L17: General Sensory Receptors and Pain Initiation

What is a sensory receptor?

Specialized cell or nerve ending that detects stimulus and converts it to electrical signal

What is an exteroceptor?

Sensory receptors that detect stimuli from the external environment (touch, temperature, light, and sound)

What are proprioceptors?

Sensory receptors in muscles, tendons, and joints that give info about body position and movement

What are sense organs?

Structures containing sensory receptors and supporting tissues that help detect specific stimuli (eye and ear)

What are interoceptors?

Sensory receptors that detect changes within internal organs and tissues (like stomach stretch and change in BP)

What is sensory adaption?

Gradual decrease in sensitivity of sensory receptors after continuous, unchanging exposure to stimulus

What are rapidly adapting receptors?

Responds mainly to changes in a stimulus, including onset and removal

What are slowly adapting receptors?

Continued signaling during sustained stimulation—firing rate may gradually decrease

What does stronger stimuli usually produce?

1. Higher firing rates

2. Greater recruitment

What is stimulus transduction?

Conversion of stimulus energy into an electrical signal in a sensory receptor

What is the transduction for mechanical energy from pressure (touch)?

Channels open—depolarization

What is the transduction for light energy (vision)?

Channels close—hyperpolarization

What is the transduction for mechanical energy from sound-induced vibrations (hearing)?

Channels open—depolarization

What are thermorecptors?

Detect temperature changes

What are mechanorecptors?

Detect mechanical deformation (pressure, stretch, vibration, etc.)

What are nociceptors?

Detect damaging/potentially damaging mechanical, thermal, or chemical stimuli (ex: thorn prick, hot surface, etc.)

What are chemoreceptors?

Detect chemical stimuli (ex: smell, taste, changes in blood O2, pH, etc.)

What are photoreceptors?

Detect light (ex: rods in retina)

What signals do free nerve endings detect?

Pain, temperature, and crude touch

Where are free nerve endings located?

Epidermis and dermis

What signals do hair receptors detect?

Light touch (hair movement)

What type of receptors are hair receptors?

Rapidly adapting

What signals do merkel discs (tactile discs) detect?

Steady pressure, texture

What type of receptors are merkel discs?

Slowly adapting

What signals do Meissner’s Corpuscles (Tactile Corpuscles) detect?

Light touch, flutter, stroking

What type of receptors are Meissner’s Corpuscles?

Rapidly adapting

What signals do Pacinican Corpuscles (Lamellar Corpuscles) detect?

High-frequency vibration and rapid changes in pressure

What type of receptors are Pacinian Corpuscles?

Rapidly adapting

What signals do Ruffini Endings (Bulbous Corpuscles) detect?

Sustained skin stretch

What type of receptors are Ruffini Endings?

Slowly adapting

What is a dermatome?

Area of skin supplied by sensory fibers associated with a single spinal nerve root

Which spinal nerve root has no dermatome?

C1

Do dermatomes overlap?

Yes, adjacent overlap—boundaries vary between individuals

What is the clinical significance of dermatomes?

Reported altered sensation can help identify sensory level of spinal cord injury

What are the types of somatic pain?

Superficial and deep

Where is superficial pain?

Skin (Ex: cut, burn)

Where is deep pain?

Muscles, joints, and bones (Ex: osteoarthritis, muscle strain)

What are characteristics of deep pain?

Dull, aching, less localized

Where is visceral pain?

Internal organs (Ex: kidney stones, peptic ulcer)

What are characteristics of visceral pain?

Poorly localized, often referred pain

What are causes of visceral pain?

Obstruction, distension, inflammation, ischemia

What is neuropathic pain?

Pain caused by lesion or disease of somatosensory nervous system (peripheral or central)

What are symptoms of neuropathic pain?

Burning, tingling, shooting, electric, allodynia (pain from non-painful stimulus), and hyperalgesia (increased pain from painful stimulus)

What is peripheral pain caused by?

Occurs when peripheral nerves are damaged (ex: diabetic neuropathy)

What is central pain caused by?

Stroke, multiple sclerosis, and spinal cord injury

What is nociplastic pain?

Pain arising from altered pain processing in CNS that occurs w/o evidence of tissue/nerve injury

What are characteristics of nociplastic pain?

Widespread pain, fatigue, sleep disturbances, cognitive issues, and mood changes

What are examples of nociplastic pain?

IBS and fibromyalgia

What does injury and inflammation generate/release?

Chemical mediators from damaged tissues, platelets, and immune cells

What do chemical mediators do?

Activate nociceptors and sensitize nociceptors (lower activation threshold or increase response)

What is peripheral sensitization?

Increased responsiveness of peripheral nociceptors

What is the axon reflex?

Local trigger activates impulse along sensory fiber and branches, which sends signal backwards without involving CNS

Where do action potentials spread in axon reflex?

Neighboring branches of same axon and spinal cord

What contributes to neurogenic inflammation?

Release of substance P and CGRP

What does CGRP effect?

Dilates blood vessels to increase blood flow and redness

What does substance P effect?

Increases vascular permeability, which allows fluid to enter tissue and contributes to swelling

What are eicosanoids?

Signaling molecules derived from fatty acids

What does inflammatory stimuli activate/what does that do?

Activates phospholipase A2, which releases arachidonic acid for eicosanoid synthesis

What is arachidonic acid?

20-carbon omega-6 polyunsaturated fatty acid incorporated into membrane phospholipids

What two pathways is arachidonic acid metabolized through?

1. Prostanoids (regulate physiological processes)

2. Leukotrienes (immune system signaling cells)

What inhibits production of prostanoids?

NSAIDs (anti-inflammatory)

What inhibits production of leukotrienes?

Zileuton

What do prostanoids promote?

Inflammation, fever, and nociceptor sensitization

What do leukotrienes promote?

Immune cells, bronchoconstriction, mucus secretion. and increased vascular permeability

What are gastric functions of prostanoids?

• Increases mucus secretion

• Reduces gastric acid secretion

• Supports blood flow

What are the renal functions of prostanoids?

• Maintains renal blood flow and filtration (especially when perfusion is low)

• Regulates sodium and water handling, which supports fluid balance and BP regulation

What does Prostacyclin do?

Relaxes vascular smooth muscle and inhibits platelet aggregation (vasodilation)

What is Prostacyclin produced by?

Vascular endothelial cells

What does Thromboxane do?

Contracts vascular smooth muscle and promotes platelet aggregation (vasoconstriction)

What is Thromboxane produced by?

COX-1-dependent pathway