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When does gastrulation begin?
- by day 15
- formation of primitive streak on post. surface of epiblast
- establishes trilaminar disc-ectoderm, mesoderm, endoderm
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When does neurulation begin?
- by day 17
- notochord derived from epiblast (mesoderm)
- notochord induces ectoderm to form neural plate
- neural plate becomes neural groove then neural tube
- neural tube forms brain and spinal cord
- neural crest cells arise adjacent to neural tube
- cranial (rostral) neuropore closes by day 25
- caudal neuropore closes by day 27
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what happens early in week four that relates to eye developement?
- prior to cranial neuropore closure
- two optic grooves from on the sides of the forebrain
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what happens late in week four that relates to eye developement?
- optic grooves form outpocketings of the forebrain call optic vesicles
- come in contact with surface ectoderm and induce formation of lens placodes
- happens with neuropore closure
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what happens in week five that relates to eye developement?
- optic vesicle and lens placode invaginate
- optic vesicle forms a double-walled optic cup with inner(neural) layer and outer(pigmented) layer
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what happens in week six that relates to eye developement?
- optic stalk connects optic cup to forebrain
- lens vesicle loses contact with surface ectoderm and lies in mouth of optic cup
- optic stalk becomes optic nerve
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Embryonic origins of structures of the eye:
- Surface ectoderm: lens
- Neural ectoderm: vitreous body, sphincter pupillae and dilator pupillae muscles, optic nerve
- Mesoderm (Neural Crest): sclera, stroma of cornea, ciliary body, iris choroid
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PAX6
Master gene for eye developement-regulates differentiation of optic cup and lens
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SHH
- secreted by prechordal plate
- inhibits PAX6 expression in midline
- upregulates PAX2 expression in the midline
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PAX2
regulates differentiation of the optic stalk
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Which cells have a concentric center-surround receptive field?
- bipolar cells
- ganglion cells
- LGN cells
- granule cells
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types of cells in primary visual cortex
- granule cells
- simple cells
- complex cells
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Posterior Column-medial lemniscus system (PCMLS)
discriminative touch (2-point localization), vibratory sense, conscious proprioception from the body
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Anterolateral System (ALS)
pain, temperature, crude touch sensation from the body
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Trigeminal pathways
all types of sensation from the face
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Spinocerebellar pathways
non-conscious proprioceptive information to the cerebellum
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somatosensory receptors
- mechanoreceptors
- proprioceptors
- nociceptors-pain
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primary sensory neuron
- pseudounipolar
- peripheral process
- cell body (PRG or trigeminal ganglion)
- central process enters spinal cord or brainstem
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secondary neuron
in spinal cord or brain stem
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tertiary (3*) neuron
in thalamic nucleus
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types of mechanoreceptors
- encapsulated: meissner's corpuscle, pacinian corpuscle, ruffini corpuscle
- non-encapsulated: merkel cell neurite complex(hair follicle), free nerve endings
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nociceptors
- all free nerve endings:
- mechanonociceptors
- themonociceptors
- chemonociceptors
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Nociceptors: peripheral (primary hyperalgesia)
- occurs at receptor
- decreased threshold, increased firing rates at lower thresholds
- axon reflex: release of NTs and neuropeptides (substance P) from sensory ending, inflammatory response
- (sun burn)
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Nociceptors: central (secondary hyperalgesia)
- increased excitability of A-delta and C-type fibers
- neuropathic pain (allodynia)
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PCMLS (posterior column-medial lemniscus system) pathway
- primary neuron: peripheral process, cell body in PRG, central process from fasciculus gracilis and fasciculus cuneatus, synapse in caudal medulla
- secondary neuron: nucleus gracilis or nucleus cuneatus, axon decussates to contralateral medial meniscus, synapse in thalamus
- tertiary neuron: ventral posterolateral nucleus (VPL) of thalamus, posterior limb of internal capsule, primary sensory cortex (SI)
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PCMLS: primary sensory cortex submodalities
- 3a, 2 - proprioceptive inputs
- 3b, 1 - cutaneous mechanoreceptors
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PCMLS primary sensory cortex
- S1
- postcentral gyrus (lateral)
- posterior paracentral gyrus (medial)
- Brodmann areas 3, 1, 2
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PCMLS secondary cortical processing sites
- parietal association complex
- Brodmann areas 5, 7
- Secondary somatosensory cortex (SII)
- lesion = astereognosis (inability to identify objects by touch)
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PCMLS blood supply?
- middle cerebral artery
- anterior cerebral artery
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