The Peripheral Nervous System, Sensory Receptors, and Pain Mechanisms

Organization of the Peripheral Nervous System

• PNS: Two Functional Directions:

  • The peripheral nervous system (PNS) links the periphery and CNS.

  • Afferent division: detects internal and external changes and conveys information to CNS.

  • CNS interprets incoming information and directs a response.

  • Efferent division: carries CNS commands to effectors.

• Efferent Division: Somatic vs Autonomic:

  • Somatic nervous system: conscious control of skeletal muscle.

  • Autonomic nervous system: subconscious regulation of smooth muscle and other organs.

  • Both divisions contribute to survival and homeostasis.

• Afferent Neurons: Function and Structure:

  • Afferent (sensory/receptor) neurons conduct action potentials from receptors toward CNS.

  • A long peripheral process, short axon and rounded cell body.

  • Many spinal sensory cell bodies cluster in dorsal root ganglia.

• Fight-or-Flight | Why Sensation Matters:

  • A perceived threat begins with sensory information about the environment.

  • Afferent input supports arousal, perception and decisions about efferent output.

  • During acute stress, resources shift toward immediate survival functions.

• Bear Encounter | Sensory Input and Physiological Adjustments:

  • Rustling leaves are detected by the auditory system.

  • Visual signals identify the bear and help judge its distance.

  • The CNS integrates sensory information to assess the threat.

  • Head turning helps orient toward the sound.

  • Accommodation allows a clear image of the bear.

  • Pupil dilation allows more light to enter the eyes.

  • Increased arousal modifies efferent output, preparing the body for fight or flight.

Clinical Connections: Diabetic Peripheral Neuropathy

• Deborah:

  • $60\text{-year-old}$ woman with long-standing type $2$ diabetes and chronically elevated blood glucose.

  • Progressive numbness in fingers and toes, impaired fine coordination, weakness and unsteadiness.

  • She also develops intermittent sharp pain in toes at night.

• Mechanism of Neuropathy:

  • Chronically elevated blood glucose is associated with microvascular injury.

  • Compromised blood supply can injure peripheral nerves or alter their function.

  • Sensory loss and abnormal pain signalling may occur together.

  • Diabetic peripheral neuropathy affects sensory receptors and the transmission of action potentials to the CNS.

  • Loss of touch sensation: Some damaged mechanoreceptor afferent neurons.

  • Loss of temperature sensation: Damage to thermoreceptor afferent neurons.

Classification of Sensory Receptors and Afferent Inputs

• Somatosensation: Receptors and CNS:

  • The somatosensory system includes peripheral receptors and central processing areas.

  • Sensory modality: The specific type of sensation detected, such as touch, temperature, pain, or light.

  • It processes modalities such as touch, temperature and pain.

  • Primary somatosensory cortex is in the parietal lobe.

• Four Broad Receptor Categories:

  • Thermoreceptors: Detect changes in temperature.

  • Mechanoreceptors: Detect mechanical deformation, such as touch, pressure, and stretch.

  • Photoreceptors: Detect light.

  • Chemoreceptors: Detect chemicals.

• External and Body Sensation:

  • Somatic sensation includes skin sensation and proprioception.

  • Proprioceptive input comes from muscles, joints, skin and inner ear.

  • Special senses include vision, hearing, taste, smell and equilibrium.

• Afferent Information to the CNS:

  • Visceral afferents:

    • Originate in internal organs.

    • Monitor blood pressure, carbon dioxide, and other internal conditions.

    • Most information remains subconscious.

    • Help determine efferent responses that maintain homeostasis.

    • Visceral pain can reach conscious awareness.

  • Sensory afferents:

    • Originate at the body surface, muscles, and joints.

    • Usually reach conscious awareness.

    • Include:

      • Somatic sensation: skin sensations and proprioception.

      • Special senses: vision, hearing, taste, smell, and equilibrium.

• Compound Sensations:

  • There is no such thing as a "wetness receptor." The perception of wetness comes from touch, pressure, and thermal receptor input.

  • Some sensations are compound sensations and perception arises from the central integration of several simultaneously activated primary sensory inputs.

• Uses of Information Detected by Receptors:

  • Sensory information reaches the CNS through afferent neurons and is used for:

    • Control of efferent output: Regulates motor behaviour and maintains homeostasis.

    • Consciousness: Supports cortical arousal through the reticular activating system.

    • Perception: Creates our perception of the world.

    • Memory: Stores selected sensory information for future reference.

    • Emotions: Evokes feelings such as happiness, fear, and sadness.

Perception, Reality, and Cortical Processing

• Perception: Conscious Interpretation:

  • Perception: Conscious awareness of our surroundings.

  • Perception is the brain’s conscious interpretation of the external world.

  • Sensory receptors detect stimuli and convert them into patterns of nerve impulses.

  • The brain processes these impulses to create our perceptual experience.

  • Processing emphasizes some features and suppresses others.

• Humans Detect Only Limited Forms of Energy:

  • Humans have receptors for only some forms of energy.

  • We can perceive: Sound, Colour, Shape, Texture, Smell, Taste, Temperature.

• Sensory Systems Create Species-Specific Realities:

  • Receptors respond only to a specific modality and within a limited detection range.

  • A stimulus may be physically present but remain undetected if it:

  • Falls outside the receptor’s sensitivity range, or

  • Cannot be transduced by the organism’s receptors.

  • Different species experience different sensory worlds because their receptors and nervous systems detect and process different information.

• Sensory Pathways Modify Information:

  • Sensory pathways do not function like high fidelity recording systems.

  • During processing before information reaches the cerebral cortex:

    • Some features of a stimulus are emphasized.

    • Other features are suppressed.

    • Some information may be ignored.

• The Cerebral Cortex Interprets Sensory Input:

  • The cerebral cortex further processes sensory information by:

  • Comparing it with other incoming information.

  • Comparing it with memories of past experiences.

  • Identifying the most significant features.

  • Suppressing irrelevant information.

• Perception Does Not Replicate Reality:

  • Our perception does not perfectly reproduce the external world because:

  1. We lack receptors for some forms of energy.

  2. Our receptors have limited response ranges.

  3. Sensory information is modified before reaching the cortex.

  4. The cortex interprets information using context and memories.

• Top-Down Attention:

  • Top-down attention increases neural gain.

  • The neural representation of the selected voice is strengthened, producing a stronger response synchronized to speech (speech envelope/syllables) over competing voices.

• Optical Illusions - How the Brain Creates Perception:

  • Sensation is the detection of a stimulus by sensory receptors, whereas perception is the brain's conscious interpretation of sensory information.

  • The brain does not merely record the external world; it actively constructs our experience of it.

  • Examples: (a) The imaginary white square, (b) The faces or wineglass.

Sensory Transduction, Potentials, and Action Potential Generation

• Receptor Physiology: Sensory Receptors and Stimuli:

  • Afferent neurons: Have receptors at their peripheral endings that respond to stimuli in external and internal environments.

  • Stimulus: A detectable change that meets a minimum threshold.

  • Stimulus energy forms: Heat, light, sound, pressure, and chemical changes.

  • Receptor structure: Varies in complexity; afferent axons may be myelinated or unmyelinated.

  • Somatosensory receptors: Include free nerve endings with exposed receptors.

    • Somatosensory receptors are the sensory nerve endings responsible for monitoring body feelings and physical stimuli (like touch, pressure, temperature, and pain)

    • Unlike specialized receptor cells in organs like the eye or ear, many somatosensory receptors feature free nerve endings

    • This means their terminal tips are bare and directly exposed to the surrounding skin or tissues, allowing them to instantly detect changes in the external or internal environment

  • Special senses receptors (ear): Convert mechanical stimulation into neural signals by synapsing onto sensory neurons.

• Transduction and Sensory Information:

  • Sensory transmission: Afferent neurons transmit information to the CNS through action potential propagation.

  • Transduction: Conversion of stimulus energy into electrical signals through changes in ion permeability at controlled ion channels.

  • Sensory coding: Although stimuli are converted into action potentials, the CNS distinguishes incoming sensory information using four basic properties:

    • Modality

    • Intensity

    • Location

    • Duration

• Adequate Stimuli and Threshold:

  • Adequate stimulus: The type of energy to which a receptor is most sensitive.

  • Eye receptors $\rightarrow$ Light

  • Ear receptors $\rightarrow$ Sound waves

  • Warmth receptors in the skin $\rightarrow$ Heat

  • Receptor specificity: Some receptors respond weakly to stimuli other than their adequate stimulus (e.g., seeing stars: mechanical pressure on the eye can stimulate photoreceptors, producing a visual sensation).

  • Sensory perception: Depends on the modality of the receptor stimulated, rather than the type of stimulus.

  • Threshold: The minimum stimulus required to activate a receptor.

• Types of Receptors According to Their Adequate Stimulus:

  • Photoreceptors: Respond to visible wavelengths of light.

  • Mechanoreceptors: Respond to mechanical energy, including muscle stretch, sound waves (hair cells in the ear), blood pressure (baroreceptors), stretch (osmoreceptors), vibration and acceleration.

  • Thermoreceptors: Respond to varying amounts of heat.

  • Chemoreceptors: Respond to specific chemicals, including smell and taste, chemical content of the digestive tract, oxygen and carbon dioxide in the blood.

• How Stimuli Change Receptor Membrane Permeability:

  • Stimulus $\rightarrow$ changes receptor membrane permeability by opening ion channels.

  • $Na^+$ influx: Usually causes depolarization of the receptor membrane.

  • Exception: Photoreceptors hyperpolarize upon stimulation.

  • Two types of receptors:

  • a) Specialized ending of an afferent neuron.

  • b) Separate receptor cell associated with an afferent neuron.

• Generator and Receptor Potentials:

  • Stimulation changes the membrane potential of a sensory receptor.

    • Generator potential: Occurs at the specialized ending of an afferent neuron.

      • In this setup, the receptor is the actual ending of the sensory (afferent) neuron itself (e.g., touch receptors in your skin or pain receptors)

      • The stimulus directly depolarizes this nerve ending, "generating" an action potential that travels along the same cell toward the central nervous system

    • Receptor potential: Occurs in a separate sensory receptor cell.

      • In this setup, the receptor is a distinct, specialized cell that is separate from the afferent neuron (e.g., photoreceptors in the eye or hair cells in the inner ear)

      • The stimulus produces a voltage change (receptor potential) in this specialized cell, which then releases neurotransmitters to stimulate an adjacent afferent neuron

    • Both are graded potentials: Their amplitude (size) and duration vary with the stimulus.    

    • Stronger stimulus: Produces a greater change in ion permeability and a larger potential.

    • No refractory period: Allows successive potentials to add together (summation) when stimuli occur rapidly.

• Mechanisms of Graded Potentials:

  • a. Specialized afferent ending (Generator potential): e.g., touch receptors in the skin.

    • The generator potential produces local current flow to the adjacent membrane.

    • This opens voltage-gated Na+ channels in the adjacent region.

    • If the adjacent membrane reaches threshold, an action potential is initiated and propagates along the afferent fibre to the CNS.

  • b. Separate receptor cell (Receptor potential): e.g., taste receptor cells in taste buds.

    • The receptor potential triggers the release of a chemical messenger.

    • The messenger diffuses across a small space to the afferent neuron, similar to a synapse.

    • Binding to specific receptors opens chemically gated $Na^+$ channels.

• Conversion to Action Potentials:

  • Receptor and generator potentials must trigger action potentials for long-distance transmission.

  • Action potentials do not occur at the receptor region because it has a very high threshold.

  • A sufficiently strong stimulus produces a graded potential that brings a nearby region of the afferent neuron to threshold.

  • Stronger stimuli usually affect a larger area, activating more receptors (e.g., a forceful touch activates more pressure receptors than a light touch applied to the same area).

  • Action potentials then propagate along the afferent fibre to the CNS.

  • The CNS distinguishes stimulus intensity via $2$ mechanisms:

  1. Frequency of action potentials generated in afferent neurons.

  2. Number of receptors activated within the stimulated area.

• Where Action Potentials Begin:

  • In afferent neurons, action potentials begin at the peripheral end of the nerve fibre, next to the receptor. This initiation site is located a long distance from the cell body.

  • In efferent neurons and interneurons, action potentials begin at the axon hillock, near the cell body.

Receptor Adaptation and Neural Coding

• Adaptation to Sustained Stimulation:

  • A stimulus of the same intensity does not always produce a receptor potential of the same magnitude.

    • Even if a physical stimulus (like the pressure of your clothes on your skin or the temperature of a room) stays at the exact same physical intensity, the receptor's electrical output can change over time

  • Some receptors gradually decrease their depolarization even when the stimulus remains constant. This decrease in response is called adaptation.

  • As the receptor potential decreases, the frequency of action potentials in the afferent neuron also decreases.

  • The receptor therefore responds less strongly to a sustained stimulus.

• Remember the difference:

  • Sensory adaptation: Sensory receptors respond less to a sustained stimulus.

  • Habituation: The CNS responds less to a repeated, familiar stimulus.

• Tonic Receptors: Slowly Adapting:

  • Tonic receptors adapt slowly or do not adapt to sustained stimulation.

  • When a stimulus begins, the receptor potential increases.

  • As the stimulus continues, the receptor potential remains elevated or decreases slowly.

  • As a result, the afferent neuron continues generating action potentials, sending information to the CNS.

  • This allows the CNS to continuously monitor a stimulus.

  • Examples:

    • Muscle stretch receptors monitor muscle length.

    • Joint proprioceptors monitor the degree of joint flexion.

• Phasic Receptors: Rapidly Adapting:

  • Phasic receptors adapt rapidly to sustained stimulation.

  • When stimulation begins, the receptor potential increases briefly, then rapidly returns toward baseline despite continued stimulation.

  • When the stimulus is removed, a brief depolarization called the off response typically occurs.

  • These receptors signal changes in stimulus intensity rather than continuously reporting an unchanged stimulus.

  • Examples include tactile receptors in the skin that detect changes in pressure (e.g., explains why we stop noticing our watch, rings, or clothing but become aware when we remove them).

Sensory Pathways, Spatial Acuity, and Cortical Processing

• Somatosensory Pathways: Labelled Lines:

  1. A first-order sensory neuron detects a stimulus through its peripheral receptor.

  2. It synapses with a second-order sensory neuron in the spinal cord or medulla, depending on the pathway.

  3. The second-order neuron then synapses with a third-order sensory neuron in the thalamus.

  4. Sensory information undergoes further processing at each stage.

  5. Each sensory modality (e.g., touch, pain) travels along a specific neural pathway called a labelled line.

  6. These pathways deliver sensory information to specific regions of the cerebral cortex, allowing the brain to identify the type of sensation.

• Phantom Pain: The Labelled-Line Principle:

  • The brain interprets sensory information according to the pathway activated, even when the body part is missing.

  • Phantom pain is pain perceived in a body part that has been amputated.

  • Sensory pathways can produce sensations even when the original receptors are no longer present.

  • Irritation of severed sensory nerve endings in the residual limb may generate action potentials.

  • When these signals reach the somatosensory cortex, the brain may interpret them as pain originating from the missing limb.

  • Changes in brain organization (cortical remapping) may also contribute to phantom pain.

• Receptive Fields and Sensory Acuity:

  • A receptive field is the area of skin from which a sensory neuron receives information.

  • Receptive field size depends on receptor density:

  1. High receptor density: Smaller receptive fields (small area monitored by one neuron), e.g., Fingertips with high tactile acuity.

  2. Low receptor density: Larger receptive fields (large area monitored by one neuron), e.g., Elbow with lower tactile acuity.

  • Sensory acuity is the ability to distinguish fine details and precisely locate a stimulus.

  • Smaller receptive fields $\rightarrow$ greater sensory acuity.

• Receptive Fields and the Sensory Homunculus:

  • Sensory acuity also depends on how much of the somatosensory cortex processes information from a body region.

  • Regions with high receptor density and small receptive fields receive greater cortical representation.

  • The sensory homunculus illustrates this unequal representation of body regions.

  • Fingertips occupy a disproportionately large area of the somatosensory cortex because of their high tactile acuity.

• Lateral Inhibition: Sharpening Sensory Localization:

  • When a stimulus presses on the skin, receptors directly beneath it are stimulated most strongly.

  • Receptors in surrounding areas are also activated, but less strongly.

  • Without further processing, these surrounding signals would make the stimulus difficult to locate precisely.

  • Lateral inhibition occurs in the CNS to sharpen sensory localization.

  • Strongly activated pathways inhibit neighbouring, less active pathways through inhibitory interneurons.

  • This reduces weaker surrounding signals and increases contrast, allowing more precise localization.

Tactile Mechanoreceptors of the Somatosensory System

• Mechanoreceptors: Receptors for Touch:

  • Mechanoreceptors respond to mechanical stimuli such as pressure, stretch, vibration, acceleration, and sound.

  • Tactile mechanoreceptors detect physical contact, including pressure, vibration, texture, stretch, and gentle touch.

  • They are found in the skin, deep tissues, muscles, joints, and organs.

• Five Important Types Associated with Somatic Touch:

  1. Pacinian corpuscles:

  • Rapidly adapting tactile mechanoreceptors.

  • Found in the skin, including deeper layers.

  • Detect deep pressure, vibration, and changes in contact.

  • Most sensitive to vibration around $250\,\text{Hz}$.

  • Have an onion-like structure with concentric connective tissue layers.

  • Rapidly adapt as these layers redistribute sustained pressure.
    2. Meissner’s corpuscles:

  • Rapidly adapt to sustained stimulation (phasic receptor).

  • Found in areas sensitive to light touch, especially fingertips and lips.

  • Detect light touch and low-frequency vibration (below $50\,\text{Hz}$).

  • Have small receptive fields (approximately $3\text{--}5\,\text{mm}$).
    3. Merkel’s discs:

  • Slowly adapting tactile mechanoreceptors (tonic).

  • Located in superficial skin layers, particularly the fingertips.

  • Detect fine touch, pressure, and small changes in skin deformation.

  • Respond to low-frequency vibration (approximately $5\text{--}15\,\text{Hz}$).

  • Have small receptive fields (approximately $2\text{--}3\,\text{mm}$).

  • Important for fine tactile discrimination.
    4. Ruffini corpuscles:

  • Slowly adapting receptors that provide ongoing information about tissue stretch.

  • Found in deeper skin layers and joints.

  • Respond to skin stretch, sustained pressure, and joint deformation.
    5. Free nerve endings:

  • The most abundant sensory endings in the skin.

  • Found in the skin, around hair roots, in the cornea, and in many other tissues.

  • May be associated with myelinated or unmyelinated sensory fibres.

  • Different free nerve endings detect: touch and pressure, temperature, pain (nociception), itching and tickling.

  • Some sensations, particularly itch, are transmitted through slow, unmyelinated fibres.

  • Adaptation varies.

• Comparison of Tactile Mechanoreceptors: Stimuli and Adaptation:

  • Pacinian corpuscle: Deep pressure changes, high-frequency vibration; Rapid adaptation.

  • Meissner’s corpuscle: Light touch, low-frequency vibration; Rapid adaptation.

  • Merkel’s disc: Fine touch, sustained pressure; Slow adaptation.

  • Ruffini corpuscle: Skin stretch, joint deformation; Slow adaptation.

  • Free nerve endings: Touch, temperature, pain, itch; Adaptation varies.

Nociception and Pain Mechanisms

• Pain: A Protective Sensory Mechanism:

  • Pain is an unpleasant sensory and emotional experience that may be associated with actual or potential tissue damage.

  • Nociception is the neural process of detecting and transmitting potentially harmful stimuli.

  • Nociceptors are sensory receptors that respond to potentially damaging stimuli.

  • Nociception is an important part of the body's defence system because it provides warning of possible injury.

  • Stimulation of nociceptors can trigger protective behaviours (e.g., withdrawal reflex) and emotional reactions (e.g., crying).

  • Pain perception is subjective and influenced by past experiences, emotions, and the current situation (e.g., fear of a dental procedure may increase the intensity of pain experienced).

• Nociceptors:

  • Nociceptors are specialized sensory free nerve endings that detect potentially damaging stimuli.

  • Found in the skin, deep tissues, joints, muscles, and bones.

  • Unlike many other sensory receptors, nociceptors generally do not adapt to sustained or repeated harmful stimulation.

• Categories of Pain Receptors (Nociceptors):

  • Mechanical nociceptors: Respond to potentially damaging mechanical forces, such as cutting, crushing, or pinching.

  • Thermal nociceptors: Respond to extreme temperatures, particularly harmful heat.

  • Chemical nociceptors: Respond to irritating chemicals, including substances released from damaged tissues.

• Thermal Pain: TRP Channels:

  • Thermal nociceptors contain temperature-sensitive ion channels called transient receptor potential (TRP) channels.

  • These channels help convert harmful temperature stimuli into electrical signals.

  • $TRPV1$ is activated by noxious heat and capsaicin, the active ingredient in hot chili peppers.

  • $TRPV2$ is associated with responses to very high temperatures.

  • Other TRP channels participate in detecting warmth and cold.

• Chemical Modulation of Pain: Prostaglandins:

  • Tissue injury causes the local release of inflammatory chemicals, including prostaglandins.

  • Prostaglandins increase nociceptor sensitivity by lowering their activation threshold.

  • As a result, nociceptors respond more strongly to stimulation, contributing to increased pain sensitivity.

  • Aspirin-like drugs (NSAIDs) reduce prostaglandin synthesis, helping relieve pain.

  • Other chemicals that influence nociceptors include histamine, potassium, serotonin, and substance P.

• Fast and Slow Afferent Pain Fibres:

  • Pain signals travel from nociceptors to the CNS through two types of afferent fibres:

  • $A\text{-delta}\ (A\delta)$ fibres: Larger, thinly myelinated fibres that conduct pain signals rapidly.

  • $C$ fibres: Smaller, unmyelinated fibres that conduct pain signals slowly (less than approximately $1\text{--}2\,\text{m/sec}$).

  • Conduction speed depends on fibre diameter and myelination.

  • Comparison of Fast Pain vs Slow Pain:

  • Sensation: Fast pain ($A\delta$) is sharp, prickling; Slow pain ($C$) is dull, aching.

  • Localization: Fast pain is easily localized; Slow pain is poorly localized.

  • Duration: Fast pain is brief; Slow pain is longer-lasting.

  • Stimuli: Fast pain is mainly mechanical or noxious heat; Slow pain is chemical, thermal, and mechanical.

  • Myelination: Fast pain is thinly myelinated; Slow pain is unmyelinated.

  • Characteristics of Pain Nerve Fibres:

    • $A\delta$: Small and myelinated; Speed $6\text{--}30\,\text{m/sec}$; Diameter $1\text{--}5\,\mu\text{m}$; Stimuli: fast pain, cold, warmth, mechanical; Description: sharp, stabbing, or acute; Receptor classification: free nerve ending.

    • $C$ fibres: Small and unmyelinated; Speed $< 1\text{--}2\,\text{m/sec}$; Diameter $0.5\text{--}2\,\mu\text{m}$; Stimuli: slow pain, heat, cold, mechanical; Description: burning, aching, throbbing; Receptor classification: free nerve ending.

• Slow Pain and Chemical Stimulation:

  • Tissue injury $\rightarrow$ Enzymes released into extracellular fluid (ECF) $\rightarrow$ Bradykinin activation $\rightarrow$ Stimulation of polymodal nociceptors $\rightarrow$ Slow pain signals transmitted via $C$ fibres $\rightarrow$ Dull, aching, prolonged pain.

• Pain Transmission in the Spinal Cord:

  1. Pain signals travel through primary afferent fibres to the dorsal horn.

  2. Action potentials trigger release of substance P and glutamate.

  3. Neurotransmitters act on dorsal horn neurons:

  • Substance P: Activates ascending pain pathways.

  • Glutamate: Pain transmission and increased sensitivity.

    • The dorsal horn transmits and modifies pain signals.

• Glutamate Mechanisms in the Dorsal Horn:

  • Glutamate released from primary afferent pain fibres acts on two receptor types:

  • AMPA receptors $\rightarrow$ Membrane depolarization $\rightarrow$ Action potentials in dorsal horn neurons $\rightarrow$ Pain transmission to higher centres.

  • NMDA receptors $\rightarrow$ $Ca^{2+}$ entry $\rightarrow$ Second-messenger activation $\rightarrow$ Increased dorsal horn excitability.

• Ascending Pain Pathways and Brain Processing:

  • Somatosensory cortex: Localizes the pain.

  • Other cortical areas: Conscious evaluation of the painful event.

  • Thalamus: Relays and processes pain information.

  • Reticular formation: Increases alertness.

  • Hypothalamus: Coordinates physiological and behavioural responses.

  • Limbic system: Emotional and unpleasant aspects of pain.

• Chronic and Neuropathic Pain:

  • Chronic pain may occur without ongoing tissue injury or a painful stimulus.

  • Unlike protective pain, abnormal chronic pain may result from damage to pain pathways in peripheral nerves or the CNS.

  • Abnormal signalling can produce persistent pain.

  • This type of pain may be classified as neuropathic pain.

  • Examples include diabetic peripheral neuropathy and pain following a stroke.

Endogenous Analgesic Systems and Pain Modulation

• The Brain’s Built-in Analgesic System:

  • Descending pathways suppress pain transmission in the spinal cord:

  • Periaqueductal grey matter (PAG) and brain-stem reticular formation

    • Periaqueductal grey matter (PAG): A region of grey matter surrounding the cerebral aqueduct in the midbrain that serves as the primary control center for descending pain modulation.

    • Reticular formation: A network of neurons in the brainstem that receives input from higher brain areas (like the cortex and limbic system) and helps activate these downward analgesic pathways.

  • $\downarrow$ Descending analgesic pathways activated

  • $\downarrow$ Endogenous opioids released: endorphins, enkephalins, dynorphins

  • $\downarrow$ Opioid receptors on afferent pain-fibre terminals activated

    • The released endorphins and enkephalins bind to specialized opioid receptors ($\mu$, $\delta$, and $\kappa$ receptors) situated on the presynaptic terminals of incoming pain fibers (primary afferent neurons coming from the body)

  • $\downarrow$ Substance P release reduced (presynaptic inhibition)

  • $\downarrow$ Reduced pain transmission to higher CNS centres

• Endogenous Opioids and Pain Modulation:

  • Endogenous opioids: Naturally occurring substances that suppress pain.

  • Three types: Endorphins, enkephalins, and dynorphins.

  • Morphine: Produces analgesia by binding to opioid receptors.

  • Exercise: Prolonged exercise may release endorphins associated with the runner’s high.

  • Stress: Certain stressful situations can temporarily suppress pain.

  • Acupuncture: May influence pain modulation.

Critical Thinking Scenarios

• Scenario 1: Intraoperative and Postoperative Signals:

  • Context: During surgery, a patient’s arterial carbon dioxide level rises. After awakening, the patient reports abdominal pain and hears the monitor alarm.

  • Classification: Carbon dioxide and abdominal pain are visceral afferent inputs, while hearing is special sensory input. Carbon dioxide is processed mainly subconsciously, whereas abdominal pain and hearing can reach conscious awareness.

• Scenario 2: Auditory Processing in Noise:

  • Context: In a noisy cafeteria, several voices activate auditory receptors and generate action potentials in sensory afferent pathways. Even so, a student can follow the words of one friend.

  • Explanation: Precortical processing emphasizes some stimulus features and suppresses others, after which the cerebral cortex compares the input with other information and