BIO133 -> Week 6: Sensory

Sensory processing

4 steps: stimulation → transduction → transmission → interpretation

Stimulation

Activation of a sensory receptor by a stimulus

Transduction

Conversion of stimulus energy into an electrical signal

Transmission

Movement of signals via neurons to the CNS

Interpretation

Brain processing that gives meaning to the signal

Stimulus-gated ion channels

Ion channels that open/close in response to stimuli

Receptor potential

Graded electrical change in a sensory receptor (similar to EPSP)

Depolarization

Decrease in membrane potential making neuron more likely to fire

Action potential

All-or-nothing electrical signal transmitted along neurons

Mechanoreceptors

Respond to physical forces (pressure, stretch, vibration)

Chemoreceptors

Respond to chemical stimuli (taste, smell, blood chemistry)

Electromagnetic receptors

Respond to light and heat energy

Cutaneous receptors

Sensory receptors in the skin

Function of cutaneous receptors

Detect pain, temperature, touch, and pressure

Nociceptors

Receptors that detect pain and tissue damage

Free nerve endings

Simple, exposed dendrites that detect harmful stimuli

TRP channels

Ion channels involved in sensing heat, pain, and chemicals

Capsaicin response

Activates TRP channels → sensation of heat/pain

Thermoreceptors

Detect changes in temperature

Structure of thermoreceptors

Naked dendritic endings

Mechanism of thermoreceptors

TRP ion channels respond to hot/cold

Phasic receptors

Adapt quickly, respond to changes (start/stop of stimulus)

Tonic receptors

Adapt slowly, respond continuously

Examples of phasic receptors

Hair follicle receptors, Meissner corpuscles, Pacinian corpuscles

Examples of tonic receptors

Ruffini corpuscles, Merkel's disks

Proprioceptors

Detect body position and movement

Function of proprioceptors

Provide awareness of limb position and motion

Muscle spindles

Detect muscle stretch (parallel to muscle fibers)

Golgi tendon organs

Detect tension in tendons

Protective reflex (Golgi)

Inhibits muscle contraction to prevent damage

Baroreceptors

Detect blood pressure via stretch in vessel walls

Location of baroreceptors

Carotid sinus and aortic arch

Low blood pressure response

↓ firing rate → ↑ heart rate + vasoconstriction

Taste (Gustation)

Detection of dissolved chemicals in food

5 basic tastes

Sweet, sour, salty, bitter, umami

Taste buds

Clusters of chemosensory cells connected to neurons

Papillae

Raised structures on tongue containing taste buds

Mechanism of salty & sour tastes

Direct ion channel activation

Mechanism of sweet, bitter, umami tastes

G protein-coupled receptors (indirect)

Olfaction

Detection of airborne chemicals

Location of olfactory receptors

Upper nasal passages

Requirement for olfaction

Molecules must dissolve in mucus

Pathway of olfaction

Receptors → axons → cerebral cortex

Sensitivity of human olfaction

Humans detect thousands of odors

Peripheral chemoreceptors

Located in carotid & aortic bodies

Central chemoreceptors

Located in medulla oblongata

Key relationship of CO₂

↑ CO₂ → ↓ pH → stimulates breathing

Vision

Detection of light energy by photoreceptors

Function of vision

Determines direction and distance of objects

Photoreceptors

Cells that convert light into neural signals

Eyespot (invertebrates)

Simple light detection, no image formation

Flatworms

Detect direction of light only

Humans' photoreceptors

3 types of photoreceptors

Mantis shrimp

16 types of photoreceptors (extreme color detection)

Stimulus → Response Flow

Stimulus → receptor potential → action potential → CNS → perception

Adaptation Concept

Phasic = change detection; Tonic = constant monitoring

Direct vs Indirect Signal Transduction

Direct: ion channels (fast, simple); Indirect: GPCRs (slower, amplified)

Structure → Function

Free nerve endings → pain/temperature; Encapsulated receptors → fine touch/pressure

CO₂ and Breathing

CO₂ controls breathing more than oxygen because it affects pH directly

Common misconceptions about receptors

All receptors behave the same (wrong); Pain = just strong stimulus (wrong); Taste is just chemical (incomplete); Oxygen controls breathing (mostly false)

Focus areas for studying

Phasic vs tonic (almost guaranteed question); Receptor types + examples; CO₂ → pH → breathing; Direct vs indirect taste pathways.

Sensory information process

Conveyed to the CNS and perceived in a four-step process: Stimulation, Transduction, Transmission, Interpretation.

Classes of receptors

Receptors can be grouped into three classes: Mechanoreceptors, Chemoreceptors, and Electromagnetic receptors.

Photoreceptors in humans

Humans have 3 types of photoreceptors.

Photoreceptors in mantis shrimp

Mantis shrimp have 16 types of photoreceptors.