Anatomy of the Central Nervous System of the Australian Echidna

摘要

Even from their gross appearance, the brain and spinal cord of the Australian echidna show unusual features. The spinal cord is one of the shortest ever recorded for any mammal, ending at the mid-thoracic level, a feature which may be related to the defensive posture of the echidna.,The pattern of termination of unmyelinated afferents in the spinal cord as revealed by lectin labelling with the B4 isolectin from Griffonia simplicifolia is also quite different from that seen in placental mammals, with termination in patches within deeper layers of the dorsal horn. Within the brainstem, specializations of the trigeminal system are apparent with great enlargement of all trigeminal nuclei. The mesencephalic trigeminal nucleus also shows an unusual aggregation of neurons in a central midline position quite unlike therian mammals. While the dorsal thalamus of therian mammals shows compartmentation related to function , the dorsal thalamus of the echidna is remarkable for its lack of cytoarchitectural differentiation. Most of the high encephalization in this mammal is attributable to the highly gyrified cerebral cortex. This cortex is further distinguished by the positioning of the major functional areas (primary motor, somatosensory, visual and auditory areas) towards the caudal pole of the brain.