General Sensory Reception

The Sensory System

• What are the senses ? • How sensory systems work• Body sensors and homeostatic maintenance• Sensing the external environment• Mechanisms and pathways to perception

General Properties of Sensory Systems

• Stimulus– Internal– External– Energy source

• Receptors– Sense organs– Transducer

• Afferent pathway• CNS integration

General Properties of Sensory Systems

Sensory Receptors

• Somatic

-- Chemoreceptors (taste, smell)-- Thermoreceptors (temperature, pain)-- Photoreceptors (vision)-- Proprioreceptors (muscle stretch)--Mechanoreceptors (touch, pain, audition, balance).

• Visceral

-- Chemoreceptors (chemicals in blood, osmoreceptors)-- Baroreceptors (bp)

Sensory Receptor Types

Special Senses – External Stimuli

• Vision• Hearing• Taste• Smell• Equilibrium

Special Senses – External Stimuli

Somatic Senses – Internal Stimuli

• Touch• Temperature• Pain• Itch• Proprioception• Pathway

Somatic Pathways

• Receptor– Threshold– Action potential

• Sensory neurons– Primary – medulla – Secondary – thalamus– Tertiary – cortex

• Integration– Receptive field– Multiple levels

Somatic Pathways

Sensory Modality

• Location– Lateral inhibition– Receptive field

• Intensity• Duration• Tonic receptors• Phasic receptors• Adaptation

Sensory Modality

Figure 10-3: Two-point discrimination

Sensory Modality

The Somatosensory System

• Types of receptors - Mechanoreceptors: -- Proprioreceptors in tendons, ligaments and muscles body position -- Touch receptors in the skin: free nerve endings, Merkel’s

disks and Meissner’s corpuscles (superficial touch), hair follicles, Pacinian corpuscles and Ruffini’s ending

- Thermoreceptors: Warm receptors (30-45oC) and cold

receptors (20-35oC)

- Nociceptors: respond to noxious stimuli

Touch (pressure)

Skin touch receptors

Sensory pathways

• The sensory pathways convey the type and location of the sensory stimulus

• The type: because of the type of receptor activated

• The location: because the brain has a map of the location of each receptor

Temperature

• Free nerve endings• Cold receptors• Warm receptors• Pain receptors• Sensory coding:– Intensity– Duration

Temperature

Pain perception

• Fast pain: sharp and well localized, transmitted by myelinated axons

• Slow pain: dull aching sensation, not well localized, transmitted by unmyelinated axons

• Visceral pain: not as well localized as pain originating from the skin pain impulses travel on secondary axons dedicated to the somatic afferents referred pain

Pain and Itching

• Nociceptors• Reflexive path• Itch• Fast pain• Slow pain

Pain and Itching

All the Preceding Modalities Culminate in the Propagation of Action Potentials

Sensory transduction

• Receptors transform an external signal into a membrane potential

• Two types of receptor cells: - a nerve cell - a specialized epithelial cell

Two types of sensory receptors

Receptor adaptation

• Tonic receptors -- slow acting, -- no adaptation:

continue to for impulses as long as the stimulus is there

(e.g., proprioreceptors)

• Phasic receptors -- quick acting, adapt: stop firing

when stimuli are constant (e.g., smell)

Sensory coding• A receptor must convey the type of information it is sending

the kind of receptor activated determined the signal recognition by the brain

• It must convey the intensity of the stimulus the stronger the signals, the more frequent will be the APs

• It must send information about the location and receptive field, characteristic of the receptor

Figure 10.16a

Referred pain

Figure 10.16b

• What is Phantom pain?