D1: Sensory Systems

Sensory receptors

Detect stimuli and convert them into signals

Sensory transduction

Conversion of stimulus into neural impulses

Sensory pathways

Route from receptors to brain

First-order neuron

Afferent neuron carrying signal to CNS

Adaptation

Reduced response to constant stimulus

Sensory coding

Encoding of stimulus intensity (rate coding)

Rate coding

Stronger stimulus → higher firing rate

Receptive field

Area where stimulus affects neuron activity

Topographic organisation

Spatial mapping in brain

Retinotopic map

Visual spatial mapping

Tonotopic map

Sound frequency mapping

Somatotopic map

Body surface mapping

Eye structures

Cornea, pupil, lens, retina, optic nerve

Photoreceptors

Rods and cones

Rods

Detect dim light

Cones

Detect bright light (RGB)

Phototransduction

Light → electrical signal

Rhodopsin

Light-sensitive receptor (GPCR)

Visual pathway

Retina → optic nerve → optic chiasm → lateral geniculate body → visual cortex

Visual processing

Left visual field → right brain

Right visual field → left brain

Sound frequency

Pitch of sound

Sound amplitude

Intensity (loudness)

Human hearing range

20 Hz – 20 kHz

Ear

Organ for hearing

Cochlea

Structure for sound transduction

Organ of Corti

Sensory structure in cochlea

Frequency tuning

Different regions detect different frequencies

Auditory pathway

Signal travels from ear to brain

Tonotopic map

Frequency-based organisation in brain

Skin

Sensory organ

Mechanoreceptors

Detect touch

Thermoreceptors

Detect temperature

Nociceptors

Detect pain

Somatosensory pathways

• Dorsal column-medial lemniscal pathway

• Spinothalamic tract

Pain fibres

A-delta fibres : Fast, myelinated
C fibres : Slow, unmyelinated

Somatotopic map

Body mapped in brain

Olfactory receptors

Detect odours

Olfactory bulb

Processes smell signals

Olfactory pathway

From nose to brain

Unique feature

Thalamus does not directly process smell

Olfactory diversity

Large variety of receptors

Taste receptors

Located in taste buds

Five basic tastes

• Sweet

• Bitter

• Sour

• Salty

• Umami

Gustatory pathway

Taste buds → nucleus of solitary tract → thalamus → insular cortex

Gustotopic map

Taste mapping in brain (likely present)

Stimulus conversion

Sensory transduction

Signal strength coding

Rate coding

Mapping principle

Topographic organisation

Vision mapping

Retinotopic

Hearing mapping

Tonotopic

Touch mapping

Somatotopic

Fast pain

A-delta fibres

Slow pain

C fibres

Smell exception

No direct thalamus involvement