Platypus brain
The brain of the platypus
See also Atlas of the forebrain of a newly hatched platypus
The brain of the platypus can be up to 16 ml in volume in large-bodied specimens.
Unlike the convoluted cerebral hemisphere of the echidnas, the surface of the platypus cerebral cortex is smooth (lissencephalic; Fig. 1). Nevertheless, the isocortex (6 layered cortex) of the platypus is thick and large (4100 mm3 and 48% of brain volume, Pirlot and Nelson 1978). Bohringer and Rowe (1977) found only a single somatosensory field in the isocortex of the platypus, but Krubitzer and colleagues (Krubitzer et al., 1995) identified three separate somatosensory fields (S1, PV and R; see coloured areas in Figs 1 and 2); as well as a manipulation field (M), where neurons respond to deep stimulation, co-extensive with motor cortex. Krubitzer also identified two visuotopic (visual world) representations in the platypus cortex (Krubitzer et al., 1995).
As has been seen for other monotremes, the proportion of iso- and periallocortical white matter in monotremes is substantially lower than that in the forebrains of therians (Ashwell and Gurovich, 2019). This suggests that the forebrains of the three monotremes have fewer association, commissural and/or projection connections than those of similarly sized forebrains of therian mammals. Ashwell and Gurovich (2019) also found a distinct caudal shift in the positioning of cortical white matter in the platypus forebrain, consistent with expansion of the posterior thalamic radiation to the somatosensory cortex.
Unlike placental mammals, the monotremes and marsupials do not have a corpus callosum connecting the two cerebral hemispheres. The major connection between the two sides of the forebrain in monotremes and marsupials is the anterior commissure (see ac in Fig. 3.).
The other distinctive external feature of the platypus nervous system is the large size of both the trigeminal ganglion and trigeminal nerve. The size of the trigeminal sensory ganglion is very different between the platypus and short-beaked echidna, about 55 mm3 for the platypus and only about 9 mm3 for the larger bodied short-beaked echidna, a difference that is reflected throughout the central parts of the trigeminal pathway in the two species. The trigeminal sensory nuclear column in the pons and medulla oblongata also produces distinct bulges (tuberculum cinereum) on the exterior of the brainstem.
The external appearance of the platypus cerebellum is broadly similar to other mammals, but the cerebellum is poorly foliated (folded; Ashwell, 2020), and wider than most mammals in the transverse dimension due to the presence of lateral extensions of the nodulus and pyramis.
References
Ashwell KWS (2020) Quantitative analysis of cerebellar morphology in monotreme, metatherian and eutherian mammals. Zoology (Jena) 139, 125753. doi: 10.1016/j.zool.2020.125753.
Ashwell KWS, Gurovich Y (2019) Quantitative analysis of forebrain pallial morphology in monotremes and comparison with that in therians. Zoology (Jena) 134, 38-57.
Bohringer RC, Rowe MJ (1977) The organization of the sensory and motor areas of cerebral cortex in the platypus (Ornithorhynchus anatinus). Journal of Comparative Neurology 174, 1–14.
Elston GN, Manger PR, Pettigrew JD (1999) Morphology of pyramidal neurons in cytochrome oxidase modules of the S-I bill representation of the platypus. Brain Behavior and Evolution 53, 87–101.
Krubitzer L, Manger P, Pettigrew J, Calford M (1995) Organization of somatosensory cortex in monotremes. In search of the prototypical plan. Journal of Comparative Neurology 351, 261–306.
Pettigrew JD, Manger PR, Fine SLB (1998) The sensory world of the platypus. Philosophical Transactions of the Royal Society. Series B. Biological Sciences 353, 1199–1210.
Pirlot P, and Nelson J (1978) Volumetric analysis of monotreme brains. In Monotreme Biology: The Australian Zoologist Special Symposium (Ed ML Augee) Published in The Australian Zoologist 20, 171–179.
Fig. 1. Superior view of the brain of the platypus, showing the smooth surface of the cerebrum (lissencephaly) and the functional areas of the cerebral cortex. Functional areas are based on Krubitzer et al. (1995). The darker bands within the platypus S1 indicate the cytochrome oxidase (CO) and myelin dense zones. Regions of CO and myelin dark bands contain neurons that respond only to cutaneous stimulation, whereas areas between the CO dark bands contain neurons that respond to both electrical and mechanical stimuli (but more to the former). It has been suggested that the banded pattern within S1 allows for processing of temporal disparities between mechano- and electrosensory input as a ranging system for prey (Pettigrew et al., 1998; Elston et al., 1999).
Fig. 2. Inferior view of the brain of the platypus, showing functional areas. Note that the image of the brain is slightly distorted because it has been photographed while inside a tubular bottle. See legend for Fig. 1 for explanation of coloured areas.
Fig. 3. View of a midline section through the brain of the platypus. Note the large size of the anterior commissure (ac).