Multisensory activation of pyramidal tract neurons in the cat

EXPERIMENTAL NEUROLOGY 30, 352-361 (1971)

Multisensory Activation of Pyramidal Tract Neurons in the Cat NAI-SHIN CHU AND L. T. t~UTLEDGE1

Department of Physiology, The University of ~Iichigan Medical School, Ann Arbor, Michigan 48104 Received October 20, 1970 Extracellular unit recordings were made of the responses of pericruciate neurons to photic, auditory, and forepaw stimulation in chloralose-anesthetized cats. More than 70% of pyramidal tract ( P T ) neurons was activated by two or more different sensory modalities, whereas about 40% of other neurons had such sensory convergence. The majority of the responsive pericruciate neurons was located in the lower one-half of the cortex. During sensory activation PT activity was found to precede the cortical-evoked response. The majority of the multisensory PT cells belonged to the "fast" group with conduction velocities between 25 and 50 m/sec. The results suggest that multisensory activation of certain pericruelate neurons provides a means of rapidly producing a state of "readiness to act" in the spinal cord. Introduction A l t h o u g h A d r i a n a n d M o r u z z i ( 1 ) m o r e than 30 y e a r s ago o b s e r v e d reflex p y r a m i d a l discharges after somesthetic, a u d i t o r y , a n d photic stimulation, v i r t u a l l y all attention since then has been d i r e c t e d to the study of m o t o r s e n s o r y c o r t e x and p y r a m i d a l tract n e u r o n s after somesthetic stimulation (2, 14, 23, 2 5 ) . I t n o w seems established that, at least for the cat, the m o t o r s e n s o r y c o r t e x is one of the n o n p r i m a r y cortical areas exhibiting m u l t i s e n s o r y responses ( 3 ) a n d n e u r o n s in the m o t o r s e n s o r y c o r t e x show m u l t i s e n s o r y convergence (4, 18). I n no k n o w n instance have cortic o f u g a l p a t h w a y s been identified in the face of m u l t i s e n s o r y activation of m o t o r s e n s o r y or other cortex. A r e the p a t h w a y s exclusively p y r a m i d a l ? D o the routes r e p r e s e n t fast or slow means of lower m o t o r n e u r o n activat i o n ? W h a t is the likely physiological function of a m o t o r s e n s o r y system receiving m n l t i s e n s o r y convergence? A n s w e r s to these questions m a y not only shed some light upon sensori-efferent integrative mechanisms in mo1 This work was in part supported by NIH-NDS Grant No. 04119 to Dr. L. T. Rutledge. Dr. Chu's present address is Laboratory of Neuropharmacology, Division of Special Mental Health Research, National Institute of Mental Health, St. Elizabeths Hospital, Washington, D. C. 20032.

352

ACTIVATION

OF

PT

NEURONS

353

torsensory cortex but may help in understanding the organization of other nonprimary association areas especially noted for multisensory convergence. We attempt to provide some of the answers in this study. Methods

Experiments were oll adult cats anesthetized with alphachloralose, (40-50 mg/kg, ip). After tracheostomy and femoral vein cannulation the animal was placed in a stereotaxic instrument. Cisternal drainage was used to reduce brain pulsations. After the motorsensory cortex was exposed a skin pouch was made to hold mineral oil kept at body temperature by a battery-operated wire loop. Concentric stimulating electrodes with tip separation of 0.8-1.0 mm were placed in the ipsilateral cerebral peduncle. Photic stimulation was a short flash via a G E 327 bulb placed about 15 cm in front of the dilated pupils. The duration of auditory click stimulation was 0.01 msec. The intensity of both photic and auditory stimulation was adjusted to give evoked responses at primary sensory areas. Somesthetic stimulation was shocks to the forepaw foot pad using inserted needles and at threshold intensity determined before the animal was immobilized with gallamine triethiodide. Animals were maintained on positive-pressure ventilation adjusted to keep expired CO2 at 3-4%. Body core temperature was maintained at 37-38 C. Insulated stainless-steel microelectrodes of about 1-/~ tip diameter were used to detect neuronal extracellular activity. The recording area was restricted to precruciate gyrus and postcruciate gyrus rostral to the dimple. Microelectrode insertions with a hydraulic micromanipulator were perpendicular to the surface. Most of the recordings were made at the lateral half of the pericruciate cortex and about 1.5 mm from the cruciate sulcus. Action potentials of cortical units were led through a cathode follower to a preampIifier with a band-pass of 100--2000 Hz. Amplifier output was displayed on an oscilloscope and monitored through a loud speaker. Only cortical units that displayed the following features were tested for their responsiveness to sensory and pyramidal tract ( P T ) stimulation: (a) constancy of spike amplitude; (b) stable spontaneous firing; (c) spike amplitude of at least 300 /~v; and (d) spike well isolated from other spikes. When a unit was isolated at least 20 min were allowed to ensure stability before testing procedures were started. The various sensory stimuli were used alternatively while hunting for responsive neurons. Data analyses were made on-line after unit responses were passed through a window discriminator and displayed as a histogram with 10msec intervals. Single-frame film records taken concomitantly were processed later for latency measurements. Mean latencies to first spikes were made from 20-50 frames.

354

c ~ v AND RUTLEDGE

Cortical slow waves were recorded monopolarly with a silver-ball electrode placed close to the site of microelectrode insertion. Preamplifier band-pass was 0.8-2000 Hz. The cerebral peduncle-stimulating electrode position was determined physiologically by the appearance of cortical surface a- and b-waves (8) to single square pulses of 0.01-msec duration and 0.2-0.8 ma. A cortical unit was classified as a P T neuron only if it responded to P T stimulation with invarient latency and followed faithfully stimulation at 100 t t z at threshold intensity. Final verification of P T electrodes' position was determined histologically. In some experiments, simultaneous recordings of the cortical-evoked response and pyramidal-tract activity in response to sensory stimulation were made. The position of recording electrodes in the cerebral peduncle was checked antidromically by the appearance of a- and b-waves (8) and orthodromically by the appearance of P T responses after bipolar surface stimulation of pericruciate cortex. These responses were not artifacts since they disappeared when the electrodes were withdrawn 1.0~-1.5 ram. Results

The study was made on 334 pericruciate neurons. Among them 89 were identified as P T neurons. The majority of the 334 pericruciate neurons had slow spontaneous rates. About three-fourths of them displayed a phasic type of response to sensory stimulation, while the remaining only randomly increased their firing after stimulation. In general, the phasic responses of P T neurons consisted of a series of high-frequency discharges. Both latency and number of spikes per stimulus varied for each of the sensory modalities. The individual neuronal responses to sensory stimulation were sometimes not correlated with the presence or the configuration of surface slow waves. A decrease in firing to any sensory stimulation was rather rare, involving about 5% of the total sample. Sensory Convergence on Pericruciate Neurons. Responsiveness of the adequately tested pericruciate neurons to the three types of sensory inputs is summarized in Table 1. The general responsiveness of the P T population to sensory input was consistently greater than that of the non-PT population. The somesthetic input was the most effective of all three sensory modalities for both P T and n o n - P T cells. Two hundred and fifty cells were tested with all types of sensory stimulation (Table 2). Most P T cells (70%) were activated by two or more different sensory modalities, whereas only 42% of n o n - P T neurons responded similarly. Few P T cells ( 5 ~ ) were not responsive to any of the sensory stimuli but one-fourth of n o n - P T cells fell into this category. One hundred and twenty-seven pericruciate cells were studied with attention to somesthetic input (Table 3). More P T neurons were found to

355

ACTIVATION OF PT NEURONS TABLE 1 RESPONSIVENESS OF PERICRUCIATE NEURONS TO SENSORY INPUTS OF DIFFERENT MODALITIES PT

non-PT

Input

N ~

%

N

%

Somatic (CFP a n d / o r IFP) b Photic Auditory

50 39 32

80.1 62.0 50.8

111 68 55

64.6 39.5 32.0

N represents the total n u m b e r of cells t h a t responded to the particular sensory inp u t in each category; % refers to the percentage of the population of t h a t category. C F P = contralateral forepaw stimulation. I F P = ipsitateral forepaw stimulation.

respond to bilateral forepaw shocks, whereas n o n - P T neurons were predominately activated by input from CFP. If n o n - P T neurons that responded to bilateral forepaw inputs are included, about 85% of the n o n - P T population was responsive to inputs from CFP, but only about 44% was responsive to I F P stimulation. On the contrary, the percentages of P T neurons that responded to either C F P or I F P stimulation were equal and high (74 and 7 6 % ) . Response Latencies of P T cbnd N o n - P T Neurons. Latencies of 214 pericruciate cells are diagrammatically presented in Fig. 1. Only those cells showing a phasic response pattern were selected. Usually the latencies of the first spike varied closely around a mean. It is evident that the latency distributions between P T and n o n - P T cells were characterized by large degrees of overlapping and dispersion. However, P T neurons, as compared with non-PT, tended to show less variability. P T neurons tended to respond earlier to auditory, visual, and I F P inputs than did non-PT, but TABLE 2 SENSORY CONVERGENCE AMONG P T AND NoN-PT PERICRUCIATE NEURONS PT

non-PT

Convergence t y p e

N ~

%

N

%

3 Modalities b 2 Modalities 1 Modality only Nonresponsive Total

30 24 19 4 77

39.0 31.2 24.7 5.1 100

31 41 56 44 173

18.0 23.8 32.6 25.6 100

N represents the n u m b e r of cells t h a t exhibited a particular type of sensory convergence; ~o refers to the percentage of the population of t h a t category. b "3 modalities" means cells responding to photie, auditory, a n d somatic inputs. "2 modalities" means ceils responding to a n y two of the three sensory modalities; "1 modality" means ceils responding to only one of the three sensory inputs.

356

CHU AND RUTLEDGE

TABLE 3 PERICRUCIATE UNITS RESPONDING TO SOMATIC SENSORY INPUT

PT

non-PT

Input

N"

%

N

%

CFP + IFP CFP only IFP only Total

25 12 13 50

50 24 26 100

22 43 12 77

28.6 55.8 15.6 100

N represents cells responding to a particular sensory input in each category; % refers to the percentage of the total responsive cells in each category. statistically the difference is not significant ( P = 0.14).25). O n the contrary, the P T n e u r o n s ' responses to C F P i n p u t tended to be slower than n o n - P T but again the difference was not significant. Multisensory P T Neurons. T h e latency m e a s u r e m e n t s of antidromic responses indicated that the m a j o r i t y of the responsive P T n e u r o n s belong to the "fast" g r o u p (20) with conduction velocities between 2 5 - 5 0 m / s e c (Fig. 2). O n the contrary, more than half of the n o n r e s p o n s i v e P T neuCFP

2.0 16

~ i

IFP

Non - PT cells(72)

[ ] Non- PT cells (53) [ ] PT ceils (25)

T cells(25)

4

[]

PHOTIC

d 2O Z

Non-PT cells (69) PT cells

.......

RY

OT cells 3) ills (25)

.16 ~2 8 4 0

0

2o

40

60

so I00

20

4o

60

8o Joo

Latency ( m s e c )

Fzd. 1. iVIean latencies of initial spikes of perieruciate neurons. CFP = contralaterm forepaw stimulation. I F P = ipsilateral forepaw stimulation. Numbers in parentheses are numbers of cells studied.

A C T I V A T I O N OF P T

357

NEURONS

rons had antidromic latencies longer than 2 msec, that is, they had fibre conduction velocities below 15 m/sec ("slow" group). The data seem to indicate that large P T neurons receive most of the sensory convergence. Caution is necessary, however, since extracellular single-unit recordings also favor sampling large P T neurons (22) and the subliminal changes are not detectable. Gross P T Activity Durin 9 Sensory Activation. By comparing surfaceevoked potentials with monopolarly recorded gross P T potentials at the cerebral peduncle, it is possible to show that some P T neuron activity preceded the cortical waves (Fig. 3). For photic stimulation, the evoked potential began at 30 msec, but the P T activity began at 20 msec and reached a peak at 37 msec (Fig. 3, P ) . The P T initial negativity was followed by a prolonged positive wave that began at the time when the surface positive potential began to turn toward negativity and lasted for 75 reset. For auditory stimulation, the evoked cortical potential began at 22 msec whereas P T activity began at 12 msec (Fig. 3, A). Again, P T activity lasted longer than the evoked cortical potential. The evoked cortical response to I F P shock sometimes displayed two separate positive potentials (Fig. 3, upper right). In this instance, the early small positive wave began at 22 msec and the late large positive wave at 42 msec, corresponding in time to two waves of P T activity. When the late positive cortical potential was absent (Fig. 3, lower right), the early positive potential of the cortex and the early P T wave were still noted. Apparently the early cortical potential was consequent to activation by callosal impulses originating in contralateral somatosensory cortex after ipsilateral cutaneous stimulation (14). Depth Distribution of Responsive Pericruciate Nem'ons. Responsive pericruciate neurons tended to be located in the lower one-half of the cortex and their distribution displayed three distinctive peaks at depths of 900, 20-

[ ] Responsive PT cells (66) [ ] Non-responsive PT cells (16)

16=

8 F2-

4I'-'l

Kk'~l

,,;,,,,,!,,,,,,,,,,,, 0

0

0.8

1.2

I6

2.0

2.4-

28

3.2

5.6

4.0

Antidromic response latency (msec)

FIG. 2. Antidromic response latencies of PT neurons. Note that the majority of responsive PT neurons have antidromic response latencies shorter than 1 msec.

358

CHU AND RUTLEDGE

Imv Imv

FIG. 3. PT and cortical responses to sensory activation. Upper trace ~ evoked cortical responses. Lower trace = PT responses recorded at ipsilateral cerebral peduncle. P = photic stimulation. A = auditory stimulation. IFP = ipsilateral forepaw stimulation. Positivity down. Total trace is 200 msec. Stimulation was applied at the onset of the trace. 1400, and 1700 ~. Nonresponsive pericruciate neurons were, however, more concentrated in the upper one-half of the cortex. Discussion

It is clear from the present study that many pericruciate neurons receive not only somatosensory but also photic and auditory inputs, and that P T neurons have a greater degree of multisensory convergence than do nonP T neurons. Many nmltisensory n o n - P T neurons are probably also corticofugal because about two-thirds of responsive n o n - P T cells were located in the lower one-half of the cortex. Layers V and VI, are the main sources of corticofugal fibers (12). The percentages of either P T or n o n - P T neurons having multisensory convergence in the present study are lower than those reported by Buser and Imbert (4) using chloralose anesthesia and observing that 92% of pericruciate neurons tested was "polysensory." In the present study, about 70~'o of P T neurons and 40% of n o n - P T neurons responded to more than two sensory modalities. Our data are similar to those obtained under curare (4). Variability in neuronal responses under chloralose probably arises from different criteria used for determining cellular evokability and from differences in depth of anesthesia. In the study by Buser and Imbert (4), deep chloralose anesthesia (90 m g / k g ) was used, whereas moderate anesthesia (40-50 m g / k g ) was maintained in our work. Dubner and Rutledge (7) found that high dosages of chloralose would significantly increase the responsiveness of cells in suprasylvian cortex and that at moderate dosages

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(35-50 m g / k g ) , the polysensory cells made up approximately the same proportion of the total population as in the unanesthetized state. It has been shown that mid- to deep-lying neurons with bilateral receptive fields contribute negligibly to the surface-evoked potentials in the pericruciate gyrus (9, 21). In the present study, the earliest evoked P T activity was also not reflected in cortical surface responses. Asynchronous afferent activation and the small number of P T cells active at that early time may be some of the responsible factors. Towe et al. (23, 24) noted that the "wide-field" pericruciate neurons tended to be located in the lower one-half of the cortex and that about 80% of P T neurons belonged to this category. The percentage of P T and non-PT neurons responding to C F P or I F P stimulation (or both) is quantitatively similar to those reported by Towe, Patton, and Kennedy (23). Since many P T neurons which responded to C F P or I F P inputs (or both) also responded to photic or auditory inputs (or both), it is reasonable to suspect that many wide-field P T neurons are multisensory. Deep P T neurons are sensitive to acetylcholine and respond with repetitive firing to iontophoretically applied or synaptically released acetylcholine (6, 11). Since an increased acetylcholine release from the cortical surface was found in arousal produced by stimulation of afferent pathways (5) or the midbrain reticular formation (15, 17, 19), an ascending cholinergic system mediating cortical arousal has been proposed (16). The extensive sensory convergence on acetylcholine-sensitive deep P T neurons as found in the present study may have some significance for the proposed cholinergic arousal mechanism. The earlier and preferential activation of P T neurons by various sensory modalities has a unique functional advantage when the spinal stage of pyrmnidal activation is considered. The P T fibers descending from the pericruciate cortex terminate mainly in the external basilar region ( E B R ) of the dorsal horn (13). The E B R neurons require repetitive volleys in the fast P T fibers to produce monosynaptic E P S P s (10). During sensory activation of the motorsensory cortex, the ability of fast P T neurons to conduct impulses rapidly, to generate more spikes per unit time and their richness of sensory convergence make them ideal candidates to mobilize a "ready state to act" in the spinal cord. References 1. ADRIAN, E. D., and G. MORUZZL1939. Impulses in the pyramidal tract. J. Physiol. London 97 : 153-199.

2. BROOKS,V. B., P. RUDO~IN, and D. L. SLAYa~AN. 1961. Sensory activation of neurons in the cat's cerebral cortex. J. Neurophysiol. 24 : 286-301. 3. BUS~R,P., and K. E. BIGNALL.1968. Non primary sensory projections on the cat neocortex. Int. Rev. Neurobiol. 1O: 111-165.

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4. BUSER, P., and M. I~BERT. 1961. Sensory projections to the motor cortex in cats: A microeleetrode study, pp. 607-626. In "Sensory Communication," W. A. Rosenblith [ed.]. Wiley, New York. 5. COLLIER, B., and J. F. MITC~IELL. 1966. The central release of acetylcholine during stimulation of the visual pathway. J. Physiol. London 184 : 239-254. 6. CRAWrORD, J. M., and D. R. CURTIS. 1966. Pharmacological studies on feline Betz cells. J. Physiol. London 186 : 121-138. 7. DUBBER, R., and L. T. Rr:TLEOGE. 1964. Recording and analysis of converging input upon neurons in cat association cortex. J. Neurophysiol. 9.7 : 620-637. 8. JABBER, S. J., and A. L. TOWE. 1961. Analysis of the antidromic cortical response ~ollowing stimulation at the medullary pyramids. J. Physiol. London 155: 148-160. 9. KENNEDY, T. T., A. L. TowE, and H. D. PATTOI'r. 1960. Contribution of pyramidal tract ( P T ) neurons to surface primary evoked response in the cat. Physiolocqist 3 : 93. 10. KOSTVL'K, P. G., and D. A. "VASILENKO. 1968. Transformation of cortical motor signals in spinal cord. Proc. IEEE 56: 1049--1058. 11. KRNJEVI6, K., and J. W. PHILLIS. 1963. Acetylcholine-sensitive cells in the cerebral cortex. J. Physiol. Lo~uton 166 : 296-327. 12. LORW~,-TEDE N6, R. 1943. Cerebral cortex: Architecture, intracortical connections, motor projections, pp. 274-301. In "Physiology of the Nervous System," J. F. Fulton [ed.]. Oxford Univ. Press, New York. 13. NVl3ERG-HANSEN, R., and A. ]3RODAL. 1963. Sites of termination of corticospinal fibers in the cat. An experimental study with silver impregnation methods. J. Co;np. Neurol. 19.0 : 369-391. 14. PATTON, H. D., A. L. TowF, and T. T. K~NrZEI~V. 1962. Activation of pyramidal tract neurons by ipsilateral cutaneous stimuli. J. Neurophysiol. 25 : 501-514. 15. PI~ILLIS, J. W. 1968. Acetylcholine release from the cerebral cortex: Its role in cortical arousal. Brain Res. 7 : 378-389. 16. St~UTE, C. C. D., and P. R. LEwis. 1967. The ascending cholinergic reticular system: Neocortical, olfactory and subcortical projections. Brain 90: 497-519. 17. SIE, G., H. H. JASPER, and L. WOLFE. 1965. Role of A C h release from cortical surface in encephaIe and cervean isole cat preparations in relation to arousal and epileptic activation of ECoG. Electroencephalogr. Clin. Neurophysiol. 18: 206. 18. SOKOLOW, A. A., and J. D. LIPENETSKAYA. 1967--1968. Microelectrode investigation of motor cortex in unanesthetized rabbits. Neurosci. Transl. No. 1, pp.

57-65. 19. SZERB, J. C. 1967. Cortical acetylcholine release and electroencephalographic arousal. J. Physiol. London 192: 329-343. 20. TAI~A~tASHI, K. 1965. Slow and fast groups of pyramidal tract cells and their respective membrane properties. J. Neurophysiol. 9.8 : 908-924. 21. TowF, A. L. 1966. On the nature of the primary evoked response. Exp. Neurol. 15 : 113-139. 22. TOWE, A. L., H. D. PATTON, and T. T. KENNEDY. 1963. Properties of the pyramidal system in the eat. Exp. Neurol. 8 : 220-238.

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23. TOWE, A. L., H. D. PATTON, and T. T. KENNEDY. 1964. Response properties o{ neurons in the pericruciate cortex of the cat following electrical stimulation o{ the appendages. Exp. Neurol. 10 : 325-344. 24. TowE, A. L., D. WaITEHORN, and J. K. NYQUIST. 1968. Differential activity among wide-field neurons of the cat postcruciate cerebral cortex. Exp. Neurol. 20 : 497-521. 25. WELT, CAROL, J. C. ASCHOFF, K. KAMEDA, and V. ]3. BROOKS. 1967. Intracortical organization of cat's motorsensory neurons, pp. 255-293. In "Neurophysiological Basis of Normal and Abnormal Motor Activities," M. D. Yahr and D. P. Purpura [eds.]. Raven Press, New York.