Spinal Synaptic Delay in Young and Aged Rats1

WAYNER, M. J., JR. AND R. EMMERS. Sf&al synaptic delay in young and aged rats. Am. J. Physiol. 194(2): 403405. q#.-Synaptic delays for the monosynaptic flexor hallucis longus reflex were measured in II~-, 276-, 449 and 822-day-old rats. The mean synaptic delays were 0.97, 1.14, 1.23 and I .36 msec. Afferent conduction velocity was approximately the same in young (66.1 m/set.) and old (56.9 m/set.) rats; however ventral root conduction velocity decreased from 64.2 m/set. in the young animals to 43.7 m/set. in the aged rats. an increase in conduction velocity in rat LOWNESS S sciatic nerve from 25 m/set. to 38 m,/sec. from in speed of response is a characteristic change in older life in 50 to IOO days, and an increase to 53 m/set. both rats and human beings. Brody (I) found that reaction time in the rat decreased from 100 to 300 days. There was little change from birth to about IOO days, remained constant between IOO and 50~ days, and increased rapidly after 2 years of age. A slight decrease and the conduction velocity remained virtually in reaction time was associated with a decrease in metabolic rate produced by hypophysecconstant from 300 to 850 days. Therefore, tomy and thyroidectomy. Thyroxin although it increased metabolic rate did not significantly affect reaction time. Birren and Wall (2) found the increase in response time after 2 years could not be attributed to a decrease in analysis, they are divided into six groups as shown in table I. The ages of the individuals in each group were: I, 116, III, 113, 122, 122; 14 269, 274, 27% 283; III, 43% 440, 443, 459; IV, 77% 7% 83% 850, 859; v, 86% 873, 882; and VI, 274 and 283 days old. The youngest animals were the smallest that could be attached to the apparatus used to support them during the experiment. Anesthesia. In the main experiment, groups and were then injected intraperitoneally with I-IV, 13 animals were anesthetized with ether during surgery, were allowed to recover 0.4 mg/kg of d-tubocurarine chloride. Animals were maintained by artificial respiration, and responses were recorded I 5 minutes after spontaneous breathing movements began to appear. Five animals, one in group I, two in in group III, and two in group IV, were spinal preparations in which the cord was peripheral conduction velocity. It seems reasonable th .at because of aging there might be a transected (under eth er anesthesia) between the eight cervical and first thoracic segments, general slowing of integrative processes in the and responses were recorded 3 hours later. central nervous system due to prolonged In groups V and VI an attempt was made to excitation at synapses. obtain synaptic delays in unanesthetized prepThe purpose of this experiment was to comarations with an intact spinal cord. The effects pare the delay in transmission of a monosynaptic reflex through the spinal cord in young and aged rats. of the ether anesthesia which was administered during surgery and while the preparatio n was attached to th .e apparatus disappeared after METHODS about 2% hours. Results were obtained during Animals. Hooded rats from our own colony the brief periods which followed when the were used, 18 males and 5 aged females. For animal was quiet and spontaneous electrical activity was at a minimum. In these preparaReceived for publication January 28, 1957. tions the hind legs and a large portion of the 1 Aided by a grant (B-776) from the National Inback was deafferented and the animal was stitute of Neurological Diseases and Blindness. fixed rigidly to the apparatus. by 10.0.33.5 on A ril 5, 2017 http://ajple.physiology.org/ D ow nladed fom 404 M. J. WAYNER, JR. AND R. EMMERS Procedure. A laminectomy was performed and the roots in the lumbar region were exposed. All the roots below Lg on both sides of the cord and L3 and L4 on the left side were cut. The animal was then fixed to a rigid support by means of clamps which were attached to the vertebral column and the skull. The biceps femoris and vastus lateralis muscles of the right leg were exposed, cut and retracted. The two heads of the gastrocnemius muscle were separated and retracted. The small nerve to the flexor hallucis longus muscle was ligated and cut at the muscle. The two heads of the gastrocnemius muscle formed a natural container for a pool of paraffin oil which insulated the stimulating electrodes and prevented evaporation. All other branches of the sciatic nerve were cut. Cord temperature was maintained at 37.5”C by means of pumping water at a constant temperature through copper tubing under the paraffin oil which insulated the recording electrodes and surrounded the exposed cord. The stimulus duration was held constant at 0.1 msec., and its amplitude was adjusted to produce a maximum afferent input. The afferent input was recorded From LsDR close to the spinal cord. Reflex discharges in LsVR were recorded by means of two pairs of electrodes. Dorsal and ventral root responses were recorded individually. By means of an elecFIG. I. A. Ventral root reflex discharge recorded from LsVR in a I I r-day-old rat in response to stimulation of the flexor hallucis longus nerve. Correction for conduction time, 0.66 msec. B. A reflex discharge recorded under similar conditions in a 873-day-old animal. Correction for conduction time, 0.84 msec. tronic switch, a 1000 cps time signal was recorded simultaneously with each response. Afferent and ventral root conduction velocities and conduction times were calculated for each animal. Synaptic delay was obtained by subtracting the conduction time, to and from the spinal cord, from the reflex delay.