Ole Kiehn

Bistability of alpha‐motoneurones in the decerebrate cat and in the acute spinal cat after intravenous 5‐hydroxytryptophan.

1. In the preceding paper (Crone, Hultborn, Kiehn, Mazieres & Wigström, 1988) it was shown that a short‐lasting synaptic excitation (‘on’ stimulus) of extensor motoneurones (primarily triceps surae) in the decerebrate cat often resulted in a maintained excitability increase, which could be reset by a short‐lasting inhibitory stimulus train (‘off’ stimulus). In the present experiments intracellular recording from triceps surae motoneurones and the electroneurogram (ENG activity) from triceps surae nerve branches were performed in parallel. 2. Sustained firing of individual triceps surae motoneurones was most often recorded in parallel with the maintained ENG activity following a synaptic ‘on’ stimulus. When the motoneurone was silenced, by a hyperpolarizing current through the microelectrode, there was no sign of on‐going synaptic excitation during the maintained ENG activity following an ‘on’ stimulus. It was therefore suggested that voltage‐dependent intrinsic properties of the motoneurones themselves could be responsible for the maintained firing. 3. In confirmation of this hypothesis it was found that short‐lasting depolarizing current pulses through the recording microelectrode could trigger a self‐sustained firing in the motoneurone provided that the bias current (i.e. the holding potential) was kept within certain limits. Hyperpolarizing current pulses terminated the firing. When the spike‐generating mechanism was inactivated (by long‐lasting excessive depolarization) similar depolarizing and hyperpolarizing current pulses could initiate and terminate plateau potentials in the motoneurones. By grading the depolarizing current pulses it was found that the plateau potentials were of all‐or‐none character, typically around 10 mV in amplitude. The two levels of excitability which can be triggered by short‐lasting excitation and inhibition of the motoneurones is referred to as ‘bistable’ behaviour of the motoneurones. 4. After an acute spinal transection, in the unanaesthetized cat, the bistable behaviour of the motoneurones disappeared. However, it reappears following intravenous injection of the serotonin precursor 5‐hydroxytryptophan (50‐120 mg/kg). 5. Individual triceps surae motor units were recorded by selective EMG electrodes during tonic stretch reflexes in the decerebrate preparations. Based on an analysis of their firing pattern during lengthening and shortening (or vibration) of the muscle it is suggested that plateau potentials in motoneurones are recruited during the tonic stretch reflex. Furthermore, it is argued that a quantitatively important part of the depolarization of motoneurones during the tonic stretch reflex indeed originates from these plateau potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Proprioceptive input resets central locomotor rhythm in the spinal cat

SummaryThe reflex regulation of stepping is an important factor in adapting the step cycle to changes in the environment. The present experiments have examined the influence of muscle proprioceptors on centrally generated rhythmic locomotor activity in decerebrate unanesthetized cats with a spinal transection at Th12. Fictive locomotion, recorded as alternating activity in hindlimb flexor and extensor nerves, was induced by administration of nialamide (a monoamine oxidase inhibitor) and L-DOPA. Brief electrical stimulation of group I afferents from knee and ankle extensors were effective in resetting fictive locomotion in a coordinated fashion. An extensor group I volley delivered during a flexor burst would abruptly terminate the flexor activity and initiate an extensor burst. The same stimulus given during an extensor burst prolonged the extensor activity while delaying the appearance of the following flexor burst. Intracellular recordings from motoneurones revealed that these actions were mediated at premotoneuronal levels resulting from a distribution of inhibition to centres generating flexor bursts and excitation of centres generating extensor bursts. These results indicate that extensor group I afferents have access to central rhythm generators and suggest that this may be of importance in the reflex regulation of stepping. Experiments utilizing natural stimulation of muscle receptors demonstrate that the group I input to the rhythm generators arises mainly from Golgi tendon organ Ib afferents. Thus an increased load of limb extensors during the stance phase would enhance and prolong extensor activity while simultaneously delaying the transition to the swing phase of the step cycle.

Serotonin‐induced bistability of turtle motoneurones caused by a nifedipine‐sensitive calcium plateau potential.

1. The effect of serotonin on the firing properties of motoneurones was studied in transverse sections of the adult turtle spinal cord in vitro with intracellular recording techniques. 2. In normal medium, turtle motoneurones adapt from an initial high frequency to a low steady firing during a depolarizing current pulse. In the presence of serotonin (4‐100 microM) motoneurones responded with accelerated firing and a frequency jump during a depolarizing current pulse followed by an after‐depolarization outlasting the stimulus. From a depolarized holding potential motoneuronal activity was shifted between two stable states by brief depolarizing and hyperpolarizing current pulses. As an expression of this bistable firing behaviour, the frequency‐current relation in response to a triangular current injection was counter‐clockwise in serotonin while clockwise in normal medium. 3. The delay to onset of the frequency jump was shortened as the amplitude of the activation pulse was increased. From a positive holding potential the after‐depolarization exceeded spike threshold and its duration increased with an increase in steady bias current. The effect of serotonin on turtle motoneurones could be blocked by methysergide (10 microM). 4. When action potentials were depressed by tetrodotoxin, a voltage‐dependent, non‐inactivating plateau potential, intrinsic to the motoneurone, was revealed. Activation of this voltage plateau provides the motoneurones with two stable states of firing. The apparent input resistance was 2‐4‐fold lower during the plateau than at rest. 5. The serotonin‐induced plateau potential was Ca2+‐dependent and was blocked when Ca2+ was replaced by either Co2+ (3 mM) or Mn2+ (3 mM). 6. The Ca2+ plateau was blocked by nifedipine (1‐15 microM). 7. Serotonin reduced the slow after‐hyperpolarization following action potentials. The change in balance between inward and outward currents seems to be sufficient to reveal the plateau response. 8. Although a small plateau response was induced by Bay K 8644 (1‐15 microM), this L‐channel agonist could not reproduce the pronounced effect of serotonin. 9. It is concluded that serotonin induces a Ca2+‐dependent and nifedipine‐sensitive plateau potential in turtle motoneurones primarily by reducing a K+‐current responsible for the slow after‐hyperpolarization.

V1 spinal neurons regulate the speed of vertebrate locomotor outputs

The neuronal networks that generate vertebrate movements such as walking and swimming are embedded in the spinal cord. These networks, which are referred to as central pattern generators (CPGs), are ideal systems for determining how ensembles of neurons generate simple behavioural outputs. In spite of efforts to address the organization of the locomotor CPG in walking animals, little is known about the identity and function of the spinal interneuron cell types that contribute to these locomotor networks. Here we use four complementary genetic approaches to directly address the function of mouse V1 neurons, a class of local circuit inhibitory interneurons that selectively express the transcription factor Engrailed1. Our results show that V1 neurons shape motor outputs during locomotion and are required for generating ‘fast’ motor bursting. These findings outline an important role for inhibition in regulating the frequency of the locomotor CPG rhythm, and also suggest that V1 neurons may have an evolutionarily conserved role in controlling the speed of vertebrate locomotor movements.

Distribution of Central Pattern Generators for Rhythmic Motor Outputs in the Spinal Cord of Limbed Vertebratesa

Abstract: Neuronal networks in the spinal cord are capable of producing rhythmic movements, such as walking and swimming, when the spinal cord itself is isolated from the brain and sensory inputs. These spinal networks, also called central pattern generators or CPGs, serve as relatively simple model systems for our understanding of brain functions. In this paper we concentrate on spinal CPGs in limbed vertebrates and in particular address the question: Where in the spinal cord, in the longitudinal and transverse planes, are they located? We will review the use of lesions to isolate the rhythm and pattern‐generating parts of the CPG network, indirect methods like activity‐dependent labeling with [14C]‐2‐deoxyglucose, c‐fos, sulforhodamine 101, and WGA‐HRP, which label presumed rhythmically active neurons en bloc, and direct methods such as calcium‐imaging, extracellular and intracellular recordings, which identify rhythmically active cells directly. With this review we hope to highlight the scientific disagreements and the consensus, which have emerged from these studies with regard to the distribution of the CPG networks in the spinal cord.

Plateau potentials in alpha‐motoneurones induced by intravenous injection of L‐dopa and clonidine in the spinal cat.

1. Intracellular recordings were made from lumbar alpha‐motoneurones in unanaesthetized decerebrate acute spinal cats. The response of motoneurones to direct current pulse injection or synaptic excitation was investigated following intravenous injection of L‐beta‐3,4‐dihydroxyphenylalanine (L‐DOPA, 20‐120 mg/kg) alone, nialamide (10‐50 mg/kg) and L‐DOPA or clonidine (0.5‐1 mg/kg). 2. The response properties of motoneurones were tested with rectangular and triangular current waveforms. Before L‐DOPA treatment motoneuronal firing during a rectangular current pulse is characterized by an initial high firing frequency which rapidly decreases to a lower steady‐state firing which is maintained only for the duration of the pulse. Following administration of L‐DOPA an acceleration in firing frequency is apparent following the initial adaptation seen with rectangular current pulses. A transient after‐depolarization or an after‐discharge often followed the termination of the pulse. The frequency‐current relation in response to a triangular current injection changed from a clockwise to a counter‐clockwise hysteresis after L‐DOPA treatment (i.e. after L‐DOPA the firing frequency was higher for any given current during the descending phase than during the ascending phase of the triangular waveform). 3. Firing acceleration during and self‐sustained firing after rectangular current pulses and counter‐clockwise hysteresis of firing frequency with triangular current pulses are causally related to the presence of plateau potentials, which can be directly visualized after inactivation of the spikes. Plateau potentials in motoneurones could be generated by short‐lasting intracellular depolarizing current pulses or brief excitatory synaptic inputs and terminated by short‐lasting hyperpolarizing current pulses or brief inhibitory synaptic inputs. Plateau potentials were demonstrated in flexor and extensor motoneurones. 4. All bistable properties described in the preceding paragraphs following L‐DOPA administration could also be seen after administration of the alpha‐receptor agonist clonidine. 5. Slow rhythmic oscillations of the membrane potential (7.5‐10 Hz) were seen superimposed on plateau potentials in a few cells after administration of L‐DOPA and clonidine. The oscillations had an amplitude in the range 10‐20 mV and represent the expression of an intrinsic property of the motoneurone. 6. It is demonstrated that plateau potentials in the motoneurones contribute to the late long‐lasting reflexes observed in L‐DOPA‐treated spinal cats. 7. It is concluded that L‐DOPA (and clonidine) change the response properties of the motoneurones in an analogous way to 5‐hydroxy‐DL‐tryptophan (5‐HTP).(ABSTRACT TRUNCATED AT 400 WORDS)

Maintained changes in motoneuronal excitability by short‐lasting synaptic inputs in the decerebrate cat.

1. During investigation of the tonic stretch reflex in the unanaesthetized decerebrate cat we observed that a short train of impulses in Ia afferents from the soleus muscle (or its synergists) may cause a prolonged activity in the soleus muscle as judged by EMG and tension recordings. This excitability increase, which outlasted the stimulus train, could stay virtually constant during long periods (even minutes), but could be terminated at any time by a train of impulses in, for example, the peroneal nerve. 2. Gradation of the strength of stimulation and the duration of the train of impulses show that the amount of maintained excitability increase depends‐within some limits‐on the total amount of Ia impulses. 3. In paralysed preparations a short train of impulses in Ia afferents from any part of the triceps surae, caused a maintained increase of the efferent activity in the nerves to triceps surae and a maintained increase of the triceps surae monosynaptic test reflex. These experiments demonstrate the existence of a central mechanism (in the spinal cord and/or the brain stem), which is responsible for the maintained excitability increase seen in motoneurones to the homonymous and synergic muscles. 4. In acute spinal preparations it was not possible to demonstrate any long‐lasting excitability increase by a train of Ia impulses. Following intravenous administration of the serotonin precursor 5‐hydroxytryptophan, mimicking the tonic activity of these pathways in the decerebrate state, it was again possible to elicit the long‐lasting excitability increase by a train of impulses in Ia afferents. A subsequent I.V. injection of methysergide (a serotonin receptor blocker) abolished the long‐lasting excitability increase. This set of experiments demonstrates that the basic mechanism responsible for the maintained excitability increase is located at segmental level, and involves serotonergic systems. 5. It was demonstrated that activation of several ipsilateral and crossed reflex pathways by trains of impulses in cutaneous or high‐threshold muscle afferents could trigger a maintained excitability increase of those motoneurone pools which were activated by the stimulation. Trains of stimuli to facilitatory regions in the brain stem could also cause a long‐lasting excitability increase of motoneurones. Furthermore, activation of all reflex pathways which mediate postsynaptic inhibition to a motor nucleus (including recurrent inhibition via Renshaw cells) could terminate the prolonged excitability increase of that particular motor nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Genetic ablation of V2a ipsilateral interneurons disrupts left-right locomotor coordination in mammalian spinal cord.

The initiation and coordination of activity in limb muscles are the main functions of neural circuits that control locomotion. Commissural neurons connect locomotor circuits on the two sides of the spinal cord, and represent the known neural substrate for left-right coordination. Here we demonstrate that a group of ipsilateral interneurons, V2a interneurons, plays an essential role in the control of left-right alternation. In the absence of V2a interneurons, the spinal cord fails to exhibit consistent left-right alternation. Locomotor burst activity shows increased variability, but flexor-extensor coordination is unaffected. Anatomical tracing studies reveal a direct excitatory input of V2a interneurons onto commissural interneurons, including a set of molecularly defined V0 neurons that drive left-right alternation. Our findings imply that the neural substrate for left-right coordination consists of at least two components; commissural neurons and a class of ipsilateral interneurons that activate commissural pathways.

Intrinsic membrane properties causing a bistable behaviour of α-motoneurones

SummaryIn decerebrate cats a train of impulses in Ia afferents may lead to a sustained increase in excitability of α-motoneurones of homonymous and heteronymous muscles. It was previously suggested that this long-lasting excitability increase reflects a maintained synaptic input to the motoneurones from excitatory interneurones. With intracellular recording from motoneurones we here demonstrate that the sustained increase of α-motoneurone activity is due to an all-or-none plateau depolarization. This plateau can be induced by a short train of excitatory synaptic potentials or a brief, intracellularly injected depolarizing current pulse and is terminated by a short train of inhibitory synaptic potentials or a hyperpolarizing current pulse. It is concluded that maintained motor unit firing triggered by a brief train of impulses in Ia afferent reflects an intrinsic bistable behaviour of α-motoneurones.

Functional identification of interneurons responsible for left-right coordination of hindlimbs in mammals.

Local neuronal networks that are responsible for walking are poorly characterized in mammals. Using an innovative approach to identify interneuron inputs onto motorneuron populations in a neonatal rodent spinal cord preparation, we have investigated the network responsible for left-right coordination of the hindlimbs. We demonstrate how commissural interneurons (CINs), whose axons traverse the midline to innervate contralateral neurons, are organized such that distinct flexor and extensor centers in the rostral lumbar spinal cord define activity in both flexor and extensor caudal motor pools. In addition, the nature of some connections are reconfigured on switching from rest to locomotion via a mechanism that might be associated with synaptic plasticity in the spinal cord. These results from identified pattern-generating interneurons demonstrate how interneuron populations create an effective network to underlie behavior in mammals.