Topographic studies on medial reticular nucleus

Abstract

Several distinct classes of neurons have been identified in the medial reticular nucleus of the medulla and pons and in proximity thereto. Neurons projecting down the spinal cord comprised the principal class with two subclasses according as the neurons did or did not receive monosynaptic inputs from the fastigial nuclei of the cerebellum. Two other classes were recognized accordings as they projected to the cerebellum or rostrally to the mesencephalon. Topographic planar maps giving the location of these neurons have been constructed by exploring the nucleus with series of microelectrode tracks in parasagittal or in transverse planes. The different classes of neurons were not arranged in large discrete nuclei. In part they appeared to be randomly distributed, but many colonies of one or another class of neurons could be recognized with 3-11 neurons in zones with dimensions of a millimeter or so. Because of the limitations of sampling by microelectrode tracks at spacings of 0.5 mm, single colonies might have an actual population of 100 or more. Many of the class of neurons projecting to the cerebellum were in the region of the perihypoglossal nucleus. However, almost as many were located deep in the medial reticular nucleus. None was found at the pontine level. Reticulospinal neurons with fast axonal conduction velocities tended to be located dorsally to those with slow velocities. Correlation with the findings of Ito et al. leads to the conjecture that the neurons with fast axons are excitatory, while those with slow axons are primary inhibitory neurons. There is a brief reference to the problems raised by the admixture of the various neuronal classes, there being discrete colonies immersed in a scattered arrangement of all classes.

Is there a brainstem substrate for action selection?

“…On the output side, Peterson (1979) proposed a crude topography of the reticulospinal projections, based on the combinations of elicited responses in motoneurons related to the neck, back, forelimb and hindlimb. However, other studies of this system found no anatomical topography of the spinal projections ( Torvik & Brodal 1957;Eccles et al 1976), and neurons responding during movement of those body parts seemed randomly intermingled (Siegel & Tomaszewski 1983).…”

“…A substrate for action selection should also be able to express the outcome of the selection competition. The majority of neurons in the mRF project extensively to all levels of the spinal cord and to the cranial nerves (Torvik & Brodal 1957;Eccles et al 1976;Jones 1995). Axons of individual reticulospinal neurons can contact multiple spinal levels on both sides of the spinal cord (Peterson 1979).…”

“…How then might the mRF neurons be functionally organized? They are not topographically organized to match patterns of sensory input; despite numerous attempts, no topographical projections to the mRF have ever been convincingly demonstrated (Segundo et al 1967;Bowsher 1970;Eccles et al 1976). Groves et al (1973) reported that tactile stimuli were encoded in rough somatotopic form in the RF, but the methods used could not distinguish between recording from neuron bodies and that from passing fibres, and their recording sites covered the whole coronal extent of the brainstem (Angel 1977).…”

Habituation of the Auditory Startle Response in Cervical Dystonia and Parkinson’s Disease

“…In case of partly impaired short-term habituation in FD, the dysfunction could hypothetically result from impaired synaptic depression by cerebellar input of brainstem interneurons localized in the pontine reticular formation. There is evidence from recent imaging studies that the process of habituation might reflect a decrease in tonic activity on reticular neurons subserving startle responses during the process of habituation [20, 21]. …”

Involvement of the human cerebellum during habituation of the acoustic startle response: A PET study

“…Because the cerebellum is not involved in the basic neural circuit for the acoustic startle response, involvement in the modulation of the response size seems likely 12. The significant decrease of cerebellar activity during the process of habituation might reflect a decrease in tonic activity on reticular neurons subserving startle responses during the process of habituation 13. In the present study, effects of short term and long term habituation were not separated with the subjects receiving a total of nine startle stimuli within one 2 hour test session.…”