Chapter 7 Understanding Pain
congenital insensitivity - a very rare genetic disorder that prevents one from
experiencing any pain at all. While it may sound good, the consequences are
somatosensory system - The part of the nervous system that carries sensory
information from various parts of the body (e.g., from the skin and muscles) to
afferent neurons - are also called sensory neurons. They relay information from
the sense organs (e.g., touch) toward the brain.
efferent neurons - are also called motor neurons. They carry neural impulses
from the brain to the muscles.
primary afferents - Are specialized sensory neurons in the skin and other
places. They respond to physical energy (e.g., touch) and convert it to neural
signals that travel to the spinal cord and brain.
action potential - is the technical term for when a neuron "fires." In response to
sufficient amounts of neurotransmitters, or physical stimulation (for the primary
afferents), a signal travels down the neuron's axon and causes release of
nocioception - is the process of perceiving pain.
nociceptors - Sensory receptors in the skin and organs that
are capable of responding to various types of stimulation that may cause tissue
damage (heat, cold, cutting, or burning).
myelin - A fatty substance that acts as insulation for the axons of some neurons.
Neural impulses are transmitted much faster by neurons with myelinated axons.
fiber size - larger fibers (of larger neurons) transmit messages faster than
smaller fibers. A fiber is basically the same thing as an "axon."
A beta fibers - are both large and myelinated. They transmit information "100
times faster" than C fibers. They rapidly transmit sensory information.
A-delta fibers - are small sensory fibers that are involved in the experience of
“fast” pain that is "sharp" or "pricking." C fibers - are smaller and unmyelinated. Their slower transmission results in
sensations that are dull, aching, or lingering. About 60% of all sensory afferents
are "C fibers."
nerve - A group or bundle of fibers (axons) that travel together (like a large
electrical cable made up of hundreds of wires). Some nerves (afferent and
efferent) are referred to as "Tracts."
the cranial Nerves - Sensory information from the head and neck area travel to
the brain via "12 cranial nerves." Information from all other body areas travels to
the brain via the spinal cord.
spinal cord - carries afferent (sensory) information to the brain and efferent
(motor) information from the brain to the muscles (see Figure).
dorsal horns - The back part of the spinal cord (away from the stomach) that
receives sensory input (via the sensory tracts). There, the sensory fibers
connect to small "secondary afferents" which, in turn, connect to efferent (motor)
fibers that leave the ventral (front) side of the spinal cord. The messages also
travel up the spinal cord via other neurons to the brain.
transmission cells - "secondary afferents" and "interneurons" are all different
names for the same thing, cells that connect neurons to other neurons.
laminae - Are different layers of the dorsal horns which are composed of cell
substantia gelatinosa - Is made up of layers 1 and 2 of the dorsal horns and is
important in modulating sensory input.
thalamus - Structure in the forebrain that acts as a relay center for incoming
sensory information and outgoing motor information (from and to the spinal cord).
somatosensory cortex - The part of the brain that receives and processes
sensory input from the body.
primary somatosensory cortex - receives information from the thalamus that
allows the entire surface of the skin to be mapped onto the somatosensory
cortex. The more sensitive areas of the skin (face and lips) take up a much
larger part of the cortex relative to less sensitive parts like the back (see Figure). emotional and physical pain - May be all the same to the brain. Eisenberg et.
al. (2003) found that experimentally induced emotional pain (social rejection) had
the same activating effect on certain brain areas as physical pain.
endorphins - Along with the enkephalins and dynorphin are naturally occurring
neurochemicals whose pain lowering effects resemble those of the opiates.
glutamate and substance p - Are neurotransmitters that sensitize (or excite) the
neurons that relay pain messages.
periaqueductal gray - An area of the midbrain that, when stimulated, decreases
pain. It sends signals, via the medula, to the substantia gelatinosa in the spine's
dorsal horn. Endorphin release blocks pain transmission (see Figure)
medulla - The structure of the hindbrain just above the spinal cord. Sensory and
motor messages travel through it.
how pain is viewed - Prior to about 1900, pain was thought to be directly
proportional to the extent of tissue damage. C. A. Strong then proposed that the
experience of pain reflected BOTH sensory information AND the person's
interpretation of it.
the IASP definition