosynaptic reflex is progressively inhibited when preceded at shorter delays by a train of PT stimuli. Similarly, PT stimulation also suppresses the long latency reflex. In contrast, the PT facilitates the DRR and repeated stimulation of the PT may evoke antidromic discharges recorded from the DRs. These data from the in vitro brainstem-spinal cord preparation indicate that the PT of the hamster exerts both inhibitory and facilitatory effects on reflex transmission in the spinal cord. The present study shows that it is possible to examine the descending control of spinal circuitry using an in vitro brainstem-spinal cord preparation.     

Effect of rocuronium on the level and mode of pre-synaptic acetylcholine release by facial and somatic nerves,and changes following facial nerve injury in rabbits

Muscles innervated by the facial nerve show differential sensitivities to muscle relaxants than muscles innervated by somatic nerves. The evoked electromyography (EEMG) response is also proportionally reduced after facial nerve injury. This forms the theoretical basis for proper utilization of muscle relaxants to balance EEMG monitoring and immobility under general anesthesia. (1) To observe the relationships between the level and mode of acetylcholine (ACh) release and the duration of facial nerve injury, and the influence of rocuronium in an in vitro rabbit model. (2) To explore the pre-synaptic mechanisms of discrepant responses to a muscle relaxant. Quantal and non-quantal ACh release were measured by using intracellular microelectrode recording in the orbicularis oris 1 to 42 days after graded facial nerve injury and in the gastrocnemius with/without rocuronium. Quantal ACh release was significantly decreased by rocuronium in the orbicularis oris and gastrocnemius, but significantly more so in gastrocnemius. Quantal release was reduced after facial nerve injury, which was significantly correlated with the severity of nerve injury in the absence but not in the presence of rocuronium. Non-quantal ACh release was reduced after facial nerve injury, with many relationships observed depending on the extent of the injury. The extent of inhibition of non-quantal release by rocuronium correlated with the grade of facial nerve injury. These findings may explain why EEMG amplitude might be diminished after acute facial nerve injury but relatively preserved after chronic injury and differential responses in sensitivity to rocuronium.     

Characterstics of motor innervation of muscle spindles in the monkey

The motor nerve supplies of four whole muscle spindles and 16 half spindles (equator and one pole) from lumbrical muscles of the monkey were reconstructed by light microscopy of serial, 1-μm-thick transverse sections. The 24 poles of spindle were innervated by 37 fusimotor (γ) axons and 18 skeletofusimotor (β) axons. Sixty-seven percent of spindle poles received γ axons only, 25% were supplied by both γ and β axons or β axons only, and 8% were not innervated by motor axons. All β axons except one and 35% of the γ axons innervated one type of intrafusal fiber only. The other 65% of γ axons coinnervated two or three different types of intrafusal fiber. Gamma axons innervated the (dynamic) bag₁ intrafusal fiber together with the (static) bag₂ and/or chain fibers in 65% of the poles of spindle in which the bag₁ fiber received motor supply. The bag₂ fiber shared γ-innervation with either the bag₁ or chain fibers in nearly every spindle pole. The chain fibers were usually coinnervated with the bag₂ fiber by γ axons. Compared to spindles in the cat tenuissimus muscle, the monkey spindle received fewer γ axons and had a higher incidence of shared γ-innervation between the dynamic and static intrafusal fibers. Unlike cat tenuissimus spindles, the monkey spindles lacked γ axons selective to the bag₂ fiber. However, the β motor system within the monkey lumbrical muscle was organized in a manner similar to the cat tenuissimus muscle. The significance of these observations is discussed relative to the general motor organization and function of spindles in different muscles and species of mammals.     

Cellular mechanisms preventing sustained activation of cortex during subcortical high-frequency stimulation

Axonal excitation has been proposed as a key mechanism in therapeutic brain stimulation. In this study we examined how high-frequency stimulation (HFS) of subcortical white matter tracts projecting to motor cortex affects downstream postsynaptic responses in cortical neurons. Whole cell recordings were performed in the primary motor cortex (M1) and ventral thalamus of rat brain slices. In M1, neurons showed only an initial depolarization in response to HFS, after which the membrane potential returned to prestimulation levels. The prolonged suppression of excitation during stimulation was neither associated with GABAergic inhibition nor complete action potential failure in stimulated axons. Instead we found that HFS caused a depression of excitatory synaptic currents in postsynaptic neurons that was specific to the stimulated subcortical input. These data are consistent with the hypothesis that axonal HFS produces a functional deafferentation of postsynaptic targets likely from depletion of neurotransmitter.     

Motor unit numbers,motor unit sizes and innervation of single muscle fibres in hyperinnervated adult mouse soleus muscle

The sural and the lateral plantar nerves were implanted simultaneously into the denervated soleus muscle of adult mice. Each of these nerves contained approximately the normal number of soleus motor axons. This procedure therefore allowed a study of how an initial excessive number of motor axons provided by two different, foreign nerves and terminating into the soleus muscle affected the final pattern of muscle innervation. In muscles examined two months or more after the implantation of the foreign nerves all muscle fibres were innervated. The fraction of the muscle innervated by either nerve varied widely from one preparation to another. However, all the motor axons which were implanted into the muscle appeared to make permanent synapses. Moreover, the distribution of motor unit sizes of each foreign nerve relative to the total number of muscle fibres innervated by that nerve was similar to the distribution of motor unit sizes in muscles cross-innervated by that nerve alone, although the absolute motor unit sizes were reduced. Estimated by intracellular recording, 20–30% of the muscle fibres were polyneuronally innervated. A similar fraction of teased muscle fibres stained for acetylcholinesterase had more than one endplate.