PSB3340 Exam 2

Sensory receptor cells

translate physical sensations to electrical signals

Sensory receptor organs

specialized to detect a certain stimulus (eyes, nose)

Receptor cells

• within organ

• convert stimulus to electrical signal

• graded potential (not AP)

• transducer

Sensory transduction

• conversion of electrical stimulus to change in membrane potential (receptor cell, graded)

Receptor potentials

local membrane potentials specific in response to adequate stimuli from receptor cells

Receptor potentials

• can result in the generation of APs OR release of a graded amount of NT

• only impact voltage in a small local area of cell

• summate

Restricted range of responsiveness of sensory systems

Responses range between individuals, lifespan, and species

like how we don’t have receptors that can pick up UV light

Range fractionation

• Different cells have different thresholds for firing over a range of stimulus intensities

• Stimulus intensities encoded by frequency of AP firing

• Different cells cover different fractions of possible intensities (like how colors are split up into RGB)

  • diff. areas of skin responding to different amounts of pressure applied

Stimulus location

• sensory touch and pain receptors in body map onto corresponding regions of the primary somatosensory cortex = somatotopic representation

• differing intensities of mechanical pressure activate nociceptors (pain)vs. fine touch receptors

Coding

patterns of APs that reflect a stimulus

• number and frequency of APs, rhythm of AP clusters

• ex. spatial, temporal summation

Adaptation

• Loss of response to sustained stimulus - reduction in AP frequency

• Highlight changes

Tonic receptors

slow/no adaptation

no decr. in AP frequency

Phasic receptors

rapid adaptation

decr. in AP frequency

Suppression

Adaptation

Accessory structures (dog ears, eyelids) - before reaches sensory receptors

Top-down processing - suppression

Higher brain centers suppress some sensory inputs + amplify others

• attention is most common!

• memory, expectations etc.

Pathways

• Each sensory system has a distinct sensory pathway through brain

• distinct senses bc APs travel along separate nerve tracts = labeled lines

• MOST pass through thalamus (NOT olfactory)

• Terminate in cerebral cortex

• General senses (somatosensory, visceral) along somatic or visceral afferent nerve fibers

• Specialized via specific cranial nerves (optic, olfactory)

General sensory pathway

Receptors → thalamic relay nuclei → primary sensory cortex (varied) → secondary sensory cortex (varied) → association cortex

*smell does NOT pass through thalamus first

Receptive fields

space in which a stimulus will alter a neuron’s firing rate

excitatory center/inhibitory surround or inhibitory center/excitatory surround = edge detection

can change by experience

• severed limb - cortical area devoted to that area shrinks, others expand

• intentional stimulation = expand in cortical representation

Non-primary (association) cortex

receives main input from primary cortical area for specific sense

responds to a large range of stimuli over a larger portion of the environment

Attention

Mainly activated: posterior parietal lobe and cingulate cortex

Synesthesia

blurring of sensory modalities

Pseudo unipolar neurons

Somatosensory receptors

All start as one appendage from soma

4 tactile receptors

Detect touch

1. Pacinian corpuscles

2. Meissner’s corpuscles

3. Ruffini’s endings

Pacinian corpusles

“onion layered”

deeper than meissners/merkels

touch moves some layers relative to others, uncovering channels

vibration, transient stimulation

fast adapting

Meissner’s corpuscles

near surface of smooth skin

layers of overlapping disks moved by pressure - open underlying ion channels

light touch

fast-adapting

help perceive object forms

prevalent in fingertips, tongue, lips

Merkel’s discs

fine touch (light, sustained)

slow-adapting

help perceive object forms

prevalent in fingertips, tongue, lips

Ruffini’s endings/corpuscles

nerve fiber branches embedded in collagen

stretching collagen opens channels in embedded dendrites

sustained, deep stimulation

stretch

slow-adapting

somatosensory AND proprioceptive (body position)

Epidermis

outermost layer

dead cells

free nerve endings

Dermis

middle

Merkel disc

Meissner corpuscle

Hypodermis

Innermost layer

fat + connective tissue

anchors skin to muscles + help shape body

Pacinian corpuscle

Ruffini ending

Mechanoreceptors

Tactile touch sensors of skin

Respond to physical deformation of their membranes by opening mechanically gated ion channels (non selective)

Piezo ion channel protein

In tactile receptors

Opens when mechanically stretches and depolarizes the cell by letting cations enter

Dorsal column system

deliver touch info to brain

up spinal cord to medulla (brainstem) → crosses over midline → thalamus → primary somatosensory cortex

Dermatome

Region of skin innervated by a particular spinal nerve

Reflect quadruped

Primary somatosensory cortex (S1)

receive touch info from opposite side of body

homunculus - amount of tissue corresponds to how sensitive/densely enervated the area is not how large it is

Secondary somatosensory cortex (S2)

maps both sides of body in registered overlay

blends together info

Free nerve endings

Temperature, pain, itch

Different nerve endings produce different receptor proteins to respond to temp changes, chemicals, pain, itch

Thermoreceptors

Free nerve endings

Cold = more numerous, superficial

• TRPM8 - alsp methol

Warm

• TRPV1 receptor - also capsaicin (chile) - polymodal receptor

• TRPM3 - even higher temps (no capsaicin)

Itch free nerve endings

• Respond to histamine (NT) or chloroquine (released by mast cells in skin)

• Activates TRP receptors + terminate in the pons = itch sensation

Nociceptors

Pain

• Intense pressure

• Hear

• Chemicals released by destroyed cells

• Extreme pain will be nociceptors AND free nerve endings

• Mutation in gene that codes for one of its voltage gated Na channels causes insensitivity another chronic pain

• High temps/pain = large, myelinated axons to conduct signals rapidly

Tissue damage

= “inflammatory soup” that sensitize or excite terminals of nociceptors

• Peptides

• Lipids

• Neurotransmitters

• Neurotrophins

Substance P

Promotes inflammation

Vasodilation → more blood flow → attract immune cells to site

Anterolateral (spinothalamic) system

Delivers pain + temp to brain

Info crosses midline in the spinal cord (FIRST) → thalamus

More poorly localized than if it ascended through the dorsal column before crossing midline (like fine touch, vibration)

Neuropathic pain

Phantom limb pain

Microglia overexciting pain pathways in limb

Anterior cingulate cortex (ACC)

Immediate emotional consequences of pain

Social rejection also activates this area

Periaqueductal gray (PAG)

Area in midbrain involved in pain perception

Main source of endogenous opioids

PAG neurons send endorphin-containing axons to medulla and spinal cord

Placebo releases endogenous opiates

Hair cells

• Sensory receptors cells for hearing

• Mechanoreceptors - increases/decreases in air pressure

• In organ of corti in cochlea

Amplitude

• Perceived as loudness

Frequency

• Perceived as pitch

Pinna

• External ear

• funnels sound

• outer ear

ear wax

antifungal

water resistant

antibacterial

Cochlea

• Fully developed at birth

Tympanic membrane

• ear drum

• beginning of middle ear

• concentrates sound energies

• connected to the oval window by 3 ossicles

Ossicles

Mechanically multiply force through lever action

Larger tympanic membrane to smaller oval window

1. Malleus - hammer

2. Incus - anvil

3. Stapes - stirrup

Acoustic reflex

2 muscles in middle ear can contract to inhibit vibration in the ossicles - help prevent damage

Have delay

• Tensor tympani - attached to malleus

• Stapedius - attaches to stapes

Cochlea

• 3 parallel canals

  • Scala vestibuli

  • Scala media - middle; contains organ of conti

  • Scala tympani

Scala vestibuli

Cochlea channel

At the oval window, vibrations from stapes transferred to fluid here

Scala media

• Isolated from other layers

• Endolymph with different ionic makeup than perilymph

• Contains receptor system → organ of Corti → hair cells convert sound to neural

Organ of Corti

• Contains hair cells - embedded in basilar membrane

• Sound vibrations cause basilar membrane to oscillate like waves

Scala tympani

Cochlea channel

• Round window - pressure release valve so sound waves can travel through perilymph

Tonotopic organization

• Diff. sound frequencies map to diff. areas of cochlea

• Low frequency - displaces near end, round window

• High frequency - displaces base of basilar membrane, near oval window

Place coding

• Near base = treble

• Near apex = bass

Temporal coding

Firing pattern of auditory neurons help encode frequency

Volley principle

groups of neurons fire in turn (during refractory period) to create high frequency sounds

mastoid process

• Bone in back of jaw

• add low frequency sound to our own voice when we hear it directly

Stereocilia - hair cells

• Extend into tectorial membrane

• Tip links connect stereocilia - tension pulls stereocilia together - opening ion channels

• don’t produce APs themselves - release graded amounts of NTs (glutamate) - afferent nerve produces AP

• K+ depolarizes (endolymph has high K+ concentration; perilymph has low) → Ca+ channels open → NT released → cochlear nerve (afferent) AP

Inner hair cells

auditory signal transduction

• lack causes deafness

• most afferent connections to brain

• each innervated with approx. 10 nerve fibers for diff. loudness levels

• efferent (top down) to inner ear cells help suppress loud noises by reducing excitability of afferent nerve (slow down rate of APs)

Outer hair cells

sound amplification

• lack causes hearing impairment

• efferent (top down) change lengths of OHCs to fine-tune cochlea to help discriminate frequencies

  • hyperpolarize - lengthen - more sensitive

  • depolarize - shorten - less sensitive

Vestibulocochlear nerve (CN VIII)

• Afferent (to brain) axons of both hair cells become cochlear nerve which combines with vestibular nerve = Vestibulocochlear nerve

• Auditary + vestibular info

Binaural

• Two ear information

• Most info from each ear projects to cortex on opp. side of brain

• Begins in the brainstem superior olivary nucleus

*Auditory pathway

• S uperior olivary nucleus

• L ateral lemniscus

• I nterior colliculus

• M edical geniculate nucleus

• To superior temporal gyrus

All levels have tonotopic organization (arranged in a map according to frequencies to which they respond)

Tuning curves

Show a cell’s response to various frequencies

Sound frequency/pitch discrimination increases along hierarchy of system

Primary auditory cortex (A1)

• Part of temporal lobes

• Located within the lateral sulcus

• Involved in lip reading - integrating sight/sound info

Dorsal auditory stream

Where?

Ventral auditory stream

What?