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Lecture 5

PSYC 302 Lecture Notes - Lecture 5: Brainstem, Substance P, Grey Matter

Course Code
PSYC 302
Jeffrey Mogil

of 11
Pain Anatomy and Physiology—Lecture 5 11/23/2014
Descartes was the first one to think of pain
Pain Anatomy
There are components involved in pain processing at the following levels
Pain relevant loci (for pain below the neck)
Periphery: Pain on the skin (or muscle, joint, viscera(internal organs))
This pain travels up the dorsal root ganglion (DRG), then the dorsal horn of the spinal cord, then the brain
Dorsal: information going up to the brain
Ventral: information going down—motor response from the brain
Ascending pathways (the “pain matrix”)—lead to perception of the pain.
Ventrobasal thalamus
S1 and S2 (somatosensory cortex 1 and 2)
Limbic cortex
Descending Pathways (besides motor responses)
Spinal cord (counts as part of the central nervous system)
Why do we have descending pathways? That’s just the way evolution developed it. It would make more
sense for things to modulate upwards, but that’s just not how we developed
Central nervous system: brain and spinal cord
Peripheral nervous system: nerves
Epidermis, then dermis, then muscle. Our skin is epidermis and dermis
Skin nerves are free—don’t have any structures around them. Just peters out to a tip
Glabrous skin: skin without hair—palms of hands and soles of feet
Same in hairy skin except there are hair follicles.
Nociceptor: free nerve endings, don’t have enlargement at the end
Can take a chunk of your skin and count how many nociceptors you have through staining
3 types of neurons
Multipolar neurons: has a cell body with many dendrites and one main axon. The axon has terminals
at the end. This is the most standard neuron
Bipolar neurons: found in the ear and eye. The cell body is in the middle and there’s a dendrite that
goes one way and the axon that goes the other way. Both branch out at the end
Unipolar neurons: take sensory info and send it up to the brain. Similar to a bipolar neuron, but the
cell body is off to the side of the one long axon. At one end it branches out and at the other end it branches
out—one end goes into the skin, one goes into the central nervous system/spinal cord. Cell body sits on the
dorsal root ganglia
Why do neuropathic pain syndromes show a “glove-and-stocking” distribution?
It starts in your toes/finger tips and goes just past your ankles or just past your wrists
The cells that go all the way down to your toes are a lot longer than the ones that go to your thigh or
shoulder or chest. Toe neurons are the longest ones you have in your body. A lot of neuropathic pain
disorders are because neurons are dying—longest ones are most fragile because they’re so big and require
so much work, so they die first
You can infer that the pain is being caused by the death of nociceptors
Primary Sensory Afferent Neurons
Nociceptors are one of these
Affterent: means “going toward” the brain
Primary because they’re the first in a chain of neurons going up to the brain
A alpha, A beta, A delta and C—difference in thickness/diameter
Outside of them is myelin, and red core is the actual neuron itself. C’s are un-myelinated (have no myelin
around them)
An action potential’s speed depends on the cell’s diameter (the bigger it is, the faster things can go) and the
amount of myelin (axons that have myelin use a different way of propagating action potentials)
If two fibers are carrying pain information, then one is going to reach the brain first—this is what produces a
A alpha=muscle control
A beta=touch and vibration
A delta=thermal and pain
C=pain and sweating
Nerves are arranged in bundles to make sure things receive blood and stay alive. There’s too many of them
to all run alone
Eventually, the individual axons covering a wide part of the body bundle together and enter the spinal cord
as a unit (for efficiency)
Spinal Cord Anatomy
Dorsal: towards the back (used for rest of body)
Ventral: towards the stomach
Rostral: toward the head (used for brain)
Caudal: toward the tail (used for brain)
Anterior: straight forward
Posterior: straight back
Dermitome: show the bunching pattern of the nerve that enter or exit the spine at the same level
Parts of the body that are served by the nerves that enter the spine at a certain area. (T11 nerves all enter
the spine at Thoracic 11, and they serve parts of the body around there)