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CHAPTER 44

NEUROSCIENCE II:

EVOLUTION AND

FUNCTION OF THE

BRAIN AND

NERVOUS SYSTEMSPrepared by

Brenda Leady, University of Toledo

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Nervous system is the product of hundreds of millions of years of evolution

Development provided advantages that promoted reproductive success

Organization ranges from simple network of a few cells to complexity of human brain

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Representative nervous systems

Except for sponges, all animals have a nervous system

Nerve net Simplest nervous system Cnidarians (jellyfish,

hydras, anemones) Neurons connect to each

other in a network

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Echinoderms Slightly more sophisticated Nerve ring around mouth

connected to larger radial nerves extending to arms

Planaria Nerve cords extend length

of animal connected by transverse nerves

Collection of neurons in head form ganglia

Perform basic integration

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Annelids Same basic structure More neurons Ventral nerve cords

have ganglia in each segment

Simple mollusks Similar to annelids Pair of anterior ganglia Paired nerve cords

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Trend toward cephalization – increasingly complex brain in the head

Drosophila Brain has several

subdivisions with separate functions

Advanced mollusks Brains with well-

developed subdivisions

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Vertebrates and simpler chordates have a CNS (brain and spinal cord) and PNS

Organization shows similarities to segmentation of invertebrates

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3 divisions of vertebrate brain

Entire nervous system develops from neural fold in embryo

HindbrainMetencephalonMyelencephalon

MidbrainMesencephalon

ForebrainTelencephalonDiencephalon

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HindbrainMedulla oblongata – coordinates many basic reflexes and bodily

functionsPons and cerebellum – responsible for monitoring and coordinating

body movements Midbrain

Processes several types of sensory inputControls sophisticated tasks

ForebrainHigher functions of conscious thought, planning and emotionCerebrumCerebral cortex – outer layer of cerebrumThalamus and hypothalamus

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Increased brain complexity

Size of cerebrum and surface area of cerebral cortex (convolutions) increased with more complex nervous systems

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Body mass and brain mass proportional with exceptions (humans and dolphins)

Greater size and folding provides more surface area for greater processing and interpretation of information

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Human nervous system

Central nervous system (CNS)Brain and spinal cord

Peripheral nervous system (PNS)Neurons and axons of neurons outside the

CNS

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Nucleus – cell bodies of neurons involved in a similar function in the CNS

Ganglion – similar structure in PNS Tract – myelinated axons that run in parallel

bundles in the CNS Nerves – similar structure in PNS Cranial nerves are connected directly to the brain Spinal nerves are connections between the PNS

and spinal cord

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White matterMyelinated axons grouped together

Gray matterNeuronal cell bodies, dendrites and some

unmyelinated axons Spinal cord gray matter forms

Dorsal horns – connects to dorsal root, part of spinal nerve, incoming information

Ventral horns – connects to ventral root, part of spinal nerve, outgoing information

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CNS encased in bone and 3 layers of meningesDura mater – outer thick layerArachnoid mater – numerous connections

to inner layerPia mater – inner thin membrane on

surface of brain and spinal cord Cerebrospinal fluid circulated through

subarachnoid spaceBetween arachnoid mater and pia materAbsorbs physical shocksTransportVentricles and central canal

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PNS divisions

Somatic nervous system Sensory neurons (afferent) receive stimuli such as heat,

vision, smell, taste, hearing, touch and transmit to CNS Motor neurons (efferent) control skeletal muscles –

voluntary Autonomic nervous system

Predominantly composed of motor neurons (efferent) control smooth muscles, cardiac muscles and glands – involuntary

Sensory neurons (afferent) detect internal body conditions

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Efferent nerves of autonomic further divided Act on same organs with opposing actions Sympathetic division

“fight or flight” Increased heart rate, faster breathing

Parasympathetic division “rest and digest” Slow heart rate, promote digestion

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Hindbrain

Medulla oblongataCoordinates many basic reflexes and bodily

functions that maintain the normal homeostatic processes of a person

Involved in the control of heart rate, breathing, cardiovascular function, digestion, swallowing, and vomiting

With pons and areas of the midbrain, collectively called the brainstem, contain additional nuclei (groups of cell bodies) whose axons project dorsally to many other parts of the brain

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Hindbrain

Cerebellum and ponsResponsible for monitoring and coordinating

body movementsPons serves as relay between cerebellum and

other areas of the brainOverall function of cerebellum to maintain

balance and coordinate hand-eye movements

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Midbrain

Forms part of the reticular formation Processes several types of sensory

inputs, including vision, smell, and hearing Tracts that pass this information to other

parts of the brain for further processing and interpretation

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Forebrain

Thalamus and hypothalamus (diencephalon)

Cerebrum (telencephalon) – basal nuclei, limbic system and cerebral cortex

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Forebrain

Thalamus Major role in relaying sensory information to

appropriate parts of the cerebrum and, in turn, sending outputs from the cerebrum to other parts of the brain

Receives input from all sensory systems Hypothalamus

Major role of production of hormones regulating pituitary gland which regulates hormone secretion from other glands

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Forebrain Basal nuclei

Involved in planning and learning movements Involved in initiating or inhibiting movementsAffected in Parkinson disease

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Limbic system Not all agree on members of limbic system Primarily involved in formation and expression of

emotions Role in learning, memory, and perception of smells Amygdala – understand and remember emotional

situations, recognize emotional expressions in others Hippocampus – establish memories for spatial

locations, facts, and sequences of events

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Forebrain

Cerebral cortexSurface layer of gray matter on the cerebrumNeocortex layer evolved most recently in

mammals with only 6 layers of cellsContains 10% of neurons in the brainSensory and motor information integrated Initiation of voluntary acts, generation of speech,

learning, memory, and production of emotions

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Frontal – conscious thought and social awareness

Parietal – attention and making association between events and incoming information

Occipital – vision Temporal – language, learning, and some

types of memory

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Sensory inputs come into the cortex and motor outputs leave the cortex in areas that stretch like a map

Amount of space proportional to sensitivity or number of muscles

May change depending on use or disuse of body part

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Cerebral hemispheres connected by corpus collosum

Severing connection used in the past to treat severe epilepsy

Hemispheres can function independently Process different types of information Left hemisphere – understanding

language and producing speech Right hemisphere – nonverbal memories,

recognizing faces, and interpreting emotions

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Several Genes Have Been Important in the Evolution of the Cerebral Cortex

Genes identified by studying developmentally disabled individuals or comparing human genes to other species

Polymicrogyria – results in mental impairment, disrupted gait and language production Abnormal surface folds and fewer layers of cells Mutations alter receptors’ ability to bind ligand

Microcephalin and ASPM genes Determinants of brain size May be involved in brain evolution – greater changes in

humans and great apes

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Learning and memory

Learning – process by which new information is acquired

Memory – involves retention of that information over time

Long-term potentiation (LTP) Long-lasting strengthening of connections between neurons Experiments with rabbits showed short, electrical

stimulations to a neuron strengthened synapses with adjacent cells

Neurons communicated more readily

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Work with California sea slug (Aplysia californica)

Has only 20,000 nerve cells

Some neurons extremely large

Can isolate proteins and mRNA to identify biochemical and genetic changes during learning

Gill-withdrawal reflex subject to learning

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Sea slugs process in 2 phases Short-term memory lasts for minutes or hours

Typically single stimulus No new proteins Intracellular second messengers make it easier for

neurons to communicate Long-term memory lasts days or weeks

Repeated stimuli Activates genes, leads to mRNA synthesis, new

proteins for additional synaptic connections Learning does not change neuron pathways but

changes strength of signals along those pathways

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Neurogenesis

Until recently, adult brain thought incapable of producing new neurons through cell division

Late 1990s, stem cells found in primate and human CNS

1998, found new hippocampal cells in recently deceased patients

Some evidence suggests that neurogenesis is involved in learning and memory

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Brain images in assessment

Computerized tomography (CT) scan – 3D image based on density but not great detail

Magnetic Resonance Imaging (MRI) – detection of structures as small as 1/10th mm

Function MRI (fMRI) – modification to assess functional activity based on oxygen in active tissue Oxygenated hemoglobin increases in metabolically

active areas

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Gaser and Schlaug Showed That the Sizes of Certain Brain Structures Differ Between Musicians and Nonmusicians

Used MRI to examine sizes of brain structures in professional musicians, amateur musicians, and nonmusicians

Brain areas involved in hearing, moving the fingers, and coordinating movements with vision and hearing were larger in professional musicians than in amateur musicians, and larger in amateurs than in nonmusicians

Have not determined underlying reason(s) for increased brain size People with increased brain size in these areas may be more likely to

become musicians or musical training may cause these areas to enlarge

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Impact on public health

Meningitis Viral or bacterial infection of meninges Increased pressure effects range from severe

headaches to death Bacterial infection can be treated with antibiotics Viral form less serious and short lasting Vaccine has reduced cases but still dangerous and

prevalent disease (especially close quarters like college dorms)

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Alzheimer’s disease (AD) Leading worldwide cause of dementia Characterized by memory loss and intellectual and

emotional function Definitive diagnosis can only be made after death 2 noticeable changes – senile plaques and

neurofibrillary tangles Not clear how changes influence function Genetics plays a role but not the only possible cause Impact on public health enormous – especially as

baby boomers age

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