Part 1 Portrait_anatomy and Physiology of the Nervous System

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, and emotion, 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 system has three overlapping function: (1) much like a sentry, it uses millions of sensory receptors to monitor changes occurring both inside and outside the body. These changes are called stimuli, and the gathered information is called sensory input. (2) It process and interprets the sensory input and makes decisions about what should he done at each moment—a 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 and see 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 (the response).The nervous system does not work alone to regulate and maintain body homeostasis: the endocrine system is a second important regulating system. While the nervous system controls with rapid electrical nerve impulses, the endocrine system organs produce hormones that are released into the blood. Thus, the endocrine system typically brings 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 same time. 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 their relationships are illustrated in Figure 7.2. It is not necessary to memorize this whole scheme now, but as you are reading the descriptions, try to get a "feel" for the major parts and how they fit together. This will make your learning task easier as you make your way through this chapter. Later you will meet all these terms and concepts again and in more detail.

a. Structural ClassificationThe structural classification which includes all nervous system organs has two subdivisions—the central nervous system 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 and act as the integrating and command centers of the nervous system. They interpret incoming sensory information and 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, consists mainly of the nerves that extend from the brain and spinal cord. Spinal nerves carry impulses to and from the spinal cord. Cranial (kra'ne-al) nerves carry impulses to and from Tie brain. These nerves serve as communication lines. They link all parts of the body by carrying impulses from the sensory

receptors to the CNS and from the CNS to the appropriate glands or muscles.

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

FIGURE 7.2 Organization of the nervous system. Organizational flowchart showing that the central nervous system receives input via sensory fibers and issues commands via motor fibers. The sensory and motor fibers together form the nerves that constitute the peripheral nervous system.

FIGURE 7.1 The nervous system's functions.The motor or efferent (ef'er-rent), division carries impulses from the CNS to effector organs the muscles and glands. These impulses activate muscles and glands; that is, they effect (bring about) a motor response.The motor division in turn has two subdivisions (see Figure 7.2):

1. The somatic (so-mat'ik) nervous system allows us to consciously, or voluntarily, control our skeletal muscles. Hence this subdivision is often referred to as the voluntary nervous system. However, not all skeletal muscle activity controlled by this motor division is voluntary. Skeletal muscle reflexes, like the stretch reflex for example are initiated involuntarily by these same fibers.

2. The autonomic (aw"to-nonfik) nervous system (ANS) regulates events that are automatic, or involuntary, such as the activity of smooth and cardiac muscles and glands. This subdivision, commonly called the involuntary nervous system itself has two parts, the sympathetic and parasympathetic, which typically bring about opposite effects. What one stimulates, the other inhibits. These will be described later.

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 of convenience only.

FIGURE 7.3 Supporting (glial) cells of nervous tissues. Astrocytes (a) form a living barrier between neurons and capillaries in the CNS. Microglia (b) are phagocytes, whereas ependymal cells (c) line the fluid- filled cavities of the CNS. The oligodendrocytes (d) form myelin sheaths around nerve fibers in the CNS. (e) The relationship of Schwann cells (myelinating cells) and satellite cells to a neuron in the peripheral nervous system.

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

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 cells—supporting cells and neurons.

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

Astrocytes: abundant star-shaped cells that 'ac¬count for nearly half of the neural tissue. Their numerous projections have swollen ends that cling to neurons, bracing them and anchoring them to their nutrient supply lines, the blood capillaries (Figure 7.3a). Astrocytes form a living barrier between capillaries and neurons and play a role in making exchanges between the two. In this way, they help protect the neurons from harmful substances that might be in the blood. Astrocytes also help control the chemical environment in the brain by picking up excess ions and recapturing released neurotransmitters.

Microglia: spiderlike phago¬cytes that dispose of debris, 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 cush¬kin around the CNS.

Oligodendrocytes: glia that wrap their flat extensions tightly around the nerve fibers, producing fatty insulating cov¬erings called myelin sheaths (Figure 7.3d).

Although they somewhat resemble neurons structurally (both cell types have cell extensions), glia are not able to transmit nerve impulses, a func¬tion that is highly developed in neurons. Another important difference is that glia never lose their ability to divide,

FIGURE 7.5 Relationship of Schwann cells to axons in the peripheral nervous system. As illustrated (top to bottom), a Schwann cell envelops part of an axon in a trough and then rotates around the axon. Most of the Schwann cell cytoplasm comes to lie just beneath the exposed part of its plasma membrane. The tight coil of plasma membrane material surrounding the axon is the myelin sheath. The Schwann cell cytoplasm and exposed membrane are referred to as the neurilemma.

whereas most neurons do. Conse¬quently, most brain tumors are gliomas, or tumors formed by glial cells (neuroglia). Supporting cells in the PNS come in two major varieties—Schwann cells and satellite cells (Figure 7.3e). Schwann cells form the myelin sheaths around nerve fibers that are found in 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 of the body to another. Although neurons differ structurally, they have many common features (Figure 7.4). All have a cell body, which contains the nucleus and is the metabolic center of the cell, and one or more slender processes extending from the cell body.

The cell body is the metabolic center of the neuron. It contains the usual organelles except for centrioles (which confirm 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 4 feet. The longest ones in humans reach from the lumbar region of the spine 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 away from the cell body are axons (ak'sonz). Neurons may have hundreds of the branching dendrites (dendr = tree), depending on the neuron type, but each neuron has only one axon, which arises from a conelike region of the cell body called the axon hillock.

An occasional axon gives off a collateral branch along its length, but all axons branch profusely at their terminal end, forming hundreds to thousands of axon terminals. These terminals contain hundreds of tiny vesicles, or membranous sacs, that contain chemicals called neurotransmitters

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

FIGURE 7.6 Neurons classified by function. Sensory (afferent) neurons conduct impulses from sensory receptors (in the skin, viscera, muscles) to the central nervous system; most cell bodies are in ganglia in the PNS. Motor (efferent) neurons transmit impulses from the CNS (brain or spinal cord) to effectors in the body periphery. Association neurons (interneurons) complete the communication pathway between sensory and motor neurons; their cell bodies reside in the CNS.

Each axon terminal is separated from the next neuron by a tiny gap called the synaptic (si-nap'tik) cleft. Such a functional junction is called a synapse (syn = to clasp or join). Although they are close, neurons never actually touch 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 themselves tightly around the axon jelly-roll fashion (Figure 7.5). When the wrapping process is done, a tight coil of wrapped membranes, the myelin sheath, encloses the axon. Most of the Schwann cell cytoplasm ends up just beneath the outermost part of its plasma mem-brane. This part of the Schwann cell, external to the myelin sheath, is called the neurilemma (nu"ri-lem'mah, "neuron husk"). Since the myelin sheath is formed by many individual Schwann cells, 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 deposits myelin around a small segment of one nerve fiber, the oligodendrocytes with their many flat extensions can coil around as many as 60 different fibers at the same time. Although the myelin sheaths formed by oligodendrocytes and those formed by Schwann cells are quite similar, the CNS sheaths lack a neurilemma. Because the neurilemma remains intact (for the most part) when a peripheral nerve fiber is damaged, it plays an important role in fiber

FIGURE 7.8 Classification of neurons on the basis of structure. (a) Multipolar. (b) Bipolar (c) Unipolar.

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.

1. Functional Classification Functional classifications groups neurons according to the direction the nerve impulse is traveling relative to the CNS .On this basis, there are sensory, motor, and association neurons (Figure 7.6). Neurons carrying impulses from sensory receptors (in the internal organs or the skin) to the CNS are sensory or afferent neurons. (Afferent literally means (“To go toward”)

The cell bodies of sensory neurons always found in a ganglion outside the CNS. Sensory neurons keep us informed about what is happening both inside and outside the body.

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

of sensory receptors seen in the skin (cutaneous sense organs) and in the muscles and tendons (proprioceptors) are shown in Figure 7 .7. The pain receptors (actually bare dendrite endings) are the least specialized of the cutaneous receptors. They are also the most numerous, because pain warns us that some type of body damage is occurring or is about to occur. However, strong stimulation of any of the cutaneous receptors (for example, by searing heat, extreme cold or excessive pressure) is also interpreted as pain.

FIGURE 7.9 The nerve impulse. (a) Resting membrane electrical conditions. The external face of the membrane is slightly positive: its internal face is slightly negative. The chief extracellular on is sodium fhlarl, whereas the chief intracellular ion is potassium (K4). The membrane is relatively impermeable to both ions. (b) Stimulus initiates local depolarization. A stimulus changes the permeability of a "patch" of the membrane, and sodium ions diffuse rapidly into the cell. This changes the polarity of the membrane (the inside becomes more positive; the outside becomes more negative). (c) Depolarization and generation of an action potential. If the stimulus is strong enough, depolarization causes membrane polarity to be completely reversed and an action potential is initiated. (d) Propagation of the action potential. Depolarization of the first membrane patch causes permeability changes in the adjacent membrane, and the events described in (b) are repeated. Thus, the action potential propagates rapidly along the entire length of the membrane. (e) Repolarization. Potassium ions diffuse out of the cell as membrane permeability changes again, restoring the negative charge on the inside of the membrane and the positive charge on the outside surface. Repolarization occurs in the same direction as depolarization. (f) The ionic conditions of the resting state are restored later by the activity of the sodium-potassium pump.

The proprioceptors detect the amount of stretch. or tension, in skeletal muscles, their tendons, and joints. They send this information to the brain so that the proper adjustments can be made to maintain balance and normal posture. Propria comes from the 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/or muscles and glands are motor, or efferent, neurons (see Figure 7.6). The cell bodies of motor neurons are always located in theCNS.

The third category of neurons is the association neurons, or interneurons. They connect the motor and sensory neurons in neural pathways. Like the motor neurons, their cell bodies are always located in the CNS.

2. Structural Classification Structural classification is based on the number of processes extending from the cell body (Figure 7.8). If there are several, the neuron is a multipolar neuron. Since all motor and association neurons are multipolar, this is the most common structural type. Neurons with two processes—an axon and a dendrite—are called bipolar neurons. Bipolar neurons are rare in adults, found only in some special sense organs (eye, nose), where they act in sensory processing as receptor cells. Unipolar neurons

FIGURE 7.10 How neurons communicate at chemical synapses. The events occurring at the synapse are numbered in order.

have a single process emerging from the cell body. However, it is very short and divides almost immediately into proximal (central) and distal (peripheral) processes. Unipolar neurons are unique in that only the small branches at the end of the peripheral process are dendrites. The remainder of the peripheral process and the central process function as axons; thus, in this case, the axon conducts nerve impulses both toward and away from the cell body. Sensory neurons found in PNS ganglia are unipolar.

Physiology

Nerve Impulses Neurons have two major functional properties: irritability, the ability to respond to a stimulus and convert it into a nerve impulse, and conductivity, 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, which means that there are fewer positive ions sitting on the inner face of the neuron's plasma membrane than there are on its outer face in the tissue fluid that surrounds it (Figure 7.9). The major positive ions inside the cell are potassium (K), whereas the major positive ions outside the cell are sodium (Na). As long as the inside remains more negative as compared to the outside, the neuron will stay inactive.

Many different types of stimuli excite neurons to become active and generate an impulse. For example, light excites the eye receptors, sound excites some of the ear receptors, and pressure excites some cutaneous receptors of the skin. However, most neurons in the body are excited by neurotransmitters released by other neurons, as will be described shortly. Regardless of what the stimulus is, the result is always the same—the permeability properties of the cell's plasma membrane change for a very brief period. Normally, sodium ions cannot diffuse through the plasma membrane to any great extent; but when the neuron is adequately stimulated, the "gates" of sodium channels in the membrane open. Because sodium is in much higher concentration outside the cell, it will then diffuse quickly into the

neuron. (Remember the laws of diffusion?) This inward rush of sodium ions changes the polarity of the neuron's membrane at that site, an event called depolarization. Locally, the inside is now more positive, and the outside is less positive, a situation called a graded potential. However if the stimulus is strong enough and the sodium in-flux is great enough, the local depolarization (graded potential) activates the neuron to initiate and transmit a long distance signal called an action potential, also called a nerve impulse in neurons. The nerve impulse is an all-or-none re- sauna like firing a gun. It is either propagated (conducted) over the entire axon, or it doesn't happen at all. The nerve impulse never 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, the membrane permeability changes again, becoming impermeable to sodium ions but permeable to potassium ions. So potassium ions are allowed to diffuse out of the neuron into the tissue fluid, and they do so very rapidly. This outflow of positive ions from the cell restores the electrical 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 and potassium ions inside and outside the neuron are restored by activation of• the sodium-potassium pump. This pump uses ATP (cellular energy) to pump excess sodium ions out of the cell and to bring potassium ions back into it. Once begun, these sequential events spread along the entire neuronal membrane.

The events just described explain propagation of a nerve impulse along unmyelinated fibers. Fibers that have myelin sheaths conduct impulses much faster because the nerve impulse literally jumps, or leaps, from node to node along the length of the fiber. This occurs because no current can flow across the axon membrane where there is fatty myelin insulation. This type of impulse is called salutatory (Sal’tah-to”re) conduction (saltare = to dance or leap)

IV. Central Nervous System

Our body is made up of biological processes. Everything we feel, think or do has biological components. Biological processes help us to understand behavior. All of the psychological phenomena covered in this topic are a direct product of these biological processes. Psychology is the study of what the nervous system does. Therefore

an understanding of this system is essential to an understanding of human psychology.

Every section of this part of the case presentation is about the brain and the nervous system. It is impossible to examine all of the major neuroanatomic structures. The points of interest here include the structures of the brain believed to be involved in the formation of thought and emotion.

The brain is defined in various ways. The definition that best suits the perspective of this case study is that the brain is that part of the central nervous system encapsulated by the skull. The brain is the core of our humanity. Intercommunication of different parts of the brain yield the experiences of love, hate, elation, joy or madness. The brain provides the underlying 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

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

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

In multicellular organism, cells specialized for receptor function are located on the surface. Other cells specialized 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, neurons integrate the diverse array of incoming information from the receptors, which in neurons may result in the firing of an action potential (when the summation is above a threshold level) rather than swimming toward a nutrient source as in the unicellular organisms. Once the threshold for generating an action potential is reached, the signal is always the same, both in amplitude and shape (a nerve consists of many neurons, it does not obey the all-or-none law).

Action potentials and voltage-gated sodium channels are present in jellyfish, which are the simplest organisms to possess nervous systems. The development of this basic neuronal mechanism set the stage for the proliferation of animal life that occurred during the Cambrian period. Among these Cambrian animals were the early chordates, which possessed very simple brains. Some of these early fish developed a unique way to insulate their axons by wrapping them with a fatty material called myelin, which greatly facilitated axonal transmission and evolution of larger brains. Some of their descendants, which also were small predators, crawled up on the muddy shores and eventually took up permanent residence on dry land. Challenged by the severe temperature changes in the terrestrial environment, some experimented with becoming warm-blooded, and the most successful became the ancestors of birds and mammals. Changes in the brain and parental care were a crucial part of the set of mechanisms that enabled these animals to maintain a constant body temperature.

The human brain can be divided into three parts: the hindbrain, which has been inherited from the reptiles; the limbic system, which was first emerged in mammals; 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 is separated into two hemispheres. Apart from a single little organ -- the pineal gland in the centre base of the brain -- every brain module is duplicated in each hemisphere. The left brain is calculating, communicative and capable of conceiving and executing complicated plans --the reductionistic brain; while the right one is considered as gentle, emotional and more at one with the natural world -- the holistic brain. The cerebral cortex is covered in a thin skin of acquiring

Table 02 - Human Brain

knowledge by the use of reasoning, intuition or perception). Table 02 below lists 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, Pituitary Gland

beneath thalamus regulates basic biological drives, hormonal levels, sexual behavior, and controls

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):

Caudate Nucleus,

Putamen, Globus Pallidus

outside the thalamus involves in movement, emotions, planning and in integrating sensory information

Ventricles and Central Canal

from tiny central canal within the spinal cord to the enlarged

hollows within the skull called ventricles

fills with cerebrospinal fluid for mechanical protection.

Cingulate Gyrus above corpus callosum concentrates attention on adverse internal stimuli such as pain, contains the feeling of

self.Corpus

Callosumunder the cingulate gyrus is a bundle of nerve fibers linking the cerebral

hemispheres, involve in language 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 focus attention, and

bestows meaning to perceptions.Parietal Lobe in top rear of the head contains the primary 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.

The parietal eye is not an eye in the traditional sense in that it does not see images, but rather is a photosensitive organ which only reacts to light and dark. The parietal eye is connected to the pineal body and is used to trigger hormone production and thermoregulation. It often shows up as either a dark spot or an opalescent 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 be broadly divided into five stages. Table 03 summarizes the significant events within each stage, the "DO" and "DON'T" to keep a healthy mind.

Stag Age Event(s) DO DON'T

e

1 0 - 10 months

Gestation

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

Mother should:* be stress-free, eats well* take folic acid and vitamin B12

* stimulate the young brain with sounds and sensations

* Mother should stay away from cigarettes, alcohol and other toxins

2 Birth - 6Childhood

* A sense of self develops as the parietal and frontal lobe circuits become more integrated.* Development of voluntary movement, reasoning, and perception* Frontal lobes become active leading to the development 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 weight and at its peak of energy consumption

* Parents should provide a nurturing environment and one-on-one interaction

* Parents should beware of the emotional consequence of neglect or harsh parenting

3 7 - 22Adolescenc

e

* Wiring of the brain is still in progress* Grey matter (neural connections) pruning* White matter (fatty tissue surrounding neurons) increase helps to speed up electrical impulses and stabilize connections* The prefrontal cortex (involving control of impulses, judgment and decision-making) is the last to mature

* Teenagers should learn to control reckless, irrational and irritable behaviors* Do learn a skill to support life in the future

* Teenagers should avoid alcohol abuse, smoking, drug and unprotected sex.

4 23 - 65Adulthood

* The brain reaches the peak power at around age 22 and lasts for about 5 years; thereafter it's downhill all the way* The last to mature and first to go brain functions are those involve executive control in the prefrontal and temporal cortices* Episodic memory for recalling events also declines rapidly* Processing speed slows down* Working memory is able to store less information

* Stay active mentally and physically* Eat healthy diet

* Avoid cigarettes, booze, and mind-altering drugs.

5 > 65Old Age

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

* Exercise to improve abstract reasoning and concentration* Learn new skill such as guitar playing to attain the

* Avoid grumpiness by eating certain foods, such as yogurt, chocolate, and almonds to

Table 07 - The Five Stages of Human Brain

same effect* Practice meditation can promote neutral emotions

get a good dose of dopamine (for promoting positive emotions)* Don't stressed out as it is related to higher risk of developing dementia.

It is well known that the brain is an electrochemical organ; a fully functioning brain can generate as much as 20 watts of electrical power. Even though this electrical power is very limited, it does occur in very specific ways that are characteristic of the human brain. Electrical activity emanating from the brain can be displayed in the form of brainwaves. There are four categories of these brainwaves, ranging from the most active to the least active. Figure 03f is produced by an EEG (ElectroEncephaloGraph) chart recorder to show the different kind of brainwave 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 among large groups of neurons. The resulting signals from the electrodes are amplified and recorded.

Brain waves originate from the cerebral cortex, but also reflect activities in other parts of the brain that influence the cortex, such as the reticular formation. Because the intensity of electrical changes is directly related to the degree of neuronal activity, brain waves vary markedly in amplitude and frequency between sleep and wakefulness. Beta wave rhythms appear to be involved in higher mental activity, including perception and consciousness. 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 are producing a wave synchronously, they are said to be coherent. Alpha waves are generated in the Thalamus (the brain 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 scientist has the "aha" experience, there's a good chance he or she is in Alpha or Theta. These two kinds of brain waves are also associated with relaxation and, stronger immune systems. Therefore, many people try to train themselves 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 the reticular formation. In slow-wave sleep, the entire brain oscillates in a gentle rhythm quite unlike the fragmented oscillations of normal consciousness. The neocortical activity is often modulated by a rhythm of 40-80 Hz, called the Gamma wave (not shown in Figure 03f). When there are strong gamma oscillations in certain parts of the neocortex, human subjects do better on learning and memory tasks.

Diagram of a cerebral capillary enclosed in astrocyte end-feet. Characteristics of the blood-brain barrier are indicated: (1) tight junctions that seal the pathway between the capillary (endothelial) cells; (2) the lipid nature of the cell membranes of the capillary wall which makes it a barrier towater-soluble molecules; (3), (4), and (5) represent some of the carriers and ion channels; (6) the 'enzymatic barrier'that removes molecules from the blood; (7) the efflux pumps which extrude fat-soluble molecules that have crossed into the cells.

b. Protection: Meninges, Blood-Brain Barrier and CSF

1. Blood-brain Barrier The main function of the blood-brain barrier (BBB) is to protect the brain from changes in the levels in the blood of

ions, amino acids, peptides, and other substances. The barrier is located at the brain blood capillaries, which are unusual in two ways. Firstly, the cells which make up the walls of these vessels (the endothelium) are sealed together at their edges by tight junctions that form a key component of the barrier. These junctions prevent water-soluble substances in the blood from passing between the cells and therefore from freely entering the fluid environment of the brain cells. Secondly, these capillaries are enclosed by the flattened ‘end-feet’ of astrocytic cells (one type of glia), which also act as a partial, active, barrier. Thus the only way for water-soluble substances to cross the BBB is by passing directly through the walls of the cerebral capillaries, and because their cell membranes are made up of a lipid/protein bilayer, they also act as a major part of the BBB.

In contrast, fat-soluble molecules, including those of oxygen and carbon dioxide, anaesthetics, and alcohol can pass straight through the lipids in the capillary walls and so gain access to all parts of the brain.

Apart from these passive elements of the BBB there are also enzymes on the lining of the cerebral capillaries 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 and can diffuse straight through capillary walls into the brain. In the capillary wall there are three classes of specialized ‘efflux pumps’ which bind to three broad classes of molecules and transport them back into the 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 other unwanted molecules in the opposite direction.

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

Although the BBB is an obvious advantage in protecting the brain, it also restricts the entry from the blood 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 overcome these problems drugs are designed to cross the BBB, by making them more fat soluble. But this also means that they might enter most cells in the body and be too toxic. Alternative approaches are to make drug molecules that can ‘ride on’ the natural transporter proteins in the cerebral capillaries, and so 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 key role in this process, by limiting the freedom of movement of water and salts from the blood into the extracellular fluid of the brain. Whereas in other body tissues extracellular fluid is formed by leakage from capillaries, the BBB in fact secretes brain extracellular fluid at a controlled rate and is thus critical in the maintenance of normal brain volume. If the barrier is made leaky by trauma or infection, water and salts cross into the brain, causing it to swell (cerebral oedema), which leads to raised intracranial pressure; this can be fatal.