RESUMO
In order to clarify the effect of age-related change in trigeminal nociception, phosphorylation of extracellular signal-regulated kinase (pERK) in trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord neurons were studied in the aged rats following subcutaneous capsaicin injection into the whisker pad. A large number of pERK-LI cells was expressed in the superficial laminae of Vc and upper cervical spinal cord in adult and aged rats following subcutaneous capsaicin injection into the whisker pad region. The number of pERK-LI cells was largest at about 2.0mm caudal from the obex and gradually decreased in their numbers in more rostral and caudal sections. The rostro-caudal distribution profile of pERK-LI cells expressed after subcutaneous capsaicin injection into whisker pad was similar in adult and aged rats. The number of pERK-LI cells was slightly, but not significantly larger in aged rats compared with that of adults. Pretreatment with naloxone significantly increased the number of capsaicin-induced pERK-LI cells in adult rats but not in aged rats. The present findings suggest that the descending modulation system impaired with advancing age, resulting in the abnormal pain sensation in aged rats.
Assuntos
Envelhecimento/fisiologia , MAP Quinases Reguladas por Sinal Extracelular/efeitos dos fármacos , Naloxona/farmacologia , Antagonistas de Entorpecentes/farmacologia , Dor/fisiopatologia , Animais , Capsaicina/farmacologia , MAP Quinases Reguladas por Sinal Extracelular/metabolismo , Face/inervação , Vias Neurais/efeitos dos fármacos , Vias Neurais/fisiopatologia , Fosforilação/efeitos dos fármacos , Ratos , Ratos Endogâmicos F344 , Fármacos do Sistema Sensorial/farmacologia , Núcleo Inferior Caudal do Nervo Trigêmeo/efeitos dos fármacos , Núcleo Inferior Caudal do Nervo Trigêmeo/metabolismoRESUMO
Electromyographic (EMG) burst patterns of m. longissimus and the kinematics of the vertebral column were assessed in cats during treadmill walking for six downslope grades (5 degrees-30 degrees). The EMG bursts during downslope walking were weak between 5 degrees and 20 degrees. At steeper grades (>20 degrees), EMG bursts were large. Bursts at T10 facilitated inward movements, and those at L1 decreased forward movements, while those at L5 decreased backward movements during downslope walking at steeper grades.
Assuntos
Músculo Esquelético/fisiologia , Coluna Vertebral/fisiologia , Caminhada/fisiologia , Animais , Fenômenos Biomecânicos , Gatos , Eletromiografia , Feminino , Membro Posterior/fisiologia , MasculinoRESUMO
To gain insight into the neural mechanisms controlling vertebral column movement and its role in walking, we performed kinematic and electromyographic (EMG) studies on cats during level and upslope treadmill walking. Kinematic data of the limbs and vertebral column were obtained with a high-speed camera synchronized with EMG recordings from levels T10, L1, and L5 of m. longissimus dorsi (Long). During a single-step cycle at all upslope angles, vertebral movement in the lateral (left-right), cranial-caudal (forward-backward), and dorsal-ventral (upward-downward) directions was observed. Lateral movements were produced by forelimb take-off and hindlimb landing, and forward and upward movements were produced by hindlimb extension. During the single-step cycle, each of the three epaxial muscles, m. multifidus, m. iliocostalis, and Long, showed two bilateral EMG bursts. The onset of the EMG bursts coincided with the left-right movements, suggesting that epaxial muscle activity depresses lateral movement. The termination of the EMG bursts correlated with the forward and downward phase of the step cycle, suggesting that contraction of the epaxial muscles produces forward and downward movements. EMG bursts of the epaxial muscles increase the stiffness and produce inwardly movements to decrease the lateral movements of the vertebral column and the termination of EMG bursts control the movements into cranial and ventral direction of the vertebral column. The results suggest that the rhythmic EMG bursts in the epaxial muscles are produced by pattern generators, and the timing of EMG bursts among the different levels of the epaxial muscles are altered by walking condition input via peripheral afferents and descending pathways.
Assuntos
Marcha/fisiologia , Movimento/fisiologia , Contração Muscular/fisiologia , Músculo Esquelético/fisiologia , Coluna Vertebral/fisiologia , Caminhada/fisiologia , Potenciais de Ação/fisiologia , Vias Aferentes/fisiologia , Animais , Fenômenos Biomecânicos , Gatos , Sistema Nervoso Central/fisiologia , Vias Eferentes/fisiologia , Eletromiografia , Teste de Esforço , Extremidades/fisiologia , Feminino , Masculino , Condicionamento Físico Animal/fisiologia , Coluna Vertebral/anatomia & histologia , Suporte de Carga/fisiologiaRESUMO
This chapter addresses the neural control of spinal-column behavior during locomotion. Kinematic and EMG measurements were obtained from the adult cat during its level and upslope treadmill walking. Increasing the grade of upslope walking augmented horizontal movements of the spinal column and decreased its lateral movements. In conjunction, there were significant increases in the amplitude, duration, and pattern of EMG bursts in relevant spinal column musculature. During even steeper upslope walking, three EMG bursts were evident. They were phase-locked to the outward, downward and backward movements of the spinal column, respectively. Our results suggest that the component of the locomotor pattern generator that produces rhythmical spinal column movements must generate a wide variety of EMG bursts in spinal column muscles, as dependent in part on sensory input from the spinal column and its musculature.
Assuntos
Abdome/fisiologia , Eletromiografia , Atividade Motora/fisiologia , Tórax/fisiologia , Animais , Fenômenos Biomecânicos , Gatos , Músculo Esquelético/fisiologia , Tempo de Reação , Medula Espinal/fisiologiaRESUMO
The effects of life-long food restriction on motoneuronal cell death with advancing age was studied in male Fischer rats, which had access to food only 3 days a week after weaning (FR rats). Motoneurons innervating the medial gastrocnemius muscle were labeled with retrogradely transported HRP. The number of labeled motoneurons in FR rats and rats fed ad libitum (AL rats) was similar at the age of 16 months (131.8 +/- 1.7 for FR rats vs. 133.8 +/- 4.5 for AL rats). However, at 28 months of age, AL rats had less labeled motoneurons compared to FR rats (117.0 +/- 2.4 for FR rats vs. 124.3 +/- 7.0 for FR rats). The number of type I muscle fibers in the medial gastrocnemius muscle increased significantly in AL rats during the period from 16 to 28 months of age, which might reflect motor unit reorganization following retraction of axons and/or death of innervating motoneurons. FR rats did not show statistically significant alteration. These findings were also true for the data compiled from several different experiments including those conducted for primarily different purposes in our laboratory. The results suggest that life-long food restriction retards motoneuronal cell death occurring with advancing age.
Assuntos
Restrição Calórica , Neurônios Motores/patologia , Músculo Esquelético/inervação , Envelhecimento , Animais , Fator Neurotrófico Derivado do Encéfalo/biossíntese , Masculino , Fibras Musculares de Contração Lenta/patologia , Óxido Nítrico Sintase/biossíntese , Ratos , Ratos Endogâmicos F344RESUMO
The aim of this study was to elucidate the neuronal pathways mediating propriospinal reflexes of the tail with special reference to the intersegmental connections between group I muscle afferents and motoneurons. Experiments were performed on 25 unanesthetized, spinalized (L1) cats of both sexes. The effects of electrical stimulation of the afferents embedded in different levels of the following muscles: m. extensor caudae medialis (ECM), m. extensor caudae lateralis (ECL), m. flexor caudae longus (FCL), and m. flexor caudae brevis (FCB), were observed in tail motoneurons in the first or second coccygeal (Co1 or Co2) spinal segment. Stimulation of group I muscle afferents from the cranial parts of the tail muscles produced predominantly excitatory postsynaptic potentials. In ECM and FCB, some motoneurons had significant excitatory effects exerted by group I afferent inputs from homonymous and/or synergistic muscles during vertical tail movement, and some motoneurons had predominately excitatory effects from antagonistic muscles during vertical tail movement. Excitatory effects in ECL and FCL motoneurons occurred more often and were larger in size after stimulating the ipsilateral tail muscles than after stimulating the contralateral tail muscles. The stimulation of group I muscle afferents from the caudal parts of the tail muscles produced predominantly inhibitory effects. The shortest neuronal pathways for excitatory postsynaptic potentials and inhibitory postsynaptic potentials were mono- and disynaptic neuronal pathways, respectively. The results of the present experiments suggest that continuous tail movements along the tail are performed by group I reflex pathways.
Assuntos
Vias Aferentes/fisiologia , Neurônios Motores/fisiologia , Movimento/fisiologia , Músculo Esquelético/inervação , Reflexo Monosináptico/fisiologia , Medula Espinal/fisiologia , Cauda/inervação , Animais , Gatos , Cóccix , Estimulação Elétrica , Potenciais Pós-Sinápticos Excitadores/fisiologia , Feminino , Lateralidade Funcional/fisiologia , Masculino , Contração Muscular/fisiologia , Fusos Musculares/fisiologia , Músculo Esquelético/fisiologia , Condução Nervosa/fisiologia , Nervos Periféricos/fisiologia , Tempo de Reação/fisiologia , Sacro , Transmissão Sináptica/fisiologia , Cauda/fisiologiaRESUMO
Peripheral nerve injury produces a hyperexcitability of primary afferents and neurons in the spinal cord that is considered important in the development of nerve injury-induced pain. The authors recently developed a nerve injury model in the trigeminal region of the rat to study the neuronal mechanism of neuropathic pain in the trigeminal system. The escape thresholds to mechanical stimulation applied to the whisker pad area were significantly lower in rats with an inferior alveolar nerve (IAN) transection than those evoked from the contralateral, sham-operated whisker pad. Also, background activity and mechanically evoked responses in infraorbital nerve (ION) afferents and hyperpolarization-activated current (Ih) in trigeminal ganglion ION neurons were increased following IAN transection. Background activity and mechanically evoked responses of wide dynamic range (WDR) neurons in trigeminal subnucleus caudalis on the ipsilateral side relative to the transection were also significantly increased after the operation. A large number of cells expressed c-Fos-like immunoreactivity in the caudal medulla and upper cervical spinal cord following non-noxious mechanical stimulation of the faces of rats with IAN transection. The effect of aging on spinal dorsal horn neurons and the involvement of nerve injury in producing abnormal pain sensation in rats with advancing age were also studied. The incidence of licking behavior in response to noxious radiant heat stimulation of the hind paw was lower in the aged rats than in younger adults, but paw withdrawal latency was shorter and the activities of spinal dorsal horn neurons were higher in the aged rats. Furthermore, the descending inhibitory systems were impaired in the aged rats. These observations suggest that the changes in neuronal activity in the aged rats likely corresponded to the changes observed in the rat model of peripheral nerve injury.
Assuntos
Envelhecimento/fisiologia , Traumatismos dos Nervos Cranianos/patologia , Células do Corno Posterior/fisiologia , Núcleo Inferior Caudal do Nervo Trigêmeo/fisiologia , Traumatismos do Nervo Trigêmeo , Animais , Traumatismos dos Nervos Cranianos/fisiopatologia , Reação de Fuga , Bulbo/metabolismo , Modelos Animais , Plasticidade Neuronal , Neurônios Aferentes/fisiologia , Nociceptores/fisiologia , Órbita/inervação , Proteínas Proto-Oncogênicas c-fos/biossíntese , RatosRESUMO
Age-related physiological and morphological changes of muscle spindles were examined in rats (male Fischer 344/DuCrj: young, 4-13 months; middle-aged, 20-22 months; old, 28-31 months). Single afferent discharges of the muscle spindles in gastrocnemius muscles were recorded from a finely split dorsal root during ramp-and-hold (amplitude, 2.0 mm; velocity, 2-20 mm s(-1)) or sinusoidal stretch (amplitude, 0.05-1.0 mm; frequency, 0.5-2 Hz). Respective conduction velocities (CVs) were then measured. After electrophysiological experimentation, the muscles were dissected. The silver-impregnated muscle spindles were teased and then analysed using a light microscope. The CV and dynamic response to ramp-and-hold stretch of many endings were widely overlapped in old rats because of the decreased CV and dynamic response of primary endings. Many units in old rats showed slowing of discharge during the release phase under ramp-and-hold stretch and continuous discharge under sinusoidal stretch, similarly to secondary endings in young and middle-aged rats. Morphological studies revealed that primary endings of aged rat muscle spindles were less spiral or non-spiral in appearance, but secondary endings appeared unchanged. These results suggest first that primary muscle spindles in old rats are indistinguishable from secondary endings when determined solely by previously used physiological criteria. Secondly, these physiological results reflect drastic age-related morphological changes in spindle primary endings.
Assuntos
Envelhecimento/fisiologia , Fusos Musculares/anatomia & histologia , Fusos Musculares/fisiologia , Animais , Axônios/fisiologia , Eletrofisiologia , Masculino , Fusos Musculares/efeitos dos fármacos , Músculo Esquelético/inervação , Condução Nervosa , Fármacos Neuromusculares Despolarizantes/farmacologia , Neurônios Aferentes/efeitos dos fármacos , Neurônios Aferentes/fisiologia , Ratos , Ratos Endogâmicos F344 , Estresse Mecânico , Succinilcolina/farmacologia , Fatores de TempoRESUMO
To elucidate the effect of chronic inflammation on spinal nociceptive neurons in the elderly, we compared nocifensive behavior, peripheral inflammatory responses, and spinal dorsal horn neuronal activities between the aged (29-34 mo) and adult (7-12 mo) male rats after injection of complete Freund's adjuvant (CFA) into the hind paw. Aged rats exhibited a significantly lower mechanical paw withdrawal threshold before inflammation. However, after CFA injection mechanical allodynia developed in both adult and aged rats after CFA injection. The changes of foot temperature and thickness after CFA injection were greater and lasted longer in aged than in adult rats. Sets of 124 wide dynamic range (WDR) neurons (aged: 59, adult: 65) and 26 nociceptive specific (NS) neurons (aged: 13, adult: 13) were recorded from the lumber spinal dorsal horn. NS neurons from the inflamed adult rats showed significantly higher responses to noxious mechanical stimulation than those in aged rats, whereas WDR neurons from inflamed adult and aged rats were similar. Background activity of WDR neurons from the adult rats increased after CFA, whereas WDR neurons of aged rats and NS neurons from either group were not. The afterdischarge followed by noxious mechanical stimulation was significantly greater for WDR neurons in both adult and aged rats, whereas no significant differences were observed in NS neurons. Two days after CFA injection, Fos expression increased similarly in aged and adult rats. Thus the aged rats showed enhanced peripheral inflammatory responses to CFA injection with only a slight change in dorsal horn neuronal activity. Together with our previous finding that nociceptive neurons in aged rats exhibit hyperexcitability, these results suggest that the dorsal horn nociceptive system becomes sensitized with advancing age and its excitability cannot be further increased by inflammation.
Assuntos
Envelhecimento/fisiologia , Inflamação/fisiopatologia , Nociceptores/fisiopatologia , Dor/fisiopatologia , Células do Corno Posterior/fisiopatologia , Medula Espinal/citologia , Potenciais de Ação/fisiologia , Potenciais de Ação/efeitos da radiação , Fatores Etários , Animais , Comportamento Animal , Temperatura Corporal/fisiologia , Mapeamento Encefálico , Doença Crônica , Relação Dose-Resposta à Radiação , Edema/patologia , Edema/fisiopatologia , Estimulação Elétrica/métodos , Extremidades/patologia , Extremidades/fisiopatologia , Adjuvante de Freund , Lateralidade Funcional , Inflamação/induzido quimicamente , Inflamação/patologia , Masculino , Dor/patologia , Medição da Dor/métodos , Limiar da Dor , Estimulação Física/efeitos adversos , Ratos , Ratos Endogâmicos F344 , Fatores de TempoRESUMO
To clarify the functional role of the dorsal column nucleus (DCN) in nociception in rats with advancing age, single neuronal activity and substance P-like immunoreactivity (SP-LI) of the gracile nucleus (GN) were studied in aged rats (29 to 34 mo old) and adult rats (9 to 12 mo old). A total of 122 neurons [aged: 34 wide-dynamic-range (WDR), two nociceptive-specific (NS), and 32 low-threshold mechanical (LTM) neurons; adult: 22 WDR and 32 LTM neurons] were recorded from GN. For WDR neurons, the latency to antidromic activation of the ventral posterior lateral nucleus of the thalamus showed no difference between the aged and adult rats. Sciatic nerve stimulation with C-fiber intensity induced responses of GN with significantly longer latency in aged rats than in adults, whereas there was no difference in the response latency to A-fiber intensity stimulation. Background activity and afterdischarges were significantly higher in the aged rats than those in the adult rats. Responses to noxious mechanical and thermal stimuli were significantly greater in the aged rats during application of graded stimuli. There were no significant differences in responses to nonnoxious mechanical stimulus, mechanical response threshold, and the size of the receptive fields between neurons in the aged and adult rats. The area occupied by SP-LI fibers in the GN and the size of SP-LI dorsal root ganglia neurons were significantly larger in aged rats than in adults. The present findings suggest that the hyperexcitability of GN neurons could be involved in abnormal noxious pain sensations with advancing age.
Assuntos
Envelhecimento/fisiologia , Neurônios/fisiologia , Dor/fisiopatologia , Medula Espinal/citologia , Potenciais de Ação/fisiologia , Fatores Etários , Animais , Mapeamento Encefálico , Relação Dose-Resposta à Radiação , Estimulação Elétrica/métodos , Potenciais Evocados/fisiologia , Potenciais Evocados/efeitos da radiação , Imuno-Histoquímica/métodos , Neurônios/classificação , Dor/etiologia , Medição da Dor/métodos , Limiar da Dor/fisiologia , Limiar da Dor/efeitos da radiação , Estimulação Física/métodos , Ratos , Ratos Endogâmicos F344 , Tempo de Reação/fisiologia , Tempo de Reação/efeitos da radiação , Substância P/metabolismo , Núcleos Talâmicos/fisiologia , Núcleos Talâmicos/efeitos da radiação , Fatores de TempoRESUMO
The mechanical properties of individual motor units in the medial gastrocnemius muscle, as well as the whole muscle properties and innervating motor nucleus, were investigated in dietary-restricted, male Fischer 344/DuCrj rats at ages of 4, 7, 12, 21/22, 27, 31, and 36 mo. The tetanic tension of the type S units continuously increased until the age of 36 mo. Those of type FF and FR units declined from 21/22 to 27 mo of age but did not change further while the whole muscle tension decreased greatly. The atrophy of muscle fibers, the decline in motoneuron number and axonal conduction velocity, and the decrease in the posttetanic potentiation of twitch contraction of motor units seemed to start after 21/22 mo of age and were accelerated with advancing age. Prolongation of twitch contraction time was evident for only type S and FR units in 36-mo-old rats. The fatigue index was greatly increased for type FF units in 36-mo-old rats. These findings indicated that the progress of changes in various properties occurring in the senescent muscle was different in terms of their time course and degree and also dependent on the types of motor unit. The atrophy and decrease in specific tension of muscle fibers affected the decline in tension output of motor units. This was effectively compensated for by the capture of denervated muscle fibers over time.
Assuntos
Envelhecimento/fisiologia , Fadiga Muscular/fisiologia , Fibras Musculares Esqueléticas/fisiologia , Músculo Esquelético/fisiologia , Animais , Estimulação Elétrica/métodos , Masculino , Ratos , Ratos Endogâmicos F344RESUMO
We examined the relationship between input resistance and amplitude of monosynaptic and polysynaptic EPSPs produced by electrical stimulation of group I muscle afferents innervating the m. longissimus lumborum (Long) at different levels (L1-L4) in Long motoneurons in L4 spinal segments to obtain an insight into the neuronal control of trunk muscles. In the Long motoneuron pool, the amplitude of monosynaptic EPSP was shown to have a close relationship to input resistance. Furthermore, the relation between the amplitude of polysynaptic EPSP after stimulating Long nerves at L3 and input resistance was statistically significant, but the relation between EPSP amplitude evoked by stimulation of Long at L1 or L2 and input resistance was not statistically significant. Our findings suggest a position-dependent control of motoneuron activity by group I muscle afferents. The motoneuron activities carried out by monosynaptic pathways and polysynaptic pathways from adjacent spinal segments are dependent on the intrinsic properties of motoneurons (input resistance, etc.), while the motoneuron activities carried out by polysynaptic pathways from the far spinal segments have independent intrinsic properties.
Assuntos
Vias Aferentes/fisiologia , Dorso/inervação , Neurônios Motores/fisiologia , Fusos Musculares/fisiologia , Músculo Esquelético/inervação , Raízes Nervosas Espinhais/fisiologia , Transmissão Sináptica/fisiologia , Vias Aferentes/citologia , Animais , Dorso/fisiologia , Gatos , Estimulação Elétrica , Potenciais Pós-Sinápticos Excitadores/fisiologia , Feminino , Vértebras Lombares , Masculino , Neurônios Motores/citologia , Fusos Musculares/citologia , Músculo Esquelético/fisiologia , Neurônios Aferentes/citologia , Neurônios Aferentes/fisiologia , Tempo de Reação/fisiologia , Reflexo de Estiramento/fisiologia , Raízes Nervosas Espinhais/citologia , Sinapses/fisiologiaRESUMO
(1) We studied the reflex actions of group I and II afferents to longissimus lumborum (Long) motoneurons in the L1-L5 spinal segments from the epaxial muscle, m. Long, and the hypaxial muscle, m. obliquus externus abdominus (OEA). (2) Postsynaptic potentials (PSPs) recorded from 140 Long motoneurons in 30 spinal cats were analyzed. Under the present experimental conditions, the stimulation of Long and OEA nerves at an intensity below 1.5 times threshold (T) activated only group I muscle afferents, while stimulations at 2-5T activated group II muscle afferents as well. (3) The incidence of PSPs was related to the proximity of the spinal segments of the nerves stimulated to the spinal segment of the motoneurons; the shorter the distance the larger the PSPs and higher incidence of PSPs. The Long motoneurons received group I afferent input mainly from the same and adjacent segments, and received group II afferent inputs from a wider range of segments. (4) A short (i.e., less than 1.0 ms) latency of excitatory PSPs (EPSPs) evoked by ipsilateral group I afferents of Long at the same or adjacent segment indicated a monosynaptic connection. In general, the central latencies became longer as the distance between spinal segments of stimulated nerves and motoneurons increased. Major PSP components were produced by polysynaptic neuronal pathways. The spatial facilitation between PSPs evoked by afferents of different nerves (i.e., ipsilateral Long (iLong) and contralateral Long (cLong) of the same segment; iLongs of different segments; and iLong and iOEA of the same segment) indicated that they shared common interneurons. (5) Although iLong and iOEA muscle afferents produced predominantly EPSPs, and contralateral muscle afferents elicited predominantly IPSPs in Long motoneurons at each spinal segment, the patterns of convergence from Long and OEA muscle afferents of different spinal segments and of different sides differed considerably among motoneurons. (6) These findings demonstrated various input patterns of individual motoneurons within the same motoneuron pool, which might reflect the complexity of neuronal control of the back muscles for various trunk movements, including lateral and dorsal bending, rotating, and fixation of the trunk.
Assuntos
Potenciais Pós-Sinápticos Excitadores , Neurônios Motores/fisiologia , Músculo Esquelético/fisiologia , Medula Espinal/fisiologia , Transmissão Sináptica , Potenciais de Ação , Vias Aferentes/fisiologia , Animais , Gatos , Eletrofisiologia , Feminino , Região Lombossacral , Masculino , Inibição NeuralRESUMO
Previous studies (Wada and Kanda 2001, Exp Brain Res 136:263-263; Wada et al. 1999, Exp Brain Res 128:543-549) demonstrated that input patterns from hindlimb muscles and cutaneous afferents vary among individual trunk muscle motoneurons. The purpose of the present study was to examine the relationship between the synaptic pattern from hindlimb afferents and the area innervated by motoneurons. Histologic study of m. longissimus lumborum (Long) indicated that the distribution of different fiber types (slow-twitch oxidative, SO; fast-twitch oxidative glycolytic, FOG; fast-twitch glycolytic, FG) depends on the area of the Long cross-section. The ventromedial area and dorsolateral area of the cross-section possess a high content of SO and FG, respectively. The motoneurons innervating the dorsolateral area receive muscle afferent inputs mainly from the ipsilateral side, while the motoneurons innervating the ventromedial area often receive bilateral afferent inputs. The motoneurons innervating the dorsolateral area receive excitatory post-synaptic potentials from cutaneous nerves on both sides. These findings indicate that the effects of afferent inputs from the hindlimbs are related to motoneuron type or the area innervated by the motoneurons.
Assuntos
Vias Aferentes/fisiologia , Neurônios Motores/fisiologia , Músculo Esquelético/inervação , Nervos Periféricos/fisiologia , Propriocepção/fisiologia , Raízes Nervosas Espinhais/fisiologia , Animais , Dorso/inervação , Dorso/fisiologia , Gatos , Estimulação Elétrica , Potenciais Pós-Sinápticos Excitadores/fisiologia , Feminino , Lateralidade Funcional/fisiologia , Membro Posterior/fisiologia , Masculino , Neurônios Motores/citologia , Fibras Musculares de Contração Rápida/fisiologia , Fibras Musculares de Contração Lenta/fisiologia , Músculo Esquelético/fisiologia , Terminações Pré-Sinápticas/fisiologia , Terminações Pré-Sinápticas/ultraestrutura , Raízes Nervosas Espinhais/citologia , Transmissão Sináptica/fisiologiaRESUMO
To understand characteristics of the pain system in the elderly, we investigated the electrophysiological properties of nociceptive neurons in the lumbar spinal dorsal horn of aged (29-34-mo old) and adult (7-13-mo old) rats. The responses of nociceptive neurons to noxious thermal stimulation, as well as the spontaneous firing rate, were significantly higher in the aged as compared with adult rats. Furthermore, the size of the high-threshold receptive field area of wide dynamic range neurons was larger (P < 0.01) and that of the low-threshold area was smaller (P < 0.05) in aged rats than in adult rats. The increased nociceptive neuronal activity in the aged rats correlated with the finding that the paw withdrawal latency was significantly shorter in the aged rats than those of the adult rats following heat stimulation of the hind paw (P < 0.05). Reversible local anesthetic block of descending pathways resulted in a dramatic increase in neuronal activity in adult rats but had little effect in aged rats. There was also a significant loss of serotoninergic and noradrenergic fibers in the spinal dorsal horn of the aged rats. These results demonstrate an age-related plasticity in spinal nociceptive processing that is related to impairment of descending modulatory pathways.