Alex Simotas, MD

Hospital for Special Surgery
Board Certified Physiatrist
Specializing In Spine & Sports Medicine

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How Pain Works

The Anatomy of Pain? What is it?

How does something that like a cut on our finger, or bruise on our back get into nervous system and our experience of pain?

What is pain? Why do we have it? Where does it occur? Why is it important? Understanding pain is important if you want to make your pain go away. To understand pain, we need to take a look at:

  • The anatomy of our nervous system.
  • How pain occurs in the nervous system when it senses injury to body tissues.
  • How the spine and the nervous system coexist, interact, and depend upon each other.
  • How the anatomy of the nervous system can change and adapt.
  • How to apply these understandings to common problems like back and neck pain to help your pain. get better and better and maybe…go away.

THE NERVOUS SYSTEM: A Quick Anatomy Lesson

Your nervous system is made up of two main parts:

  • The Central Nervous System (CNS)
  • The Peripheral Nervous Systems (PNS)

The brain and the spinal cord form the Central Nervous System (CNS). They have special nerve cells or pain neurons that produce pain sensations.

Sensory and motor nerves form the Peripheral Nervous System (PNS). This system consists of the nerves outside of the brain and the spinal cord. The main function of the PNS is to connect the Central Nervous System (CNS) to the head, trunk (which includes the spine), limbs and organs.

The Stages Or Hubs That Relay Pain Information In The Nervous System

Moving from hub to hub until you experience pain.
The brain and spinal cord have processing centers that act like relay hubs. The hubs process and then stimulate another set of nerves to the next hub.

One concept is these hubs work like gates that block further transmission until they load up to certain level. The gates may be open or closed depending on conditions within the hub. The degree to which gates are open or closed or the threshold at which they open may be controlled or influenced by other factors. When a gate opens the messages are relayed further up the chain. Eventually messages transmit from hub to hub and reach the conscious parts of the brain that create the unpleasant experience we call pain.


What Happens When You’re Injured?

Let’s say you cut your finger. Many things start to happen, resulting in feeling. It may seem automatic but here are some of the steps that occur to excite the nervous system and render the pain experience you feel:

  1. Peripheral pain cells, called nociceptors, create impulses.
  2. These impulses pass into other cells in the spinal cord, called spinal pain neurons.
  3. A new set of impulses are produced and transmitted towards a part of the brain, called the thalamus.
  4. The thalamus then sends these impulses to other parts of the brain.
  5. Some of these signals arrive in the conscious part of our brain, the cerebral cortex. Here, we figure out where the pain came from, and how it compares to other kinds of pain we have experienced in the past.
  6. Other signals are sent to the emotional center of our brain, called the limbic system, and each feeling generates a response.
    a. Your heart rate may increase.
    b. You may break into a sweat.
    c. You might even cry.
    d. Some of these reactions happen almost instantaneously. Others evolve over time.

To quickly review these are the steps, from hub to hub, that result in the acute pain experience that occurs after an injury:

Nociceptors ⇒ spinal pain neurons ⇒ thalamus ⇒ cerebral cortex

⇒limbic system


The Same Pain Can Occur From A Lot Of Tissue Injury Or Minimal, To No Tissue Injury!

A hub in the spinal cord may transmit messages to the next hub only when:

  • It receives a large amount of stimulation from nociceptors that sense tissue injury.
  • Less stimulation is present but the gate threshold is lower.
  • Or where there is little or no injury present and the hub has been chronically activated and gates are constantly open.

This is one of the explanations for the occurrence of chronic pain.


All pain occurs in the brain, no matter where in the body it may appear to be. When you cut you finger the injury occurs to the tissues (the skin, the blood vessels, and connective tissue). The pain occurs in your brain. Remember that pain is the result of your nervous system relaying information to you about the injury.

Phantom Pain: Have you ever heard of phantom limb pain?

The phenomenon of phantom limb pain illustrates this well. Phantom pain occurs when a person with an amputation of a limb experiences pain in a limb that is no longer attached to their body.

How is this possible?

Phantom limb pain is possible because the network of pain neurons in the spinal cord and brain that are responsible for arm or leg pain have been erroneously activated by other indirect connections, resulting in pain that is very real. This is an example of how pain can be trained or influenced in a disadvantageous way to produce unwarranted pain experiences.

Why Do We Have Pain?

Pain is a key part of our survival and protection system. It works to monitor harm that might be a threat to our well-being. In an attempt to control our experience of pain, the nervous system is constantly adjusting to changes in our environment and within our bodies.

A Life Without Pain: Why Not?

What would life be like without pain? Why not just figure out how to eliminate the experience of pain altogether?

There are individuals who are born with a genetic defect that renders them without the capacity to feel pain. They are unaware that they have injured themselves when they suffer serious burns, fractures, and other serious or even potentially lethal injuries. They must be monitored and cared for closely particularly when they are young. Read about individuals born with this disease in this NY Times article: Ashlyn Blocker, The Girl Who Feels No Pain, by Justin Heckert. Published November 15, 2012.

Spinal Degeneration Is A Natural Function Of Aging. It Doesn’t Necessarily Lead To Pain.

In the previous articles we review some of the processes that occur to the spine as we age. As we get older, all of our spines start to naturally degenerate to varying degrees: the discs, facet joints,  ligaments and the muscles. This includes:

  • Disc degeneration
  • Bulges
  • Herniation without neurologic injury
  • Facet joint arthritis

As you may notice, some or all of these items may be identified as an injury to the spine. In each case there is a gradual change with degeneration of the spinal tissues and structure. Why don’t we just get more and more back pain as we get older?

Our Nervous System Adapts To Our Changing Bodies And Spinal Degeneration As We Age.

Our nervous system can adapt to the aging process, and degenerative changes to the spine frequently occur without stimulating our spinal pain neurons.

Most people with spinal degeneration, are still able to function normally. We can bend, twist, and rotate our spines. We can perform everyday tasks such as lifting and carrying. We can be very physical, often without any pain or neurological symptoms.

On The Other Hand, Maladaptive Changes In The Nervous System Functioning May Be A Major Cause Of Chronic Back Pain.

This is a new way of thinking about back pain that has gained more support in recent years.

X-rays and MRI scans often appear the same in people with or without chronic back pain. New developments in neuroscience suggest that changes in the way the nervous system works is a primary cause of a lot of chronic pain.

One simple way to understand this is that, in chronic pain, our pain gates in the nervous system hubs are overactive. One possibility is that we have actually inadvertently trained them to be overactive.

Changing the brain to treat pain.

There is evidence that while persistent pain is occurring, in particular chronic pain, dynamic changes are occurring to the way the brain processes pain experiences and related information. New brain imaging techniques, known as a functional MRI, have shown that people with chronic pain process physical stimuli differently from people who are not in pain.

Brain research has also shown that the areas of the brain involved with processing pain interact with the brain centers responsible for our thoughts and emotions. This demonstrates that thoughts, beliefs and emotions affect our experience of chronic pain.

The exciting implications of this research is that:

  • Real changes in the brain may be at play when individuals develop chronic back pain.
  • A lot of these changes may be influenced by maladaptive thoughts, beliefs and behaviors that individuals with chronic back pain develop.
  • Changing these to more positive and well-adapted thoughts and behaviors may also lead to a reversal of these changes in the brain.

Individualized pain systems

Everybody is different and so is his or her pain system. These differences depend greatly on the individual interaction of a person’s unique nervous system with their own degeneration process. Differences may be due to:

  • Genetic factors
  • Environment
  • Personal experiences

Genetics may influence an individual’s pain sensitivity, affecting the ability to adapt efficiently to degeneration. Scientists are performing studies to better understand the impact of genetics on spinal degeneration.

Individuals who suffer with back pain appear to suffer with a persistent pain sensitivity, which has been attributed to something called low threshold persistent pain (LTP).

Low-threshold pain

The explanation of chronic pain in many cases appears to be related to a Low Threshold Pain or LTP. Low threshold pain is defined as a lower than average capacity to withstand pain.

LTP is caused by a heightened sensitivity of the spinal pain neurons:

  • Pain neurons are the nerve cells in the spinal cord.
  • These cells monitor the spine for significant injury and danger.
  • Activation of these cells leads to the experience of pain.
  • Persistent activation of these cells results in LTP.

This heightened sensitivity also affects the sensory neurons that monitor touch, joint movement, and muscle function. So in a state of LTP you may also experience:

  • Pressure-sensitive pain, sometimes referred to as trigger points.
  • Pain in distant muscles such as in the back, shoulder or pelvis.
  • Pain may be associated with prolonged sitting or standing.

When the LTP spreads to other nearby spine neurons, other uninjured and neighboring parts of the body become painful. This is called central sensitization.

Everybody Gets LTP Temporarily Right After Any Injury

Actually LTP is what we get right after any injury. It’s a normal response and just part of normal functioning nervous system. It’s the nervous system’s way of putting things on hold for just enough time to figure out how bad and dangerous every injury is and how much we need to protect ourselves from further harm.

Just think of the painful sensitivity to pressure or touch at the site of a cut or bruise even though these stimuli are not really harmful. Temporary low threshold pain sensitivity is normal and disappears as you heal.

Persistent LTP

Persistent LPT state is a primary cause of chronic pain. This is a dysfunction of the pain system. In effect the nervous system forgets to switch back to more normal threshold levels. In this state:

  • Pain no longer serves any physiologic purpose and can reach difficult to control states.
  • Pain is persistent although there is no damage or harm.
  • Incorrect messaging occurring between parts of the nervous system may be a cause of the LTP.

Why knowing about LTP is so important

  • Since such pain is not a sign of injury, it is safe to continue activities in the presence of pain.
  • Low threshold pain is more of a neurological problem than a problem of spine degeneration.
  • Therefore, addressing the neurologic part of the pain problem must be part of a successful treatment plan.


  • Pain occurs by the transmission of impulses through series of stages or hubs in the spinal cord and brain.
  • After acute tissue injury, local nociceptive nerves stimulate the spinal cord neurons- the first step in the cascade of messages.
  • By contrast, chronic pain often occurs in the absence of tissue injury
  • The readiness (thresholds) of nervous system “hubs”, may be stimulated directly or indirectly by other factors: muscle tension, previous pain experience, emotional states,  fearful thoughts, etc.
  • In states of lower threshold pain, hubs may transmit with relatively few impulses. This provides some explanation for chronic pain..
  • The nervous system, with its complex anatomy, is best able to adapt to spinal degeneration changes that occur with age.