Nervous System: B3M1 Cases 5-6 (PDF)

NERVOUS SYSTEM SYSTEM COORDINATOR: DR. CECILLE ESPINO BLOCK 3 MODULE 1 (CASES 5 AND 6) BASIC BIOMEDICAL SCIENCES I NOT FOR SALE | DO NOT UPLOAD IN ONLINE SITES...

NERVOUS SYSTEM SYSTEM COORDINATOR: DR. CECILLE ESPINO BLOCK 3 MODULE 1 (CASES 5 AND 6) BASIC BIOMEDICAL SCIENCES I NOT FOR SALE | DO NOT UPLOAD IN ONLINE SITES ------------------------------------------------------------------------------------------------------------------------------------------------------- CASE 5 process of a primary afferent neuron. This leads to a depolarizing SOMATIC SENSATIONS graded membrane potential across the membrane of the neuron. Classification of somatic senses  If this potential depolarizes the trigger zone, located at the first o When you reach into your pocket to determine the types of coins myelin segment of the axon, to threshold, an action potential is present, you are gathering information through the activation of produced. specialized receptors of the somatosensory system.  In most receptors, transduction occurs between the  Specifically, the size of a coin is determined by noting the joint mechanoreceptor and the subjacent primary afferent membrane. angles when the coin is held between the forefinger and thumb. However, in some cases (i.e., Merkel cells), the nonneural cells of  “Heads and tails” may be identified with the use of slowly adapting the receptor complex may influence their associated primary receptors sensitive to stimuli that indent the skin. afferent axon by vesicular release of neurotransmitters or  Dimes can be distinguished from pennies by stroking their edges neuromodulators (e.g., glutamate, serotonin (5-hydroxytryptamine with the fingertips and activating rapidly adapting receptors. [5-HT], substance P, vasoactive polypeptide [VIP]). Upon release  This information is transmitted to the cerebral cortex by a from the Merkel cell, these substances bind to specialized multisynaptic pathway called the posterior column-medial membrane receptor complexes and can alter sensory lemniscal system. transmission. o In general, the somatosensory system transmits and analyzes touch  Each morphologic type of mechanoreceptor responds to different or tactile information from external and internal locations on the body tactile stimuli. Cutaneous tactile receptors are located in the basal and head. epidermis and dermis of glabrous (palms, soles, lips) and hairy skin.  The result of these processes leads to the appreciation of somatic sensations, which can be subdivided into the submodalities discriminative touch, flutter-vibration, proprioception (position sense), crude (nondiscriminative) touch, thermal (hot and cold) sensation, and nociception (tissue damage).  The following anatomically and functionally discrete pathways transmit these signals:  The posterior column-medial lemniscal pathway  The trigeminothalamic pathways  The spinocerebellar pathways  The anterolateral system The different receptors o Types of sensory receptors and the stimuli they detect  Mechanoreceptors- detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor  Thermoreceptors- detect changes in temperature, with some receptors detecting cold and others detecting warmth  Nociceptors (pain receptors)- detect physical or chemical damage occurring in the tissues  Free nerve endings- found everywhere in the skin and in many  Although touch, pressure, and vibration are frequently classified other tissues, can detect touch and pressure. as separate sensations, they are all detected by the same types  Most endings detect pain of receptors. There are three principal differences among them:  For example, even light contact with the cornea of the eye, which  Touch sensation generally results from stimulation of tactile contains no other type of nerve ending besides free nerve receptors in the skin or in tissues immediately beneath skin; endings, can nevertheless elicit touch and pressure sensations  Pressure sensation generally results from deformation of  Meissner’s corpuscles- elongated encapsulated nerve ending deeper tissues; that excite a large (type Aβ) myelinated sensory nerve fiber.  Vibration sensation results from rapidly repetitive sensory  A touch receptor with great sensitivity signals.  Inside the capsulation are many branching terminal nerve  Electromagnetic receptors- detect light on the retina of the eye filaments. These corpuscles are present in the non-hairy parts  Chemoreceptors- detect taste in the mouth, smell in the nose, of the skin and are particularly abundant in the fingertips, lips, oxygen level in the arterial blood, osmolality of the body fluids, and other areas of the skin where a person’s ability to discern carbon dioxide concentration, and other factors that make up the spatial locations of touch sensations is highly developed. chemistry of the body  They adapt in a fraction of a second after they are stimulated, which means that they are particularly sensitive to movement of objects over the surface of the skin, as well as to low-frequency vibration.  Merkel’s discs- expanded tip tactile receptors found in the fingertips and other areas where there are large numbers of Meissner’s corpuscles.  The hairy parts of the skin also contain moderate numbers of expanded tip receptors, even though they have almost no Meissner’s corpuscles.  These receptors differ from Meissner’s corpuscles in that they transmit an initially strong but partially adapting signal and then a continuing weaker signal that adapts only slowly. Therefore, they are responsible for giving steady-state signals that allow one to determine continuous touch of objects against the skin.  Merkel discs are often grouped together in a receptor organ o Tactile Receptors or Peripheral Mechanoreceptors called touch domes, which project upward against the  The first step in evoking somatic sensations of touch is the activation underside of the epithelium of the skin. of peripheral mechanoreceptors. Mechanical pressure, such as skin deformation, is transduced into an electrical signal in the peripheral   This upward projection causes the epithelium at this point to  Finally, pain receptors in the skin are almost never stimulated by protrude outward, thus creating a dome and constituting an usual touch or pressure stimuli but do become highly active the extremely sensitive receptor. moment tactile stimuli become severe enough to damage the tissues.  The entire group of Merkel’s discs is innervated by a single Mechanisms of Receptor Potentials large myelinated nerve fiber (type Aβ). These receptors, along o All sensory receptors have one feature in common. Whatever the type with the Meissner’s corpuscles, play extremely important roles of stimulus that excites the receptor, its immediate effect is to change in localizing touch sensations to specific surface areas of the the membrane electrical potential of the receptor. This change in body and in determining the texture of what is felt. potential is called a receptor potential.  Hair-end organ- a nerve in each hair and its basal nerve fiber and o Different receptors can be excited in one of several ways to cause is stimulated by slight movement of any hair on the body receptor potentials:  A receptor adapts readily and, like Meissner’s corpuscles, 1. By mechanical deformation of the receptor, which stretches the detects mainly the following: receptor membrane and opens ion channels; 1. Movement of objects on the surface of the body 2. By application of a chemical to the membrane, which also opens 2. Initial contact with the body ion channels;  Ruffini’s end organ- located in the deeper layers of the skin and 3. By change of the temperature of the membrane, which alters the also in still deeper internal tissues permeability of the membrane;  These endings adapt very slowly and therefore, are important 4. By the effects of electromagnetic radiation, such as light on a for signaling continuous states of deformation of the tissues, retinal visual receptor, which either directly or indirectly changes the such as heavy prolonged touch and pressure signals. receptor membrane characteristics and allows ions to flow through  They are also found in joint capsules and help to signal the membrane channels. degree of joint rotation. o These four means of exciting receptors correspond in general to the  Pacinian corpuscles- lies immediately beneath the skin and deep different types of known sensory receptors. In all cases, the basic in the facial tissues of the body. cause of the change in membrane potential is a change in membrane  They are stimulated only by rapid movement of the tissues permeability of the receptor, which allows ions to diffuse more or less because they adapt in a few hundredths of a second. readily through the membrane and thereby to change the  Therefore, they are particularly important for detecting tissue transmembrane potential. vibration or other rapid changes in the mechanical state of the o Maximum Receptor Potential Amplitude tissues.  The maximum amplitude of most sensory receptor potentials is  Thermal receptors- cold and warm receptors are located about 100 mV, but this level occurs only at an extremely high immediately under the skin at discrete but separated spots, each intensity of sensory stimulus. having a stimulatory area of about 1 sq mm. They are presumed  This is about the same maximum voltage recorded in action to be free nerve ending potentials and is also the change in voltage when the membrane  These low-threshold mechanoreceptors may be encapsulated becomes maximally permeable to sodium ions. (Meissner, Pacinian, and Ruffini corpuscles) or unencapsulated o Relation of the Receptor Potential to Action Potentials (Merkel cell-neurite complexes, commonly referred to as Merkel  When the receptor potential rises above the threshold for eliciting cells, and hair follicle receptors) action potentials in the nerve fiber attached to the receptor, then o Transmission of Tactile Signals in Peripheral Nerve Fibers action potentials occur.  Almost all specialized sensory receptors, such as Meissner’s  Note that the more the receptor potential rises above the threshold corpuscles, Iggo dome receptors, hair receptors, Pacinian level, the greater becomes the action potential frequency. corpuscles, and Ruffini’s endings, transmit their signals in type Aβ Adaptation of Receptors nerve fibers that have transmission velocities ranging from 30 to 70 o Another characteristic of sensory receptors is that they adapt either m/sec. partially or completely to any constant stimulus after a period of time.  Conversely, free nerve ending tactile receptors transmit signals  That is, when a continuous sensory stimulus is applied, the receptor mainly via the small type Aδ myelinated fibers that conduct at responds at a high impulse rate at first and then at a progressively velocities of only 5 to 30 m/sec. slower rate until, finally, the rate of action potentials decreases to  Some tactile free nerve endings transmit via type C unmyelinated very few or to none at all. fibers at velocities from a fraction of a meter up to 2 m/sec.  Note that the Pacinian corpuscle adapts very rapidly, hair receptors  These nerve endings send signals into the spinal cord and lower adapt within a second or so, and some joint capsule and muscle brain stem, probably subserving mainly the sensation of tickle. spindle receptors adapt slowly.  Thus, the more critical types of sensory signals- those that help to determine precise localization on the skin, minute gradations of intensity, or rapid changes in sensory signal intensity- are all transmitted in more rapidly conducting types of sensory nerve fibers.  Conversely, the cruder types of signals, such as pressure, poorly localized touch, and especially tickle, are transmitted via much slower, very small nerve fibers that require much less space in the peripheral nerve bundle than the fast fibers. o Furthermore, some sensory receptors adapt to a far greater extent Differential Sensitivity of Receptors than others. o How do two types of sensory receptors detect different types of  For example, the Pacinian corpuscles adapt to “extinction” within a sensory stimuli? The answer is “by differential sensitivities.” few hundredths of a second, and the receptors at the bases of the  That is, each type of receptor is highly sensitive to one type of hairs adapt to extinction within a second or more. stimulus for which it is designed and yet is almost nonresponsive to  It is probable that most mechanoreceptors eventually adapt almost other types of sensory stimuli. completely, but some require hours or days to do so, and they are  Thus, the rods and cones of the eyes are highly responsive to light called “non-adapting” receptors. but are almost completely nonresponsive to normal ranges of heat,  The longest measured time for almost complete adaptation of a cold, pressure on the eyeballs, or chemical changes in the blood. mechanoreceptor is about 2 days, which is the adaptation time for  The osmoreceptors of the supraoptic nuclei in the hypothalamus many carotid and aortic baroreceptors; however, some detect minute changes in the osmolality of the body fluids but have physiologists believe that these specialized baroreceptors never never been known to respond to sound. fully adapt.  Some of the non-mechanoreceptors (the chemoreceptors and pain receptors) never adapt completely. o Mechanisms by Which Receptors Adapt  For example, when a person is running, information from the  The mechanism of receptor adaptation is different for each type of joint rate receptors allows the nervous system to predict where receptor in much the same way that development of a receptor the feet will be during any precise fraction of the next second. potential is an individual property.  Therefore, appropriate motor signals can be transmitted to the  In the case of the mechanoreceptors, the receptor that has been muscles of the legs to make any necessary anticipatory studied in greatest detail is the Pacinian corpuscle. Adaptation corrections in position so that the person will not fall. occurs in this receptor in two ways.  Loss of this predictive function makes it impossible for the  First, the Pacinian corpuscle is a viscoelastic structure, so that person to run. when a distorting force is suddenly applied to one side of the Mechanisms of Transmission of Signals of Different Intensity in corpuscle, this force is instantly transmitted by the viscous Nerve Tracts component of the corpuscle directly to the same side of the central o One of the characteristics of each signal that always must be nerve fiber, thus eliciting a receptor potential. conveyed is signal intensity, for example, the intensity of pain. The  However, within a few hundredths of a second, the fluid within the different gradations of intensity can be transmitted either by using corpuscle redistributes, and the receptor potential is no longer increasing numbers of parallel fibers or by sending more action elicited. Thus, the receptor potential appears at the onset of potentials along a single fiber. These 2 mechanisms are called, compression but disappears within a small fraction of a second, respectively, spatial summation and temporal summation. even though the compression continues. o Spatial Summation  The second, much slower mechanism of adaptation of the Pacinian  The phenomenon of spatial summation is that increasing signal corpuscle results from a process called accommodation, which strength is transmitted by using progressively greater numbers of occurs in the nerve fiber itself. fibers. Each of these fibers arborizes into hundreds of minute free  That is, even if by chance the central core fiber should continue to nerve endings that serve as pain receptors. be distorted, the tip of the nerve fiber gradually becomes  The entire cluster of fibers from one pain fiber frequently covers an accommodated to the stimulus. area of skin as large as 5 centimeters in diameter. This area is called  This probably results from progressive “inactivation” of the sodium the receptor field of that fiber. channels in the nerve fiber membrane, which means that sodium  The number of endings is large in the center of the field but current flow through the channels causes them to close gradually, diminishes toward the periphery. One can also see from an effect that seems to occur for all or most cell membrane sodium the figure that the arborizing fibrils overlap those from other pain channels. fibers.  Presumably, these same two general mechanisms of adaptation  Therefore, a pinprick of the skin usually stimulates endings from also apply to the other types of mechanoreceptors. That is, part of many different pain fibers simultaneously. When the pinprick is in the adaptation results from readjustments in the structure of the the center of the receptive field of a particular pain fiber, the receptor, and part results from an electrical type of accommodation degree of stimulation of that fiber is far greater than when it is in in the terminal nerve fibril. the periphery of the field because of the greater number of free o Slowly Adapting Receptors Detect Continuous Stimulus nerve endings in the middle of the field. Strength- the Tonic Receptors  Slowly adapting receptors continue to transmit impulses to the brain as long as the stimulus is present (or at least for many minutes or hours). Therefore, they keep the brain constantly apprised of the status of the body and its relation to its surroundings.  For example, impulses from the muscle spindles and Golgi tendon apparatuses allow the nervous system to know the status of muscle contraction and load on the muscle tendon at each instant.  Other slowly adapting receptors include the following: 1. Receptors of the macula in the vestibular apparatus; 2. Pain receptors; 3. Baroreceptors of the arterial tree; and 4. Chemoreceptors of the carotid and aortic bodies.  Because the slowly adapting receptors can continue to transmit  Thus, the lower part of Figure 47-7 shows three views of the cross information for many hours, or even days, they are called tonic section of the nerve bundle leading from the skin area. receptors.  To the left is the effect of a weak stimulus, with only a single nerve o Rapidly Adapting Receptors Detect Change in Stimulus Strength- fiber in the middle of the bundle stimulated strongly (represented the Rate Receptors, Movement Receptors, or Phasic Receptors. by the red-colored fiber), whereas several adjacent fibers are  Receptors that adapt rapidly cannot be used to transmit a stimulated weakly (half-red fibers). continuous signal because they are stimulated only when the  The other two views of the nerve cross section show the effect of stimulus strength changes. Yet, they react strongly while a change a moderate stimulus and a strong stimulus, with progressively is actually taking place. Therefore, these receptors are called rate more fibers being stimulated. Thus, the stronger signals spread to receptors, movement receptors, or phasic receptors. more and more fibers, a phenomenon called spatial summation.  Predictive Function of the Rate Receptors o Temporal Summation  If the rate at which some change in the body’s status is taking  A second means for transmitting signals of increasing strength is place is known, the state of the body a few seconds or even a few by increasing the frequency of nerve impulses in each fiber, called minutes later can be predicted. temporal summation.  For example, the receptors of the semicircular canals in the  Figure 47-8 demonstrates this phenomenon, showing a changing vestibular apparatus of the ear detect the rate at which the head strength of signal in the upper part and the actual impulses begins to turn when a person runs around a curve. transmitted by the nerve fiber in the lower part  Using this information, a person can predict how much he or Modality of Sensation- The “Labeled Line” Principle she will turn within the next 2 seconds and can adjust the o Each of the principal types of sensation that we can experience- pain, motion of the legs ahead of time to keep from losing balance. touch, sight, sound, and so forth- is called a modality of sensation.  Likewise, receptors located in or near the joints help detect the Yet, despite the fact that we experience these different modalities of rates of movement of the different parts of the body. sensation, nerve fibers transmit only impulses. Therefore, how do different nerve fibers transmit different modalities of sensation?  o The answer is that each nerve tract terminates at a specific point in  The lateral branch enters the dorsal horn of the cord gray matter the central nervous system, and the type of sensation felt when a and then divides many times to provide terminals that synapse with nerve fiber is stimulated is determined by the point in the nervous local neurons in the intermediate and anterior portions of the cord system to which the fiber leads. gray matter. These local neurons in turn serve three functions:  For example, if a pain fiber is stimulated, the person perceives pain 1. A major share of them give off fibers that enter the dorsal regardless of what type of stimulus excites the fiber. The stimulus columns of the cord and then travel upward to the brain. can be electricity, overheating of the fiber, crushing of the fiber, or 2. Many of the fibers are very short and terminate locally in the spinal stimulation of the pain nerve ending by damage to the tissue cells. cord gray matter to elicit local spinal cord reflexes. In all these cases, the person perceives pain. 3. Others give rise to the spinocerebellar tracts.  Likewise, if a touch fiber is stimulated by electrical excitation of a o Dorsal Column–Medial Lemniscal Pathway. touch receptor or in any other way, the person perceives touch  Nerve fibers entering the dorsal columns pass uninterrupted up to because touch fibers lead to specific touch areas in the brain. the dorsal medulla, where they synapse in the dorsal column  Similarly, fibers from the retina of the eye terminate in the vision nuclei (the cuneate and gracile nuclei). From there, second-order areas of the brain, fibers from the ear terminate in the auditory neurons decussate immediately to the opposite side of the brain areas of the brain, and temperature fibers terminate in the stem and continue upward through the medial lemnisci to the temperature areas. thalamus. o This specificity of nerve fibers for transmitting only one modality of  In this pathway through the brain stem, each medial lemniscus is sensation is called the labeled line principle. joined by additional fibers from the sensory nuclei of the trigeminal DIFFERENT SENSORY PATHWAYS nerve; these fibers subserve the same sensory functions for the Almost all sensory information from the somatic segments of the body head that the dorsal column fibers subserve for the body. enters the spinal cord through the dorsal roots of the spinal nerves.  In the thalamus, the medial lemniscal fibers terminate in the However, from the entry point into the cord and then to the brain, the thalamic sensory relay area, called the ventrobasal complex. sensory signals are carried through one of two alternative sensory  From the ventrobasal complex, third-order nerve fibers project pathways: (1) the dorsal column-medial lemniscal system; or (2) the mainly to the postcentral gyrus of the cerebral cortex, called anterolateral system. somatic sensory area I (these fibers also project to a smaller o Dorsal Column-Medial Lemniscal system- carries signals upward area in the lateral parietal cortex called somatic sensory area II). 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 via the medial lemniscus.  Composed of large myelinated nerve fibers that transmit signals to the brain at velocities of 30 to 110 m/sec.  Has a high degree of spatial orientation of the nerve fibers with respect to their origin  Where sensory information that must be transmitted rapidly with temporal and spatial fidelity is transmitted mainly  Limited to discrete types of mechanoreceptive sensations o Anterolateral system- immediately after entering the spinal cord from the dorsal spinal nerve roots, synapse in the dorsal horns of the spinal gray matter and 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.  Composed of smaller myelinated fibers that transmit signals at velocities ranging from a few meters per second up to 40 m/sec  Has much less spatial orientation  Where sensory information which does not need to be transmitted rapidly or with great spatial fidelity is transmitted mainly  Has the ability to transmit a broad spectrum of sensory modalities, such as pain, warmth, cold, and crude tactile sensations o These two systems come back together partially at the level of the thalamus. o Anatomy of the Anterolateral Pathway o Anatomy of the Dorsal Column-Medial Lemniscal System  The anterolateral pathway for transmitting sensory signals up the  On entering the spinal cord through the spinal nerve dorsal roots, spinal cord and into the brain, in contrast to the dorsal column the large myelinated fibers from the specialized mechanoreceptors pathway, transmits sensory signals that do not require highly divide almost immediately to form a medial branch and a lateral discrete localization of the signal source and do not require branch. discrimination of fine gradations of intensity.  The medial branch turns medially first and then upward in the  These types of signals include pain, heat, cold, crude tactile, tickle, dorsal column, proceeding via the dorsal column pathway all the itch, and sexual sensations way to the brain.   The spinal cord anterolateral fibers originate mainly in dorsal if the temperature remains above this level indefinitely. Therefore, it horn laminae I, IV, V, and VI. These laminae are where many of is immediately apparent that pain resulting from heat is closely the dorsal root sensory nerve fibers terminate after entering the cord. correlated with the rate at which damage to the tissues is  The anterolateral fibers cross immediately in the anterior occurring and not with the total damage that has already occurred. commissure of the cord to the opposite anterior and lateral white  The intensity of pain is also closely correlated with the rate of columns, where they turn upward toward the brain via the anterior tissue damage from causes other than heat, such as bacterial spinothalamic and lateral spinothalamic tracts. infection, tissue ischemia, tissue contusion, and so forth.  The upper terminus of the two spinothalamic tracts is mainly twofold: Pain transmission (1) throughout the reticular nuclei of the brain stem; and (2) in two o The fast-sharp pain signals are elicited by either mechanical or different nuclear complexes of the thalamus, the ventrobasal thermal pain stimuli. They are transmitted in the peripheral nerves to complex and the intralaminar nuclei. the spinal cord by small type Aδ fibers at velocities between 6 and  In general, the tactile signals are transmitted mainly into the 30 m/sec. ventrobasal complex, terminating in some of the same thalamic  Conversely, the slow-chronic type of pain is elicited mostly by nuclei where the dorsal column tactile signals terminate. chemical types of pain stimuli but sometimes by persisting  From here, the signals are transmitted to the somatosensory mechanical or thermal stimuli. This slow-chronic pain is transmitted cortex, along with the signals from the dorsal columns. to the spinal cord by type C fibers at velocities between 0.5 and 2  Conversely, only a small fraction of the pain signals project directly m/sec. to the ventrobasal complex of the thalamus. o Because of this double system of pain innervation, a sudden painful  Instead, most pain signals terminate in the reticular nuclei of the stimulus often gives a “double” pain sensation: a fast-sharp pain that brain stem and from there are relayed to the intralaminar nuclei is transmitted to the brain by the Aδ fiber pathway, followed a second of the thalamus where the pain signals are further processed. or so later by a slow pain that is transmitted by the C fiber pathway. PAIN  The sharp pain plays an important role in making the person react Types of pain and its characteristics immediately to remove himself or herself from the stimulus. FAST PAIN SLOW PAIN  The slow pain tends to become greater over time, eventually Felt within about 0.1 second after a Begins only after 1 second or more producing intolerable pain and making the person keep trying to pain stimulus is applied and then increases slowly over relieve the cause of the pain. many seconds and sometimes o On entering the spinal cord from the dorsal spinal roots, the pain fibers even minutes terminate on relay neurons in the dorsal horns. Here again, there are Also called as sharp pain, pricking Also called as slow burning pain, 2 systems for processing the pain signals on their way to the brain. On pain, acute pain, and electric pain aching pain, throbbing pain, entering the spinal cord, the pain signals take 2 pathways to the brain, nauseous pain, and chronic pain through (1) the neospinothalamic tract and (2) the This type of pain is felt when a This type of pain is usually paleospinothalamic tract. needle is stuck into the skin, when associated with tissue destruction. o Neospinothalamic Tract for Fast Pain the skin is cut with a knife, or when It can lead to prolonged, almost the skin is burned acutely. It is also unbearable suffering  The fast type Aδ pain fibers transmit mainly mechanical and acute felt when the skin is subjected to thermal pain. They terminate mainly in lamina 1 (lamina marginalis) electric shock. of the dorsal horns, and there they excite second-order neurons of Fast-sharp pain is not felt in most Can occur both in the skin and in the neospinothalamic tract. deep tissues of the body. almost any deep tissue or organ  These second-order neurons give rise to long fibers that cross o Pain Receptors immediately to the opposite side of the cord through the anterior  The pain receptors in the skin and other tissues are all free nerve commissure and then turn upward, passing to the brain in the endings. They are widespread in the superficial layers of the skin, anterolateral columns. as well as in certain internal tissues, such as the periosteum, the o Termination of the Neospinothalamic Tract in the Brain Stem and arterial walls, the joint surfaces, and the falx and tentorium in the Thalamus cranial vault.  A few fibers of the neospinothalamic tract terminate in the reticular  Most other deep tissues are only sparsely supplied with pain areas of the brain stem, but most pass all the way to the thalamus endings; nevertheless, any widespread tissue damage can without interruption, terminating in the ventrobasal complex along summate to cause the slow, chronic, aching type of pain in most with the dorsal column-medial lemniscal tract for tactile sensations. of these areas.  A few fibers also terminate in the posterior nuclear group of the  Pain can be elicited by multiple types of stimuli, classified as thalamus. mechanical, thermal, and chemical pain stimuli. In general, fast  From these thalamic areas, the signals are transmitted to other pain is elicited by the mechanical and thermal types of stimuli, basal areas of the brain, as well as to the somatosensory cortex. whereas slow pain can be elicited by all three types. o It is believed that glutamate is the neurotransmitter substance  Some of the chemicals that excite the chemical type of pain are secreted in the spinal cord at the type Aδ pain nerve fiber endings. bradykinin, serotonin, histamine, potassium ions, acids, o Paleospinothalamic Pathway for Transmitting Slow-Chronic Pain acetylcholine, and proteolytic enzymes. In addition,  The paleospinothalamic pathway is a much older system and prostaglandins and substance P enhance the sensitivity of pain transmits pain mainly from the peripheral slow-chronic type C pain endings but do not directly excite them. fibers, although it also transmits some signals from type Aδ fibers.  The chemical substances are especially important in stimulating  In this pathway, the peripheral fibers terminate in the spinal cord the slow suffering type of pain that occurs after tissue injury. almost entirely in laminae II and III of the dorsal horns, which  In contrast to most other sensory receptors of the body, pain together are called the substantia gelatinosa. receptors adapt very little and sometimes not at all. In fact, under  Most of the signals then pass through one or more additional short some conditions, excitation of pain fibers becomes progressively fiber neurons within the dorsal horns before entering mainly lamina greater, especially for slow, aching, nauseous pain, as the pain V, also in the dorsal horn. stimulus continues. This increase in sensitivity of the pain receptors  Here, the last neurons in the series give rise to long axons that is called hyperalgesia. mostly join the fibers from the fast pain pathway, passing first  One can readily understand the importance of this failure of pain through the anterior commissure to the opposite side of the cord and receptors to adapt because it allows the pain to keep the person then upward to the brain in the anterolateral pathway apprised of a tissue-damaging stimulus as long as it persists o Substance P, the Probable Slow-Chronic Neurotransmitter of  The average person begins to perceive pain when the skin is heated Type C Nerve Endings above 45°C. This is also the temperature at which the tissues begin  Type C pain fiber terminals entering the spinal cord release both to be damaged by heat; indeed, the tissues are eventually destroyed glutamate transmitter and substance P transmitter.   The glutamate transmitter acts instantaneously and lasts for only a  Branches of visceral pain fibers are shown to synapse in the spinal few milliseconds. cord on the same second-order neurons (1 and 2) that receive pain  Substance P is released much more slowly, building up in signals from the skin. concentration over a period of seconds or even minutes.  When the visceral pain fibers are stimulated, pain signals from the o Projection of Paleospinothalamic Pathway (Slow-Chronic Pain viscera are conducted through at least some of the same neurons Signals) Into the Brain Stem and Thalamus. that conduct pain signals from the skin, and the person has the  The slow-chronic paleospinothalamic pathway terminates widely in feeling that the sensations originate in the skin. the brain stem. Only 10% to 25% of the fibers pass all the way to Some Clinical Abnormalities of Pain and Other somatic sensations the thalamus. Instead, most terminate in one of three areas: o Hyperalgesia (Hypersensitivity to Pain) 1. The reticular nuclei of the medulla, pons, and mesencephalon;  A pain nervous pathway sometimes becomes excessively excitable, 2. The tectal area of the mesencephalon deep to the superior and which gives rise to hyperalgesia. Possible causes of hyperalgesia inferior colliculi; or are the following: 3. The periaqueductal gray region surrounding the aqueduct of  Primary hyperalgesia- excessive sensitivity of the pain receptors Sylvius.  Example: The extreme sensitivity of sunburned skin, which  These lower regions of the brain appear to be important for feeling results from sensitization of the skin pain endings by local tissue the suffering types of pain. products from the burn (histamine, prostaglandins, and others)  From the brain stem pain areas, multiple short-fiber neurons relay  Secondary hyperalgesia- facilitation of sensory transmission the pain signals upward into the intralaminar and ventrolateral nuclei  Secondary hyperalgesia frequently results from lesions in the of the thalamus and into certain portions of the hypothalamus and spinal cord or the thalamus. Several of these lesions are other basal regions of the brain. discussed in subsequent sections. Pain Suppression and Referred Pain o Thalamic syndrome o The analgesia system consists of 3 major components:  When an artery supplying the posteriventral portion of the thalamus 1. The periaqueductal gray and periventricular areas of the is blocked or thrombosed, the nuclei in this area degenerate and the mesencephalon and upper pons surround the aqueduct of Sylvius person usually experiences several sensory phenomenon including and portions of the third and fourth ventricles. loss of almost all sensation on the opposite side of the body, loss of 2. Neurons from the previous area send signals to the raphe magnus kinesthetic and position sense, poorly localized and lancinating nucleus, a thin midline nucleus located in the lower pons and upper pains, or other "unpleasantness" of sensations medulla, and the nucleus reticularis paragigantocellularis, located o Herpes Zoster (Shingles) laterally in the medulla.  Occasionally, herpesvirus infects a dorsal root ganglion. 3. From these nuclei, second-order signals are transmitted down the  This infection causes severe pain in the dermatomal segment dorsolateral columns in the spinal cord to a pain inhibitory subserved by the ganglion, thus eliciting a segmental type of pain complex located in the dorsal horns of the spinal cord. that circles halfway around the body.  At this point, the analgesia signals can block the pain before it is  The disease is called herpes zoster, or shingles, because of a skin relayed to the brain. eruption that often ensues.  Several transmitter substances, especially enkephalin and o Tic Douloureux serotonin, are involved in the analgesia system. The enkephalin is  A lancinating or stabbing type of pain occasionally occurs in some believed to cause both presynaptic and postsynaptic inhibition of people over one side of the face in the sensory distribution area (or incoming type C and type Aδ pain fibers where they synapse in the part of the area) of the fifth or ninth nerves dorsal horns  The pain feels like sudden electrical shocks, and it may appear for o About a dozen such opiate-like substances have now been found at only a few seconds at a time or may be almost continuous. different points of the nervous system. All are breakdown products of  Often, it is set off by exceedingly sensitive trigger areas on the three large protein molecules: pro-opiomelanocortin, surface of the face, in the mouth, or inside the throat- almost always proenkephalin, and prodynorphin. Among the more important of by a mechanoreceptive stimulus rather than a pain stimulus. these opiate-like substances are β-endorphin, met-enkephalin, leu-  For example, when the patient swallows a bolus of food, as the food enkephalin, and dynorphin. touches a tonsil, it might set off a severe lancinating pain in the o The two enkephalins are found in the brain stem and spinal cord and mandibular portion of the fifth nerve. β-endorphin is present in both the hypothalamus and the pituitary o Headache gland. Dynorphin is found mainly in the same areas as the  A type of pain referred to the surface of the head from deep head enkephalins, but in much lower quantities. structures. o Referred Pain  Some headaches result from pain stimuli arising inside the cranium, but others result from pain arising outside the cranium, such as from the nasal sinuses CASE 6 EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERE Surface o The cerebral hemispheres are developed from the telencephalon and form the largest part of the brain. Each hemisphere has a covering of gray matter, the cortex and internal masses of gray matter, the basal nuclei, and a lateral ventricle. o The cerebrum is the largest part of the brain, situated in the anterior  Often, a person feels pain in a part of the body that is fairly remote and middle cranial fossae of the skull and occupying the whole from the tissue causing the pain. This phenomenon is called concavity of the vault of the skull. It may be divided into two parts: the referred pain. diencephalon, which forms the central core, and the telencephalon,  For example, pain in one of the visceral organs often is referred to which forms the cerebral hemispheres. an area on the body surface. o The cerebral hemispheres are the largest part of the brain; they are  Knowledge of the different types of referred pain is important in separated by a deep midline sagittal fissure, the longitudinal cerebral clinical diagnosis because, in many visceral ailments, the only fissure (Fig. 7-6). The fissure contains the sickle-shaped fold of dura clinical sign is referred pain. mater, the falx cerebri, and the anterior cerebral arteries. In the depths of the fissure, the great commissure, the corpus callosum, connects the hemispheres across the midline. A second horizontal fold  of dura mater separates the cerebral hemispheres from the cerebellum and is called the tentorium cerebelli. o To increase the surface area of the cerebral cortex maximally, the surface of each cerebral hemisphere is thrown into folds or gyri, which are separated from each other by sulci or fissure. Each hemisphere is divided into lobes, which are named according to the cranial bones under which they lie.  The central and parieto-occipital sulci and the lateral and calcarine sulci are boundaries used for the division of the cerebral hemisphere into frontal, parietal, temporal, and occipital lobes (Fig. 7-7; Fig. 7-10). Different lobes and boundaries of each lobe on the dorsolateral surface o The frontal lobe occupies the area anterior to the central sulcus and superior to the lateral sulcus (Fig. 7-10).  The superolateral surface of the frontal lobe is divided by 3 sulci into 4 gyri.  The precentral sulcus runs parallel to the central sulcus, and the precentral gyrus lies between them (see Fig. 7-7).  Extending anteriorly from the precentral sulcus are the superior and inferior frontal sulci.  The superior frontal gyrus lies superior to the superior frontal sulcus, the middle frontal gyrus lies between the superior and inferior frontal sulci, and the inferior frontal gyrus lies inferior to the inferior frontal sulcus. The inferior frontal gyrus is invaded by the anterior and ascending rami of the lateral sulcus. o The parietal lobe occupies the area posterior to the central sulcus and superior to the lateral sulcus; it extends posteriorly as far as the parieto-occipital sulcus (see Figs. 7-7 to 7-10).  The lateral surface of the parietal lobe is divided by 2 sulci into 3 gyri.  The postcentral sulcus runs parallel to the central sulcus, and the postcentral gyrus lies between them.  Running posteriorly from the middle of the postcentral sulcus is the intraparietal sulcus (see Fig. 7-7).  Superior to the intraparietal sulcus is the superior parietal lobule (gyrus), and inferior to the intraparietal sulcus is the inferior parietal lobule (gyrus). o The temporal lobe occupies the area inferior to the lateral sulcus (see Figs. 7-7 to 7-10).  The lateral surface of the temporal lobe is divided into 3 gyri by 2 sulci.  The superior and middle temporal sulci run parallel to the posterior ramus of the lateral sulcus and divide the temporal lobe into the superior, middle, and inferior temporal gyri; the inferior temporal gyrus is continued onto the inferior surface of the hemisphere (see Fig. 7-7). o The occipital lobe occupies the small area behind the parieto- occipital sulcus (see Figs. 7-7 to 7-10). Different sulci/fissures and gyri on each surface o The central sulcus (see Fig. 7-7) is of great importance because the gyrus that lies anterior to it contains the motor cells that initiate the movements of the opposite side of the body; posterior to it lies the general sensory cortex that receives sensory information from the opposite side of the body.  The central sulcus indents the superior medial border of the hemisphere about 0.4 in (1 cm) behind the midpoint (Fig. 7-8).  It runs downward and forward across the lateral aspect of the hemisphere, and its lower end is separated from the posterior ramus of the lateral sulcus by a narrow bridge of cortex.  The central sulcus is the only sulcus of any length on this surface of the hemisphere that indents the superomedial border and lies between two parallel gyri. o The lateral sulcus (see Fig. 7-7) is a deep cleft found mainly on the inferior and lateral surfaces of the cerebral hemisphere.  It consists of a short stem that divides into 3 rami. The stem arises on the inferior surface, and on reaching the lateral surface, it divides into the anterior horizontal ramus and the anterior ascending ramus and continues as the posterior ramus (see Fig. 7-7).   An area of cortex called the insula lies at the bottom of the deep derived from the apical dendrites of the pyramidal cells and lateral sulcus and cannot be seen from the surface unless the lips of fusiform cells, the axons of the stellate cells, and the cells of the sulcus are separated (Fig. 7-9). Martinotti. o The parieto-occipital sulcus begins on the superior medial margin of  Afferent fibers originating in the thalamus and in association with the hemisphere about 2 in (5 cm) anterior to the occipital pole (see Fig. commissural fibers also are present. 7-8). It passes downward and anteriorly on the medial surface to meet  Scattered among these nerve fibers are occasional horizontal the calcarine sulcus (see Fig. 7-8). cells of Cajal. o The calcarine sulcus is found on the medial surface of the  This most superficial layer of the cortex clearly is where large hemisphere (see Fig. 7-8). numbers of synapses between different neurons occur  It commences under the posterior end of the corpus callosum and 2. External granular layer arches upward and backward to reach the occipital pole, where it  This layer contains large numbers of small pyramidal cells and stops. In some brains, however, it continues for a short distance onto stellate cells. the lateral surface of the hemisphere.  The dendrites of these cells terminate in the molecular layer, and  The calcarine sulcus is joined at an acute angle by the parieto- the axons enter deeper layers, where they terminate or pass on occipital sulcus about halfway along its length. to enter the white matter of the cerebral hemisphere. INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERE 3. External pyramidal layer Cerebral Cortex- Gray Matter  This layer is composed of pyramidal cells, whose cell body size o Different Types of Cortex increases from the superficial to the deeper borders of the layer.  Allocortex (Archiocortex/Heterogenic)  The apical dendrites pass into the molecular layer, and the  Found in the limbic system axons enter the white matter as projection, association, or  Consists of 3 layers of cells commissural fibers.  Paleopallium (olfactory cortex) and archipallium (hippocampal 4. Internal granular layer formation and dentate gyrus) do not show 6 layers in either the  This layer is composed of closely packed stellate cells with a high developing or adult stage, and together constitute the allocortex or concentration of horizontally arranged fibers known collectively as heterogenetic cortex. the external band of Baillarger.  Paleocortex- found in the olfactory lobe 5. Ganglionic layer (internal pyramidal layer)  Paleopallium  This layer contains very large and medium-size pyramidal cells.  Isocortex (Neocortex/Homogenetic)  Scattered among the pyramidal cells are stellate cells and cells  More common of Martinotti.  Consists of 6 layers of cells  In addition, a large number of horizontally arranged fibers form the o Layers of the cerebral cortex and structure of each layer inner band of Baillarger.  In the motor cortex of the precentral gyrus, the pyramidal cells of this layer are very large and are known as Betz cells. These cells account for about 3% of the projection fibers of the corticospinal or pyramidal tract. 6. Multiform layer (layer of polymorphic cells)  Although the majority of the cells are fusiform, many of the cells are modified pyramidal cells, whose cell bodies are triangular or ovoid.  The cells of Martinotti also are conspicuous in this layer.  Many nerve fibers are present that are entering or are leaving the underlying white matter. o Principal Cells of the Cerebral Cortex  The neocortex consists of 6 layers and share a common basic structure with the neuron arranged in 6 layers or laminae oriented parallel to the surface of the cortex.  Another feature is the arrangement of the neuron in rows or columns oriented perpendicular to the cortical surface.  The density of the perikaryal differs among the layers.  About two-thirds of the neuron are cortical pyramidal cells.  A typical pyramidal cell has a long apical dendrite that extends  The pyramidal cells are named from the shape of their cell bodies. towards the cortical surface- it thus extends through several layers  Most of the cell bodies measure 10 to 50 um long. However, giant superficial of the layer in which the perikaryon is located. pyramidal cells, also known as Betz cells, whose cell bodies  These cells are mainly found in layers 3 and 5. measure as much as 120 um, are found in the motor precentral  The largest of which is called the giant cell of Betz and is found in gyrus of the frontal lobe. the precentral gyrus.  The apices of the pyramidal cells are oriented toward the pial  The rest of the cortical neurons constitute a heterogenous group surface of the cortex. whose neuron have in common that their perikaryal are not  From the apex of each cell, a thick apical dendrite extends pyramidal; such neurons are therefore lumped together as upward toward the pia, giving off collateral branches. nonpyramidal cells.  From the basal angles, several basal dendrites pass laterally 1. Molecular layer (plexiform layer) into the surrounding neuropil.  This is the most superficial layer; it consists mainly of a dense  Each dendrite possesses numerous dendritic spines for network of tangentially oriented nerve fibers. These fibers are synaptic junctions with axons of other neurons.   The axon arises from the base of the cell body and either dysarthria in that the latter pertains to a problem with articulation. terminates in the deeper cortical layers or, more commonly, enters Dysarthrias are disorders of the motor pathways with weakness, the white matter of the cerebral hemisphere as a projection, slowness, or incoordination producing characteristic speech association, or commissural fiber. disturbances through alteration of respiration, phonation,  The stellate cells, sometimes called granule cells because of their resonance, articulation, or speech rhythm. small size, are polygonal in shape, and their cell bodies measure  This involves lesion of the motor pathways- either pyramidal about 8 pm in diameter. (corticospinal tract) or extrapyramidal (basal ganglia). In this  These cells have multiple branching dendrites and a relatively case, the motor area of the frontal lobe. short axon, which terminates on a nearby neuron.  Prefrontal areas (areas 9 and 10)- located in the most anterior  The fusiform cells have their long axis vertical to the surface and part of the frontal lobe and are active not only in programming are concentrated mainly in the deepest cortical layers. motor function but also in some aspects of memory, emotion, and  Dendrites arise from each pole of the cell body. The inferior intellectual functions. dendrite branches within the same cellular layer, while the  Orbitofrontal area (area 11)- located at the base of the frontal superficial dendrite ascends toward the surface of the cortex and lobe and is concerned with visceral and emotional activities. branches in the superficial layers.  Parietal lobe  The axon arises from the inferior part of the cell body and enters  The parietal lobe is bounded anteriorly by the central sulcus and the white matter as a projection, association, or commissural fiber. its posterior margin merges indistinctly with the occipital lobe  The horizontal cells of Cajal are small, fusiform, horizontally behind and the temporal lobe below. oriented cells found in the most superficial layers of the cortex.  It has two major areas, the primary and the association cortex.  A dendrite emerges from each end of the cell, and an axon runs  Primary sensory cortex (area 1,2,3)- is in the postcentral gyrus parallel to the surface of the cortex, making contact with the and receives afferent fibers from the thalamus, particularly from dendrites of pyramidal cells. the spinothalamic, trigeminal thalamic, and medial lemniscal  The cells of Martinotti are small, multipolar cells that are present pathways. throughout the levels of the cortex.  It has topographic homuncular organization similar to that of  The cell has short dendrites, but the axon is directed toward the the motor cortex. This is concerned especially with the pial surface of the cortex, where it ends in a more superficial layer, integration of sensory experience and with the discriminative commonly the most superficial layer. qualities of sensation.  The axon gives origin to a few short collateral branches en route.  With the exception of olfaction, impulses reach the localized o Important Areas of Brodmann in each Major Lobe (Functional areas via the thalamocortical radiations coming from the Areas) ventral tier of the thalamus.  The cerebral hemispheres are subdivided into frontal, parietal,  Sensory association cortex (area 5,7)- receives input from the occipital, temporal and limbic lobes. Each of these has been primary sensory cortex and coordinates, integrates, and refines subdivided into histological and functional areas by the German the perception of the external sensory input. anatomist Brodmann, hence the term Brodmann’s areas  Cortical analysis here deals primarily with such discriminative  Frontal lobe aspects of tactile sensations as localization and recognition of  This makes up one-third of the hemisphere, extending from the spatial relations, texture, shape, size and recognition of frontal pole to the central sulcus. It has 7 functional areas differences.  Primary motor area (Area 4)- is located on the anterior wall of  Lesions in this region produce impairment of the ability to the precentral sulcus, extending onto the mesial surface of the recognize objects by palpation (astereognosis), of two-point hemisphere. discrimination, of touch localization, to recognize a number  It has giant pyramidal cells of Betz in the fifth layer, whose written on the hand (graphesthesia) and of weight axons form part of the corticobulbar and most of the discrimination (barognosis). corticospinal tracts.  Gustatory area is poorly defined but is believed to be in the  There is somatotropic arrangement of the contralateral half of parietal lobe (area 43) the body along the gyrus, with the degree of representation  Occipital lobe proportional to the discreteness of the movement required of the  The occipital lobe forms the posterior pole of the cerebral body. hemisphere and contains the primary visual cortex (area 17).  Destructive lesions of this area cause contralateral paralysis  The primary visual cortex in the banks of the calcarine fissure on  Premotor area (area 6)- located immediately in front of area 4 and the medial aspect of the occipital lobe receive the optic gives rise to fibers in the direct activation pathways to the red radiations from the lateral geniculate body (LGB). nucleus, caudate nucleus, and reticular formation, which  The optic radiation projects topographically upon the striate coordinate movement and control gross or postural movements. cortex so that cells in:  Lesions involving in this area also produces contralateral  The medial half of the lateral geniculate body, representing paralysis and also motor apraxia. upper retinal quadrants (lower quadrants of the visual field)  Supplementary motor area (area 6a)- located in front of the motor project on the superior bank of the calcarine sulcus area on the mesial surface of the hemisphere and is a secondary  The lateral half of the LGB project to the inferior bank. representation of motor function.  The macular fibers terminate in the caudal third of the calcarine  Seizures in this area produce a characteristic posturing with area, and those from the paracentral and peripheral retinal areas elevation of the arm and deviation of the head and eyes end in respectively more rostral portions. towards the elevated arm.  Complete unilateral destruction of the visual cortex produces a  Frontal eye field (area 8)- anterior to the premotor area and is homonymous hemianopsia in which there is blindness in the concerned with voluntary eye movements. ipsilateral nasal field and the contralateral temporal field.  Seizures in this area produce a conjugate deviation of the  Adjacent to the primary visual area are the visual association eyes to the opposite side, whereas destructive lesions (such as areas (area 18, 19) which organize and integrate visual stimuli. hemorrhage or tumors) produce deviation of the eyes toward  Temporal lobe the side of the lesion.  The temporal lobe is located on the lateral aspect of the cerebral  Motor speech areas (areas 44 and 45)- located in the inferior hemispheres inferior to the lateral (sylvian) fissure. frontal gyrus of the dominant hemisphere, and they control the  It is continuous posteriorly with the parietal and occipital lobes. programming of speech.  The superior temporal gyrus (Heschl’s) contains the primary  This is also called Broca’s area and lesion produce motor auditory cortex (areas 41 and 42), which receives auditory speech apraxia or Broca’s aphasia. This differs from fibers from the medial geniculate body.   Sounds coming into either ear reach the auditory cortex are not known. However, the fibers from the pretectal nuclei bilaterally. involved in the pupillary light reflex are believed to cross in this  Unilateral lesions of the auditory cortex cause some difficulty in commissure on their way to the parasympathetic part of the sound localization, but there is no significant hearing loss. oculomotor nuclei.  Bilateral ablation of the auditory cortex does not prevent reaction  The fornix is composed of myelinated nerve fibers and constitutes sounds but does reduce greatly or abolish the ability to the efferent system of the hippocampus that passes to the discriminate different patterns of sound. The dominant temporal mammillary bodies of the hypothalamus. lobe also has a primary role in language function.  The nerve fibers first form the alveus, which is a thin layer of white  Homuncular arrangement- the various regions of the body area matter covering the ventricular surface of the hippocampus, and represented in specific portions of the postcentral gyrus, the pattern then converge to form the fimbria. corresponding to that of the motor area.  The fimbriae of the 2 sides increase in thickness and, on reaching  Thus, the face area lies in the most ventral part, while above it is the posterior end of the hippocampus, arch forward above the the sensory (motor areas) for the hand, arm, trunk, leg and foot in thalamus and below the corpus callosum to form the posterior the order named; the lower extremity extends into the paracentral columns of the fornix. lobule.  The two columns then come together in the midline to form the  The cortical areas representing the hand, face and mouth regions body of the fornix. are disproportionately large.  The commissure of the fornix consists of transverse fibers that  The digits of the hand, particularly the thumb and index finger, are cross the midline from one column to another just before the well represented. formation of the body of the fornix.  The cortical area related to sensations from the face occupies  The function of the commissure of the fornix is to connect the almost the entire lower half of the postcentral gyrus. hippocampal formations of the two sides.  It should also be noted that association fibers abound between  The habenular commissure is a small bundle of nerve fibers that lobes. crosses the midline in the superior part of the root of the pineal stalk.  There are also projection fibers such as corticoreticular,  The commissure is associated with the habenular nuclei, which corticopontine, and corticothalamic fibers connecting the cortex to are situated on either side of the midline in this region. the reticular activating system, pons, and thalamus respectively.  The habenular nuclei receive many afferents from the amygdaloid Cerebral White Matter nuclei and the hippocampus. These afferent fibers pass to the o The white matter is composed of myelinated nerve fibers of different habenular nuclei in the stria medullaris thalami. diameters supported by neuroglia. The nerve fibers may be classified  Some of the fibers cross the midline to reach the contralateral into three groups according to their connections: (1) commissural nucleus through the habenular commissure. The function of the fibers, (2) association fibers, and (3) projection fibers. habenular nuclei and its connections in humans is unknown. o Commissure Fibers o Association Fibers  Commissure fibers essentially connect corresponding regions of  Association fibers are nerve fibers that essentially connect various the two hemispheres. They are as follows: the corpus callosum, the cortical regions within the same hemisphere and may be divided into anterior commissure, the posterior commissure, the fornix, and the short and long groups. habenular commissure.  The short association fibers lie immediately beneath the cortex  The corpus callosum, the largest commissure of the brain, and connect adjacent gyri; these fibers run transversely to the long connects the two cerebral hemispheres. It lies at the bottom of the axis of the sulci. longitudinal fissure. For purposes of description, it is divided into  The long association fibers are collected into named bundles that the rostrum, the genu, the body, and the splenium. can be dissected in a formalin-hardened brain.  The rostrum is the thin part of the anterior end of the corpus  The uncinate fasciculus connects the first motor speech area and callosum, which is prolonged posteriorly to be continuous with the the gyri on the inferior surface of the frontal lobe with the cortex of upper end of the lamina terminalis. the pole of the temporal lobe.  The genu is the curved anterior end of the corpus callosum that  The cingulum is a long, curved fasciculus lying within the white bends inferiorly in front of the septum pellucidum. matter of the cingulate gyrus. It connects the frontal and parietal  The body of the corpus callosum arches posteriorly and ends as lobes with parahippocampal and adjacent temporal cortical regions. the thickened posterior portion called the splenium.  The superior longitudinal fasciculus is the largest bundle of nerve  Traced laterally, the fibers of the genu curve forward into the fibers. It connects the anterior part of the frontal lobe to the occipital frontal lobes and form the forceps minor. The fibers of the body and temporal lobes. extend laterally as the radiation of the corpus callosum. They  The inferior longitudinal fasciculus runs anteriorly from the intersect with bundles of association and projection fibers as they occipital lobe, passing lateral to the optic radiation, and is distributed pass to the cerebral cortex. to the temporal lobe.  Some of the fibers form the roof and lateral wall of the posterior  The fronto-occipital fasciculus connects the frontal lobe to the horn of the lateral ventricle and the lateral wall of the inferior horn occipital and temporal lobes. It is situated deep within the cerebral of the lateral ventricle; these fibers are referred to as the tapetum. hemisphere and is related to the lateral border of the caudate Traced laterally, the fibers in the splenium arch backward into the nucleus. occipital lobe and form the forceps major. o Projection Fibers  The anterior commissure is a small bundle of nerve fibers that  Afferent and efferent nerve fibers passing to and from the brainstem crosses the midline in the lamina terminalis. to the entire cerebral cortex must travel between large nuclear  When traced laterally, a smaller or anterior bundle curves forward masses of gray matter within the cerebral hemisphere. on each side toward the anterior perforated substance and the  At the upper part of the brainstem, these fibers form a compact band olfactory tract. known as the internal capsule, which is flanked medially by the  A larger bundle curves posteriorly on each side and grooves the caudate nucleus and the thalamus and laterally by the lentiform inferior surface of the lentiform nucleus to reach the temporal nucleus. lobes.  Because of the wedge shape of the lentiform nucleus, as seen on  The posterior commissure is a bundle of nerve fibers that crosses horizontal section, the internal capsule is bent to form an anterior the midline immediately above the opening of the cerebral aqueduct limb and a posterior limb, which are continuous with each other at into the third ventricle; it is related to the inferior part of the stalk of the genu. the pineal gland.  Once the nerve fibers have emerged superiorly from between the  Various collections of nerve cells are situated along its length. The nuclear masses, they radiate in all directions to the cerebral cortex. destinations and functional significance of many of the nerve fibers These radiating projection fibers are known as the corona radiata.   Most of the projection fibers lie medial to the association fibers, but  Cortical branches supply the entire lateral surface of the they intersect the commissural fibers of the corpus callosum and the hemisphere, except for the narrow strip supplied by the anterior anterior commissure. The nerve fibers lying within the most posterior cerebral hemisphere, the occipital pole, and the inferolateral part of the posterior limb of the internal capsule radiate toward the surface of the hemisphere (supplied by the posterior cerebral calcarine sulcus and are known as the optic radiation. artery). BLOOD SUPPLY OF THE BRAIN, MENINGES, AND SPINAL CORD  The MCA actually supplies the motor area except for the “leg area”. It is essential that there is a continued blood flow to the brain to ensure  Central branches supply the lentiform and caudate nuclei and the normal brain function. The average person will lose consciousness if the internal capsule. brain deprived of blood for 10 to 12 seconds. After 3 to 5 minutes, there Vertebral Artery is irreversible irreparable brain damage or even death. o The vertebral artery branches from the first part of the subclavian The common carotid artery is a large vessel supplying the head and artery and ascends toward the head through the foramina of the neck, dividing into the: transverse processes of the upper six cervical vertebra and enters o External carotid artery- which supplies the exterior of the head, face the skull through the foramen magnum, passing upward, forward and and most of the neck medially on the medulla oblongata. At the lower pons, it joins its o Internal carotid artery- supplying the cranial and orbital contents counterpart on the opposite side to form the basilar artery. o The right common carotid artery originates from the brachiocephalic o The branches of the cranial portion of the vertebral artery are: trunk which is a direct branch of the aorta.  Meningeal artery- supply the bone and dura in the posterior cranial o The left common carotid artery originates directly from the aortic fossa arch.  Posterior spinal artery- it may arise directly from the vertebral o The vertebral artery arises from the supero-posterior aspect of the artery or the posterior inferior cerebellar artery, descending close to subclavian artery. the posterior roots of the spinal nerves. Blood is supplied to the brain via the internal carotid artery (ICA) and  Anterior spinal artery- formed from a contributory branch from the vertebral arteries.