Neurophysiologic studies have demonstrated the existence of very sensitive, rapidly adapting mechano-receptive free nerve endings that elicit only the tickle and itch sensations. Furthermore, these endings are found almost exclusively in superficial layers of the skin, which is also the only tissue from which the tickle and itch sensations usually can be elicited. These sensations are transmitted by very small type C, unmyeli-nated fibers similar to those that transmit the aching, slow type of pain.
The purpose of the itch sensation is presumably to call attention to mild surface stimuli such as a flea crawling on the skin or a fly about to bite, and the elicited signals then activate the scratch reflex or other maneuvers that rid the host of the irritant. Itch can be relieved by scratching if this removes the irritant or if the scratch is strong enough to elicit pain. The pain signals are believed to suppress the itch signals in the cord by lateral inhibition, as described in Chapter 48.
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. However, from the entry point into the cord and then to the brain, the sensory signals are carried through one of two alternative sensory pathways: (1) the dorsal column-medial lemniscal system or (2) the anterolateral system. These two systems come back together partially at the level of the thalamus.
The dorsal column-medial 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 column-medial 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.
Another difference between the two systems is that the dorsal column-medial lemniscal system has a high degree of spatial orientation of the nerve fibers with respect to their origin, while the anterolateral system has much less spatial orientation. These differences immediately characterize the types of sensory information that can be transmitted by the two systems. That is, sensory information that must be transmitted rapidly and with temporal and spatial fidelity is transmitted mainly in the dorsal column-medial 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; most of these are discussed in detail in Chapter 48. The dorsal system is limited to discrete types of mechanoreceptive sensations.
With this differentiation in mind, we can now list the types of sensations transmitted in the two systems.
Dorsal Column-Medial Lemniscal System
1. Touch sensations requiring a high degree of localization of the stimulus
2. Touch sensations requiring transmission of fine gradations of intensity
3. Phasic 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
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
Transmission in the Dorsal Column-Medial Lemniscal System
Anatomy of the Dorsal Column-Medial Lemniscal System
On entering the spinal cord through the spinal nerve dorsal roots, the large myelinated fibers from the specialized mechanoreceptors divide almost immediately to form a medial branch and a lateral branch, shown by the right-hand fiber entering through the spinal root in Figure 47-2. The medial branch turns medially first and
Tract of Lissauer
Dorsal spinocerebellar tract
Ventral spinocerebellar tract
Ventral spinocerebellar tract
Anterolateral spinothalamic pathway
then upward in the dorsal column, proceeding by way of the dorsal column pathway all the way to the brain.
The lateral branch enters the dorsal horn of the cord gray matter, then divides many times to provide terminals that synapse with local neurons in the intermediate and anterior portions of the cord gray matter. These local neurons in turn serve three functions: (1) A major share of them give off fibers that enter the dorsal columns of the cord and then travel upward to the brain. (2) Many of the fibers are very short and terminate locally in the spinal cord gray matter to elicit local spinal cord reflexes, which are discussed in Chapter 54. (3) Others give rise to the spinocerebellar tracts, which we will discuss in Chapter 56 in relation to the function of the cerebellum.
The Dorsal Column-Medial Lemniscal Pathway. Note in Figure 47-3 that nerve fibers entering the dorsal columns pass uninterrupted up to the dorsal medulla, where they synapse in the dorsal column nuclei (the cuneate and gracile nuclei). From there, second-order neurons
Cross section of the spinal cord, showing the anatomy of the cord gray matter and of ascending sensory tracts in the white columns of the spinal cord.
The dorsal column-medial lemniscal pathway for transmitting critical types of tactile signals. (Modified from Ranson SW, Clark SL: Anatomy of the Nervous System. Philadelphia: WB Saunders Co, 1959.)
decussate immediately to the opposite side of the brain stem and continue upward through the medial lemnisci to the thalamus. In this pathway through the brain stem, each medial lemniscus is joined by additional fibers from the sensory nuclei of the trigeminal nerve; these fibers subserve the same sensory functions for the head that the dorsal column fibers subserve for the body.
In the thalamus, the medial lemniscal fibers terminate in the thalamic sensory relay area, called the ventrobasal complex. From the ventrobasal complex, third-order nerve fibers project, as shown in Figure 47-4, mainly to the postcentral gyrus of the cerebral cortex, which is called somatic sensory area I (as shown in Figure 47-6, these fibers also project to a smaller area in the lateral parietal cortex called somatic sensory area II).
Spatial Orientation of the Nerve Fibers in the Dorsal Column-Medial Lemniscal System
One of the distinguishing features of the dorsal column-medial lemniscal system is a distinct spatial orientation of nerve fibers from the individual parts of the body that is maintained throughout. For instance, in the dorsal columns of the spinal cord, the fibers from the lower parts of the body lie toward the center of the cord, whereas those that enter the cord at progressively higher segmental levels form successive layers laterally.
In the thalamus, distinct spatial orientation is still maintained, with the tail end of the body represented by the most lateral portions of the ventrobasal complex and the head and face represented by the
Projection of the dorsal column-medial lemniscal system through the thalamus to the somatosensory cortex. (Modified from Brodal A: Neurological Anatomy in Relation to Clinical Medicine. New York: Oxford University Press, 1969, by permission of Oxford University Press.)
medial areas of the complex. Because of the crossing of the medial lemnisci in the medulla, the left side of the body is represented in the right side of the thalamus, and the right side of the body in the left side of the thalamus.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.