sense transduction

Light hits photoreceptors (rods and cones)

Photopigments inside rods and cones are activated

Photopigment structure

Consists of opsin (protein) and retinal (vitamin A-derived lipid)

Photopigment in rods

Rhodopsin = rod opsin + retinal

Effect of light on rhodopsin

Isomerization: shape changes, bleaching (color change), opsin and retinal separate

Ion channel changes in photoreceptors

Light causes Na⁺ channels to close, K⁺ channels stay open → hyperpolarization

Ca²⁺ effect in light

Ca²⁺ channels close, reducing neurotransmitter release

Glutamate in the dark

Photoreceptors release glutamate continuously (inhibitory via mGluRs)

Glutamate in light

Less glutamate released → less inhibition of bipolar cells

Activation of bipolar cells

Disinhibition activates ON bipolar cells → more glutamate released (excitatory)

Ganglion cell stimulation

Ganglion cells fire action potentials → form optic nerve → send signal to brain

ON bipolar cells in light

Disinhibited by less glutamate → depolarize and activate

OFF bipolar cells in light

Lose excitatory glutamate → hyperpolarize and become inactive

Dark state photoreceptors

Depolarized → release glutamate → inhibit ON bipolar cells, excite OFF bipolar cells

Light state photoreceptors

Hyperpolarized → reduce glutamate → ON bipolar cells activate, OFF bipolar cells silent

Sound wave path

Outer ear → tympanic membrane → ossicles → oval window → cochlear fluid

Basilar membrane response

Fluid waves move basilar membrane → hair cells in Organ of Corti activated

Hair cell cilia

Bending of cilia opens mechanically gated ion channels

Ion entry into hair cells

K⁺ and Ca²⁺ enter due to high K⁺ in cochlear fluid → depolarization

Hair cell neurotransmitter release

Depolarization causes glutamate release

Activation of auditory nerve

Spiral ganglion neurons (CN VIII) activated by neurotransmitter

Auditory pathway to brain

Auditory nerve → cochlear nucleus → superior olive → inferior colliculus → MGN → auditory cortex

Mechanoreceptors

Respond to mechanical pressure/stretch via ion channels

Merkel’s disks

Texture/form; slow adapting, small receptive field

Meissner’s corpuscles

Light touch; fast adapting, small receptive field

Ruffini endings

Skin stretch; slow adapting, large receptive field

Pacinian corpuscles

Vibration; fast adapting, large receptive field

Temperature receptors

Warm = TRPV1–4; Cool = TRPM8

Pain (nociceptors)

Free nerve endings detect mechanical, thermal, chemical damage

Action potentials in somatosensation

Depolarization → AP travels via dorsal root to spinal cord

Touch pathway

Dorsal column-medial lemniscus → thalamus → somatosensory cortex

Pain/temp pathway

Spinothalamic tract → thalamus → somatosensory cortex

Taste detection

Taste molecules in saliva bind to receptors in taste buds

Salty taste

Na⁺ enters ion channels → depolarization → AP

Sour taste

H⁺ enters channels → depolarization

Sweet receptors

T1R2 + T1R3 (GPCRs)

Umami receptors

T1R1 + T1R3 (GPCRs)

Bitter receptors

T2R (GPCRs)

GPCR cascade in taste

Activates G-protein (gustducin) → 2nd messenger → ion channels open → depolarization

Neurotransmission in taste

Depolarized receptor cells release ATP (not typical neurotransmitters)

Taste pathway

Facial (VII), Glossopharyngeal (IX), Vagus (X) → NST → thalamus → gustatory cortex

Odorant detection

Odorant binds GPCR on cilia of olfactory receptor neuron

Golf protein cascade

Odorant → Golf → adenylate cyclase → ↑cAMP → Na⁺ channels open → depolarization

Action potential path in smell

AP travels through cribriform plate → olfactory bulb

Synapse location in bulb

Olfactory neuron axons synapse at glomeruli onto mitral cells

Olfactory pathways

Mitral cell axons project to amygdala, piriform, entorhinal, and orbitofrontal cortices

Unique feature of olfaction

Only sense that bypasses thalamus and connects directly to limbic system