Chapter 48: Somatic Sensations. II.
Pain, Headache, and Thermal
Sensations
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Pain is a Protective Mechanism
• Pain
a. Most, if not all, ailments of the body cause pain
b. Pain occurs when tissues are being damaged,
and it causes the individual to react to remove
the stimulus
Types of Pain and Their Qualities
• Fast Pain
a. Also called sharp pain, pricking pain, acute pain,
and electric pain
b. Felt within o.1 sec after the stimulus is applied
c. Not felt in most deeper tissues of the body
Types of Pain and Their Qualities
• Slow Pain
a. Also called slow burning pain, aching pain,
throbbing pain, nauseous pain, and chronic pain
b. Felt only after one second or more and then
increases slowly over many seconds or even minutes
c. Usually associated with tissue destruction
d. Can lead to prolonged, almost unbearable suffering
Types of Pain and Their Qualities
• Pain Receptors and Their Stimulation
a. Pain receptors are free nerve endings; widespread
in the superficial layers of the skin and certain
internal tissues (periosteum, arterial walls, joint
surfaces, falx and tentorium in the cranial vault)
b. Types of Stimuli
1. Mechanical
2. Thermal
3. Chemical
Types of Pain and Their Qualities
• Pain Receptors and Their Stimulation (cont.)
c. Fast pain is elicited by mechanical and thermal
d. Slow pain by all three types
e. Chemicals include: bradykinin, serotonin, K ions,
histamine, acids, AcH, proteolytic enzymes
f. Prostaglandins and substance P enhance the
sensitivity but do not directly excite them
Types of Pain and Their Qualities
• Non-Adapting Nature of Pain Receptors- adapt very
little, and sometimes not at all
• Rate of Tissue Damage as a Stimulus for Pain
a. Pain resulting from heat is closely correlated with
the rate at which damage to the tissues is occurring
and not with the total damage that has already
occurred
b. Intensity of pain is also closely correlated with the
rate of tissue damage from causes other than heat
(bacterial infection, tissue ischemia, tissue
contusion, etc.)
Types of Pain and Their Qualities
Fig. 48.1 Distribution curve showing the minimal skin temperature that
will cause pain
Types of Pain and Their Qualities
• Importance of Chemical Pain Stimuli During
Tissue Damage
a. Bradykinin thought to be the agent most responsible
for causing pain following tissue damage
b. Intensity of pain felt correlates with local increase in
K ion concentration or increase in proteolytic
enzymes
Types of Pain and Their Qualities
• Tissue Ischemia as a Cause of Pain
a. When blood flow is blocked, the tissue often
becomes very painful and the greater the rate of
metabolism, the more rapidly the pain occurs
b. May be due to the large accumulation of lactic
acid as well as bradykinin and proteolytic
enzymes
Types of Pain and Their Qualities
• Muscle Spasm as a Cause of Pain
a. Results partially from the direct effects of muscle
spasm in stimulating mechanosensitive pain
receptors
b. Also the indirect effects of muscle spasm to compress
blood vessels and cause ischemia
c. Spasm increases the rate of metabolism making the
relative ischemia even greater
Dual Pathways for Transmission of Pain Signals
• Peripheral Pain Fibers- Fast and Slow Fibers
Fig. 48.2 Transmission of both “fast-sharp”
and “slow-chronic” pain signals into
and through the spinal cord on their
way to the brain
Dual Pathways for Transmission of Pain Signals
• Peripheral Pain Fibers- Fast and Slow Fibers
Fig. 48.3 Transmission of pain signals into the brain
stem, thalamus, and cerebral cortex by way
of the fast pricking pain pathway and the
slow burning pain pathway
Dual Pathways for Transmission of Pain Signals
• Dual Pain Pathways
a. Neospinalthalmic Tract
1.
2.
3.
4.
Tract for fast pain
Most terminate in the thalamus
Fast pain is localized more exactly
Glutamate is the primary neurotransmitter
Dual Pathways for Transmission of Pain Signals
• Dual Pain Pathways
b. Paleospinothalamic Pathway
1.
2.
3.
4.
Transmission of slow-chronic pain
Substance P is the main neurotransmitter
Projects into the brain stem and thalamus
Localized only to a major part of the body
Pain Suppression
• Pain Suppression
Fig. 48.4 Analgesia system of the brain and spinal cord
Pain Suppression
• Pain Suppression- analgesia system has three
components
a. Periaqueductal gray and periventricular areas of
mesencephalon and pons
b. Raphe magnus nucleus in the pons and the nucleus
reticularis in the medulla
c. Pain inhibitory complex in the dorsal horns of the
spinal cord
Pain Suppression
• Transmitter Substances
a. Enkephalin-cause both pre- and post-synaptic
inhibition of incoming pain fibers
b. Serotonin-released from the dorsal horn and causes
the release of enkephalin
Pain Suppression
• Brain’s Opiate System
a. Endorphins
b. Enkephalins
Referred Pain
• Mechanism
Fig. 48.5 Mechanism of referred pain and referred hyperalgesia
Thermal Sensations
• Thermal Receptors and Their Excitation
a. Thermal gradations are perceived by 3 types of
receptors: cold, warm, and pain
b. Cold and warm are located at discrete spots
immediately under the skin
Thermal Sensations
• Stimulation of Thermal Receptors
Fig. 48.10 Discharge frequencies at different skin temperatures
Thermal Sensations
• Adaptation of Thermal Receptors
• Spatial Summation of Thermal Sensations
• Transmission in the Nervous System
a. Thermal signals parallel pain signals
b. Most go to the reticular areas of the brain and
the thalamus