Figure 1.
Description of the parameters used in the footprint analyses (labeled a–d). a: area of the print (calculated by counting the colored pixels within a frame drawn by the user). b: lengths of the entire print. c: track widths: distance between left and right paw. d: stride lengths, distance between one print and the corresponding following one. Print area and lengths of the prints can be used to characterize limb-specific features of walking, track widths and stride lengths describe global walking properties.
Figure 2.
(A) We used a fixed set of 4 electromagnetic stimulators positioned along the proximal-distal axis of the hindpaw. The first one was positioned on a digit (p1), stimulators 2 and 3 were positioned on the pads and the 4th stimulator was on the more proximal portion of the hindpaw (p4). Tactile stimulation was carried out one after the other at each position. Only RFs were analyzed whose RF were according to hand-plotting localized on a digit. (B) The neuronal response after stimulation at position 1 was set as 100%. The neuronal responses obtained from stimulation at the other positions were normalized and plotted versus the distance from the RF-center. The RF-length was obtained for that distance where neuronal response reached 50% (pink line).
Figure 3.
Example of footprints of fore- and hindpaws of young and old rats.
The upper panel shows typical prints of fore- (left) and hindpaws (right) of a young rat (4months). Presumably, such prints result from a walking pattern, where only the distal parts of the paw contact the ground. The lower panel shows prints of fore- and hindpaws of an old rat (30 months). The prints of the forepaws (left side) are comparable in young and old animals. In contrast, the prints of the hindpaw of the old animals are considerably larger than those of the young animal indicative of limb-specific effects of aging on walking behavior. Such prints result from a walking behavior where also more distal parts of the paws and the heels are placed on the ground due to reduced muscle forces.
Figure 4.
Footprint characteristics of the forepaw.
Mean values±SD of the gait parameters print area (A), lengths of the print (C), stride-lengths (B) and track-widths (D) for young (black) and aged rats (red), * p<0.01. (A+C): The limb-specific parameter print area was reduced and print lengths remained unchanged in aged rats, which was not the case for the hindpaw (Fig. 5). (B+D): The global walking parameters stride-lengths decreased and track-widths increased in aged rats. This was also observed for the hindpaw (Fig. 5).
Figure 5.
Footprint characteristics of the hindpaw.
Mean values±SD of the gait parameters print area (A), lengths of the print (C), stride-lengths (B) and track-widths (D) for the young (black) and the aged rats (red), * p<0.01. (A+C): In contrast to the prints obtained from the forepaws (Fig. 4), the limb-specific parameters print area and the lengths of the prints of the hindpaw were significantly increased in old animals. (B+D): As in the forepaw (Fig. 4), for the global parameters we found decreasing stride-lengths and increasing track-widths for the hindpaw in the aged animals.
Figure 6.
Cutaneous receptive fields (RFs) of the fore- and hindpaw.
Typical examples of cutaneous receptive fields (RFs) of neurons recorded in the fore- and hindpaw representation of a rat aged 4 months (A, upper panel) and a rat aged 30 months (B, lower panel). (A) RFs on the forepaw of young rats were very small (left) usually comprising only small parts of a digit or a single digit or pad. RFs on the hindpaw of young rats (right) were slightly larger than on the forepaw, typically consisting of a single digit or parts of a digit. On the more proximal area of the hindpaw, RFs comprised single pads and larger skin areas in the range of the heel. (B) RFs on the forepaw of old rats (left) were only slightly enlarged as compared to young animals. Typical RFs comprised parts of a digit or a single digit or pad. In contrast to the forepaw, RFs located on the hindpaw (right) of old rats were severalfold enlarged. RFs in old rats were characterized by representations of multiple digits and pads and by substantially enlarged RFs in the proximal parts of the paw.
Figure 7.
Average RF-sizes of young and old rats.
(A) Average RF-sizes of the fore- and hindpaw of young (black) and aged (red) animals are shown. Error bars indicate SEM. * p<0.01, ** p<0.001. (B) Normalized age-related RF-enlargement. The increase of RF-size was 22% for the forepaw and 190% for the hindpaw. Although we found that RFs were enlarged on both the fore- and the hindpaw of old animals, the effects of aging on the RF-size was substantially stronger on the hindpaw than on the forepaw. (C) Mean values (±SEM) of RF-length. ** p<0.001. RF-lengths was only measured for hindpaw RFs of young (black) and old (red) rats. Analogous to the RF-size determined by handplotting we found a significant increase of RF-lengths for the old animals.
Figure 8.
Response latencies of cortical neurons recorded in the fore- and hindpaw map.
Examples of neuronal responses in form of post-stimulus time histograms (PSTHs) after tactile stimulation applied to the center of a RF. The number of spikes/bin (binwidth = 1 ms) is plotted against time. The red dotted line marks the time of maximal cell response (peak latency) in the PSTHs of fore- (left) and hindpaw (right) neurons of the young animal (upper panel). (A) Examples of PSTHs recorded in the fore- (left) and in the hindpaw representation (right) of a young rat (5 months). (B) Examples of PSTHs recorded in the fore- (left) and in the hindpaw representation (right) of an old rat (29 months). Response latencies were lengthened in aged animals by approximately the same amount for neurons of both representations.
Figure 9.
Averaged latencies of young and old rats.
(A) Averaged peak response latencies of neurons from fore- and hindpaw-representations of young (black) and aged (red) animals are shown. Error bars indicate the SEM. * p<0.01. (B) Normalized age-related lengthening of peak response latencies. The lengthening was 24.4% for the forepaw and 36.2% for the hindpaw. In contrast to the parameter RF-size the lengthening of latencies was similarly affected in cortical neurons of fore- and hindpaw-representation.
Figure 10.
Timecourse of age-related changes.
(A) Averaged RF-size for each individual animal plotted as a function of age. RFs on the hindpaw (light blue squares), RFs on the forepaw (dark blue squares). Until an age of approximately 24 months we found no age related alterations of RF-size of fore- and hindpaw cortical neurons. Beyond that age RFs on the hindpaw increasingly enlarged with advancing age, with the largest RFs found in the animals with the highest age. For the forepaw no comparable increase was observed. (B) Averaged peak-latencies for each individual animal plotted as a function of age. Light blue squares Latencies of neurons recorded in the representation of the hindpaw (light blue squares), latencies of neurons from the forepaw representation (dark blue squares). Similar to RF-size, we found no age related lengthening of peak latencies until an age of about 24 months. From then on latencies of neurons from both the fore- and the hindpaw representation were increasingly lengthened with advancing age.
Table 1.
Correlation analysis between fore- and hindpaw parameters.