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NEUROSCIENCE |
1 Department of Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, PA 19129, USA
2 Spinal Cord Research Centre and Department of Physiology, University of Manitoba, Winnipeg, Manitoba R3E 3J7, Canada
3 A. B. Kogan Research Institute for Neurocybernetics, Rostov State University, Rostov-on-Don 344090, Russia
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Abstract |
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(Received 4 August 2006;
accepted after revision 24 September 2006;
first published online 28 September 2006)
Corresponding author D. A. McCrea: Spinal Cord Research Centre, University of Manitoba, 730 William Avenue, Winnipeg, Manitoba, R3E3J7 Canada. Email: dave{at}scrc.umanitoba.ca
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionAppendixReferences |
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Perhaps the most studied hindlimb afferent system controlling locomotor activity is that from extensor group Ia muscle spindle and Ib tendon organ afferents (collectively referred to as group I afferents). Activation of extensor group I afferents, and particularly those from ankle muscle nerves, results in a strong excitation of extensor motoneurones during extension (Conway et al. 1987; Pearson & Collins, 1993; Guertin et al. 1995). This proprioceptive feedback contributes to a substantial portion of stance-phase extensor activity in cats during treadmill locomotion (e.g. Hiebert & Pearson, 1999; Donelan & Pearson, 2004), and in man (Sinkjaer et al. 2000). In reduced preparations, activity in extensor group I afferents can also control the transition from stance to swing (Duysens & Pearson, 1980), regulate the duration of the stance phase, and entrain the step cycle period (references in Pearson, 2004; Rossignol et al. 2006). In contrast, activation of group II afferents in the same extensor nerves appears to contribute little to the control of locomotion (Conway et al. 1987; Guertin et al. 1995; Donelan & Pearson, 2004).
Cutaneous reflexes also have an important role in controlling locomotion (e.g. Zehr & Duysens, 2004; Rossignol et al. 2006). For example, stimulation of tibial nerve cutaneous afferents during the extensor phase of fictive locomotion enhances the activity of extensor motoneurones and prolongs the ongoing extensor phase. The same stimulation during flexion produces a premature initiation of the extension phase (Conway et al. 1994; Guertin et al. 1995). In addition to these widespread effects on hindlimb motoneurones, specialized cutaneous reflexes such as the stumbling correction reaction produce a specific and more limited pattern of motoneurone excitation and inhibition in intact cats (e.g. Buford & Smith, 1993) and during fictive locomotion (Quevedo et al. 2005a) evoked through both restricted cutaneous reflex pathways and through CPG circuitry (Quevedo et al. 2005b).
Activation of hindlimb flexor muscle group I afferents during the flexor phase of locomotion can enhance hindlimb flexor activity in a manner analogous to the extension-promoting effects of extensor group I afferents (e.g. Perreault et al. 1995; Stecina et al. 2005). However, unlike group II fibres in extensor nerves, group II afferents in flexor nerves evoke powerful effects on the activity of motoneurones and on step cycle timing during fictive locomotion. Depending on the nerve tested and the preparation used, activation of flexor group II fibres during the flexor phase can either prolong flexion (e.g. stimulation of the extensor digitorum longus (EDL) nerve; Stecina et al. 2005) or terminate flexion and initiate extension (e.g. tibialis anterior (TA); Perreault et al. 1995; Stecina et al. 2005). An important feature of the reflex actions of flexor group II afferents evoked during fictive locomotion is that spontaneous reflex reversals can occur. In those cases, TA stimulation prolongs flexion and EDL stimulation resets the step cycle to extension for a few stimulus trials (Stecina et al. 2005). During treadmill walking, activation of group II afferents in both nerves (TA or EDL) usually enhances flexor activity (Hiebert et al. 1996) although variability in flexor group II actions has also been reported (Lam & Pearson, 2002). Thus the effects of hindlimb flexor muscle afferents on locomotion can be more variable than those from extensor nerves and depend upon the involvement of group II fibres, the particular nerve stimulated and the preparation.
Based primarily on observations obtained during fictive locomotion, it would appear that most hindlimb reflex pathways are altered with the onset of locomotion. For example, monosynaptic Ia excitation of homonymous and synergist motoneurones (Gosgnach et al. 2000) and transmission from group II afferents (Perreault et al. 1999) are reduced by a presynaptic inhibition. The nature of reflexes may also change with the onset of locomotion. For example, group I non-reciprocal inhibition of motoneurones is suppressed during fictive locomotion (Gossard et al. 1994; McCrea et al. 1995; Angel et al. 1996, 2005) and in many preparations replaced by a group I-evoked, phase-dependent, disynaptic excitation of extensor (McCrea et al. 1995; Angel et al. 1996, 2005) and flexor (Degtyarenko et al. 1998; Quevedo et al. 2000) motoneurones. Both in cats and in man, the control of motoneurone activity by sensory feedback during locomotion involves significant changes in the reflex circuitry operating in quiescent or non-locomoting motor states (McCrea, 2001; Rossignol et al. 2006).
The ability of afferent stimulation to simultaneously affect activity in flexor and extensor motoneurone pools throughout the limb and to control locomotor cycle timing is strong evidence for afferent actions exerted through a common network, such as the CPG, rather than through a system of private reflex pathways acting on a limited set of motoneurones (Conway et al. 1987; Gossard et al. 1994; McCrea, 2001; Pearson, 2004; Rossignol, 2006). Accordingly, it appears that CPG and reflex circuits are deeply integrated within the spinal cord and can influence and modify the performance of each other (Jankowska et al. 1967; McCrea, 2001; Angel et al. 2005).
According to classical views, the locomotor CPG consists of two half-centres that reciprocally inhibit each other and directly excite antagonist groups of motoneurones (e.g. Lundberg, 1981). In this organization, a single network controls the locomotor rhythm and patterns of motoneurone activity during locomotion (discussed in Lafreniere-Roula & McCrea, 2005; and the accompanying paper, Rybak et al. 2006). Therefore, any afferent stimulation that produces a premature or delayed phase switching would be expected to change the ongoing step cycle duration and hence shift the phase of the following locomotor rhythm (i.e. produce resetting). However, the effects of afferent stimulation observed during fictive locomotion are often inconsistent with this expectation. For example, stimulation of extensor group I afferents during flexion can produce a premature switching to extension with a compensatory shortening of the subsequent extensor phase so that there is no change in step cycle duration and timing of the following step cycles (Guertin et al. 1995). Thus despite changes in phase duration, the system can ‘remember’ and maintain the original cycle period timing. Similarly, when extensor afferent activation prolongs the extensor phase, step cycle duration is often maintained by a corresponding shortening of the subsequent flexion phase (Guertin et al. 1995; see also Kriellaars et al. 1994).
In the present report we propose a neuronal organization of the mammalian spinal cord circuitry that can accommodate the variety of sensory-evoked changes in locomotor activity described above including spontaneous and preparation-dependent reflex reversals of group II actions (see Stecina et al. 2005). This spinal circuitry incorporates our recently developed computational model of the mammalian spinal cord circuitry that incorporates the locomotor CPG (Rybak et al. 2006) that has a two-level architecture consisting of a half-centre rhythm generator (RG) and a pattern formation (PF) network with reciprocal inhibitory interactions between antagonist groups of neurones at several levels. The model is presently limited to describing the activity of only one pair of antagonist motoneurone pools. Here we incorporate hindlimb afferent inputs into this model to examine how these inputs control the locomotor CPG. Specifically, we describe the results of modelling the effects of stimulation of extensor group I afferents, cutaneous posterior tibial nerve afferents and flexor muscle nerve group I and II afferents on the firing patterns of motoneurones, the timing of phase transitions and the locomotor cycle period. We begin by discussing the reorganization of group I reflex pathways that occurs with the transition to the locomotor state. This reorganized circuitry is then incorporated into our CPG model to reproduce the actions of hindlimb nerve stimulation observed in experimental studies during fictive locomotion. Some results have been presented in abstract form (McCrea et al. 2004; Rybak & McCrea, 2005).
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Methods |
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In the present paper, additional interneurone populations were incorporated in the model to mediate effects of afferent stimulation. These included interneurones meditating afferent input to extensor and flexor portions of the rhythm generator and pattern formation networks (Irg-E, Irg-F and Ipf-E, Ipf-F, respectively; see Fig. 2 and 6) and interneurones mediating locomotor-dependent disynaptic excitation of extensor motoneurones from extensor group I afferent (Iab-E; see Figs. 1B and 2). The schematic of interactions between neural populations in the model used for simulation of the effects of extensor and cutaneous afferent stimulation is shown in Fig. 2 and for flexor afferent stimulation in Fig. 6. The weights of connections to and from interneurone populations mediating the effects of afferent stimulation (e.g. Irg-E, Irg-F, Ipf-E, Ipf-F, In-E and Iab-E) were adjusted to replicate afferent-evoked changes in electroneurogram (ENG) activity recorded during midbrain locomotor region (MLR)-evoked fictive locomotion in decerebrate adult cats. The relative weights of all synaptic connections, including the connections to the populations incorporated in the extended model, are shown in Table 1 in the Appendix.
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In the model, afferent stimuli were applied as rectangular pulses with the amplitudes and durations indicated in the corresponding figures and legends. To mimic the experimental conditions in which electrical stimulation of peripheral nerves at group I intensity recruits both Ia and Ib fibre types and stimulation at group II intensity recruits type Ia, Ib, and II sensory afferents, afferent stimuli were applied simultaneously either to Ia and Ib fibres of extensors (Ia(e) and Ib(e), respectively; Ia(e) = Ib(e)), or to Ia, Ib and group II fibres of flexors (Ia(f), Ib(f) and II(f), respectively; Ia(f) = Ib(f)), or to cutaneous afferents (Cut). Activation of flexor group II afferents required stimulus amplitude to exceed some threshold (Thr(II)). Specifically,
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0, and 0 if x < 0. All simulations were performed using a simulation package NSM 2.0 (for Windows XP) developed at Drexel University by I. A. Rybak, N. A. Shevtsova and S. N. Markin. Differential equations were solved using the exponential Euler integration method (MacGregor, 1987) with a step of 0.1 ms (further details in Rybak et al. 2003).
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Results |
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In the cat, activation of group Ia muscle spindle afferents evokes a reflex monosynaptic excitation of synergist motoneurones (Eccles et al. 1957) and a disynaptic inhibition of antagonist motoneurones mediated by Ia interneurones (Jankowska, 1992). Activation of Ib tendon organ or group Ia muscle spindle afferents in non-locomoting preparations evokes a short latency inhibition of synergist motoneurones, the non-reciprocal group I inhibition (Jankowska et al. 1981; Jankowska, 1992). These and other reflex actions were incorporated into the scheme shown in Fig. 1. Figure 1A represents a simplified scheme of basic reflex circuitry under non-locomoting conditions. It shows two motoneurone (an extensor and a flexor) populations and six interneurone populations mediating reflex interactions. The Ia-E and Ia-F interneurone populations are excited by primary muscle spindle afferents from extensors and flexors, respectively, and mediate Ia reciprocal inhibition of antagonist motoneurones. They are also connected by mutual inhibition (see Jankowska, 1992). R-E and R-F are the populations of Renshaw cells which are excited by axon collaterals of the corresponding motoneurones and inhibit these motoneurones and the corresponding Ia inhibitory interneurones (Jankowska, 1992). In Fig. 1A, the group I afferents (from both tendon organs and spindles) activate pathways providing the non-reciprocal inhibition of extensors and flexors mediated, respectively, by the Ib-E and Ib-F populations of inhibitory interneurones, which also mutually inhibit each other (Brink et al. 1983; Jankowska & McCrea, 1983; Jankowska, 1992). Therefore during non-locomoting conditions, the reflex circuits described above produce (1) Ia-evoked monosynaptic excitation of homonymous and close synergist motoneurone pools, (2) Ia-evoked disynaptic reciprocal inhibition of antagonist motoneurone populations, and (3) group I (Ib and Ia)-evoked disynaptic non-reciprocal inhibition of homonymous motoneurone pools.
Figure 1B shows the spinal cord circuitry operating during fictive locomotion. The two-level locomotor CPG (described in our preceding paper, Rybak et al. 2006) and several additional interneurone populations (see areas enclosed by dashed lines) have been incorporated into the circuitry of Fig. 1A to provide interactions between the CPG and reflex circuits during locomotion. Activation of tonic excitatory drive to the RG and PF populations (e.g. by continuous MLR stimulation) initiates the generation of the basic locomotor rhythm (for details, see Rybak et al. 2006). During locomotion, alternating activity in the RG populations provides periodic alternating inhibition of excitatory interneurone populations in the extensor and flexor portions of the PF network (PF-F and PF-E, respectively), which in turn alternately excite flexor and extensor motoneurone populations as well as several interneurone populations including Ia-E and Ia-F that provide rhythmic inhibition of antagonist motoneurone populations.
With the onset of locomotion, the state of the spinal circuitry changes and the operation of the reflex circuitry is reorganized. First of all during fictive locomotion in decerebrate cats, the non-reciprocal inhibitory reflexes are suppressed (e.g. Gossard et al. 1994; McCrea et al. 1995). To reproduce this effect, the tonic MLR drive in the model provides a sustained excitation of the hypothetical interneurone population In (see centre portion of Fig. 1B) which inhibits the Ib-E and Ib-F populations thus suppressing the group I-evoked non-reciprocal inhibition of motoneurones during locomotion.
The Iab-E population has been included in the model (see Fig. 1B) to provide a disynaptic reflex excitation of extensor motoneurones from extensor group I afferents during extension. This group I-evoked disynaptic excitation of extensor motoneurones can be evoked during the extensor phase of fictive locomotion (Schomburg & Behrends, 1978; McCrea et al. 1995; Angel et al. 1996; Degtyarenko et al. 1998) and is mediated by a population of lumbar interneurones that cannot be activated in quiescent preparations (Angel et al. 2005). To reproduce these data, the hypothetical In-E population which inhibits the Iab-E population, has been included in the model (Fig. 1B). Under non-locomoting conditions (i.e. when the locomotor drive is zero), excitatory external drive produces tonic activity of this population that prevents sensory activation of the excitatory Iab-E interneurones (see Fig. 1B). However during the extensor phase of locomotion (i.e. when the PF-E population and the inhibitory Inpf-F population are active), the Inpf-F population inhibits In-E thereby removing the In-E inhibition of Iab-E. This disinhibition permits disynaptic excitation of extensors by group I extensor afferents. In keeping with experimental observations (e.g. McCrea et al. 1995; Angel et al. 1996, 2005), inhibition of Iab-E during flexion prevents the group I disynaptic excitation of extensor motoneurones during the flexion phase of locomotion. Finally, direct excitation of the In-E population from the PF-E population creates rhythmic extensor-phase activity in Iab-E interneurones. Such rhythmic activity has been found in candidate Iab-E interneurones recorded during fictive locomotion in the cat in the absence of sensory stimulation (Angel et al. 2005).
The organization of flexor nerve-activated reflex circuitry operating during fictive locomotion has several similarities to that of extensor reflex circuitry. Flexor afferent stimulation can also evoke disynaptic excitation in flexor motoneurones that is modulated with cycle phase (Degtyarenko et al. 1998; Quevedo et al. 2000). However, compared to the disynaptic excitation of extensors by extensor afferents, the disynaptic excitation of flexors has a more complex phase dependency and a more limited distribution to hindlimb flexor motoneurones (Quevedo et al. 2000). Therefore, a disynaptic excitation of flexors has not been included in the present simplified model containing only two motoneurone pools.
An additional change in the model in the locomotor state (not seen in Fig. 1B) is that the weights of monosynaptic excitatory inputs of Ia extensor and flexor afferents to the corresponding motoneurone populations (Mn-E and Mn-F, respectively) were set relatively small (see Appendix Table 1) to simulate the tonic presynaptic depression of Ia monosynaptic excitation of motoneurones occurring during fictive locomotion (Gosgnach et al. 2000; Rossignol et al. 2006). The additional circuitry incorporated into the model to mediate sensory actions on the CPG is discussed below (see Figs. 2 and 6).
In summary, the reorganization of reflex circuits in our model during locomotion includes: (1) reduction in the amplitude (i.e. weighting) of monosynaptic excitation of extensor and flexor motoneurones by the corresponding Ia afferents; (2) suppression of the non-reciprocal inhibition of extensor and flexor motoneurones by the corresponding group I extensor and flexor afferents; (3) emergence of the phase-dependent disynaptic excitation of extensor motoneurones by the same group I extensor afferents; and (4) emergence of the phase-dependent polysynaptic excitation of extensor motoneurones by extensor group I afferents via the CPG.
Taken together, the last three issues represent a ‘global reorganization’ of the afferent feedback from group I afferents to extensor motoneurones during locomotion. This feedback therefore switches from a negative type via the non-reciprocal inhibition under non-locomotor conditions, to a phase-dependent positive type during locomotion (see Pearson & Collins, 1993; McCrea, 2001; Pearson, 2004; Rossignol et al. 2006).
In the remainder of this paper, only the locomotor state is considered. Therefore the Ib-E, Ib-F and In populations and all connections involving the suppressed non-reciprocal inhibitory pathways have been removed (see Figs 2 and 6).
Control of the CPG at the PF and RG level: effects of extensor group I stimulation
Figure 2 shows the reduced circuitry used to model the effects of extensor group I afferent stimulation during locomotion. The locomotor rhythm is generated by the rhythm generator (RG) network. The mechanism for rhythm generation (described in detail in the accompanying paper, Rybak et al. 2006) is based on a combination of intrinsic bursting properties of neurons comprising RG-E and RG-F populations, mutual excitation and inhibition between these populations, and their excitation by external (MLR) drive. The rhythm generated by the RG causes alternating activity of PF-F and PF-E populations (within the PF network) which transmits the rhythm to the motoneurone level producing alternating activation of flexor and extensor motoneurones, respectively. One aspect of the model important for understanding the effects of afferent activation is that although both the RG and PF populations have intrinsic bursting properties, the locomotor pattern generated under normal conditions is critically dependent on the mutual inhibition within both the RG and PF networks. This inhibition is important for burst termination and causes the alternating activity bursts of antagonist populations at each level of the network to be tightly coupled.
In Fig. 2, two hypothetical interneurone populations (Irg-E and Ipf-E) have been included in the model to mediate the effects of extensor group I afferent stimulation on the CPG (at the RG and PF levels, respectively) and produce the afferent-evoked excitation of motoneurones that has been postulated to be mediated by the CPG (Conway et al. 1987; Gossard et al. 1994; Guertin et al. 1995; McCrea, 2001; Pearson, 2004; Rossignol et al. 2006).
An important feature of our model is that the extensor afferent stimulation can excite extensor motoneurones via both the RG and PF levels of the CPG as well as through local circuits producing disynaptic and, in the case of Ia afferents, monosynaptic motoneurone excitation. Afferent access to the RG-E and PF-F populations is mediated by the Irg-E and Ipf-E populations, respectively (see Fig. 2). According to the hypothesis explored here, the synaptic weighting of group I input (i.e. combining the Ia and Ib inputs) to the PF-E population (controlling extensor activity at the PF level) is stronger than that to RG-E (the extensor half-centre of the RG) (see Appendix Table 1).
Figure 3A shows a result of modelling the effects of group I extensor afferent stimulation delivered during flexion. All traces except the top one are average activity histograms of the labelled neuronal populations (see Fig. 2). In this example, a brief group I extensor afferent stimulation (top trace) is delivered to the RG-E and PF-E populations (via Irg-E and Ipf-E populations, respectively) when the flexor motoneurone population (Mn-F) is active (i.e. during the flexion phase). During flexion, both the RG-E and PF-E populations are inhibited by activity in the flexor half-centres (RG-F and PF-F, mediated by the Inrg-E and Inpf-E populations, see Fig. 2). The relatively weak synaptic input from group I extensor afferents to Irg-E is unable to overcome this flexion-related inhibition and excite the RG-E population. Thus rhythm generator activity (second and third traces in Fig. 3A) is unaffected by the afferent stimulation. But at the same time, the stronger synaptic weight of afferent input to Ipf-E (1.0 versus 0.4 to Irg-E, see Appendix Table 1) evokes a brief excitation of the PF-E population, which in turn inhibits PF-F activity (fourth and fifth traces from the top in Fig. 3A). As a result there is a resetting of the PF network to extension (without affecting the RG) that evokes a brief burst of extensor motoneurone activity and a corresponding inhibition (cessation of activity) of flexor motoneurones (see the two bottom traces in Fig. 3A).
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During real locomotion, extensor group I afferent feedback would occur mainly during the extension phase when Ib afferents are active during extensor muscle contractions (Prochazka & Gorassini, 1998). Figure 4Aa and Ab shows the results of our simulations of the effects of stimulation of extensor group I afferents during the extension phase when both the RG-E and PF-E populations are active. The simulation in Fig. 4Aa had the same amplitude (dmax = 0.8) as in Fig. 3. This moderate stimulation had little effect on the activity of the RG populations and hence did not change the ongoing locomotor rhythm generated by the RG (see the second and third traces in Fig. 4Aa). The applied stimulation did, however, enhance and prolong PF-E population activity. This increased PF-E activity delayed the switching to the flexion phase at the PF level (see fourth and fifth traces in Fig. 4Aa) and enhanced and prolonged extensor motoneurone firing (see the bottom trace in Fig. 4Aa). Because the rhythm generator was not affected, the subsequent flexion phase was shortened and the step cycle duration remained constant. This simulation is consistent with the experimental data shown in Fig. 4Ba where plantaris (Pl) nerve stimulation enhanced and prolonged extensor motoneurone activity (hip, AB; knee, Quad; and ankle, medial gastrocnemius (MG)). As in our simulation, there was a corresponding shortening of the subsequent flexor phase such that the step cycle period remained unchanged (see arrows at the bottom of Fig. 4Ba).
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Based on the results of these simulations, we conclude that stimulation of group I extensor afferents during extension may prolong the current extension phase with or without changing the duration of ongoing locomotor cycle and the phase of post-stimulation rhythm. The exact effect in the model depends on how strongly the applied stimulation influences the rhythm generator.
Dominant control of the CPG at the RG level: effects of cutaneous tibialis nerve stimulation
In the cat, stimulation of cutaneous afferents in the tibial nerve (Tib, innervating plantar foot structures) during the extension phase of fictive locomotion promotes hindlimb extensor activity (Guertin et al. 1995). The same stimulation during flexion terminates the ongoing flexion phase and initiates extension (Guertin et al. 1995). To simulate these effects, we suggest that stimulation of Tib nerve afferents (the Cut lines in Fig. 2) activates the extensor part of the CPG via the corresponding interneurone populations (Inrg-E and Inpf-E) with an equal effect on the RG-F and PF-E populations (Appendix Table 1). In the model, these afferents have no monosynaptic or disynaptic actions on motoneurones or Ia inhibitory interneurones. Figure 5Aa and Ab shows simulations of the effects of delivering such stimulation during flexion and extension, respectively. Stimuli applied during flexion (Fig. 5Aa) reset the RG (and the entire pattern) to extension. Note the shortening of activity in the RG-F population and the advanced start of activity of the RG-E populations with the onset of stimulation (vertical dash-dot lines). The same stimuli applied during extension prolong activity in the RG-E and PF-E populations which then causes prolonged and increased extensor motoneurone activity with each stimulus presentation (Fig. 5Ab). The effect in both cases (resetting or prolongation) is produced at the level of RG with a resetting of the locomotor rhythm (see arrows at the bottom of Fig. 5Ab, Ba and Bb). The simulations closely reproduce the results of corresponding experimental studies (see Fig. 5Ba and Bb).
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As outlined in the Introduction, activation of group II afferents in flexor nerves during fictive locomotion can either terminate the ongoing flexor phase and switch the step cycle to extension (e.g. following TA nerve stimulation) or prolong ongoing flexor activity (e.g. EDL nerve stimulation; Perreault et al. 1995; Stecina et al. 2005). Occasionally these actions spontaneously reverse (Stecina et al. 2005). In the same preparations, flexor group I afferent activation can enhance and prolong flexor motoneurone activity in a manner analogous to the extensor promoting actions of extensor group I afferents (Perreault et al. 1995; Stecina et al. 2005). These observations support the suggestion that the group I flexor afferents are excitatory to the flexor part of the CPG (i.e. to RG-F and PF-F populations in the model). Our hypothesis (see Stecina et al. 2005) is that flexor group I afferents excite the flexor part of the CPG (both RG-F and PF-F) while flexor group II afferents are excitatory to the extensor part of the CPG. Accordingly, the effects of simultaneous activation of group II and group I flexor afferents work against each other. The resulting effect is therefore dependent upon the interplay between the two influences on the CPG.
The schematic of the model used for flexor afferent stimulation is shown in Fig. 6. As in Fig. 2, hypothetical interneurone populations (Irg-F, Ipf-F, Irg-E and Ipf-F) have been added to mediate the effect of group I and II flexor afferent stimulation on the CPG. Similar to the organization postulated for extensor afferents, there is a relatively strong excitatory effect of flexor group I afferents on PF-F (via Ipf-F) and a weak effect on RG-F (via Irg-F) (see Appendix Table 1). In accordance with the above hypothesis, the flexor group II afferents are connected to the Irg-E and Ipf-E populations which mediate their excitatory effects on the extensor part of the CPG (RG-E and PF-E, respectively). Because of the strong weighting of group II afferent input to RG-E (via Irg-E), activation of these afferents may strongly affect the RG (e.g. reset the RG to extension).
Figure 7A shows an example of our simulation of the effects of flexor afferent stimulation during flexion. To match the experimental procedure, all stimuli were applied to both flexor group I (Ia and Ib) and group II inputs. The first stimulus (left, top trace in Fig. 7A) strongly recruits both group I and II afferents. This stimulus terminated the ongoing flexion phase and reset the cycle to extension (see RG-F and RG-E activities in Fig. 7A). In order to mimic a less effective recruitment of group II afferents, the threshold for activation of group II flexor afferents was increased prior to the application of the second stimulus. This time the stimulation prolonged the ongoing flexor phase. This occurred because the relatively stronger group I-evoked excitation of PF-F (i.e. weaker group II activation of RG-F) prevented phase resetting at the level of pattern formation (see the fourth and fifth traces in Fig. 7A) and hence at the level of motoneurones. Therefore, the net result of strong activation of flexor group I and relatively weak activation of group II afferents was a prolongation of the ongoing flexion phase. It is important to notice that both stimuli reset the RG oscillator to extension (see the second and third traces in Fig. 7A).
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Figure 8Aa–Ae shows the effects of a progressive increase in the influence of flexor group II afferents on RG-E and PF-E populations in the model. This was produced by a sequential reduction of the threshold for group II flexor afferent activation from panel Aa to Ae (see threshold value at the top of each panel). At the lowest level of group II activation (highest threshold, Aa), the stimulation had little effect on the RG populations and produced a mild enhancement and prolongation of flexor motoneurone activity due to the activation of PF-F by flexor group I afferents (see third and fifth traces in Fig. 8Aa). In both Fig. 8Ab and Ac, the group II activation reset the RG (see the first and second traces in Fig. 8Ab and Ac) but this resetting was not expressed at the PF level (see the third and fourth traces in Fig. 8Ab and Ac). As a result, flexor motoneurone activity was prolonged even though activity in the flexor portion of the rhythm generator (RG-F) was shortened. In Fig. 8Ad, the increased group II input (further threshold reduction) produced further excitation of the extensor side of the CPG. This partially overcame the group I actions on the flexor side, and a phase resetting occurred at both RG and PF levels. There was, however, only a small brief burst of extensor motoneurone activity and a short break in flexor activity. Further increase in group II flexor afferent activation (further reduction of their threshold) resulted in a much stronger effect of group II afferents on the CPG. In Fig. 8Ae, the threshold for group II flexor afferent activation was maximally reduced (to the same value as during the first stimulus application in Fig. 7A). As a result, stimulation terminated the flexion phase and produced a premature resetting to extension. Importantly, in all but one (Fig. 8Aa) of the simulations shown in Fig. 8Aa–Ae, the prolongation of flexion or resetting to extension was accompanied by a resetting of the rhythm at the RG level.
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Discussion |
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The reorganization of reflex circuits during locomotion in our model results from interactions of these circuits with the locomotor CPG (Fig. 1) and includes the suppression of non-reciprocal inhibition (Gossard et al. 1994; McCrea et al. 1995; Angel et al. 1996; McCrea, 2001) and the emergence of phase-dependent, disynaptic excitation of extensor motoneurones by group I extensor afferents (McCrea et al. 1995; Angel et al. 1996, 2005). This reorganization results in the replacement of the non-reciprocal inhibition operating under non-locomoting conditions with a phase-dependent positive feedback to extensors during locomotion (Pearson & Collins, 1993; Gossard et al. 1994; McCrea, 2001; Donelan & Pearson, 2004; Rossignol et al. 2006). In the present simulations, the weights of Ia afferent inputs to extensor and flexor motoneurones were set at a relatively weak level to account for the presynaptic depression of monosynaptic Ia reflexes during locomotion (e.g. Gosgnach et al. 2000; Rossignol et al. 2006). We did not attempt to simulate all of the known hindlimb reflexes operating during fictive locomotion. For example, we did not consider the flexor afferent-evoked disynaptic excitation of flexor motoneurones (Degtyarenko et al. 1998; Quevedo et al. 2000) and the short-latency cutaneous reflexes involved in the stumbling correction reaction (Burke, 1999; Quevedo et al. 2005a,b). Nor did we simulate cutaneous reflexes evoked from nerves other than the tibial nerve (e.g. Guertin et al. 1995). These issues will be considered in our future modelling studies.
An important advantage of the two-level CPG structure considered here is that it allows sensory feedback to separately control (1) the amplitude and timing of flexor and extensor motoneurone activities (via the PF level) and (2) the frequency and phase of locomotor oscillations (through the RG level). Specifically, the model suggests and identifies the conditions in which there can be a premature phase switching or a prolongation of the ongoing phase without (e.g. Figs 3 and 4Aa) or with (e.g. Figs 4Ab and 5) an alteration of the phase of post-stimulation rhythm. Furthermore, by regulating the relative degree to which flexor group I and group II afferents affect flexor and extensor components of the RG and PF, we could also simulate and suggest an explanation for the complex reflex actions of flexor afferents and the occasional reflex reversal observed during fictive locomotion (e.g. Figs 7 and 8; see Stecina et al. 2005).
The effects of extensor afferent stimulation
Modelling the effects of group I extensor afferent stimulation delivered during flexion shows that these afferents can produce a premature switching of ongoing flexion to extension without affecting rhythm generation (Fig. 3A). These simulation results are quite similar to the experimental data obtained during MLR-evoked fictive locomotion (e.g. see Fig. 3B). To our knowledge, such phase switching that does not affect the locomotor rhythm has not been considered before. The reciprocal inhibition between flexor and extensor components within the PF level as suggested by our model is critical for this behaviour.
There is substantial evidence (see Introduction) that during real locomotion, the activity of group I extensor afferents provides strong activation of extensor motoneurones and significantly contributes to the weight support during stance as well as to the control of the timing of stance–swing transition. In the context of our model, we suggest that these afferents contribute to weight support and the control of stance–swing transitions via separate pathways within the CPG. We hypothesize that the contribution of the activity of group I extensor afferents to weight support during stance is provided by a positive feedback loop via the PF network (in the model, via the activation of PF-E population) and, at a lower level, through the disynaptic excitation of extensor motoneurones. At the same time, the control of the transition from stance to swing (e.g. prohibiting swing until the limb is unloaded; see Duysens & Pearson, 1980) operates via an extensor group I afferent-evoked activation of the extensor half-centre of the RG (RG-E population). Our model also suggests that the effect of group I afferents on the RG is weaker than their effect on the PF. Therefore, moderate activation of extensor group I afferents would enhance and prolong the ongoing extension phase via actions at the PF level without resetting the locomotor rhythm (see Fig. 4Aa). At the same time, a stronger activation of RG-E may delay (or even ‘prohibit’) the transition from stance to swing at the RG level and shift the timing of the following step cycles. Our model predicts that such effects can be produced with an increase in the intensity of group I extensor afferent stimulation (see Fig. 4Ab) or with stimulus delivery closer to the expected time of transition to flexion. These predictions await experimental confirmation.
The effects of cutaneous afferent stimulation
In our model, stimulation of the tibial nerve was used as an example of reflex control exerted principally at the RG level of the CPG and not at the level of motoneurones and short-latency reflexes. Tibial nerve stimulation during the extension phase of fictive locomotion enhances the activity of extensor neurones and prolongs extension while the same stimulation during flexion terminates the ongoing flexion and initiates extension (Conway et al. 1994; Guertin et al. 1995). In both cases the stimulation strongly affects the duration of the ongoing locomotor period (see Fig. 5). Our simulation predicts that cutaneous afferents in the tibial nerve have direct access to the spinal rhythm-generating circuitry to control the timing of locomotor phase switching and increase extensor motoneurone activity throughout the limb. According to both the experimental data and our simulation, the effects of stimulation of these afferents on step cycle period are similar but stronger than the actions of group I extensor muscle afferents. In keeping with the important contribution of cutaneous afferents to the control of locomotion in both normal and spinal cats (Rossignol et al. 2006), the flexible organization of the model could also incorporate the reflex actions of other cutaneous afferents (e.g. superficial peroneal) postulated to have actions on the flexor side of the CPG (see Quevedo et al. 2005a,b).
The effects of flexor afferent stimulation
Our modelling studies on the effects of flexor nerve stimulation were motivated by the need to explain the variable, often opposing, and spontaneously reversing reflex actions of these afferents on the locomotor pattern (see Introduction). Here we explored the hypothesis (Stecina et al. 2005) that group I afferents in flexor nerves provide excitatory input to the flexor part of the CPG (RG-F and PF-F) whereas group II afferents in the same nerves are excitatory to the extensor part of the CPG (see Fig. 6). Based on our model, lower intensity stimulation that predominately activates group I flexor afferents should prolong the ongoing flexion phase and higher intensity stimulation, activating group II afferents, may evoke a resetting to extension. This idea appears to be generally consistent with the fictive locomotion data in which raising stimulus intensity in some flexor nerves from 2T to 5T to recruit group II afferents changes the effect of flexor afferent stimulation from a flexor-phase prolongation to a resetting to extension (e.g. TA, posterior biceps combined with semitendinosis (PBSt) and Sart nerves; Perreault et al. 1995; Stecina et al. 2005).
According to our hypothesis, higher intensity electrical stimulation activating both group I and II flexor afferents causes a competition between opposing actions of these afferents on the CPG (see Figs 6, 7 and 8). Therefore, the opposite effect produced by activation of TA (and PBSt and Sart) versus EDL (and PerL and Psoas) afferents seen experimentally (see Fig. 7B) may be because of differences in the relative effectiveness of the inputs from group I and group II fibres in these nerves to central circuitry (as in Fig. 7A) and not because of qualitative differences in their anatomical projections to subpopulations of spinal interneurones. In addition, we suggest that the effectiveness of both group I and II afferent synaptic connections to the CPG are subject to control and variation during locomotion. Experimental support for this suggestion is that there is a strong centrally generated presynaptic control of synaptic transmission from group II afferents during MLR-evoked fictive locomotion in select spinal locations (Perreault et al. 1999; Stecina et al. 2002). Spontaneous variations in the effectiveness of group II and group I inputs to the CPG could explain the occasional reversal of reflex effects (e.g. those seen during EDL stimulation) (Fig. 8Ba–Be).
The two-level CPG model was able to reproduce the flexor nerve-evoked enhancement and prolongation of flexion (Fig. 7A, second stimulus, and Fig. 8Aa–Ad) as well as the resetting to extension (Fig. 7A, first stimulus, and Fig. 8Ae). We show that the resultant effect depends on the relative influences of group I and group II afferents on the CPG. An important conclusion from our simulations is that both effects (shortening or prolonging the current cycle period, as seen in the activity of motoneurones) may be produced with (and are a consequence of) the resetting of the RG by the group II afferents (see the RG-E and RG-F traces in Figs 7A and 8Ab–Ae).
The complex reflex effects of activation of group I and II afferents in flexor nerves require further analysis. The exact neuronal types and pathways operating during locomotion and their control remain unidentified. The variability of responses between and within preparations (see Introduction) may depend on the intensity of applied stimulation (and the number of group II afferents involved and the distribution of their thresholds) and on the timing of stimulus delivery (with respect to the anticipated phase switching). These factors will be subjects for future experimental and modelling studies.
The role of flexor afferent feedback during normal locomotion is poorly understood. It is known that feedback from hip flexor muscle afferents, particularly from the Sart muscle, can enhance flexor activity during treadmill locomotion (Lam & Pearson, 2001) and that hip position contributes to swing–stance transitions (McVea et al. 2005). Our simulations have demonstrated that simultaneous activation of both group I and group II afferents of flexors may enhance and prolong flexion (see Fig. 7, second stimulus, and Fig. 8Aa–Ad), which is consistent with the findings of Lam & Pearson (2001). On the other hand, and similar to the experimental data on fictive locomotion (Perreault et al. 1995; Stecina et al. 2005), the model shows that a stronger activation of group II afferents during flexion may promote a switch to the extension phase. Sorting out flexor afferent control of CPG operation will require an understanding of when these afferents are active during the step cycle as well as an appreciation of the presynaptic control of inputs from these afferents to their target interneurones.
Conclusion
In the preceding paper (Rybak et al. 2006), we proposed a model of mammalian spinal circuitry with the two-level locomotor CPG composed of RG and PF networks. Despite the relatively simple schematic, this model is able to generate locomotor oscillations with step cycle periods and phase durations spanning the range observed during fictive locomotion (Yakovenko et al. 2005) and reproduce various types of spontaneous deletions of motoneurone activity occurring during fictive locomotion (Lafreniere-Roula & McCrea, 2005). Here we show that this model can accommodate the reorganization of reflex circuits during locomotion and realistically reproduce and explain several experimentally observed effects of extensor, flexor and cutaneous afferent stimulation upon locomotor rhythm and motoneurone firing. The ability to separately control the durations of locomotor phases and the step cycle and the degree of motoneurone activity is an important feature of this model allowing it to reproduce experimental data that cannot be easily accommodated within the classical half-centre or the coupled unit-burst generator organizations. Efforts are underway to extend the model to the more complex locomotor activities of bifunctional motoneurones (Chakrabarty et al. 2004). We consider this model as a basis for future interactive experimental and modelling studies.
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