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Chapter 2&3

Chapter 2&3 Notes

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Zachariah Campbell

PSYB65-Chapter 2 & 3 Notes Chapter 2: Origins of the Human Brain and Behaviour • Cladogram: a graph that shows the relative time of origin of various closely related groups. Each branch point in a cladogram distinguishes animals positioned before that time point from animals positioned after it by one or more physical or behavioural traits. • Hominids, our humanlike ancestors, diverged from this ancestral ape lineage by acquiring characteristics that distinguished them from other apes. Hominids were taller, and there was less difference in height between males and females. They were bipedal, had long legs, and were such great travelers that their descendants have populated every habitable continent. • Genes are the functional units that control the transmission and expression of traits from one generation to the next. • According to Claudia Hetzer- Egger and coworkers, a gene called Pax is responsible for eye development in all seeing animals, demonstrating a much closer relationship among very diverse kinds of animals than had been suspected previously. Similar genes, called homeobox genes, dictate body segmentation in both fruit flies and humans. Thus, segmentation of the human nervous system into the spinal cord, brainstem, and forebrain is produced by genes first discovered in fruit flies. The differences in the structure of the eye and the nervous system in different animal species are the products of slight alterations, called mutations, in genes such as Pax and in the way in which the products of those genes interact with the products of other genes. • Parkinson’s disease is associated with aging in humans and can affect as many as 1% of the population older than 65 years of age. The symptoms include rigidity that impedes voluntary movement, balance problems, and tremors of the head, hands, and limbs. The cause of Parkinson’s dis-ease is unknown, however, and there is no cure. • Quasi- evolutionary sequence, a hypothetical sequence of animals that represent consecutive stages in evolutionary history. In some cases, an animal can be chosen because it is the living descendant of an extinct ancestor. • The three general lines of research through which scientists attempt to reconstruct the story of human evolution are archaeological, bio-chemical and genetic, and behavioural. • The relatedness of different species can also be determined by comparing their deoxyribonucleic acid (DNA), the genetic material in the nucleus of the cell. Genes are segments of DNA that specify what proteins a cell should make. Each gene is a long chain of four kinds of nucleotide bases. Through mutations, the sequence of bases can change to some extent and still leave a functional gene. • As progress in describing the genome, the full set of genes of a species, improves, an ideal description of human evolution would include information on what genetic modifications led to the evolution of modern humans. • Investigators agree that four general steps led from a chimpanzee- like common ancestor to modern humans. These steps were the evolution in hominids of 1. An upright posture in which the hands were free; 2. Extensive tool use; 3. A traveling life style; and 4. An elaborate culture. • Two versions of how the evolution of hominids took place vie for prominence. The down- from- trees hypothesis proposes that the trees being farther apart required apes to adopt bipedal locomotion. The accompanying change in posture reduced the area of the body exposed to the sun and permitted the loss of body hair. The water- baby hypothesis, proposed by Alister Hardy, suggests a different order of events, beginning with a hypothetical naked ape swimming and foraging on ocean beaches and later forced to abandon its semiaquatic habitat when the ocean receded. In this scenario, the animal is described as finding bipedalism and lack of body hair advantageous in swimming; it then retains these features when it adapts to the land. • the Encephalization quotient ( EQ) for mammals— the ratio of actual brain size to expected brain size. Expected brain size is based on an average for living mammals that takes body size into account. • This increase in brain size was due to two changes: ( 1) member species in the hominid lineage were becoming larger and ( 2) their brains were becoming larger. • Neoteny: the rate of maturation slows down enough that some juvenile features of predecessor species become the adult features of descendant species. Many features of the human anatomy resemble juvenile stages of other primates— a small face, a vaulted cranium, and a large brain- to- body- size ratio; an unrotated big toe, up-right posture, and primary distribution of hair on the head, armpits, and pubic areas. • The typical mammalian cortex can be divided into areas that are specialized for movement, body senses, audition, vision, and olfaction. In general, the frontal (movement), parietal (body senses), temporal ( audition), and occipital ( vision) lobes subserve these functions in humans • At least three factors determine cortical size: absolute increases in size, the addition of new skills, and the relative changes in sensory and motor abilities. • Nineteenth- century investigators attempted to correlate gross human brain size and behaviour with three questions in mind. They asked whether brain size was related to 1. a person’s intelligence. 2. intelligence differences between sexes. 3. intelligence differences among nationalities and races. • As Darwin pointed out first, two good scientific reasons reveal this line of inquiry into gross brain size and intelligence as superficial: 1. Even though between- species differences in brain size may be correlated with between- species differences in behaviour, to apply the correlation within a species is faulty, because within- species behaviour is much more uniform. For example, no chimpanzee can read and all normal- functioning modern humans can read. 2. IQ tests are a biased measure of intelligence. IQ tests largely measure the function of the left- hemisphere cortex and ignore the rest of the brain. Howard Gardner has proposed that an adequate measure of human intellectual abilities would have to consider seven different kinds of intelligence: verbal, mathematical, musical, spatial, motoric, interpersonal, and extrapersonal. • IQ test scores are also sensitive to many outside variables, including time. When IQ tests that were given to young adults 50 years ago are given to young adults today, today’s subjects score as much as 25 points higher (a phenomenon called the Flynn effect). Taken at face value (though it shouldn’t be), the increase suggests that human intelligence has risen to such a degree in two generations that most young adults fall in the superior IQ category relative to their grand- parents. Chapter 3: Organization of the Nervous System • Ischemia: Deficiency of blood flow to the brain due to functional constriction or to the actual obstruction of a blood vessel, such as by a clot. • Unlike the more severe haemorrhagic stroke that results from a burst ves-sel bleeding into the brain, ischemic stroke can be treated with a drug called tissue plasminogen activator ( t- PA) that breaks up clots and allows the return of normal blood flow to the affected region. R. S. was not given the drug within the required 3 hours of suffering his stroke, however, because the attending physician was unsure whether the fall from the garage roof had caused a haemorrhagic stroke as a result of a concussion and burst blood vessel. An anticlotting drug decreases tissue death in ischemic stroke but aggravates cell death in haemorrhagic stroke. • The neurons in the brain are organized in layers as well as in groups called nuclei ( from the Latin nux, meaning “ nut”), groups of cells forming clusters that can be visualized with special stains to identify a functional grouping. Some brain nuclei are folded, and others have distinctive shapes and colors. Within nuclei, cells that are close together make most of their connections with one another. Describing Locations in the Brain • The locations of the layers, nuclei, and pathways of the brain can be described by their placement with respect to other body parts of the animal, with respect to their relative locations, and with respect to a viewer’s perspective. The most frequently used sets of terms are illustrated in Figure 3.1: • Figure 3.1A describes brain structures in relation to other body parts. In Latin, rostum is “ beak,” caudum is “ tail,” dosum is “ back,” and ventrum is “ stomach.” Accordingly, rostral, caudal, dorsal, and ventral parts of the brain are located toward those body parts. Occasionally, the terms superior and inferior are used to refer to structures that are located dorsally or ventrally. • Figure 3.1B illustrates how brain parts are described in relation to one another from the frame of reference of the face. Anterior or frontal is in front, posterior structures are located behind, lateral structures are at the side, and medial structures are located at the center or between. • Figure 3.1C illustrates terms that describe the direction of a cut, or section, through the brain from the perspective of a viewer. A coronal section is cut in a vertical plane, from the crown of the head down. A horizontal section ( because the view or cut is along the horizon) is usually viewed looking down on the brain from above. A sagittal section is cut lengthways, front to back, and viewed from the side (imagine the brain oriented as an arrow— in Latin, sagitta). • The nervous system, like the body, is symmetrical, with a left side and a right side. Structures that lie on the same side are ipsilateral; if they lie on opposite sides, they are contralateral to each other. If one of them lies in each hemi-sphere, the structures are bilateral. • Structures that are close to one another are proximal; those far from one another are distal. And any movement toward a brain structure is afferent, whereas movement away from it is efferent. • The precentral gyrus, a part of the brain damaged by stroke in R. S. and responsible for his diminished motor ability, has many other names. It is called gyrus precentralis in Latin and “ the motor strip” in colloquial English. It is also called “ Jackson’s strip,” after Hughlings- Jackson, who noted that, in epileptic attacks, the limbs of the body convulse in an orderly arrangement, suggesting to him that the representation of the body in the brain also is orderly. • Electrophysiologists refer to the precentral gyrus as the primary motor cortex or M1, to distinguish it from other motor regions of the cortex. Because they can obtain movements of different body parts after stimulating this area, as was first found by Fritsch and Hitzig ( see Chapter 1), they have also called it the “ somatomotor strip” or “ the motor homunculus” ( motor human). Additionally, because anatomists such as Gall found that the pyramidal tract that extends from the cortex into the spinal cord comes mainly from this cortical region, they called it “ area pyramidalis.” • From an anatomical viewpoint, the central nervous system (CNS) consists of the brain and the spinal cord, and the peripheral nervous system (PNS) encompasses everything else. • The (CNS) consists of the brain and spinal cord. • The somatic nervous system (SNS) consists of all the spinal and cranial nerves to and from the sensory organs and the muscles, joints, and skin. The SNS produces movement and transmits incoming sensory information to the CNS, including vision, hearing, pain, temperature, touch, and the position and movement of body parts. • The autonomic nervous system ( ANS) balances the body’s internal organs to “ rest and digest” through the parasympathetic ( calming) nerves or to “ fight and flee” or engage in vigorous activity through the sympathetic ( arousing) nerves. Support and Protection The brain and spinal cord are supported and protected from injury and infection in four ways: 1. The brain is enclosed in a thick bone, the skull, and the spinal cord is encased in a series of interlocking bony vertebrae. Thus, the CNS lies within bony encasements, whereas the PNS, although connected to the CNS, lies outside them. The PNS, although more vulnerable to injury because it lacks bony protection, can renew itself after injury by growing new axons and dendrites, whereas self- repair is much more limited within the CNS. 2. Within the bony case enclosing the CNS is a triple- layered set of membranes, the meninges, shown in Figure 3.3. The outer dura mater ( from the Latin, meaning “ hard mother”) is a tough double layer of tissue enclosing the brain in a kind of loose sack. The middle arachnoid membrane ( from the Greek, meaning “ resembling a spider’s web”) is a very thin sheet of delicate tissue that follows the contours of the brain. The inner pia mater ( from the Latin, meaning “ soft mother”) is a moderately tough tissue that clings to the surface of the brain. 3. The brain and spinal cord are cushioned from shock and sudden changes of pressure by the cerebrospinal fluid that circulates in the four ventricles inside the brain, in the spinal column, and within the subarachnoid space in the brain’s enclosing membranes. Cerebral spinal fluid is continually being made and drained off into the circulatory system. If the outflow is blocked, as occurs in a congenital condition called hydrocephalus ( literally, water brain), severe mental retardation and even death can result. 4. The brain and spinal cord are protected from many chemical substances circulating in the rest of the body by the blood– brain barrier. To form this barrier, the cells of the capillaries the very small blood vessels form tight junctions with one another, thus preventing many blood- borne substances from crossing from the capillaries into the CNS tissues. • The brain receives its blood supply from two internal carotid arteries and two vertebral arteries that course up each side of the neck. The four arteries connect at the base of the brain, where they enter the skull. From there, the cerebral arteries branch off into several smaller arteries that irrigate the brainstem and cerebellum and give rise to three arteries that irrigate the forebrain. • The anterior cerebral artery ( ACA) irrigates the medial and dorsal part of the cortex, the middle cerebral artery ( MCA) irrigates the lateral surface of the cortex, and the posterior cerebral artery ( PCA) irrigates the ventral and posterior sur-faces of the cortex. • The brain has its origin in a single undifferentiated cell called a neural stem cell ( also called a germinal cell). Not only does this stem cell and its progeny produce the various specialized cells that make up the adult brain, they also produce additional stem cells that persist into adulthood. • In the developing embryo, stem cells give rise to progenitor cells that migrate and act as precursor cells, giving rise to nondividing, primitive types of nervous system cells called blasts. Some blasts differentiate into neurons; others differentiate into the glia. These two basic brain- cell types — neurons and glia— take many forms and make up the entire adult brain. • Somatosensory neurons, which project from the body’s sensory receptors into the spinal cord, are modified so that the dendrite and axon are connected, which speeds information conduction because messages do not have to pass through the cell body. Interneurons within the brain and spinal cord link up sensory- and motor- neuron activity in the CNS. There are many kinds of interneurons and all have many dendrites that branch extensively but, like all neurons, a brain or spinal- cord inter-neuron has only one axon, although it can branch as well. Motor neurons located in the brainstem project to facial muscles, and motor neurons in the spinal cord project to other muscles of the body. Together, motor neurons are called the final common path because all behaviour produced by the brain is produced through them. • Gray matter acquires its characteristic gray- brown color from the capillary blood vessels and neuronal cell bodies that predominate there. White matter consists largely of axons that extend from these cell bodies to form connections with neurons in other brain areas. These axons are covered with an insulating layer of glial cells that are composed of the same fatty substance (lipid) that gives milk its white appearance. As a result, an area of the nervous system rich in axons covered with glial cells looks white. • Reticular matter contains a mixture of cell bodies and axons from which it acquires its mottled gray and white, or netlike, appearance. Thus, with respect to our analogy equating brain regions with communities and roads, communities are gray, roads are white, and reticular matter is suburbia. • A large collection of axons projecting to or away from a nucleus or layer in the CNS is called a tract or, sometimes, a fiber pathway. • Fibers and fiber pathways that enter and leave the CNS are called nerves, such as the auditory nerve or the vagus nerve; but, after they have entered the central nervous system, they, too, are called tracts. • The adult brain of a fish, amphibian, or reptile is roughly equivalent to this three- part brain: the prosencephalon (“ front brain”) is responsible for ol- faction, the mesencephalon (“ middle brain”) is the seat of vision and hearing, and the rhombencephalon (hindbrain) controls movement and balance. Here, the spinal cord is considered part of the hindbrain. • In mammals, the prosencephalon develops further to form the cerebral hemispheres ( the cortex and related structures), which are known collectively as the telencephalon (“ endbrain”). The remaining part of the old prosencephalon is referred to as the diencephalon (“ between brain”) and includes the thalamus. The back part of the brain also de
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