Topographic map (neuroanatomy)

A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems. A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems. The visual system refers to the part of the central nervous system that allows an organism to see. It interprets information from visible light to build a representation of the world. The ganglion cells of the retina project in an orderly fashion to the lateral geniculate nucleus of the thalamus and from there to the primary visual cortex(V1); adjacent spots on the retina are represented by adjacent neurons in the lateral geniculate nucleus and the primary visual cortex. The term for this pattern of projection is topography. There are many types of topographic maps in the visual cortices, including retinotopic maps, occular dominance maps and orientation maps. Retinotopic maps are the easiest to understand in terms of topography. Retinotopic maps are those in which the image on the retina is maintained in the cortices (V1 and the LGN). In other words, if a specific region of the cortices was damaged that individual would then have a blind spot in the real world, they would not be able to see the bit of the world that corresponded to the retina damage. Orientation maps are also topographic. In these maps there are cells which have a preference to a certain orientation, the maximum firing rate of the cell will be achieved at that preference. As the orientation is moved away from the firing rate will drop. An orientation map is topographic because neighboring neural tissues have similar orientation preferences. The term retinotopic refers to the maintenance of the particular order of afferent connections from the retina along the afferent pathway via sub-cortical structures to V1 and other cortical visual areas. The primary visual cortex (V1, Brodmann's area 17) is the first cortical area to receive visual input. The stria of Gennari – a set of heavily myelinated, horizontally projecting axons within the termination zone of lateral geniculate nucleus (LGN) input to V1 – provides an anatomical marker particular to V1. According to the Chemoaffinity hypothesis, chemical labels are distributed in a graded fashion across the retina and tectum. This allows each retinal ganglion cell to recognize its proper termination site. Experiments with artificially created compound eyes in Xenopus demonstrate that not only the ganglion cells but also their axons carry these specificities. Axons must be able to communicate with each other to ensure that ones with the same positional tags innervate the same area of the superior colliculus. First-order representations are those in which adjacent points of the same hemifield always map to adjacent columns in the contralateral cortex. An example of this would be the map in primary visual cortex (V1). Second-order representations, also known as a field discontinuity map, are maps that are organized such that it appears that a discontinuity has been introduced in either the visual field or the retina. The maps in V2 and other extrastriate cortex are second-order representations. The auditory system is the sensory system for hearing in which the brain interprets information from the frequency of sound waves, yielding the perception of tones. Sound waves enter the ear through the auditory canal. These waves arrive at the eardrum where the properties of the waves are transduced into vibrations. The vibrations travel through the bones of the inner ear to the cochlea. In the cochlea, the vibrations are transduced into electrical information through the firing of hair cells in the organ of Corti. The organ of Corti projects in an orderly fashion to structures in the brainstem (namely, the cochlear nuclei and the inferior colliculus), and from there to the medial geniculate nucleus of the thalamus and the primary auditory cortex. Adjacent sites on the organ of Corti, which are themselves selective for the sound frequency, are represented by adjacent neurons in the aforementioned CNS structures. This projection pattern has been termed tonotopy. The tonotopic layout of sound information begins in the cochlea where the basilar membrane vibrates at different positions along its length depending upon the frequency of the sound. Higher frequency sounds are at the base of the cochlea, if it were unrolled, and low frequency sounds are at the apex. This arrangement is also found in the auditory cortex in the temporal lobe. In areas that are tonotopically organized, the frequency varies systematically from low to high along the surface of the cortex, but is relatively constant across cortical depth. The general image of topographic organization in animals is multiple tonotopic maps distributed over the surface of the cortex. The somatosensory system comprises a diverse range of receptors and processing centers to produce the perception of touch, temperature, proprioception, and nociception. Receptors are located throughout the body including the skin, epithelia, internal organs, skeletal muscles, bones, and joints. The cutaneous receptors of the skin project in an orderly fashion to the spinal cord, and from there, via different afferent pathways (dorsal column-medial lemniscus tract and spinothalamic tract), to the ventral posterior nucleus of the thalamus and the primary somatosensory cortex. Again, adjacent areas on the skin are represented by adjacent neurons in all aforementioned structures. This projection pattern has been termed somatotopy.