Differences in the excitability of two populations of trigeminal primary afferent central terminals

Mastication-induced modulation of the jaw-opening reflex during different periods of mastication in awake rabbits

The present study aimed to determine if sensory inputs from the intraoral mechanoreceptors similarly contributed to regulating the activity of the jaw-opening muscles throughout the masticatory sequence. We also aimed to determine if sensory inputs from the chewing and non-chewing sides equally regulated the activity of the jaw-opening muscles. Electromyographic (EMG) activities of jaw muscles (digastric and masseter) and jaw movements were recorded in awake rabbits. The entire masticatory sequence was divided into preparatory, rhythmic-chewing and preswallow periods, based on jaw muscles activity and jaw movements. The jaw-opening reflex (JOR) was evoked by unilateral low-intensity stimulation of the inferior alveolar nerve (IAN) on either the chewing or non-chewing side. Amplitude of the JOR was assessed by measuring peak-to-peak EMG activity in the digastric muscle, and was compared among the masticatory periods and between the chewing and non-chewing sides. The JOR was strongly suppressed during the jaw-closing phase in the rhythmic-chewing and preswallow periods, but this effect was transiently attenuated during the late part of the jaw-opening phase in these periods. However, modulation of the JOR varied from strong suppression to weak facilitation during the preparatory period. The patterns of JOR modulation were similar on the chewing and non-chewing sides in all masticatory periods. The results suggest that the sensory inputs from the intraoral mechanoreceptors regulate the activity of the jaw-opening muscles differently during the preparatory period compared with the other masticatory periods. Sensory inputs from both the chewing and non-chewing sides similarly regulate the activity of the jaw-opening muscles.

An explanation for reflex blink hyperexcitability in Parkinson's disease. II. Nucleus raphe magnus

Hyperexcitable reflex blinks are a cardinal sign of Parkinson's disease. The first step in the circuit linking the basal ganglia and brainstem reflex blink circuits is the inhibitory nigrostriatal pathway (Basso et al., 1996). The current study reports the circuits linking the superior colliculus (SC) to trigeminal reflex blink circuits. Microstimulation of the deep layers of the SC suppresses subsequent reflex blinks at a latency of 5.4 msec. This microstimulation does not activate periaqueductal gray antinociceptive circuits. The brainstem structure linking SC to reflex blink circuits must suppress reflex blinks at a shorter latency than the SC and produce the same effect on reflex blink circuits as SC stimulation, and removal of the structure must block SC modulation of reflex blinks. Only the nucleus raphe magnus (NRM) meets these requirements. NRM microstimulation suppresses reflex blinks with a latency of 4.4 msec. Like SC stimulation, NRM microstimulation reduces the responsiveness of the spinal trigeminal nucleus. Finally, blocking the receptors for the NRM transmitter serotonin eliminates SC modulation of reflex blinks, and muscimol inactivation of the NRM transiently prevents SC modulation of reflex blinks. Thus, the circuit through which the basal ganglia modulates reflex blinking is (1) the substantia nigra pars reticulata inhibits SC neurons, (2) the SC excites tonically active NRM neurons, and (3) NRM neurons inhibit spinal trigeminal neurons involved in reflex blink circuits.

Central connections of trigeminal primary afferent neurons: topographical and functional considerations

This article reviews literature relating to the central projection of primary afferent neurons of the trigeminal nerve. After a brief description of the major nuclei associated with the trigeminal nerve, the presentation reviews several early issues related to theories of trigeminal organization including modality and somatotopic representation. Recent studies directed toward further definition of central projection patterns of single nerve branches or nerves supplying specific oral and facial tissues are considered together with data from intraaxonal and intracellular studies that define the projection patterns of single fibers. A presentation of recent immunocytochemical data related to primary afferent fibers is described. Finally, several insights that recent studies shed on early theories of trigeminal input are assessed.