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Wednesday, 9 August 2017

sensory system - The somatosensory cortex

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The somatosensory cortex

Functional anatomy:
       Somatosensory area is that part of the cereberal cortex to which sensory signals are projected
       Somatosensory cortex occupies Brodmann’s area 1,2,3,5,7 & 40. all are behind the central sulcus in the parietal cortex
       It comprise two areas that receive direct afferent fibers from the specific nuclei of the thalamus:
      Somatosensory 1 is the primary sensory area & lies in the postcentral gyrus
      Somatosensory11 lies in the wall of the sylvian fissure
      association sensory area
      Somatic sensory area1 (SS1):
      Site: post central gyrus (area 1, 2, 3)
      Characteristic of SS1:
      1- receives sensory impulses from opposite side (contralateral) of the body, with exception of the face which bilateraly represented in both hemisphere
      2-shows spatial orientation ie different parts of the body are separately represented on SS1. this body representation characterized by
      1/ it is contralateral: from opposite side of the body
      11/ it’s inverted : body is represented upside-down except the face
      111/the area of representation is directly proportional to the no of specialized sensory receptors in this area not to its size. Thus the thumb & lips have larger representation area while back & trunk have smaller area à In human the lips is represented by the greatest area followed by face & thumb
       Somatic sensory area 11 (SS11):
       Site: occupy area 40, behind & below SS1
      Receive sensory signals from both sides of the body + SS1
      Spatial orientation is not as detailed or as complete as SS1. face is represented anteriorly, arms centrally & legs posteriorly
       Function: begins to give meaning of sensory signals
Functions of somatic sensory cortex
The somatic cortex is concerned with 3 discriminative faculties:
       1- spatial recognition: this include localization of the site of the stimulus & two points discrimination. The later is define as the ability of person to perceive two touch stimuli applied simultaneously as two separate points while both eyes are closed.
        The ability of tactile discrimination varies according to the site. normally 2 mm of separation of points can be recognized as two separate stimuli at fingertip but 30mm of separation of points are needed in the back region to perceive 2 separate stimuli 0924614363
       The acuity of two points discrimination increase with increase 1/ no of touch receptor per unit area of skin & 2/ with the increase in width of area of representation in the sensory cortex.
       The ability of tactile discrimination is lost in dorsal column & parietal cortical (somatosensory cortex) lesion
       2- recognition of relative intensities of different stimuli: increases in intensity is transmitted to the brain in the form of an increase in the no of afferent fibers stimulated & increased frequency of APs in these fibers. These two features are perceived as indication of the strength of the stimuli 
       3- Stereognosis: is  define as the ability to recognize objects with touch without the aid of vision
       Loss of this ability called astereognosis, occurs due to dorsal column or cortical lesions
       In addition to astereo- dorsal column lesion cause loss of other sensations carried by the tract, include other aspects of fine touch (ie two points discrimination & localization)& proprioceptive & vibration senses.
       In cortical lesion, somatic sensation are not abolished since some perception is possible at the level of thalamus. But pure cortical function such as localization, tactile discrimination & stereognosis are lost
       C- Somatic association area:
       Site: behind the lower part of SS1(area 5,7 of cerebral cortex)
       Connection: it receive signals from:
       SS1 & SS11  
       Ventrobasal nuclei of the thalamus
       Other area of the thalamus
       Visual cortex
       Auditory cortex
       Functions: combine information from multiple points in somatic association areas to understanding the meaning of sensory signals
Disordered function of the sensory system
       Remember that at the level of SC
       1- spinothalamic (anterolateral) tracts cross
       2- dorsal column is uncrossed. The crossing occurs higher up in the medulla
       Due to the crossing of the two major sensory tracts, sensory information from one half of the body goes to cerebral hemisphere of the opposite side.
       Lesions can occurs at any level of a sensory tracts from afferent peripheral nerve to somatosensory cortex
       Localization of the site of lesion in disorders of sensory system:
       1- lesion of a peripheral nerve: all sensations are lost in the area supplied by the nerve. When many peripheral nerves are diffusely affected as in polyneuritis or polyneuropathy à all forms of common sensation are impaired in the distal parts of the limbs (eg glove-and-stocking aneathesia).
       2- lesion of dorsal root: all sensations are lost in the relative dermatome ie area of the skin supplied by the dorsal root. The tendon reflexes mediated by fibers in the root are also lost 
       3- spinal cord lesion: the features in SC lesions depend on the location of damage. The 3 commonest lesions are:
       1/ brown-sequard syndrome (hemisection of SC):
       Here one half of SC is damaged. The patient will show the followings:
       A- sensory disturbance at the level of the lesion: loss of all sensations from the area supplied by the dorsal root that enter the SC at the damaged segments on the ipsilateral side
       b/ sensory disturbance below the level of lesion:
       Loss of position, vibration & tactile discrimination sense on the ipsilateral side of the lesion
       Loss of pain & temperature sense on contralateral side
       Touch is not lost on either sides owing the dual pathway for touch ie one crossed & other uncrossed at SC level
       C- motors disturbances including:
       Lower motor neuron lesion manifestation at the level of the lesion on the same side
       Ipsilateral upper motor neuron lesion below the level of the lesion
       2/ syringomyelia: damage is to the central part of SC, where the crossing fiber of pain, temperature & touch decussate. This lead to loss of these sensations on both sides of the body at the affected segments. However fine touch including tactile discrimination & position sense are not affected as they are carried in the dorsal column pathway . Thus the result is dissociated sensory loss
       3/ tabes dorsalis: it is neurosyphilitic disease that damage which confined to the dorsal root central to the dorsal root ganglia. It attack thin pain fibers at first then thick fiber leading to :
       a/ irritation of pain fibers à attack of sever pain. then
       b/ degeneration of pain fiber à loss of pain sensation
       c/ degeneration & atrophy of dorsal column leading to
       Loss of touch sensation
       Loss of vibration sense
       Loss of position sensation leads to sensory ataxia, which is characterized by high steppage (stamping) gate. Can be confirmed by testing for the positive Romberg’s sign in which the patient will be unable to stand steadily when closing his eye.
       4- brain stem lesions: at the level of the brain stem, the main sensory tracts have already crossed the midline, therefore lesion in this area lead to loss of all sensations on the contralateral side
       5- thalamic lesions: spontaneous pain of most of unpleasant  quality together with exaggerated response to painfull stimuli (ie secondary hyperplasia) is characteristic of this condition. Others sensations is lost on the opposite side of the body 
       6- Cortical lesion: lead to loss of topognosis ( ie ability to localize the site of the stimulus), loss of two points discrimination & astereognosis.
       Small area lying posterior to the main sensory area in C cortex called secondary somatic area à play an important role in interpreting the meaning of sensations perceived by SS1. damage to this area may produce a disturbance of the body image & spatial orientation. The patient may ignore one side of his body a condition called amorphosynthesis or sensory inattention
       When lesion occurs at higher cortical level the condition a rising is known as hysterical sensory loss. In this condition sensory loss does not correspond to the anatomical nerve supply of the part. There are sharp lines of demarcation between normal & abnormal area of sensation. The areas of sensory loss may change in response to suggestions made by the doctors

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