Lecture 5.5: Sensory Function I: Olfaction & Gustation

General Senses

Receptors distributed throughout body (particularly on body surfaces)

General senses are not collected within specialized ‘sense organs’

Four receptor types:

• Pain – nociceptors

• Temperature – thermoreceptors

• Touch, pressure, body position – mechanoreceptors

• Chemical stimuli – chemoreceptors (for O2, CO2, etc.)

Somatic vs. visceral senses

Somatic = body surface

• Surface temperature, touch, pain, muscle soreness

Visceral = internal organs

• Stomach ache, ‘gut cramps,’ etc.

The Special Senses

Receptors are congregated in specialized ‘sense organs’

Five receptor types:

• Smell (olfaction) – nose

• Taste (gustation) – tongue

• Balance (equilibrium) – ear

• Hearing (audition) – ear

• Sight (vision) – eye

Specificity of Sensation

All sensations are ‘read’ in the CNS as electrical signals (action potentials)

• Regardless of stimulus type

Sensory discrimination: Different receptors specialize in specific stimuli

• Mechanoreceptors detect touch

• Photoreceptors detect light

Sensory Transduction

Common Steps:

1. Stimulus alters membrane potential of receptor

• Produces graded potential

• Graded potentials can be depolarizing (EPSP) or hyperpolarizing (IPSP)

• A graded potential at a receptor is called a receptor potential

2. Receptor potentials influence rate of action potential production in sensory (afferent) neuron

3. Action potentials travel to CNS along afferent pathway

4. CNS interprets/processes these incoming signals

Sensory Transduction (2)

Interpretation of afferent (sensory) input:

• Brain ‘assumes’ signals from a particular receptor represent the appropriate stimulus for that receptor

  • e.g., pain from nociceptor; photon from photoreceptor

All other characteristics of the stimulus (intensity, duration) are conveyed through frequency and pattern of incoming signals

Receptive Fields

Allow for discrimination between two stimuli of the same nature

• e.g., two points on body surface

Discrimination is dependent on receptor density

• Receptor density = the number of receptors within an area of ‘sensory surface’

Receptive Field Properties

Stimulation anywhere in the receptive field activates the same sensory neuron

Smaller receptive fields = higher receptor density

• High receptor density → high resolution discrimination

Areas with small receptive fields require more neurons to process info

Explains the shape of the sensory homunculus

Sensation by Olfactory Receptors

The resulting EPSP (depolarization) leads to the triggering of an afferent action potential