1. Unit Nine: The Nervous System: A
Unit Nine: The Nervous System: A. General Principles and Sensory Physiology
Chapter 48: Somatic Sensations. II. Pain, Headache, and Thermal Sensations
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Pain is a Protective Mechanism
Most, if not all, ailments of the body cause pain
Pain occurs when tissues are being damaged,
and it causes the individual to react to remove
the stimulus

3. Types of Pain and Their Qualities
Fast Pain
Also called sharp pain, pricking pain, acute pain,
and electric pain
Felt within o.1 sec after the stimulus is applied
Not felt in most deeper tissues of the body

4. Types of Pain and Their Qualities
Slow Pain
Also called slow burning pain, aching pain,
throbbing pain, nauseous pain, and chronic pain
Felt only after one second or more and then
increases slowly over many seconds or even minutes
Usually associated with tissue destruction
Can lead to prolonged, almost unbearable suffering

5. Types of Pain and Their Qualities
Pain Receptors and Their Stimulation
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)
Types of Stimuli
Mechanical
Thermal
Chemical

6. Types of Pain and Their Qualities
Pain Receptors and Their Stimulation (cont.)
Fast pain is elicited by mechanical and thermal
Slow pain by all three types
Chemicals include: bradykinin, serotonin, K ions,
histamine, acids, AcH, proteolytic enzymes
Prostaglandins and substance P enhance the
sensitivity but do not directly excite them

7. 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
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
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.)

8. Types of Pain and Their Qualities
Fig Distribution curve showing the minimal skin temperature that
will cause pain

9. Types of Pain and Their Qualities
Importance of Chemical Pain Stimuli During
Tissue Damage
Bradykinin thought to be the agent most responsible
for causing pain following tissue damage
Intensity of pain felt correlates with local increase in
K ion concentration or increase in proteolytic
enzymes

10. Types of Pain and Their Qualities
Tissue Ischemia as a Cause of Pain
When blood flow is blocked, the tissue often
becomes very painful and the greater the rate of
metabolism, the more rapidly the pain occurs
May be due to the large accumulation of lactic
acid as well as bradykinin and proteolytic
enzymes

11. Types of Pain and Their Qualities
Muscle Spasm as a Cause of Pain
Results partially from the direct effects of muscle
spasm in stimulating mechanosensitive pain
receptors
Also the indirect effects of muscle spasm to compress
blood vessels and cause ischemia
Spasm increases the rate of metabolism making the
relative ischemia even greater

12. Dual Pathways for Transmission of Pain Signals
Peripheral Pain Fibers- Fast and Slow Fibers
Fig Transmission of both “fast-sharp”
and “slow-chronic” pain signals into
and through the spinal cord on their
way to the brain

13. Dual Pathways for Transmission of Pain Signals
Peripheral Pain Fibers- Fast and Slow Fibers
Fig 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

14. Dual Pathways for Transmission of Pain Signals
Dual Pain Pathways
Neospinalthalmic Tract
Tract for fast pain
Most terminate in the thalamus
Fast pain is localized more exactly
Glutamate is the primary neurotransmitter

15. Dual Pathways for Transmission of Pain Signals
Dual Pain Pathways
Paleospinothalamic Pathway
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

16. Pain Suppression Pain Suppression
Fig Analgesia system of the brain and spinal cord

17. Pain Suppression
Pain Suppression- analgesia system has three
components
Periaqueductal gray and periventricular areas of
mesencephalon and pons
Raphe magnus nucleus in the pons and the nucleus
reticularis in the medulla
Pain inhibitory complex in the dorsal horns of the
spinal cord

18. Pain Suppression
Transmitter Substances
Enkephalin-cause both pre- and post-synaptic
inhibition of incoming pain fibers
Serotonin-released from the dorsal horn and causes
the release of enkephalin

19. Pain Suppression
Brain’s Opiate System
Endorphins
Enkephalins

20. Referred Pain Mechanism
Fig Mechanism of referred pain and referred hyperalgesia

21. Thermal Sensations
Thermal Receptors and Their Excitation
Thermal gradations are perceived by 3 types of
receptors: cold, warm, and pain
Cold and warm are located at discrete spots
immediately under the skin

22. Stimulation of Thermal Receptors
Thermal Sensations
Stimulation of Thermal Receptors
Fig Discharge frequencies at different skin temperatures

23. Thermal Sensations
Adaptation of Thermal Receptors
Spatial Summation of Thermal Sensations
Transmission in the Nervous System
Thermal signals parallel pain signals
Most go to the reticular areas of the brain and
the thalamus