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PENN / Biological Basis of Behavior / BIBB 109 / What is the neurotransmitter system?

What is the neurotransmitter system?

What is the neurotransmitter system?


School: University of Pennsylvania
Department: Biological Basis of Behavior
Course: Introduction to Brain and Behavior
Professor: Michael kane
Term: Spring 2016
Tags: brain, neuroscience, nervous system, Structure and Function of the Nervous System, taste, tastebuds, Olfactory System, olfaction, Visual System, and cerebral cortex
Cost: 50
Name: BBB 109 Midterm 2 Study Guide
Description: Chapters 6.5-12
Uploaded: 03/22/2018
26 Pages 235 Views 2 Unlocks

Biological Basis of Behavior 109: Midterm 2 Study Guide 

What is the neurotransmitter system?

Chapters 6.5-12


∙ Important numbers - red

∙ Important physiology – purple

∙ Important processes – blue

∙ Important people – green

∙ Important concepts - orange

Chapter 6: Neurotransmitter (NT) Systems

I. Amino acid NTs

a. Glutamate and glycine (produced from glucose in all cells), GABA

II. Glutamate

a. Vesicular transporters: VGLUT1, 2, 3 – good markers of glutamatergic neurons b. Membrane transporter: EAATs

c. Rapidly removed by other glutamate receptors on neurons and astrocytes, which  may break it down into glutamine to be reused in glutamate production

What is the meaning of glutamate?

d. Glutamate excitotoxicity: too much glutamate (prolonged depolarization)  causes cell death


a. Main inhibitory NT

b. Glutamic acid decarboxylase (GAD): enzyme that synthesizes GABA from  glutamate; good marker of GABAergic neuron

c. Vesicular transporters: vGATs

d. Membrane transporter: GAT

e. Metabolized by: GABA-T

IV. Endocannabinoids

a. Anandamide and 2-AG

b. Retrograde messengers

i. Goes from post to pre

ii. Binds to CB1 (brain, blood vessels) and CB2 (immune) receptors

V. Gaseous NTs

What is the meaning of gaba?

a. Nitric oxide (NO): not stored in vesicles; synthesized from a.a. arginine by NO  synthase

i. Retrograde messenger: small and permeable We also discuss several other topics like What did hershey and chase discover in 1952?

VI. Receptors

a. Acetylcholine












b. Glutamate

i. AMPA – Na+, K+


ii. Kainate

iii. NMDA – Na+, K+, and Ca2+ 

1. Blocked by positively charged Mg2+ ???? ligand and voltage gated  

because glutamate must be bound and V > -65 mV

2. Typically co-expressed with AMPA, so that buildup of Ca2+ 

positive charge causes NMDA to open

iv. NMDA pathway activated protein kinases, including CaMKII ???? phosphorylates existing AMPA receptors, enhancing their sensitivity to  glutamate and inserting more AMPA receptors into postsynaptic  We also discuss several other topics like Can you grow things in dirt?


1. Believed to be synaptic process for learning and memory 


i. GABAA receptor gates a Cl channel ???? inhibitory

ii. Target of many drugs, i.e. Vallium, Ambien, Lunesta, etc.

d. GPCRs – enable signal amplification

i. When unactivated, GTP-binding proteins slide along membrane. Only  activated when bound to G-protein-coupled receptor

ii. First messenger: NT

iii. First effector/second messenger: ion channel or enzyme

VII. Protein Kinases

We also discuss several other topics like What does it mean for a market to fail?

a. GS: stimulatory G-protein (activates adenylyl cyclase, increasing cyclic AMP) b. GI: inhibitory G-protein (inhibits adenylyl cyclase)

c. GQ: Activates phospholipase C and leads to two diff processes: diacylglycerol or  IP3


Chapter 7: Nervous System Organization

I. Spinal Cord

a. Gray matter (inner): collection of cell bodies in shape of butterfly that are  divided into dorsal horn (receives input from dorsal roots) and ventral horn (send output to ventral roots)

b. Intermediate zone: interneurons between dorsal and ventral neurons that shape  motor output in response to sensory output

c. White matter (outer): myelinated axons that surround “butterfly”

d. Spinal canal: filled with CSF

e. Dorsal ganglia: collection of cell bodies of sensory neurons We also discuss several other topics like What is the function of proprioception?
We also discuss several other topics like What are the differences between public health and medicine?

II. CNS - Meninges

a. Three membranes: (from outer to inner) dura mater, arachnoid trabeculae,  [subarachnoid space between w CSF and blood vessels] and pia mater Don't forget about the age old question of What is the content of the civil rights act of 1875?

III. CNS – Development

a. Embryo has three germ cell layers:

i. Ectoderm: the nervous system and skin

ii. Endoderm: lining of internal organs

iii. Mesoderm: bones and muscles

b. Neurulation: neural plate becomes neural tube (basis of CNS); neural crest is  basis of PNS 

c. Defects:

i. Anencephaly: failure of anterior tube to close

ii. Spina bifida: failure of posterior tube to close

iii. Increased risk from folic acid deficiency

IV. Forebrain: includes optic vesicles and rostral


a. Cerebral cortex: perception, cognition, consciousness, voluntary action b. Basal ganglia (caudate and putamen): voluntary movement, procedural  memory


c. Limbic system (i.e. hypothalamus, hippocampus, amygdala): emotion,  memory, location


a. Thalamus: “gateway to the cortex”; receives contralateral sensory input b. Hypothalamus: regulates feeding, fighting, f**king; controls the autonomic  nervous system, i.e. fight or flight, hormone release by pituitary gland, circadian  rhythms

V. Midbrain

a. Descending axons from cortex to brain stem and spinal cord

i. Corticospinal tract: motor cortex to spinal motor neurons

b. Ascending axons from spinal cord and brain stem to forebrain

c. Periaqueductal gray: heavily involved in analgesia (pain relief)  


d. Superior colliculus: receives sensory info from eye

e. Inferior colliculus: receives sensory info from ear


f. Substantia nigra (“black substance”): where dopaminergic neurons degenerate g. Red nucleus: control voluntary movement (rubrospinal tract)

VI. Hindbrain


a. Cerebellum: coordination of fine movements; receives info about proprioception;  movement from pons

i. Ataxia: impaired voluntary muscle control, uncoordinated speech

ii. Ipsilateral motor control because pons ???? cerebellum ???? pons (double  cross-over)

b. Pons: 90% of descending axons synapse in contralateral manner


c. Medulla: corticospinal fibers decussate (cross) at medullary pyramids VII. Neocortex Structure

a. 6 layers (4th layer has sublayers, 1st layer has dendrites but not cell bodies) b. 52 regions of the cytoarchitecture (Brodmann’s areas)

c. Association cortices: primary motor, supplementary motor and premotor,  prefrontal, P-T-O

i. Give rise to feeling, thought, action

d. Prefrontal cortex: associated with higher cognitive function, impulse control,  etc.

e. 50% of cortex devoted to vision, 10% to hearing, 10-20% to motor, remaining to  reasoning

VIII. Ventricular System



Chapter 8: Taste and Smell


I. Five Basic Tastes

1. Salty

2. Sour

3. Sweet

4. Bitter

5. Umami (“delicious”; amino acids, i.e. glutamate/MSG)

II. Papillae: taste sensitive structures of 3 types, each innervated by different cranial nerve a. 1-100s taste buds per papilla (2000-5000 per person) ???? 50-150 taste receptor  cells per taste bud  

b. Taste cells: polarized epithelial cells with microvilli at apical end; transduce  signals via gustatory nerves; regenerate

i. Apical surface contains ion channels and/or GPCRs

ii. Basolateral surface contains VG ion channels, intracellular Ca2+ stores,  2nd messengers, and synaptic vesicles

iii. May respond to multiple tastes, but highest response indicates type of taste  cell  

c. Convergence of taste cell receptors onto afferent axons to create a flavor



Gustatory  nerves


microvilli apical


IV. Central Taste Pathways




∙ Na+through Na+-selective channel  depolarizes (blocked by amiloride) ∙ NT = serotonin


∙ Protons block K+-selective  

channels, decreasing K+ 

permeability and depolarizing 

∙ Protons enter transient receptor  potential (TRP) channels and  


∙ NT = serotonin


∙ Both T1R2 and T1R3 GPCRs  required to activate phospholipase  cascade and depolarize 

∙ NT = ATP


∙ T1R and T2R (30 types) GPCRs  activate phospholipase cascade and  depolarize 

∙ NT = ATP


∙ Both T1R1 and T1R3 GPCRs  required to activate phospholipase  cascade and depolarize 

∙ NT = ATP

a. Brain stem: control of vomiting, swallowing, digestion


b. Hypothalamus: feeding

c. Basal telencephalon: taste memory

V. Neural Coding of Taste

a. Labelled line hypothesis (“single-fiber”): individual taste receptor cells that are  narrowly tuned to specific stimuli and signal to non-overlapping gustatory axons b. Population coding (“across fiber” – accepted theory): taste receptors are broadly  tuned and converge onto gustatory afferents that combine tastes from many cells


I. Olfactory Epithelium

a. Organized into zones of receptor cells expressing different receptor gene subsets b. Olfactory receptor cells (odorant receptor neurons): transduce odorant signals  and regenerate

i. Single dendrite from which cilia protrude

ii. Axons (thin, unmyelinated) – CN1 olfactory nerve – do not form classic  “bundle,” but penetrate cribriform plate and then travel into olfactory bulb, iii. ORNs synapse on 2nd order neurons in glomeruli, which receive input  only from receptor cells expressing a particular receptor gene

iv. Odorants (airborne volatile chemicals - ) dissolve in mucus and bind to  ORNs at cilia

c. Supporting cells: produce mucus from Bowman’s gland

d. Basal cells: stem cells that serve as source of new receptor cells



II. Olfactory Transduction

a. GOLF GPCR binds odorant

b. αOLF stimulates adenylyl cyclase to form cAMP

c. cAMP opens cAMP-gated cation channel, leading to Na+ and Ca2+ influx d. Increased Ca2+ gates a Ca2+-activated Cl channel, leading to Cl- efflux and  depolarizing cell (due to high normal concentration of Cl inside these cells) III. Olfactory Receptor Proteins

a. One receptor per cell

b. Population coding (broadly tuned): each odorant activates multiple receptors and each receptor responds to multiple odorants

IV. Central Olfactory Pathways

a. Olfactory epithelium ???? olfactory bulb ???? olfactory cortex (piriform cortex) ???? thalamus and limbic system (amygdala and entorhinal cortex)

b. Olfactory maps (sensory maps): ordered arrangement of neurons that correlates  with features of the environment

i. Different olfactants evoke different spatial patterns

Chapter 9: The Eye

I. Optics

a. Reflection: bouncing of light rays off a surface

b. Absorption: transfer of light energy to a particle or surface

c. Refraction: bending of light rays from one medium to another

II. Vision

a. 50% of neocortex processes information

b. 30% cortical areas just concerned with a different aspect of vision (color, form  and shape, relative distance or depth, movement)


III. Gross Anatomy of the Eye

a. Pupil: opening where light enters the eye; continuously adjusts for light  parasympathetically

b. Sclera: white of the eye

c. Iris: gives color to eyes

d. Cornea: glassy transparent external surface of the eye; relevant for diffraction e. Extraocular muscles: 3 pairs that move the eye in its orbit

f. Optic nerve: bundle of axons from the retina

g. Lens: suspended by ciliary muscles; divides interior of eye into two compartments with different fluids:

i. Aqueous humor: watery

ii. Vitreous humor: jelly-like, contains phagocytic cells (source of “floaters,”  or debris too large to be phagocytosed, leading to blurs in vision)

1. Glaucoma: caused by intraocular pressure

IV. Image Formation:

a. Refraction by the Cornea

i. Eye collects light, focuses on retina, and forms image

ii. Cornea performs most of eye’s refraction

b. Accommodation by the Lens

i. Change in shape of lens (accommodation) caused by ciliary muscle  contraction allows for focusing

ii. Lens is involved in forming sharp images of objects closer than 9 m



c. Pupilary Light Reflex

i. Consensual: shining of light into one eye causes constriction of both  pupils

V. Retina

a. Blood vessels are prominent everywhere except the optic disc (Blind spot) b. Origin of vessels, no photo receptors  

c. Macula – part of retina for central vision  

d. fovea (“pit”) – anatomical reference as the center of retina

i. Where outer layers are “pushed aside” such that light strikes  

photoreceptors directly, maximizing visual acuity

e. Choroid: capillary bed – main source of blood supply to photo receptors f. Anatomical terms of the retina

i. Nasal –closer to nose

ii. Temporal –near the temples

iii. Superior –above fovea

iv. Inferior – below fovea

VI. Ocular diseases

a. Strabismus –imbalance of the extraocular muscles in both eyes

b. Cataract – clouding of the lens

c. Glaucoma –increased pressure in aqueous humor

d. Retinitis pigmentosa –progressive degeneration of photoreceptors; “tunnel vision” e. Macular degeneration –central vision is lost; peripheral vision normal VII. Visual field and visual acuity

a. Visual field: amount of space viewed by retina when the one eye is fixated  straight ahead

i. Image is inverted (L-R, and up-down) on the retina

b. Visual acuity: ability of the eye to distinguish two points close to each other i. Depends on spacing of photoreceptors

ii. In periphery, visual acuity is poor. When looking straight, easier to focus VIII. Microscopic Anatomy of the Retina

a. Five types of neurons in the retina: photoreceptors, bipolar cells, ganglion cells,  amacrine cells, and horizontal cells


b. Direct (vertical) pathway:

i. Ganglion cells: fire APs (solely), only  

output source to optic nerve

ii. Bipolar cells

iii. Photoreceptors: light-sensitive (solely)

c. Indirect (lateral) pathway:

i. Horizontal cells: receive input from  

photoreceptors and project two other  

photoreceptors and bipolar cells

ii. Amacrine cells: receive input from  

bipolar cells and project to ganglion  

cells, bipolar cells, and other amacrine  

cells (may act to inhibit other  


d. Laminar Organization of the Retina

i. Light passes through vitreous humor, ganglion cells and bipolar cells  before reaching photoreceptors

ii. Tapetum lucidum: reflective layer beneath photoreceptor layer; makes  animals more sensitive to low light at loss of acuity

IX. Photoreceptor Structure

a. Four main regions: outer segment, inner segment,  

cell body, and synaptic terminal

b. Type of photoreceptors:



∙ Long, cylindrical ∙ Many disks

∙ Light sensitive

∙ Rhodopsin

∙ ~90 million (18x)

∙ Short, tapering

∙ Fewer disks

∙ Color sensitive

∙ Opsins

∙ ~5 million

X. “Duplex Retina”

a. Scotopic (nighttime lighting): rods

b. Photopic (daytime lighting): mostly cones

c. Mesopic (twilight): both contribute

XI. Regional Differences in Retinal Structure


Central retina

Peripheral retina

∙ Most cones in fovea (within 10  degrees)

∙ No rods in central fovea

∙ Best for high-resolution vision

∙ Higher ratio of rods to cones

∙ Higher ratio of photoreceptors to  ganglion cells

∙ Best at detecting dim light

XII. Phototransduction: conversion of light energy into photoreceptor membrane  potential

a. Visual pigment molecules: located in photoreceptor disks, absorb light energy and  convert; formed by combining a chromophore (11-cis retinal) with opsin i. 11-cis retinal: changes shape when it absorbs light ???? all-trans retinal ii. Opsin: large, membrane bound protein; tunes the molecule’s absorption of  light to part of the visual spectrum (red, blue and green)

b. Phototransduction in Rods 

∙ Dark current: rods are depolarized in the dark because of steady influx of  Na+. Na+channels are opened by cGMP, produced by guanylyl cyclase,  causing sustained depolarization to ~-40 mV and release of NT glutamate

∙ Light-induced pathway: light interacts with rhodopsin (an opsin G-protein  coupled receptor) to activate transducin, which activates phosphodiesterase effector enzyme (PDE). PDE hydrolyzes (reduces) cGMP, closing the Na+ channels and hyperpolarizing the cell to ~-70 mV


c. Phototransduction in Cones 

i. Same, but…

1. Bright light – rods become saturated and cannot hyperpolarize  

further (so day vision depends on cones!)

2. Various opsins – not rhodopsin; red (long wavelength), green  

(medium), blue (short)

3. Different spectral sensitivities

4. Require more energy to become bleached

d. Color Blindness (dichromacy): X-linked recessive gene, so more common in men XIII. The Receptive Field (RF)

a. Area of retina where light changes a neuron’s firing rate

b. Center-surround organization that is mutually antagonistic



∙ More sensitive

∙ Governed by vertical pathway  from photoreceptors > bipolar  

cells > ganglion cells

∙ Larger area

∙ Governed by lateral pathway  between photoreceptors and  

horizontal cells

∙ Mediates an inhibitory response

c. Bipolar cells: OFF-center and ON-center based on differing response to  glutamate released by photoreceptors

i. Different responses to light determined by type of glutamate receptor 1. OFF-bipolar: sign-conserving synapse mediated by ionotropic  

receptors causes hyperpolarization 

2. ON-bipolar: sign-inverting synapse mediated by metabotropic  

receptors causes depolarization 

ii. Direct input from photoreceptors = center

iii. Indirect input from photoreceptors via horizontal cells = surround 1. Horizontal cells receive input from surround photoreceptors and  

inhibit neighboring center photoreceptors (lateral inhibition)

d. Retinal ganglion cells (RGCs): same center-surround RF field organization as  bipolar cells generally, but fire APs


XIV. Ganglion Action Potential Strength

a. Determined by differences in illumination of center and surround (luminance  contrast)

i. Neural response emphasizes the contrast at light-dark edges

b. Contrast signal: signal leaving retina is related to difference between center and  surround

XV. Structure-Function Relationships

a. Besides center-surround RFs, ganglion cells are characterized by appearance,  connectivity, and electrophysiological properties

b. Three major types:



∙ 5% of population

∙ Large RFs

∙ Rapid AP conduction

∙ Sensitive to low-contrast (low-resolution vision) ∙ Fire via transient bursts




∙ 90% of population

∙ Small RFs

∙ High-resolution vision

∙ Color opponency

∙ Fire via sustained discharge


∙ Remaining 5%

∙ Light/dark or color opponency

c. Three spatial comparisons:

i. Light vs. dark

ii. Color opponency: response to one color is cancelled by another color 1. Red vs. green

2. Blue vs. yellow

d. Some ganglion cells transduce light: intrinsically photosensitive retinal ganglion  cells (ipRGCs)

i. Melanopsin = opsin

ii. Send input to subcortical visual areas that control circadian rhythms iii. Sum light input over much larger areas than photoreceptors do

XVI. Parallel Processing

a. Different visual attributes are processed simultaneously using distinct pathways i. Simultaneous input from two eyes compared in cortex to determine depth  and distance

ii. Light and dark from independent streams via ON-center and OFF-center  ganglion cells

iii. Different receptive fields and response properties of RGCs

Chapter 10: Optic Pathways

I. Central Projections of Retinal Ganglion Cells

a. Ganglion axons travel through optic nerve ???? cross at optic chiasm ???? terminate  in the lateral geniculate nucleus (LGN) of the thalamus, the superior colliculus,  the pretectum (between thalamus and midbrain), and the hypothalamus

i. LGN ???? primary visual  

cortex (striate cortex) in  

back of brain

ii. Hypothalamus: circadian  


iii. Pretectum: pupil size

iv. Superior colliculus:

saccades (rapid eye  

movement between fixed  

points); orients eyes in  

response to stimuli. 10% of


ganglion cells terminate here

II. Visual Field

a. Nasal retinas cross, not temporal

i. Left hemifield: left nasal, right temporal ???? right brain

ii. Right hemifield: right nasal, left temporal ???? left brain

b. Lesions to … removes …

i. Left optic nerve… left peripheral vision  

ii. Left optic tract… right hemifield

iii. Optic chiasm… all peripheral vision

iv. Anopsia: large visual field deficit

v. Scatoma: small visual field deficit

III. Retinal Inputs in the LGN Layers

a. LGN has 6 layers

b. Input from two eyes is kept separate

i. Right LGN: right eye (ipsilateral)  

synapses in layers 2, 3, and 5. Left eye  

(contralateral) synapses in layers 1, 4,  

and 6

ii. Left LGN: left eye (ipsilateral)  

synapses in layers 2, 3, and 5. Right  

eye (contralateral) synapses in layers  

1, 4, and 6

c. Layers 1, 2: magnocellular (input from M-type GCs)

d. Layers 3-6: parvocellular (input from P-type GCs)

e. Koniocellular layers: input from non-M-non-P cells from same eye as  overlapping M or P layer

f. LGN Receptive Fields: same as RFs of GCs that feed them (see chapter 9) IV. Anatomy of the Striate Cortex (primary visual/BA 17/V1)

a. Retinotopy: neighboring cells in retina send info to neighboring places in targets  (LGN and V1)


i. Cortical magnification: more GCs with RFs in fovea, thus more neurons  in V1 that receive input from central retina

b. Layer 4: 3 subdivisions (A,B,C)

i. 4C is primary recipient of LGN afferents; sub

divided into α, β

1. Magnocellular LGN neurons: layer IVCα

(insensitive to color)

2. Parvocellular LGN neurons: layer IVCβ


3. IVC neurons receive input from one eye


ii. Intracortical connections: perpendicular through layers; layer IVCα to  IVB; IVCβ to layer III

c. Koniocellular LGN axons: synapses in layers I and III (primarily)

d. Cell Types

i. Spiny stellate cells: layer 4C; connections within cortex

ii. Pyramidal cells (glutamatergic): layers 2, 3, 4B, 5, 6; only cells that send  axons out of V1

1. Layers 2, 3, 4B cells project to other cortical areas

2. Layer 5 projects to superior colliculus and pons

3. Layer 6 projects back to LGN

V. Ocular Dominance Columns

a. Neurons over layer 4 are monocular

b. Most neurons outside of layer 4 are binocular, but often dominated by one eye VI. Cytochrome Oxidase Blobs

a. Cytochrome oxidase: mitochondrial enzyme used for cell metabolism b. Blobs: cytochrome oxidase-stained pillars in striate cortex

c. Centered on an ocular dominance column in layer 4 (receive input from  koniocellular LGN, magno and parvo), run through layers 2, 3, 5, 6

d. No way to distinguish RF properties of blob cells from interblob cells VII. RFs in Striate Cortex

a. Layer IVC: similar to LGN cells that project to them. Monocular RFs b. Outside layer IVC: RFs are not circular

i. Many neurons in V1 respond best to an elongated bar moving across the  RF

c. Orientation selectivity: highest response is given to a bar with a particular orientation:  

i. Orientation columns: radial columns from layer II to VI

ii. Selectivity shifts across cortex


d. Direction selectivity: fire action potentials in a direction-dependent response to  moving bar of light

e. Simple cell RFs: binocular, orientation-selective, with an elongated ON or OFF  RF area flanked with antagonistic surround

f. Complex cell RFs: give ON and OFF responses to stimuli throughout the RF,  responding best to particular orientation but responding to light ON or OFF  overall

VIII. Parallel Pathways

a. Magnocellular Pathway  

i. M-type GCs>>Magnocellular layer of LGN>>layer IVCα >> IVB

1. Critical for tasks that require high temporal resolution (evaluating  

location, speed, and direction of objects)

b. Parvocellular Pathway  

i. P-type GCs>>Parvocellular layer of LGN>>layer IVCβ>>layer II and III  interblob  

ii. Critical for tasks that require high spatial resolution (analysis of size,  shape, and color of objects)

c. Koniocellular Pathway  

i. nonM-nonP GCs>>koniocellular layer of LGN>>blobs in layers II and III ii. Analysis of color?

IX. Beyond the Striate Cortex

a. Dorsal stream: analysis of motion; V1, V2, V3, MT, MST, others i. MT: most cells are direction-selective, responding more to motion than  shape. Large RFs

ii. MST: navigation, directing eye movements, motion perception

b. Ventral stream: object recognition; V1, V2, V3, V4, IT, others:

i. Area V4: shape and color perception

1. Achromatopsia: caused by damage to area V4—partial or  

complete loss of color vision

ii. Area IT: major output of V4

1. Farthest extent of processing in ventral stream

2. Receptive fields respond to a wide variety of colors and abstract  shapes

3. May be important for both visual perception and visual memory

c. Fusiform Face Area: area in the human brain that is most responsive to faces i. Prosopagnosia: difficulty recognizing faces


Chapter 11: Auditory and Vestibular Systems

Auditory System

I. Audible Sound

a. 20-20,000 Hz

b. Frequency = pitch

c. Amplitude/intensity = volume

II. Auditory Transduction

1. Sound waves move tympanic membrane

2. Sound force amplification (necessary to create pressure to move fluid = 20x  pressure at TM)

a. Ossicles move oval window  

b. Stapes: footplate – bottom portion of ossicles that moves in and out at  oval window, vibrating fluid in cochlea  

o Scala vestibula and scala tympani: filled with perilymph (low K+, high Na+; EK = -80 mV)

o Scala media: filled with endolymph (high K+, low Na+; EK = 0 mV ???? endocochlear potential: endolymph > perilymph)

o Cochlea narrows from base to apex, basilar membrane widens

▪ Helicotrema: hole at apex that connects scala vestibuli and tympani




3. Structural characteristics of basilar membrane allow sound to be  discriminated, creating a tonotopic map. High frequency vibrates stiff base  and dissipates, low frequency travels to floppy, wide apex

4. Organ of Corti (contains auditory receptor cells) converts mechanical energy  into membrane potential

a. Epithelial hair cells contain 10-300 stereocilia each that extend into  endolymph.  

b. Outer hair cells are bent by tectorial membrane and inner hair cells  are bent by moving endolymph (3 outer : 1 inner)

c. Movement of tip links causes mechanically gated ion channels to  close, hyperpolarizing the cell, or open further, depolarizing the cell by  K+influx from high [K+] endolymph (at rest, tension in tip links keeps  membrane at -45 mV)

5. Inner hair cells release NT onto spiral ganglion neurons (1:10 communication) 6. Outer hair cells release NT onto one spiral ganglion (many:1 communication)

a. Prestin protein changes length of hair cell to increase flexing of  basilar membrane

b. Myosin protein may enhance response to weak sounds

c. Ototoxicity: damage to cochlear amplifier, causing deafness


III. Central Auditory Pathways

a. Each cochlear nucleus receives ipsilateral

input from one ear

b. All other auditory nuclei in brain stem receive  

bilateral input (binaural), starting at superior  


c. Auditory neurons tend to have characteristic  

frequencies to which they are most sensitive

IV. Encoding Sound Frequency

a. Intensity determined by: 1) firing rate, 2)  

number of active neurons…

b. At low frequencies (<200 Hz), phase  

lockingConsistent firing at same phase of

every APAt intermediate frequencies (200 Hz  

– 5 kHz): phase locking and tonotopy

i. Volley principle: intermediate  

frequencies (up to 5 kHz) are  

represented by activity of a  

combination of neurons firing for  

different Nth cycles

c. At high frequencies (>5 kHz): tonotopy

Vestibular System

I. Balance, equilibrium, posture; head, body, eye movement

II. Use hair cells to transduce movement

III. Two structures:

a. Otolith organs (utricle and saccule): linear accelerations (X, Y, Z)

b. Semicircular canals (3): rotational accelerations (roll – X, pitch – Y, yaw – Z)

Chapter 12: The Somatic Sensory System

I. Differences from other systems:

a. Receptors are broadly distributed


b. Responds to many kinds of stimuli

c. 4 senses: temperature, touch, pain and body perception

II. Somatic Nervous System

III. Sensory Neurons

a. “Pseudounipolar” primary afferent axons – one axon extends to periphery,  another to CNS via dorsal root, with cell body in dorsal root ganglion (DRG) i. Axon diameter: determines conduction velocity


b. Receptors:  

i. Free nerve endings: unmyelinated, widely branched

1. Thermoreceptors (temperature)

2. Nociceptors (pain)

ii. Encapsulated: lower thresholds for AP generation???? more sensitive 1. Mechanoreceptors

IV. Touch

a. Hairy and glabrous/hairless skin

b. Membrane-encapsulated mechanoreceptors:














Meissner’s  corpuscles

Low freq  


















c. Pacinian corpuscles: capsule of connective tissue layers provides large receptor  potential at onset and offset and rapid adaptation (bare axon adapts slower)

d.e. Mechanical stimuli

i. Stretch of lipid membrane

ii. Force applied on extracellular membrane

iii. Deformation and stress on cell’s cytoskeleton

V. Two-Point Discrimination: determined by…

a. Density of mechanoreceptors

b. Receptive fields of receptor types

c. Area of brain tissue

d. Special neural mechanisms devoted to high resolution discriminations e. Most 2nd order neurons have surround inhibition in their RFs to enhance spatial  resolution

VI. Segmental Organization of Spinal Cord

a. 30 segments with paired dorsal and ventral roots corresponding to dermatomes (area of skin innervated by dorsal roots)

i. Dermatomes overlap, so cutting of dorsal root does not cause loss of all  sensation

ii. Shingles infects only one DRG and therefore one dermatome

b. 4 divisions: (rostral) cervical, thoracic, lumbar, sacral (caudal)  

VII. Dorsal Column-Medial Lemniscal Pathway

a. Mediates tactile sensation, vibration and proprioception (sense of relative location  of body parts)


b. Aβ axons: synapses on 2nd order neurons in dorsal horn ???? ipsilateral dorsal  column ???? dorsal column nuclei in medulla ????

decussate at medial lemniscus ???? ventral  

posterior nucleus (VPN) of thalamus ????

primary somatosensory cortex (S1)

c. other branch ascends directly to brain  


VIII. Trigeminal Touch Pathway

a. Somatosensory info from face enters brainstem  

at pons and decussates to medial VPN  

thalamus, and then to S1

b. Three branches: ophthalamic (V1), maxillary  

(V2), mandibular (V3)

IX. Somatosensory Cortex (SMC)

a. Located in parietal lobe: postcentral gyrus (BA1,  

2, 3A, 3B)

b. 3B: primary SMC because receives direct input from VPN thalamus. Also,  stimulation evokes somatic sensation and lesions impair

c. Laminar structure (like the rest of the neocortex!)

i. Layer IV: input region from thalamus; stacked columns

X. Cortical Somatotopy: mapping of body’s surface sensations onto structure of the brain a. Homunculus (“little man”): figure that represents the relative representation of  body parts in the brain

b. Relative size of cortex devoted to each body part is determined by density of  sensory input

c. Phantom limbs: patients who lose limbs may still have cortical stimulation for that limb

XI. Somatotopic Map Plasticity

a. Removal of body part: cortex reorganization causes takeover by adjacent regions b. Increased stimulation of a body part: cortex reorganization causes expansion of  that area

XII. Posterior Parietal Cortex (BA 5 and 7)

a. Involved in somatic sensation, visual stimuli, movement planning, attentiveness b. Neurons with large RFs

c. Lesions cause interesting neurological disorders

i. Agnosia: inability to recognize object even with normal sensation

ii. Astereognosia: inability to identify objects be feeling

iii. Neglect syndrome: a part of body or visual field is ignored or existence is  denied (not consciously aware)

1. Often following right hemisphere damage

2. Often recovery over time

XIII. Pain


a. Somatic sensory depends strongly on nociception - sensory process that provides  signals that trigger pain  

b. Nociceptors: free, branching, unmyelinated nerve endings that signal pain i. Ion channels opened by:

1. Strong mechanical stimulation, temperature extremes, and  

chemicals (i.e. oxygen deprivation)

2. Substances released by damaged cells, i.e. proteases, ATP, K+ion  

channels, histamine

ii. Via C fibers (unmyelinated) and Aδ fibers (thinly myelinated)

c. Hot Peppers: active ingredient is capsaicin, which binds to TRPV1 (also  activated by 43+°C – mimics effects of heat!)

i. In large quantities, can be used as an analgesic to desensitize pain fibers  XIV. Itch

a. ~15% suffer from chronic itch caused by disease/medications/psychosomatic b. Travels along C fibers that are selectively responsive to histamine, released by  mast cells in response to inflammation

c. Histamine binds to receptors to activate TRPV1 channels

XV. Primary Afferents and Spinal Mechanisms

a. Aδ – first pain ???? fast and sharp

b. C – second pain ???? duller and longer lasting (i.e. throbbing)

i. Glutamate is primary NT

XVI. Spinothalamic Pathway

a. Pain and temperature primary  

afferents synapse in substantia  

gelatinosa of dorsal horn

b. 2nd order neurons decussate in  

spinal cord and ascend to  

thalamus via spinalthalamic  


XVII. Trigeminal Pain Pathway

a. Trigeminal nerves in face and  

head synapse on 2nd order  

neurons in trigeminal nucleus

b. Decussate and ascend to  

thalamus via trigeminal  


XVIII. Trigeminal Neuralgia: “suicide disease”

Spinothalamic Trigeminal

a. Intense burning or shock-like unilateral facial pain

b. Usually caused by blood vessel pressing on the trigeminal nerve as it exits the  brain stem, causing damage to myelin sheath

c. Microvascular decompression (MVD): surgery that moves away vessel  compressing the nerve and places cushion between


XIX. Pain Regulation

a. Pain perception is highly variable, but chronic pain affects ~20% of adult  population

b. Referred Pain: pain felt from somewhere other than its actual source (i.e. angina  pain – felt at chest and left arm), due to different tissues developing from same  dermomyotome

c. Why rub a bruise?

i. Gate Theory of Pain: nociceptor axon stimulation alone maximally  

excites projection neuron, while simultaneous excitation of interneuron by  large sensory axon causes suppression

d. Hyperalgesia: increased sensory to pain at an injured site, caused by infiltration  of response cells at the site  

e. “Top-down” – periaqueductal gray matter (PAG) in midbrain is pain suppressant.  Sends axons to dorsal raphe (serotonin source – endogenous opioid!) ???? dorsal  horns ???? suppress nociceptors

f. Opioids: opiates, synthetics, and endogenous peptides

i. Pain killer whose active ingredient is morphine

ii. Opiate: alkaloid derived from the opium poppy

XX. Thermoreceptors

a. 6 TRP channels that are sensitive to different temperature ranges

b. Cold = Aδ fibers and C fibers; warm = C fibers

c. Adaptation during long-duration stimuli. Most responsive to sudden changes d. Same as pain pathway

XXI. Two Ascending Pathways of Somatic Sensation




Ascends ipsilaterally to  medulla

Ascends contralaterally from level  of spinal cord

Damage to half of  spinal cord

Ipsilateral touch damage

Contralateral pain/temperature  damage



↑ + ipsilateral motor  


Nerve endings



Axon diameter

Large, myelinated

Thin, lightly/unmyelinated


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