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

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 removethe 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 thenincreases 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; widespreadin the superficial layers of the skin and certaininternal tissues (periosteum, arterial walls, jointsurfaces, falx and tentorium in the cranial vault)

b. Types of Stimuli

1. Mechanical2. Thermal3. Chemical

Types of Pain and Their Qualities

• Pain Receptors and Their Stimulation (cont.)

c. Fast pain is elicited by mechanical and thermald. Slow pain by all three typese. Chemicals include: bradykinin, serotonin, K ions,

histamine, acids, AcH, proteolytic enzymesf. 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 verylittle, and sometimes not at all

• Rate of Tissue Damage as a Stimulus for Pain

a. Pain resulting from heat is closely correlated withthe rate at which damage to the tissues is occurringand not with the total damage that has alreadyoccurred

b. Intensity of pain is also closely correlated with therate of tissue damage from causes other than heat(bacterial infection, tissue ischemia, tissuecontusion, 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 DuringTissue Damage

a. Bradykinin thought to be the agent most responsiblefor causing pain following tissue damage

b. Intensity of pain felt correlates with local increase inK 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 ofmetabolism, the more rapidly the pain occurs

b. May be due to the large accumulation of lacticacid 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 musclespasm in stimulating mechanosensitive painreceptors

b. Also the indirect effects of muscle spasm to compressblood vessels and cause ischemia

c. Spasm increases the rate of metabolism making therelative 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. Tract for fast pain2. Most terminate in the thalamus3. Fast pain is localized more exactly4. Glutamate is the primary neurotransmitter

Dual Pathways for Transmission of Pain Signals

• Dual Pain Pathways

b. Paleospinothalamic Pathway

1. Transmission of slow-chronic pain2. Substance P is the main neurotransmitter3. Projects into the brain stem and thalamus4. 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 threecomponents

a. Periaqueductal gray and periventricular areas ofmesencephalon and pons

b. Raphe magnus nucleus in the pons and the nucleusreticularis in the medulla

c. Pain inhibitory complex in the dorsal horns of thespinal 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 causesthe 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 signalsb. Most go to the reticular areas of the brain and

the thalamus