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Stimulation of the MLR inhibits the discharge of dorsal horn neurons responsive to muscular contraction
Brain Research 880 (2000) 178–182 www.elsevier.com / locate / bres
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Stimulation of the MLR inhibits the discharge of dorsal horn neurons responsive to muscular contraction Alexandr M. Degtyarenko*, Marc P. Kaufman Division of Cardiovascular Medicine, Departments of Internal Medicine and Human Physiology, TB 172, University of California, Davis, CA 95616, USA Accepted 18 July 2000
Abstract We found that electrical stimulation of the mesencephalic locomotor region (MLR) inhibited the discharge of deep dorsal horn neurons receiving group III afferent input from the triceps surae muscles. In contrast, contraction of these muscles induced by electrical stimulation of the tibial nerve activated these dorsal horn neurons. Our findings show that descending central motor commands can inhibit dorsal horn interneurons receiving input from group III afferents during exercise. 2000 Elsevier Science B.V. All rights reserved. Keywords: Exercise; Group III muscle afferent; Cat; Cardiovascular control
Static and moderately intense dynamic exercise is well known to increase cardiovascular and ventilatory function. These increases are widely believed to be caused by two neural mechanisms, namely central command [17] and the exercise pressor reflex [11]. The first mechanism, central command is defined as the parallel activation of somatomotor, autonomic and ventilatory circuits at the onset of exercise. The second mechanism, the exercise pressor reflex, arises from the contraction-induced stimulation of group III and IV afferent endings in skeletal muscle. Substantial evidence has been presented in humans and animals showing that both mechanisms play significant roles in evoking the cardiovascular and ventilatory responses to exercise. Both mechanisms, moreover, are likely to be engaged simultaneously. This likelihood prompted two prior studies in which central command and the exercise pressor reflex were evoked separately as well as simultaneously in anesthetized cats [16,18]. These studies found that the two mechanisms evoked simultaneously caused cardiovascular increases that were less than the algebraic sum of the two mechanisms evoked separately. This finding was not attributable to a ceiling effect because the cats were shown to be able to generate pressor, cardioaccelerator and ventilatory responses of a
*Corresponding author. Fax: 11-530-752-3264. E-mail address: [email protected] (A.M. Degtyarenko).
magnitude greater than those evoked by the algebraic sum of central command and the reflex evoked separately. Little is known about how the central nervous system integrates the two mechanisms. The dorsal horn of the spinal cord is a central site where central command and the exercise pressor reflex might interact. For example, the dorsal horn is the site of termination of group III and IV muscle afferents [4,14], whose stimulation by contraction is responsible for the exercise pressor reflex [1,13]. Moreover, the activity of dorsal horn neurons receiving nociceptive input has been shown to be inhibited by input from brainstem structures whose stimulation caused analgesia [2,3]. In addition, a central motor command has been shown to depress inhibitory motor reflexes arising from group II and III muscle afferents as well as from thin fiber cutaneous afferents [8]. Despite these findings, little is known about how central command interacts with group III and IV afferent input onto dorsal horn neurons. In decerebrated cats we examined the effect of central command on the discharge of dorsal horn cells responsive to muscular contraction. Each of these neurons, whose activity was recorded extracellularly, was shown to receive group III muscle afferent input. We found that contraction-induced input onto cells located in laminae V and VI of the dorsal horn was inhibited by central command. Cats (2.2–3.5 kg) were anesthetized with a mixture of halothane (3–4%) and oxygen. A jugular vein, a common
0006-8993 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 00 )02738-4
A.M. Degtyarenko, M.P. Kaufman / Brain Research 880 (2000) 178 – 182
carotid artery and the trachea were cannulated. The lungs were then ventilated with the anesthetic / oxygen mixture through the trachea cannula. The remaining carotid artery was ligated, and the cat was placed in a Kopf stereotaxic and spinal unit. The right tibial nerve was dissected free and placed on a stimulating electrode. The right triceps surae muscles were exposed and the right calcaneal (Achilles) tendon was cut. The L4–S2 spinal segments were exposed, after which a pre-collicular post-mammillary decerebration was performed. All neural tissue rostral to the section was removed, and the cranial vault was filled with warm (378C) agar. After the decerebration was completed, the lungs were ventilated with a mixture of room air and oxygen. Arterial blood gases were measured and maintained at normal limits either by adjusting ventilation or by injecting sodium bicarbonate (8.5%; i.v.). A stainless steel monopolar electrode (Kopf, tip impedance at 1000 Hz of 10–60 kV) was placed over the inferior colliculus and was advanced into the mesencephalic locomotor region (MLR; coordinates: P2, L4, HC1) for the delivery of monophasic square wave electrical pulses (25–50 Hz; 0.1–0.5 ms; 80–120 mA). The MLR electrode was judged to be positioned correctly when stimulation evoked either locomotion in unparalyzed cats or efferent activity in the peroneal and / or posterior biceps semitendinosus (PBST) nerves in paralyzed cats [6]. The intensity of the current applied to the tibial nerve was expressed, as multiples of the threshold current needed to evoke a muscle twitch. Extracellular impulses from dorsal horn neurons were recorded with tungsten microelectrodes (FHC; tip impedance at 1000 Hz of 4–12 MV) from either the L7 or S1 spinal segments. Arterial blood pressure was measured from a carotid arterial catheter, which in turn was connected, to a Statham transducer (P23XL). All signals were written on a chart recorder (Gould) and were also recorded on videotape after being digitized (Vetter). The impulse activity of dorsal horn neurons was displayed on a storage oscilloscope. The latency of response of the dorsal horn neurons to electrical stimulation of the tibial nerve was measured from stimulus onset. Next, with the cat paralyzed with either vecuronium bromide (0.01 mg / kg; i.v.) or rocuronium bromide (0.01– 0.1 mg / kg; i.v.), we determined the effect of MLR stimulation (i.e., central command) on the discharge of the dorsal horn neuron if it was spontaneous active. The MLR was stimulated for 5–30 s. If an effect on the discharge of the dorsal horn neuron was observed, the stimulus was turned off and then repeated. This sequence was repeated 2–4 times until a clear pattern of response emerged. If the neuron was not spontaneously active, we activated it by stimulating the tibial nerve (|1 Hz; 10–20 times motor threshold) and then examined the effect of MLR stimulation on its evoked discharge. Stimulation of the MLR evoked two different discharge patterns from the peroneal and PBST nerves. The first consisted of a pattern representative of static contraction, and the second consisted of
179
fictive locomotion. The paralytic agents in the doses used, were short lasting (i.e. no more than 30 min), and had graded effects on the neuromuscular junction. This allowed us to record the discharge of dorsal horn neurons during muscular contraction while the MLR was stimulated. We advanced the recording microelectrode through the dorsal horn while stimulating the tibial nerve at 2–3 times motor threshold (1 Hz; pulse duration: 0.025 ms). In this manner we were able to search for dorsal horn neurons responsive to repetitive muscle twitch. In some cases we searched for neurons responsive to static contraction. To accomplish this, the tibial nerve was still stimulated at 2–3 times motor threshold but the frequency was increased from 1 to 40 Hz (pulse duration: 0.025 ms). Static contractions usually lasted 5–30 s. In some instances, the L7 ventral root was identified, cut and placed on a stimulating electrode. Both twitch and static contractions were evoked by stimulating the ventral root (pulse duration: 0.1 ms) instead of the tibial nerve. We determined if the neurons whose impulse activity was recorded in the dorsal horn received group III afferent input from the tibial nerve. While the cats were paralyzed, we stimulated (1 Hz; 0.1–0.5 ms) the tibial nerve at multiples of motor threshold to determine the current threshold needed to activate the neuron. Only neurons having a latency of at least 7 ms, which was evoked by a current intensity of at least 10 times motor threshold, were classified as having group III afferent input; those having shorter onset latencies were discarded. While the cat was paralyzed we recorded the neuron’s response to both stretching the calcaneal tendon and pinching in a noxious manner the triceps surae muscles. All values are reported as means6standard errors. When appropriate, paired t-tests were performed to determine statistical significance. The criterion level was set at P,0.05. We recorded the impulse activity of 31 dorsal horn neurons, each of which received input from group III afferents in the tibial nerves. The current intensity of the electrical pulse applied to the tibial nerve that was required to activate these neurons when the cats were paralyzed averaged 18.162.8 times motor threshold (range: 10–30); their onset latency averaged 9.961.3 ms (range: 7–33 ms). The depth from the surface of the spinal cord for the 31 neurons averaged 1.8660.07 mm. Each of the neurons responded either to stretching the calcaneal tendon or to pinching the triceps surae muscles. Specifically, fourteen of the neurons responded to tendon stretch, ten responded to pinching, and seven responded to both tendon stretch and to pinching. Twenty-one of the dorsal horn neurons were stimulated by repetitive twitch contractions of the triceps surae muscles. The discharge of all but two of these was recorded during MLR stimulation while the cat was paralyzed. This stimulation inhibited the discharge of 18 neurons (Fig. 1A) and had no effect on one. Six other neurons were stimulated by static contraction of the triceps
180
A.M. Degtyarenko, M.P. Kaufman / Brain Research 880 (2000) 178 – 182
Fig. 1. Electrical stimulation of the mesencephalic locomotor region (MLR) inhibits the discharge of a dorsal horn neuron in a paralyzed cat. (A) The tibial nerve was electrically stimulated at 20 times motor threshold (|0.8 Hz; 0.5 ms) to activate a previously silent neuron. Note that MLR stimulation, initiated at downwards pointing arrow, inhibited the response of the neuron to tibial nerve stimulation. Also, note that this response returned when the MLR stimulation ended (upwards pointing arrow). (B) Thirty minutes after the end of A, response of dorsal horn neuron to stimulation of L7 ventral root (0.1 ms; 2 times motor threshold) after the effects of the paralyzing agent had partly worn off. Note that the ventral root was stimulated with a short train (40 Hz) of impulses, lasting about 50 ms. Note also that amplifier gain was increased in B over that in A. ABP, arterial blood pressure; c-Per, activity recorded from cut central end of contralateral peroneal nerve; EC, extracellular impulse activity from dorsal horn neuron.
surae muscles (Fig. 2). The discharge of six was recorded during MLR stimulation while the cat was paralyzed; five were inhibited and one was not affected. The discharge of five dorsal horn neurons was recorded during MLR stimulation while the cat was not paralyzed. In this situation, MLR stimulation caused the triceps surae muscles to exercise in either a dynamic (n53) or static (n52) manner. The discharge of each of the five neurons was inhibited by MLR stimulation while the triceps surae muscles contracted (Figs. 2 and 3). One of these neurons
was the same as one of the 21 tested with repetitive twitch contractions and by MLR stimulation during paralysis (see above). We have shown that both the spontaneous and evoked discharge of dorsal horn neurons were inhibited by stimulation of the MLR, a maneuver which evoked a central motor command to exercise. We have also shown that these dorsal horn neurons responded to contraction of the triceps surae muscles, had receptive fields in these muscles, and received group III afferent input from the tibial nerve. The
A.M. Degtyarenko, M.P. Kaufman / Brain Research 880 (2000) 178 – 182
181
Fig. 2. MLR stimulation inhibited the response of a dorsal horn neuron to static contraction of the triceps surae muscles. Note that neuron responded to contraction (middle) only when the MLR was not stimulated. Downward and upward arrows represent onset and offset of MLR and tibial nerve stimulation (2 times motor threshold, respectively). Also, note that stimulation of the tibial nerve at 2 times threshold did not activate the dorsal horn neuron when the cat was paralyzed. EC, extracellular activity recorded from dorsal horn neuron; Ten, tension recorded from triceps surae muscles; TIB 23T, tibial nerve stimulation at 2 times motor threshold.
Fig. 3. MLR stimulation inhibited the response of a dorsal horn neuron to electrical activation of group III afferents in the tibial nerve. MLR stimulation started at the downwards arrow and ended at the upwards arrow. Note that the MLR stimulation caused the triceps surae muscles to contract statically. Also note that the tibial nerve was stimulated at 0.5 Hz at 10 times motor threshold to evoke a brisk response from the dorsal horn neuron. ABP, arterial blood pressure; PER, electrical activity from cut central end of peroneal nerve; EC, extracellular activity from dorsal horn neuron; Ten, tension recorded from triceps surae muscles. Large vertical lines represent stimulus artifact from tibial nerve stimulus.
182
A.M. Degtyarenko, M.P. Kaufman / Brain Research 880 (2000) 178 – 182
impulses recorded from these neurons were at depths suggesting that they were located in laminae V and VI of the dorsal horn. We have no evidence that the dorsal horn neurons described in our experiments received monosynaptic input from thin fiber afferents innervating the triceps surae muscles. Nevertheless, the location of the neurons was consistent with existing knowledge about the termination of group III muscle afferents in the lumbar dorsal horn. Specifically, electrophysiological evidence indicates that group III afferents innervating the triceps surae muscles project both monosynaptically and polysynaptically to laminae I, IV and V [7,10,15]. Likewise, anatomical evidence indicates that group III afferents terminate in lamina V [9]. Previously, we found that stimulation of the MLR inhibited the discharge of lamina V neurons receiving thin fiber input from the tibial nerve. In contrast, MLR stimulation had no effect on the discharge of laminae II through IV neurons which also received this input [5]. The present report demonstrates that these lamina V neurons are stimulated by muscular contraction, a maneuver known to activate group III muscle afferents [12]. In addition, the present report demonstrates that MLR stimulation, a maneuver that evoked central command, inhibited this input while the cat was exercising. Finally, the present report may provide an electrophysiological basis for previous reports [16,18] showing that the cardiovascular and ventilatory responses to central command and the exercise pressor reflex evoked simultaneously were less than the algebraic sum of these responses evoked individually.
Acknowledgements This work was supported by NIH Grant HL 30710. We thank Eileen English for typing the manuscript.
References [1] C.M. Adreani, J.M. Hill, M.P. Kaufman, Responses of group III and IV muscle afferents to dynamic exercise, J. Appl. Physiol. 86 (1997) 1811–1817. [2] E. Carstens, H. Gilly, H. Schrieber, M. Zimmerman, Effects of midbrain stimulation and iontophoretic application of serotonin, noradrenaline, morphine, and GABA on electrical thresholds of afferent C- and A-fiber terminals in cat spinal cord, Neuroscience 21 (1987) 395–406.
[3] E. Carstens, D. Klumpp, M. Zimmermann, Differential inhibitory effects of medial and lateral midbrain stimulation on spinal neuronal discharges to noxious skin heating in the cat, J. Neurophysiol. 43 (1980) 332–342. [4] A.D. Craig, S. Mense, The distribution of afferent fibers from the gastrocnemius-soleus muscle in the dorsal horn of the cat as revealed by the transport of horseradish peroxidase, Neurosci. Lett. 41 (1983) 233–238. [5] A.M. Degtyarenko, M.P. Kaufman, Fictive locomotion and scratching inhibits dorsal horn neurons receiving thin fiber afferent input, Am. J. Physiol. 279 (2000) R394–R403. [6] F.L. Eldridge, D.E. Millhorn, T.G. Waldrop, Exercise hyperpnea and locomotion: Parallel activation from the hypothalamus, Science 211 (1981) 844–846. [7] R.D. Foreman, R.F. Schmidt, W.D. Willis, Effects of mechanical and chemical stimulation of fine muscle afferents upon primate spinothalamic tract cells, J. Physiol. 286 (1979) 215–231. [8] S. Grillner, M.L. Shik, On the descending control of the lumbosacral spinal cord from the ‘mesencephalic locomotor region’, Acta Physiol. Scand. 87 (1973) 320–333. [9] U. Hoheisel, E. Lehmann-Willenbrock, S. Mense, Termination patterns of identified group II and III afferent fibres from deep tissues in the spinal cord of the cat, Neuroscience 28 (1989) 495–507. [10] U. Hoheisel, S. Mense, Response behaviour of cat dorsal horn neurons receiving input from skeletal muscle and other deep somatic tissues, J. Physiol. 426 (1990) 265–280. [11] M.P. Kaufman, H.V. Forster, Reflexes controlling circulatory, ventilatory and airway responses to exercise, in: L.B. Rowell, J.T. Shepherd (Eds.), Exercise: Regulation and Integration of Multiple Systems. II. Control of Respiratory and Cardiovascular Systems, Handbook of Physiology, Vol. Section 12, Oxford University Press, New York, NY, 1996, pp. 381–447. [12] M.P. Kaufman, J.C. Longhurst, K.J. Rybicki, J.H. Wallach, J.H. Mitchell, Effects of static muscular contraction on impulse activity of groups III and IV afferents in cats, J. Appl. Physiol. 55 (1983) 105–112. [13] D.I. McCloskey, J.H. Mitchell, Reflex cardiovascular and respiratory responses originating in exercising muscle, J. Physiol. 224 (1972) 173–186. [14] S. Mense, A.D.I. Craig, Spinal and supraspinal terminations of primary afferent fibers from the gastrocnemius-soleus muscle in the cat, Neuroscience 26 (1988) 1023–1035. [15] B. Pomeranz, P.D. Wall, W.V. Weber, Cord cells responding to fine myelinated afferents from viscera, muscle, and skin, J. Physiol. 199 (1968) 511–532. [16] K.J. Rybicki, R.W. Stremel, G.A. Iwamoto, J.H. Mitchell, M.P. Kaufman, Occlusion of pressor responses to posterior diencephalic stimulation and muscular contraction, Brain Res. Bull. 22 (1989) 305–312. [17] T.G. Waldrop, F.L. Eldridge, G.A. Iwamoto, J.H. Mitchell, Central neural control of respiration and circulation during exercise, in: L.B. Rowell, J.T. Shepherd (Eds.), Exercise: Regulation and Integration of Multiple Systems, Handbook of Physiology, Vol. Section 12, Oxford University Press, New York, NY, 1996, pp. 333–380. [18] T.G. Waldrop, D.C. Mullins, D.E. Millhorn, Control of respiration by the hypothalamus and by feedback from contracting muscles in cats, Respir. Physiol. 64 (1986) 317–328.
Short communication
Stimulation of the MLR inhibits the discharge of dorsal horn neurons responsive to muscular contraction Alexandr M. Degtyarenko*, Marc P. Kaufman Division of Cardiovascular Medicine, Departments of Internal Medicine and Human Physiology, TB 172, University of California, Davis, CA 95616, USA Accepted 18 July 2000
Abstract We found that electrical stimulation of the mesencephalic locomotor region (MLR) inhibited the discharge of deep dorsal horn neurons receiving group III afferent input from the triceps surae muscles. In contrast, contraction of these muscles induced by electrical stimulation of the tibial nerve activated these dorsal horn neurons. Our findings show that descending central motor commands can inhibit dorsal horn interneurons receiving input from group III afferents during exercise. 2000 Elsevier Science B.V. All rights reserved. Keywords: Exercise; Group III muscle afferent; Cat; Cardiovascular control
Static and moderately intense dynamic exercise is well known to increase cardiovascular and ventilatory function. These increases are widely believed to be caused by two neural mechanisms, namely central command [17] and the exercise pressor reflex [11]. The first mechanism, central command is defined as the parallel activation of somatomotor, autonomic and ventilatory circuits at the onset of exercise. The second mechanism, the exercise pressor reflex, arises from the contraction-induced stimulation of group III and IV afferent endings in skeletal muscle. Substantial evidence has been presented in humans and animals showing that both mechanisms play significant roles in evoking the cardiovascular and ventilatory responses to exercise. Both mechanisms, moreover, are likely to be engaged simultaneously. This likelihood prompted two prior studies in which central command and the exercise pressor reflex were evoked separately as well as simultaneously in anesthetized cats [16,18]. These studies found that the two mechanisms evoked simultaneously caused cardiovascular increases that were less than the algebraic sum of the two mechanisms evoked separately. This finding was not attributable to a ceiling effect because the cats were shown to be able to generate pressor, cardioaccelerator and ventilatory responses of a
*Corresponding author. Fax: 11-530-752-3264. E-mail address: [email protected] (A.M. Degtyarenko).
magnitude greater than those evoked by the algebraic sum of central command and the reflex evoked separately. Little is known about how the central nervous system integrates the two mechanisms. The dorsal horn of the spinal cord is a central site where central command and the exercise pressor reflex might interact. For example, the dorsal horn is the site of termination of group III and IV muscle afferents [4,14], whose stimulation by contraction is responsible for the exercise pressor reflex [1,13]. Moreover, the activity of dorsal horn neurons receiving nociceptive input has been shown to be inhibited by input from brainstem structures whose stimulation caused analgesia [2,3]. In addition, a central motor command has been shown to depress inhibitory motor reflexes arising from group II and III muscle afferents as well as from thin fiber cutaneous afferents [8]. Despite these findings, little is known about how central command interacts with group III and IV afferent input onto dorsal horn neurons. In decerebrated cats we examined the effect of central command on the discharge of dorsal horn cells responsive to muscular contraction. Each of these neurons, whose activity was recorded extracellularly, was shown to receive group III muscle afferent input. We found that contraction-induced input onto cells located in laminae V and VI of the dorsal horn was inhibited by central command. Cats (2.2–3.5 kg) were anesthetized with a mixture of halothane (3–4%) and oxygen. A jugular vein, a common
0006-8993 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 00 )02738-4
A.M. Degtyarenko, M.P. Kaufman / Brain Research 880 (2000) 178 – 182
carotid artery and the trachea were cannulated. The lungs were then ventilated with the anesthetic / oxygen mixture through the trachea cannula. The remaining carotid artery was ligated, and the cat was placed in a Kopf stereotaxic and spinal unit. The right tibial nerve was dissected free and placed on a stimulating electrode. The right triceps surae muscles were exposed and the right calcaneal (Achilles) tendon was cut. The L4–S2 spinal segments were exposed, after which a pre-collicular post-mammillary decerebration was performed. All neural tissue rostral to the section was removed, and the cranial vault was filled with warm (378C) agar. After the decerebration was completed, the lungs were ventilated with a mixture of room air and oxygen. Arterial blood gases were measured and maintained at normal limits either by adjusting ventilation or by injecting sodium bicarbonate (8.5%; i.v.). A stainless steel monopolar electrode (Kopf, tip impedance at 1000 Hz of 10–60 kV) was placed over the inferior colliculus and was advanced into the mesencephalic locomotor region (MLR; coordinates: P2, L4, HC1) for the delivery of monophasic square wave electrical pulses (25–50 Hz; 0.1–0.5 ms; 80–120 mA). The MLR electrode was judged to be positioned correctly when stimulation evoked either locomotion in unparalyzed cats or efferent activity in the peroneal and / or posterior biceps semitendinosus (PBST) nerves in paralyzed cats [6]. The intensity of the current applied to the tibial nerve was expressed, as multiples of the threshold current needed to evoke a muscle twitch. Extracellular impulses from dorsal horn neurons were recorded with tungsten microelectrodes (FHC; tip impedance at 1000 Hz of 4–12 MV) from either the L7 or S1 spinal segments. Arterial blood pressure was measured from a carotid arterial catheter, which in turn was connected, to a Statham transducer (P23XL). All signals were written on a chart recorder (Gould) and were also recorded on videotape after being digitized (Vetter). The impulse activity of dorsal horn neurons was displayed on a storage oscilloscope. The latency of response of the dorsal horn neurons to electrical stimulation of the tibial nerve was measured from stimulus onset. Next, with the cat paralyzed with either vecuronium bromide (0.01 mg / kg; i.v.) or rocuronium bromide (0.01– 0.1 mg / kg; i.v.), we determined the effect of MLR stimulation (i.e., central command) on the discharge of the dorsal horn neuron if it was spontaneous active. The MLR was stimulated for 5–30 s. If an effect on the discharge of the dorsal horn neuron was observed, the stimulus was turned off and then repeated. This sequence was repeated 2–4 times until a clear pattern of response emerged. If the neuron was not spontaneously active, we activated it by stimulating the tibial nerve (|1 Hz; 10–20 times motor threshold) and then examined the effect of MLR stimulation on its evoked discharge. Stimulation of the MLR evoked two different discharge patterns from the peroneal and PBST nerves. The first consisted of a pattern representative of static contraction, and the second consisted of
179
fictive locomotion. The paralytic agents in the doses used, were short lasting (i.e. no more than 30 min), and had graded effects on the neuromuscular junction. This allowed us to record the discharge of dorsal horn neurons during muscular contraction while the MLR was stimulated. We advanced the recording microelectrode through the dorsal horn while stimulating the tibial nerve at 2–3 times motor threshold (1 Hz; pulse duration: 0.025 ms). In this manner we were able to search for dorsal horn neurons responsive to repetitive muscle twitch. In some cases we searched for neurons responsive to static contraction. To accomplish this, the tibial nerve was still stimulated at 2–3 times motor threshold but the frequency was increased from 1 to 40 Hz (pulse duration: 0.025 ms). Static contractions usually lasted 5–30 s. In some instances, the L7 ventral root was identified, cut and placed on a stimulating electrode. Both twitch and static contractions were evoked by stimulating the ventral root (pulse duration: 0.1 ms) instead of the tibial nerve. We determined if the neurons whose impulse activity was recorded in the dorsal horn received group III afferent input from the tibial nerve. While the cats were paralyzed, we stimulated (1 Hz; 0.1–0.5 ms) the tibial nerve at multiples of motor threshold to determine the current threshold needed to activate the neuron. Only neurons having a latency of at least 7 ms, which was evoked by a current intensity of at least 10 times motor threshold, were classified as having group III afferent input; those having shorter onset latencies were discarded. While the cat was paralyzed we recorded the neuron’s response to both stretching the calcaneal tendon and pinching in a noxious manner the triceps surae muscles. All values are reported as means6standard errors. When appropriate, paired t-tests were performed to determine statistical significance. The criterion level was set at P,0.05. We recorded the impulse activity of 31 dorsal horn neurons, each of which received input from group III afferents in the tibial nerves. The current intensity of the electrical pulse applied to the tibial nerve that was required to activate these neurons when the cats were paralyzed averaged 18.162.8 times motor threshold (range: 10–30); their onset latency averaged 9.961.3 ms (range: 7–33 ms). The depth from the surface of the spinal cord for the 31 neurons averaged 1.8660.07 mm. Each of the neurons responded either to stretching the calcaneal tendon or to pinching the triceps surae muscles. Specifically, fourteen of the neurons responded to tendon stretch, ten responded to pinching, and seven responded to both tendon stretch and to pinching. Twenty-one of the dorsal horn neurons were stimulated by repetitive twitch contractions of the triceps surae muscles. The discharge of all but two of these was recorded during MLR stimulation while the cat was paralyzed. This stimulation inhibited the discharge of 18 neurons (Fig. 1A) and had no effect on one. Six other neurons were stimulated by static contraction of the triceps
180
A.M. Degtyarenko, M.P. Kaufman / Brain Research 880 (2000) 178 – 182
Fig. 1. Electrical stimulation of the mesencephalic locomotor region (MLR) inhibits the discharge of a dorsal horn neuron in a paralyzed cat. (A) The tibial nerve was electrically stimulated at 20 times motor threshold (|0.8 Hz; 0.5 ms) to activate a previously silent neuron. Note that MLR stimulation, initiated at downwards pointing arrow, inhibited the response of the neuron to tibial nerve stimulation. Also, note that this response returned when the MLR stimulation ended (upwards pointing arrow). (B) Thirty minutes after the end of A, response of dorsal horn neuron to stimulation of L7 ventral root (0.1 ms; 2 times motor threshold) after the effects of the paralyzing agent had partly worn off. Note that the ventral root was stimulated with a short train (40 Hz) of impulses, lasting about 50 ms. Note also that amplifier gain was increased in B over that in A. ABP, arterial blood pressure; c-Per, activity recorded from cut central end of contralateral peroneal nerve; EC, extracellular impulse activity from dorsal horn neuron.
surae muscles (Fig. 2). The discharge of six was recorded during MLR stimulation while the cat was paralyzed; five were inhibited and one was not affected. The discharge of five dorsal horn neurons was recorded during MLR stimulation while the cat was not paralyzed. In this situation, MLR stimulation caused the triceps surae muscles to exercise in either a dynamic (n53) or static (n52) manner. The discharge of each of the five neurons was inhibited by MLR stimulation while the triceps surae muscles contracted (Figs. 2 and 3). One of these neurons
was the same as one of the 21 tested with repetitive twitch contractions and by MLR stimulation during paralysis (see above). We have shown that both the spontaneous and evoked discharge of dorsal horn neurons were inhibited by stimulation of the MLR, a maneuver which evoked a central motor command to exercise. We have also shown that these dorsal horn neurons responded to contraction of the triceps surae muscles, had receptive fields in these muscles, and received group III afferent input from the tibial nerve. The
A.M. Degtyarenko, M.P. Kaufman / Brain Research 880 (2000) 178 – 182
181
Fig. 2. MLR stimulation inhibited the response of a dorsal horn neuron to static contraction of the triceps surae muscles. Note that neuron responded to contraction (middle) only when the MLR was not stimulated. Downward and upward arrows represent onset and offset of MLR and tibial nerve stimulation (2 times motor threshold, respectively). Also, note that stimulation of the tibial nerve at 2 times threshold did not activate the dorsal horn neuron when the cat was paralyzed. EC, extracellular activity recorded from dorsal horn neuron; Ten, tension recorded from triceps surae muscles; TIB 23T, tibial nerve stimulation at 2 times motor threshold.
Fig. 3. MLR stimulation inhibited the response of a dorsal horn neuron to electrical activation of group III afferents in the tibial nerve. MLR stimulation started at the downwards arrow and ended at the upwards arrow. Note that the MLR stimulation caused the triceps surae muscles to contract statically. Also note that the tibial nerve was stimulated at 0.5 Hz at 10 times motor threshold to evoke a brisk response from the dorsal horn neuron. ABP, arterial blood pressure; PER, electrical activity from cut central end of peroneal nerve; EC, extracellular activity from dorsal horn neuron; Ten, tension recorded from triceps surae muscles. Large vertical lines represent stimulus artifact from tibial nerve stimulus.
182
A.M. Degtyarenko, M.P. Kaufman / Brain Research 880 (2000) 178 – 182
impulses recorded from these neurons were at depths suggesting that they were located in laminae V and VI of the dorsal horn. We have no evidence that the dorsal horn neurons described in our experiments received monosynaptic input from thin fiber afferents innervating the triceps surae muscles. Nevertheless, the location of the neurons was consistent with existing knowledge about the termination of group III muscle afferents in the lumbar dorsal horn. Specifically, electrophysiological evidence indicates that group III afferents innervating the triceps surae muscles project both monosynaptically and polysynaptically to laminae I, IV and V [7,10,15]. Likewise, anatomical evidence indicates that group III afferents terminate in lamina V [9]. Previously, we found that stimulation of the MLR inhibited the discharge of lamina V neurons receiving thin fiber input from the tibial nerve. In contrast, MLR stimulation had no effect on the discharge of laminae II through IV neurons which also received this input [5]. The present report demonstrates that these lamina V neurons are stimulated by muscular contraction, a maneuver known to activate group III muscle afferents [12]. In addition, the present report demonstrates that MLR stimulation, a maneuver that evoked central command, inhibited this input while the cat was exercising. Finally, the present report may provide an electrophysiological basis for previous reports [16,18] showing that the cardiovascular and ventilatory responses to central command and the exercise pressor reflex evoked simultaneously were less than the algebraic sum of these responses evoked individually.
Acknowledgements This work was supported by NIH Grant HL 30710. We thank Eileen English for typing the manuscript.
References [1] C.M. Adreani, J.M. Hill, M.P. Kaufman, Responses of group III and IV muscle afferents to dynamic exercise, J. Appl. Physiol. 86 (1997) 1811–1817. [2] E. Carstens, H. Gilly, H. Schrieber, M. Zimmerman, Effects of midbrain stimulation and iontophoretic application of serotonin, noradrenaline, morphine, and GABA on electrical thresholds of afferent C- and A-fiber terminals in cat spinal cord, Neuroscience 21 (1987) 395–406.
[3] E. Carstens, D. Klumpp, M. Zimmermann, Differential inhibitory effects of medial and lateral midbrain stimulation on spinal neuronal discharges to noxious skin heating in the cat, J. Neurophysiol. 43 (1980) 332–342. [4] A.D. Craig, S. Mense, The distribution of afferent fibers from the gastrocnemius-soleus muscle in the dorsal horn of the cat as revealed by the transport of horseradish peroxidase, Neurosci. Lett. 41 (1983) 233–238. [5] A.M. Degtyarenko, M.P. Kaufman, Fictive locomotion and scratching inhibits dorsal horn neurons receiving thin fiber afferent input, Am. J. Physiol. 279 (2000) R394–R403. [6] F.L. Eldridge, D.E. Millhorn, T.G. Waldrop, Exercise hyperpnea and locomotion: Parallel activation from the hypothalamus, Science 211 (1981) 844–846. [7] R.D. Foreman, R.F. Schmidt, W.D. Willis, Effects of mechanical and chemical stimulation of fine muscle afferents upon primate spinothalamic tract cells, J. Physiol. 286 (1979) 215–231. [8] S. Grillner, M.L. Shik, On the descending control of the lumbosacral spinal cord from the ‘mesencephalic locomotor region’, Acta Physiol. Scand. 87 (1973) 320–333. [9] U. Hoheisel, E. Lehmann-Willenbrock, S. Mense, Termination patterns of identified group II and III afferent fibres from deep tissues in the spinal cord of the cat, Neuroscience 28 (1989) 495–507. [10] U. Hoheisel, S. Mense, Response behaviour of cat dorsal horn neurons receiving input from skeletal muscle and other deep somatic tissues, J. Physiol. 426 (1990) 265–280. [11] M.P. Kaufman, H.V. Forster, Reflexes controlling circulatory, ventilatory and airway responses to exercise, in: L.B. Rowell, J.T. Shepherd (Eds.), Exercise: Regulation and Integration of Multiple Systems. II. Control of Respiratory and Cardiovascular Systems, Handbook of Physiology, Vol. Section 12, Oxford University Press, New York, NY, 1996, pp. 381–447. [12] M.P. Kaufman, J.C. Longhurst, K.J. Rybicki, J.H. Wallach, J.H. Mitchell, Effects of static muscular contraction on impulse activity of groups III and IV afferents in cats, J. Appl. Physiol. 55 (1983) 105–112. [13] D.I. McCloskey, J.H. Mitchell, Reflex cardiovascular and respiratory responses originating in exercising muscle, J. Physiol. 224 (1972) 173–186. [14] S. Mense, A.D.I. Craig, Spinal and supraspinal terminations of primary afferent fibers from the gastrocnemius-soleus muscle in the cat, Neuroscience 26 (1988) 1023–1035. [15] B. Pomeranz, P.D. Wall, W.V. Weber, Cord cells responding to fine myelinated afferents from viscera, muscle, and skin, J. Physiol. 199 (1968) 511–532. [16] K.J. Rybicki, R.W. Stremel, G.A. Iwamoto, J.H. Mitchell, M.P. Kaufman, Occlusion of pressor responses to posterior diencephalic stimulation and muscular contraction, Brain Res. Bull. 22 (1989) 305–312. [17] T.G. Waldrop, F.L. Eldridge, G.A. Iwamoto, J.H. Mitchell, Central neural control of respiration and circulation during exercise, in: L.B. Rowell, J.T. Shepherd (Eds.), Exercise: Regulation and Integration of Multiple Systems, Handbook of Physiology, Vol. Section 12, Oxford University Press, New York, NY, 1996, pp. 333–380. [18] T.G. Waldrop, D.C. Mullins, D.E. Millhorn, Control of respiration by the hypothalamus and by feedback from contracting muscles in cats, Respir. Physiol. 64 (1986) 317–328.