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A small system of neurons in the mammalian spinal cord
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SHORI- COMMUNICATIONS
A small system of neurons in the mammalian spinal cord G r o u p la afferent fibers from primary endings of muscle spindles are k n o w n to make direct excitatory connections with motoneurons supplying the muscle containing the spindles or synergistic muscles4,5, tl. Several workers have studied the effects of single group la fibers upon individual motoneurons'~,8, 9. However, the techniques of each have drawbacks which limit their usefulness. In Kuno's experiments, the afferent fibers were dissected in a peripheral nerve. This approach does not always guarantee that a single fiber of the appropriate type is stimulated. In Burke's experiments. unitary excitatory postsynaptic potentials attributable to single impulses in group la fibers were recorded in motoneurons during muscle stretch. This technique has at least two defects. The activity of the afferent fiber is not monitored, and so failure of the response can be due to failure of an impulse rather than to the absence of transmitter release. Furthermore, it is impossible to control the exact timing of the impulse and thus its excitatory postsynaptic potential. An alternative method has therefore been developed. This involves stimulation of the dorsal root ganglion cell of a group la fiber while recording both its action potential and the monosynaptic excitatory postsynaptic potential produced by it in a motoneuron. Cats anesthetized with sodium pentobarbital were used. The spinal cord and one or more dorsal root ganglia were exposed by laminectomy, and the cord was transected at the thoracolumbar junction. Several peripheral nerves, generally branches of the hamstring and gastrocnemius-soleus nerves, were prepared for stimulation. One microelectrode was used to impale a motoneuron which received monosynaptic excitation from one of the peripheral nerves. The unit could be identified as a motoneuron by the antidromic invasion of an action potential when the appropriate ventral root was stimulated 1. The generation of a monosynaptic excitatory postsynaptic potential when a peripheral nerve was stimulated established that the motoneuron supplied the muscle innervated by that peripheral nerve or a synergist". A second microelectrode was inserted into a dorsal root ganglion celt belonging to the same nerve. This was accomplished by tracking in the ganglion at the appropriate segmental level until a cell was impaled which was fired at short latency by stimulation of the nerve. The type of receptor to which the afferent fiber of the ganglion cell was attached was identified in the following way. Afferent fibers whose action potentials propagated at group I conduction velocities were considered to arise either from muscle spindle primary endings or from Golgi tendon organs. In general, the lower threshold fibers of this category were held to belong to muscle spindles and the higher threshold ones to Golgi tendon organs 4,~°. A further aid in identification came in several experiments in which the gastrocnemius-soleus nerve was left intact and the Achilles tendon fastened to a myograph. The response of the receptors connected with particular ganglion cells could then be tested using muscle stretch and contraction 12. When a dorsal root ganglion cell belonging to a group la fiber had been impaled, the cell was stimulated through the microelectrode. A bridge circuit was used. allowing observation of the action potential of the ganglion cell without overloading the amplifier. The effects of the nerve impulses in single group la afferenl fibers upon Brain Research, 9 (1968) 152--155
SHORT COMMUNICATIONS
153
Fig. 1. A and B show action potentials from a dorsal root ganglion cell and the tension developed at the Achilles tendon. A white dot has been placed above the top of each spike. The records in C - E show intracellular potentials from a motoneuron and cord dorsum responses to volleys in the gastrocnemius-soleus nerve, in addition to the ganglion cell spikes. In F and G, the effect of stimulation of the ganglion cell through the microelectrode is shown. The stimulus is above threshold in F and H and below threshold in G. The microelectrode was withdrawn from the motoneuron for the record in H. Averaged responses are shown in I and J corresponding to the filmed records of F and G. The calibration pulses are 1 mV in C F and 100 ItV in G-J. Each lasts 1 msec.
motoneurons could be studied in this fashion. It was often convenient to improve the signal to noise ratio in these experiments by the use of a signal averaging device (Nuclear Chicago Data Retrieval Computer). Sample records from one such experiment are shown in Fig. 1. The upper traces of Fig. 1A and B are intracellular records from a dorsal root ganglion cell belonging to the gastrocnemius-soleus nerve (GS). The lower trace shows muscle tension as recorded by a myograph. In A, the repetitive discharge evoked by stretch of the Achilles tendon is seen. In B, the discharge pauses during a muscle twitch evoked by stimulation of the GS nerve at a strength above threshold for some of its motor axons. In C-E there are three traces. The upper ones are from the same ganglion cell. The Brain Research, 9 (1968) 152-155
154
SHORT COMMUNICAI'IONS
middle ones are intracellular records from a GS motoneuron. The lower traces were recorded from the dorsum of the spinal cord by a ball-tipped platinum electrode. When the GS nerve was stimulated, a monosynaptic excitatory potential could be recorded from the motoneuron (Fig. 1C). This potential exceeded threshold when the stimulus strength included some of the higher threshold afferents of the group la volley, including the ganglion cell under study (Fig. 1D and E). In F - H . there are two traces. The upper ones were recorded by the m o t o n e u r o n electrode and the lower ones by the ganglion cell electrode. The ganglion cell was stimulated through the microelectrode; the stimulus was above threshold for the cell in F and H and below threshold in G. The record in H was made after the microelectrode was withdrawn to a position just extracellular to the motoneuron. It is evident that a small excitator.~ postsynaptic potential was produced in the m o t o n e u r o n when the ganglion cell was stimulated. This is even clearer when the averaged records in I and J are examined. The potential in I is preceded by an artifactual downward deflection, but it clearly has the form and latency of an excitatory postsynaptic potential. The same artifact can be seen in the extracellular record, H. The calibration pulse in I shows the size of the excitatory potential to be about 100 #V. The record in J was taken with the stimulus below threshold for activation of the ganglion cell. Apart from artifact the baseline is flat. The approach described here, coupled with experiments such as t h o s e d o n e by K u n o and Burke, gives the opportunity for the kind of simplified experimental conditions which have been so useful in the study of neuromuscular transmission ~. Studies have already been initiated on the effects of single motoneurons in generating muscle tension s. Thus, it is now feasible to analyze activity in the mammalian central nervous system in terms of a complete and identified chain of receptor, afferent fiber. and m o t o r unit. The study of small systems of nerve cells 7 need not be confined to invertebrates. This work was supported by U.S. Public Health Service Research G r a n t NB 04779. We would like to thank Mrs. Barbara Corbin for her expert technical assistance. Department of Anatomy, The University of Texas Southwestern Medical School at Dallas, Dallas, Texas (U.S.A.)
W, D; WILLIS W. D. LETBETTER W. M. THOMPSON
1 BROCK, L. G., COOMBS, J. S., AND ECCLES, J. C., Intracellular recording from antidromically activated motoneurones, J. Physiol. (Lond.), 122 (1953) 429-461. 2 BURKE, R. E., Composite nature of the monosynaptic excitatory postsynaptic potential, J. Neurophysiok, 30 (1967) 1114-I137. 3 DEVANANDAN,M. S,, ECCLES,R, M., AND WESTERMAN,R. A,, Single motor units of mammalian. muscle, J. Physiol. (Lond.), 178 (1965) 359-367. 4 ECCLES,J. C., ECCLES, R. M., AND LUNDBERG,A., Synaptic actions on motoneurones in relation to the two components of the group I muscle afferent volley, J. Physiol, (Lond.), 136 (1957) 527-546.
Brain Research, 9 (1968) 152-155
SHORT COMMUNICATIONS
155
5 ECCLES,J. C., ECCLES, R. M., AND LUNDBERG,A., The convergence of monosynaptic excitatory afferents on to many different species of alpha motoneurones, J. Physiol. (Lond.), 137 (1957)22-50. 6 KAT7, B., The transmission of impulses from nerve to muscle, and the subcellular unit of synaptic action, Proc. roy. Soc., B, 155 (1962) 455-477. 7 KENNEDY,D.. Small systems of nerve cells, Sci. Amer., 216 (1967) 44-52. 8 KUNO, M., Quantal components of excitatory synaptic potentials in spinal motoneurones, J. Physiol. (Lond.), 175 (1964) 81-99. 9 KUNO, M., Mechanism of facilitation and depression of the excitatory synaptic potential in spinal motoneurones, J. Physiol. (Lond.), 175 (1964) 100-112. l0 LAPORTE, Y., ET BESSOU,P., l~tude des sous-groupes lent et rapide du groupe I (fibres aff6rentes d'origine musculaire de grand diam6tre) chez le chat, J. Physiol. (Paris), 49 (1957) 1025-1034. 11 LLOYD,D. P. C., Conduction and synaptic transmission of the reflex response to stretch in spinal cats, J. Neurophysiol., 6 (1943) 317-326. 12 MATTHEWS,B. H. C., Nerve endings in mammalian muscle, J. Physiol. (Lond.), 78 (1933) 1-53. (Accepted March 27th, 1968)
Brain Research, 9 (1968) 152-155
SHORI- COMMUNICATIONS
A small system of neurons in the mammalian spinal cord G r o u p la afferent fibers from primary endings of muscle spindles are k n o w n to make direct excitatory connections with motoneurons supplying the muscle containing the spindles or synergistic muscles4,5, tl. Several workers have studied the effects of single group la fibers upon individual motoneurons'~,8, 9. However, the techniques of each have drawbacks which limit their usefulness. In Kuno's experiments, the afferent fibers were dissected in a peripheral nerve. This approach does not always guarantee that a single fiber of the appropriate type is stimulated. In Burke's experiments. unitary excitatory postsynaptic potentials attributable to single impulses in group la fibers were recorded in motoneurons during muscle stretch. This technique has at least two defects. The activity of the afferent fiber is not monitored, and so failure of the response can be due to failure of an impulse rather than to the absence of transmitter release. Furthermore, it is impossible to control the exact timing of the impulse and thus its excitatory postsynaptic potential. An alternative method has therefore been developed. This involves stimulation of the dorsal root ganglion cell of a group la fiber while recording both its action potential and the monosynaptic excitatory postsynaptic potential produced by it in a motoneuron. Cats anesthetized with sodium pentobarbital were used. The spinal cord and one or more dorsal root ganglia were exposed by laminectomy, and the cord was transected at the thoracolumbar junction. Several peripheral nerves, generally branches of the hamstring and gastrocnemius-soleus nerves, were prepared for stimulation. One microelectrode was used to impale a motoneuron which received monosynaptic excitation from one of the peripheral nerves. The unit could be identified as a motoneuron by the antidromic invasion of an action potential when the appropriate ventral root was stimulated 1. The generation of a monosynaptic excitatory postsynaptic potential when a peripheral nerve was stimulated established that the motoneuron supplied the muscle innervated by that peripheral nerve or a synergist". A second microelectrode was inserted into a dorsal root ganglion celt belonging to the same nerve. This was accomplished by tracking in the ganglion at the appropriate segmental level until a cell was impaled which was fired at short latency by stimulation of the nerve. The type of receptor to which the afferent fiber of the ganglion cell was attached was identified in the following way. Afferent fibers whose action potentials propagated at group I conduction velocities were considered to arise either from muscle spindle primary endings or from Golgi tendon organs. In general, the lower threshold fibers of this category were held to belong to muscle spindles and the higher threshold ones to Golgi tendon organs 4,~°. A further aid in identification came in several experiments in which the gastrocnemius-soleus nerve was left intact and the Achilles tendon fastened to a myograph. The response of the receptors connected with particular ganglion cells could then be tested using muscle stretch and contraction 12. When a dorsal root ganglion cell belonging to a group la fiber had been impaled, the cell was stimulated through the microelectrode. A bridge circuit was used. allowing observation of the action potential of the ganglion cell without overloading the amplifier. The effects of the nerve impulses in single group la afferenl fibers upon Brain Research, 9 (1968) 152--155
SHORT COMMUNICATIONS
153
Fig. 1. A and B show action potentials from a dorsal root ganglion cell and the tension developed at the Achilles tendon. A white dot has been placed above the top of each spike. The records in C - E show intracellular potentials from a motoneuron and cord dorsum responses to volleys in the gastrocnemius-soleus nerve, in addition to the ganglion cell spikes. In F and G, the effect of stimulation of the ganglion cell through the microelectrode is shown. The stimulus is above threshold in F and H and below threshold in G. The microelectrode was withdrawn from the motoneuron for the record in H. Averaged responses are shown in I and J corresponding to the filmed records of F and G. The calibration pulses are 1 mV in C F and 100 ItV in G-J. Each lasts 1 msec.
motoneurons could be studied in this fashion. It was often convenient to improve the signal to noise ratio in these experiments by the use of a signal averaging device (Nuclear Chicago Data Retrieval Computer). Sample records from one such experiment are shown in Fig. 1. The upper traces of Fig. 1A and B are intracellular records from a dorsal root ganglion cell belonging to the gastrocnemius-soleus nerve (GS). The lower trace shows muscle tension as recorded by a myograph. In A, the repetitive discharge evoked by stretch of the Achilles tendon is seen. In B, the discharge pauses during a muscle twitch evoked by stimulation of the GS nerve at a strength above threshold for some of its motor axons. In C-E there are three traces. The upper ones are from the same ganglion cell. The Brain Research, 9 (1968) 152-155
154
SHORT COMMUNICAI'IONS
middle ones are intracellular records from a GS motoneuron. The lower traces were recorded from the dorsum of the spinal cord by a ball-tipped platinum electrode. When the GS nerve was stimulated, a monosynaptic excitatory potential could be recorded from the motoneuron (Fig. 1C). This potential exceeded threshold when the stimulus strength included some of the higher threshold afferents of the group la volley, including the ganglion cell under study (Fig. 1D and E). In F - H . there are two traces. The upper ones were recorded by the m o t o n e u r o n electrode and the lower ones by the ganglion cell electrode. The ganglion cell was stimulated through the microelectrode; the stimulus was above threshold for the cell in F and H and below threshold in G. The record in H was made after the microelectrode was withdrawn to a position just extracellular to the motoneuron. It is evident that a small excitator.~ postsynaptic potential was produced in the m o t o n e u r o n when the ganglion cell was stimulated. This is even clearer when the averaged records in I and J are examined. The potential in I is preceded by an artifactual downward deflection, but it clearly has the form and latency of an excitatory postsynaptic potential. The same artifact can be seen in the extracellular record, H. The calibration pulse in I shows the size of the excitatory potential to be about 100 #V. The record in J was taken with the stimulus below threshold for activation of the ganglion cell. Apart from artifact the baseline is flat. The approach described here, coupled with experiments such as t h o s e d o n e by K u n o and Burke, gives the opportunity for the kind of simplified experimental conditions which have been so useful in the study of neuromuscular transmission ~. Studies have already been initiated on the effects of single motoneurons in generating muscle tension s. Thus, it is now feasible to analyze activity in the mammalian central nervous system in terms of a complete and identified chain of receptor, afferent fiber. and m o t o r unit. The study of small systems of nerve cells 7 need not be confined to invertebrates. This work was supported by U.S. Public Health Service Research G r a n t NB 04779. We would like to thank Mrs. Barbara Corbin for her expert technical assistance. Department of Anatomy, The University of Texas Southwestern Medical School at Dallas, Dallas, Texas (U.S.A.)
W, D; WILLIS W. D. LETBETTER W. M. THOMPSON
1 BROCK, L. G., COOMBS, J. S., AND ECCLES, J. C., Intracellular recording from antidromically activated motoneurones, J. Physiol. (Lond.), 122 (1953) 429-461. 2 BURKE, R. E., Composite nature of the monosynaptic excitatory postsynaptic potential, J. Neurophysiok, 30 (1967) 1114-I137. 3 DEVANANDAN,M. S,, ECCLES,R, M., AND WESTERMAN,R. A,, Single motor units of mammalian. muscle, J. Physiol. (Lond.), 178 (1965) 359-367. 4 ECCLES,J. C., ECCLES, R. M., AND LUNDBERG,A., Synaptic actions on motoneurones in relation to the two components of the group I muscle afferent volley, J. Physiol, (Lond.), 136 (1957) 527-546.
Brain Research, 9 (1968) 152-155
SHORT COMMUNICATIONS
155
5 ECCLES,J. C., ECCLES, R. M., AND LUNDBERG,A., The convergence of monosynaptic excitatory afferents on to many different species of alpha motoneurones, J. Physiol. (Lond.), 137 (1957)22-50. 6 KAT7, B., The transmission of impulses from nerve to muscle, and the subcellular unit of synaptic action, Proc. roy. Soc., B, 155 (1962) 455-477. 7 KENNEDY,D.. Small systems of nerve cells, Sci. Amer., 216 (1967) 44-52. 8 KUNO, M., Quantal components of excitatory synaptic potentials in spinal motoneurones, J. Physiol. (Lond.), 175 (1964) 81-99. 9 KUNO, M., Mechanism of facilitation and depression of the excitatory synaptic potential in spinal motoneurones, J. Physiol. (Lond.), 175 (1964) 100-112. l0 LAPORTE, Y., ET BESSOU,P., l~tude des sous-groupes lent et rapide du groupe I (fibres aff6rentes d'origine musculaire de grand diam6tre) chez le chat, J. Physiol. (Paris), 49 (1957) 1025-1034. 11 LLOYD,D. P. C., Conduction and synaptic transmission of the reflex response to stretch in spinal cats, J. Neurophysiol., 6 (1943) 317-326. 12 MATTHEWS,B. H. C., Nerve endings in mammalian muscle, J. Physiol. (Lond.), 78 (1933) 1-53. (Accepted March 27th, 1968)
Brain Research, 9 (1968) 152-155