Lecture 5: Pain

Pain

A physiologic response that serves as a warning

Nociception

The perception of pain

Nociceptors

Pain receptors

Gate control theory

Pain transmission occurs because of the signals sent to the spinal cord where the cells in the substantia gelatinosa work as a gate.

1. Neurochemistry

2. neuroanatomy

The gate control theory of pain involves what? [2]

Thalamus

Major integration center for pain

Higher brain centers: Brainstem, reticular area, thalamus

Synapse happens at the substantia gelatinosa, where do messages get sent?

C-fibres

Fibres that send slower, duller pain messages associated typically with chronic pain

Hypoxia

Lack of oxygen to tissues

Lactic acid. This stimulates nociceptors

When the body switches to anaerobic metabolism, what is created as a result?

Prostaglandins

These are synthesized from exposed phospholipid of broken cell membranes, and stimulate nociceptors and increase sensitivity to pain

Inhibitory

Signal that tries to stop pain message from getting sent higher, or reduce severity

1. Substance P

2. Glutamate

3. aspartate

4. calcitonin

5. pG

6. Histamine

7. Bradykinin

Excitatory neurotransmitters: [7]

1. chemical mediators

2. pressure

3. temperature

How are pain receptors stimulated? [3]

1. Serotonin

2. GABA

3. glycine

4. norepinephrine

5. endogenous opiods

Inhibitory neurotransmitters [5]

1. depolarization, AP generated

2. impulse transmission in nerve along a delta or c fibers

Stimulation of pain receptors initiates what? [2]

A-delta fibres

Large, myelinated pathway for fast, sharp, localized pain

C fibres

small, unmyelinated pathway for slow, dull, burning, diffuse pain

Dorsal horn

Impulses from a-delta/C fibres are sent to this relay station between peripheral and central nervous systems

Synapse in the substantia gelatinosa

for impulses to travel up the spinothalamic tract, it needs to travel across where?

Release in inhibitory or excitatory neurotransmitters.

When an impulse reaches the end of a presynaptic neuron, what does that cause?

Neurotransmitters cross the synapse in the substantia gelitanosa and bind to receptors on post-synaptic neurons to send message to CNS

What initiates depolarization of post-synaptic neurons in dorsal horn?

Neurotransmitter binding to receptors

What initiates deplarization of post synaptic neurons?

1. Neo: sharp, fast

2. Paleo: slow, dull

Two tracts from dorsal horn to CNS

1. reticular system

2. limbic system

3. cerebral cortex

Pain perception centres whre learned behaviours, cultural aspects, and previous experiences shape emotions associated with pain: [3]

HPA axis views pain as a stressor, and HPA gets activated

Why might patients in pain have increased pulse, sweating, tachypnea?

Motor response

Different than a reflex arc, normally if touching something painful for longer than a millisecond. It removes the body part that is painful at the time

1. Reticular and limbic system

2. cerebral cortex

3. HPA axis

4. Muscles

The thalamus sends messages to various effectors: [4]

Descending pathway of pain modulation

Increases or decreases the transmission of pain signals in the periphery, spinal cord, and brain

The synapse of pain. If we stop the synapse (aka modulating pain) then we stop the brain from perceiving there is pain

Where is the “gate” in gate control theory?

Segmental pain inhibition.

How do a-beta fibres modulate pain?

Once pain is perceived in CNS, message is sent back for a motor response and stimulation.

A-beta fibres take over and send non-painful messages (squeezing, tapping, etc) to the brain via the ascending pathway and inhibitory neurotransmittors are released.

pain gate gets closed

How segmental pain inhibition inhibits pain: [3]

PAG (periaqueductal grey)

Region of the midbrain that receives pain messages has axons connecting it to dorsal horn in spinal column. Stimulates NRM area to release endorphnis in dorsal horn

Endorphines

the feel-good morphine in he body

Endorphines bind to and inhibit ion channels in post-synaptic neurons OR

Inhibitory neurotransmitters inhibit the ion channels and

Post-synaptic transmission of pain impulses from periphery decreases so pain decreases

How do endorphines decrease pain?

Diffuse noxious inhibitory control (DNIC)

Activates when another area is receiving noxious (painful) stimuli at the same time as the original painful area, and A-beta fibres are stimulated and help with the pain (ex: deep massage, acupuncture, heating pad).

1. Somatic

2. Visceral

3. Referred

three classifications of acute pain:

a-delta or C-fibres

Somatic pain (joints, skin, muscles) sends signals along which fibres?

C-fibres

Visceral pain sends signals along which fibres?

Chronic Pain

Pain lasting longer than 3-6 months. Can be ongoing or intermittent

Intractable pain

Chronic pain associated with metastasis in cancer

Neuropathic pain

Chronic pain caused b damage to/dysfunction of nerves/nervous system. Described as burning, shooting, shock-like, tingling

1. Phantom limb pain

2. diabetic neuropathy

3. post-herpetic neuralgia

4. multipl sclerosis

5. parkinson’s disease pain

Examples of neuropathic pain:

Post-herpetic neuralgia

When herpes zoster virus lives in nerve ganglia after the chicken pox. When reactivates, causes inflammation f neuron with prolonged impulses

Cutaneous

“superficial”

Dermatome

Region of the body wall supplied by a single pair of dorsal root ganglia

Pain threshold

Lowest amount of pain a person can recognize

Pain tolerace

Greatest amount of pain a person can endure

1. before treatments

2. after treatments

3. with vitals (are you feeling any pain?)

When should pain be assessed? [3]

1. mild

2. moderate

3. severe

Descriptive scale of pain: [3]

Stimulates a-beta fibres to close pain gate

How do massage and tens machines treat pain?

Numbs nociceptors so the don’t fire

How can cold treat pain?

Stops lactic acid production

How can oxygen treat pain?

It removes stimuli

How can increased circulation (massage, heat, ultrasound) tret pain?