• Today– Sensory receptors

• General properties

– Skin receptors

Sensory receptors

• Vision• Taste• Smell• Hearing• Touch

• Internal– Proprioception - limb position and movement– Baroreceptors - blood pressure– Osmoreceptors - osmolarity– Chemoreceptors – chemical concentration

Chemical

Mechanical

Light

Sensory receptors

General Properties:

1. Specialized structures to receive sensory information

2. Amplification and Transduction of sensory stimuli to electrical activity

3. Coding of information by the number and frequency of APs

Types of Sensory Neurons

Sensory receptor

SpikeInitiatingZone

Action Potentials

Action Potentials

Chemical synapse

Action Potentials

Sensory receptor

SpikeInitiatingZone

Stimulus reaches receptor cells

Activation of receptors

Protein interactions &Second messengers

Ion channels open or close

Generates a receptor current

Changes in VmSpread to spike initiating zone

Changes in the amount of neurotransmitter release

Influence number and frequency of APsin sensory neuron

Sensory Energy

Electrical Energy

GradedEvents

All or noneAPs

Sensory Reception Cascade

Common Physiological Properties

• Dynamic Range– The range of stimulus intensity the sensory

system can respond to

• Adaptation1. Phasic receptors

• Fire APs for only one part of stimulus

2. Tonic receptors• Fire APs for duration of stimulus

Dynamic RangeF

requ

ency

of

AP

s (H

z)

Log Stimulus Intensity1 10 100 1000 10,000

Sensory receptor responds in this range

Upper limit set by refractory period

Threshold for detection Sensory receptor

can’t respond further

How to overcome limited range?• For each type of receptor, there are

individual receptors specialized to respond to specific parts of the range

Log Stimulus Intensity1 10 100 1000 10,000

Range of the whole sensory system

Fre

quen

cy o

f A

Ps

(Hz)

Range of individual receptors

Receptors of the skin

Pacinian Corpuscle Deep touchMeissner’s Corpuscle Light touchRuffini’s Corpuscle stretching

Free nerve endings pain & temperature

• Pain– Nociceptors –respond to painful stimulus– Carried by non-myelinated C fiber sensory

neurons– Painful heat, acids, mechanical damage all

activate non-specific cation channels

Na+

Ca++

Painful heatAcidsMechanical damage

Substances released from Damaged cells:ATP

BradykininSubstance P

Pain

Free nerve ending

Receptors of the Skin

• Tactile sensory input– Respond to pressure and movement of skin– Specialized receptors that respond to

particular types of inputs

Pacinian – heavy pressure, rapid vibration (300Hz)Meissner’s – light pressure, slow vibration (50 Hz)

Use mechanically gated ion channels

Pacinian Corpuscle

Mechanical Stimulus

Fluid filled layers

Extracellular recording

Nerve axon

Saline bath

Mechanical stimulusOpens ion channels

Receptor currentsflow passively within axon

If receptor potential islarge enough APs produced

axon myelin

First NodeOf Ranvier

Pacinian Corpuscle

• Example of a rapidly adapting receptor– Only gives an ‘on’ and an ‘off’ response– Epithelial layers filter out steady pressure but

transfer rapid changes in pressure

Receptive Field

The region of the skin in which a stimulus evokes a response in a single sensory neuron

Discrimination depends on the density of receptors

Fingers 1-4 mm

Thigh 45 mm

skin

Sensory neuron

dendrites

Sensory neuron

dendrites

AP generated in one neuron

AP generated in two different neurons

• Good Luck!