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Ratunculus
The brown rat has evolved sensory and motor adaptations that serve it well in its environment. This set of sensory and motor adaptations differs from humans in a variety of ways which we shall explore here. Let's start with the familiar first so that when we come to it we can understand the concept of Ratunculus. Homunculus is a visual approach to describing the movement and exchange of human sense and motor information. Lips, hands, feet, and sex organs have finer motor skills and sense nerves than other areas and will be displayed as distortedly large. This image represents sensory (left side) and motor (right side) homunculi. Homunculus is Latin for 'little man' so it only seems fitting to apply the name ratunculus for the brown rat sensory and motor image.
One interesting change in the ratunculus comes when a rat has given birth and is feeding her young with milk. On a ratunculus image the size around where the nipples are located doubles. In addition, approximately 66% less stimulation is needed within this area to produce a somatosensory cortex response (Stern, J, 1997). This implies that suckling kittens need apply a lot less pressure around the nipple area to get a response (dinner) than under non lactating circumstances.
I had a little time left after completing a lesson and introduced the mole-ratunculus and ratunculus to my class. It was fun to introduce touch and tactile sense through recognising an object using hands and elbows. The image above and to the right shows the creativity young minds instinctively display. My image on the left is certainly not proportionally accurate but it does provide a general impression and maybe too much in the case of the whiskers. I wanted to show a break down of the percentage of somatosensory cortex devoted to the body parts but have been unable to track down any free resources. There appear to be a number of journal articles devoted to the topic but I'm certainly not going to pay for access. However, the following mole-ratunculus will allow you to make your own inferences. Mole-Ratunculus Here's a mole-ratunculus for the mole rat. The incisors are an amazing adaptation with over 30% of the somatosensory cortex devoted to them. The lower incisors are able to move independently due to flexibility of the two halves of the lower jaw (Catania and Remple, 2002). The incisors are an indispensable tool to the mole rat and its hardly surprising they have such a large chunk of the somatosensory cortex pie. (illustration by Lana Finch)
References Kenneth C. Catania and Michael S. Remple, 'Somatosensory cortex dominated by the representation of teeth in the naked mole-rat brain' PNAS, Apr 2002; 99: 5692 - 5697. http://www.pnas.org/cgi/content/full/99/8/5692 (15 March, 2008)
Royl, George: 'Neurovascular coupling in the somatosensorischen
Kortex of the rat: Investigations to temporal kinetics by means of optical
procedures and functional magnet resonance tomography' February 2003,
Judith M. Stern, 'Maternal Behavior II: Slide Commentary' Hormones &
Behavior Course Outline Index, Nov 1997, And thank you to Dr. Ian Whishaw for taking the time to point me in the right direction for the resources I needed. |
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We
can apply the same understanding to ratunculus and any other
'animalunculus' we care to create. The ratunculus image on the left was
drawn by George Royl as part of his doctorate thesis. The head is enormous
and the cheek patterns (for whisker barrels), forepaw, hindpaw, and dental
area clearly show distortions. The whisker barrels themselves take almost
20% of the
