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THE CNS: GENERAL ORIENTATION Protection & Nourishment.

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1 THE CNS: GENERAL ORIENTATION Protection & Nourishment

2 Protection and Nourishment  In addition to the bony protective housing of the brain and spinal cord, we have a layered system of membranes called the meninges that cover them.  Within certain layers of these meninges, there is a cushioning layer of fluid called cerebrospinal fluid (CSF).  Those associated specifically with the cord are known as the spinal meninges.  Those associated specifically with the brain are known as the cranial meninges.

3 The Meninges  The outer cranial and spinal meninx is called the dura mater—“tough mother.”  The dura mater forms a tube from the level of the second sacral vertebra to the foramen magnum, where it is continuous with the dura mater of the brain.

4 The Meninges  In the spinal meninges, between the dura mater (#3) and the wall of the vertebral canal is the epidural space (#4), which is filled with fat, connective tissue and blood vessels.  The spinal epidural space serves as padding around the cord and also as the site for injection of anesthetics.

5 The Meninges  Unlike the spinal dura mater, the cranial dura mater is marked by complex folds that divide the contents of the cranial cavity into different cerebral subdivisions.

6 The Meninges  These folds are the falx cerebri, between the cerebral hemispheres, and the tentorium cerebelli, between the superior surface of the cerebellum and the inferior occipital lobes.  These folds actually serve to brace the brain against rotary displacement.

7 The Meninges  The cranial dura mater (green) actually consists of two layers that are closely united except in certain regions where they separate to form the venous sinuses (dark blue).

8 The Meninges  The thicker, outer layer, the endosteal layer, adheres tightly to the cranial bones and serves as periosteum (for bone growth, repair, and nutrition).  The thinner, inner layer, the meningeal layer, corresponds to the spinal dura mater.

9 The Meninges  Under normal circumstances, there is not an epidural space between the skull (cranium) and the dura mater.  In this slide, a bleed from a dural vessel resulted in a epidural hematoma.

10 The Meninges  The middle meninx is the arachnoid (purple), lying deep to the dura.  The arachnoid mater is a delicate connective tissue membrane that does not contain blood vessels.

11 The Meninges  It bridges over the sulci of the brain--it does not dip into those shallow grooves.  In the spinal cord, this membrane forms a tube inside the spinal dura mater.

12 The Meninges  In some areas, fingerlike projections of the arachnoid, called arachnoid villi, push into the dural venous sinuses, especially the superior sagittal sinus, to form the arachnoid granulations where cerebrospinal fluid diffuses into the blood stream.

13 The Meninges  The inner meninx is known as the pia mater, or “delicate mother.”  This transparent fibrous membrane adheres closely to the surface of the spinal cord and brain.  It contains numerous blood vessels.

14 The Meninges  It also fuses with the ependyma of the ventricles to form the choroid plexuses of the ventricles.

15 The Meninges  Between the arachnoid mater and the pia mater is the subarachnoid space.  This is the area through which cerebrospinal fluid circulates.  In addition to CSF, cerebral arteries and veins, as well as cranial nerves (spinal nerves) pass through this space.

16 The Ventricular System  The ventricles are located deep in the brain and are derived embryologically from the central canal.  Cerebrospinal fluid is formed within the ventricles by the choroids plexuses.  It circulates through the ventricular system, into the subarachnoid space, before it is removed by the arachnoid villi into the venous system.

17 Ventricular System  There is a lateral ventricle in each cerebral hemisphere that communicates with third ventricle, located medially, by a narrow, oval opening, called the interventricular foramen (left and right foramina of Monro).

18 The Ventricular System  The cerebral aqueduct (aqueduct of Silvius) courses from the 3 rd ventricle down the length of the midbrain and connects with 4 th ventricle, dorsal to the pons and medulla, but ventral to the cerebellum.

19 The Ventricular System  From the roof of the 4 th ventricle, CSF exits the ventricular system to enter subarachnoid space of the brain and spinal cord.  These openings are called the median aperture of Magendie (arrow) and the two lateral apertures of Lushka.

20 The Ventricular System  The entire CNS contains 3-5 oz of CSF.  CSF is a clear colorless liquid of watery consistency, containing proteins, glucose, urea, and salts.  CSF serves two principle functions: protection and circulation.  The fluid serves as a shock-absorbing medium to protect the brain and spinal cord from jolts that would otherwise cause them to crash against the bony walls of the vertebral and cranial cavities.  The fluid also “buoys” the brain so that it “floats” in the cranial cavity.

21 The Ventricular System  Its circulatory function is nutritive—substances filtered from the blood are delivered by the CSF to the brain and spinal cord.  It also removes waste and toxic substances produced by the brain and spinal cord cells.

22 The Ventricular System  Hydrocephalus is a condition that results from impaired circulation or re- absorption of the CSF.  The result is dilation of the ventricles and compression of nearby brain structures.

23 The Ventricular System  The relationship between the CSF producing choroid plexuses and the network of blood capillaries is the blood-CSF barrier that permits certain substances to enter the fluid but prohibits others.  Infections in the CNS, neoplasm, or bleeding, can upset the blood-CSF barrier and produce changes in the composition of the CSF.

24 The Vascular Supply  The basic arterial blood supply to the brain can be divided into an anterior and posterior system.  The anterior system begins with the paired internal carotid arteries.  They ascend in the neck and enter the base of the brain at the carotid canal of the temporal bone.

25 The Vascular Supply  Once through the temporal bone, the internal carotid arteries bifurcate into the anterior and middle cerebral arteries.  The anterior cerebral arteries supply mostly medial structures in each hemisphere.anterior cerebral arteries  The right and left anterior cerebral arteries communicate with each other via the anterior communicating artery.  Each middle cerebral artery supplies most of the lateral surface of each hemisphere through its extensive collaterals.middle cerebral artery

26 The Vascular Supply  The posterior system begins with the paired vertebral arteries, each of which is a branch of the subclavian artery.  The vertebral arteries ascend through the cervical vertebrae and enter the brain through the foramen magnum.

27 The Vascular Supply  They continue to ascend along the ventrolateral sides of the medulla.  At the junction of the pons and medulla, the two vertebral arteries merge into a single basilar artery.

28 The Vascular Supply  The basilar artery continues to ascend to the level of the midbrain, where it bifurcates into the left and right posterior cerebral arteries.

29 The Vascular Supply  The posterior cerebral arteries give off numerous branches that collectively supply the brainstem, parts of the diencephalon, the cerebellum, and parts of the posterior cerebral hemispheres.  The posterior cerebral arteries communicate with each other via the posterior communicating artery.

30 The Vascular Supply  The cerebral arterial circle or Circle of Willis is formed by the union of anterior and posterior cerebral arteries.  The posterior cerebral arteries are connected with the internal carotid arteries by the posterior communicating artery.  The anterior cerebral arteries are connected by anterior communicating arteries.

31 The Vascular Supply  The joining of these vessels is termed an anastomosis— end to end union of two vessels.  The function of the cerebral arterial circle is to equalize blood pressure to the brain and provide alternative routes for blood to the brain should arteries become damaged.

32 The Vascular Supply  Blood vessels that enter brain tissue first pass along the surface of the brain.  As they penetrate inward, they are surrounded by a loose fitting layer of pia mater.

33 The Vascular Supply The space between the penetrating vessel and the pia mater is called the perivascular space.

34 The Vascular Supply  The blood-brain barrier (BBB) is the specialized system of capillary endothelial cells that protects the brain from harmful substances in the blood stream, while supplying the brain with the required nutrients for proper function.  Unlike peripheral capillaries that allow relatively free exchange of substance across/ between cells, the BBB strictly limits transport into the brain through both physical (tight junctions) and metabolic (enzymes) barriers.

35 The Vascular Supply  Thus the BBB is often the rate-limiting factor in determining permeation of therapeutic drugs into the brain.  Additionally, BBB breakdown is theorized to be a key component in central nervous system (CNS) associated pathologies.

36 THE CNS: GENERAL ORIENTATION Neurological Histology

37 Classes of Cells  Histology is the microscopic study of the structure of tissues.  In the nervous system, the two broad classes of cells are neurons and glia.  Within each class there are many different types of cells that differ based on their structure, chemistry, and function.  Neurons are the most important cells of the unique functions of the brain.  They sense changes in the environment, communicate these changes to other neurons, and command the body’s responses to these sensations.

38 Classes of Cells  Glia are thought to contribute to brain function mainly by insulating, supporting, and nourishing neighboring neurons.  In fact, the term glia is derived from the Greek word for “glue,” giving the impression that the main function of these cells is to keep the brain from running out of our ears.  If your brain were a chocolate-chip cookie and the neurons the chocolate chips, the glia would be the cookie dough that fill all the other spaces and ensures that the chips are suspended in their appropriate locations.

39 Cranial Nerves  We’re going to look more thoroughly at the cranial nerves and their nuclei, ascending and descending tracts, specific nuclei of the reticular formation, and other select special nuclei.  Cranial nerves (CN) I Olfactory and II Optic are part of the forebrain.  The other 10 cranial nerves originate in the brainstem.  Some cranial nerves serve only sensory functions; other serve only motor functions.  Many of the nerves are mixed, serving both sensory and motor functions.

40 Ventrally Located Cranial Nerves  Cranial nerves III, VI, and XII are seen exiting from the ventral side of the brainstem, close to the midline.  The oculomotor nerve (III) emerges at the caudal border of the midbrain.  It innervates the extraocular muscles

41 Ventrally Located Cranial Nerves  The abducens nerve (VI) emerges at the caudal border of the pons.  It also innervates the extraocular muscles  The hypoglossal nerve (XII) emerges from the medulla just lateral to the medullary pyramids.  It innervates the intrinsic muscles of the tongue

42 Dorsally Located Cranial Nerve  Only one cranial nerve, the trochlear nerve (IV) exits from the dorsal aspect of the brainstem.  The trochlear nerve exits from the midbrain just below the inferior colliculus near the midline to innervate the superior oblique muscle of the eye.

43 Laterally Located Cranial Nerves  The 8 remaining cranial nerves exit laterally from the brainstem.  The trigeminal nerve (V) exits the pons.  It mediates sensation from facial skin and innervates the muscles of mastication.  The facial (VII) and the vestibulocochlear (VIII) nerves originate at the junction between the pons and medulla.

44 Laterally Located Cranial Nerves  The glossopharyngeal (IX), vagus (X), and accessory (XI) nerves arise in the medulla as a series of fine rootlets just dorsal to the inferior olive.

45 Functional Classification of CNs  Cranial nerves serve general motor and general sensory functions as well as special functions resulting from the use of special receptors and neurons.  The general and special function of CNs can be further classified by: whether the nerve innervates somatic muscles or visceral structures; Whether it is involved in sensory(afferent) or motor (efferent) information.

46 Functional Classification of CNs: Efferent GeneralSpecial SomaticVisceralSomaticVisceral GSE controls muscles derived from somites that include skeletal, extaocular, glossal muscles; innervate ocular (CNs III, IV, VI) and tongue muscles (XII). GVE regulates autonomic innervation of smooth muscles, cardiac muscles, glands; innervates muscles responsible for pupillary constriction (CN III), gland secretion (CNs VII, IX) regulation of the muscles of the heart, trachea, bronchi, esophagus, and lower viscera (CN X). SVE controls gill-related muscles of the face, pharynx, larynx, neck that evolve from the branchial arches; innervates muscles controlling the face(CNs V, VII), pharynx (CN XI), larynx (CN X), and neck (CN XI) for facial expression, mastication, phonation, deglutition, head turning, and shoulder elevation,

47 Functional Classification of CNs: Afferent GeneralSpecial SomaticVisceralSomaticVisceral GSA mediates somesthetic input of pain, temperature, touch from muscles, skin, ligaments, joints; mediates pain, temperature, touch sensation from skin and muscles of head, neck, and face (CN V). GVA mediates sensations of pain and temperature from visceral organs; mediates pain and temperature from pharynx (CN IX), palate, larynx, aorta, and abdomen (CN X). SSA mediates special sensations of vision from retina; audition, equilibrium from inner ear; regulates special senses of vision(CN II) audition, equilibrium, and proprioception CN VIII). SVA mediates sensations of taste from tongue and olfaction from nose; mediates smell (CN I) and taste (CNs VII, IX, and X).

48 Cranial Nerve Nuclei  As in the spinal cord, the cell bodies of motor and sensory neurons differ in location from one another.  The cell bodies of motor neurons that send their axons into the cranial nerves are located within the brainstem.  The cell bodies of the afferent fibers in the cranial nerves lie outside the brainstem, either in ganglia or in specialized end-organs such as the eye.  The cranial motor nuclei of all general somatic, general visceral, and special somatic motor neurons (shown in red) are located in the brainstem.  These cranial motor neurons are called lower motor neurons.

49 Cranial Nerve Nuclei  The cranial motor nuclei of all general somatic, general visceral, and special somatic motor neurons (shown in red) are located in the brainstem.  These cranial motor neurons are called lower motor neurons.  These nuclei are organized into seven longitudinal columns rostrocaudally in the brainstem according to function.

50 Somatic Efferent Column  CN nuclei III, IV, VI, and XII that innervate somatic muscles in the head derived from myotomes are situated close to the midline and immediately ventral to the floor of the fourth ventricle (red).

51 Special Visceral Efferent Column  CN nuclei V, VII, IX, X, and XI that innervate the branchiomeric muscles are displaced ventrally and laterally from the somatic motor column (orange).  The cell bodies of CNs V and VII in the pons and IX and X in the medulla are clustered in a single group called the nucleus ambiguus.

52 Nucleus Ambiguus  The nucleus ambiguus is so named because it is penetrated by fibers running from the inferior olive to the cerebellum and is consequently difficult to identify in sections stained for cell bodies.  Neurons in the nucleus ambiguus innervate striated muscles in the larynx and pharynx and are critical for speech and swallowing.

53 General Visceral Efferent Column  The parasympathetic neurons of CNs III, VII, IX, X are found immediately lateral to the somatic motor column (yellow).  They regulate specific autonomic functions.

54 GVE Column Functions  The oculomotor cranial nerve (III) innervates the smooth muscle of the eyelid, the pupillary constrictor, and Mueller’s muscle, which holds the eye forward in the orbit.  Damage to this autonomic component of the CN III results in eyelid droop (ptosis), pupil dilation (mydriasis) and the eye being drawn forward in the orbit (exophthalmos).

55 GVE Column Functions  The superior salivatory and inferior salivary nuclei are another group of general visceral column motor nuclei.  The axons from the superior salivatory nucleus run in the root of the facial nerve (VII).  The axons from the inferior salivary nucleus run in the glossopharyngeal (IX) nerve.  Together they innervate various salivary and mucous glands. Finally, the dorsal motor nucleus of the vagus (X) CN innervates the viscera of the body: the heart, the lungs, and the gut.  In the gut, the CN X promotes peristalsis.

56 Cranial Sensory Nuclei  The cell bodies of the afferent fibers supplying the cranial nerves lie outside the brainstem.  The sensory nuclei in the brainstem are composed of second order neurons that receive input from the primary sensory neurons.

57 General Visceral Afferent Column  Second order neuronal cell bodies of the general visceral afferent column (blue check) lie adjacent to the general visceral efferent column.  They receive fibers conveying the sense of taste, as well as those carrying input from the larynx and pharynx, and the heart, lungs, and gut.

58 Nucleus Tractus Solitarius  In the medulla, this column of cells is called the nucleus tractus solitarius (NTS).  The neurons conveying sensory input to the NTS (CNs VII Facial, IX Glossopharyngeal, X Vagus) have their cell bodies in ganglia that lie outside the brainstem.  The axons from these cell bodies run into the brainstem and join the solitary tract which terminate in the NTS.

59 Nucleus Tractus Solitarius  The rostral end of the NTS is the relay for taste sensation.  Axons from these nuclei synapse in the thalamus.  From the thalamic nuclei, information about taste is relayed to the cerebral cortex.  The other regions of the nucleus deal with cardiovascular functions.  They have local connections with the reticular formation, and indirect connections with the limbic system in regulating autonomic tone.

60 Special Somatic Afferent (SSA) Column  The SSA column (purple dots) lies lateral to the general visceral afferent column.  The SSA nuclei in the caudal part of the pons and the rostral part of the medulla receive fibers of the vestibulocochlear nerve (VIII).

61 Special Somatic Afferent (SSA) Column  The cochlear nuclei (CN) receive input from the cochlear division of CN VIII.  The vestibular nuclei (VN) receive input from the visceral division of CN VIII.

62 General Somatic Afferent Column  The general somatic afferent column is displaced ventrolaterally (green dots).  It is composed of the three separate divisions of the sensory trigeminal nucleus (V).  The portion of the nucleus which lies in the midbrain modulates proprioception from the muscles and joints of the face.

63 General Somatic Afferent Column  The main sensory nucleus lies in the pons.  The portion of the nucleus which lies in the medulla receives input from the muscles, skin, joints of the face, and mucous membranes of the mouth.

64 Cranial Nerve Organization  Three principles underlie the organization of the cranial nerves.  First, most of the motor nuclei in the brainstem are associated with individual cranial nerves.  For example, the trigeminal nerve has it own motor nucleus.  Each motor nucleus receives input from motor areas of the cerebral cortex and send its axons to muscles in the periphery.

65 Cranial Nerve Organization  Second, afferent nuclei in the brainstem often receive fibers from several cranial nerves.  The medullary nucleus of the trigeminal nerve receives sensory input from several cranial nerves.  The NTS, for example, collects fibers carrying information about taste from the facial (VII), glossopharyngeal (IX), and vagus (X) nerves.  The interesting point is that sensory information of a particular type, such as taste, is forwarded to a single nucleus, no matter which cranial nerve pathway it takes.

66 Cranial Nerve Organization  The third principle is related to location: neurons with different functional properties occupy consistently different positions in the brainstem.  This specificity of localization arises during development.

67 Cranial Nerve Organization  Neurons destined for different functions arise from distinctive parts of the neuroepithelial lining of the neural tube.  They differentiate at characteristic times in development, and migrate to specific positions in the brainstem.

68 Assessment of Cranial Nerves  An assessment of the functioning of the cranial nerves is an extremely important part of a clinical neurological examination.  Disease states in the brain are often reflected in functional abnormalities of one or more of the cranial nerves.  Since the cranial nerves originate from different regions in the brainstem, disorder in the function of one or more nerves can provide valuable information about the site of lesion.

69 Reticular Neurons and Processes  Aside from the major cranial nerve nuclei and the communicative pathways with which they are associated, and nuclei related to cerebellar function, the rest of the brainstem is composed of reticular neurons and their processes.  Found outside the major nuclear groups of the brainstem, these neurons and processes constitute the reticular formation.  The reticular formation represents an extensive interneuronal network distributed throughout the medulla, pons, and midbrain.

70 Reticular Nuclei and Axons  Lying in the midline are the raphe nuclei, so named because of their proximity to the midline seam or raphe.  Adjacent to the raphe is the large-cell region of the reticular formation and more laterally still is the small-group region.

71 Reticular Projections  Nearly all reticular neurons have far-flung distribution of their axons in both caudal and rostral directions along the brainstem.  Through its direct and indirect projections, the reticular formation influences all nervous system functions.  Reticular nuclei send input to brain structures such as the cochlear and vestibular nuclei, tectal (colliculi), and pretectal structures (red nucleus), geniculate nuclei, and thalamic nuclei that process specialized and general sensory input.

72 Reticular Projections  Reticular afferents consist of all collaterals from the ascending and descending spinal tracts (pain, proprioception, tactile, temperature, vibration), cranial nerve nuclei, cerebellum, midbrain, thalamus, subthalamus, hypothalamus, striatum, limbic lobe, and various cortical areas.  They project to autonomic and somatic motor nuclei of cranial nerves in the brainstem, and they relay information to interneuronal pools in the spinal cord.

73 Reticular Projections  There also project directly and indirectly to the cerebellum, red nucleus, substantia nigra, midbrain tectum, subthalamic nuclei, hypothalamus, thalamus, and limbic lobe (septum, hippocampus, amygdala, and cingulate gyrus).  For example, most coma-causing lesions are found at the midbrain, hypothalamic, and thalamic junctions.  Impairment at the level of the midbrain reticular formation has been implicated.

74 Reticular Influences  Direct reticular projections influence information processing by either accentuating or attenuating the sensory (audition, vision, olfaction, pain, temperature, and tactile) stimuli.  For example, reticulospinal projections modulate the quantity and quality of sensory information.  They employ gating mechanisms at both pre- and post-synaptic terminals to affect sensory impulse transmission at both spinal and thalamic levels.

75 Reticular Influences  Within the reticular formation, specific nuclear cell aggregates form closed-loop circuits and serve as the reticular centers for regulating various sensorimotor, visceral, and cortical activating functions.  Reticular centers may combine to form reticular networks that regulate complex sensorimotor and visceral behaviors, such as eating, swallowing, vomiting, coughing, sneezing, copulation, and fighting.

76 Reticular Influences  Integrated motor functions involving brainstem reticular nuclei regulate vasomotor activities that are vital to survival.  The reticular nuclei regulating vasomotor functions of the heart extend from the rostral medulla to the mid pons.  These nuclei receive extensive projections from peripheral receptors and are in part controlled by the hypothalamus.  They project information via fibers of the vagus nerve.

77 Reticular Influences  Stimulation of the lateral reticular pressor center in the upper medulla increases heart rate and causes vasoconstriction.  Stimulation of the lateral reticular depressor center in the lower medulla decreases heart rate and causes vasodilation.  Lesions of the medullary cardiovascular center can cause cardiac irregularities and blood pressure changes, both of which are life-threatening.

78 Respiratory Functions  The automatic brainstem respiratory center consists of several groups of scattered neurons in the reticular formation.  These groups can be divided into two distinct centers: the pontine centers and the medullary respiratory center.  The medullary respiratory group consists of the dorsal respiratory group and the ventral respiratory group of neurons.

79 Respiratory Functions  The fibers descending from the dorsal medullary respiratory neurons cross to activate spinal cord nerves that cause contraction of the diaphragm to begin the cycle of inspiration.  The ventral group of respiratory nuclei are inactive during the normal respiratory cycle.

80 Respiratory Functions  The ventral group is activated when high levels of pulmonary ventilation are required and projections from these nuclei cross to activate the needed thoracic muscles.

81 Respiratory Functions  The pontine respiratory centers generally control the duration, depth, and rate of the respiratory cycle.  Because exhalation is normally passive, the pneumotaxic area constantly inhibits the medullary respiratory centers to limit the duration of the inspiratory phase of the lungs.  A lesion in the ponto-medullary respiratory center causes asphyxia and eventually death if artificial respiration is not administered.

82 Swallowing Function.  As with respiration, the reticular formation regulates the acts of swallowing by integrating the function of several nerve pathways.  Specifically, the reticular swallowing center directs projections to the adjacent respiratory center and to the nuclei of cranial nerves V, VII, IX, X, and XII, to initiate a series of fixed action motor patterns that ensure proper breathing management while the bolus passes through the pharynx.

83 Swallowing Function.  As the bolus passes out of the pharynx, the reticular respiratory center regulates the reopening of the respiratory passageway.  Brainstem lesions interrupting afferent and efferent projections of the reticular formation can alter the integrity of the swallow response.

84 Vomiting Function  Vomiting is regulated in the medullary reticular formation where it receives input from the oropharynx and gastrointestinal tract. Vomiting  Noxious impulses mediating irritations of the intestinal tract and oropharynx initiate reflexive vomiting.  Projections from the vomiting center in the medulla descend through the fibers of cranial nerves X and IX and coordinate the contraction of the abdominal, diaphragmatic, and intercostal muscles.  The oropharyngeal musculature, which facilitates vomiting, works with all of these muscles.

85 Coughing Function  Coughing is a reflexive response to irritation of laryngeal and tracheal tissues.  Afferents mediating irritation initiate the coughing reflex via cranial nerve X and the NTS.  Diaphragm, abdominal, and intercostal muscles are involved, but they contract alternately, not simultaneously, as with vomiting.

86 Other Reticular Functions  In addition to visceral integrated activity, the reticular formation is also involved in cortical arousal and sensorimotor elaboration.  Cortical arousal is the best known function of the reticular formation.  The reticular formation uses various neurotransmitters to communicate with the brain, spinal cord, and neighboring reticular regions to contribute to cortical arousal.  These neurotransmitters are synthesized by a specialized population of the cells.

87 Other Reticular Functions  Studies of the brain using histochemical techniques have shown that many reticular neurons contain biogenic amines and neuroactive peptides that are believed to be used as chemical messengers.  In sections of a brain that have been exposed to radio- labeled transmitter substances, it is possible to identify the distinctive fluorescence emitted by each of these biogenic amines and to map their distribution throughout the reticular formation.  The three most prominent groups of reticular neurons are those that contain norepinephrine, dopamine, or serotonin.

88 Noradrenergic System  Those neurons that contain norepinephrine are part of the noradrenergic system.  In humans, each locus coeruleus (LC) lying on either side of the caudal midbrain and upper pons, is made up of approximately 12,000 noradrenergic neurons that have extensive axonal connections with the entire CNS.  At least five noradrenergic tracts have been identified, fanning out from the LC to innervate just about every part of the brain.

89 Noradrenergic System  All of the cerebral cortex, the thalamus, the hypothalamus, the olfactory bulb, the hippocampus, the midbrain, the cerebellum and the spinal cord receive LC projections.

90 Noradrenergic System  LC cells seem to be involved in the regulation of attention, arousal, and sleep-wake cycles, as well as in learning, memory, anxiety and pain, mood, and brain metabolism.  Because of its widespread connections, the LC can influence virtually all parts of the brain.

91 Noradrenergic Functions  The effects of norepinephrine are varied, depending on the part of the brain that it activates.  Studies with rats and monkeys have shown that LC neurons are activated by new, unexpected, non- painful, sensory stimuli in the animal’s environment.  They are least active when the animals are not vigilant, just sitting around quietly digesting a meal.  Therefore, the LC neurons participate in a general arousal of the brain in situations that are startling or interesting or that call for watchfulness, playing a role in maintaining attention and vigilance.

92 Noradrenergic Functions  Noradrenergic projections are known also to regulate brain tone by inhibiting background activity to enhance the signal-to-noise ratio in the brain increasing efficiency and responsiveness to salient sensory stimuli.  Behaviorally, the LC contributes to the generation of REM sleep.

93 What’s love go to do with it?  Norepinephrine is associated also with imprinting, the curious animal habit of doggedly concentrating attention on another and following this individual everywhere that he or she wanders.  Infatuation may be a human form of imprinting.  Moreover, when levels of norepinephrine increase, there is enhanced memory for new stimuli.

94 What’s love go to do with it?  Increasing levels of norepinephrine could explain why the lover can remember the smallest details of the beloved’s actions and vividly remember novel moments spent together.  Increasing levels of norepinephrine also produce exhilaration, excessive energy, sleeplessness, and loss of appetite—some of the basic characteristics of romantic love.

95 Serotonergic System  Clusters of serotonergic neurons are found along the raphe.  Each of these nine nuclei clusters projects to different regions of the brain.  The more caudal nuclei, in the medulla, innervate the spinal cord, where they modulate pain-related sensory signals.  Those more rostral, in the pons and midbrain, innervate most of the brain much the same way as do the LC neurons.

96 Serotonergic System  Specifically, serotonin pathways supply the hippocampus, the frontal lobes, the caudate and putamen, the hypothalamus, and thalamus.  Raphe nuclei cells fire most rapidly during wakefulness, when the animal is aroused and active and they are quietest during sleep.

97 Serotonergic System  These neurons seem to be intimately involved in the control of sleep-wake cycles as well as the different stages of sleep.  Serotonin raphe neurons have also been implicated in the control of mood and certain types of emotional behavior.  Human studies suggest that having normal to slightly higher levels of serotonin function may tend to translate into a number of socially useful, salutary effects on mood and behavior.

98 Serotonergic System  High levels of serotonin can be useful for multi-tasking, but they can play havoc on one’s libido.  In contrast, low serotonin levels correlate with depression and well as impulsivity.  Doctors who treat individuals with most forms of obsessive-compulsive disorder prescribe selective serotonin reuptake inhibitors (SSRIs) like Prozac or Zoloft to elevate levels of serotonin in the brain.

99 What’s love got to do with it?  A striking symptom of romantic love is incessant thinking about the beloved; lovers cannot turn off their racing thoughts.  Their obsessive cogitation about the beloved is thought to be due to decreased brain serotonin.  All those countless hours when your mind races like a mouse upon a treadmill is probably due to a negative relationship between serotonin and its relatives, dopamine and norepinephrine.

100 What’s love got to do with it?  As levels of dopamine and norepinephrine climb, serotonin levels plummet.  This could explain why lovers experience the compulsion to daydream, fantasize, muse, ponder, obsess about their romantic partner.

101 Dopaminergic System  The dopaminergic (DA) nuclei of the reticular formation are located in the midbrain.  The substantia nigra, with its darkly pigmented cell bodies, projects to the motoric nuclei of the basal ganglia to facilitate the initiation of voluntary movement.  Specifically, the projections from the substantia nigra to the caudate nucleus and the putamen (collectively known as the striatum) are referred to as nigrostriatal or mesostriatal projections reflecting their midbrain origin.

102 Dopaminergic System  The second dopaminergic source is the ventral tegmental area (VTA).  The VTA is a mother lode for dopamine-making cells.  With their tentacle-like axons, these nerve cells distribute dopamine to many brain regions including the caudate nucleus.  It has projections to limbic structures such as the amygdala via mesolimbic fibers and to the cerebral cortex via mesocortical fibers.

103 Dopaminergic System  The nigrostriatal projection and the mesocortical projections to the motor cortex are both consistent with the idea that the DA system is involved in the initiation of movement.  Disruption in these pathways, or degeneration of DA- producing cells, is instrumental in the movement deficits of Parkinson’s disease.

104 Dopaminergic System  However, the extensive DA projections to limbic structures and other cortical areas suggest that this system is also involved in motivation and cognition.  Elevated levels of dopamine in the brain produce extremely focused attention, as well as unwavering motivation and goal-directed behavior.  When a reward is delayed, dopamine-producing cells in the brain increase their work, pumping out more of this natural stimulant to energize the brain, focus attention, and drive the pursuer to strive even harder to acquire a reward.

105 Dopaminergic System  Dopamine has been associated also with learning about novel stimuli.  Dopamine levels rise when people are in novel situations.  Imbalances in the DA system are thought to play a role in certain forms of mental illness, such as schizophrenia and thought disorders.  Very high levels of dopamine can make one feel anxious, fearful, even panicky.  Many drugs of abuse enhance the action of DA release in limbic structures producing a sense of pleasure.

106 What’s love got to do with it?  Dependency and craving, symptoms of addiction, as well as romantic love, are associated with elevated levels of dopamine.  Dopamine produces focused attention, as well as fierce energy, concentrated motivation to attain a reward, and the feelings of elation, even mania—the core feelings of romantic love.  Lovers intensely focus on the beloved, often to the exclusion of all around them; they concentrate so relentlessly on the positive qualities of the adored one that they easily overlook his/her negative traits.

107 What’s love got to do with it?  Ecstasy is another trait of lovers that appears to be associated with dopamine: elevated concentrations of dopamine in the brain produce exhilaration, as well as many of the feelings that lovers report—increased energy, hyperactivity, sleeplessness, loss of appetite, trembling, a pounding heart, accelerated breathing, and sometimes mania, anxiety, or fear.  Dopamine may explain why love-stricken individuals become so dependent on their romantic relationship and why they crave emotional union with their beloved.  It probably also fuels the intense motivation to see, talk with, and be with the beloved.

108 Love’s Complex Chemistry  Given the properties of these three related chemicals in the brain, they probably all play a role in human romantic passion:  The feelings of euphoria, sleeplessness, and loss of appetite;  The lover’s intense energy, focused attention, driving motivation, and goal-oriented behaviors;  The lover’s tendency to regard the beloved as novel and unique; and  The lover’s increased passion in the face of adversity.

109 Love’s Complex Chemistry  Nonetheless, passionate romantic love takes a variety of graded forms, from pure elation when one’s love is reciprocated to feelings of emptiness, despair, and often rage when one’s love is thwarted.  These chemicals undoubtedly vary in their concentrations and combinations as the relationship ebbs and flows.  When one’s passion is returned, the brain tacks on positive emotions, such as elation and hope.  When one’s love is spurned or thwarted instead, the brain links this motivation with negative feelings, such as despair and rage.

110 The Drive to Love  Neuroscientist Don Pfaff (1999) defines a drive as a neural state that energizes and directs behavior to acquire a particular biological need to survive or reproduce.  Drives involve primary motivation systems oriented around planning and pursuit of a specific want or need.  We need food. We need water. We need warmth.  Like all other drives, romantic love is a need, a craving, and the lover feels he/she needs the beloved.

111 The Drive to Love  All drives have two components:  a generalized arousal system; and  a specific constellation of brain systems to produce the feelings, thoughts, and behaviors associated with each particular biological need.  The general arousal component of all drives is associated with the action of dopamine, norepinephrine, serotonin, acetylcholine, several histamines, and perhaps other brain chemicals.

112 The Drive to Love  The specific constellation of brain regions and systems associated with each particular drive varies considerably.  In their fMRI study, Helen Fisher and her colleagues (2003) found the general arousal component of romantic love associated with the VTA and the distribution of central dopamine.  They also found activation in the caudate body and tail, the septum, and the white matter of the posterior cingulate; several brain regions were also deactivated.

113 The Drive to Love  These areas may constitute part of the system specific to intense, early stage romantic love.  In addition to brain system chemistry, the prefrontal cortex is also hypothesized to be involved.  This “central executive” collects data from our senses, weighs them, integrates thoughts with feelings, makes choices, and controls our basic drives.  Because romantic love is focused on a specific reward—the beloved--it constitutes a primary motivation system.

114 The Drive to Love  Several regions of the prefrontal cortex are associated with monitoring rewards (Schultz, 2000).  With respect to love, the orbitofrontal and the medial prefrontal cortices are specifically involved.  The orbitofrontal cortex is involved in detecting, perceiving, and expecting rewards, as well as discriminating between rewards and making preferences.  The medial prefrontal cortex experiences emotions, bestows meaning to our perceptions, guides our reward- related behaviors, creates our mood, and also makes preferences.

115 The Drive to Love  The drive to love, then, is an elegant design.  The passion of love emanates from the motor of the mind, the caudate nucleus.  It is fueled by at least one of nature’s most powerful stimulants, dopamine.  When one’s passion is returned, the brain tacks on positive emotions, such as elation and hope.  When one’s love is spurned or thwarted instead, the brain links this motivation with negative feelings, such as despair and rage.

116 The Drive to Love  And all the while, regions of the prefrontal cortex monitor the pursuit, planning tactics, calculating gains and losses, and registering one’s progress toward the goal: emotional, physical, even spiritual union with the beloved.  Indeed, this three-pound blob can generate a need so intense that all the world has sung of it: romantic love.  And to make our lives even more complex, romantic passion is intricately enmeshed with two other basic mating drives, the sex drive and the urge to build a deep attachment to a romantic partner (Fisher, 2004).

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