Vcll

Ventral complex of the lateral lemniscus

component of the auditory pathway. In the human midbrain and most mammalian species, the central nucleus is defined by the presence of fibrodendritic laminae. The fiber component represents the ascending afferent axons from virtually all parts of the auditory system in the lower brain stem. Individual laminar axons form parallel sheets, several hundred micrometers wide, that extend in a ventrolateral to dorsomedial and rostrocaudal plane. These laminae represent an anatomical substrate for the tonotopic organization of the inferior colliculus. Acoustic stimuli at the lowest frequencies stimulate laminae in the dorsolateral part of the central nucleus, and successively higher frequencies stimulate laminae ventrome-dially (Fig. 2).

In the fibrodendritic laminae, the dendrites arise from the principal neurons in the central nucleus and are defined by their morphology. Disk-shaped neurons have flat, narrow, planar dendritic fields that parallel the planes of the laminar axons. The dendritic field of the large disk-shaped cell in humans is up to 1 mm long, and small disk-shaped cells have dendritic fields 600 mm long. In contrast, the width of the dendritic field is only around one-fourth the width of the axonal lamina (50 mm). This means that the disk-shaped dendrites are usually contained within a laminar plexus. A second, less common cell type is defined by a stellate or less flat dendritic morphology. Stellate cells can be small or large and have dendritic fields whose longest axis is either parallel or perpendicular to the fibrodendritic laminae.

Cell types in the central nucleus also are defined on the basis of their neurotransmitter content and by the target of their axons. A majority of the disk-shaped neurons in the inferior colliculus probably use glutamate as the neurotransmitter; however, a significant proportion of these cells use g-aminobutyric acid (GABA) as the neurotransmitter (about 20% in the cat). Stellate cells can also synthesize GABA. Both GABAergic and non-GABAergic neurons can send their axons to other parts of the brain to form connections in the auditory pathway.

Neurons in the central nucleus are characterized by their membrane properties. At least six types of neurons can be defined using electrophysiological and pharmacological criteria in brain slice preparations in rats. The data suggest the presence of one or more unique potassium or calcium currents in each cell type, and these are likely related to unique combinations of potassium and calcium ion channels. Most cells in the central nucleus have membrane properties that tend to modify the inputs, e.g., act like filters. For

INPUTS FROM:

Cerebral Cortex

Auditory primary cortex (temporal) Auditory association cortex (temporal) Mesencephalon

Dorsal nucleus of the lateral lemniscus Ventral complex of the lateral lemniscus Mesencephalic Reticular Formation (MRF) Cuneiform nucleus Nucleus sagulum Contralateral inferior colliculus

ASCENDING PROJECTIONS TO:

Diencephalon

Medial geniculate body: Ventral, medial, and dorsal divisions, suprageniculate. Posterior group Zona incerta Fields of Forel

Mesencephalon

Nucleus sagulum Cuneiform nucleus Contralateral Inferior Colliculus

Cuniform Nucleus

INPUTS FROM: CONTRALATERAL

Pons

Lateral superior olive Medulla

Ventral cochlear nucleus Dorsal cochlear nucleus

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