قراءة
عرض

Physiology

Lec.(3)
-----------------------------------------------------------------------
Sensory Pathways for Transmitting Somatic Signals into the Central Nervous System
Almost all sensory information from the somatic segments of the body enters the spinal cord through the dorsal roots of the spinal nerves. from the entry point into the cord and then to the brain, the sensory signals are carried through one of two sensory pathways:
1- dorsal columnmedial lemniscal system.
2-anterolateral system.
These two systems come back together partially at the level of the thalamus. The dorsal columnmedial lemniscal system, as its name implies, carries signals upward to the medulla of the brain mainly in the dorsal columns of the cord. Then, after the signals synapse and cross to the opposite side in the medulla, they continue upward through the brain stem to the thalamus by way of the medial lemniscus. Conversely, signals in the anterolateral system, immediately after entering the spinal cord from the dorsal spinal nerve roots, synapse in the dorsal horns of the spinal gray matter, then cross to the opposite side of the cord and ascend through the anterior
and lateral white columns of the cord. They terminate at all levels of the lower brain stem and in the thalamus. The dorsal columnmedial lemniscal system is composed of large, myelinated nerve fibers that transmit signals to the brain at velocities of 30 to 110 m/sec, whereas the anterolateral system is composed of smaller myelinated fibers that transmit signals at velocities ranging from a few meters per second up to 40 m/sec., sensory information that must be transmitted rapidly and with temporal and spatial fidelity is transmitted mainly in the dorsal columnmedial lemniscal system; that which does not need to be transmitted rapidly or with great spatial fidelity is transmitted mainly in the anterolateral system. The anterolateral system has a special capability that the dorsal system does not have: the ability to transmit a broad spectrum of sensory modalities pain, warmth, cold, and crude tactile sensations.

Dorsal ColumnMedial Lemniscal System transmit

1. Touch sensations requiring a high degree of
localization of the stimulus
2. Touch sensations requiring transmission of fine
gradations of intensity
3. Physic sensations, such as vibratory sensations
4. Sensations that signal movement against the skin
5. Position sensations from the joints
6. Pressure sensations having to do with fine degrees
of judgment of pressure intensity


Anterolateral System transmit
1. Pain
2. Thermal sensations, including both warmth and
cold sensations
3. Crude touch and pressure sensations capable only
of crude localizing ability on the surface of the
body
4. Tickle and itch sensations
5. Sexual sensations





Pain Is a Protective Mechanism

Pain occurs whenever any tissues are being damaged, and it causes the individual to react to remove the pain stimulus.

Types of Pain and Their QualitiesFast Pain and Slow Pain

Pain has been classified into two major types: fast pain and slow pain. Fast pain is felt within about 0.1 second after a pain stimulus is applied, whereas slow pain begins only after 1 second or more and then increases slowly over many seconds and sometimes even minutes. Fast pain is also described by many alternative names, such as sharp pain, pricking pain, acute pain, and electric pain. This type of pain is felt when a needle is stuck into the skin, when the skin is cut with a knife, or when the skin is acutely burned. It is also felt when the skin is subjected to electric shock. Fast-sharp pain is not felt in most deeper tissues of the body. Slow pain also goes by many names, such as slow burning pain, aching pain, throbbing pain, nauseous pain, and chronic pain. This type of pain is usually associated with tissue destruction. It can lead to prolonged, unbearable suffering. It can occur both in the skin and in almost any deep tissue or organ.

Pain Receptors Are Free Nerve Endings. The pain receptors in the skin and other tissues are all free nerve endings. They are widespread in the superficial layers of the skin as well as in certain internal tissues, such as the periosteum, the arterial walls, the joint surfaces, and the falx and tentorium in the cranial vault. Most other deep tissues are only sparsely supplied with pain endings.


Transmission of Pain Signals into the Central Nervous System Peripheral Pain FibersFast and Slow Fibers.
The fast sharp pain signals are elicited by either mechanical or thermal pain stimuli; they are transmitted in the peripheral nerves to the spinal cord by small type A delta fibers at velocities between 6 and 30 m/sec. Conversely, the slow-chronic type of pain is elicited mostly by chemical types of pain stimuli but sometimes by persisting mechanical or thermal stimuli. This slow chronic pain is transmitted to the spinal cord by type C fibers at velocities between 0.5 and 2 m/sec. Because of this double system of pain innervations, a sudden painful stimulus often gives a double pain sensation: a fast-sharp pain that is transmitted to the brain by the A delta fiber pathway, followed a second or so later by a slow pain that is transmitted by the C fiber pathway. The sharp pain apprises the person rapidly of a damaging influence and, therefore, plays an important role in making the person react immediately to
remove himself or herself from the stimulus. The slow pain tends to become greater over time.. On entering the spinal cord from the dorsal spinal roots, the pain fibers terminate on relay neurons in the dorsal horns.

Dual Pain Pathways in the Cord and Brain StemThe Neospinothalamic Tract and the Paleospinothalamic Tract
On entering the spinal cord, the pain signals take two pathways to the brain, through
1-the neospinothalamic tract
2- the paleospinothalamic tract.

Neospinothalamic Tract for Fast Pain

The fast type A delta pain fibers transmit mainly mechanical and acute thermal pain. They terminate mainly in lamina I (lamina marginalis) of the dorsal horns, as shown in Figure and they excite second-order neurons of the neospinothalamic tract. These give rise to long fibers that cross immediately to the opposite side of the cord through the anterior commissure and then turn upward, passing to the brain in the anterolateral columns.

 SHAPE \* MERGEFORMAT 



Termination of the Neospinothalamic Tract in the Brain Stem and Thalamus. Paleospinothalamic Pathway for Transmitting Slow-Chronic Pain.
The paleospinothalamic pathway transmits pain mainly from the peripheral slow-chronic type C pain fibers, although it does transmit some signals from type Adelta fibers as well. In this pathway, the peripheral fibers terminate in the spinal cord almost entirely in laminae II and III of the dorsal horns, which together are called the substantia gelatinosa, as shown in Figure Most of the signals then pass through one or more additional short fiber neurons within the dorsal horns themselves before entering mainly lamina V, also in the dorsal horn. Here the last neurons in the series give rise to long axons that mostly join the fibers from the fast pain pathway, passing first through the anterior commissure to the opposite side of the cord, then upward to the brain in the anterolateral pathway

Substance P, the Probable Slow-Chronic Neurotransmitter of Type C Nerve Endings.

Research experiments suggest that type C pain fiber terminals entering the spinal cord secrete both glutamate transmitter and substance P transmitter. The glutamate transmitter acts instantaneously and lasts for only a few milliseconds. Substance P is released much more slowly, building up in concentration over a period of seconds or even minutes. In fact, it has been suggested that the double pain sensation one feels after a pinprick might result partly from the fact that the glutamate transmitter gives a faster pain sensation, whereas the substance P transmitter gives a more lagging sensation. , it seems clear that glutamate is the neurotransmitter most involved in transmitting fast pain into the central nervous system and substance p-is concerned with slow-chronic pain


Referred Pain
Often a person feels pain in a part of the body that is fairly remote from the tissue causing the pain. This is called referred pain. For instance, pain in one of the visceral organs often is referred to an area on the body surface. The best example is referred of cardiac pain to the inner aspect of the left arm.other example include pain in the tip of the shoulder caused by irritation of the central portion of diaphragm and pain in the testicle due to distension of ureter

Dermatomal rule

When pain is referred,it is usually to astructure that developed from the same embryonic segment or dermatome as the structure in which the pain originates this principle is called the dermatomal rule for example the diaphgragm during embryonic development it migrates from the neck region to its adult location between the chest and abdomen and takes its nerve supply

Mechanism of Referred Pain.

Figure shows the probable mechanism by which most pain is referred. In the figure, branches of visceral pain fibers are shown to synapse in the spinal cord on the same second-order neurons (1 and 2) that receive pain signals from the skin.When the visceral pain fibers are stimulated, pain signals from the viscera are conducted through at least some of the same neurons that conduct pain signals from the skin, and the person has the feeling that the sensations originate in the skin itself.












PAGE 


PAGE 1




رفعت المحاضرة من قبل: Sultan Alsaffar
المشاهدات: لقد قام 9 أعضاء و 216 زائراً بقراءة هذه المحاضرة








تسجيل دخول

أو
عبر الحساب الاعتيادي
الرجاء كتابة البريد الالكتروني بشكل صحيح
الرجاء كتابة كلمة المرور
لست عضواً في موقع محاضراتي؟
اضغط هنا للتسجيل