S. J. Shefchyk

Disynaptic group I excitation of synergist ankle extensor motoneurones during fictive locomotion in the cat.

1. Intracellular recording from medial gastrocnemius (MG) motoneurones was used to examine postsynaptic potentials produced by electrical stimulation of the plantaris nerve at group I strength at rest and during fictive locomotion. Fictive locomotion was evoked by stimulation of the midbrain locomotor region (MLR) in decerebrate cats or in decerebrate, acute low‐spinal cats by perineal stimulation following intravenous administration of clonidine and naloxone. 2. In both MLR and spinal fictive locomotor preparations, stimulation of plantaris nerve group I afferents at rest evoked short‐latency (< 2 ms) IPSPs in MG motoneurones. During the extensor phase of MLR‐evoked locomotion, the same stimulation produced short‐latency (1.6‐1.8 ms) EPSPs. Such latencies suggest mediation by one interneurone interposed between plantaris nerve group I afferents and MG motoneurones. Non‐monosynaptic, short‐latency excitation was not seen at rest nor during the flexion phase of the step cycle. 3. Group I EPSPs during the extensor phase of MLR‐evoked locomotion were evoked by stimulation at intensities ranging from 1.4‐2 times threshold (T). The effectiveness of stimulation intensities < 1.5 T suggests that activation of group II afferents is not required to evoke disynaptic excitation. Selective activation of group Ia afferents by stretches of the Achilles tendon also produced disynaptic EPSPs during extension. 4. During fictive locomotion in spinal animals pretreated with clonidine, short‐latency group I EPSPs were not seen but group I IPSPs recorded at rest disappeared or were greatly attenuated. The failure of depolarizing current to reveal group I IPSPs suggests that fictive locomotion involves an inhibition of the inhibitory interneurones that operate at rest. In both clonidine‐treated spinal and MLR preparations, trains of stimuli at group I strength evoked longer‐latency and slowly rising potentials that were more prominent during the flexor phase of fictive locomotion. 5. These results show a reduction in short‐latency group I inhibition of synergists in both MLR and clonidine‐treated spinal preparations during fictive locomotion. In addition, activation of group I afferents evokes short‐latency excitation of synergists during extension in the MLR preparation. Such excitatory reflexes activated by ankle extensor group Ia and Ib afferents may form an excitatory feedback system, reinforcing on‐going extensor activity during the stance phase of the step cycle.

On the regulation of repetitive firing in lumbar motoneurones during fictive locomotion in the cat

SummaryRepetitive firing of motoneurones was examined in decerebrate, unanaesthetised, paralysed cats in which fictive locomotion was induced by stimulation of the mesencephalic locomotor region. Repetitive firing produced by sustained intracellular current injection was compared with repetitive firing observed during fictive locomotion in 17 motoneurones. During similar interspike intervals, the afterhyperpolarisations (AHPs) during fictive locomotion were decreased in amplitude compared to the AHPs following action potentials produced by sustained depolarising current injections. Action potentials were evoked in 10 motoneurones by the injection of short duration pulses of depolarising current throughout the step cycles. When compared to the AHPs evoked at rest, the AHPs during fictive locomotion were reduced in amplitude at similar membrane potentials. The post-spike trajectories were also compared in different phases of the step cycle. The AHPs following these spikes were reduced in amplitude particularly in the depolarised phases of the step cycles. The frequency-current (f-I) relations of 7 motoneurones were examined in the presence and absence of fictive locomotion. Primary ranges of firing were observed in all cells in the absence of fictive locomotion. In most cells (6/7), however, there was no relation between the amount of current injected and the frequency of repetitive firing during fictive locomotion. In one cell, there was a large increase in the slope of the f-I relation. It is suggested that this increase in slope resulted from a reduction in the AHP conductance; furthermore, the usual elimination of the relation is consistent with the suggestions that the repetitive firing in motoneurones during fictive locomotion is not produced by somatic depolarisation alone, and that motoneurones do not behave as simple input-output devices during this behaviour. The correlation of firing level with increasing firing frequency which has previously been demonstrated during repetitive firing produced by afferent stimulation or by somatic current injection is not present during fictive locomotion. This lends further support to the suggestion that motoneurone repetitive firing during fictive locomotion is not produced or regulated by somatic depolarisation. It is suggested that although motoneurones possess the intrinsic ability to fire repetitively in response to somatic depolarisation, the nervous system need not rely on this ability in order to produce repetitive firing during motor acts. This capability to modify or bypass specific motoneuronal properties may lend the nervous system a high degree of control over its motor output.

The role of Renshaw cells in locomotion: antagonism of their excitation from motor axon collaterals with intravenous mecamylamine

SummaryThe contribution of Renshaw cell (RC) activity to the production of fictive locomotion in the mesencephalic preparation was examined using the nicotinic antagonist mecamylamine (MEC). After the i.v. administration of 3 doses of MEC (1.0 mg/kg) the following observations were made: 1) ventral root (VR) evoked discharge of RCs was decreased by up to 87.7%, 2) recurrent inhibitory postsynaptic potentials recorded in alpha motoneurons were greatly reduced or abolished, and 3) the rhythmic firing of RCs during the fictive step cycle was abolished in 83% of the cells examined. Locomotor drive potentials (LDPs) in motoneurons persisted during the fictive step cycle after MEC administration. Bursts of motoneuron firing during each fictive step cycle were characterized by increased frequency and number of spikes after MEC, although the burst duration was unaltered for similar step cycle lengths. A greater number and frequency of spikes per burst was also observed in Ia inhibitory interneurons (IaINs), which remained rhythmically active after MEC administration. It is concluded that Renshaw cells are not an integral part of the spinal central pattern generator for locomotion, nor do they control the timing of the motoneuron or IaIN bursts of firing during fictive locomotion. The data are consistent with a role for RCs in limiting the firing rates of motoneurons and IaINs during each burst.

Reversible cooling of the brainstem reveals areas required for mesencephalic locomotor region evoked treadmill locomotion

SummaryThe evidence suggests that the mesencephalic locomotor region (MLR) may not be a unitary region since anatomical and functional variations in the descending projections are clearly indicated. Reversible cooling of midline reticular structures can effectively block locomotion evoked by stimulation of lateral MLR (L3.5–4) sites while not significantly affecting the locomotion evoked from more medial MLR (L2–2.5) sites. In contrast, locomotion evoked by stimulation of the medial MLR sites is blocked by cooling of the ipsilateral lateral brainstem region which corresponds to the pontomedullary strip (PLS). Ipsilateral PLS cooling was not effective for blocking lateral MLR evoked locomotion, and contralateral PLS cooling was not effective for blocking either medial or lateral MLR evoked stepping. The evidence indicates that the lateral MLR relays through medial reticular nuclei while the medial MLR sites relay largely through the lateral brainstem structures often referred to as the PLS.

Urethral pudendal afferent-evoked bladder and sphincter reflexes in decerebrate and acute spinal cats

Electrical stimulation of the urethral sensory pudendal nerve in decerebrate or acute spinal cats was used to evoke micturition reflexes in animals that failed to respond to bladder distension. In the decerebrate animals, stimulation of urethral afferents evoked voiding characterized by a large bladder pressure increase coordinated with a simultaneous decrease in external urethral sphincter activity. In animals in which the spinal cord was transected between T10 and L6, electrical stimulation of the urethral afferents evoked small increases in bladder pressure that were insufficient to expel fluid but the contractions were coordinated with a decrease in external urethral sphincter activity. It was concluded that in addition to interacting with spinobulbospinal micturition pathways, urethral pudendal afferents may have direct access to a spinal circuitry that can coordinate bladder and sphincter activity.

Sacral spinal interneurones and the control of urinary bladder and urethral striated sphincter muscle function

Normally, during bladder filling (continence) and expulsion (micturition) there is a reciprocity between the pattern of activity in the urinary bladder sacral parasympathetic efferents and the somatic motoneurones innervating the striated external urethral sphincter muscle. The co‐ordination of this pattern of reciprocal activity appears to be determined by excitatory and inhibitory actions of a variety of segmental afferents and descending systems with sacral spinal actions. These actions may in part be mediated through lower lumbar and sacral excitatory and inhibitory spinal interneurones. Over the past 30 years, both neuroanatomical and electrophysiological approaches have been used to reveal an ever‐increasing richness in the neuronal network in the lower spinal cord related to the bladder and striated external urethral sphincter muscle. The purpose of this review is to present an overview of the identified excitatory and inhibitory spinal interneurones hypothesized to be involved in the central networks controlling the sacral bladder parasympathetic preganglionic neurones and striated urethral sphincter motoneurones during continence and micturition.

Depression of muscle and cutaneous afferent‐evoked monosynaptic field potentials during fictive locomotion in the cat

1 Monosynaptic extracellular field potentials evoked by electrical stimulation of ipsilateral hindlimb nerves carrying muscle group I, II and cutaneous afferents were examined during fictive locomotion. Fifty‐eight field potentials were recorded in the dorsal and intermediate laminae throughout the mid‐lumbar to first sacral segments and fictive locomotion was evoked by mesencephalic locomotor region (MLR) stimulation in paralysed decerebrate cats. 2 The majority (96 %) of group I, II and cutaneous‐evoked field potentials were decreased during fictive locomotion. Group I, cutaneous and dorsal group II potentials were reduced on average to about 80 % of control values. Group II field potentials recorded in the intermediate laminae were reduced to a mean of 49 % of control values. Cyclic variations in field potential amplitude between the flexion and extension phases were observed in 24 of 45 cases analysed. Of those 24 field potentials, the two group I and four cutaneous field potentials were smaller during the flexion phase. All eleven group II and the remaining seven cutaneous fields were smaller during extension. In all but two cases, these cyclic variations were smaller than the tonic depression upon which they were superimposed. 3 In 7/9 group II field potentials examined, reductions (on average to 85 % of control) began with the onset of MLR stimulation that produced tonic activity in the motor nerves before the onset of rhythmic alternating, locomotor discharges. In six of the seven cases the field potential depression increased with the establishment of fictive locomotion. This observation and the cyclic modulation of field potentials during fictive locomotion suggests that the depression was strongly linked to the operation of the spinal locomotor circuitry. 4 Depression of the monosynaptic components of the field potentials suggests a reduction in synaptic transmission from primary afferents to first‐order spinal interneurones during fictive locomotion. Accordingly, the larger depression of intermediate group II field potentials may indicate a preferential reduction in transmission from group II afferents to interneurones located in intermediate spinal laminae. 5 Flexion reflexes evoked by group II and cutaneous afferents were also depressed during MLR‐evoked fictive locomotion. The possibility that this depression results from a reduction in transmission from primary afferents, and in particular from group II afferents, ending on interneurones in the intermediate laminae is discussed.

An intracellular study of perineal and hindlimb afferent inputs onto sphincter motoneurons in the decerebrate cat.

SummaryThe external urethral sphincter (EUS) and external anal sphincter (EAS) are striated muscles that function to maintain urinary and fecal continence respectively. This study examines the short-latency synaptic input from a variety of cutaneous perineal and muscle/cutaneous hindlimb afferents to the motoneurons innervating these muscles. Intracellular recordings from anti dromically identified EUS and EAS motoneurons provided records of the postsynaptic potentials (PSPs) produced by electrical stimulation of peripheral afferents in decerebrate or chloralose anesthetized cats. Excitatory postsynaptic potentials (EPSPs) were produced in most EUS and EAS motoneurons by stimulation of ipsilateral and contralateral sensory pudendal (SPud) and superficial perineal (SPeri) cutaneous nerves. The shortest cen tral latencies in the study (1.5 ms) suggest that there are disynaptic excitatory, in addition to tri-and oligosynap tic, connections within these reflex pathways. EPSPs mixed with longer latency inhibitory potentials (E/I PSPs) were observed in both motoneuron populations but were found more frequently in EAS motoneurons. These E/I PSPs were evoked more often from contralat eral afferents than from ipsilateral afferents. Cutaneous nerves innervating the hindlimb had weaker if any synaptic effects on sphincter motoneurons. Stimulation of ipsilateral hindlimb muscle nerves rarely produced PSPs in EUS motoneurons and had weak synaptic actions on EAS motoneurons. In 2 of 22 animals (both decerebrate), large inhibitory potentials predominated over early small EPSPs suggesting that inhibitory pathways from these afferents to sphincter motoneurons can be released under certain circumstances. The relation between the segmental afferents to EUS and EAS motoneurons and the neural circuitry influencing them during micturition and defecation are discussed.

Spinal cord neural organization controlling the urinary bladder and striated sphincter.

The storage and elimination of urine requires the coordination of activity between the autonomic nervous system (thoracolumbar sympathetic and sacral parasympathetic divisions) controlling the urinary bladder and urethra and the lumbosacral somatic motoneurons innervating the striated sphincter and pelvic floor muscles. These three efferent systems involved in the control of lower urinary tract function receive segmental sensory information from various visceral organs and the perineum, as well as inputs from supraspinal regions. Ascending and descending connections between the various spinal segments levels and supraspinal regions provide the reflex substrates participating in normal bladder continence and micturition reflexes. Many of the actions of descending and segmental reflexes are mediated by excitatory and inhibitory sacral spinal interneurons located within the region of the parasympathetic preganglionic autonomic neurons and the sphincter ventral horn motoneurons. This review will: (1) discuss the basic organization and spinal elements of the reflex pathways subserving continence and micturition; (2) describe features of the identified sacral interneuronal circuitry contributing to the control of the bladder and sphincter function; and (3) discuss how changes in the control of these reflex pathways and neurons may contribute to abnormal patterns of bladder and sphincter function commonly observed following spinal cord injury.

Evidence for a strychnine-sensitive mechanism and glycine receptors involved in the control of urethral sphincter activity during micturition in the cat

Abstractu2002Micturition in the decerebrate cat is characterized by a coordinated bladder contraction and a simultaneous decrease in external urethral sphincter (EUS) efferent activity. Without the suppression of EUS activity, voiding is significantly impaired, resulting in a state sometimes referred to as bladder-sphincter dyssynergia. The aim of the present study was to determine whether glycinergic inhibition contributes to the suppression of EUS activity during micturition evoked by bladder distension or electrical stimulation of the pontine micturition center (PMC) in decerebrate cats. Using subconvulsive intravenous doses of strychnine (0.1–0.24 mg/kg), we examined changes in bladder and EUS electroneurographic (ENG) activity during micturition. Following subconvulsive doses of strychnine, tonic EUS ENG activity increased during bladder filling in five of six animals. In the presence of strychnine, it was possible to evoke reflex bladder contractions of similar duration and peak pressure to those observed before strychnine administration. However, there was an absence of suppression of EUS ENG activity during the bladder contractions in all the animals. To determine whether the changes in sphincter activity could be due to strychnine acting at glycine receptors on EUS motoneurons, sacral spinal tissue was processed for a structural protein (gephyrin) associated with the glycine receptor. Motoneurons in Onuf′s nucleus in S1 were identified using choline acetyltransferase immunohistochemistry and subsequently processed with a gephyrin monoclonal antibody. Abundant gephyrin labeling was evident throughout Onuf′s nucleus. Since Onuf′s nucleus is made up of EUS and other motoneuron populations, a sample of antidromically identified urethral and anal sphincter motoneurons were intracellularly labeled with tetramethylrhodamine dextran (TMR-D) and then processed with the gephyrin antibody. Using dual-beam confocal microscopy, gephyrin immunoreactivity was observed on the soma and proximal processes of individual EUS motoneurons in both male and female animals. It was concluded that a strychnine-sensitive mechanism contributes to the suppression of sphincter activity normally observed during voiding. Although glycinergic inhibition may affect several components of the circuitry responsible for micturition, it appears that the suppression of EUS motoneurons during micturition may be partly due to a direct glycinergic inhibition of the EUS motoneurons.