Sensory System - General Senses

Different Senses

General - Touch, temp, pain, etc

Special - vision, hearing, smell, taste

Visceral - pH, osmolarity, chemoreceptors, etc

Proprioceptors - stretch, position, over-contraction

Types of Sensory Receptors

Specialized endings of neuron - eg. Touch

Separate cell that signals to afferent neuron - eg. Rods and cones

Receptor Field

Area of skin that a sensory receptor innervates, size will vary

Characteristics of Sensory Receptors

Modality, intensity, adaptation, localization

Modality of Sensory Receptors

Chemoreceptors, mechnoreceptors, proprioceptors, thermoreceptors - each responds to one type of stimulus only (except pain)

Intensity Of Sensory Receptors

Coded by frequency of stimulus - higher stimulus stimulates more fibres (fires more AP’s)

Adaptation of Sensory Receptors

When a neuron stops sending AP’s due to continuous stimulus - two different receptors

Eg. Adapting to temperature of shower

Fast-adapting Receptors / Phasic

Responds to change in stimulus

Eg. Touch, temp, smell

slow-adapting receptors / tonic

Continues to send AP’s in response to constant stimulus - slow to adapt or doesn’t adapt at all

Eg. Pain, vision, proprioceptors

Localization/Acuity (What it is and what it depends on)

Ability to distinguish between two stimulus points

Depends on: receptor field size, receptor field overlap, area of representation in cortex, lateral inhibition

Receptor Field Effect

If receptor field size increases the ability to localize/the acuity decreases (eg. Back)

More overlap of receptor fields increases localization/acuity (eg. Fingers)

Area of Representation in Cortex

Greater area representation - greater ability to localize

Lateral Inhibition

Receptors fields are continuous, increases ability to localize - decreases firing rate of neighbouring neurons in overlapping sensory receptors

Pain characteristics

Protective mechanism - behavioural response and emotional reactions, memory helps avoid harmful events in future

Subjective perception - influenced by past experiences, some people have higher pain thresholds

Nociceptors

Don’t adapt to sustained stimulation (pain receptor)

Cytokines in pain reception

Lowers nociceptor’s threshold - enhances receptor response to noxious stimuli (eg. Hyper-algesia - increased sensation to pain)

Nociceptors types

Mechanical - respond to cutting, crushing, pinching, etc

Thermal - respond to temperature extremes

Polymodal - respons to all kinds of damaging stimuli

Fast pain vs. Slow pain

Fast - mechanical/thermal Nociceptors, carried by myelinated A-delta fibres, occurs first, easily localized, sharp prickling sensation

Slow - polymodal Nociceptors, carried by unmyelinated C fibres, occurs second, poorly localized, dull aching burning sensation

Pain neurotransmitters

Substance P - bigger molecule = longer response, activates ascending pathways

Glutamate - smaller molecule, major excitatory neurotransmitter

Brain Analgesic System

Suppresses transmission in pain pathways, depends on presence of opiate receptors (eg. Endorphins blocks release of NT at presynaptic neuron)