Topographic mapping

In most of the sensory pathways primary afferents are wired to particular subsets of more central neurons in a strictly ordered fashion therefore nearest neighborhood relations are conserved. This means that the information about stimulus position is not lost in more proximal sections of a pathway. This arrangement is known as topographic mapping. The Receptive fields are aligned to generate an ordered map across the brain structures like thalamus or the cerebral cortex. Such maps are neural representations of a sensory surface or a few feature of a sensation. The key illustrations are: somatotopic maps that symbolize skin surface; retinotopic maps that reflect the visual fields; and tonotopic maps that symbolize the pitch of a sound. Additionally there are numerous motor maps, mainly in the cerebral and cerebellar cortices in which the movements are represented in a systematic way. The motor mapping is conserved in descending pathways therefore connections with motor neurons are accurately those required to execute the mapped movement.

Three broad kinds of map are recognized, cosidered to be determined by the extent of the connections among the neurons included in the mapping:

- The Discrete maps like somatotopic or retinotopic maps are anatomically precise and complete representations of a sensory surface; however they are generally distorted, in that the region of the surface is not faithfully proportioned. The Fingers and lips get far more than their fair share of the cortex in somatotopic maps. The Discrete maps arise as neurons are connected mostly to their neighbors, permitting local interactions among cells. In another words, most of the comparisons the CNS requires to make of, say an image, are among adjacent pixels of retina.

- Patchy maps consist of numerous domains within each of which the body is precisely represented. Though, adjacent domains map areas which are not anatomically close or which are disoriented. The Cerebellar motor maps are of this type and said to display fractured somatotopy. Patchy maps arise since while a few groups of neurons are locally connected to others and wired to distant neurons permitting global interactions to occur. Serving a tennis ball needs the coordination of movements in distant parts of the body.

- The Diffuse maps are those that have no underlying topography. Different smells are mapped to particular sites in the olfactory bulb but not in any arranged fashion. The Smells are not arranged within the brain in any systematic way by property.