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The five senses. Or are they?
Over the past several months this blog has talked a lot about structure and function within the brain, with particular attention paid to how information gets into and out of the brain. The primary senses listed above are the major forms of input to the brain, but this list is somewhat deceptive in its simplicity.
We know from the multiple processing sites in the brain that vision and hearing are not uniform, monolithic inputs. We can subdivide the visual sense into color vs black & white, lines vs. shapes, moving vs. stationary objects. We are justified in thinking of each as a separate visual "senses" because the output from the retina goes to several different brain nuclei, and those separate lines do not necessarily converge again. For example, visual information goes from retinal ganglion to lateral geniculate (in the Thalamus) to V1 visual cortex (Occipital Lobe) to support the detection of lines and shapes. On the other hand, visual information goes from retinal ganglion to superior colliculus (in the midbrain) to cerebellum and back to Frontal Lobe for tracking of movement.
A similar subdivision can be made for auditory sense - single tones vs. harmony, melody, rhythm and stereo localization. As with the visual sense, information flows from cochlea to medial geniculate (Thalamus) to auditory cortex for sound identification, but it also travels from cochlea to inferior colliculus (midbrain) to cerebellum and back to Frontal Lobe for integration with vision to track moving objects. There are also brainstem structures that receive sensory information. The Superior Olive (visual) and Inferior Olive (auditory) in the Medulla compare the information received from the left-right visual or auditory fields to assist in determining location for *unmoving* objects.
And what about the sense of balance? Isn't that associated with the inner ear and the auditory system? Shouldn't it be a "sense" as well?
Well, No and Yes. While the semicircular canals of the inner ear share a structure with the cochlea, their functions are quite different. Many neuroscientists consider the sense of balance to be part of the sixth sensory classification called "proprioception." This is the sense of position that combines pressure on the soles of the feet, with angle information from the joints, stretch information from muscles and balance information to inform the brain about the position of the body. Anyone who has ever had to close their eyes and walk or touch their nose with an outstretched finger has used the proprioceptive sense.
Unlike visual and auditory systems in which one type of sensor (retina photoreceptors or cochlear hair cells) has its output divided into many functions, the proprioceptive sense is a combination from many different sensory neuron types that are combined into one sense. The semicircular canals rely on "otoliths" which sense movement and position changes of the head using a system very similar to the hair cells of the cochlea. pressure sensors in the feet use the same neurons as the sense of touch elsewhere - neurons that become more active when the cells are pushed, compressed or stretched - in other words, if the cell changes shape, it sounds off! The stretch receptors in the muscles and tendons are in fact specialized neurons similar to muscle cells that detect when a muscle is either actively or passively stretched.
Tactile sense comes from a vast number of these "mechanoreceptors" located in the skin, muscle and all of teh tissues in between. So in another sense, the "proprioceptive sense" could be considered just a subdivision of tactile sensation. However, there is more to the tactile sense than just "touch" as we will learn in the *next* blog - "The *Six* Senses (and maybe more)"
So tune in next Sunday for Taste, Smell and hot & cold running senses!