This is an extract of our Dorsal Coloumn Pathway document, which we sell as part of our Neuroscience 1 Notes collection written by the top tier of Oxford students.
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Somatosensory pathways Somatosenosry system three major functions
-Proprioception - sense of oneself, receptors in skeletal muscle, joint capsules, skin enable us to have conscious awareness of posture, movements of body
-Exteroception: Sense of direct interaction with external world, e.g sense of touch ( contact, pressure, stroking, motion, vibration) Sense of hot and cold, Sense of pain/ nociception
-Interocetion: Sense of function of major organ systems of the body and its internal state, primarily chemoreceptors Ascending Somatosensory pathways
-Somatosensory information is conveyed to the thalamus and cerebral cortex by 2 asending pathways
a) Dorsal column medial lemniscal system: tactile and proprioceptive information- wide diameter Aa, AB fibres
-Tactile sensation can be divieded into nondiscriminative (Crude) touch and discriminative (fine touch)
-Crude touch is transmitted via anterolateral system and is sensed following gentle stroking of skin with fine cooton strand but this sensation doesn't include detailed information about stimulus
-Tactile examination of an object is dependent on discriminative fine touch sense- enables to detect fine detail regarding location size, shape and texture of an object even when eyes are closed.
-Conscious Proprioception may be categorised into static proprioception (awareness of position of a body part such as a limb) and dynamic proprioception (awareness of movement of a body part and balance) Discriminative fine touch and pressure sense from the body Receptors
-Free nerve endings responding to touch, pressure and proprioception in skin, muscles and joint capsules
-Merke's discs- respond to touch and pressure in skin
-Peritrichial endings stimulated by touch of hair follicles
-Meissner's corpuscles activated by touch of skin
-Pancinian corpuscle- stimulated by touch, pressure, vibration and proprioception in deep layers of skin and in visceral structures Receptive fields
-To stimulate a fibre, stimulus must lie within particular area - receptive field
-In case of fibres innervating hair follicles- one finds that each fibre innervate as many as hundred follicle and each follicle in turn receives branches from several fibres
-So receptive fields of individual fibres are large and show overlap- if stimulus lies within area of overalp then it will stimulate two fibres in different proportions depending on exact psotion- so by comparing frequency of firing of one with that of the other, CNS can determine position of stimulus much more accurately than if fields were discrete
-Overlap has further advantage that it makes the system less vulnerable to damage
-Receptors converge onto first order pseudounipolar neurons whose cell bodies are located in the dorsal root ganglia.
-Neurons are medium size Ab and larger size Aa fibres i) SPINAL CORD.
-Ascending branch of large sensory dorsal root ganglion axons (Aa, Ab) enters the ipsilateral dorsal coloumn of the spinal cord (white matter tract medial to the dorsal horn) via the medial division of dorsal roots of spinal nerves
-Upon entry into posterior funiculus of spinal cord, afferent fibres bifurcate into long ascending and short descending fibres. These fibres give rise to collateral branches that synapse with several distinct cell groups of dorsal horn interneurons and with ventral horn motor neurons. These fibres collectively form dorsal coloumn pathways -either the fasciculus gracilis/ fasciculus cuneatus Below T6: Central processes that enter spinal cord below T6 (lower thoracic, lumbar and sacral levels) which bring info from lower limb and lower half of trunk- enter ipsilateral fasciculus gracilis and ascend to medulla and terminate in ipsilateral nucleus gracilis. Fasciculus gracilis present at all levels. Level T6 and above
-Bring information from upper thoracic and cervical levels- enter ipsilateral fasciculus cuneatus, situated laterally in spinal cord, and ascend to medulla to synapse with second order neurons in the ipsilateral nucleus cuneatus.
-fasciculus cuneatus is present only at upper 6 thoracic and at all cervical spinal cord levels and is separated from fasciculus gracilis by septum Information transferred through posterior coloumns: Discriminate touch (ability to localise the area of the body touched, aware points touched simultaneously, 2 point discrimination. Information from muscles and joints pertaining to movement and osition of different parts of the body. Vibratory sensations ii) MEDULLA:
-At the junction between the medulla and the spinal cord : Neurons from each fascicle synapse on the second order neurons in the ipsilateral dorsal column nulcuei: gracile and cuneate nuclei-medulla, dorsal coloumn nuclei
-Terminates in the ipsilateral dorsal column nuclei- 90% axons of the dorsal root ganglion cells, remaining 10% are axons of spinal neurons. Post synaptic dorsal column axons course along midline adjacent to medial septum provide a specialised rapid pathway for visceral pain
-Primary afferent fibres terminate on neurons throughout rostrocaudal extend of each nucleus
-Rostral 1/3rd of the dorsal column nuclei-dominated by neurons that process proprioceptive information (75% receive sensory information from muscle afferents), tactile inputs predominate in middle 1/3 rd where 90% of neurons process cutaneous info, neurons in caudal third are evenly divided between cutaneous and proprioceptive
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