The function of sensory information from the first somatosensory cortex for facial movements during ingestion in cats

Cortical control of appropriate tongue protrusion during licking in cats--increase in regional cerebral blood flow (rCBF) of the contralateral area P and in tongue protrusion after the unilateral area P lesion

Adequate tongue protrusion may be regulated by cat bilateral area P (the motor cortex for jaw and tongue movements) (Hiraba and Sato, Somatosens Mot Res 2005b;22:183-192). The ICMS (intracortical microstimulation) in the unilateral area P evoked motor effects of tongue protrusion without deviation (Hiraba and Sato, Somatosens Mot Res 2004;23:1-12), and cats with the unilateral lesion of area P showed abnormal tongue protrusion without deviation during licking (Hiraba and Sato, Somatosens Mot Res 2005b;22:183-192). Further, the measurements of the regional cerebral blood flow (rCBF) in the bilateral jaw and tongue motor cortical areas were shown to have the same activity rate during the lateral licking (Hiraba and Sato, Somatosens Mot Res 2005c;22:307-317). We assumed from these results that cortical control for tongue protrusion was executed by networks between the bilateral area P including inhibitory interneurons. We prepared the measurable cats of the rCBF in the contralateral side after the unilateral area P lesion. Changes in the rates of rCBF and tongue protrusion during licking were examined over a long time course of about 1-2 months after the unilateral area P lesion. All cats after the unilateral area P lesion showed increased rate (double or triple in comparison with the normal ones) of rCBF of the contralateral area P in the early (0-20 days) phase. On the other hand, increased rates of tongue protrusion were about 120% in the early phase, and about 180% in the middle (21-35 days) and late (36-last days) phases. The results support the organization of networks between bilateral area P including the inhibitory interneurons.

Cortical control for mastication in cats: Changes in masticatory movements following lesions in the masticatory cortex

In a previous paper (Hiraba and Sato 2004) we reported that an accurate mastication might be executed by the cortical processing in bilateral masticatory area (MA)and motor cortices. The aim of this study was to determine if cats with lesion of either unilateral or bilateral MA showed changes in mastication. After exploring mechanoreceptive fields and motor effects of mastication-related neurons (MRNs) in MA using the single unit recording and intracortical microstimulation methods, we made various lesions in MAs with injections of kainic acid (0.1%, 2.0 microl). Since the MA was divided into facial (F) and intraoral (I) projection areas as reported in the previous paper, cats with the unilateral lesion in F or I, and with the bilateral lesion in F and F, I and I or F and I (F on one side and I on other side) were prepared. Cats with unilateral lesion in F or I and with bilateral lesion in F and I showed no changes in mastication except for prolongation of the food intake and masticatory periods. Cats with bilateral lesion into F and F, or I and I showed wider jaw-opening during mastication. Particularly, the latter group showed enormous jaw-opening, delay in the start of mastication and difficulty in manipulating food on the tongue. In all cats with lesions of each type, masticatory and swallowing rhythms remained normal. These findings suggest that accurate mastication is executed by the close integration between F and F and I and I of the bilateral MA.

Cerebral control of face, jaw, and tongue movements in awake cats: Changes in regional cerebral blood flow during lateral feeding

Mastication is achieved by cooperation among facial, masticatory, and lingual muscles. However, cortical control in cats for the masticatory performance is processed by two systems: facial movement processed by facial SI (the first somatosensory cortex), area C, and area M (motor areas), and jaw and tongue movements performed by intraoral SI, masticatory area, and area P (motor area). In particular, outputs from area P organized in the corticobulbar tract are projected bilaterally in the brainstem. In this present study, the aim is to explore changes in the regional cerebral blood flow (rCBF) in the facial SI, area M, and area P during trained lateral feeding (licking or chewing from the right or left side) of milk, fish paste, and small dry fish. The rCBF in area M showed contralateral dominance, and rCBF in area P during chewing or licking from the right or left side was almost the same value. Furthermore, activities of genioglossus and masseter muscles in the left side showed almost the same values during licking of milk and of fish paste, and chewing of small dry fish during lateral feeding. These findings suggest that the cortical process for facial, jaw, and tongue movements may be regulated by the contralateral dominance of area M and the bilateral one of area P.

Organization of cortical processing for facial movements during licking in cats

We proposed that cortical organization for the execution of adequate licking in cats was processed under the control of two kinds of affiliated groups for face and jaw & tongue movements (Hiraba H, Sato T. 2005A. Cerebral control of face, jaw, and tongue movements in awake cats: Changes in regional cerebral blood flow during lateral feeding Somatosens Mot Res 22:307-317). We assumed the cortical organization for face movements from changes in MRN (mastication-related neuron) activities recorded at area M (motor cortex) and orofacial behaviors after the lesion in the facial SI (facial region in the primary somatosensory cortex). Although we showed the relationship between facial SI (area 3b) and area M (area 4delta), the property of area C (area 3a) was not fully described. The aim of this present study is to investigate the functional role of area C (the anterior part of the coronal sulcus) that transfers somatosensory information in facial SI to area M, as shown in a previous paper (Hiraba H. 2004. The function of sensory information from the first somatosensory cortex for facial movements during ingestion in cats Somatosens Mot Res 21:87-97). We examined the properties of MRNs in area C and changes in orofacial behaviors after the area C or area M lesion. MRNs in area C had in common RFs in the lingual, perioral, and mandibular parts, and activity patterns of MRNs showed both post- and pre-movement types. Furthermore, cats with the area C lesion showed similar disorders to cats with the area M lesion, such as the dropping of food from the contralateral mouth, prolongation of the period of ingestion and mastication, and so on. From these results, we believe firmly the organization of unilateral cortical processing in facial SI, area C, and area M for face movements during licking.

Cortical control of mastication in cats. 2. Deficits of masticatory movements following a lesion in the motor cortex

Our previous study suggested that area P in the lateral wall of the presylvian sulcus and MA (masticatory cortex) in the rostral part of the orbital gyrus play an important role in execution of mastication. The aim of this present study is to examine if changes in orofacial behaviors and masticatory movements occur in cats with lesions of area P. First, we explored the locations in area P through the use of single unit recording and ICMS (intracortical microstimulation). Since mastication related neurons (MRNs) with the mechanical receptive field (RF) in facial or intraoral region were intermingled in area P, we performed either a partial or entire lesion in area P by injections of 2 microl or 4 microl of 0.1% kainic acid. Cats with the entire lesion in area P showed a decrease of food intake rates associated with abnormal tongue protrusion and wide jaw-opening, fluctuation of masticatory start, and prolongation of masticatory and food intake periods. Abnormal movements of tongue and jaw did not recover, although their prolongation and fluctuation returned to normal levels in one month. On the other hand, all deficits evoked by cats with the partial lesion recovered in about one month. In cats with the partial and entire lesions, masticatory rhythm remained normal. These findings suggest that area P may regulate accurate and suitable tongue and jaw movements during mastication depending on cortical processing.

Coordination of cranial motoneurons during mastication

Mastication is the first stage of digestion and involves several motor processes such as food intake, intra-oral food transport, bolus formation and chewing in its broad sense. These complicated motor functions can be accomplished by the well-coordinated activities in various cranial motoneurons innervating the jaw, hyoid, tongue and facial muscles. The brainstem masticatory central pattern generator (CPG) plays a crucial role in generating basic activity patterns of these cranial motoneuron groups. However, descending inputs from higher brain (e.g., cerebral cortex) and mastication-generated peripheral sensory inputs also play important roles in modulating the activity pattern of each motoneuron so that the final motor outputs fit the environmental demand. In this review, we focus on the coordination of the trigeminal, facial and hypoglossal motoneurons during mastication. We first summarize findings showing the activity patterns of muscles innervated by these motoneurons during natural mastication, and then discuss the possible neural mechanisms underlying their coordinated activities during mastication.