ABSTRACT
Well-defined myotactic reflexes (retractor-extensor thrusts) can be elicited from the retractor and extensor musculature when these muscles contract against an external resistance. A de-afferentated limb cannot exert a retractor-extensor thrust.
A retractor-extensor thrust arising in a forelimb evokes an extensor response in the diagonal hindlimb and a protractor response in the contralateral forelimb. In order that these responses should display themselves with regularity it is necessary to de-afferentate the responding limb; in an intact limb they are often masked by the control exerted by the limb’s own proprioceptive mechanism.
In an intact animal a retractor-extensor thrust in a forelimb evokes a ‘placing reaction’ from the ipsilateral hindlimb. This phenomenon is only partially displayed if the hindlimb is de-afferentated.
When an intact forelimb responds, by protraction, to passive stretch, the contralateral hindlimb flexes and the ipsilateral hindlimb extends if the latter limbs have been de-afferentated.
The individual proprioceptor limb responses integrate to form an adequate picture of the co-ordinated limb movements seen when an intact toad ceases to swim and begins to walk on land.
The role of proprioceptor activity in the maintenance of an ambulatory rhythm in a single limb is discussed.
The term ‘retractor-extensor thrust’ is used descriptively and is not meant to indicate any close relationship to the ‘extensor thrust reflex’ of other authors.
