The spinal locomotor generator.

Abstract

In several vertebrate species it has been clearly demonstrated that neural circuits within the spinal cord are capable of generating the signals underlying locomotory rhythms. In the present paper a halfcentre model is proposed for the spinal locomotor generator which is based entirely on the connections and properties of known spinal neurones. In its simplest form the model involves six groups of neurones: Two sets of alpha motorneurones (α-MN), Renshaw cells (RC) and Ia inhibitory interneurones (Ia-IN), associated, respectively, with flexor and extensor muscles. Mutual inhibition between antagonist Ia-INs provides the basis for a neuronal ‘flip-flop’ in which one group is active and the other depressed. A novel element in the model is the RC which completes a feedback pathway from a-MNs to Ia-INs. This pathways converts the ‘flip-flop’ into a free running oscillator. Reciprocal inhibition betwen groups of RCs is necessary to prevent the oscillator stopping under certain circumstances. The model has been simulated using electronic analogue neurones. Variation in the tonic excitation of α-MNs changes the frequency of the oscillator. Reduction of the tonic excitation of Ia-INs causes the model to lock into a state resembling standing, or even decerebrate rigidity or spasticity. Afferent excitation of Ia-INs produces prolongation of the current step. The oscillator may be entrained by inhibitory inputs to the Ia-INs from another oscillator. The broader implications of the model in relation to the control of locomotion, posture and other types of movement are discussed.