in some kinds of rhythmic jaw movements, we can then clearly identify patterns of both excitation and inhibition which these motor neurons experience during that particular behavior.

If we can then derive those particular behaviors from other parts of the nervous system, for example from cortical stimulation, we then have a chance to move from the motor neuron back step by step into the nervous system, into the premotor cells, and to try to find those cells which are influencing the motor neurons involved in a particular behavior. Are the interneurons or premotor cells for speech the same as for mastication, for example? It's an operational approach without a concept in a sense. However, within the methodology you have using a behaving or semibehaving animal, you have the opportunity to ask, "Is it significant to that cell?" and then, "Does this significance reflect itself into the behavior?"

I think as we move along, concepts will develop that may be different from the "non-concepts" we now have, in a sense.

DR. STELMACH: I share the view that as our ideas of heterarchies and multiple feedback notions become increasingly popular, as we tone down the time reference in terms of how it is used, we'll view the data we're collecting a little differently. That is, we almost have become descriptive, but at a multiple recording technique so that when we put the pieces together we can note these heterarchies. Implicit are questions such as, do we really want to record at the single neuronal level or to work at the reflex level. It seems to me we gain some information, but on the other hand, we need a more descriptive outlook now as we look at the plasticity of systems. We've been caught up with the reductionistic techniques we borrowed from the physical sciences and I make the plea for us to step back, as Lou (Goldberg) was suggesting, and be more cognizant of what's happening --how ideas are changing, but our experiments really aren't.

DR. HANNAM: This is the sort of feeling one gets looking at the literature, surveying all the work on reflexes. I don't mean to polarize the group; I'm not saying that reflexes don't exist, or they're not important. Obviously reflexes matter. But when you go through the literature and try to put it all together, it's inescapable that what's developed since Sherrington is to look at very simple reflex phenomena. It's terrible to bias our thinking that this is the basic structure on which everything has to be built. It has clouded our thinking. Both of the recent speakers were driving at that. Perhaps what we're looking at is a set of motor neurons in the fifth nucleus which is receiving a great many inputs from a variety of causes. And depending on the contraints operating at that point in time, that's what you're going to see. Some of these reflexes under functional conditions may behave utterly differently. So our concept of the jaw reflex, for example, as being a constant substrate superimposed on something else probably isn't true. I think I'm very much in agreement with the last speakers.

Causes and Effects of Hyperactivity of Jaw Muscles

R. Yemm

For a number of years there has been an increasing tendency to attribute several oral conditions to abnormal and hyperactivity of the jaw muscles. The present paper will attempt to review evidence for this hypothesis.

To this end, it is proposed to review first the current concepts of the role of the muscles when the mouth is at rest. Second, the ways in which it is thought that muscle activity can occur other than required for normal function will be examined, followed by an assessment of the evidence for the mechanisms whereby this occurs. Third, an evaluation will be made of the evidence that abnormality of muscle activity is indeed an etiological factor in recognized clinical conditions such as periodontal disease, mandibular dysfunction and persistent mucous membrane soreness underlying dentures. Finally it is intended to examine the possibility that identifiable individuals are more susceptible to the clinical conditions where there seems to be valid evidence of abnormal muscle activity as an etiological factor.

Muscle Function In The Resting State

Chewing, speech, and swallowing occupy only a very small proportion of the time. (Brewer, 1963). The state of the muscles at other times constitutes the baseline for any consideration of levels of muscle activity. As such, the nature of the neuromuscular mechanisms when the muscles are "at rest" is clearly of importance.

The existence of a resting, or postural position of the mandible, in association with which there is a freeway space (inter-occlusal clearance) between the upper and lower teeth, was recognized a number of years ago, together with the controlling function of the masticatory muscles. When the electromyographic (EMG) technique became available, it was recognized that when the mandible is in the postural position the jaw muscles are relatively inactive.

Some years ago it was thought that all skeletal muscles exhibited continuous activity (muscle tone). This view was derived, it seems, from observations of decerebrate animals (decerebrate rigidity). This concept, applied to the jaw muscles and supported by the observation of low and variable levels of activity in some human jaw muscles in apparently relaxed human subjects led to the widely held hypothesis that the jaw-closing muscles are tonically active and responsible for maintenance of the mandibular postural position. By analogy with the situation in decerebrate rigidity, the muscle spindles of these muscles are considered to provide the sensory element in a feedback system

responsible for initiating muscle activity to counteract displacing influences upon the mandibular posture.

In more recent years, work on other muscles of the body has led to the alternative, and now generally accepted hypothesis that complete inactivity is often a characteristic of skeletal muscles not in use. Reexamination of the mandibular postural position in this light suggests that a similar mechanism may apply to the jaw-closing muscles. The posture does not show a constancy and reproducibility such as might be expected from the earlier hypothesis, it having been recognized that there is a range of position rather than a single unchanging posture. (Fish, 1961; Nairn and Cuttress, 1967; Christensen, 1970; Griffiths, 1975).

Human and animal experiments have shown that elastic properties of muscle and other tissue can account for stability of relaxed posture (Clemmesen, 1951; Yemm, 1969a; Yemm and Nordstrom, 1974). Furthermore, examination of jaw muscles for motor units exhibiting tonic activity in relaxed subjects has failed to reveal such units (Yemm, 1977), as have similar searches in other muscles (Hoefer, 1941; Ralston and Libet, 1953; Joseph, Nightingale, and Williams, 1955). According to the original hypothesis of antigravity activity of the jaw-closing muscles, it would be expected that the stretch reflex system would be sensitive to the stimulus of a brief stretch by a chin tap in the relaxed subject. Experiments have shown that this is usually not the case for masseter and temporal muscles, except with large stimuli (Lewis, Pilcher, and Yemm, 1978). Once again this finding is similar to that for other muscles of the body (e.g. ankle jerk response, Bierman and Ralston, 1965).

It seems, therefore, more reasonable to regard the baseline of jaw muscle activity to be the fully relaxed state. Occurrence of activity could be caused by an increase in excitatory inputs, ultimately to the alpha motor neurons, derived either from peripheral sensory mechanisms of an excitatory nature, or of central origin. The postural position is thus to be regarded as a fundamental position, subject to variation as a consequence of:

(1)

changes in the elastic properties of the associated
tissues and their equilibrium

(2)

variation in external factors such as gravity

(3)

variations in the excitability of the neuromuscular
system.

The extent to which, during the day, the jaw muscles are active in modifying mandibular posture remains to be defined. It seems certain that this varies greatly among individuals. Some may employ muscle activity much of the time in the form of habitual activity. This will be discussed in more detail in a later section.

Muscle Hyperactivity

The preceding section shows evidence that muscle activity when the mouth is ostensibly at rest is not precisely controlled, nor possessed of a rigid postural role. The muscles do not respond only to prevent or minimize postural change.

It is generally accepted that the jaw muscles can exhibit increases in activity of a nonfunctional nature, the most easily recognized being, perhaps, that associated with bruxism. Any such tendency of a subject to develop hyperactivity not associated with a recognized oral function must result from an effective excitation of the neuromuscular system.

There are two possible sources of an excitatory stimulus. The first is a primary stimulation of receptors in or around the mouth, resulting in a reflex activation of the jaw-closing muscles. The second is that the excitation might originate more centrally in the nervous system.

Reflex effects of oral receptor stimulation

A number of articles have, in the past, speculated upon the role of oral receptors and the possibility of their providing the afferent input to initiate prolonged reflex increases in jaw muscle activity. At present, it seems that experimental evidence is against this possibility. The following section will summarize the research conclusions with regard to the reflex effects elicited, in animals and in man, by stimulation of the following receptors:

In animals and in man the predominant reflex effect of stimulation of these receptors appears to be to inhibit jaw closure, and in animals at least to exert an excitatory affect upon jaw opening.

In animals, principally the cat, this is shown by observations of appropriate inhibitory and excitatory effects on motor neurons (e.g. Kidokoro, Kubota, Shuto, and Sumino, 1968) and also by changes in the electrical activity of the muscles, or by actual jaw movements (Hannam and Matthews, 1969).

Although the principle effect is of stimulating jaw opening, there are some reports of an excitatory pathway to the jaw closing muscles.

Until recently these have been limited to observations on decerebrate animals, where the effects are attributable to hypersensitivity of muscle spindles to mechanical stimuli applied to teeth (Harrison and Corbin, 1942), when the closing responses are similar to the "jar" reflex first observed by Sherrington (1898). An early exception (Sherrington, 1906) was the reported jaw-closing response to tooth tapping in animals given strychnine or tetanus toxoid. recently several papers describe excitation of jaw-closing muscles following tooth stimulation either preceding, (Goldberg, 1976; Sessle, 1977) or following (Funakoshi and Amano, 1974) the major inhibitory influence.

In man,

More

mechanical stimulation of single teeth has been shown to result in a silent period in ongoing jaw-closing muscle activity. The interpretation of this observation is complicated by a number of ancillary observations. First, there is no consensus of whether this response survives the administration of local anesthetic to the teeth. Second, some studies have shown the silent period to be preceded by evidence of synchronization or excitation of the motor units of the closing muscles immediately prior to the silent period. This, together with the survival of the response under local anesthesia would be consistent with the jaw-reflex mechanism (Hannam, Matthews and Yemm, 1970). Abolition of the response by local anesthesia (Sessle and Schmidt, 1972) or evidence of a similar sequence of excitation and inhibition with electrical stimulation (Goldberg, 1971) would be consistent with the existence of a reflex excitatory pathway.

The observation of a jaw-opening reflex in animals has not been paralleled in man. Attempts have been made under what would be expected to be the most favorable to elicit such a response, including application of stimuli when the opening muscles are already active.

In summary to the evidence of the effects of periodontal ligament mechanoreceptor stimulation, it seems that, at the reflex level, there are no grounds for supposing that their afferent input could be the primary initiating factor in prolonged and forceful contraction of the jaw-closing muscles.

Any reversal of the dominance of the jaw-opening reflex effects over the apparent jaw closing effects (very short or very long latency) would require a central effect upon neuron excitability, rather than being a "local" effect (Storey, 1979).

(2) Muscle spindles

Muscle spindles are present in numbers in the jaw-closing muscles, but are few or absent in the jaw-openers. Although closing muscle spindle afferent discharge is an excitatory input to the muscles, it is difficult to see how peripheral stimulation could lead to maintained activity without concurrent activation of the gamma motor system from higher centers. In the absence of the latter, a brief stretch (chintap) initiates the familiar synchronized jaw-jerk response, or vibrational