part 1 landscape_anatomy and physiology of the nervous system

8/14/2019 Part 1 Landscape_anatomy and Physiology of the Nervous System

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ANATOMY AND PHYSIOLOGY OF THE NERVOUS SYSTEM

I. Introduction

The nervous system is the master controlling and communicating system of the body. Every thought, action, andemotion, reflects its activity. Its signaling device, or means of communicating with body cells, is electrical impulses,which are rapid and specific and cause almost immediate response. To carry out its normal role, the nervous systemhas three overlapping function: (1) much like a sentry, it uses millions of sensory receptors to monitor changesoccurring both inside and outside the body. These changes are called stimuli, and the gathered information is calledsensory input. (2) It process and interprets the sensory input and makes decisions about what should he done ateach momenta process called integration. (3) It then effects a response by activating muscles or glands(effectors) via motor output. An example will illustrate how these functions work together. When you are driving andsee a red light just ahead (sensor input), your nervous system integrates this information (red light means "stop")and sends motor output to the muscles of your right leg and foot, and your foot goes for the brake pedal (theresponse).

The nervous system does not work alone to regulate and maintain body homeostasis: the endocrine system is asecond important regulating system. While the nervous system controls with rapid electrical nerve impulses, theendocrine system organs produce hormones that are released into the blood. Thus, the endocrine system typicallybrings about its effects in a more leisurely way.

II. Classification

We have only one nervous system, hut, because of its complexity, it is difficult to consider all its parts at the sametime. So, to simplify its study, we divide it in terms of its structures (structural classification) or in terms of its

activities (functional classification). Each of these classification schemes is described briefly below, and theirrelationships are illustrated in Figure 7.2. It is not necessary to memorize this whole scheme now, but as you arereading the descriptions, try to get a "feel" for the major parts and how they fit together. This will make your


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learning task easier as you make your way through this chapter. Later you will meet all these terms and conceptsagain and in more detail.

a. Structural ClassificationThe structural classification which includes all nervous system organs has two subdivisionsthe central nervoussystem and the peripheral nervous system (see Figure 7.2).The central nervous system (CNS) consists of the brain and spinal cord, which occupy the dorsal body cavity andact as the integrating and command centers of the nervous system. They interpret incoming sensory informationand issue instructions based on past experience and current conditions.The peripheral (p0-rifer-al) nervous system (PNS), the part of the nervous system outside the CNS, consistsmainly of the nerves that extend from the brain and spinal cord. Spinal nerves carry impulses to and from thespinal cord. Cranial (kra'ne-al) nerves carry impulses to and from Tie brain. These nerves serve ascommunication lines. They link all parts of the body by carrying impulses from the sensory receptors to the CNSand from the CNS to the appropriate glands or muscles.

b. Functional Classification

The functional classification scheme is concerned only with PNS structures. It divides them into two principalsubdivisions (see Figure 7.2).The sensory, or afferent (afferent), division consists of nerve fibers that convey impulses to the central nervoussystem from sensory receptors located in various parts of the body. Sensory fibers delivering impulses from theskin, skeletal muscles, and joints are called somatic (soma = body) sensory (afferent) fibers, whereas thosetransmitting impulses from the visceral organs are called visceral sensory fibers, or visceral afferents. Thesensory division keeps the CNS constantly informed of events going on both inside and outside the body.


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FIGURE 7.1 The nervous system's functions.

The motor or efferent (ef'er-rent), division carries impulsesfrom the CNS to effector organs the muscles and glands. Theseimpulses activate muscles and glands; that is, they effect(bring about) a motor response.The motor division in turn has two subdivisions (see Figure7.2):

1. The somatic (so-mat'ik) nervous system allows us toconsciously, or voluntarily, control our skeletal muscles. Hence

this subdivision is often referred to as the voluntary nervoussystem. However, not all skeletal muscle activity controlled by


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this motor division is voluntary. Skeletal muscle reflexes, like the stretch reflex forexample are initiated involuntarily by these same fibers.

2. The autonomic (aw"to-nonfik) nervous system (ANS) regulates events that areautomatic, or involuntary, such as the activity of smooth and cardiac muscles andglands. This subdivision, commonly called the involuntary nervous system itself

has two parts, the sympathetic and parasympathetic, which typically bring aboutopposite effects. What one stimulates, the other inhibits. These will be describedlater.

Although it is simpler to study the nervous system in terms of its subdivisions,you should recognize that these subdivisions are made for the sake ofconvenience only. Remember that the nervous system acts as a coordinated unit,both structurally and functionally.

III. Neurons and Nerves

Even though it is complex, nervous tissue is made up of just two principal types of cellssupporting cells andneurons.

a. Supporting CellsSupporting cells in the CNS are "lumped together" as neuroglia (nu-rog'le-ah), literally "nerve glue." Neurogliaincludes many types of cells that generally support, insulate, and protect the delicate neurons (Figure 7.3). Inaddition, each of the different types of neuroglia, also simply called glia (gle'ah) or ghat cells has specialfunctions. The CNS glia include:

FIGURE 7.2 Organization of the nervou

Organizational flowchart showing that nervous system receives input via senand issues commands via motor fsensory and motor fibers together nerves that constitute the periphersystem.


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FIGURE 7.3 Supporting (glial) cells of nervoustissues. Astrocytes (a) form a living barrier betweenneurons and capillaries in the CNS. Microglia (b) arehagocytes, whereas ependymal cells (c) line the fluid-

filled cavities of the CNS. The oligodendrocytes (d) formmyelin sheaths around nerve fibers in the CNS. (e) Therelationship of Schwann cells (myelinating cells) andatellite cells to a neuron in the peripheral nervous

Astrocytes: abundant star-shaped cells that 'accountfor nearly half of the neural tissue. Their numerousprojections have swollen ends that cling to neurons,bracing them and anchoring them to their nutrientsupply lines, the blood capillaries (Figure 7.3a).

Astrocytes form a living barrier between capillaries andneurons and play a role in making exchanges betweenthe two. In this way, they help protect the neurons fromharmful substances that might be in the blood.

Astrocytes also help control the chemical environment inthe brain by picking up excess ions and recapturingreleased neurotransmitters.

Microglia: spiderlike phagocytes that dispose ofdebris, including dead brain cells and bacteria tFigure

7.3b). Ependymal: these glial cells line the cavities of the brain and the spinal cord (Figure 7.3c). The heating of

their cilia helps to circulate the cerebrospinal fluid that fills those cavities and forms a protective cushkinaround the CNS.

Oligodendrocytes: glia that wrap their flat extensions tightly around the nerve fibers, producing fattyinsulating coverings called myelin sheaths (Figure 7.3d).

Although they somewhat resemble neurons structurally (both cell types have cell extensions), glia are not ableto transmit nerve impulses, a function that is highly developed in neurons. Another important difference is thatglia never lose their ability to divide, whereas most neurons do. Consequently, most brain tumors are gliomas,


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FIGURE 7.4 Structure of a typical motor neuron. (a)Diagrammatic view. (b) Photomicrograph (265x ).

or tumors formed by glial cells (neuroglia). Supporting cells in the PNS come in two major varietiesSchwanncells and satellite cells (Figure 7.3e). Schwann cells form the myelin sheaths around nerve fibers that are foundin the PNS. Satellite cells act as protective, cushioning cells.

b. Neurons

Neurons, also called nerve cells, are highly specialized to transmit messages (nerve impulses) from one part ofthe body to another. Although neurons differ structurally, they have many common features (Figure 7.4). All havea cell body, which contains the nucleus and is the metabolic center of the cell, and one or more slenderprocesses extending from the cell body.

The cell body is the metabolic center of the neuron. It contains the usual organelles except for centrioles (whichconfirm the amitotic nature of most neurons). The rough ER, called Nissl (nisi) substance, and neurofibrils,intermediate filaments that are important in maintaining cell shape, are particularly abundant in the cell body.The armlike processes, or fibers, vary in length from microscopic to 3 to 4feet. The longest ones in humans reach from the lumbar region of thespine to the great toe. Neuron processes that convey incoming messages(electrical signals) toward the cell body are dendrites (den'dritz), whereas

those that generate nerve impulses and typically conduct them awayfrom the cell body are axons (ak'sonz). Neurons may have hundreds ofthe branching dendrites (dendr = tree), depending on the neuron type,but each neuron has only one axon, which arises from aconelike region of the cell body called the axon hillock.

An occasional axon gives off a collateral branch along itslength, but all axons branch profusely at their terminalend, forming hundreds to thousands of axon terminals. These terminalscontain hundreds of tiny vesicles, or membranous sacs, that containchemicals called neurotransmitters

As we said, axons transmit nerve impulses away from the cell body.When these impulses reach the axon terminals, they stimulate therelease of neurotransmitters into the extracellular space.


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FIGURE 7.5 Relationship of Schwanncells to axons in the peripheral nervoussystem. As illustrated (top to bottom), Schwann cell envelops part of an axon in trough and then rotates around the axonMost of the Schwann cell cytoplasm comes tlie just beneath the exposed part of itplasma membrane. The tight coil of plasmamembrane material surrounding the axon ithe myelin sheath. The Schwann cecytoplasm and exposed membrane ar

Each axon terminal is separated from the next neuron by a tiny gap called the synaptic (si-nap'tik) cleft. Such afunctional junction is called a synapse (syn = to clasp or join). Although they are close, neurons never actuallytouch other neurons. We will learn more about synapses and the events that occur there a bit later.

Most long nerve fibers are covered with a whitish, fatty material, called myelin (mi'e-lin), which has a waxy

appearance. Myelin protects and insulates the fibers and increases the transmission rate of nerve impulses.Axons outside the CNS are myelinated by Schwann cells, specialized supporting cells that wrap themselvestightly around the axon jelly-roll fashion (Figure 7.5). When the wrapping process is done, a tight coil of wrappedmembranes, the myelin sheath, encloses the axon. Most of the Schwann cell cytoplasm ends up just beneath theoutermost part of its plasma mem-brane. This part of the Schwann cell, external to the myelin sheath, is calledthe neurilemma (nu"ri-lem'mah, "neuron husk"). Since the myelin sheath is formed by many individual Schwanncells, it has gaps or indentations called nodes of Ranvier (rahn-ver), at regular intervals (see Figure 7.4).

Myelinated fibers are also found in the central nervous system.However, there it is oligodendrocytes that form CNS myelin sheaths(see Figure 7.3d). In contrast to Schwann cells, each of which depositsmyelin around a small segment of one nerve fiber, the

oligodendrocytes with their many flat extensions can coil around asmany as 60 different fibers at the same time. Although the myelinsheaths formed by oligodendrocytes and those formed by Schwanncells are quite similar, the CNS sheaths lack a neurilemma. Becausethe neurilemma remains intact (for the most part) when a peripheralnerve fiber is damaged, it plays an important role in fiber regeneration,an ability that is largely lacking in the central nervous system.

Neurons may be classified either according to boa they function or according to their structure.


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FIGURE 7.6 Neurons classified by function. Sensoryneurons conduct impulses from sensory receptors (in the skmuscles) to the central nervous system; most cell bodies arin the PNS. Motor (efferent) neurons transmit impulses fro(brain or spinal cord) to effectors in the body periphery. neurons (interneurons) complete the communication pathwsensory and motor neurons; their cell bodies reside in the CNS

1. Functional Classification Functional classifications groups neurons according to the direction the nerveimpulse is traveling relative to the CNS .On this basis, there are sensory, motor, and association neurons

(Figure 7.6). Neurons carrying impulses from sensoryreceptors (in the internal organs or the skin) to theCNS are sensory or afferent neurons. (Afferentliterally means (To go toward)

The cell bodies of sensory neurons always found in aganglion outside the CNS. Sensory neurons keep usinformed about what is happening both inside andoutside the body.

The dendrite endings of the sensory neurons areusually associated with specialized receptors that areactivated by specific changes occurring nearby. Thevery complex receptors of the special sense organs(vision, hearing, equilibrium. taste, and smell) arecovered separately in Chapter 8. The simpler types of

sensory receptors seen in the skin (cutaneous senseorgans) and in the muscles and tendons(proprioceptors) are shown in Figure 7 .7. The painreceptors (actually bare dendrite endings) are the

least specialized of the cutaneous receptors. Theyare also the most numerous, because pain warns usthat some type of body damage is occurring or isabout to occur. However, strong stimulation of any ofthe cutaneous receptors (for example, by searingheat, extreme cold or excessive pressure) is alsointerpreted as pain.


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FIGURE 7.8 Classification of neurbasis of structure. (a) Multipolar.

(c) Unipolar.

The proprioceptors detect the amount of stretch. or tension, inskeletal muscles, their tendons, and joints. They send thisinformation to the brain so that the proper adjustments can bemade to maintain balance and normal posture. Propria comes fromthe Latin word meaning "one's own," and the proprioceptors

constantly advise our brain of our own movements.

Neurons carrying impulses from the CNS to the viscera and/ormuscles and glands are motor, or efferent, neurons (see Figure7.6). The cell bodies of motor neurons are always located in theCNS.

The third category of neurons is the association neurons, orinterneurons. They connect the motor and sensory neurons inneural pathways. Like the motor neurons, their cell bodies arealways located in the CNS.

2. Structural Classification Structural classification is based on thenumber of processes extending from the cell body (Figure 7.8). Ifthere are several, the neuron is a multipolar neuron. Since allmotor and association neurons are multipolar, this is the mostcommon structural type. Neurons with two processesan axonand a dendriteare called bipolar neurons. Bipolar neurons arerare in adults, found only in some special sense organs (eye,nose), where they act in sensory processing as receptor cells.

Unipolar neurons have a single process emerging from the cell body. However, it is very short and dividesalmost immediately into proximal (central) and distal (peripheral) processes. Unipolar neurons are unique inthat only the small branches at the end of the peripheral process aredendrites. The remainder of the peripheral process and the central process

function as axons; thus, in this case, the axon conducts nerve impulses bothtoward and away from the cell body. Sensory neurons found in PNS gangliaare unipolar.


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FIGURE 7.9 The nerve impulse. (a) Resting membrane electrical

conditions. The external face of the membrane is slightly positive: itsinternal face is slightly negative. The chief extracellular on is sodium fhlarl,whereas the chief intracellular ion is potassium (K4). The membrane isrelatively impermeable to both ions. (b) Stimulus initiates localdepolarization. A stimulus changes the permeability of a "patch" of themembrane, and sodium ions diffuse rapidly into the cell. This changes the

olarity of the membrane (the inside becomes more positive; the outsidebecomes more negative). (c) Depolarization and generation of an action

otential. If the stimulus is strong enough, depolarization causesmembrane polarity to be completely reversed and an action potential isinitiated. (d) Propagation of the action potential. Depolarization of the firstmembrane patch causes permeability changes in the adjacent membrane,

and the events described in (b) are repeated. Thus, the action potentialropagates rapidly along the entire length of the membrane. (e)

Repolarization. Potassium ions diffuse out of the cell as membraneermeability changes again, restoring the negative charge on the inside of

Physiology

Nerve Impulses Neurons have two major functional properties: irritability,the ability to respond to a stimulus and convert it into a nerve impulse, andconductivity, the ability to transmit the impulse to other neurons, muscles,

or glands. We will consider these functional abilities next.

The plasma membrane of a resting, or inactive, neuron is polarized, whichmeans that there are fewer positive ions sitting on the inner face of theneuron's plasma membrane than there are on its outer face in the tissuefluid that surrounds it (Figure 7.9). The major positive ions inside the cellare potassium (K), whereas the major positive ions outside the cell aresodium (Na). As long as the inside remains more negative as compared tothe outside, the neuron will stay inactive.


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FIGURE 7.10 How communicate at chemical The events occurring at the sy

Many different types of stimuli excite neurons to become active and generate an impulse. For example, lightexcites the eye receptors, sound excites some of the ear receptors, and pressure excites some cutaneousreceptors of the skin. However, most neurons in the body are excited by neurotransmitters released by otherneurons, as will be described shortly. Regardless of what the stimulus is, the result is always the samethe

permeability properties of the cell's plasma membrane change for a very brief period. Normally, sodium ionscannot diffuse through the plasma membrane to any great extent; but when the neuron is adequatelystimulated, the "gates" of sodium channels in the membrane open. Because sodium is in much higherconcentration outside the cell, it will then diffuse quickly into the neuron. (Remember the laws of diffusion?) Thisinward rush of sodium ions changes the polarity of the neuron's membrane at that site, an event calleddepolarization. Locally, the inside is now more positive, and the outside is less positive, a situation called agraded potential. However if the stimulus is strong enough and the sodium in-flux is great enough, the localdepolarization (graded potential) activates the neuron to initiate and transmit a long distance signal called anaction potential, also called a nerve impulse in neurons. The nerve impulse is an all-or-none re- sauna like firing agun. It is either propagated (conducted) over the entire axon, or it doesn't happen at all. The nerve impulsenever goes partway along an axon's length, nor does it die out with distance as do graded potentials.

Almost immediately after the sodium ions rush into the neuron, themembrane permeability changes again, becoming impermeable tosodium ions but permeable to potassium ions. So potassium ions areallowed to diffuse out of the neuron into the tissue fluid, and they doso very rapidly. This outflow of positive ions from the cell restores theelectrical conditions at the membrane to the polarized, or resting,state, an event called repolarization. Until repolarization occurs, a

neuron cannot conduct another impulse. After repolarization occurs, the initial concentrations of the sodium andpotassium ions inside and outside the neuron are restored by activation of thesodium-potassium pump. This pump uses ATP (cellular energy) to pump excesssodium ions out of the cell and to bring potassium ions back into it. Oncebegun, these sequential events spread along the entire neuronal membrane.

The events just described explain propagation of a nerve impulse along unmyelinated fibers. Fibers that havemyelin sheaths conduct impulses much faster because the nerve impulse literally jumps, or leaps, from node to


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node along the length of the fiber. This occurs because no current can flow across the axon membrane wherethere is fatty myelin insulation. This type of impulse is called salutatory (Saltah-tore) conduction (saltare = todance or leap)

IV. Central Nervous System

Our body is made up of biological processes. Everything we feel, think or dohas biological components. Biological processes help us to understandbehavior. All of the psychological phenomena covered in this topic are adirect product of these biological processes. Psychology is the study of whatthe nervous system does. Therefore an understanding of this system isessential to an understanding of human psychology.

Every section of this part of the case presentation is about the brain and thenervous system. It is impossible to examine all of the major neuroanatomic

structures. The points of interest here include the structures of the brainbelieved to be involved in the formation of thought and emotion.

The brain is defined in various ways. The definition that best suits theperspective of this case study is that the brain is that part of the centralnervous system encapsulated by the skull. The brain is the core of ourhumanity. Intercommunication of different parts of the brain yield theexperiences of love, hate, elation, joy or madness. The brain provides theunderlying biology for will, determination, hopes and dreams. Without the

brain to integrate experience, people would neither enjoy the wonder nor fear the horror of life.

a. Brain


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Brains exist because the distribution of resources necessary for survival and the hazards that threaten survivalvary in space and time. There would be little need for a nervous system in an immobile organism or an organismthat lived in regular and predictable environment. Brains are informed by the senses about the presence ofresources and hazards; they evaluate and store this input and generate adaptive responses executed by themuscles.

Some of the most basic features of brains can be found in bacteria because even the simplest motile organismsmust solve the problem of locating resources and avoiding toxins. They sense their environment through a largenumber of receptors, which are protein molecules embedded in the cell wall. The action taken in response to theinputs usually depends on the gradient of the chemicals. Thus memory is required to compare the inputs fromdifferent locations. The strength of the signal is modulated by immediate past experience. This in turn regulatesthe strength of the signal sent by chemical messengers from the receptor to the flagellar motors. Thus even atthe unicellular level, the bacteria have already possessed the ability to integrate numerous analog inputs andgenerate a binary (digital) output of stop or go.


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In multicellular organism, cells specialized for receptor function are located on the surface. Other cellsspecialized for the transmission and analysis of information are located in the protected interior and are linked to

effector cells, usually muscles, which produce adaptive responses. As do unicellular organisms, neuronsintegrate the diverse array of incoming information from the receptors, which in neurons may result in the firingof an action potential (when the summation is above a threshold level) rather than swimming toward a nutrientsource as in the unicellular organisms. Once the threshold for generating an action potential is reached, thesignal is always the same, both in amplitude and shape (a nerve consists of many neurons, it does not obey theall-or-none law).


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Action potentials and voltage-gated sodiumchannels are present in jellyfish, which arethe simplest organisms to possess nervoussystems. The development of this basicneuronal mechanism set the stage for theproliferation of animal life that occurredduring the Cambrian period. Among theseCambrian animals were the early chordates,which possessed very simple brains. Some ofthese early fish developed a unique way toinsulate their axons by wrapping them with afatty material called myelin, which greatlyfacilitated axonal transmission and evolutionof larger brains. Some of their descendants,which also were small predators, crawled upon the muddy shores and eventually took uppermanent residence on dry land.Challenged by the severe temperature

changes in the terrestrial environment, someexperimented with becoming warm-blooded,and the most successful became theancestors of birds and mammals. Changes inthe brain and parental care were a crucialpart of the set of mechanisms that enabledthese animals to maintain a constant bodytemperature.

The human brain can be divided into threeparts: the hindbrain, which has been inherited from the reptiles; the limbic system, which was first emerged inmammals; and the forebrain, which has its full development in human. Different views of the human brain are

shown in Figure 03c, d, and e. Tables 01 lists the functions of the different parts of the human brain. The brain isseparated into two hemispheres. Apart from a single little organ -- the pineal gland in the centre base of thebrain -- every brain module is duplicated in each hemisphere. The left brain is calculating, communicative and


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Table 02 - Human Brain

capable of conceiving and executing complicated plans --the reductionistic brain; while the right one isconsidered as gentle, emotional and more at one with the natural world -- the holistic brain. The cerebral cortexis covered in a thin skin of acquiring knowledge by the use of reasoning, intuition or perception). Table 02 belowlists the location and functions of the major components in the human brain.

Structure Location Functions

Limbic System

(Mammalian Brain)Thalamus in the middle of the limbic system relays incoming information (except smell) to the appropriate part of the

brain for further processing.

Hypothalamus, PituitaryGland

beneath thalamus regulates basic biological drives, hormonal levels, sexual behavior, andcontrols autonomic functions such as hunger, thirst, and body

temperature.

Optic Chiasm in front of the pituitary gland left-right optic nerves cross-over point.

Septum adjacent to hypothalamus stimulates sexual pleasure

Hippocampus within the temporal lobe mediates learning and memory formation.

Amygdala in front of the hippocampus responsible for anxiety, emotion, and fear

Mammillary Body, Fornix linked to the hippocampus have a role in emotional behavior, learning, and motivation.

Basal Ganglia(Striatum): CaudateNucleus, Putamen,

Globus Pallidus

outside the thalamus involves in movement, emotions, planning and in integrating sensoryinformation

Ventricles and CentralCanal

from tiny central canal within the spinal cord to theenlarged hollows within the skull called ventricles

fills with cerebrospinal fluid for mechanical protection.

Cingulate Gyrus above corpus cal losum concentrates attent ion on adverse internal st imul i such as pain, conta insthe feeling of self.

Corpus Callosum under the cingulate gyrus is a bundle of nerve f ibers linking the cerebral hemispheres, involve inlanguage learning.

Forebrain(Human Brain)

Frontal Lobe(Conscious Brain)

in front of the head controls voluntary movement, thinking, and feeling.

Prefrontal Cortex in front of the frontal lobe inhibits inappropriate actions, forms plans and concepts, helps focusattention, and bestows meaning to perceptions.

Parietal Lobe in top rear of the head contains the pr imary somatosensory area that manages skin sensation.

Occipital Lobe in the back of the head contains the visual cortex to manage vision.

Temporal Lobe on each side of the head above the temples contains the auditory cortex to manage hearing and speech.
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The parietal eye is not an eye in the traditional sense in that it does not see images, but rather is aphotosensitive organ which only reacts to light and dark. The parietal eye is connected to the pineal body and isused to trigger hormone production and thermoregulation. It often shows up as either a dark spot or anopalescent spot. Opsin proteins sensitive to blue and green light has been identified in the cell.

Throughout its lifetime, the human brain undergoes more changes than any other part of the body. They can bebroadly divided into five stages. Table 03 summarizes the significant events within each stage, the "DO" and"DON'T" to keep a healthy mind.

Stage

Age Event(s) DO DON'T

1 0 - 10months

Gestation

* Growing neurons and connections* Making sure each section of the brain growsproperly and in the right place

Mother should:* be stress-free, eats well* take folic acid and vitamin B12* stimulate the young brain withsounds and sensations

* Mother should stay away fromcigarettes, alcohol and othertoxins

2 Birth - 6Childhood

* A sense of self develops as the parietal andfrontal lobe circuits become more integrated.* Development of voluntary movement,reasoning, and perception* Frontal lobes become active leading to thedevelopment of emotions, attachments,planning, working memory and attention* Life experiences shape the emotional well-being in adulthood* At age 6, the brain is 95% of its adult weightand at its peak of energy consumption

* Parents should provide a nurturingenvironment and one-on-oneinteraction

* Parents should beware of theemotional consequence of neglector harsh parenting

3 7 - 22Adolescen

ce

* Wiring of the brain is still in progress* Grey matter (neural connections) pruning

* White matter (fatty tissue surroundingneurons) increase helps to speed up electricalimpulses and stabilize connections* The prefrontal cortex (involving control of

* Teenagers should learn to controlreckless, irrational and irritable

behaviors* Do learn a skill to support life in thefuture

* Teenagers should avoid alcoholabuse, smoking, drug and

unprotected sex.
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Table 07 - The Five Stages of Human Brain

impulses, judgment and decision-making) is thelast to mature

4 23 - 65Adulthood

* The brain reaches the peak power at aroundage 22 and lasts for about 5 years; thereafterit's downhill all the way* The last to mature and first to go brainfunctions are those involve executive control inthe prefrontal and temporal cortices

* Episodic memory for recalling events alsodeclines rapidly* Processing speed slows down* Working memory is able to store lessinformation

* Stay active mentally and physically* Eat healthy diet

* Avoid cigarettes, booze, andmind-altering drugs.

5 > 65Old Age

* Losing brain cells in critical areas such as thehippocampus where memories are processed

* Exercise to improve abstractreasoning and concentration* Learn new skill such as guitar playingto attain the same effect* Practice meditation can promoteneutral emotions

* Avoid grumpiness by eatingcertain foods, such as yogurt,chocolate, and almonds to get agood dose of dopamine (forpromoting positive emotions)* Don't stressed out as it is relatedto higher risk of developingdementia.

It is well known that the brain is an electrochemical organ; a fully functioning brain can generate as much as 20watts of electrical power. Even though this electrical power is very limited, it does occur in very specific waysthat are characteristic of the human brain. Electrical activity emanating from the brain can be displayed in theform of brainwaves. There are four categories of these brainwaves, ranging from the most active to the leastactive. Figure 03f is produced by an EEG (ElectroEncephaloGraph) chart recorder to show the different kind ofbrainwave according to the different state of the brain. These are all oscillating electrical voltages in the brain,but they are very tiny voltages, just a few millionths of a volt. Electrodes are placed on the outer surface of the

head to detect electrical changes in the extracellular fluid of the brain in response to changes in potential amonglarge groups of neurons. The resulting signals from the electrodes are amplified and recorded.
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Brain waves originate from the cerebral cortex, but also reflect activities in other parts of the brain that influencethe cortex, such as the reticular formation. Because the intensity of electrical changes is directly related to thedegree of neuronal activity, brain waves vary markedly in amplitude and frequency between sleep andwakefulness. Beta wave rhythms appear to be involved in higher mental activity, including perception andconsciousness. It seems to be associated with consciousness, e.g., it disappears with general anesthesia. Other

waves that can be detected are Alpha, Theta, and Delta. When the hemispheres or regions of the brain areproducing a wave synchronously, they are said to be coherent. Alpha waves are generated in the Thalamus (thebrain within the brain), while Theta waves occur mainly in the parietal and temporal regions of the cerebrum.The Alpha and Theta waves seem to be associated with creative, insightful thought. When an artist or scientisthas the "aha" experience, there's a good chance he or she is in Alpha or Theta. These two kinds of brain wavesare also associated with relaxation and, stronger immune systems. Therefore, many people try to trainthemselves to enter such states through various biofeedback7 techniques (with varying degree of success).Delta Waves occur during sleep. They originate from the cerebral cortex when it is not being activated by thereticular formation. In slow-wave sleep, the entire brain oscillates in a gentle rhythm quite unlike the fragmentedoscillations of normal consciousness. The neocortical activity is often modulated by a rhythm of 40-80 Hz, calledthe Gamma wave (not shown in Figure 03f). When there are strong gamma oscillations in certain parts of theneocortex, human subjects do better on learning and memory tasks.


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Diagram of a cerebral capillary enclosed in astrocyte end-feet. Characteris

the blood-brain barrier are indicated: (1) tight junctions that seal the pabetween the capillary (endothelial) cells; (2) the lipid nature of thmembranes of the capillary wall which makes it a barrier towater-smolecules; (3), (4), and (5) represent some of the carriers and ion channethe 'enzymatic barrier'that removes molecules from the blood; (7) the pumps which extrude fat-soluble molecules that have crossed into the cells

b. Protection: Meninges, Blood- BrainBarrier and CSF

1. Blood-brain Barrier

The main function of the barrier (BBB) is to protect the brainfrom changes in the levels inthe blood of ions, amino acids,peptides, and othersubstances. The barrier is located atthe brain blood capillaries,which are unusual in two ways.Firstly, the cells which make upthe walls of these vessels (theendothelium) are sealed together attheir edges by tight junctions

that form a key componentof the barrier. These junctionsprevent water-soluble substances in the blood from passing between the cells and therefore from freelyentering the fluid environment of the brain cells. Secondly, these capillaries are enclosed by the flattenedend-feet of astrocytic cells (one type of glia), which also act as a partial, active, barrier. Thus the onlyway for water-soluble substances to cross the BBB is by passing directly through the walls of the cerebralcapillaries, and because their cell membranes are made up of a lipid/protein bilayer, they also act as amajor part of the BBB.

In contrast, fat-soluble molecules,including those of oxygen and carbon


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dioxide, anaesthetics, and alcohol can pass straight through the lipids in the capillary walls and so gainaccess to all parts of the brain.

Apart from these passive elements of the BBB there are also enzymes on the lining of the cerebralcapillaries that destroy unwanted peptides and other small molecules in the blood as it flows through the

brain.

Finally, there is another barrier process that acts against lipid-soluble molecules, which may be toxic andcan diffuse straight through capillary walls into the brain. In the capillary wall there are three classes ofspecialized efflux pumps which bind to three broad classes of molecules and transport them back intothe blood out of the brain.

However, in order for nourishment to reach the brain, water-soluble compounds must cross the BBB,including the vital glucose for energy production and amino acids for protein synthesis. To achieve this

transfer, brain vessels have evolved special carriers on both sides of the cells forming the capillary walls,which transport these substances from blood to brain, and also move waste products and otherunwanted molecules in the opposite direction.

The successful evolution of a complex brain depends on the development of the BBB. It exists in allvertebrates, and also in insects and the highly intelligent squid and octopus. In man the BBB is fullyformed by the third month of gestation, and errors in this process can lead to defects such as spinabifida.

Although the BBB is an obvious advantage in protecting the brain, it also restricts the entry from theblood of water-soluble drugs which are used to treat brain tumours or infections, such as the AIDS virus,which uses the brain as a sanctuary and hides behind the BBB from body defence mechanisms. To


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overcome these problems drugs are designed to cross the BBB, by making them more fat soluble. Butthis also means that they might enter most cells in the body and be too toxic. Alternative approaches areto make drug molecules that can ride on the natural transporter proteins in the cerebral capillaries, andso be more focused on the brain, or to use drugs that open the BBB.

Since the brain is contained in a rigid, bony skull, its volume has to be kept constant. The BBB plays a keyrole in this process, by limiting the freedom of movement of water and salts from the blood into theextracellular fluid of the brain. Whereas in other body tissues extracellular fluid is formed by leakage fromcapillaries, the BBB in fact secretes brain extracellular fluid at a controlled rate and is thus critical in themaintenance of normal brain volume. If the barrier is made leaky by trauma or infection, water and saltscross into the brain, causing it to swell (cerebral oedema), which leads to raised intracranial pressure;this can be fatal.

part 1 landscape_anatomy and physiology of the nervous system

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