NEUROPHYSIOLOGY of Pain

What is pain? According to the International Association for the Society of Pain, Pain is “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”. (Loeser, 2011)

Aetiology: What causes pain? “Pain can be due to a wide variety of diseases, disorders and conditions that range from a mild injury to a debilitating disease” (Williams, 2011)

Acute Pain “The terms acute and chronic refer exclusively to the time course of the pain, irrespective of aetiology” Acute Pain: Usually lasts less than 3 months Sudden onset Usually know the cause of the pain Usually well defined Predicable ending (healing) Can lead to chronic pain if left untreated Examples: cut to the finger, broken bone

Chronic Pain Chronic Pain: Persistent or recurring pain Continues for more than 3 months May last for months or even years Can be difficult to diagnose and treat Primary goal is not total pain relief but reducing pain relief Examples include: arthritis and back pain

Categories of pain Another way to categorise pain is on the basis of origin: Nociceptive Neuropathic Psychogenic

Nociceptive Pain External / Somatic Nociceptive pain is directly related to tissue damage and can be either external (somatic) or internal (visceral) External / Somatic Most common type of pain Can be superficial -in the skin but may extend to the underlying tissues. Usually described as: sharp, shooting, throbbing, burning, stinging well defined area Usually lasts from a few seconds to a few days Examples include: paper cut, sprained ankle

Nociceptive Pain Internal / Visceral (Deep) Less common and usually more severe Originates in the walls of visceral organs Poorly defined area Described as: deep, aching, pressing or aching Usually lasts a few days to weeks Virtually a symptom of all diseases at some point during disease progression. Often associated with feeling sick Examples include: Major surgery, labour pain, irritable bowel.

Neuropathic Pain Injury or disease of the central nervous system rather than the peripheral tissue. May be due to nerve compression, inflammation or trauma Usually lasts between a few months to many years. Difficult to treat due to the lack of knowledge of the underlying cause. Often associated with paraesthesia, hyperalgesia and allodynia Burning, shooting or pins and needles (not sharp like nociceptive).

Psychogenic Pain Psychological, psychiatric or psychosocial are the primary causes Severe and persistent pain Appears to have no underlying pathology. Less common now due to medical technology Pain experienced (Headaches, abdominal pain, back pain) is indistinguishable from that experienced by people with identifiable injuries or diseases. This kind of pain can be very frustrating to sufferers and can interfere with their ability to function normally.

Clinical Manifestations “No two people are likely to experience the same level of pain for a given painful stimulus” Pain Tolerance: The maximum level of pain that a person is able to tolerate without seeking avoidance of the pain or relief What affects Pain Tolerance? Fatigue, anger, boredom, apprehension, sleep deprivation. Alcohol consumption, medication, hypnosis, warmth, distracting activities and strong beliefs or faiths.

Location It is important record a patients pain location to be able to monitor any changes. Pain can feel like it is coming from one part of the body but in fact it is another, this type of pain is called referred pain.

PATHOPHYSIOLOGY Pain is not a disorder or disease. Pain incorporates A consequential reaction by the body to noxious stimuli. Injury Disease Pain incorporates Cognition Emotion Behaviour Simple pathway to the brain; Transduction Transmission Perception Modulation

PATHOPHYSIOLOGY Transduction Process by which afferent nerve endings participate in translating noxious mechanical, chemical or thermal impulses into nociceptive impulses. Strong physical stimuli and disease processes cause chemical release. Once activated the chemicals bind to specific receptors. chemicals such as bradykinin, cholecystokinin and prostaglandins, activate or sensitize nearby nociceptors Lead to the generation of Action Potentials (AP)

transduction

PATHOPHYSIOLOGY Transmission 1st Order Sensory Neurons Located in the dorsal root ganglia in the posterior of the spinal cord. AP’s are conducted to the CNS primarily via two types of primary afferent neurons A delta Fibres "Epricritic Pain" C Fibres "Protopathic Pain" 2nd Order Sensory Neurons The impulse crosses the spinal cord and ascends to the thalamus and branches to the brainstem nuclei via central transmission. Messages cross the cord and ascend to the thalamus via the Spinothalamic pathway, heading to the somatosensory cortex, the insula, frontal lobes and limbic system.

A-delta and c fibres A delta Fibres "Epricritic Pain" Nerve fibre Aδ C Appearance Type of Pain Epicritic Protopathic Information carried Sharp pain (‘fast pain’) Temperature Dull pain (‘slow pain’) Itch Diameter (micrometres) 1-5 0.2-1.5 Speed of signal conduction 5-35 m/sec 0.5-2.0 m/sec A delta Fibres "Epricritic Pain" Mechanical message Sharp, Fast pain Thin Myelinated fibres increase speed of processing C Fibres "Protopathic Pain" Mechanical and Thermal Stimuli Slow, dull, long lasting pain Unmyelinated fibres, slower response

Peripheral transmission An electron micrograph showing large myelinated Aβ small lightly myelinated Aδ fibres unmyelinated fibers C Fibres.

Synaptic transmission Action potential synapse at the dorsal horn of the spinal cord Neuroactive excitatory and inhibitory neurotransmitters are released Lead to generation of action potentials and central transmission of pain signals to higher centres.

PATHOPHYSIOLOGY Perception When noxious stimuli is recognised. Multiple areas of the brain 3rd Order Sensory Neurons To the higher brain centres of m Limbic system Frontal cortex, primary sensory cortex of the post central gyrus of parietal lobe Sensory-Discriminative Response result of activity in the somatosensory and the insular cortex allows the person to identify the type, intensity and bodily location of the noxious event. Affective-Emotional Response Mediated by the limbic system. Defines the response and associated behaviour.

PATHOPHYSIOLOGY Modulation Dampening or amplifying pain-related neural signals. Descending input from the brainstem influences central nociceptive transmission in the spinal cord. Descending inhibition of nociception through the release of neurotransmitters such as serotonin, norepinephrine and endogenous opioids. Gate Control Theory (Melzack and Wall, 1965) The body can reduce or increase the degree of perceived pain through modulation of incoming impulses at a gate located in the dorsal horn of the spinal cord. The integration determines whether the gate will be opened or closed, either increasing or decreasing the intensity of the ascending pain signal. Psychological variables in the perception of pain, including motivation to escape pain, and the role of thoughts, emotions, and stress reactions in increasing or decreasing painful sensations.