The action of alpha-beta-dihydroxy-gamma-(2-methylphenoxy)-propane (myanesin) on the spinal cord of the cat

Recurrent facilitation of spinal reflexes

Antidromic volleys in muscle nerves may facilitate monosynaptic reflexes originating from neighboring motoneurons. This facilitation has been studied in spinal cats. It is at its peak with a conditioning-test interval of 20 to 30 msec., and can last 50 to 100 msec. The threshold of facilitation is about the same as that of recurrent inhibition. Both phenomena appear to be activated by stimulation of the large motor axons. The latency of facilitation seems to be longer than that of recurrent inhibition by approximately 1 msec., suggesting the presence of at least one more synaptic delay. Facilitation often follows an inhibition of variable depth and duration. Frequently, however, the facilitation is not preceded by inhibition, and therefore it cannot be a rebound effect. The pharmacological properties of facilitation resemble those of recurrent inhibition. Dihydro-beta-erythroidine(1) partially blocks facilitation; the peak is decreased and occurs earlier, and the duration is shortened. Eserine increases the duration of facilitation and inhibition and sometimes enhances their magnitude. It is concluded that recurrent facilitation is mediated by the cholinergic axon collaterals, and that at least two interneurons are located between collateral and motoneuron. Possible mechanisms of facilitation are discussed.

Responses of motoneurons undergoing chromatolysis

The delayed and asynchronous firing of chromatolytic motoneurons in response to group I afferent volleys is shown to be evoked monosynaptically, there being an abnormally long and variable delay between onset of monosynaptic action and generation of impulse discharge. Intensity of monosynaptic excitatory action is reduced, and considerable variability in the form of successively evoked postsynaptic potentials is often observed. No evidence has been found for the development of excitatory group I polysynaptic pathways. Reduction in responsiveness of finer dendrites is indicated by the feeble "d" response evoked by an antidromic volley in a chromatolytic motor nucleus. Antidromic impulses appear to invade the cell bodies and coarse dendrites, but die out at points short of the normal extent of dendritic invasion. Vigorous firing of Renshaw cells can be elicited by antidromic volleys. Chromatolytic motoneurons appear to maintain reasonably normal resting membrane potentials, but are more susceptible to damage than are normal cells. Action potentials are large and usually overshoot the resting potential level. Post spike potentials are similar to those of normal cells except for a less prominent, or absent, early phase of depolarisation. In contrast with the reduced responsiveness of peripheral dendrites, there is a lowered threshold for antidromic and segmental reflex synaptic activation of the more central regions, probably the cell bodies and nearby coarse dendrites, of motoneurons undergoing chromatolysis.

Action of meprobamate on spinal monosynaptic reflexes and on inhibitory pathways

Meprobamate was administered intravenously to spinal cats, in cumulative doses of 30 to 40 mg./kg. each. Initial doses may have a variable action on monosynaptic reflexes. At times some reflexes are depressed, while others are enhanced or unaffected. When dose levels of 100 mg./kg. or higher are reached, monosynaptic reflexes, both flexor and extensor, are depressed. Monosynaptic reflexes can be strongly depressed by meprobamate, their input-output relations often remaining unchanged. In such cases there is thus no change in the spatial summation requirements of those motoneurons remaining in the excitable zone. Inhibitory pathways, both direct and disynaptic, are highly resistant to the action of meprobamate. The drug does not distinguish between the direct and disynaptic pathways. It is suggested that meprobamate acts as a general depressant of excitatory synaptic transmission.

The action of local anaesthetics on the spinal cord of the cat

The effects of intravenously injected lignocaine, procaine and amethocaine on the monosynaptic and polysynaptic reflexes and the cord dorsum potentials elicited by selective electrical stimulation of appropriate afferent fibres have been investigated in decerebrate cats before and after strychnine. The three drugs depress the amplitude of the reflex responses in the basal preparation for 40 to 50 min. Smaller doses eliminate the reflex augmentation produced by strychnine for comparable periods. Very small doses selectively abolish the augmentation of the P component of the cord dorsum response induced by strychnine. The drugs have no effect on post-tetanic potentiation of the monosynaptic reflex response evoked from the gastrocnemius muscle in the cat. Their activity on the spinal cord is roughly proportional to their local anaesthetic potency. The ratios of the doses which produce the same central activity are amethocaine 3: lignocaine 10: procaine 20.

The pharmacology of 2-amino-4-methyl-6-phenyl-amino-1,3,5-triazine, a centrally acting muscle relaxant

2-Amino-4-methyl-6-phenylamino-1, 3, 5-triazine (CB.2487) was found to resemble mephenesin both qualitatively and quantitatively in its actions in mice, and could be distinguished from meprobamate and pentobarbitone. In this species CB.2487 antagonised strychnine competitively but had no effect against leptazol or nicotine. In the anaesthetised cat, the most pronounced action of CB.2487 was to block multi-synaptic crossed extensor and flexor reflexes in doses which did not suppress the mono-synaptic patellar reflex. The compound also depressed selectively the crossed extensor reflex and not the patellar reflex in the spinal animal. CB.2487 had no effect on the myoneural junction. The results indicate that CB.2487 is a centrally acting muscle relaxant. Unfortunately the drug was poorly active by mouth and had a short duration of action. It is considered to have no advantage over mephenesin.

Temporal fluctuation in excitability of spinal motoneurons and its influence on monosynaptic reflex response

OBSERVATIONS ON TEMPORAL VARIATION IN MONOSYNAPTIC REFLEX RESPONSE IN THE ACUTELY DECAPITATE CAT INDICATE THE FOLLOWING: 1. Frequency distribution of response amplitude has a nearly normal form often with some degree of negative skewness. Response variation differs only moderately in form and magnitude from one preparation to another. 2. Temporal variation remains essentially constant at different levels of drive above that level required to complete the zone of variation. 3. The role of response variation in the determination of mean response amplitude is considered. 4. One of the major sources of excitability fluctuation in the "resting" cord is variation in background activity of interneurons.