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 (30-45 o C) and cold receptors (20-35 o C) - 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?