Russell G. Durkovic

Classical conditioning, sensitization and habituation in the spinal cat

Abstract Changes in the flexion reflex of the tibialis anterior muscle of acute spinal cats were examined during conditioning, sensitization and habituation paradigms. Experimental animals were classically conditioned by pairing electrical stimulation of the saphenous nerve (CS) with stimulation of the superficial peroneal nerve (US). Recordings from these nerves assured known and constant stimulus inputs. The response observed was an increase in the magnitude of the reflex response to the CS over training. Habituation (CS only) and sensitization (CS and US presentations, unpaired) control animals exhibited no such reflex facilitation. The results of post-tetanic potentiation studies indicated that the intertrial intervals used were not a factor in the differences observed between experimental and control groups. The results give positive support to the concept of spinal conditioning and emphasize the potential of this model system for the study of neural correlates of learning.

d-2-Amino-5-phosphonovalerate, an NMDA receptor antagonist, blocks induction of associative long-term potentiation of the flexion reflex in spinal cat

The effect of the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (APV) on classical conditioning of the flexion reflex in spinal cat was examined. Animals perfused intrathecally with artificial cerebral spinal fluid (CSF) containing APV exhibited flexion reflex potentiation during the conditioning period that was not significantly different from cats receiving artificial CSF alone. However, the APV group exhibited no signs of reflex potentiation during the 2.5 h retention period, in contrast to the CSF alone group. The results suggest that NMDA receptor activation plays a critical role in the induction of associative long-term potentiation of flexion reflexes in spinal cat.

Classical conditioning of the flexion reflex in spinal cat: features of the reflex circuitry.

Classically conditioned facilitation of the flexor withdrawal reflex of spinal cat occurs in knee and ankle flexor muscles but not in a flexor muscle of the toes. Furthermore, the spinal circuitry activated by a component of the conditioned stimulus (A alpha cutaneous fibers) is not by itself involved in the reflex conditioning. The results suggest that increases in both cutaneous afferent output and motoneuron excitability may be eliminated as mechanisms contributing to conditioning and point to certain interneuronal pools as the locus of learning in this preparation.

Retention of a classically conditioned reflex response in spinal cat

Retention of classically conditioned flexion reflex facilitation was examined in unanesthetized, decerebrate, acute spinal cats. Flexion reflex facilitation, recorded from the tibialis anterior muscle, was obtained by pairing saphenous nerve stimulation (the conditioned stimulus) with superficial peroneal nerve stimulation (the unconditioned stimulus). The flexion reflex declined in control animals receiving the same number of nerve stimuli over the same time span, but in an explicitly unpaired sequence. To investigate retention, conditioned stimuli were presented at 5-min intervals following acquisition for a 2 1/2-h period. During this time a significant difference between conditioning and control groups was maintained even to the last trial, with no indication that the difference was subsiding over time. The results support the possibility that a classical conditioning paradigm applied to the spinal cord can induce alterations in spinal reflexes of long duration. Furthermore, the results appear to rule out post-tetanic potentiation as a mechanism producing the observed long-term effects.

Aftereffects of static or dynamic fusimotor activation on primary afferent discharge

Abstract An analysis is presented of the effects of previous static or dynamic fusimotor activity on various phases of primary afferent discharge before and during ramp stretch of the soleus muscle of anesthetized cat. Short term effects (under 1 sec) following termination of fusimotor activity are generally opposite for static and dynamic fusimotor fibers. Compared to control levels, there is a pronounced depression of primary afferent discharge shortly after termination of static fusimotor activation, and a facilitation following termination of dynamic fusimotor activation. Facilitatory and depressive effects on afferent discharge at periods beyond 1 sec following cessation of fusimotor activation are similar for static and dynamic fusimotor fibers. Possible mechanisms of these changes are discussed in terms of changes in muscle spindle actin-myosin crossbridges, receptor adaptation, and the innervation of nuclear bag and nuclear chain spindle fibers.

Classically conditioned alterations in single motor unit activity in the spinal cat

Muscle tension and single motor unit EMG recordings from a flexor muscle of acute spinal cats were obtained during presentation of classical conditioning and control paradigms. Conditioned increases in muscle tension were similar to previously obtained results. Motor unit recordings suggested that this conditioned reflex facilitation is brought about by an increased probability firing of motoneurons initially responsive to the conditioned stimulus, as well as orderly, size-dependent recruitment of initially non-responsive motoneurons. These results indicate that the same physiological mechanisms for grading motor output may apply for conditioned responses as have been demonstrated for reflexive and voluntary movements.

US intensity and blood pressure effects on classical conditioning and sensitization in spinal cat

Acute spinal cats were immobilized with lesions which produced either high or low mean arterial blood pressures. These animals were assigned to classical conditioning or sensitization paradigms in which the conditioned and unconditioned stimuli (CS and US) were shocks to two different cutaneous nerves of the left hindlimb. Conditioned and unconditioned responses were flexion reflex tensions recorded from the isolated tibialis anterior muscle of the same leg. Three different US intensities were employed for both conditioning and sensitization groups. While conditioning animals received paired stimuli, sensitization animals received either a pseudorandom or an alternating schedule of unpaired CS and US presentations. Spinal cats receiving US intensities which activated at least Aα and Aδ cutaneous afferents showed greater flexion reflex facilitation during the conditioning paradigm than in the sensitization paradigm. The addition of C fiber activation to the Aαδ US produced no additional influence on net conditioning. A US intensity which activated only Aα fibers did not produce facilitation in either conditioning or sensitization paradigms. High- and low-blood-pressure groups behaved similarly, demonstrating the independence of spinal conditioning from this physiological factor over the range observed. No differences were observed between alternating and pseudorandom sensitization control groups.

Functional consequences of motor unit recruitment order reversals following spinal cord transection in cat

Motoneuron recruitment order determinations were made for acute, 2-week chronic, and 3-month chronic spinal cats by comparing cutaneous nerve stimulation thresholds for evoking single unit tibialis anterior (TA) electromyogram (EMG) spikes of different sizes. Recruitment order was largely (∼80%) orderly (small spikes recruited at lower stimulus intensities than large spikes) in acute and 3-month chronic spinal animals. However, in 2-week chronic spinal animals recruitment order was reversed, with large units more often recruited at lower stimulus intensities than small units (∼65%). Morphological analyses of TA muscle fibers suggested that fiber size changes were unlikely to account for the dramatic alterations in recruitment order results of the 2-week chronic spinal animals. Additional studies suggested that the recruitment order reversal in the 2-week chronic animals coincided with an enhanced reflex neural output (increased recruitment or reflex gain) for the flexion reflex which compensated for disuse atrophy related decreases in flexor muscle force generation capability in these animals. The data from 2-week chronic spinal animals represent a functionally significant example of deviation from the normal size principle of motoneuron recruitment order as the corresponding reflex gain increases can enhance the rapidity of motor function recovery (standing, locomotion) following spinal injury.