Nociceptive Pathway By Sitthichai Wanachantararak

Pain A A There are several aspects associated with pain: A A Sensory-discriminative aspects: location, intensity, duration of the pain A A Motivational-affective dimension: unpleasant feeling associated with pain. A A Pain has an urgency. A A Pain can be modulated by behavioral experiences. A A There are neural mechanisms that can modulate pain transmission and the emotional reaction to pain.

“The subject’s conscious perception of modulated nociceptive impulses that generate an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (IASP definition, 1994) “The subject’s conscious perception of modulated nociceptive impulses that generate an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (IASP definition, 1994) “The subject’s conscious perception of modulated nociceptive impulses that generate an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (IASP definition, 1994) “The subject’s conscious perception of modulated nociceptive impulses that generate an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (IASP definition, 1994) Pain

4 processes of pain sensation 1.Transduction: receptors 2.Transmission: –Primary afferent neuron (first order) –Secondary order neuron –Interaction of neuron netween the thalamus, cortex and limbic system 3.Modulation: 4.Perception:

Sensory Receptors Exteroceptor:Exteroceptor: surface of skin and mucosa – –Merkel’s corpuscle, Meissner’s corpuscle, Kruse’s corpuscle or end bulbs, Free nerve ending Proprioceptors:Proprioceptors: skeletal muscle for control movement – –Muscle spindle, golgi tendon organ, pacinian corpuscle, periodontal mechanoreceptors, free nerve ending Interoceptors:Interoceptors: viscera

Structure of peripheral nociceptors and primary afferents A A What is a nociceptor? A A The peripheral sense organs or receptors, that respond to noxious stimuli are called nociceptors. A A Definition of a noxious stimulus? A A A stimulus (e.g., chemical, thermal, mechanical) that can potentially produce tissue damage. A A What do nociceptors look like? A A Free-nerve endings A A Nociceptors are widely distributed.

Pain vs Nociception A A Pain is the perception of the aversive or unpleasant sensation that is thought to arise in a specific region of the body. A A Nociception refers to the reception of action potentials in the CNS evoked by the activation of specialized sensory receptors (called nociceptors) that provide information about tissue damage.

Pain vs Nociception A A Not all noxious stimuli that activate these nociceptors and produce action potentials that travel towards the CNS, necessarily produce the experience of pain. A A The nociceptive impulses can be gated out and prevented from accessing higher centers of the brain.

Functions of nociceptors A A Provide information about the location, intensity and duration of a noxious stimulus to the body A A Nociceptors are connected to primary afferent nerve fibers. A A Nociceptors fire action potentials because they are responding to changes in their chemical environment.

First Order Neuron Attached to receptorAttached to receptor diameter of axon & conduction velocitydiameter of axon & conduction velocity –A fiber: –alpha (α): –alpha (α):  m, m/s –beta (β): –beta (β): 6-13  m, m/s –gamma (  ) : –gamma (  ) : 3-8  m, m/s –delta(  ): –delta(  ): 1-5  m, 5-15 m/s –C fiber: –C fiber:  m, m/s

Generation of action potential

Impulse Propagation

The Erlanger/Gasser classification of peripheral nerve fibres (1939)

Characteristics of pulpal sensory fibres (Trowbridge and Kim, 1991)

Pain Superficial Pain Superficial Pain – Sharp, pricking sensation, – brief duration(<min.) – 1 st pain or initial pain Deep pain Deep pain – Dull, aching, burning, diffuse, – long duration (min., hrs., days) –second pain

Nociceptive first order neuron Mechanothermal afferentsMechanothermal afferents – –A delta fibers c.v m/s response to heat and mechanical stimuli Polymodal afferentsPolymodal afferents – –C fiber c.v. 0.5 m/s response to mechanical, heat and chemical stimuli High-threshold mechanoreceptive afferentsHigh-threshold mechanoreceptive afferents – –mostly A delta response to mechanical, noxious thermal and chemical stimuli

Synapse

Synapses at Cell Body

Second Order Neuron Low threshold mechanoreceptive neuron (LTM)Low threshold mechanoreceptive neuron (LTM) – –tactile, pressure and proprioception Nociceptive specific neuron (NS)Nociceptive specific neuron (NS) – –Pain and noxious stimuli Wide dynamic range neuron (WDR)Wide dynamic range neuron (WDR) – –non noxious to noxious stimuli

Substantia gelatinosa

Brain Stem and Brain Brain stem: medulla oblongata, pons, mesencephalon (midbrain) diencephalonBrain stem: medulla oblongata, pons, mesencephalon (midbrain) diencephalon –reticular formation of medulla oblongata can modulate sensory impulses –Thalamus relay station for almost all signal –hypothalamus --> produce hormone and interact with sympathetic --> nociceptive impulse

Cerebrum Cerebral cortexCerebral cortex –thinking process, memory, skill –area 3, 1, 2 Basal ganglionBasal ganglion –caudate nu, putamen, globus pellidus –control gross movement of skeletal Limbic systemLimbic system –emotion and behavior

Trigeminal Sensory Pathway NociceptorNociceptor –Trigeminal nu. TactileTactile –Motor nu. of V ProprioceptiveProprioceptive –Mesencephalic nu. Primary Neurons

Second Order Neuron of V Low threshold mechanoreceptive neuron (LTM)Low threshold mechanoreceptive neuron (LTM) – –tactile, pressure and proprioception Nociceptive specific neuron (NS)Nociceptive specific neuron (NS) – –Pain and noxious stimuli Wide dynamic range neuron (WDR)Wide dynamic range neuron (WDR) – –non noxious to noxious stimuli

Proprioceptive : Golgi tendon organ Muscle spidle Proprioceptive receptor in periodontium Mesencephalic nucleus of V Pain : Trigeminal ganglion TMJ Sensory Pathway

References Textbook of Physiology Textbook of Pain by Wall, P and Melzack, R 4 th edition Churchill and Livingstone Disorders and Orofacial Pain. by Klineberg, I Craniomandibular Wright, Oxford Basic Neuroscience for Health Professions. by Littell, EH SLACK corporation, New Jersey

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