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Transmitting Pain Signals to the Spinal Cord

As mentioned, the free nerve endings are not the only sensors in the body that can signal pain. All the other types of sensors (for temperature, movement, etc.) can create a pain message if they receive a strong enough stimulation. A little heat is not sensed as pain, but a lot of heat will be; a little pressure doesn't hurt, but a lot of pressure does. This is because the sensors, whether simple or sophisticated, send signals at a rate corresponding to the strength of the sensation. Each sensor is like a beating drum. The sensors are not capable of causing soft beats or loud beats; they can only change how fast they send signals. A little heat causes a temperature sensor to send a few slow signals. The temperature sensor sends the exact same signal rapidly and repeatedly when it is exposed to a lot of heat.
The brain and spinal cord are able to decipher this message by determining the type of sensor sending signals (heat, cold, pain, etc.), where the sensor is located, and how fast the signals are arriving. Brushing your finger over a candle flame sends a few heat signals to the brain, telling you it is hot and you should probably keep that finger moving. Resting the same finger in the flame causes a barrage of heat signals that the brain senses as severe pain. At times, this message is misinterpreted, such as when you place your hand in cold water, but immediately jerk it out again, thinking the water was too hot.
Each pain signal travels to the spinal cord along a sigle sensory nerve fiber. The cells that create the nerve fiber actually are located inside the bones of the spine in collections of neurons (nerve cells) called, the dorsal root ganglion. Even though each cell is microscopically small, its "transmission line" is long enough to reach all the way out to whatever part of the body that the cell monitors.
These long nerve filaments are not all the same thickness, nor do they all have the same structure. The two most important types of pain-sensng fibers are called C-fibers and A-delta fibers. The A-delta fibers transmit pain signals to the brain very quickly because they are covered with a special insulation called myelin, which speeds up conduction. C-fibers lack this covering and transmit their signals more slowly.
How fast do these nerves conduct the signal? The slow C-fibers transmit their signals about one and 1.5 to 6 feet per second, while the A-delta fibers transmit at 40-90 feet per second. This defference is significant enough to that most people can clearly sense the defferent kinds of pain sensation carried after injury. This pain typically feels sharper and "brighter." The C-fibers message comes a second or so later and tends to feel more dull and aching.
This has sometimes been termed first and second pain. After hitting your thumb with a hammer there is an immediate, sharp pain (the A-delta sensation), but this soon gives way to a deeper, throbbing, aching pain (the C-fiber sensation). If you don't remember the last time you hit your thumb with a hammer, trust us on this one.

Processing Pain Signals in the Spinal Cord

The system gets more complicated once the pain signal reaches the spinal cord, because a lot of different nerve cells all have to work together to get the pain message through to the brain. There is not just one nerve that carries pain information from your toe to your conscious brain, for example. The signal must be passed from the original sensory nerve to other nerves in the spinal cord.

Transmitting Signals from One Nerve Cell to Another

The place where two nerve cells interact with each other is called a synapse. At a synapse, the fiber from one nerve cell will come close to, but not quite touch, a fiber from another nerve. When a signal travels up the first nerve fiber to the synapse, the fiber releases a small burst of chemical messagers, called neurotransmitters. The neurotransmitters drift across the small gap between the cells and bind to special receptors located on the second nerve fiber.