Exam 1 Flashcards
What is neurulation?
• Central part of ectoderm differentiates into the neural plate; neural plate forms neural tube in 4th week; neural tube differentiates along dorsal/ventral (primary and secondary neurulation) axis by growth signaling factors (Shh from notochord and BMP from ectoderm)
What is primary neurulation?
• Involves columnarization of an existing epithelium, and the rolling or folding the epithelium
What is secondary neurulation?
• Characterized by condensation of mesenchyme to form a rod, which then undergoes an epithelial transition to form the neural tube
How does neural tube close?
• As a result of five separate waves of closure (rather than a simple two-way zipper-like action) during days 19-21
Where does neural tube closure begin?
• In the region of brain stem and upper spinal cord, followed by head and neck
Where does the final closure of the neural tube occur?
• In caudal region where the sacral part of the spinal cord (formed by secondary neurulation) fuses with the rest of the spinal cord (formed by primary neurulation)
What must close for complete CNS formation?
• Rostral and caudal neuropores
What is anencephaly?
• Lack of skull and cerebrum formation, with a brain stem intact; due to failure of wave two closure
What is spina bifida?
• Incomplete formation of both the spinal cord and the overlying vertebrae which remain unfused and open; variable degreed of severity (occulta, meningocele, myelomeningocele); due to incomplete closure of caudal neuropore, located at the junction of waves 1 and 5 (representing the junction of primary and secondary neurulation)
What differentiates the CNS regions?
• Dilations and flexures of neural tube
What are the CNS regions?
- Hindbrain: medulla, pons; myelencephalon, metencephalon
- Midbrain: mesencephalon
- Diencephalon: thalamus, hypothalamus, epithalamus
- Telencephalon: cerebral hemispheres
What becomes of the neural tube space?
• Spinal canal and ventricles of brain stem and cerebral hemispheres
What do rostral and caudal mean?
• Rostral (toward front of brain); caudal (toward spinal cord)
What forms the 3 vesicle stage?
• Prosencephalon (forebrain); mesencephalon; rhombencephalon (hindbrain); spinal cord
What form the 5 vesicle stage?
• Prosencephalon (telencephalon, optic vesicle, diencephalon); mesencephalon; pons (metencephalon); medulla (myelencephalon); spinal cord
What is cephalic flexure?
• Related to cranial base flexion; ensure that optical axes are at right angles to vertebral column
What is pontine flexure?
• Areas of 4th ventricle and pons enlarge; cerebellum derived from edge of pons
What are the subdivisions of the spinal cord?
• Dorsal and ventral roots; Grey matter: divided into sensory, autonomic, and motor areas; while matter: divided into ascending and descending tracts
How is the grey matter of the spinal cord designated?
• Rexed’s lamina are functionally specific areas of gray matter: dorsal (sensory); intermediate (autonomic); ventral (motor)
What are the neurons of the dorsal horn?
• Sensory; afferents convey tactile, proprioceptive, pain and temperature sensations to neurons into lamina 2-5; second order neurons send information to local spinal areas as well as ascend to brain stem and thalamus
What are the neurons of the intermediate region?
• Autonomic; site of preganglionic autonomic neurons (visceral motor, WM)
What are the neurons of the ventral horn?
• Motor; efferent motor neurons project to skeletal muscle groups
How is the white matter of the spinal cord designated?
• Separated into dorsal, central, and lateral funiculi; spinal tracts within white matter are formed by axons of ascending and descending neurons
What is the dorsal funiculus?
• Dorsal columns (cuneate and gracile fasciculi) carry tactile info to brain stem and thalamus
What is the lateral funiculus?
• Lateral corticospinal tract- major descending motor tract from cortex; spinocerebellar tracts- tactile and proprioceptive information to cerebellum; anterolateral system conveys pain and temperature to thalamus
What is the ventral funiculus?
• Anterior (ventral) corticospinal- descending motor pathways from cortex; vestibulospinal and reticulospinal- descending motor pathways from brain stem
What is the propiospinal tract?
• Surrounds gray matter and interconnects various spinal levels
What are the parts of the brain stem?
• Medulla, pons, midbrain
What are the nervous centers/ structures in the midbrain?
• Regulatory centers for respiratory, cardiovascular, GI systems, etc; cranial nerves; sensory and motor pathways; reticular formation
What are the physical features of the cerebellum?
• Located on dorsal side of pons and medulla; cerebellar peduncles: input and output tracts between cerebellum and pons
What is the function of the cerebellum?
• different regions regulate muscle coordination, motor planning and procedural memory, as well as balance and eye movements
What are the structures in the midbrain?
• Substantia nigra (DA modulation of motor control); periaqueductal grey (PAG; regulates pain/ stress response); superior (vision) and inferior (hearing) colliculi; red nucleus (part of descending motor pathway)
What is a cerebral peduncle?
• Found in midbrain; sensory and motor pathways to and from spinal cord, brain stem and cortex
What are the nuclear regions of the diencephalon?
• Thalamus and hypothalamus are both paired structures that flank the third ventricle; epithalamus contains pineal gland
What is the function of the thalamus?
• Several nuclei that process and distribute sensory and motor information to and from the cerebral cortex
What are the structures of the hypothalamus?
• Nuclei; pituitary (anterior and posterior)
What is the anterior pituitary?
• Aka adenohypophysis; derived from ectoderm primordial (Rathke’s pouch from primitive oral cavity); portal system of vessels extend from hypothalamus into anterior pituitary
What is the posterior pituitary?
• Aka neurohypophysis; derived from neural tube; neurosecretory neuronal axons extend into posterior pituitary to release hormones into blood
What are the structures of the cerebral cortex?
• Frontal, parietal, temporal and occipital lobes; insula; central sulcus (separates frontal and parietal lobes); lateral fissure (separates frontal and temporal lobes
What does the frontal lobe do?
• Motor cortex: primary, premotor, supplementary, Broca’s speech area
What does parietal lobe do?
• Somatosensory cortex: primary, secondary, association, Wernicke’s language area
What does temporal lobe do? Occipital?
- Auditory cortex: primary, secondary, association
* Visual cortex: primary, secondary, association
What does the insula do?
• Gustatory, visceral, emotional cortex within lateral sulcus/fissure
What is the reticular formation?
• Nuclei along medial axis of brain stem; neurons receive general sensory input; project to cortex, limbic structures and spinal cord (pretty much everything); often characterized by use of particular neurotransmitter; associated with arousal, attention, motivation and wakefulness, etc (reticular activating system)
What are the structures of the basal ganglia?
• Caudate and putamen (striatum); globus pallidus (GP; both striatum and GP are embedded within central white matter); substantia nigra and subthalamus (located in midbrain)
What are the structures of the limbic system?
• C-shaped cluster of structures that extends into temporal lobe: limbic cortex (orbital and medial prefrontal cortex; cingulate gyrus; parahippocampal gyrus); anterior and medial dorsal thalamic nuclei; hippocampus; amygdala; ventral striatum (ventral basal ganglia) includes nucleus accumbens
Where does the hippocampus project?
• In the temporal lobe, projects medially toward hypothalamus and other structures
What structures of white matter axon bundles interconnect cortical regions?
• Bundles such as superior longitudinal and occipitofrontal fasciculi interconnect cortices, along longitudinal axis; arcuate fibers interconnect local gyri; corpus callosum interconnects left and right hemispheres (visualized by diffusion tensor imaging (MRI of subtle water currents around axons))
What is the relations of sensation and perception?
• Closely related, but have distinct qualities that set them apart; information obtained through collector, receptor, transmission, and coding mechanisms; complement each other to create meanings from what we experience, but two completely different ways of how we interpret out world
What is the definition of sensation?
• Stimulation of a sensory receptor which produces neural impulses that brain interprets as a specific sense, etc; when sensory organs absorb energy from a physical stimulus in environment; sensory receptors then convert this energy into neural impulses and send to brain
What is the definition of perception?
• When the brain organizes the information and translates/interprets into something meaningful (selective attention) or that can be rationalized by us; how one “receives” this feeling or thought, and gives meaning to it through memories and emotions; how our brain interprets a sensation
What does the survival of organisms depend on?
• Having adequate information about both the external and internal environments and how to respond or adapt to any changes; the old rule is “adapt or die”
What does sensation involve?
• Sensory receptors sampling small amounts of energy from the environment including mechanical (pressure, vibration, sound), temperature, light and chemical qualities, such as acidity
How is sensory info processed?
• At different levels of the CNS where it forms an internal representation of specific aspects of the external and internal world
What does the CNS activity do?
• produce feedback to regulate sensory activity at all levels including the receptors themselves; one reason for this is to maintain a tolerable range of sensory stimulation
What are the layers of the eyeball (or orbit)?
• Fibrous coat homologous to dura; vascular coat homologous to arachnoid and pia; nervous coat homologous to CNS
What is the fibrous coat?
• Sclera (dense, white CT where extrinsic eye muscles insert); cornea, continuous with sclera, transparent o allow light to project to retina (avascular, draws its nutrients from aqueous humor by diffusion)
What is the vascular coat/ Uvea / Uveal tract?
• Characterized by prolific vasculature; choroid layer of numerous anastomosing blood vessels; ciliary body/muscle that controls refraction of light by the lens; iris controls amount of light entering eye; uveitis
What is the neural coat?
• Retina
How do cornea and lens refract light from objects onto the retina?
• Have curvatures that refract radially separating rays of light at different degrees; causes the light rays to converge back to a point; most refraction don by cornea (2/3; but this is a fixed structure); remaining refraction done variably by lens
What are the layers of the cornea?
• Epithelium (stratified squamous, non-keratinizing); Bowman’s (basement) membrane: reduces spread of infections, can’t regenerate; Stroma: fibroblasts that generate orthogonal lamellae of collagen fibers; Descemet’s (basement) membrane; endothelium: simple squamous, conveys metabolic substances and water from aqueous humor into the cornea
Where does the corneal epithelium come from?
• Epithelial cells arise from stem cells in adjacent corneoscleral limbus;
How is the cornea replenished?
• daughter stem/ transient amplifying cells (TACs) divide and migrate towards the central cornea to replenish the epithelium, which rests on Bowman’s layer;
How is the cornea protected from UV light?
• DNA protected from UV by nuclear ferritin
How does LASIK work?
• Laser beam reshapes the cornea
Describe the physical of the lens. How is it held in place?
• Clear, avascular and dependent on diffusion of nutrients; held in place by suspensory ligaments: zonule fibers extend from ciliary body to the equatorial perimeter of the lens, maintain resting tension
How is the resting convexivity of the lens maintained?
• Outward pull of suspensory ligaments from the ciliary body; inward pull by intrinsic elastic fibers
What do the intrinsic elastic fibers of the lens capsule do?
• Produce an inherent tendency to bulge, i.e. increase the convexivity of the lens
What are the structures of the lens?
• Capsule is elastic (collagen IV and glycoprotein); germinal zone produces new cells during life; after migration from germinal zone, cells lose their nuclei and become transparent
What is presbyopia?
• Far-sightedness; the age-related loss of resting convexivity caused by loss of elasticity
What are cataracts?
• Reduction of vision due to opaqueness of the lens; oxidative damage, iron-catalyzed free radical reactions; higher iron levels in older and cataractous lenses
How does the lens control light refraction?
• By changing its convexivity; increased convexivity focuses light from a closer source for “near vision”, while decreased from a more distant source (far vision)
What is the ciliary muscle?
• Accommodation=alters convexivity of lens; actively relax tension of suspensory ligs
How does ciliary muscle change the convexivity of lens?
• It is a sphincter, so contraction will increase convexivity
How does convexivity of lens impact close or far vision?
• Convexivity helps see up close; needs to be less convex to see far away
Where does the lens focus light to?
• Onto a point on the retina; light from nearby sources diverges more and requires greater refraction and lens contraction; light from far sources is close to parallel and requires lees refraction by lens
What does eyeball length determine?
• Where the lens focuses the light; corrected by extra lenses
What is myopia and hyperopia?
- Myopia: near sighted; eyeball is too long and light is focused in front of the retina
- Hyperopia: far sighted; eyeball is too short and light is focused behind retina
Where does the aqueous humor come from?
• Ciliary processes secrete humor into anterior and posterior chambers
What is schlemm’s canal?
• Aka scleral venous sinus; absorbs humor into venous system
What is glaucoma?
• Buildup of fluid pressure due to inadequate drainage into schlemm’s canal; one cause is adherence of iris to lens blocking flow and causing iris to press against corneoscleral angle blocking access of aqueous to schlemm’s canal; intraocular pressure can damage optic nerve
What are the structures of the iris?
• Contains pigmented striations of CT, blood vessels and smooth muscle; stroma: CT with melanocytes, melanin absorbs and refracts different frequencies (colors); constrictor and dilator pupillae; pigmented epithelium: continuous on posterior surface of iris, completely absorbs all light restricting incoming light to within pupil
What is the stroma?
• Consists of CT plus radiating and circular patterns of blood vessels
Why is the sky blue?
• Rayleigh scattering (and red sunsets); atmospheric particles scatter blue light during day; at sunset, light takes longer path through atmosphere
What causes eye color?
• Genetic trait that involves distribution of melanin pigments in the iris; determined by different patterns of light refraction; there are no blue or green iris pigments
What causes blue eyes?
• Melanin is mostly on the deep surface of the iris; turbid medium of the stroma causes refraction of the blue part of spectrum and undergoes Rayleigh scattering; so eyes appear sky blue (but not direct effect of melanin)
How do you get brown or green eyes?
• Melanin is more evenly distributed through the iris CT changing the refractive indices of the stroma, plus causing more absorption and reflection of light by the pigments; varied refraction and reflection of the light spectrum produces more color possibilities
What are pigments?
• Absorb some part of the light spectrum, and reflect the rest, giving its color; if everything is absorbed, nothing is reflected=black
Why is melanin brown?
• Brown is a mix of black, red and yellow; melanin absorbs blue/green side, allowing reflection of yellow and red; the more pigment, the more absorption leading to very dark brown or black
What is the purpose of the iris?
• Determines how much light enters the eye by controlling the aperture of the pupil; sphincter pupillae (constricts pupil and reduces incoming light, parasympathetic); dilator pupillae (opens pupil and increases incoming light; sympathetic)
What does pupil size determine?
• Regulate light intensity; focal range: smaller pupils increase the range of focus, wider pupils decrease it
What is the parasympathetic activity of the lens and iris?
• Parasympathetic neurons travel along the oculomotor nerve (CN III); ciliary ganglion is a parasympathetic ganglion located near the posterior surface of the eyeball (project postganglionic neurons along eyeball)
What is the sympathetic activity of lens and iris?
• Arises from thoracic levels of the spinal cord; superior cervical ganglion consists of postganglionic neurons that project axons along arteries to the iris
How does the oculomotor nerve get to the eye?
• Enters superior orbital fissure along with CN IV and VI
What do parasympathetics do?
• Constrict the pupil and contract the lens
Where are parasympathetic preganglionic neurons?
• Located within Edinger-Westphal nucleus, one of the nuclei of the oculomotor nerve in midbrain
What do the postganglionic neurons in the ciliary ganglion activate?
• Sphincter pupillae to constrict pupil; ciliary muscle to contract lens
What is the pupillary light reflex?
• Maintains homeostatic level of light entering eye; melanopsin ganglion cells in retina respond to ambient light; ganglion cells activate pretectum and Edinger-Westphal nuclei in midbrain; pretectal nucleus coordinates both eyes; reflexive constriction of both pupils maintains homeostatic level of light entering the eye
What is the consensual pupillary response?
• Constriction of both pupils in response to light shone in one eye: Light melanopsin ganglion cells pretectal n. E-W nucleus ciliary ganglion pupil (both??)
What does sympathetic activity do? Via what pathway?
• Increases light entry into eye; preganglionic neurons in T1,2 innervate superior cervical ganglion (SCG) cells; postganglionic SCG neurons activate dilator pupillae to open pupil (NB: no sympathetic control of ciliary muscle/lens) and tarsal muscle to raise upper eyelid
How is sympathetic control of the iris and upper eyelid regulated?
• Local reflexes; descending influences from the limbic system and hypothalamus during an emotional state
What is the tarsal muscle?
• Smooth muscle just deep to the levator palpebrae superioris and attaches to the tarsal plate in the eyelid; innervated sympathetically to raise upper eyelid in emotional states;
What can inhibit the tarsal muscle?
• Damage to some elements of the sympathetic nervous system, causing a drooping eyelid (ptosis). This is seen in Horners’s syndrome
What are eye floaters?
• Black or gray specks, strings or cobwebs that drift when you move your eyes; age-related changes as the vitreous humor becomes more liquid leading to depolymerization of collagen; collagen fiber bundles within the vitreous humor clump together and cast shadows on your retina, i.e floaters
What is the vitreous body and humor?
• Vitreous body is clear gel-like fluid contains hyaluronic acid and type II collagen fibrils that are slowly replaced (99% water)
What is the hyaloid canal?
• The vestige of hyaloid artery used to nourish embryonic lens; blood and cell debris is removed by local phagocytes, but residual pieces of hyaloid artery remain as a type of “floater”
What is the eyeball derived from?
• Embryonic brain; neural tube is derived and separated from ectoderm and extends the length of the embryo; rostral part forms the cerebral hemispheres including the eyes; neural tube contacts ectoderm from which lens placode forms
What is the neural retinal layer derived form? Pigmented retinal layer? Lens?
- Neural: Optic vesicle; neurons and photoreceptors
- Pigmented: Optic vesicle; retinal pigment epithelium, PRE
- Lens from ectoderm, not neural tissue
What is the sclera and choroid derived form?
• Embryonic meningeal tissues
What surrounds the optic nerve?
• Dura, arachnoid, pia and subarachnoid space
What is the hyaloid artery?
• Nourishes vitreous body and lens; degenerates postnatally, leaving a hyaloid canal and remnants of the artery
What is the retina?
• Nervous coat of orbit; innermost layer receives light that has passed through the cornea, aqueous humor, lens and vitreous body
What is the fovea centralis?
• Central point of retina with only cones and no rods, imparting high levels of visual acuity; center of the visual field
What is the macula lutea?
• Yellow area surrounding fovea; yellow from accumulated lutein and zeaxanthin, carotenoids derived from the diet; both pigments are antioxidants and absorb excess blue and ultraviolet protective role), leaving the yellow reflected and hence visible
What is the optic disc?
• (“blind spot”); contains axons of “ganglion cells”, but no receptor cells
Where is the central artery of retina? What does it do?
• Radiates out and surrounds the macular region; blood supply to retina; branch off the internal carotid; courses through optic tract and exits onto inner surface of retina
What are the retinal layers, from outer to inner eye?
• Retinal pigmented epithelium; photoreceptors (rods and cones) absorb visible light; neural cells integrate light info: bipolar, horizontal and amacrine cells; ganglion cells project visual info to thalamus
How does light reach the photoreceptors?
• By passing through the other retinal layers (inverted retina)
What are the photoreceptors?
• Rods and cones; outer segment with stack of discs contain pigments (iodopsin in cones, rhodopsin in rods); inner segment with nucleus and synaptic terminal that release glutamate
What is visible light?
• Part of the frequency spectrum of electromagnetic radiation, the same stuff that makes radio and gamma waves
Where are the pigments found? What are they?
- Rhodopsin (rods) and iodopsin (cones) are located in vesicular discs
- Forms of opsin, respond to different parts of spectrum of visible light (450-700 nm)
How do photoreceptors respond to light?
• By hyperpolarizing the cells and releasing less transmitter (glutamate)
How does this work?
• light converts 11-cis retinal (aldehyde form of Vit A) to trans-retinal, cause opsin to activate PDE via transducing (a G protein); PDE reduces the background levels of cGMP which closes Na+ and Ca2+ channels, hyperpolarizing the cell; net effect is to release less glutamate
What is the function of cones?
- Photopic: vision in bright light; low sensitivity (less pigment)
- High acuity (concentrated in fovea); sensitive to direct axial light; good spatial resolution
- Chromatic: color vision: 3 types of pigmented cells responsive to different parts of the spectrum
What is the function of rods?
- Scotopic: vision in dim light; high sensitivity (more pigment; night vision)
- Low acuity (rods absent in fovea); sensitive to scattered light; poor spatial resolution; better to detect visual motion
- Achromatic: only one type of pigment (rhodopsin)
Where is the general location of rods and cones?
• Rods primarily in peripheral part of retina and do not code for color; cones concentrated in fovea, but spread out thinly to provide peripheral color vision
How is the fovea modified for highest possible acuity?
• Only cones, favoring acuity and color; cones are more exposed to incoming light by the outward dispersal of ganglion and other integrative cells; each cone activates a single ganglion cell increasing the acuity of vision
Why must photoreceptors be very close to the choroid blood supply?
• Pigment turnover requires high levels of oxygen; inner layer cells draw nutrients from smaller retinal arteries
Where is the choroid located?
• Behind the retina, because light cannot penetrate blood vessels well
How can light pass through the ganglion and bipolar cells without distortion?
• They have the same refractive index as the vitreous humor
Where is the retinal pigment epithelium (RPE) located?
• Pigmented cell layer is deep to the photoreceptors
What are the functions of the RPE?
- Visual acuity: RPE absorbs light passing through retina in order to limit reflection of light back into photoreceptors; reflected light would blur the image
- Antioxidant: pigments absorb blue light (blue light increases levels of free radical in retinal cells)
- Maintain photoreceptor excitability: trans-retinal formed by light absorption is transported from photoreceptor to RPE, reformed to 11-cis-retinal and redelivered to photoreceptors (visual cycle of retina)
- Nutrient: RPE transports nutrients such as glucose and retinol (source of retinal), to photoreceptors
- Phagocytosis of photoreceptor cell debris that result from light absorption: deficient uptake of membrane fragments accumulate, separate the receptors from the choroid, leading to anoxia and cell death (retinitis pigmentosa)
Why is the fovea avascular?
• Reduces vascular interferes with high acuity; capillaries encircle fovea, but there are no veins; extravasated salt and water is transported from the extracellular space into the choroid veins via membrane transporters in the RPE
Why does retinal detachment occur?
• The contact between the neural retina and the RPE is mechanically unstable (neural retina tears away from the RPE)
What can cause retinal detachment?
• Macular degeneration due to rupture of blood vessel or buildup of cellular waste material
What is exudative macular degeneration?
• Macular degeneration with subretinal bleeding
What are the integrative neurons forming the remaining layers of the retina?
• Bipolar cells, horizontal and amacrine cells integrate info from receptor cells and activate ganglion cells
What are ganglion cells?
• Output cells of the retina transmit visual info to the thalamus, superior colliculus, and to the brain stem areas; axons converge at the optic disc to form the optic “nerve” (it’s a CNS structure so should be called a “tract”)
What does the optic nerve/tract consist of?
• Axons of ganglion cells that transmit visual info to the thalamus; CNS ganglia; central artery and veins of the retina
What are the meninges of the optic nerve? Glia?
- Dura, subarachnoid space with CSF; pia mater
* Oligodendrocytes myelinate the axons; astrocytes surround cell bodies and dendrites; contact blood vessels
What is papilledema?
• Increased CSF pressure limits venous return from retina; causes edema under optic disc (bulging optic disc); visible with ophthalmoscope; blurred disc margins and dilated tortuous veins; axoplasmic stasis of ganglion cells
Why is the optic disc called the blind spot?
• Has no receptor cells and hence has no structures to respond to light
What happens normally in binocular vision to account for blindspot?
• Visual info within blind spot is interpolated from adjacent areas of the retina during saccadic movements or by activity in visual cerebral cortex
What is the retinal processing of visual input?
• Light triggers hyperpolarization in photoreceptor cells, leads to decreased release of glutamate; change bipolar cell membrane potentials, trigger differential activity in ganglion cells
How are on/off bipolar and ganglion cells activated?
• Light or absence stimulates different rod and cone glutamate receptors to generate paths of ON and OFF responses; light inhibits glutamate release from rods/cones; darkness enhances it
How are ON cells activated?
• ON bipolar cells have glutamate receptors that response to decreased glutamate with depolarization and excitation of the ganglion cell; ON ganglion cells are activate din the presence of light in center of the RF; OFF cells are inhibited
How are OFF cells activated?
• With darkness in center of the RF, photoreceptors release more glutamate; OFF bipolar cells have glutamate receptors that respond to increased glutamate with depolarization the excites the OFF ganglion cell
So happens when there is no light?
• OFF bipolar-ganglion cells are excited; ON bipolar-ganglion cells are inhibited; hence there is an active response to darkness in the retina
What happens to cell polarizations in light?
• Light hyperpolarizes cone receptor, less glutamate release; ON bipolar cell depolarizes, so ON ganglion cell fires Aps; OFF bipolar cell hyperpolarizes, so OF ganglion cell does NOT fire Aps
What happens to polarizations in darkness?
• Darkness on cone provides constant release of glutamate; ON bipolar cell hyperpolarizes, so ON ganglion cells does NOT fire Aps; OFF bipolar cell hyperpolarizes, so OFF ganglion cell fires Aps
What are the center-surround receptive fields of ON and OFF cells?
• Circular areas of retina are organized into concentric rings where inner receptors activate ganglion cells one way, the surrounding region activates it in the opposite way; each ganglion cell responds to light shining on the center of field opposite to that of peripheral field
What do on-center and off-center retinal ganglion cells respond to?
- On-center: light spots surrounded by dark backgrounds like a star in a dark sky
- Off-center: dark spots surrounded by light backgrounds like a fly in a bright sky
What is lateral inhibition?
• Center-surround receptive fields are created; affects both ON and OFF cells; activity of one cell inhibits another, for increased perception; creates contrast and sharpness; HORIZONTAL cells activated by peripheral part to reverse activity of center receptors, so center now inhibits bipolar and ganglion cells
How does lateral inhibition affect both ON and OFF cells?
- ON: inhibited by horizontal cells, which ON cells normally respond to light in the center
- OFF: excited by differential release of glutamate; and activates horizontal cells
How do ganglion cells encode contrast?
• With center-surround fields; ON and OFF cells response best when their receptive fields subtend light-dark edge; each type responds oppositely to light and dark sides; they enhance contrast, so a moderately bright region may appear brighter or darker depending on the background; higher rate of activity of ganglion cells with more contrast between center-surround receptive fields
What do retinal ganglion cells respond to?
• Edges (contrast); center-surround receptive fields emphasize edges
What are mach bands?
• Contrasting shades of edges make one part of a shade appear darker or lighter than it really is; may cause artifacts is xrays, where contrasts “appear” where they shouldn’t
How do ganglion cells code color?
• Respond to a specific color by reciprocal excitation-inhibition by various cones with different pigments differentiates colors (i.e. they are stimulated by one color and inhibited by others)
What are the types of colorblindness measured with dots, colors and numbers?
• Red-green (74 read as 21); red (protanope) reads 2 instead of 42; green (deuteranope) reads 4
How is visual info projected to cortex for conscious perception?
• Visual field is bisected into right and left sides, each projecting to opposite side of brain; done by partial decussation (of ganglion cell axons in optic chiasm); right and left go to opposite LGN and primary visual cortex; center of field (at fovea) is sent to both sides of brain
How does total blindness in one eye occur?
• Lesion of one optic nerve distal to the chiasm
What is hemianopsia, and its types, heteronymous, homonymous?
- Hemianopsia: loss of half of visual field
- Heteronymous: loss of opposite visual fields due to lesion of central optic chiasm (pituitary tumors)
- Homonymous: loss of same visual fields due to lesion of optic system b/w optic chiasm and visual cortex
What is the retina-LGN-cortex pathway?
• Includes functionally distinct pathways that originate from different types of ganglion cells; conveys different types of visual info along each route and underlies reconstructing the visual world and perception
What cells are responsible for projecting to the LGN?
• Parvocellular (P, midget, parvo=small) and magnocellular (M, parasol, magno=large) ganglion cells; LGN then projects to visual cortex
What does the parvocellular pathway do?
• Transmits color and shape info for object perception and identification; 90% of axons in optic nerve; receive input from 1 or a few photoreceptors (esp cones in fovea) and their ON and OFF bipolar cells; high acuity with smaller center-surround receptive fields; sensitive to color and shape
What does the magnocellular pathway do?
• Transmits mov’t related info for perception of motion and direction to enable visual attention, alerting, grasping, etc; 5% axons in optic nerve; input from larger group of photoreceptors (esp rods in periphery) and ON and OFF cells; low acuity with large center-surround RFs; respond to mov’t with low spatial resolution; dark or night vision
What is the koniocellular pathway?
• Transmits some low acuity and color info to primary visual cortex, but mostly to extrastriate cortex for visual behavior outside of the usual visual consciousness; related to blindsight
What is the lateral geniculate nucleus?
• Visual thalamus; six-layered structure that receives input from each eye in alternating layers; parvo- and magnocellular pathways project their small and large center-surround RFs to different sectors of the LGN
What is the visual cortex?
• Primary visual cortex (V1) on medial surface of occipital lobe; concentric cortical areas extend out from V1 to association visual areas V2, V3, V5; LGN neurons project the retinal center-surround RFs to V1
What is V1 responsible for?
• Develops perception of form, color, direction of mov’t and binocular vision through activity of simple and complex cells organized in columns
What are simple cells?
• Respond to stimuli such as edges; combine input from several geniculate cells that individually respond to contiguous points along the bar of light; respond to bars of light with specific orientation, directions of mov’t, and exact location within the RF; RFs are more rectangular/oval than round compared to retinal or geniculate RFs
What are complex cells?
• Similar to simple cells: they respond to a properly oriented edge or bar of light, but anywhere within the RF; combine the info of several simple cells and detect the position and orientation of a structure
Why may the concept of simple and complex cells be not so useful?
• Cortical RFs are malleable depending on visual context, so no fixed RF can be attributed to cortical cells (simple or complex)
What are hyper-complex cells?
• Detect endpoints and crossing lines from info about position and orientation
What is the association cortex?
• Aka secondary cortex; integrates all the info of simple, complex, and hyper-complex cells to generate a sensibility of familiar objects
What do the cortices do to an image?
• Deconstruct a visual image into variants of orientation, contrast, shape
What are visual cortical columns?
• Vertical zones of cortex that use combos of simple and complex cells to sort info into various properties: orientation of edges, color, shape/size, direction of mov’t, R or L eye (ocular dominance)
How is color registered?
• Color opposite pairs of red-green or blue-yellow center-surround RFs from retina and LGN are sorted into regions of cortex called “blobs”; blobs are inserted within columns and process the various relative activities of color cells to form the perceptual palette
What is binocularity or stereopsis?
• Depth perception comes from binocular input of complex cells in the upper and lower layers of primary and association cortices
How is depth perception determined?
• Binocularly, by retinal disparity and convergence of the eyes; although there is some depth perception using one eye using various visual cues
What are binocular and monocular visual cues?
- Binocular: convergence, retinal disparity
- Monocular: accommodation, interposition, linear perspective, texture gradient, shading, relative size, relative height, relative motion
How do cortical columns provide feedback?
• Generate complex activity that is sent to sensory pathways; so as to focus on particular aspects of the environment; sent to LGN, thus shaping sensory input according the behavior, affecting what when, and how visual signals are transferred to the cortex
What other way modulates LGN cells?
• By brainstem using NE, 5HT, ACh, NO (reticular formation)
What is the purpose of cortical feedback to LGN?
• Enhances acuity of their responses to bars of light by altering lateral-inhibition-like mechanisms among LGN neurons; makes responses more discrete, sharpened in the LGN; increases the number of LGN cells responding to the same stimulus; generate synchrony of cortical neurons
How do hormones from brainstem affect LGN cells?
• NO and ACh (parabrachial nucleus) enhance transmission of visual info by stimulating LGN and cortex (via basal forebrain, which also releases ACh and NO); shifts b/w sleep and waking; affects cortical feedback and waking up thalamus for alertness; may cause intense emotions and flash of light??
Where does V1 project to, doing what?
• Dorsal and ventral streams of association cortices; integrates cognition and visual input with touch, proprioception, audition, etc.
What is the ventral stream?
• Parvocellular; takes info from fovea (cones, acuity, color), to diff parts of association cortices, like inferotemporal gyrus; for high levels of visual perception and cognition (identify form, color, orientation, and understanding WHAT you are looking at); color, texture, pictorial detail, shape, size (object processing)
What is the dorsal stream?
• Magnocellular; takes info from peripheral retina (rods) to posterior parietal cortex; integrates with motor cortex for what action to take; answers the question HOW and WHERE; location, mov’t spatial transformations, spatial relations (spatial processing)
Where do both dorsal and ventral streams converge?
• Into prefrontal cortex to form working memory
What are the visual association cortices?
V2, V4, IT (inferotemporal), MT (middle temporal)
What does V2 do?
• More complex properties; angles b/w lines, illusion outlines, whether something is part of figure or ground
What does V4 do?
• Color and complex shapes, like corners and outlines inferred by the mind
What does inferotemporal cortex do?
- recognize faces, hands, complex objects; responds to both halves of visual field via corpus callosum; emotional expression
- Convergence of simple and complex cells; neuron activity modulated by attention; both short and long term memory, modified by experience; impacted by hippocampus and amygdala (responsible for making memory in cortex)
- Déjà vu: Penfield and neurosurgery for epilepsy, cortical stimulation can elicit déjà vu, real or imagined
What does the middle temporal cortex do?
• Detects motion; part of dorsal stream; complex and global motion
What is visual neglect syndrome?
• Loss of function in the right visual association cortices; cannot see or comprehend left visual field in vision or mind
What is the fusiform gyrus?
• Involved in face recognition, especially connected with emotions (130 msec latency); dysfunction may cause hallucinations
What is synesthesia?
• Hereditary; experience different modalities/senses simultaneously; possible cross-activation of color-coded and number neurons in fusiform gyrus (Ramachandran)
What are the secondary visual pathways?
• For non-conscious visual responses and behavior; includes koniocellular and melanopsin ganglion cell pathways
Where do koniocellular ganglion cells project to?
• Superior colliculus, and involved in visual orientation, saccades, blindsight, emotion; have moderate spatial resolution, large RFs, no center-surround fields
Where to melanopsin ganglion cells project to?
• Pretectum for the pupillary reflex; SCN (suprachiasmatic nucleus) for circadian rhythms
What is the superior colliculus pathway?
- Receives info from retinal koniocellular cells, and from visual, auditory, association, and motor areas of cerebral cortex
- Projects to cortices for eye mov’t and saccades, and to brain stem and spinal cord to turn head
- Functions to orient head and eyes toward visual stimuli to answer “where is it?”
- SC also modulates CN III, IV, VI (eye movement muscles)
What are saccadic eye mov’ts?
- Quick , simultaneous mov’ts of both eyes in same directions; initiated by cortices or SC
- Reflexive: triggered by stimulus to orient your vision
- Scanning: triggered from within in to explore environment or read; like a movie of fast frames
What is blindsight?
• Lesion of V1 causes no consciousness of visual info, as it goes to parietal cortex; although still have some responses of info, and even emotions; generated by “extrastriate” midbrain pathway
What is the pathway proposed for blindsight?
• Koniocellular, which project to SC pulvinar posterior parietal cortex (dorsal stream); pulvinar also projects to amygdala (emotions); may also be involved in “auto pilot,” when you drive while thinking and not consciously paying attention to road
What is the pulvinar nucleus?
• Large thalamic nucleus; interconnects several cortical areas; projects aversive emotional stimuli to amygdala, and also to orbitofrontal (prefrontal) cortex)
What is the visual-emotion pathway?
• Non-conscious retinal koniocellular SC pulvinar amygdala and orbitofrontal (prefrontal) cortex; pathway involved in blindsight
What does the amygdala do?
• Stimulated NE pathways (from LC) from reticular formation to cortex for mental arousal and awareness
What is the different between visual perceptions and hallucinations?
- Externally perceived: activate specialized visual areas
* Internally perceived: activate frontal and parietal areas
What are the common types of hallucinations (syndromes)?
• CBS; ACh and 5HT deficiency
What is CBS?
• Charles Bonnet or Deafferentation syndrome, hallucination come from parietal, superior or ventral temporal cortex; hallucinations of faces involves fusiform face area, colors in V4
How do hallucinations arise?
- Activation of specific cortex or intercortical connections of occipital, temporal, parietal cortex
- Disturbance of cortical area by ACh and 5HT pathways from reticular formation
What types of hallucinations does CBS most influence?
• Occipital visual areas more than in anterior ventral temporal lobe, resulting in mostly simple rather than complex hallucinations
What types of hallucination are due to ACh and 5HT deficiency?
• influences anterior ventral temporal regions more than occipital; results in more complex hallucinations
What are melanopsin-containing retinal ganglion cells?
- Aka ipRGC (intrinsically photosensitive retinal ganglion cells); contain pigment melanopsin, sensitive to blue light (reduce ROS); light absorption initiates action potentials
- Stimulate subcortical visual pathways: SCN (circadian) and Pretectum (pupillary reflex)
What is the SCN?
• Hypothalamic nucleus (just above optic chiasm); pacemaker of circadian rhythms, influenced by light and dark cycles; projects axons to PVN (paraventricular hypothalamic nucleus, affects sympathetic (IML) and parasympathetic (DMV) systems); subpopulation of SCN cells have peak activity at different times; cells target peripheral clocks of skin, liver, pineal, adrenal, more
What is melatonin?
• SCN regulates pineal release of melatonin via sympathetic nervous system; regulates various organs for sleep-wake cycles, temperature, cortisol release (directly or via SCN); released with a decrease in light to promote sleep
What is the pupillary light reflex?
• Reflexive constriction of both pupils, maintains neutral/homeostatic amount of light entering eye; melanopsin ganglion cells in retina respond to ambient light, activate pretectum (coordinates both eyes) and Edinger-Westphal nuclei in midbrain
What is the consensual pupillary response?
• Constrict both pupils by shining light in one eye: light melanopsin ganglion cells pretectum E-W nucleus ciliary ganglion pupil
What is the E-W nucleus?
• From oculomotor nerve; supplies constricting muscles of iris and ciliary muscle
What does sympathetic activity do to eye?
• Increases light into eye; preganglionic neurons in T1,2 innervate superior cervical ganglion (SCG) cells; postganglionic SCG neurons activate dilator pupillae (there is no sympathetic control of ciliary muscle/lens) and tarsal muscle
What regulates sympathetic control of iris and upper eyelid?
• Local reflexes; descending influence from limbic system and hypothalamus during emotional state
What is the tarsal muscle?
• Smooth muscle, just deep to levator palpebrae superioris, attaches to tarsal plate in eyelid; innervated sympathetically to raise upper eyelid in emotional states; inhibited by damage to sympathetics, cause droopy eyelid (ptosis, Horner’s syndrome)
- What is the vitreous body?
Clear gel-like fluid; contains hyaluronic acid and type II collagen fibrils that are slowly replaced; 99% water
What is the hyaloid artery and what did it do?
Used to nourish the embryonic lens; and became the hyaloid canal
What are floaters?
The blood and cell debris of the hyaloid artery is removed by local phagocytes, but residual pieces remain as a type of floater
- Compare the sclera to the cornea.
Sclera is dense white CT, where intrinsic eye muscles insert; cornea is continuous with sclera, but transparent to allow light to project to retina, and is avascular, drawing its nutrients from aqueous humor by diffusion
- How much do the cornea and the lens each contribute to light refraction?
Most (2/3) refraction is done by the cornea, but it is a fixed structure; the remaining refraction is variably controlled by the lens
- What is the cellular and CT makeup of the lens and how is it attached within the orbit?
- capsule is intrinsic elastic fibers (type IV collagen and glycoprotein); germinal zone produces new cells during life, lose nuclei and become transparent
- held in place by suspensatory ligaments (zonule fibers extend from ciliary body to the equatorial perimeter of the lens)
How does the lens accommodate to near and far vision?
Near: becomes more convex
Far: becomes less convex
What muscles and nerves are involved?
Near: ciliary muscle (sphincter) contracts to actively relax tension allow intrinsic elastic fibers of lens to increase convexivity
Far: ciliary muscle to make lens less convex
What are cataracts?
Reduction of vision due to opaqueness of the lens; oxidative damage, probably from iron-catalyzed free radical reactions (responsible for virtually all oxidative tissue damage), and iron levels are higher in older at cataractous lenses
- Describe aqueous humor and its circulation.
Ciliary processes secrete humor into anterior and posterior chambers; schlemm’s canal (scleral venous sinus) absorbs humor into venous system
How does it compare to the vitreous body?
?? vitreous is gel-like with no circulation?? While aqueous humor is not a gel, but a thinner liquid, that is secreted and reabsorbed
What is glaucoma?
Buildup of fluid pressure due to inadequate drainage into schlemm’s canal; intraocular pressure can damage optic nerve; one cause is adherence of iris to lens blocking flow and causing iris to press against corneoscleral angle blocking access of aqueous to schlemm’s canal
- Describe the iris in terms of CT, blood vessels, smooth muscles, innervation and pigments.
The iris stroma is CT with melanocytes (pigmented striations of CT) and radiating and circular patterns of blood vessels; smooth muscle (constrictor and dilator pupillae); pigment epithelium is continuous on the posterior side of iris,
How does it affect entry of light into the orbit?
Pigmented epithelium completely absorbs all light restricting incoming light to within pupil; the sphincter pupillae is parasympathetic (ciliary ganglion), constricts pupil and reduces incoming light; the dilator pupillae is sympathetic (superior cervical ganglion), opens pupil and increases incoming light
Account for the different colors of the eye.
Color is determined by different patterns of light refracted through stroma (turbid medium, Rayleigh scattering); blue eyes, melanin is mostly on the deep surface of the iris, and deeper refraction through stroma causes iris to appear blue; brown and green eyes, melanin is more evening distributed through iris CT, so refractive index is lower, more absorption and reflection of light by the pigments
- Describe the circuit and action of the consensual pupillary reflex.
Constriction of both pupils in response to light shone in one eye; light melanopsin ganglion cells pretectal n. E-W nucleus ciliary ganglion pupil
what is the ciliary ganglion?
Parasympathetic ganglion from E-W nucleus; postganglionics innervate sphincter pupillae (dilate pupil) and ciliary muscle (convexivity of lens, accommodation)
- Describe the embryonic development of the eye: retina, meninges, lens
Retina: neural and pigmented retinal layers are derived from the optic vesicle: neural layer becomes neurons and photoreceptors; pigment layer becomes RPE
Meninges: sclera and choroid are derived from embryonic meningeal tissues; optic nerve is surrounded by dura, arachnoid, pia and a subarachnoid space
Lens: lens is derived from ectoderm; ectoderm merges with the neural tube (optic vesicle), and the invaginating lens placode is pinched off from the ectoderm, then developing and becoming highly differentiated
- Describe the blood supplies associated with the retina: choroid and central artery of the retina.
Choroid: branched from the central artery of the retina??; located behind retina because light cannot pass through blood vessels well; pigments have high turnover, so choroid must be close to the RPE
Central artery of the retina: blood supply to retina, branch off the internal carotid; courses through optic tract and exits onto inner surface of retina; radiates out and surround the macular region
- Describe the fovea, macula and optic disc.
Fovea centralis: central point of retina with only cones and no rods, high level of acuity
Macula lutea: yellow area surrounding fovea, due to accumulated lutein and zeaxanthin (carotenoids) antioxidants which absorb blue/UV light
Optic disc: has no photoreceptors, but contains axons of ganglion cells; where optic nerve enters retina
What is the blind spot?
Optic disc, because has no photoreceptors
- Describe the layers of the retina and what the different cells do.
RPE: pigmented cell layer deep to photoreceptors; visual acuity (absorbs light so not reflected); antioxidant (absorb blue light); maintain photoreceptor excitability (RPE reforms cis-retinal and sends back to photoreceptors); nutrient; phagocytosis of cell debris
Photoreceptors: rods and cones absorb visible light; outer segment with stacks of discs contain pigments (iodopsin in cones, and rhodopsin in rods); inner segment with nucleus and synaptic terminal that release glutamate
Neural cells: integrate light information; bipolar, horizontal, and amacrine
Ganglion cells: project visual information to thalamus
- What are the differences between rods and cones?
Rods: Scotopic (vision in dim light), high sensitivity, more pigment, night vision, low acuity (no rods in fovea), sensitive to scattered light, poor spatial resolution, better to detect visual motion, achromatic (only rhodopsin)
Cones: Photopic (vision in bright light), low sensitivity (less pigment), high acuity (concentrated in fovea), sensitive to direct axial light, good spatial resolution, chromatic (color vision with three types of pigmented cells respond to different parts of spectrum)
How does light activate rods and cones?
Light hyperpolarizes the cells to release less glutamate; light converts 11-cis retinal to trans-retinal, causing opsin to activate PDE via transducin (a G protein); PDE reduces cGMP which closes Na+ and Ca2+ channels, hyperpolarizing the cells; net effect is less glutamate released (the transmitter)
- Describe the fovea in terms of receptor cells and retinal organization.
Fovea is concentrated with cones, by has no rods (favoring acuity and color). Located at the posterior orbit, but not at the optic disc. The center of the visual field
How does this differ from the peripheral retina?
Rods are primary in the peripheral part, with a much higher number than cones, although cones are still present throughout the retina
- Describe the functions of the retinal pigmented epithelium in regards to retinal support.
Absorbs light passing through retina in order to limit reflection of light back into photoreceptors (visual acuity; reflected light would blur the image); antioxidant by pigments absorbing blue light (and UV) which would increase ROS formation; maintains photoreceptor excitability (turns trans-retinal back into 11-cis-retinal and takes back to photoreceptors); transports nutrients such as glucose and retinol to photoreceptors; phagocytosis of photoreceptor cell debris from light absorption (that would separate receptors from choroid, leading to anoxia and cell death (retinitis pigmentosa))
- What kinds of retinal detachment are there?
Retinitis pigmentosa and macular degeneration; “wet” MD is due to bleeding and is a fluid buildup, vision loss can be sudden; “dry” MD is due to waste product buildup under retina, and vision loss is gradual
Between which layers does retinal detachment occur?
The neural retina tears away from the RPE
- Describe the optic nerve/tract in terms of neurons, glia, meninges and CSF.
Consists of axons of ganglion cells that transmit visual info to the thalamus; has CNS glia (astrocytes and Oligodendrocytes); intertwined with central artery and veins of retina; surrounded by CSF, subarachnoid space, pia mater, and dura
What is papilledema?
Increased CSF pressure limits venous return from retina; causes edema under optic disc (visible with ophthalmoscope: blurred disc margins and tortuous veins)
- Describe the neural sequence from retina to primary visual cortex.
Parvocellular and magnocellular ganglion cells project their info to thalamic lateral geniculate neurons, which then project to the primary visual cortex. Visual field is bisected into right and left side, each side projecting to opposite side of brain (by partial decussation)
What is partial decussation?
Of ganglion cell axons in the optic chiasm; right and left visual fields project to opposite lateral geniculate body and primary visual cortex (each eye detects both left and right visual fields, but all left field goes to right brain, and all right field goes to left brain)
- What is the difference between homonymous and heteronymous hemianopsia?
Hemianopsia: loss of half visual field
Heteronymous: loss of opposite visual fields due to lesion of central optic chiasm (pituitary tumors)
Homonymous: loss of same visual fields due to lesion of optic system b/w optic chiasm and visual cortex
- Describe the differences between ON and OFF retinal cells.
ON: have glutamate receptors, respond to low glutamate with depolarization and excitation of cell; activated in presence of light in center of RF; inhibited in no light
OFF: photoreceptors release more glutamate with darkness in center of RF; OFF cells depolarized and excited by high glutamate levels; inhibited by presence of light in center of RF
What are center-surround receptive fields in the retina and how do they affect perception of visual boundaries?
Circular areas of retina, concentric rings, where inner ones activate ganglion cells in one way, and surround activates it in opposite way; on cells respond to center light surrounded by dark; off cells respond to dark center with light surround
- What is the difference between the parvocellular and magnocellular pathways from retina to LGN and visual cortex?
Parvocellular: make up 90% of axons in optic nerve; receive input from 1 or a few photoreceptors (esp cones in fovea) and their ON and OFF bipolar cells inferotemporal gyrus prefrontal cortex
Magnocellular: 5% of axons in optic nerve; receive input from large group of photoreceptors (esp. rods in periphery) and their ON and OFF bipolar cells posterior parietal cortex prefrontal cortex
How do they relate to ventral and dorsal streams cortical and processing of visual information?
Parvocellular: is ventral stream
Magnocellular: is dorsal stream
- Give a brief description of simple and complex cells in the primary cortex.
Simple: combine input from several geniculate cells that individually respond to contiguous points along the bar of light; respond to bars of light with specific orientations, directions of mov’t, and exact location within the RF (receptive field);
Complex: respond to a properly oriented edge or bar of light, but anywhere within the RF; combine the info of several simple cells and detect the position and orientation of a structure; hyper-complex cells detect endpoints and crossing lines from info about position and orientation
How do simple/complex cells differ from ganglion and lateral geniculate cells?
RFs are more rectangular/oval than round; receive info from geniculate cells, which receive info from ganglion cells
What characterizes visual cortical columns?
Vertical zones of cortex that use combinations of simple and complex cells to sort info into various properties: orientation of edges, color, shape/size, direction of mov’t, R or L eye (ocular dominance)
- What impact does cortical feedback have on incoming visual input?
Cortical feedback to LGN influences the sensory input; affecting what, when, and how visual signals are transferred to the cortex; enhances acuity of responses to bars of light by altering later-inhibition-like mechanisms of LGN neurons, making response more discrete; increases the number of LGN cells responding to the same stimulus, generating synchrony among cortical neurons
- What impact does neuromodulation by the reticular formation have on incoming visual information?
LGN modulated by brainstem (reticular formation) by NE, 5HT, ACh, NO; NO and ACh enhance transmission of visual info by stimulating LGN and Cortex (whose neurons also release ACH and NO); changes with sleep or waking; may cause intense emotions and flash of light?
- What are the visual association cortices?
V1: projects orientation, spatial frequency, color to association cortices for integration into objects V2: tuned to complex properties: angles b/w lines, orientation of illusory contours and whether the stimulus is part of the figure or the ground V4: tuned to color and more complex shape attributes, like corners that can be built into complex outline shapes Inferotemporal cortex (IT): scale and position invariance, indicating sensitivity to global form, and responsive to objects like faces; shape/color of complex shape; hand images; emotional expression MT (middle temporal): part of dorsal stream for detecting complex, global motion
How do the inferotemporal and fusiform cortices differ from the other visual association cortices?
IT: responds to both halves of visual field, info transmitted by interhemispheric axons via corpus callosum; activity can be modulated by animal’s attention; short and long term memory for visual stimuli (Déjà vu)
Fusiform: face recognition ; may lead to hallucinations; associated with synesthesia
- Describe visual neglect and its impact on perception of one’s visual field.
Loss of function of the right visual association cortices; can’t see left visual field, also can’t conjure left field of a picture from memory; not simply a sensory problem, but a problem of consciousness
- What does Wernicke’s area do?
Involved in understanding of written and spoken language; cerebral cortex linked to speech
- Describe the visual pathways mediated by the superior colliculus (SC).
Modulates CN III. IV, and VI; initiates saccades; projects to pulvinar nucleus and then to posterior parietal cortex (dorsal stream)
Visual-emotional pathway: non-conscious retinal koniocellular SC pulvinar aversive emotional stimuli to amygdala, cingulate and orbitofrontal cortex
What role does the SC have in eye movements?
Modulates CN III (oculomotor), IV (trochlear), and VI (abducens); initiates saccades
- What are saccadic eye movements and how do they impact vision?
Saccade: quick simultaneous mov’t of both eyes in the same direction
Reflexive saccade: triggered exogenously by peripheral stimulus to orient your vision
Scanning saccade: triggered endogenously to explore surroundings or reading (ratchet mov’t, you only see during the pauses; vision is like a movie of many frames)
- What is blindsight and how does it relate to the SC?
Lesion of primary visual cortex leads to complete lack of consciousness of visual info; generated by “extrastriate” midbrain pathway: visual input to parietal instead of primary visual cortex (which is striates, this “extrastriate”) Koniocellular pathway (proposed): koniocellular ganglion cells SC pulvinar nucleus posterior parietal cortex (dorsal stream), amygdala (emotions)
- What does the suprachiasmatic nucleus do?
A hypothalamic nucleus (just above optic chiasm); pacemaker of circadian rhythms; project axons to paraventricular hypothalamic nucleus
Describe the pathway how light impacts the SCN.
Melanopsin ganglion cells project to SCN (melanopsin most sensitive to blue light, light absorption initiates action potentials; intrinsically photoreceptive retinal ganglion cells (ipRGCs)); light and dark cycles entrain SCN activity
What are circadian rhythms?
Sleep-wake cycle
- How is melatonin released by the SCN?
SCN regulated pineal release of melatonin via the sympathetic nervous system
What impacts does melatonin have on the systems covered in class?
Regulates various organs, including sleep-wake cycles, temperature, and cortisol release; released with a decrease in light; it is a sleep promoting factor
