Understanding Sensory Receptors and Pain Mechanisms

Sensation

The subjective responses of the brain to various stimuli.

Sensory receptors

Specialized neural structures that detect internal and external stimuli and relay to the CNS.

Exteroreceptors

Sensory receptors that detect stimuli from outside the body.

Interoreceptors

Sensory receptors that detect internal bodily stimuli.

Somatic senses

Nervous mechanisms that collect sensory info from the body.

Mechanoreceptive

Type of somatic sense detecting touch, pressure, vibration, tickle, and position.

Thermoreceptive

Type of somatic sense detecting temperature.

Nociceptive

Type of somatic sense responsible for detecting pain.

Free nerve endings

Simplest sensory receptor that detects touch and pressure.

Meissner's corpuscle

Aβ receptor found in non-hairy skin, abundant in fingertips and lips, sensitive to touch and vibration.

Expanded tip tactile receptor (Merkel's discs)

Receptor that gives a strong initial signal then a slow adapting one; allows for continuous touch detection.

Iggo dome receptor

Merkel's discs grouped together and innervated by a single Aβ fiber.

Hair end-organ

Rapidly adapting receptor that detects movement or initial contact on skin.

Ruffini's endings

Deep, slowly adapting receptor in skin and joint capsules, detects stretch and deformation.

Pacinian corpuscle

Receptor beneath the skin, sensitive to vibration from rapid compression.

Thermoreceptor

Receptor type responsible for detecting hot and cold.

Nociceptor

Pain receptor classified as a free nerve ending.

Electromagnetic receptor

Receptor for light, includes rods and cones.

Rods

Photoreceptors that function in low light.

Cones

Photoreceptors responsible for color vision.

Taste buds

Chemoreceptor for taste.

Olfactory epithelium

Chemoreceptor for smell.

Arterial oxygen receptors

Receptors in the aorta and carotid for detecting oxygen.

Medullary chemoreceptors

Brain receptors for blood oxygen and carbon dioxide.

Hypothalamic chemoreceptors

Hypothalamic receptors for detecting amino acids, fatty acids, and BP.

Exteroceptive

Sensation from the body's surface.

Proprioceptive

Sensation of physical state (position, pressure, equilibrium).

Visceral sensation

Sensation from visceral organs.

Deep sensation

Sensation from deep tissues like fascia, muscles, bones.

Stimulation of tactile receptors in skin

What causes touch sensation?

Deformation of deeper tissues

What causes pressure sensation?

Rapid, repetitive sensory signals

What causes vibration sensation?

Nerve fiber with fast transmission (30-70 m/s) for Meissner's, Pacinian, etc.

Type of nerve fiber associated with certain receptors.

Type Aβ fibers

Myelinated fiber with moderate speed (15-30 m/s) for free nerve endings.

Type Aδ fibers

Myelinated fiber with moderate speed (15-30 m/s) for free nerve endings.

Type C fibers

Unmyelinated fiber with slow transmission (≤2 cm/s), carries tickle sensation.

Primary Sensory Coding

The process of converting a physical stimulus (e.g., pressure, light) into an electrical signal (action potential) that the nervous system can interpret.

Stimulus Modality

Refers to the type of stimulus, such as pressure, temperature, pain, sound, or light, detected by specific sensory receptors.

Stimulus Intensity

Indicates how strong a stimulus is. It influences whether an action potential is generated and how frequently neurons fire.

Stimulus Location

The spatial property of a stimulus—how precisely it can be pinpointed, such as distinguishing one touch point from two nearby touches.

Lateral Inhibition

A neural mechanism that enhances contrast and sharpens sensory perception by inhibiting signals from receptors adjacent to the stimulus center.

Central Control of Afferent Information

Sensory input can be modulated before reaching the brain via presynaptic inhibition, interneuron modulation, or descending brain pathways.

Presynaptic Inhibition

A mechanism where an inhibitory neuron acts directly on the axon terminal of an afferent neuron, reducing neurotransmitter release and altering perception.

Ascending Sensory Pathway

A multi-neuron chain where afferent neurons relay information from receptors to the central nervous system, often via specific and nonspecific tracts.

Specific Sensory Pathway

Pathways that carry one type of sensory information (e.g., touch, vision) through dedicated neural routes to defined areas in the brain.

Nonspecific Sensory Pathway

Pathways that carry multiple types of sensory information and project to areas like the reticular formation, contributing to general arousal.

Association Areas

Regions of the cerebral cortex that integrate and process complex sensory information after it has reached the primary sensory cortices.

Somatic Sensation

Sensation arising from skin, muscles, joints, bones, and tendons, involving receptors for touch, pressure, temperature, and pain.

Somatic Receptors

A diverse group of receptors located in body tissues that detect mechanical, thermal, or nociceptive stimuli.

Pain

A protective sensory experience triggered by noxious stimuli; unlike touch, it can be modified by emotion, memory, or context.

Referred Pain

A type of pain felt in a region different from the actual site of the stimulus due to shared ascending sensory pathways in the spinal cord.

Pain Modulation

Pain perception is not fixed—it can be altered by previous experiences, emotional states, and even simultaneous stimuli like pressure or vibration.

Afferent Neuron

A sensory neuron that carries signals from a receptor to the CNS, forming the first link in a sensory pathway.

Sensory Integration

The brain's process of organizing and interpreting multiple sensory inputs, allowing accurate perception, awareness, and response.

Fast Pain

Sharp, pricking, acute, electric-like pain felt within 0.1 seconds after stimulation, mainly in superficial or somatic tissues.

Slow Pain

Burning, aching, throbbing, or nauseous pain that develops slowly and is often chronic, usually associated with tissue destruction.

Pain Receptors (Nociceptors)

Free nerve endings located in superficial skin layers, periosteum, arterial walls, joints, and cranial dura mater that detect harmful stimuli.

Pain Stimuli Types

Pain receptors respond to mechanical (fast and slow), thermal (fast and slow), and chemical stimuli, with chemicals like bradykinin and histamine mainly causing slow pain.

Chemical Pain Mediators

Substances such as bradykinin, serotonin, histamine, potassium ions, acids, acetylcholine, and proteolytic enzymes excite pain receptors and increase sensitivity.

Non-adapting Pain Receptors

Pain receptors continue responding without diminishing over time, ensuring ongoing awareness of tissue damage.

Hyperalgesia

Increased sensitivity of pain receptors due to substances like prostaglandins and substance P, which amplify pain signals during tissue injury.

Pain Intensity Correlation

The intensity of pain closely matches the rate of tissue damage rather than the total damage already sustained.

Ischemic Pain

Pain arising from blocked blood flow leading to tissue oxygen deprivation; associated with lactic acid accumulation and chemical irritants like bradykinin.

Pain Due to Muscle Spasm

Caused by direct activation of mechanosensitive pain receptors and ischemia from compressed blood vessels during sustained muscle contraction.

Pain and Potassium Ions

Elevated potassium levels in damaged tissue increase nerve membrane permeability, directly exciting pain receptors.

Role of Proteolytic Enzymes in Pain

These enzymes attack nerve endings in damaged tissues, increasing pain receptor activation and sensitivity.

Location of Pain Receptors

Found in skin, periosteum, arteries, joint surfaces, and cranial dura mater structures like falx and tentorium.

Pain Receptors Excited by Mechanical Stimuli

Both fast and slow pain can result from mechanical stimuli such as pressure or injury.

Pain Receptors Excited by Thermal Stimuli

Both fast and slow pain can be triggered by extreme heat or cold sensations damaging tissues.

Importance of Non-adapting Pain Receptors

Ensures continued pain sensation during ongoing injury to prevent further tissue damage and promote protective behaviors.

Visceral Pain

Usually slow, burning type of pain associated with internal organ damage or dysfunction.

Summation of Widespread Tissue Damage

Extensive injury can cause cumulative activation of pain receptors leading to slow, persistent pain sensations.

Neospinothalamic Tract

A fast pain pathway transmitting sharp mechanical and thermal pain via Aδ fibers to the thalamus for precise localization.

Paleospinothalamic Tract

A slow pain pathway transmitting chronic pain through type C fibers to brainstem reticular areas and thalamic nuclei, involved in poorly localized pain.

Glutamate (Pain Transmission)

Primary excitatory neurotransmitter released by type Aδ fibers at synapses in the dorsal horn during fast pain transmission.

Substance P

Neuropeptide neurotransmitter released by type C fibers that facilitates slow, chronic pain signaling and increases pain sensitivity.

Lamina I of Dorsal Horn

The termination site for fast Aδ fiber pain signals in the spinal cord, part of the neospinothalamic pathway.

Substantia Gelatinosa

The region of laminae II and III in the dorsal horn where slow pain (type C fiber) signals terminate before ascending.

Anterior Commissure (Spinal Cord)

The location where pain fibers cross from one side of the spinal cord to the other in both fast and slow pain pathways.

Anterolateral System

The ascending spinal cord tract that carries both fast and slow pain signals to the brain, including neospinothalamic and paleospinothalamic tracts.

Pain Localization

Primarily achieved through the neospinothalamic tract requiring stimulation of both tactile and pain receptors for precise spatial identification of pain.

Reticular Formation (Pain Pathway)

Brainstem region receiving slow pain signals via the paleospinothalamic tract, contributing to diffuse, poorly localized pain perception.

Ventrobasal Thalamic Complex

The main thalamic relay center for fast pain signals ascending via the neospinothalamic tract to the somatosensory cortex.

Multisynaptic Pain Connectivity

Characteristic of slow pain pathways (paleospinothalamic), involving multiple synapses and diffuse brain regions, resulting in difficult localization of pain.

Tectal Area and Periaqueductal Gray

Midbrain structures involved in processing slow pain signals and modulating pain perception via descending pathways.

Chronic Pain Transmission

Slow pain conducted by type C fibers releasing substance P, leading to prolonged pain sensation often associated with tissue injury or inflammation.