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Lecture 6
Nervous System: Sensory (Vision, Hearing, Balance, Taste, Smell)
Undergraduate 3

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The Eye
  • Layers: Sclera and Cornea, Choroid, Ciliary Body, Iris, Retina.
  • Reflection, Refraction, Accomodation.
  • the ability of a lens to adjust its refractive power for viewing near objects.
  • increasing lens curvature in order to focus on near objects.
  • under parasympathetic (involuntary) control [ciliary muscle contracts].
  • decreases tension on zonular fibers, causing the lends to become rounder and the refractive index to increase.
Regulation of Light Entering Eye
  • the size of pupil regulates the amount of light entering the eye.
  • Iris: two layers of smooth muscle.
  • Inner circular muscle=constrictor (parasympathetic); outer radial muscle=dilator (sympathetic)
  • Hole in center=pupil.
The Retina
  • composed of neural tissue and contains photoreceptors (the rods and cones), which communicate with bipolar cells that talk to ganglion cells, which form the optic nerve.
  • 3 layers: 
    • Outer: photoreceptors
    • Middle: bipolar cells, modulators (amacrine and horizontal cells)
    • Inner: ganglion cell
  • Light passes through the inner and middle layers before striking rods/cones.
Rods and Cones
  • Rods: sense black and white; mostly located in the periphery of the retina; high sensitivity to light; high abundance; low visual acuity; high convergence with bipolar cells.
    • normally in the cis form.
    • when light hits it, it will turn into the trans form.
    • the retina must realign with the rhodopsin.
  • Cones: sense color; mostly located in the fovea; low sensitivity to light; low abundance, high visual acuity; low convergence with bipolar cells (usually targets only a single bipolar cell).
  • Rods and Cones undergo graded potentials.
  • Communicate with bipolar cels (interneurons), which communicate with ganglion cells of the optic nerve.
  • No rods or cones at the optic disk (where the optic nerve connects)=blind spot.
  • Conversion of light energy to nerve signals
  • 4 Photopigments: retinal (form of Vitamin A that binds to opsin) and opsin
  • 4 opsins: one in rods (black/white), 3 in cones (color vision)
Dark Adaptation
  • In total dark, there is a "dark current"; tendency to produce cGMP
  • Sodium channels then open.
  • Sodium enters the cell, causing a depolarization.
  • Calcium channels open, calcium enters --> exocytosis of the transmittter.
  • Transmitter causes graded potentials in the bipolar cell.
Light Adaptation
  • Rhodopsin is sensitive to light waves; light is absorbed by the photopigment.
  • Activates the G Protein.
  • Alpha Subunit leaves and induces phosphodiesterase.
  • This causes cGMP to breakdown, decreasing the levels in the cytosol.
  • Sodium Channels close.
  • Cell undergoes hyperpolarization, causing the calcium channels to close.
  • Transmitter rlease decreases, graded potential in bipolar cells diminish.
  • Bipolar cells need graded potentials in order to have the optic nerve cells fire.
Bipolar Cells
  • Usually found only in the retina.
  • Create a refinement of graded potential that is released by the rod (glutamate).
  • Dark Current: Constant Release of Glutamate (excitatory) by cones.
    • ON Bipolar Cell: stimulation of the mGluRG receptor leads to inhibition of the cation channel and no cation conductance so it hyperpolarizes. This leads to less glutamate release and the ON ganglion cell then does not fire APs.
    • OFF Bipolar Cell: stimulation of ionotropic receptors, so cation conductance, so depolarization. MOre glutamate release, so OFF ganglion cell fires an AP.
  • Light: Hyperpolarizes the cone receptor, leading to less glutamate release.
    • ON bipolar Cell: less stimulation of mGluRb receptor, so less inhibition of cation channel, so depolarization. Then more glutamate releases, causing the ON ganglion to fire an AP. 
    • OFF bipolar Cell: less stimulation of ionotropic receptors, so less cation conduction, so hyperpolarization. Then less glutamate releases, and OFF ganglion cell does not fire an AP.


Neural Pathways for Vision
  • parallel pathways transfer different types of visual information (color, shape, movement)
  • coding is distinct all the way to the visual cortex. 
  • visual cortex integrates everything.
Binocular Visual Field
  • two perspectives of visual field
  • one from each eye
  • gives us depth perception
  • the brain can construct a 3D image
Localization in Hearing and Olfaction
determine location by differntial quality and intensity of smell, or pitch and loudness of sound, as they arrive at the 2 nostrils or ears at slightly different times.
Anatomy of the Ear
  • External Ear: sound waves enter
    • pinna
    • external auditory meatus
  • Middle Ear: amplifies sound waves
    • tympanic membrane
    • ossicles (malleus, incus, stapes)
    • oval window
    • round window
  • Inner Ear: transduces sound energy
    • cochlea
    • vestibular apparatus (also for equilibrium/balance)
  • Eustachian Tube: equilibrates pressure, connects middle ear with pharynx
Sound Amplification in the Middle Ear
  • sound waves strike tympanic membrane
  • movement of ossicles is like a lever system. gives first level amplification.
  • this makes the oval window (smaller than tympanic membrane) move. gives second level amplification.
Sound Signal Transduction
  • conversion of sound energy to action potentials occurs in cochlea of inner ear
  • within the inner ear there are stereocilia that are receptors for sound. 
  • these stereocilia are embedded in the tectorial membrane, and are oriented in a line from short cilia to tall cilia.
  • Each stereocilia is surrounded by ECF with a large [K+], called endolymph.
  • The stereocilia are connected to each other with protein bridges, that when stimulated allow potassium channels to open and close.
  • When hair cells bend towards taller, DEPOLARIZATION
    • Since [potassium] is high in ECF, it flows into the stereocilia, causing depolarization.
    • This causes increased calcium, and the activation of more APs
  • When hair cells bend towards shorter, HYPERPOLARIZATION.
    • channels close.
Coding for the Quality of Sound
  • Intensity Coding: The loudness of the sound is coded for by the degree of deflection of the stereocilia and the opening/closing of ion channels.
  • Frequency Coding: The pitch of sound is determined by how far along the basilar membrane the deflection occurs (high frequency stimulates sooner).
Neural Pathways for Sound
  • Hair Cells are receptors=modified neurons.
  • Synapse on afferent axons of the cochlear nerve.
  • one hair cell per cochlear nerve fiber.
  • Cochlear nerve enters brainstem, and synapses with second-order neuron.
  • Second-order neuron goes to the medial geniculate nucleus of the thalamus.
  • Synapses with a third-order neuron, which goes to the auditory cortex, which has a frequency map.
Clinical Defects to Hearing
  • Conductive Deafness: inadequate conduction of sound waves through external and/or middle ear.
  • Sensorineural Deafness: Rinne Test (inadequate transduction of sound waves to electrical signals in the inner ear).
  • Central Deafness: damage to the neural pathway for sound.
The Vestibular Apparatus
  • 3 Semicircular Canals perpendicular to each other.
  • Anterior: detects head movement up and down.
  • Posterior: detects head movement up and down to the side.
  • Lateral: detect head movement side to side. 
  • Transduction of Rotation: the receptor cells are hair cells located in the ampullae. 
  • Turning head towards higher cilia produces more action potentials, and vice versa.
Utricle and Saccule
  • Utricle and Sacculae: bulges between the semicircular canals and the cochlae that are oriented to detect linear acceleration and head tilt.
  • Utricle: forward and backward motion.
  • Saccule: detects up and down motion.
  • Cilia and Cells have a 1:1 relationship.
  • Transduction of Linear Acceleration: Forward acceleration causes head to tilt towards the higher cilia (more AP) and vice-versa. 
  • Kinocilium=longest of the stereocilia.
    • mechano-gated.
    • as head moves and the gelatinous fluid moves, the way the cilia move is affected.
Neural Pathways for Equilibrium
  • Cranial Nerve VIII: vestibular nerve for equilibrium and cochlear nerve for hearing.
  • Vestibular Fibers and Cochlear Fibers fuse at the vestibular-cochlear nerve.
  • depends on chemicals in food binding to chemoreceptors.
  • over 10K taste buds, with 50-150 receptors per bud (modified epithelial cells).
  • 4 primary tastes.
  • Sour: due to H+
  • Salty: due to Na+
  • Sweet: due to ligands
  • Bitter: due to ligands
Transduction of Sour of Salty
  • Sour: H+ binds and blocks K+ channels, cell depolarizes, Calcium ion channels open, transmitter released.
  • Salty: Na flow increases by chemical gradient due to more salt in food, causing voltage-gated calcium channel to open, causing neurotransmitter release.
Transduction of Sweet and Bitter
  • Sweet: organic sucrose-like ligand binds, activates G-protein, which causes production of cAMP. Phosphorylation of a K channel, which closes it, causing depolarization, causing calcium channels to open, causing neurotransmitter release.
  • Bitter: N-containing compound binds, causing a blockage of a K channel by protective quinine. depolarization. calcium. neurotransmitter.
Neural Coding for Taste
  • each taste receptor cell responds to all 4 primary tastes, but it generally responds to one more strongly than the others.
  • different types of recepter cells within each bud; differential distribution.
  • Coding of taste complex, pattern theory, requires sense of smell.
  • Sensory Neurons=CN VII, IX, and X.
  • Terminate in brainstem gustatory nucleus.
  • 2nd order neurons to thalamus.
  • 3rd order neurons to gustatory cortex (in parietal lobe near mouth region of somatosensory cortex).
  • depends on chemicals in air binding to chemocreceptors in olfactory epithelium.
  • olfactory epithelium (in nasal cavity) has 3 cell types:
    • supporting cells (mucus)
    • basal cells (precursors for new receptor cells)
    • receptor cells (neurons that respond to odorants)
  • Olfactory neurons are the only neurons that are replaced continuously.
  • Cilia project into mucus (serve as chemoreceptors).
  • Olfactory binding proteins (located in mucus, transport odorants to receptors); dissolve air-borne chemicals.
  • Cribriform plate: holes in base of skull, axons of receptor cells travel through it.
  • Chemical binds to receptor, activating G protein called "Golf"; activates adenylate cyclase, which produces cAMP, which directly binds cation channels and opens them.
  • Na and Calcium enter cell, so depolarization.
Neural Pathway for Olfaction
  • Olfactory Receptor Cell: specific ones for each type of odorant-binding protein. specialized endings of afferent neurons.
  • Axon of receptor cells goes to CN I, olfactory nerve.
  • 2nd order neurons are mitral cells; synapse b/w 1 and 2 occcurs in glomeruli.
  • 2nd order neurons form olfactory tract, relaying in the olfactory tubercle to the cerebral cortex.
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