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

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

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

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

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

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

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.)

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 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

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

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

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 Neospinalthalmic Tract 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 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

Pain Suppression Pain Suppression Fig. 48.4 Analgesia system of the brain and spinal cord

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

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

Pain Suppression Brain’s Opiate System Endorphins Enkephalins

Referred Pain Mechanism Fig. 48.5 Mechanism of referred pain and referred hyperalgesia

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

Stimulation of Thermal Receptors 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 Thermal signals parallel pain signals Most go to the reticular areas of the brain and the thalamus