Understanding Nerve Fibers and Receptors

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How Pain Works

Two Views of Pain Close Window A. The

big picture. When you bang your finger, the signal starts at the very tips of

nerve cells, then travels to and up the spinal cord, and into a part of the

brain called the thalamus. The thalamus sends the signals out to several parts

of the brain, including those that control touch, emotion, physical reaction,

and memory. B. Up close. Pain signals are carried by two types of nerve

fibers, A-delta and C fibers. The A-delta fiber carries the first, sharp pain.

The C fiber conveys the dull, throbbing pain that follows. The signals travel

through the spinal cord through a dense array of nerve cells known as the dorsal

horn. The dorsal horn sends the signals up to the brain’s thalamus, which then

distributes them to many different parts of the brain.

Pain Signals Close Window A pain message travels

through the body from one nerve cell (neuron) to the next. The signal passes

down the axon of one neuron and must travel across a small gap called a synapse

to reach the next neuron. To transmit signals, the neuron releases chemical

messengers called neurotransmitters into the synapse between it and the next

neuron. The neurotransmitters attach to receptors on neighboring neurons and

allow the message to continue on its way. This process is repeated in rapid

succession between nerve cells throughout the body, up the spinal cord, and into

the brain.

Understanding Nerve Fibers and Receptors

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Nerve cells, called neurons, resemble spiders with a small central body and

several long, leglike protrusions. Neurons bundle together to form nerve fibers

(what we commonly call nerves) that extend throughout the body. Sensory nerves

carry information from the outside world to the brain. At the ends of these

nerves are specialized sensors, called nociceptors (pronounced no-seh-SEP-ters).

They play a key role in receiving painful stimuli and transmitting pain signals.

Nociceptors respond to bangs, bumps, burns, and other assaults to the body, as

well as to inflammation and other tissue changes. When magnified, nociceptors

look like the frayed end of a rope. A pain-sensitive area of the body, such as

the skin or tooth pulp, has thousands of nociceptors in a tiny fraction of a

square inch. Muscles, joints, and some organs have nociceptors, but the liver,

kidneys, and functional parts of the lungs have none. Once a nociceptor is

activated by some type of unpleasant

stimulus, it sends the pain message along a nerve fiber in the form of an

electrical impulse. Two kinds of nerve fibers carry pain signals. Each carries a

different type of signal and relays it at a different speed. A-delta fibers

carry the first sharp pain and transmit signals at about 40 miles per hour.

Slower and thinner C fibers carry the dull, throbbing pain that follows, sending

these signals along at only 3 miles per hour.

When the signal reaches the nerve ending, specialized chemicals known as

neurotransmitters are called into action. Different types of neurotransmitters

are involved in the transmission of pain signals. Certain neurotransmitters

dampen or block a pain signal from being sent on, while others convey the pain

signal to neighboring nerve cells (see Pain Signals).

The pain impulse is transmitted in this manner along nerve fibers and into the

spinal cord. The transfer point for pain information from the peripheral nerves

to the spinal cord is a dense array of nerve cells known collectively as the

dorsal horn. In some sense, this is the “thinking” part of the spinal cord. It’s

a network of nerves and nerve connections where incoming messages can be

accentuated, dampened, or blocked altogether.

Once through the dorsal horn, the pain signals journey over nerve tracks to

multiple regions of the brain. Some signals reach the part of the brain

responsible for spatial awareness, while others arrive in the limbic system,

where emotions arise. Still others travel to the hypothalamus, which controls

hormonal responses and such functions as sleep, body temperature, and appetite.

Because the brain simultaneously processes pain information in so many disparate

regions, human beings have an understandably complex and multilayered response

to painful stimuli.

Sometimes this signaling system goes awry. For example, scientists believe

that the cells of the dorsal horn can become overly stimulated — which can

heighten pain or increase its frequency. If nerve fibers continually barrage the

dorsal horn with pain signals, the nerve cells there can become more sensitive

and excitable. When a weak signal, or one that would not ordinarily be a pain

signal at all, comes in, the now jittery dorsal horn nerve cells overreact. What

would normally be experienced as mild pain is instead very painful

(hyperalgesia); in some cases, simply being touched becomes painful (allodynia).

Just as the forward motion of a row of tumbling dominos can be interrupted by

removing one tile, pain signals can be thwarted by disrupting the elaborate

electrochemical communication system of the nerve cells. This basic principle is

the foundation for nearly all pain treatment approaches. The three principal

ways of relieving pain can be boiled down to the

following:

blocking the ability of a nerve to carry pain signals by interfering with the

electrical impulses traveling through the nerve fiber

blocking the action of the neurotransmitters that relay pain signals between

nerves

enhancing the action of systems in the body that inhibit pain signals from

being passed on.

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