CNS

Question 1

Central Nervous System

Answer 1

• PNS comprises of somatic and autonomic nervous system
• Gray matter: Nerve cell bodies, unmyelinated axons, in clusters called nuclei. White matter: Consists of myelinated axons running in bundles called tracts. PNS: Clusters of neurons called ganglia and bundles of axons called nerves
• Dorsal root carriers afferent(incoming) signals, ventral root carries efferent(outgoing) signals from CNS to body
• Ascending tracts carry signal to the brain(dorsal), descending tracts carry signal from the brain(ventral)
• Brain stem is control center for autonomic functions

Question 2

Introduction to Senses

Answer 2

• Labeled lines: indicate stimulus modality
• Phasic: Respond briefly then stop firing, Tonic: maintain activity when stimulus not changing, Phasic-tonic: react to change but don’t return all the way to zero firing
• Lateral inhibition: Cells inhibit their neighbor -> accentuate edges
• Sensory pathways run via thalamus to cortex, except olfactory, equilibrium pathway project to cerebellum

Question 3

Optics of the Eye

Answer 3

• Lens is transparent disk that focuses light -> light passes from cornea to lens through the pupil
• Parasympathetic contract constrictor muscle to shrink pupil in bright light -> lens round for near vision
• Sympathetic signals contract dilator muscles to dilate pupil in dark light -> lens flatter for far vision
• When pupil constrict -> full depth of field(everything in focus). When dilate -> shallow depth of field(objects near specific distance are in focus)
• Near point of accommodation: closest point a person can focus -> old people have farther near point of accommodation
• Hyperopia: focal point falls behind retina -> convex lens helps out
• Myopia: Focal point fall in front of retina(light bend too much) -> concave lens helps

Question 4

Visual Processing

Answer 4

• When light hits a photoreceptor, they hyperpolarize to reduce release of glutamate(more active in darkness)
• Cones for bright light, rods for dim light. Fovea mostly contains cones, peripheral contains rods
• Photoreceptors synapse onto bipolar cells which synapse onto ganglion cells
• On center cell: excited by light in center and inhibited by light in surround. Off center: inhibited by light in center and excited by light in surround. Bipolar cells react when light is contrasted(not uniform)
• Fovea good at spatial detail, peripheral is not
• Magnocellular: movement of objects, Parvocellular: fine details, Melanopsin: Project to SCN for circadian rhythm
• LGN has 2million neurons

Question 5

Color Vision

Answer 5

• Wavelength we normally see are from 400nm - 700nm
• Trichromats: Have red(63%), green(31%), and blue(6%) cones. Red and green cones see yellow/yellow-green light, Blue cones see blue light, rhodopsin sees blue-green, Melanopsin sees blue
• Spectral colors: Evoked by single wavelength, Extraspectral: Evoked by mix of wavelengths
• Yellow channel(R + G cells): excited by red and green light, Red-green opponent channel(R - G or G - R): Excited by red/green, inhibited by red/green
• Blue-yellow opponent channel(B - R - G): Excited by blue light and inhibit by red and green
• Color constancy: Brains can infer reflectance, see ripe banana even in green light

Question 6

Hearing and Equilibrium

Answer 6

• External ear: pinna and ear canal, sealed at its end by tympanic membrane(eardrum). Middle ear: Connected to pharynx by Eustachian tube. Inner ear: Contains sensor, cochlea for hearing and vestibular apparatus for equilibrium
• At peaks, pressure is high, at troughs, pressure is low. Frequency is perceive as pitch(range: 16-20,000 Hz, acuity highest 1000-3000 Hz). Amplitude determines perception of loudness
• Eardrum vibrates ossicles(malleus to incus to stapes -> pushes against oval window)
• Cochlea: Vestibular and tympanic duct contain perilymph(communicate at helicotrema). Cochlear duct contains endolymph. Wave energy enter through oval window and exit back into middle ear through round window
• Organ of corti contains auditory receptor cells(around 20k per cochlea). Each hair cell has 50-100 stiff hairs called stereocilia
• When cilia bend towards longest cilium, hair cell depolarizes and release transmitter. Axons form auditory nerve, branch of cranial nerve VIII
• High frequency displace at oval window end, low frequency displace at helicotrema
• Primary auditory cortex in temporal lobe
• Conductive hearing loss: sound cant be transmitted through external/middle ear, Sensorineural: Damage to hair cells. Presbycusis is sensorineural. Central: Damage to cortex or pathway from cochlea t cortex.
• Vestibular apparatus sense head position and motion -> project to cerebellum

Question 8

Smell and Taste

Answer 8

• Olfactory epithelium contains olfactory receptors -> has 10 million receptors
• 400 kinds of receptor cells: Golf increases cAMP -> cation channel open to depolarize receptor neurons and trigger an action potential that travels to olfactory bulb
• Sends axon through cribriform plate -> projection to olfactory nerve(cranial nerve 1)
• Olfactory cortex in frontal and temporal lobes)
• Limbic system: old brain region concerned with motivation and emotion -> odors can call up emotional memories
• Pheromones: chemicals released by animal that affect behavior or physiology. Rodents have vomeronasal organ involved in responses to sex pheromones
• 5000 taste buds, babies have 10000 taste buds -> each contain 100 receptor cells
• 5 kinds of receptor cells: Sweet(detect sugar), umami(detect glutamate), Bitter(detect poison), Salty(detect Na+), Sour(detect H+)
• Type 2: include sweet, bitter, and umami -> release ATP, Type 3: sense sour -> form synapses with sensory neurons activating by serotonin, Type 1: sense salt
• Nerve endings in wall of mouth have TRP channels sensitive to temperature and chemicals.
• Receptor cells in taste buds excite fibers of cranial nerves VIII, IX, and X(facial, glossopharyngeal, and vagus nerves) -> synapse in medulla and thalamus en route to cortex
• TRP receptors excite cranial nerve V

Question 9

Hypothalamic Control

Answer 9

• Hypothalamus contains control centers for feeding, plasma osolality, blood temperature and sexual/stress response.
• Lesions in ventromedial hypothalamus -> obese, Lesions in lateral hypothalamus -> thin. Controlled by arcNPY and arcPOMC
• ArcNPY: inhibit PVN -> inhibit sympathetic nervous system, stimulate LH -> release orexin to inhibit PVN -> LH stimulates feeding
• ArcPOMC: cleave POMC to make alpha-MSH -> excite neurons in PVN and VMH and inhibits DMH -> PVN and VMH excite sympathetic nervous system to inhibit feeding
• Leptin inhibits ArcNPY and stimulates PVN -> or excite ArcPOMC and VMH -> inhibit DMH -> inhibit feeding
• Rising blood glucose excited ArcPOMC to inhibit feeding
• CCK, PYY, and GLP-1 excite vagus nerve to excite VMH via the nucleus tractus solitarius(NTS)
• Ghrelin stimulates ArcNPY and LH to stimulate feeding

Question 10

Circadian Rhythm

Answer 10

• In fruit flies: per transcribed at night -> mRNA abundant at 10pm -> PER most abundant 6 hours later. PER represses transcription of per, drives transcription-translation feedback loop. tim and TIM oscillate like per and PER. TIM binds PER to form PER/TIM dimer -> repress transcription of tim and per. high levels of PER/TIM at 4am shut off per/tim -> PER/TIM gradually falls -> per/tim rise to peak in late evening
• clk codes for CLK, cyc codes for CYC. CLK-CYC dimer stimulates transcription of per and tim in morning. PER/TIM block CLK-CYC binding to DNA at night to repress per/tim transcription.
• DBT breaks down PER, levels of PER rise slowly than normal, result in cycle length of 24 hours
• Mammals use cry/CRY instead of TIM. clk, cyc, and dbt are called clk, bmal1 and ck1e. CLK/BMAL1 dimer stimulates transcription of per and cry
• Internal clocks kept in sync by zeitgeber: light, temperature, feeding cues. Zeitgeber is light sensed by melanopsin retinal ganglion cells that project to suprachiasmatic nucleus(SCN) of hypothalamus
• Retinal signals lead to breakdown of PER/CRY. If drop occurs after 4am, set clock forward a little, if in evening, set clock back
• Nudging a clock into synchrony with another rhythm is called entrainment. Other clocks entrained to SCN
• Pineal body secretes melatonin in darkness -> peaks at 2am then fall back down at 8am. Melatonin acts on SCN to reset clock to night
• In day: SCN excite LH to secrete orexin -> cause arousal. In dark: release MCH to induce sleep
• Breakdown of ATP cause adenosine buildup to make us sleepy. Caffeine blocks adenosine receptors -> when caffeine wears off we crash. Half life of 6 hours
• In REM sleep -> erratic 30-40 Hz brain waves. In NREM -> dreamless with slower brain wave. Stage 3 has regular 2-4Hz brain waves
• First REM stage occurs after 90 min -> as night progress -> sleep shallower and REM stages longer