Reasons for thinking so, according to von Frey:

The experiment consisted ofsticking a pig bristle against the eye.

... and a warm and cold bristle elicited thesame response -- pain, rather than asense of warmth or cold.

Even with fairly light contact,the bristle was painful ...

It contains several other kinds of endings, and when properly tested, it can sense light touch, vibration and temperature.

Also, it is now known that the ear lobecontains only free nerve endings, but itcan sense all submodalities of skin sensation.

However, Iggo (mid ‘30s) found that about 30% of small, unmyelinated axons began firing at the point where thestimulus was judged to be painful. This is the kind ofaxon which generally innervates the skin as a free nerve ending.

Also, after WWII, Waddell studied pain syndromes after battlefield injuries.

lowered threshold to paine.g. can trigger with light touch

pain spreads to other skin areas(complex regional pain syndrome)

denervated dermatome

normal dermatome

Classical view -- through the ventrolateral sector of thespinal cord on the contralateral side, or the ipsilateral, or both.

mid ‘60s

But Melzack and Wall formulated a new concept ofthe method for pain transmission from the spinal cord.

It is common for amputees,such as war veterans, to suffersevere pain.

Note motor effectsNote motor effects

Even transecting the entire spinal cord did not provide permanent relief from pain.

Melzack and Wall argued that a new concept was needed for how pain could be transmitted to the brain.Melzack and Wall argued that a new concept was needed for how pain could be transmitted to the brain.

The theory which they offered was known as the

They argued that pain and other skin sensationsinteract at a common junction, and then are sentby various routes to higher brain areas.

SG T

excitatoryinhibitory

to brain

A fiber

c fiber

There are various ways of diagramming their concept, so don’t worry about details.

SG: substantia gelatinosaT: transmission cell

The basic concept is that a pain message will cause ahigh firing rate in a common transmission pathway.

SG T

to brain

Pain

to brain

SG T

SG T

Their theory spurred a lot of research, and pain gates were found to exist at at leasttwo locations:

Electrodes implanted into these sites (in humans)is now being used to treat chronic pain.Electrodes implanted into these sites (in humans)is now being used to treat chronic pain.

Patients with frontal lobotomyoften are less sensitive topain, i.e. it doesn’t bother them.

The intralaminar nucleiconnect to the frontal lobes.

Just a few words about ...

-- short chain polypeptides-- used as neural transmitters

to gate pain at various sites

-- protein, released by the pituitary gland and at synapses

-- carried by blood to manypain-relay sites (likelymimicking enkephalins)

to brain

substance P

enkephalin

pain gate

pain message

Ch. 48 Pain Sensations (Reading Homework)• Purpose of pain: protection of body• Types of pain: fast pain and slow pain• Pain receptors are free nerve endings• Three types of stimuli excite pain receptors:

mechanical, thermal, and chemical• Pain receptors are non-adapting• Fig. 48-3: pain pathways• Pain suppression (“analgesia”) system in the brain and

spinal cord– Analgesia system: Fig. 48-4– Inhibition of pain signals– Enkephalin-secreting neurons– Morphine– Opiates: morphine-like agents– Typical opiate substances: endorphine &

enkephalin