Neurophysiology
The larger fibres turn upwards soon after entering the dorsal horn of the spinal cord, to form a pair of large ascending tracts called the dorsal columns( see left side of above diagram).These continue up to the level of the medulla and terminate in the dorsal column nuclei( called the gracile and cuneate). The gracile recieves fibres from the sacral (back leg and bottom), lumbar (front leg and waist) and lower thoracic (Belly) segments, and cuneate, from higher regions. Second-order fibres then cross to the other side and continue up as the medial lemniscus to the other posterior nuclei of the thalamus and thence realy through the internal capsule to a region of cerebral cortex called the somatoscensory cortical area. It is this system, called the lemnsical system which keeps the topographical relationship between the representations of different areas of skin and thus enable the somatosensory cortex to maintain a map of the other side of the skin surface. This map is the sensory homunculus or 묩ttle man'.
The smaller fibres, concerned with temperature, pain and light touch, take a rather different route due to their nature, synapsing in central grey matter with interneurones to excite third-order neurones whose axons cross to the other side of the cord before they too proceed upwards as part of what is known as 볰inothalamic projection( see right side of above diagram). The two pathways of this system, the anterior and the lateral, are both evolunarily older than the lemnmsical system, the anterior projects only to the border of the other posterior nuclei of the thalumus, and to nearby regions that are not wholly somatoscensory wheras the lateral afferents project to central regions of the thalamus and are more concerned with pain and temperature than with touch.
Both of these routes end up in the somatosensory cortex and hence create the sensory humunculus, giving us proprioception.
[b]The sonsory homunculus itself is rather distorted in shape with areas such as the hands and lips, with the greatest sensitivity having a much larger area devoted to them. However, due to visual information and information regarding the space which our body takes up, our body schema is usually intact.
| When we feel any kind of sensation on our bodies, we are doing so through sensory receptors which are situated all over our bodies. These are the same receptors which are used in Proprioception. There are many different types of sensory receptors, just as we feel many different sensations, each responding to different things. They are generally classified into mecahnoreceptors which contain skin, viceral, muscle and vascular receptors, Thermoreceptors which deal with hypothalamic and the warm and cold skin receptors, photoreceptors in the retina and chemoreceptors which deal with vascular, visceral,gustatory recpors and are used in hearing.The Pacinian Corpsucles, for example, are mechanoreceptors which respond to pressure and are mainly found distributed in visceral structures, in joints and deep conective tissue. Free or naked endings are abundant in hairy skin and serve both warmth and pain and are also sensitive to mechanical stimuli, encaponated endings usually serve the sensation of cold. Merkel's discs are sensitive in deformation of the skin and indicate contact and Meissner's Corpsicules take in side-ways shearing of the skin experienced for example when we are holding an object in the fingertips. Due to these receptors, if we close our eyes and run our hands over a table, we are able to create a visual impression of the said table by it's texture, crack's and other surface properties. In order for these sensory receptors to actually give us information regarding our bodily sensations and help to keep our body schema correct, they must undertake quite a complicated journey. Afferent fibres from receptors are actually bipolar cells, whose bodies lie in the dorsal root ganglia near the spinal cord.Their axons run all the way from sensory endings in the skin to their terminals within the central nervous system. These fibres are seen to be divided into two groups; small fibres, mostly from free endings, and large fibres, from encapsulated which is reflected in the two different ways they make their way to termination.  |
The larger fibres turn upwards soon after entering the dorsal horn of the spinal cord, to form a pair of large ascending tracts called the dorsal columns( see left side of above diagram).These continue up to the level of the medulla and terminate in the dorsal column nuclei( called the gracile and cuneate). The gracile recieves fibres from the sacral (back leg and bottom), lumbar (front leg and waist) and lower thoracic (Belly) segments, and cuneate, from higher regions. Second-order fibres then cross to the other side and continue up as the medial lemniscus to the other posterior nuclei of the thalamus and thence realy through the internal capsule to a region of cerebral cortex called the somatoscensory cortical area. It is this system, called the lemnsical system which keeps the topographical relationship between the representations of different areas of skin and thus enable the somatosensory cortex to maintain a map of the other side of the skin surface. This map is the sensory homunculus or 묩ttle man'.
The smaller fibres, concerned with temperature, pain and light touch, take a rather different route due to their nature, synapsing in central grey matter with interneurones to excite third-order neurones whose axons cross to the other side of the cord before they too proceed upwards as part of what is known as 볰inothalamic projection( see right side of above diagram). The two pathways of this system, the anterior and the lateral, are both evolunarily older than the lemnmsical system, the anterior projects only to the border of the other posterior nuclei of the thalumus, and to nearby regions that are not wholly somatoscensory wheras the lateral afferents project to central regions of the thalamus and are more concerned with pain and temperature than with touch.
Both of these routes end up in the somatosensory cortex and hence create the sensory humunculus, giving us proprioception.
[b]The sonsory homunculus itself is rather distorted in shape with areas such as the hands and lips, with the greatest sensitivity having a much larger area devoted to them. However, due to visual information and information regarding the space which our body takes up, our body schema is usually intact.
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