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The survival of motoneurons requires a trophic factor derived from the target muscles, but the nature of this trophic factor is not known. We examined whether skeletal muscle expresses mRNAs encoding survival-promoting factors for motoneurons. Messenger RNAs were purified from hindlimbs of chick embryos and injected into Xenopus oocytes for translation. The media conditioned by incubation of the injected oocytes were applied to chick embryos during the natural cell death period of lumbar motoneurons. This procedure significantly reduced the magnitude of natural motoneuronal death. The conditioned media also promoted the survival of motoneurons cultured from chick embryos.

The spinal nucleus of the bulbocavernosus (SNB) in the rat is a cluster of sexually dimorphic motoneurons that innervate perineal muscles. In adult male rats, the size of SNB neurons is reduced following castration, and this effect is reversed by treatment with testosterone. However, androgen receptors are present in the perineal muscles as well as in SNB neurons. Therefore, it is not clear whether the neuronal size is regulated by direct action of the hormone on SNB neurons or by a target-derived factor that may be controlled via hormonal action on the innervated muscle. To address this question, the peripheral (pudendal) nerve of SNB neurons on one side was cut and united to the grafted soleus muscle, which lacks androgen sensitivity. On the control side, the pudendal nerve was similarly cut but was allowed to reinnervate the perineal muscles. The size of SNB neurons was measured on both sides after a postoperative period of 10 weeks, during which the animal had been castrated or treated with testosterone after castration. The size of SNB neurons that had reinnervated perineal muscles was reduced following castration and enlarged by testosterone treatment. In contrast, the size of SNB neurons that had innervated the soleus muscle remained unaltered in response to testosterone manipulation. It is concluded that hormonal regulation of the size of SNB neurons in adult rats is mediated by their target muscles.

1. The electrophysiological properties were compared between sexually dimorphic motoneurones in the spinal nucleus of the bulbocavernosus (SNB) and those innervating hindlimb muscles by the whole-cell recording technique in thin slices of the neonatal rat spinal cord. 2. The mean duration of action potentials in SNB motoneurones was significantly longer than that in hindlimb motoneurones. 3. The spike duration of motoneurones was inversely related to the magnitude of transient K+ currents (IA), and in this relation, there was a continuous gradation between SNB and hindlimb motoneurones. 4. The mean duration of spike after-hyperpolarization (AHP) in female SNB motoneurones was significantly longer than that in male SNB motoneurones. 5. In both SNB and hindlimb motoneurones, sustained, Ca(2+)-dependent K+ currents (IAHP) appear to be responsible for the generation of AHP. When IAHP was recorded as a tail current following an identical depolarizing pulse, the magnitude and time course of IAHP were relatively uniform, regardless of the type of motoneurone. 6. In both SNB and hindlimb motoneurones, voltage-gated Ca2+ currents showed an initial transient phase followed by a sustained phase. The mean magnitude of sustained Ca2+ currents was larger in female SNB motoneurones than in male SNB motoneurones, whereas the mean magnitude of transient Ca2+ currents showed no significant different between male and female SNB motoneurones. 7. It is concluded that SNB and hindlimb motoneurones cannot be classified into two distinct neurone types in terms of their electrophysiological properties. 8. It is suggested that the predominant occurrence of natural cell death in female SNB motoneurones during early development may be due in part to high densities of sustained Ca2+ channels in these neurones.

A hypothesis for neural control of the contractile properties of muscle is presented. The hypothesis is based on two assumptions: i) the motoneurons innervating both the fast- and slow-twitch muscles in the mammal possess an identical neurotrophic substance which accelerates the speed of muscle contraction, and ii) slow contractions of the slow-twitch muscle are induced by the low frequency discharges of the innervating motoneurons. The presence of such a trophic substance in the motoneurons is supported by the prolongation of contraction time in both the fast- and slow-twitch muscles following denervation. Furthermore, a given slow-twitch muscle dually reinnervated by the fast and slow muscle nerves contracts at the same speed regardless of which nerve is stimulated, if identical neural activity patterns are chronically imposed upon the two nerves. The presence of effective neural activity patterns in the motoneurons innervating the slow-twitch muscle is supported by an increase in the contraction speed of the slow-twitch muscle following virtual elimination of motoneuron activity by transection of the spinal cord. Under such conditions, contraction time of the fast-twitch muscle remains unchanged. Also, cord transection of newborn animals does not affect the postnatal acceleration in contraction time of the fast-twitch muscle, while the postnatal changes of contraction of the slow-twitch muscle mimic those of the fast-twitch muscle. The contraction time of slow-twitch muscle fibers is correlated with the duration of after-hyperpolarization (AHP) in the motoneurons, which regulates the discharge frequency. The duration of AHP increases with age in the motoneurons innervating the slow-twitch muscle but not in those innervating the fast-twitch muscle. It is proposed that the maintenance of slow contractions in the slow-twitch muscle during muscle differentiation is due to the appearance of low frequency discharges in the innervating motoneurons as a result of the postnatal prolongation of the AHP. The implications and limitations of the proposed hypothesis are discussed.

1. The spinal cord of kittens, 3--5 days of age, was transected at the lower thoracic level. Isometric contractions of the medial gastrocnemius and soleus muscles as well as intracellular potentials of their motoneurones were recorded after varying post-operative periods of up to 110 days. Similar observations were made 52--59 days after cord transection in adult cats. 2. In cord-transected kittens, contraction time of the gastrocnemius muscle showed normal development, whereas the soleus muscle failed to maintain slow contraction. In adult cats, cord transection increased the speed of contraction in the soleus muscle without significant changes in contraction times of the gastrocnemius muscle. 3. Soleus motoneurones showed a normal post-natal increase in the duration of afterhyperpolarization (a.h.p.) up to a certain stage (61--71 days in age) following cord transection. However, the subsequent increase in the duration of a.h.p. of soleus motoneurones observed in normal kittens was lacking in cord-transected kittens. It is suggested that soleus motoneurones show two stages of differentiation in terms of the duration of a.h.p. 4. In adult cats, cord transection caused a decrease in the duration of a.h.p. of soleus motoneurones approximately to the value observed at the end of the first stage of differentiation in kittens. 5. The duration of a.h.p. of gastrocnemius motoneurones remained virtually unchanged follwoing cord transection in both kittens and adult cats. 6. The positive correlation between the duration of a.h.p. of soleus motoneurones and contraction time of the innervated muscle fibres normally observed in kittens and adult cats was absent following cord transection. 7. It was assumed that alteration s in contraction time of the muscle following cord transection are due to virtual elimination of motoneurone discharge and that the duration of a.h.p. reflects the discharge pattern of motoneurones under normal conditions. Based on these assumptions, a possible process for normal post-natal differentiation of motoneurone and muscle is proposed.

1. Monosynaptic excitatory post-synaptic potentials (e.p.s.p.s.) were recorded from triceps surae motoneurones of the cat 2-8 months after section of the medial gastrocnemius nerve whose regeneration into the muscle was prevented. In other animals, the cut nerve was reunited to the muscle with a post-denervation delay varying from 2 to 6 months, and the e.p.s.p.s were recorded 2 months later. The e.p.s.p.s were evoked by stimulation of the medial gastrocnemius nerve central to the site of the section or reunion. 2. The mean amplitudes of the e.p.s.p.s decreased with time after nerve section, virtually disappearing by the eighth post-operative month. The decrement of the e.p.s.p. amplitude occurred more quickly in soleus motoneurones than in medial or lateral gastrocnemius motoneurones. 3. The e.p.s.p.s evoked in medial or lateral gastrocnemius motoneurones from the cut medial gastrocnemius nerve returned to normal levels 2 months after reunion of the cut nerve even following a post-denervation delay of 6 months. However, in soleus motoneurones the e.p.s.p. recovery was incomplete. 4. The degree of functional motor reinnervation of the medial gastrocnemius muscle depended upon the post-denervation delay preceding the reunion operation. With a delay of 6 months before the reunion, the muscle showed no or only weak contractions in response to nerve stimulation. 5. Axotomized medial gastrocnemius motoneurones showed a significant decrease in conduction velocity and significant increase in the amplitude of overshoot of action potentials. The changes persisted even when the cut nerve was reunited to the muscle 2-6 months after denervation. 6. When the cut medial gastrocnemius nerve was reunited to the muscle 4 months after denervation, most of the sensory fibres, tested 2 months later, failed to respond to muscle stretch. 7. It is concluded that recovery of monosynaptic e.p.s.p.s following reconnexion of the cut nerve with its muscle does not require recovery of muscle activity, full restoration of sensory activity or the recovery of normal motoneurone properties. 8. It is also suggested that the degree to which central synaptic efficacy declines and recovers following section and regeneration of a peripheral nerve depends partly upon the type of motoneurone (fast phasic or slow tonic) with which the sensory fibres form synaptic connexions.

1. The properties of medial gastrocnemius (m.g., fast alpha) and soleus (sol., slow alpha) motoneurones of the cat were examined with intracellular electrodes 8-119 days after section of the muscle nerves.2. The axonal conduction velocity was significantly decreased in both m.g. and sol. motoneurones after chronic section of the muscle nerves.3. The amplitude of overshoot of action potentials was significantly increased in both m.g. and sol. motoneurones following section of the muscle nerves.4. No significant changes in the resting membrane potential or the input resistance were observed for sol. motoneurones, whereas m.g. motoneurones showed a slight decrease in the resting potential and a slight increase in the input resistance.5. The duration of after-hyperpolarization was significantly decreased in sol. motoneurones, whereas that in m.g. motoneurones remained virtually unchanged or increased slightly following section of the muscle nerves.6. The changes described above were not seen in the preparations examined 29-46 days after section of the lumbosacral dorsal roots, suggesting that alterations in the motoneurone properties observed after section of the muscle nerves resulted from axotomy of the motoneurones rather than from sensory deprivation.7. The differences in electrophysiological properties between m.g. and sol. motoneurones were less prominent in axotomized animals than in control, unoperated cats.8. It is concluded that fast (m.g.) and slow (sol.) alpha-motoneurones have qualitatively different properties. A possible ;dedifferentiation' of fast and slow alpha-motoneurones by axotomy is discussed.

1. The fast-twitch flexor digitorum (the lateral head; equivalent to the flexor hallucis longus) and slow-twitch soleus muscles of the cat were denervated, and the two nerves immediately reunited to one or the other muscle. Contraction times of the dually reinnervated muscle were examined 9 weeks post-operatively in response to separate stimulation of its own and the foreign nerve. Over a 5 week period before the terminal experiment, a variety of artificial activity patterns was imposed on the two nerves.2. Following the dual-union operation, the flexor digitorum muscle was preferentially reinnervated by its own nerve. In contrast, the soleus muscle showed no evidence of preferential reinnervation.3. When neural activity was not artificially modified, the dually reinnervated flexor digitorum or soleus muscle showed faster contractions in response to stimulation of the flexor digitorum nerve than to stimulation of the soleus nerve.4. Following a 5 week period in which neural activity was virtually eliminated by cord transection or in which the two nerves were stimulated at the same frequency, the contraction times of the dually reinnervated soleus muscle were the same in response to stimulation of either nerve.5. In contrast, under the experimental conditions described above (cord transection or nerve stimulation), the dually reinnervated flexor digitorum muscle showed a significantly faster contraction in response to stimulation of its own nerve than to stimulation of the soleus nerve.6. It is concluded that, when neural activity is absent or identical in pattern, motoneurones normally innervating the fast- or slow-twitch muscles exert the same influence on contraction times of the soleus muscle.7. The dependence of contraction times of the dually reinnervated flexor digitorum muscle upon the type of the innervating motoneurone may be explained either by selective reinnervation of a particular group of muscle fibres or by different trophic substances emanating from the motoneurones.

1. Electrophysiological properties of soleus motoneurones in adult cats were examined with intracellular electrodes following alterations of activity of the soleus muscle induced by transection of the thoracic spinal cord or by conduction block of the muscle nerve with tetrodotoxin (TTX) cuffs. Attempts were also made to maintain muscle activity by daily stimulation of the maintain muscle activity by daily stimulation of the peripheral nerve. 2. Within 8 days after transection of the thoracic cord, soleus motoneurones showed a significant decrease in the duration of afterhyperpolarization following action potentials. This change in motoneurone properties induced by cord transection was prevented by daily stimulation of the sciatic nerve. 3. Soleus motoneurones showed a significant decrease in the duration of after-hyperpolarization within 8 days after conduction block of the soleus nerve with TTX. This change in montoneurone properties was prevented by daily stimulation of the nerve peripheral to the TTX cuff but not central to the cuff. 4. The soleus muscle showed a significant decrease in weight relative to body weight within 8 days after transection of the thoracic cord. This decrease in muscle weight following cord transection was prevented by daily stimulation of the sciatic nerve. 5. No fibrillation was detected in the soleus muscle 8 days after conduction block of the soleus nerve with TTX. The maximum twitch tension of the soleus muscle evoked by nerve stimulation showed no significant difference between the two sides treated and untreated with TTX. Fast axoplasmic transport measured with cholinesterase as a marker was not affected by TTX. Thus, there was no sign of functional although morphological abnormalities were found in some nerve fibres. 6. It is concluded that motoneurone properties in an adult depend partly upon some factors associated with activity of the innervated muscles and that such trophic signals are retrogradely carried by the motor axons.

1. The soleus muscle of adult cats was fixed in a shortened or lengthened position for 2 weeks by immobilizing the leg with a plaster cast. To eliminate the effects of reflex activity arising from the muscle, the lumbosacral dorsal roots were sectioned on the side of leg immobilization.2. The soleus muscle showed a significant decrease in weight after immobilization in a shortened position. Immobilization of the muscle in a lengthened position produced no significant changes in its weight.3. Two weeks after transection of the throacic spinal cord, the soleus muscle showed a significant decrease in weight. This decrease was prevented by immobilization of the muscle in a lengthened position. Immobilization in a shortened position caused no further muscle atrophy in cord-transected animals.4. Soleus motoneurones showed a significant decrease in the duration of after-hyperpolarization (a.h.p.) 2 weeks after immobilization of the muscle in a shortened position, whereas immobilization of the muscle in a lengthened position caused no significant changes in the duration of a.h.p.5. A decrease in the duration of a.h.p. of soleus motoneurones induced by cord transection was significantly prevented by immobilization of the muscle in a lengthened position but not by immobilization in a shortened position.6. In a variety of experimental conditions, the mean duration of a.h.p. of soleus motoneurones was significantly correlated with the soleus muscle: body wt. ratio.7. It is concluded that certain motoneurone properties depend upon the condition of the innervated muscle and that the primary factor responsible for the trophic influence is metabolic change in the muscle rather than contractile activity itself.

1. Discharge of neurones at the region of Clarke's column of the cat was recorded intracellularly or extracellularly in response to electrical stimulation of various hind limb nerves or to adequate stimuli applied to a variety of sensory receptors.2. According to the type of excitatory sensory input, the neurones were classified into four groups. Their relative proportions were: muscle group, 55-65%; cutaneous group, 15-25%; joint group, 5-15%; convergent group, 10-20%.3. The location of the neurone cell bodies was examined by marking the site of extracellular recording with fast green. The neurones of the muscle or cutaneous group were found to be within Clarke's column. One out of four neurones tested for the joint group and two out of six cells examined for the convergent group were found to be located outside the column.4. There was no difference in axonal conduction velocity among the neurones of different groups.5. The neurones of different groups could be distinguished in terms of the pattern of discharges recorded under ;resting' conditions and during activation of sensory receptors. It is suggested that information carried by dorsal spinocerebellar tract neurones to the cerebellum has a lower signal-to-noise ratio for cutaneous inputs than for muscle or joint inputs.