Healing Pain Class: Neurological Pathways of Pain
Presented by Dr. Grant de la Motte (Neurology)
1. Basic pain pathway
pain nerve ⟷ spinal cord ⟷ thalamus ⟷ parietal lobe ⟷ thalamus ⟷ spinal cord ⟷ pain nerve
“Pain nerves” are specialized nerves located all over the body. The source of pain, such as pricking a finger, activates the pain nerve. Pain nerves only senses “pain”, not the other aspects of sensations such as smoothness, softness, wetness, dryness, etc.
The impulse travels from the point of where the pain nerve is activated to the spinal cord, then to the thalamus.
The thalamus is the brain relay center, but processes information as well. The thalamus helps regulate sleep and wakefulness, arousal and consciousness. In addition, the thalamus is functionally connected to the hippocampus, which plays an important role with memory.
The pain impulse of information continues to the parietal lobe, which is where we feel and integrate sensations, located at the top back of the skull.
The impulse and its accumulated information travels through the neurological pathway in both directions.
2. Modification of the pain pathway
a. Modifiers
i. Attention (didn’t notice pricked finger)
ii. Memory (pricked finger in the past, hurts even more the 2nd time)
iii. Emotion (hurts even more when experiencing a negative emotion)
The brain can send information along both directions of the pathway to the nerves and relay centers to modify the experience of pain. Our actual sensation is affected by ways in which the pain impulse is modified. Where our awareness is focused (attention), remembering another similar experience, and our current emotional state all play a role in the experience of pain.
b. Gate control theory (the chain of command)
The “gate control theory” describes how the occurrence of pleasant or pleasurable input into the pain pathway closes the “gate” to the painful input, which prevents the pain sensation from traveling through the central nervous system and thereby suppress the experience of pain.
c. Feedback loops
i. Negative feedback loop (inhibition) – most biologic systems us this (thyroid, glucose)
ii. Positive feedback loop (facilitation) – commonly used by the nervous system (stampede, child birth: oxytocin increasingly being released to increase contractions, seizures, memories)
The term “feedback loop” refers to the continuous path that the input (or impulse, sensation, information, etc.) travels through the body, from the point of stress or pain, through the nervous system to the brain and back again. The experience of pain is modified by 2 different kinds of feedback loops.
Negative feedback loop describes the effect of inhibition, which means that when the brain detects stress or pain it sends a a response (ex. a hormone) to inhibit or decrease the cause. In the case of the thyroid gland, a low metabolic rate which leads to a lower than normal body temperature, can signal the release of thyroid hormone. When the body temperature increases, the release of the hormone decreases (negative feedback). Low blood sugar triggers the hormone glucagon. If the body responds and the stress of low blood sugar is decreased, the release of the hormone will also be decreased.
Positive feedback loop describes the effect of facilitation, which means that the effect increases, such as in the case of a stampede. An alarmed or panicked animal in a herd of animals can trigger other animals in the herd to panic, which in turn increases the overall level of panic in the herd, and so on. An example in human biology is the release the hormone oxytocin during labor. The release, which is triggered by the increase in pressure on the uterine wall, facilitates contractions and the overall birth process, which in turn facilitates more oxytocin being released. Seizures, which are abnormal electrical activities in the brain, also are an example of a positive feedback loop in that seizures seem to beget more seizures, both in intensity and occurrence. Memories, including cellular memory, also increases in occurrence when one activated.
3. Central sensitization (i.e. “central pain)
a. Positive feedback loop of pain ex. stroke, phantom limb
b. Neuropathic pain medications
“Pain itself often modifies the way the central nervous system works, so that a patient actually becomes more sensitive and gets more pain with less provocation. That sensitization is called “central sensitization” because it involves changes in the central nervous system (CNS) in particular — the brain and the spinal cord. Victims are not only more sensitive to things that should hurt, but also to ordinary touch and pressure as well. Their pain also “echoes,” fading more slowly than in other people. Indeed, this neurological meltdown is such a consistent complication of other painful problems that some researchers now believe central sensitization is actually a major common denominator in most difficult pain problems. That is, it may be the nearly universal factor that puts the “chronic” in chronic pain, giving all such problems shared characteristics regardless of how it got started — not the cause of the pain, but perhaps the cause of its chronicity.”
– from Paul Ingraham’s article “Pain Changes How Pain Works”
4. Pathways to chronic pain
a. Structural (Cancer, nerve pinch)
b. Psychological or physical trauma (car accident, PTSD)
c. Spontaneous via mind-body connection (Fibromyalgia)
Take Home Point:
Central pain is modifiable and sometimes even eliminated by breaking the positive feedback loop