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Tactile Sensation and Proprioception
- Sensory stimuli - light touch, vibration, and proprioception
- discriminative or two-point touch is an important collective property of this pathway
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Types of receptors
- cutanous mechanoreceptors: Meissner's, Pacinian, Ruffini Corpuscles, Merkel receptors, hair receptors
- Muscle and skeletal mechanoreceptors: muscle spindles, joint capsule receptors, Golgi Tendon Organ
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Cutaneous mechanoreceptors differ in
- location in the skin : hairy vs non-hairy; superficial vs. deep
- Receptive field: size of skin area
- Adaptation: duration of action potential activity in their axons
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stereognosis
- when several different types of receptors are activated simultaneously
- collectively info is processed by synaptic interactions in the cortex to enable us to identify objects based on tactile sensation
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Primary axon diameters of mechanoreceptors
- alpha: 10-20um
- beta: 5-15um
- gamma 3-10um
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First-Order neuron cell body/Nucleus
Dorsal Root Ganglion (DRG)
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Second-Order neuron cell body/Nucleus
Nuclei: Gracilis (lower body above and below T6, below T6 lower body ONLY) and Cuneatus (upper body above T6)
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Third-Order neuron cell body/Nucleus
Ventral posterior Lateral (VPL) nucleus in the thalamus
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First-Order neuron axon/tract
spinal nerve/fasciculus gracilis and cuneatus
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Second-Order neuron axon/tract
Medial lemniscus (turns as go up like "elbows/forearms"
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Third-Order neuron axon/tract
posterior limb of the internal capsule
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primary axons branch as they enter the spinal cord
- pathway to cortex for sensory perception
- locatl connections in spinal cord for reflexes
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Cortical localization of tactile sensation and proprioception
- postcentral gyrus: is somatosensory cortex
- also paracentral lobule
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loss of any of the complex tactile functions while simple, peripheral tactile awareness is intact
agnosia: indicates a parietal cortex lesion
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Different tactile agnoaias
- loss of 2-point discrimination
- astereognosis
- agraphesthesia
- extinction on double simultanous stimulation - damage to the parietal lobe can cause misperception of or inattention to tactile stimuli
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a lesion
- an area of localized dysfunction in the nervous system
- May be caused by: trauma, loss of blood supply, tumors, congenital malformations, genetic disorders, viral and bacterial infection, disease, etc.
- lesions typically block conduction/function, but in some situations they can irritate adjacent tissue and inched activity (e.g. seizures)
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loss of tactile sensibility: lesion could be in
- spinal cord
- brainstem
- thalamus
- internal capsule
- postcentral gyrus
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5 deficits: key symptoms that imply damage to the cortex
- inability to localize tactile stimuli
- loss of 2-point discrimination
- astereognosis
- agraphesthesia
- extinction on double simultanous stimulation (inattention to bilateral stimuli)
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