Pacinian corpuscle

1. % of hand's mechanoreceptors
2. size
3. level of acuity
4. sensitive to what?
5. adaptation to stimuli

1. 10-15% of hand's mechanoreceptors
2. largest mechanoreceptor
3. low spatial acuity (10+ mm)
4. sensitive to high frequency vibration
5. rapid

Ruffini's endings

1. % of hand's mechanoreceptors
2. size
3. level of acuity
4. sensitive to what?
5. adaptation to stimuli

1. 20% of hand's mechanoreceptors
2. 2nd largest mechanoreceptor
3. low spatial acuity (7 mm)
4. sensitive to skin stretch from finger; provides info about finger/hand position
5. slow

Meissner's corpuscle

1. % of hand's mechanoreceptors
2. size
3. level of acuity
4. sensitive to what?
5. adaptation to stimuli

1. 40% of hand's mechanoreceptors
2. 1/10th the size of Pacinian corpuscles
3. intermediate spatial acuity (3 mm)
4. sensitive to low frequency vibration
5. rapid

Merkel's disk

1. % of hand's mechanoreceptors
2. size
3. level of acuity
4. sensitive to what?
5. adaptation to stimuli

1. 25% of hand's mechanoreceptors (especially at fingertips)
2. smallest receptor
3. high spatial acuity (0.5 mm)
4. sensitive to edges, points, curves; detects form & texture
5. slow

dorsal column-medial lemniscal pathway

• describe steps in pathway

1. cell bodies of sensory Aβ axons (1st order neurons) in DRG
2. these axons enter at dorsal roots, ascend the spinal cord via ipsilateral dorsal columns
1. Lower body axons in medial part ascend via graciLe tract
2. Upper body axons in lateral part ascend via cUneate tract
3. both axons synapse on 2nd order neurons in dorsal column nuclei
1. pathways from lower body synapse on gracile nucleus
2. pathways from upper body synapse on cuneate nucleus
4. from here, 2nd order neurons crossover as internal arcuate fibers to form the medial lemniscus, which ascends thru the medulla, pons, midbrain
5. thes 2nd order neurons synapse on cells in the ventral posterior (VP) nucleus of the thalamus
6. VP neurons (3rd order) project via internal capsule to primary somoatosensory cortex (S1)

trigeminothalamic tract

• describe steps in pathway

1. cell bodies for mechanoreceptors in skin of face are in the trigeminal ganglion (primary afferents are Aβ neurons)
2. these 1st order neurons project to cells in ipsilateral trigeminal nucleus
3. 2nd order neurons from here crossover, forming the trigeminal lemniscus, which ascends to the thalamus
4. these axons synapse on cells in the VP medial nucleus of the thalamus
5. VPM neurons (3rd order) project to primary somatosensory cortex (S1)

somatosensory cortex

1. location in cortex
2. Brodmann's area __
3. posterior to _______ _______, along ________ ________
4. part of cortex involved (Brodmann areas?)
5. Brodmann areas for texture, size/shape

1. parietal lobe of cerebral cortex
2. Brodmann's area 3b
3. posterior to central sulcus, along postcentral gyrus (3a, 1 & 2)
4. posterior parietal cortex involved (5 & 7)
5. 3b → 1; 3b → 2

pain: the perception of unppleasant sensation; doesn't require activation of nociception and is subjective to individual

nociception: the result of activation of nociceptors & associated neural pathways; doesn't necessarily lead to perception of pain

nociceptors

1. location
2. fiber characteristics
3. modality

1. located in free nerve endings of Aδ fibers and unmyelinated C fibers
2. these fibers have the smallest diameters, slowest conduction velocity
3. most are polymodal (have multiple stimuli; mechanical, chemical, thermal)

bradykinin: synthesized by enzyme released from damaged cells, directly activates nociceptors via depolarization via G-protein coupled receptors

ATP: same synthesis process, directly activates P2X receptors (ionotropic)

K⁺: released from damaged cells, depolarizes nociceptors

describe characteristics of:

1st and 2nd pain

1st pain: fast and sharp; result of firing of Aδ fiber

2nd pain: slow and dull; result of firing of unmyelinated C fiber

spinothalamic pathway (of nociception)

• describe steps of pathway

1. nociceptive cell bodies located in the dorsal root ganglion, from 1st order neurons (Aδ & C fibers)
2. these axons travel up and down via Lissauer's tract, synapsing onto 2nd order neurons in the substantia gelatinosa of the dorsal horn
3. the 2nd order axons crossover in the spinal tract (unlike the dorsal column-medial lemniscus pathway), and ascend via the spinothalamic tract
4. then, they synapse onto cells in the VP lateral nuclei of the thalamus & intralaminar nuclei
5. VPL neurons (3rd order) project to S1

trigeminal (nociceptive) pathway

a.k.a the spinal trigeminal tract

1. cell bodies (Aδ and C; 1st order neurons) in trigeminal ganglion
2. axons travel up and down from pons to caudal medulla via spinal trigeminal tract
3. these axons then synapse on cells in the spinal trigeminal nucleus (2nd order neurons), which then crossover, froming the trigeminal lemniscus
4. this tract ascends to the thalamus, where axons synapse on cells in the VP medial nucleus of the thalamus
5. VPM neurons (3rd order) project to S1

descending pain pathways

(periaqueductal grey)

• describe characteristics of PAG (location, pain, influences), pathway

gate theory of pain

1. explain theory
2. explain when a "gate" will be closed or open

1. some 2nd order projection neurons excited by both a nociceptive neuron (C-fiber) and a mechanosensory neuron (Aβ fiber)
2. innervating the 2nd order projection neuron is an inhibitory interneuron, which is inhibited by a nociceptive neuron and excited by a mechanosensory (Aβ) neuron
3. key: coactivation of mechanosensory neurons by touch can reduce the strength of pain signalling; helps "close" the gate on pain

heat: TRPV3, TRPV4

cold: TRPM8

noxious heat: TRPV1, TRPV2

noxious cold: TRPA1

3 types of papillae covering the tongue

- describe each

• fungiform (anterior two-thirds); few taste buds on ea. papilla
• foliate (posterior lateral edge); 12-600 taste buds on ea.
• circumvallate (back); 250 taste buds on ea. papilla

salty and sour TRC transduction mechanisms

- describe them

salt TRC transduction:

1. Na⁺ (tastant) enters thru amiloride-sensitive Na+ channels, causes depolarization
2. V-gated Na⁺ and Ca²⁺ channels open → NT release

sweet, bitter & umami TRC transduction mechanism

- describe

1. γtastant (glucose, quinine, or glutamate (respectively)) binds GPC receptor
2. activates gustducin (G-protein)
3. βγ subunit of gustducin stimulates phospholipase C, which hydrolyzes PIP₂ to IP₃ & DAG
4. IP₃ activates IP₃ receptors → release of Ca²⁺ from intracellular stores
5. Ca²⁺ activates TRPM5 channel
6. increased Na⁺ depolarizes cell  NT is released (onto gustatory afferents)

• bitter: 30 diff. types of receptors (T2Rs)
• receptors consist of two T2R proteins
• sweet & umami: receptors are heterodimers of 2 diff. subunit types of T1Rs
• only one combo for each type:
• sweet = T1R2 + T1R3
• umami = T1R1 + T1R3

taste encoding

• explain labeled-line and across-fiber theories of taste encoding

• labeled-line: in support of this theory, axons were grouped according to the stimulus that generated the maximal response
• idea was that info about a particular taste was selectively carried by those groups
• single axons tuned for response to a specific taste stimulus
• TRCs of 1 type synapse onto 1 sensory neuron
• across-fiber: in support of this theory, the activity of all axons to all tastes was evaluated and taste was attributed to specific patterns of activity across the whole population
• different taste stimuli have distinctive patterns of population activity
• a form of population coding
• TRCs are multimodal (have many receptor types) and synapses onto a sensory neuron OR more than one kind of TRC (ea. w/ 1 receptor type) that synapses onto one sensory neuron

cranial nerves VII, IX and X carry taste signals, project ipsilaterally to the gustatory nucleus in the medulla

axons then synapse on cells in VPM nucleus of the thalamus, which project to the gustatory and frontal cortex

1. odorant binds to odorant receptor, activates G_olf-protein
2. G_olf-protein activates adenylyl cyclase, synthesizes cAMP
3. cAMP binds to cyclic nucleotide-gated (CNG) channels → influex of Na⁺ & Ca²⁺
4. Ca²⁺ binds to Ca²⁺-activated Cl^- channels (leading to an efflux of Cl^- ions, and further depolarization)

• 7TM receptors
• G-protein coupled
• bind odorants in their extracellular domains
• G-protein interaction occurs via intracellular C-terminal
• most ORNs express single odorant receptor type
• ea. ORN is broadly tuned, responds to multiple odorants even if it expresses only one type of odorant receptor protein

1. ORNs (1st order axons) project to olfactory bulb via small bundles which make up cranial nerve I
2. ORNs synapse onto mitral cells (2nd order) in glomeruli
3. mitral cell axons form bundle called lateral olfactory tract, which projects to several brain structures (including olfactory cortex)
4. olfactory tubercle → medial dorsal nucleus (MDN) of the thalamus → orbitofrontal cortex

vomeronasal organ:

• has 2 types of specialized G-protein coupled receptors
• V1Rs (volatile compounds)
• V2Rs (non-volatile protein compounds)

list cranial nerves and their involvement in sensory systems

I

III

IV

VI

V

VIII

VII

IX

X

I. olfactory nerve (olfactory system)

III. oculumotor nerve

IV. trochlear nerve

VI. abducens

(III, IV, VI involved in the vestibu-ocular reflex)

V. trigeminal (touch, pain/temperature system)

VIII. auditory-vestiubular nerve (vestibular system)

VII. facial

IX. glossopharyngeal

X. vagus

(VIII, IX, X, involved in the gustatory reflex)

• inner ear
• two structures are:
• two otoloith organs (detects head tilt, linear acceleration)
• three semicircular organs (detects angular acceleration (rotation))

• sensory epithelium is the macula
• the utricle is a structure w/in the otoloith organs that contains a macula oriented  to the ground parallel
• connected to semicircular canals
• sensitive to motion in the horizontal plane
• the saccule is the other structure w/in the otolith
• its macula is oriented perpendicular to the ground
• its connected to the cochlea
• sensitive to gravity & vertical field

• sensory epithelium is the crista
• 3 canals: superior, posterior, horizontal/lateral
• each canal joins the utricle at an enlargment called the ampulla, which houses the crista & a gelatinous structure called the cupula
• summary:
• each semicircular canal has an ampulla, and each ampulla has: 1) a crista, 2) a hair bundle (cilia of hair cells), and 3) a cupula

1. stretch-activated cation channels open when cilia are deflected towards kinocilium (cilia are all linked at tips, deflection is ubiquitous)
2. Ca influx²⁺
3. membrane depolarization
4. V-gated Ca²⁺ channels open
5. NT is released at synapses w/ vestibular ganglion cells

1. in left horizontal canal: cupula moves towards kinocilia, hair cells depolarize, and vestibular axons increase firing rate
1. (opposite happens iin right horizontal canal)
2. left canal has: 1) increased firing during acceleration, 2) adaptative firing rate during constant velocity, and 3) decreased firing during decelration
1. right canal has opposite

central vestibular pathways

1. vestibular axons project to: cerebellum, lateral vestibular nucleus (input from otolith organs) & medial vestibular nucleus (input from semicircular canals)
2. LVN projects to limb motor neurons (act to orient posture
3. MVN projects to: 
1. neck motor neurons (act to orient head)
2. extraocular motor nuclei III, IV, VI (involved in vestibulo-ocular reflex)

vestibulo-ocular reflex

1. two key points are ?
2. describe pathway during leftward turn of head

1. key points
1. mantains focus of vision during head motion
2. direction of eye movement is opposite to direction of head movement
2. pathway
1. left horizontal canal hair cells are excited → excite muscles that move the eyes rightward (left medial rectus (nasal side of eye), right lateral rectus muscle (temporal sidde of eye); inhibits muscles that move eyes leftwards (left lateral rectus, right medial rectus muscle)
2. right horizontal canal hair cells are inhibited → won't inhibit OR excite muscles needed for complementary  response