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Anatomy and Cell Biology
Anatomy and Cell Biology 3319
Peter Merrifield

Anatomy NotesOct 1311 Elements of ascending sensory systems Afferent neuronal signals from skin and deeper structures are segregated in the spinal cord ex subthalamic tract vs dorsal funiculi Axons of pain and temperature cross the midline in the spinal cord Axons of discriminative touch and conscious proprioception ascend ipsilaterally to the medulla After synapsing in the medulla the axons cross the midline The ventral posterolateral nuclei VPL in the thalamus is the primary site of termination for somatosensory information to the postcentral gyrus It acts as a relay station for the sensory info ascending to the primary sensory areas of the cerebral cortexThe dorsal column pathway or medial lemniscus sensory system located at dorsal funiculus carries information on discriminative touch pressure vibratory sensation and conscious joint position These are senses that can be localized very precisely on the body surface Information from the lower limbs enters the lumbar spinal cord at the dorsal horn then ascends via the fasiculus gracilis medial Info from the upper limbs enters the cervical spinal cord at the dorsal horn and ascends via the fasiculus cuneatus lateral This happens on the ipsilateral side no crossing over has occurred yet There is segregation of upper and lower info These first order sensory neuron axons ascend to the medulla and terminate and synapse with second order neurons in the nucleus gracilis and nucleus cuneatus Axons from these nuclei form a tract called the medial lemniscus tract which decussates in the medulla This then travels up the pons and midbrain to the VPL in the thalamus The neurons synapse here with third order neurons and the axons are sent via the internal capsule to the primary somatosensory cortex on the postcentral gyrus where the sensory info is processed resulting in awareness of precisely localized sensations Dorsal column system summary Afferents enter dorsal horn and ascend through the spinal cord in gracile and cuneate fasciculi First order neurons terminate in the gracile a
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