this lecture dealt with some of the higher order processing of the somatosensory cortex; namely, the cortex's ability for localization and discrimination of fine textures and resolutions. localization refers to how accurately the somatosensory cortex can distinguish a stimuli from a certain location on the body. the two point discrimination test measures the smallest distance between two simultaneous stimuli which will indeed produce a sensation of two stimuli.
lateral inhibition is introduced as a means of increasing the resolution and sensitivity to stimuli, a mechanism which is used widely in the cortex, thalamus, and dorsal column nuclei. the neuron at the center of a given receptive field essentially produces the strongest signal because of its inhibition of neighboring nuclei, which are also inhibiting neighboring nuclei. this inhibition occurs via inhibitory interneurons, which are stimulated by recurrent branches of the dorsal column nuclei.
the cortex can also regulate lateral inhibition to enhance the selectivity of the sensory input even further; this is seen especially during movement, where the cortical neurons will directly stimulate or inhibit dorsal column nuclei to select for stimuli that is relevant to the movement, allowing for greater discrimination of complex textures.
the next section looked at the somatosensory association cortex, which lies posterior to the primary somatosensory cortex. it is responsible for deciphering the meaning behind tactile input, formation of an egocentric space via the integration of tactile, proprioceptive, auditory, and visual information, and language comprehension via Wernicke's area. lesions within this area can produce such syndromes as astereognosis, which is the inability to identify an object by touch, or neglect syndrome, which is a loss of cognition in half of the sensory field.
finally, two conditions are discussed related to spinal cord injury: syringomyelia is the formation of cysts in spinal grey matter which blocks decussation of anterolateral second order neurons and thus the bilateral loss of pain and temperature sensation. brown-sequard syndrome results from a hemisection of the spinal cord, an injury that slices through the dorsal and anterolateral columns on one side, that results in loss of ipsilateral motor control and discriminative touch sensation, and loss of contralateral pain and temperature sensation.
questions
localization...
1. cortical localization requires...
2. how are receptive fields differentiated in peripheral structures?
3. what is the relationship between localization and receptive field size?
4. how are receptive fields differentiated in the CNS?
lateral inhibition...
5. what is lateral inhibition?
6. describe what happens with stimulation of the center and peripheral region of a receptive field for a given dorsal column nuclei neuron.
7. what are inhibitory interneurons and what are their function in lateral inhibition?
8. what is the net effect of lateral inhibition?
two point discrimination, cortical regulation...
9. what is meant by "two point discrimination"?
10. describe how two point discrimination is related to localization and lateral inhibition.
11. how does syphilis affect the two point discrimination test?
12. how does the cortex regulate lateral inhibition?
13. what is the purpose of cortical regulation of lateral inhibition?
14. why does somatosensory perception depend on movement?
somatosensory association cortex...
15. somatosensory association cortex lies...
16. what is the somatosensory association cortex involved in?
17. what is the region within the somatosensory cortex involved in language comprehension?
pathologies...
18. what is astereognosis?
19. what is neglect syndrome?
20. most neglect is due to...
21. why do lesions to the left parietal lobe not necessarily lead to neglect syndrome?
22. what is syringomyelia?
23. what is brown-sequard syndrome?
answers
1. differentiation of neural input from receptive fields at many different levels of processing including the dorsal column nuclei, thalamus, and somatosensory cortex.
2. by the specific activity of sensory neurons that innervate each RF.
3. the smaller receptive fields have a greater degree of localization.
4. via neural interactions between pathways between different RF's.
5. a neural mechanism employed in the cortex, thalamus, and dorsal column nuclei that enhances the difference between somatosensory receptive fields.
6. stimulation at the center will produce a stimulatory effect on the dorsal column nuclei neuron and stimulation at the periphery of the receptive field will produce an inhibitory effect.
7. inhibitory interneurons are the neurons that are stimulated by the recurrent branches of dorsal column nuclei which inhibit action of neighboring dorsal column nuclei neurons.
8. lateral inhibition enhances the signal received at the central region of activation and decreases the signal away from the center.
9. the ability to distinguish between separate but simultaneous pinpricks to the skin.
10. lateral inhibition refines the localization of sensory stimulation, which is what is being measured in two point discrimination.
11. the dorsal column degeneration interferes with the reception of discriminative touch and thus reduces one's two point discrimination ability.
12. via descending pathways from the primary somatosensory cortex and motor cortex which modulate afferent inputs.
13. to focus on the afferents that are delivering useful information.
14. during movement, motor cortex sends messages to dorsal column nuclei to facilitate sensory stimuli related to the movement, in a way that increases the discrimination of complex textures.
15. posterior to the primary somatosensory cortex.
16. deciphering meaning of tactile stimuli, integration of touch, proprioception, vision, audition (formation of ego-centric space), language comprehension.
17. Wernicke's area.
18. the inability to identify an object by touch.
19. loss of cognition of half of a sensory field.
20. lesions in the right inferior parietal lobe.
21. the functioning right hemisphere has the capacity to compensate for the left's dysfunction but not vice versa.
22. formation of cysts in the center of the spinal grey area that prevents decussation of second order neurons in the anterolateral system, leading to bilateral loss of pain and temperature sensation.
23. a hemisection of spinal cord which cuts through the dorsal and anterolateral columns on one side, leading to loss of ipsilateral discriminative touch and motor control and loss of contralateral pain and temperature.
No comments:
Post a Comment