Physiology

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Physiology Psychology of Pain
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What is Pain??
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Introductory Ideas

• Sensation of the affected level of unpleasantness
• Perception of actual or threatened damage
• Perception based on expectations, past
  experience, anxiety, suggestions, cognitive
  factors
• Acute
• Chronic
• Pain is Subjective
• Simple Spinal Reflex Arc

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First Order Neurons

• Stimulated by sensory receptors
• End in the dorsal horn of the spinal cord
• Types
• A-alpha non-pain impulses
• NCV - 70-120m/sec
• A-beta non-pain impulses
• NCV 36-72m/sec
• A-delta pain impulses due to mechanical
  pressure
• Large diameter, myelinated, NCV 4-30m/sec
• Short duration, sharp, prickling, localized
• C pain impulses due to chemicals or mechanical
• Small diameter, unmyelinated, NCV - .5-2m/sec
• Delayed onset, diffuse, aching, throbbing

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Neurotransmitters

• Chemical substances that allow nerve impulses to
  move from one neuron to another
• Found in synapses
• Norepinephrine
• Substance P
• Acetylcholine
• Enkephalins
• Endorphins
• Serotonin
• Can be either excitatory or inhibitory

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Descending Neurons

• Transmit impulses from the brain (corticospinal
  tract in the cortex) to the spinal cord (lamina)
• Periaquaductal gray area (PAG) release
  enkephalins
• Nucleus Raphe Magnus (NRM) release serotonin
• The release of these neurotransmitters inhibit
  ascending neurons

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Assessment of pain

• Visual analogue scale
• Picture
• McGill pain questionnaire
• Part I is used to localize the pain and identify
  whether the perceived source of the pain is
  superficial (external), internal, or both.
• Part II incorporates the visual analogue scale.
• Part III is the pain rating index, a collection
  of 76 words grouped into 20 categories. Patients
  are to underline or circle the words in each
  group that describes the sensation of pain being
  experienced.
• Groups 1-10 somatic in nature
• Groups 11-15 affective
• Group 16 evaluative
• Group 17-20 miscellaneous words that are used
  in the scoring process.

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Pain Scales

• Visual Analog Scale
• Locate area of pain on a picture
• McGill pain questionnaire
• Evaluate sensory, evaluative, affective
  components of pain
• 20 subcategories, 78 words

None
Severe
0
10
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Scoring

• Add up the total number of words chosen, up to
  the maximum of 20 words (one for each category)
• The level of intensity of pain is determined by
  the value assigned to each word.
• 1st word 1 point
• 2nd word 2 point
• And so on
• Pt could have a high score of 20, but have a
  low-intensity score by selecting the 1st word in
  each category.

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Submaximal Effort Tourniquet Test

• In 1966, Smith et al described a method of
  matching a patients pain using a SETT.
• The SETT is performed by inflating a BP cuff to
  above systolic pressure on the pt elevated arm.
• Once the cuff is inflated, the pt is instructed
  to open and close the hand or fist rhythmically.
• A handgrip dynamometer and a metronome can be
  used for standardization.
• The pt should continue opening and closing the
  hand or fist until the cramping sensation that he
  or she feels matches the pain from the
  original pathology.
• The amount of time that elapses from onset to
  fruition of matched pain is the recorded
  objective measure.
• The SETT can be repeated at every tx session to
  gauge tx progress and is effective in matching
  all types of pain

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• Pain Threshold level of noxious stimulus
  required to alert an individual of a potential
  threat to tissue
• Pain Tolerance amount of pain a person is
  willing or able to tolerate

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Pain Control Theories

• Where have we been?
• Where are we now?

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Where have we been?

• Specificity theory
• 4 types of sensory receptors heat, cold, touch,
  pain
• A nerve responded to only one type
• Nerve was continuous from the periphery to the
  brain
• Pattern theory
• A single nerve responded to each type of
  sensation by creating a code (i.E. Different
  telephone rings)
• Gate control theory
• Melzack wall, 1965 the basis for theories
  today
• Non-painful stimulus can block the transmission
  of a painful stimulus

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Pain

• Of all the components of the injury response,
  none is less consistent or less understood than
  an individuals response to pain
• The sensation of pain is a diffuse entity
  inherent to the nervous system and basic to all
  people
• It is a personal experience that all humans
  endure
• Acute pain is the primary reason why people seek
  medical attention and the major complaint that
  they describe on initial evaluation.

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• Chronic pain may be more debilitating than the
  trauma itself and, in many instances, is so
  emotionally and physically debilitating that it
  is a leading cause of suicide.
• Pain serves as one of the bodys defense
  mechanisms by warning the brain that its tissues
  may be in jeopardy, yet pain may be triggered
  without any physical damage to tissues.
• The pain response itself is a complex phenomenon
  involving sensory, behavioral (motor), emotional,
  and cultural components.

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• Once the painful impulse has been initiated and
  received by the brain, the interpretation of pain
  itself is based on interrelated biological,
  psychological, and social factors.
• What are the nerve fibers that stimulate pain?
• Nociceptors.
• Once these are stimulated, pain impulses are
  sent to the brain as a warning that the bodys
  integrity is at risk.
• The emotional response may be expressed by
  screaming, crying, fainting, or just thinking
  _at_, that hurts!

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• When the pain is intense or unexpected, an
  immediate reflex loop activates the behavioral
  response by sending instructions to motor nerves
  to remove the body part from the stimulus.
• Sticking your finger with a needle
• Placing your hand on a hot stove
• These stimulis activate specialized nerve fibers
  to send signals through a peripheral nerve
  network
• Routing the impulses up the spinal cord to the
  brain

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• When the afferent impulse reach the spinal cord,
  a reflex loop is formed within the tract to
  activate the muscles necessary to remove your
  hand or finger from the stimulus.
• The remaining impulses of the reflex continue on
  to the brain, where they are translated as pain,
  and you respond by saying ouch! or other choice
  words.
• If an individual has knowledge about a
  potentially painful stimulus, such as receiving
  an injection, cognitive mechanisms can inhibit
  the reflex loop and block portions of the
  behavioral response.
• As a the painful stimulus increases, so does the
  conscious effort required to keep from trying to
  escape from the stimulus.

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• The emotional component may still be in place as
  you grimace, make a fist, or think what the _at_
  is this jerk doing to me.
• The cultural components of pain are almost too
  complex to define.
• However, pain perception has been linked to
  ethnicity and socioeconomic status.
• Example
• Italian patents are less inhibited in the
  expression of pain than are the Irish or
  Anglo-Saxon patients
• Ultimately, cultural components can be viewed as
  any variable that relates to the environment in
  which a person was raised and how that
  environment deals with pain and responses to pain.

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Pain Process

• Noxious input or nociceptive stimulus causes the
  activation of pain fibers.
• The painful impulse is triggered by the initial
  mechanical force of the injury (whether sudden or
  gradual onset) and is continued by chemical
  irritation resulting from the inflammatory
  process
• In subacute and chronic conditions, pain may be
  continued by reflex muscle spasm in a positive
  feedback loop or through the continued presence
  of chemical irritation

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• The pain response is initiated by stimulation of
  nociceptors
• Nociceptors- specialized nerve endings that
  respond to painful stimuli
• Mechanical stress or damage to the tissues excite
  mechanosensitive nociceptors
• Chemosensitive nociceptors are excited by various
  chemical substances released during the
  inflammatory response
• Chemical irritation of nerve endings may produce
  a severe pain response without true tissue
  destruction

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• Unlike other types of nerve receptors,
  nociceptors display a sensitization to repeated
  or prolonged stimulation
• During the inflammatory process, the threshold
  required to initiate an action potential is
  lowered, and the continued stimulation of the
  chemosensitive receptors perpetuates the cycle

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Modulation of Pain

• Acute pain response begins with a noxious
  stimulus.
• IE. A burn or cut externally or internally a
  muscle strain or ligament sprain
• After trauma chemicals are released in and around
  the surrounding tissues.
• Immediately after the trauma, primary
  hyperalgesia occurs
• Lowers the nerves threshold to noxious stimuli
  and magnifying the pain response

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Pain fibers

• A-delta fibers- a type of nerve that transmits
  painful information that is often interpreted by
  the brain as burning or stinging pain
• C-fibers- a type of nerve that transmits painful
  information that is often interpreted by the
  brain as throbbing or aching

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• After an injury, A-delta and C fibers carry
  noxious stimuli from the periphery to the spinal
  cord.
• The noxious stimuli activates 10-20 of the
  A-delta fibers and 50-80 of the C-fibers.
• Triggered by strong mechanical pressure or
  intense heat, A-delta fibers produce a fast,
  bright, localized pain sensation.
• C-fibers are triggered by thermal, mechanical,
  and chemical stimuli and generate a more diffuse,
  nagging sensation

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• After an injury, such as a sprained ankle, an
  athlete feels
• Sharp, well-localized, stinging or burning
  sensation coming from which fibers??
• A-delta fibers
• This initial reaction allows an individual to
  realise that trauma has occurred and to recognize
  the response as pain
• Very quickly, the stinging or burning sensation
  becomes an aching or throbbing sensation, which
  indicates activation of which fiber
• C-fibers
• A third type of peripheral afferent nerve fiber
  warrants mention. A-beta fibers, respond to light
  touch and low intensity mechanical information.
• Rubbing and injured area
• These interrupt nociception to the dorsal horn

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• The brains limbic system aids in integrating
  higher brain function with motivational and
  emotional reactions.
• Contains afferent nerves from the hypothalamus
  and the brain stem.
• Receives descending influence from the cortex.
• This communication is responsible for the
  emotional response to painful experiences.
• When an injury occurs, the neural communication
  between the limbic system, thalamus, RF, and
  cortex produces reactions such as fear, anxiety,
  or crying.
• In short , the limbic system is responsible for
  the bodys affective qualities of reward,
  punishment, aversive drives, and fear reactions
  to pain
• AKA motivational-affective system.

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• The integration of the cortex is an important
  component in both the ascending and descending
  aspects of pain modulation.
• Via axons, ascending pain stimuli are transmitted
  from the thalamus to the central sulcus in the
  parietal lobe (somatosensory cortex), where the
  pain is discriminated and localized.
• Because of the proliferation of nerve cells and
  the cortexs functions
• Consciousness
• Speech
• Hearing
• Memory
• Thought
• It is unlikely that the afferent synapses that
  occur during noxious stimulation affect only one
  efferent neuron.
• Thus, many areas of the cortex can be stimulated
  during a painful experience.

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• The notion of central control and descending
  inhibition of pain is based on the bodys ability
  to use and produce various forms of endogenous
  opiates.
• Each having a distinct function and a specific
  receptor affinity.
• The enkephalins are found throughout the central
  nervous system, but particularly in the dorsal
  horn.
• Thus, the aggregation of noxious stimuli may
  cause both presynaptic and postsynaptic control
  of nociception in the dorsal horn via enkephalin
  release

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Review of the process of Pain Transmission

• Much decision making in the tx of pain can be
  based on the understanding of the physiological
  and chemical interaction that occurs after
  trauma.
• In simple terms, pain transmission appears to be
  fairly straightforward.
• The acute pain response is initiated when
  substances are released form injured tissues,
  causing a noxious stimulus to be transmitted via
  A-delta and C fiber to the dorsal horn

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Pain Theory Historical Perspectives

• Theories regarding the cause, nature, and purpose
  of pain have been debated since the dawn of
  humankind.
• Most early theories were based on the assumptions
  that pain was related to a form of punishment.
• The word pain is derived from the Latin word
  poena meaning fine, penalty, or punishment.

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• The ancient Greek believed that pain was
  associated with pleasure because the relief of
  pain was both pleasurable and emotional.
• Aristotle reassessed the theory of pain and
  declared that the soul was the center of the
  sensory processes and that the pain system was
  located in the heart

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• The Romans, coming closer to contemporary
  thought, viewed pain as something that
  accompanied inflammation.
• In the 2nd century, Galen offered the Romans his
  works on the concepts of the nervous system.
• However, the views of Aristotle weathered the
  winds of time.
• In the 4th century, successors of Aristotle
  discovered anatomic proof that the brain was
  connected to nervous system
• Despite this, Aristotles belief prevailed until
  the 19th century, when German scientist provided
  irrefutable evidence that the brain is involved
  with sensory and motor function

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Specificity Theory of Pain Modulation

• Modern concepts of pain theory continue to
  advance from the ideas of Aristotle.
• However, controversy still exists as to which
  theories are correct.
• The theories accepted at the turn of the century
  were the specificity theory and the pattern
  theory, two completely different and seemingly
  contradictory views

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• The specificity theory suggests that there is a
  direct pathway from peripheral pain receptors to
  the brain.
• The pain receptors are located in the skin and
  are purported to carry pain impulses via a
  continuous fiber directly to the brains pain
  center
• The pathway includes the peripheral nerves, the
  lateral STT (spinothalamic tract) in the spinal
  cord and the hypothalamus (the brains pain
  center)
• This theory was examined and refuted using
  clinical, psychological, and physiological
  evidence by Melzack and Wall in 1965.
• They discussed clinical evidence describing pain
  sensations in severe burn patients, amputee
  patients, and patients with degenerative nerve
  disease.

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• These syndromes do not occur in a fixed, direct
  linear system
• Rather in the quality and quantity of the
  perceived pain are directly related to a
  psychological variable and sensory input.
• This theory had been previously addressed by
  Pavlov, who inflicted dogs with a painful
  stimulus, then immediately gave them food.
• The dogs eventually responded to the stimulus as
  a signal for food and showed no responses to the
  pain

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• The psychological aspect of pain perception was
  later addressed by Beecher, who studied 215
  soldiers seriously wounded in the Battle of
  Anzio, finding that only 27 requested
  pain-relieving medication (Morphine).
• When the soldiers were asked if they were
  experiencing pain, almost 60 indicated that they
  suffered no pain or only slight pain, and only
  24 rated the pain as bad.
• This was most surprising because 48 of the
  soldiers had received penetrating abdominal
  wounds.
• Beecher also noted that none of the men were
  suffering from shock or were insensitive to pain
  because inept intravenous insertions resulted in
  complaints of acute pain.

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• The conclusion was drawn that the pain
  experienced by these men was blocked by emotional
  factors.
• The physical injuries that these men had received
  was an escape from the life-threatening
  environment of battle to the safety of a
  hospital, or even release form the war.
• This relationship suggests that it is possible
  for the central nervous system to intervene
  between the stimulus and the sensation in the
  presence of certain psychological variables.
• No physiological evidence has been found to
  suggest that certain nerve cells are more
  important for pain perception and response than
  others therefore, the specificity theory can be
  discounted.

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Contemporary Pain Control Theories

• Although both the specificity and pattern
  theories of pain transmission were eventually
  refuted, they did provide some lasting principles
  that are still present in contemporary pain
  modulation theories
• The strengths of these 2 theories, plus findings
  obtained through additional research, were
  factored together to for the basis of the current
  perspective regarding pain transmission and pain
  modulation.
• Still, there is much to be learned and studied
  before the exact mechanisms of pain transmission
  and perception are understood.

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Pattern Theory of Pain

• States that there are no specialized receptors in
  the skin.
• Rather, a single generic nerve responds
  differently to each type of sensation by creating
  a uniquely coded impulse formed by a
  spatiotemporal pattern involving the frequency
  and pattern of nerve transmission.

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• An analysis of the words elements
• Spatio- the distance between the nerves
  impluses
• temporal- the frequency of the transmission
• An example of this type of coding can be found
  with most institutional phone systems.
• A call from inside a university has a different
  ring from an outside call.
• Although this theory was closer to being
  neurological correct there were still
  shortcomings
• Melzack and Wall refuted this theory as well,
  based on the physical evidence of physiological
  specialization of receptor-fiber units.
• Plus this theory failed to account for the brains
  role in pain perception.

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Gate Control Theory

• Implies a non-painful stimulus can block the
  transmission of a noxious stimulus.
• Is based on the premise that the gate, located in
  the dorsal horn of the spinal cord, modulates the
  afferent nerve impulses.

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• The SG (substantia gelatinosa) acts as a
  modulating gate or a control system between the
  peripheral nerve fibers and central cells that
  permits only one type of nerve impulse (pain or
  no pain) to pass through.
• Serving in a capacity similar to that of a
  switch operator in a railroad yard, the SG
  monitors the amount of activity occurring on both
  incoming tracts in a convergent system
• Opening and closing the gate to allow the
  appropriate information to be passed along to the
  T cell.
• Impulses traveling on the fast, non-pain fibers ?
  activity in the SG.
• Impulses on the slower pain fibers exert an
  inhibitory influence.
• When the SG is active, the gate is in its
  closed position and a non-painful stimulus is
  allowed to pass on to the T cell.

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• Example
• Bumping the head
• The initial trauma activates the A-delta and,
  eventually, C fibers
• Rubbing the traumatized area stimulates the
  A-beta fibers, which activate the SG to close the
  spinal gate
• Thus inhibiting transmission of the painful
  stimulus

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Placebo Effect

• Placebo stems from the Latin word for I shall
  please
• Used to describe pain reduction obtained from a
  mechanism other than those related to the
  physiological effects of the tx.
• Linked to psychological mechanisms
• All Treatments have some degree of placebo
  effect
• Most studies involving TM involving the use of a
  sham TM (ultrasound set at the intensity of 0)
  and an actual treatment have shown ? levels of
  pain in each group.

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Two main categories of pain

• 1. Acute - is a relatively brief sensation,
  usually less than six months duration - usually a
  response to a specific trauma - forms the basis
  for danger warnings and subsequent learning.

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Two main categories of pain

• 2. Chronic - lasts more than six months - exists
  beyond the time for normal organic healing The
  pain begins to impair other functions Patients
  may begin to experience learned helplessness and
  hopelessness this leads to the classic signs of
  depression (lethargy, sleep disturbance, weight
  loss) May quit work and adopt a self imposed
  invalid existence.

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Chronic Pain

• Characteristics of
• Symptoms last longer than 6 months
• Few objective medical findings
• Medication abuse
• Difficulty sleeping
• Depression
• Manipulative behavior
• Somatic preoccupation

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Categories of Chronic Pain

• Chronic recurrent pain -- benign condition
  consisting of intense pain alternating with
  pain-free periods. eg, migraine, tension
  headaches, endometriosis.
• Chronic intractable-benign pain -- benign
  condition where pain is persistent with no pain
  free periods, although the pain may vary in
  intensity eg low back pain.

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Categories of Chronic Pain

• Chronic progressive pain --malignant condition
  where pain is continuous and increases in
  intensity as the organic condition (disease)
  worsens eg. Cancer and rheumatoid arthritis.

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Congenital Analgesia

• A well-known case of congenital insensitivity to
  pain is a girl referred to as 'miss C' who was a
  student at McGill university in Montreal in the
  1950s. She was normal in every way, except that
  she could not feel pain. When she was a child she
  had bitten off the tip of her tongue and had
  suffered third-degree burns by kneeling on a
  radiator.

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Congenital Analgesia

• When she was examined by a psychologist (Charles
  Murray) in 1950 she did not feel any pain when
  she was given strong electric shocks or when
  exposed to very hot and very cold water. When
  these stimuli were presented to her she showed no
  change in heart rate, blood pressure or
  respiration. She did not remember ever having
  coughed or sneezed, and did not show a blinking
  reflex. She died at the age of 29 as a result of
  her condition.

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Congenital Analgesia

• Although during a post-mortem there were no
  obvious signs of what had caused the analgesia in
  the first place, she had damaged her knees, hips
  and spine. This damage was due to the fact that
  she did not shift her weight when standing or
  sitting, did not turn over in bed and did not
  avoid what would normally be considered to be
  uncomfortable postures. This caused severe
  inflammation in her joints.

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Congenital Analgesia

• Although there is some evidence that this
  condition may be inherited, there are other
  causes such as neurological damage. However, some
  cases cannot be explained in this way. Most
  people with this condition learn to avoid causing
  themselves too much harm but, as in the case of
  'Miss C, may die as a result of the problems
  caused by the analgesia.

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Episodic Analgesia

• Serious injury (e.g. loss of limb) - little pain
  felt.
• 6 characteristics (Melzack and Wall 1988).
• The condition has no relationship to the severity
  or the location of the injury.
• No simple relationship to circumstances - occurs
  in battle or at home.
• Victim fully aware of injury but feels no pain
• Analgesia is instantaneous
• Analgesia lasts for a limited time

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Episodic Analgesia

• 6 Analgesia is localised, pain can be felt in
  other parts of the body (arm blown off is not
  felt, but injection is!)

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Episodic Analgesia

• Carlen et al (1978) - Israeli soldiers - Yom
  Kippur War. Loss of arm - 'bang', 'thump' or
  'blow'.
• Melzack, Wall and Ty (1982) - 37 of accident
  victims reported the experience of episodic
  analgesia.
•  

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Fibromyalgia Pain Without Injury

• The occurrence of body-wide pain in the absence
  of tissue damage, as in fibromyalgia, interferes
  with all aspects of a person's life and
  undermines their credibility. The problem is that
  normal activities can be exhausting, sleep is
  disturbed, the ability to concentrate is
  impaired, gastrointestinal function is often
  abnormal, persistent headaches are common, and
  the unrelenting pain that no one can see is often
  detrimental to their personal and professional
  lives--as it creates a "credibility gap."

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Pain - Injury

• Neuralgia - sharp pain along a nerve pathway.
  Causalgia - burning pain Both develop after wound
  or disease has ended. Triggered by a simple
  stimulus e.g. breeze or vibration. Physiological
  cause of headaches not known. Melzack and Wall
  (1988) report that migraine causes dilation of
  blood vessels, not the other way around! Pain out
  of proportion to the injury Some cancers produce
  little pain until they are advanced. (Serious
  illness, little pain). Kidney stones are not
  serious, but produce excruciating pain.

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Purpose of pain

• 1.      Prevents serious damage. If you touch
  something hot, you are forced to withdraw your
  hand before it gets seriously burnt.
• Teaches one what to avoid
• If pain is in joints, pain limits the activity,
  so no permanent damage can occur.
• but pain can become the problem, and cause people
  to want to die.

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Phantom limb pain

• Melzack (1992) 7 features
• Phantom limb feels real. Sometimes amputees try
  to walk on their phantom limb.
• Phantom arm hangs down at the side when resting.
  Appears to swing in time with other arm, when
  walking.
• Sometimes gets stuck in awkward position. If
  behind the patients back, then patient feels
  obliged to sleep on stomach.
• Artificial limb appears to fit like a glove. See
  artificial limb as part of their body.

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Phantom limb pain

• 5 Phantom limbs give impression of pressure and
  pain
• 6 Even if phantom limb is experienced as
  spatially detached from the body, it is still
  felt to belong to the patient.
• 7 Paraplegic people experience phantom limbs.
  They can even experience continually cycling
  legs.

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Phantom limb pain

• Not just the cut nerve endings (neuromas) sending
  messages to the brain, because cuts made along
  the neural pathways only produce a temporary
  relief from pain.

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Phantom limb pain

• Melzack believes - brain contains a neuromatrix
  of the body image - neurosignature - like a
  hologram.

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Phantom limb pain

• Connections to this neuromatrix - sensory
  systems, emotional and motivational systems. It
  is the emotional and motivational systems that
  cause the phantom limb experience.

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Phantom limb pain

• Neuromatrix pre-wired - young amputees experience
  phantom limb pain.
• People born without limbs also experience phantom
  limb pain.

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Gate Control Theory Proposed by Melzack and Wall
in the 1960's
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Gate opened or closed by 3 factors

• Activity in the pain fibres - opens the gate
• Activity in other sensory nerves - closes the
  gate
• Messages from the brain - concentrating on the
  pain or trying not to think about it

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Conditions that open or close the gate
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Three variables control this gate

• A-Delta fibres (sharp pain)
• C fibres (dull pain)
• A-Beta fibres that carry messages of light touch

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Pain Gate Theory

• Special neurons located in the grey matter of the
  spinal cord make up the gate This gate has the
  ability to block the signals from the a-delta and
  c-delta fibres preventing them from reaching the
  brain.

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Pain Gate Theory

• The special neurons in the spinal cord are
  inhibitory ie they keep the gate closed. These
  special neurons make a pain blocking agent called
  enkephalin. This is an opiate substance similar
  to heroin which can block Substance P the
  neurotransmitter from the C fibres and the
  A-delta fibres and this keeps the gate closed.

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Pain Gate Theory

• C-Fibres and A-Delta fibres obstruct (inhibitory)
  the special gate neurons and tend to open the
  gate. A-beta fibres are irritable (excitatory) to
  the special gate neurons and tend to keep the
  gate closed.

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Pain Gate Theory

• If impulses in the C and A-Delta Fibres are
  stronger than the A-beta Fibres the gate opens.
  A-delta fibres are always stronger.

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Pain Gate Theory

• Specialised nerve impulses arise in the brain
  itself and travel down the spinal cord to
  influence the gate. This is called the central
  control trigger and it can send both obstructive
  and irritable messages to the gate sensitising it
  to either C or A-beta fibres.