Understanding Sensory Receptors and Pain Pathways

Tonic receptor

Slow Adapting, always active, slows and maintains firing as long as the stimulus is present.

Phasic receptor

Fast adapting, normally inactive, cease firing if the strength of the stimulus remains constant.

Thermoreceptors

Respond to temperature.

Photoreceptors

Respond to light.

Chemoreceptors

Respond to chemicals; odor, taste; can be exteroceptors or interoceptors.

Nociceptors

Respond to pain; trauma, ischemia; not found in the brain.

Mechanoreceptors

Respond to distortion of a plasma membrane.

Baroreceptors

Respond to pressure changes.

Tactile receptors

Respond to touch, vibration, pressure.

Proprioceptors

Respond to stretch of muscles and joints.

Proprioception

Interpreted in cerebellum/brainstem; body's ability to sense position w/o visual.

Acuity

Precision with which a stimulus is perceived.

Two-point discrimination

Two stimuli activate separate pathways to the brain and are perceived separately; determines acuity.

Nociceptor modality

Detect painful stimuli including pressure, temperature, and chemicals.

Purpose of a nociceptor

Protect body by detecting harmful pain and sending it to brain.

Pain pathway to the brain

Nociceptors send signals to the spinal cord → The spinal cord relays these signals to the thalamus in the brain → The thalamus sends the signals to the brain's sensory cortex, where pain is perceived.

Neurotransmitters for pain pathways

Substance P, glutamate; can affect pain perceived.

Fast pain

Ad (A-delta) fibers, Small myelinated fibers, Sharp, pricking sensation, Easily localized.

Slow pain

C fibers, Nonmyelinated fibers, Dull aching, Poorly localized.

High pain tolerance

Neurotransmitters affect pain levels— (glutamate or substance P) amounts released can determine the amount of pain perceived which may be out of proportion with the painful stimuli or tissue damage.

Neuromodulators

Affect pain levels— endorphins and enkephalins (types of opioids) inhibit activity along pain pathways by preventing the release of substance P; pain perception decreases even though the stimulus is still there.

Inhibitory interneurons

Suppress pain pathway in absence of pain.

Alternate sensation

Can happen from inhibitory interneurons weakening pathway.

Referred pain

Occurs when visceral and somatic sensory pain inputs converge on a single ascending tract.