Development of the Nervous System Chapter 9

Phases of Neurodevelopment

• Zygote: a single cell formed by the amalgamation of an ovum and a sperm that divides to form two daughter cells, then four, then eight.
• Three other processes occur simultaneously:
  • Cell differentiation: cells become specialized (e.g., muscles, multipolar neurons).
  • Cell migration: cells move to their proper locations.
  • Cell-cell interactions: cells establish functional relationships with each other.

Induction of the Neural Plate

• Neural plate: a small patch of ectodermal tissue on the dorsal surface of the developing embryo, the first stage of neurodevelopment in all vertebrates.
• Ectoderm → mesoderm → endoderm: the neural plate develops from the ectoderm.
• Organizer: an area of the underlying mesoderm that sends chemical signals to induce the development of the neural plate.

Neural Proliferation

• Neural tube: the neural plate folds to form the neural tube, which eventually becomes the cerebral ventricles and spinal canal.
• Ventricular zone: the region where most cell division occurs in the neural tube.
• Proliferation patterns: different parts of the neural tube proliferate at different rates, leading to the characteristic shape of the brain.

Migration and Aggregation

• Migration: cells move to their target destinations.
• Radial migration: cells move in a straight line outward from the ventricular zone.
• Tangential migration: cells move parallel to the neural tube's walls.
• Glia-mediated migration: glial cells guide migrating neurons.
• Aggregation: neurons align with other developing neurons to form the nervous system's structure.

Axon Growth and Synapse Formation

• Axon growth: growth cones at the tip of axons extend and retract filopodia to guide growth.
• Chemoaffinity hypothesis: each postsynaptic surface releases a specific chemical label that attracts growing axons.
• Revised version: growing axons follow a series of chemical signals along the route.
• Pioneer growth cones: the first growth cones to travel along a particular route, guided by guidance molecules.
• Fasciculation: developing axons follow the paths established by preceding axons.

Synapse Formation

• Glial cells: particularly astrocytes, are necessary for synapse formation.
• Synaptogenesis: the formation of synapses, which depends on the presence of glial cells.

Neuron Death and Synapse Rearrangement

• Neuron death: about 50% of neurons produced are eliminated, occurring in waves throughout development.
• Apoptosis: active, programmed cell death, removing excess neurons in a safe and orderly way.
• Synapse rearrangement: as neurons die, the space they leave is filled by the sprouting axon terminals of surviving neurons.

Postnatal Cerebral Development in Human Infants

• Brain growth: the brain's volume quadruples between birth and adulthood.
• Synaptogenesis: a major burst of synaptogenesis occurs in the human cortex shortly after birth.
• Myelination: increases the speed of axonal conduction, and its pattern parallels functional development.
• Dendritic branching: progresses from deeper to more superficial layers, and can reconfigure quickly.

Development of the Prefrontal Cortex

• Prefrontal cortex: involved in working memory, planning, inhibiting responses, and following social rules.
• Development: young children do not demonstrate these cognitive functions until the prefrontal cortex has developed.
• Piaget studies: infants between 7 and 12 months exhibit perseveration errors due to underdeveloped prefrontal cortex.### Neurodevelopment and Experience
• Permissive experiences are necessary for information in genetic programs to be manifested.
• Instructive experiences contribute to the direction of development.
• Experience has a time-dependent effect on development, with critical periods and sensitive periods.
• Critical periods: essential for an experience to occur within a particular interval to influence development.
• Sensitive periods: experiences have a great effect on development when they occur during a particular interval, but can still have weak effects outside the interval.

Early Studies of Experience and Neurodevelopment

• Research focused on sensory and motor deprivation and enrichment.
• Sensory deprivation: reduced synapses and dendritic spines in primary visual cortex, and deficits in depth and pattern vision.
• Enrichment: thicker cortex, more dendritic spines, and more synapses per neuron.

Competitive Nature of Experience and Neurodevelopment

• Depriving one eye of input for a few days early in life has a lasting adverse effect on vision in the deprived eye.
• Ocular dominance columns in layer IV of the primary visual cortex are almost fully developed at birth.
• Monocular deprivation changes the pattern of synaptic input into layer IV of the primary visual cortex.
• If just one eye is deprived of light for several days, the system is reorganized.

Effects of Experience on Topographic Sensory Cortex Maps

• Three demonstrations:
  • Surgically altered axons of ferret's retinal ganglion cells to synapse in the medial geniculate nucleus of the auditory system.
  • Raised barn owls with vision-displacing prisms over their eyes.
  • Early music training influences the organization of human cortex.

Experience Fine Tunes Neurodevelopment

• Neurons begin to fire spontaneously and interact with the environment before the nervous system is fully developed.
• The resulting neural activity fine-tunes subsequent stages of neurodevelopment.

Neuroplasticity in Adults

• Neurogenesis in adults: new neurons are created in the rat hippocampus and olfactory bulbs.
• Substantial neurogenesis is restricted to the olfactory bulbs and hippocampuses.
• New neurons come from different places: subventricular zone of the lateral ventricles for olfactory bulbs, and dentate gyrus of the hippocampus for hippocampal neurons.
• Enriched environments produce more new hippocampal neurons.
• Aerobic exercise improves cognitive function in older adults.

Effects of Experience on the Reorganization of the Adult Cortex

• Experience in adulthood can lead to reorganization of sensory and motor cortical maps.
• Examples: tinnitus, adult musicians who play stringed instruments.

Disorders of Neurodevelopment

• Autism:
  • Complex neurodevelopmental disorder.
  • Core symptoms: reduced ability to interpret emotions and intentions, reduced social interaction and communication, and preoccupation with a single subject.
  • Characteristics: 80% male, 50% with mental retardation, 35% with seizures.
• Autism spectrum disorders:
  • Delayed development of language and social interaction.
  • Many display minor anomalies of ear structure.
• Autistic savants:
  • Intellectually handicapped individuals displaying amazing cognitive or artistic abilities.
  • About 10% of individuals with autism display some savant abilities.

William Syndrome

• Neurodevelopmental disorder associated with mental retardation and a heterogeneous pattern of abilities and disabilities.
• Patients are sociable, empathetic, and talkative.
• Language skills are remarkable despite low IQs.
• Cognitive strengths: verbal and social skills, music.
• Cognitive weaknesses: spatial abilities.
• Health problems: involving the heart.

Brain Damage and Neuroplasticity

• Causes of brain damage:
  • Brain tumors.
  • Cerebrovascular disorders: strokes.
• Brain tumors:
  • Meningiomas: tumors that grow between the meninges.
  • Infiltrating tumors: grow diffusely through surrounding tissue.
• Cerebrovascular disorders:
  • Strokes: sudden onset cerebrovascular disorders.
  • Infarct: area of dead or dying tissue produced by a stroke.
  • Penumbra: surrounding area of dysfunctional tissue.
  • Cerebral hemorrhage: bursting aneurysm.
  • Cerebral ischemia: disruption of blood supply to an area of the brain.

Note: Please let me know if you would like me to make any changes or add anything.### Ischemia Induced Brain Damage

• Takes a while to develop, with little or no evidence of brain damage immediately after a temporary cerebral ischemic episode
• Does not occur equally in all parts of the brain, with susceptible neurons in certain areas of the Hippocampus
• Mechanisms of damage vary from structure to structure within the brain, and astrocytes have been implicated in at least some areas

Close Head Injuries

• Brain injuries produced by blows that do not penetrate the skull are called closed-head injuries
• Contusions involve damage to the cerebral circulatory system, producing internal hemorrhaging and a hematoma
• Contusions can occur when the brain slams against the inside of the skull, resulting in blood accumulation in the subdural space
• Contusions frequently occur on the side of the brain opposite the side struck by a blow, known as Contrecoup Injuries
• A concussion is diagnosed when there is a disturbance of consciousness following a blow to the head, but no evidence of a contusion or other structural damage
• The Punch Drunk Syndrome is a type of dementia and cerebral scarring observed in boxers and other individuals who experience repeated concussions

Infections of the Brain

• Encephalitis is inflammation of the brain, and can be caused by bacterial or viral infections
• Bacterial infections can lead to the formation of cerebral abscesses, and are also the major cause of meningitis
• Penicillin and other antibiotics can eliminate bacterial infections of the brain, but cannot reverse brain damage that has already been produced
• Viral infections, such as rabies, have a particular affinity for neural tissue and can be fatal

Neurotoxins

• Toxic psychosis is a type of chronic insanity produced by a neurotoxin
• Some drugs used to treat neurological disorders can have toxic effects, such as Tardive Dyskinesia (TD)
• Endogenous neurotoxins can be produced by the body, with antibodies attacking particular components of the nervous system
• Genetic factors can also play a role in neurotoxin-related disorders

Genetic Factors

• Most neuropsychological diseases of genetic origin are caused by abnormal recessive genes that are passed from parent to offspring
• Inherited neuropsychological disorders are rarely associated with dominant genes, as they tend to be eliminated from the gene pool
• Inherit one abnormal gene, but do not develop the disorder, and the gene is passed on
• Down syndrome is an example of a genetic disorder, caused by an extra chromosome 21

Programed Cell Death

• Cell death is not an either-or situation, with some dying cells displaying signs of both necrosis and apoptosis
• Apoptosis is a more adaptive form of cell death, occurring gradually and without inflammation
• Necrosis is a quick and destructive form of cell death, leading to inflammation and damage to nearby cells

Neuropsychological Diseases

• Epilepsy is a chronic brain dysfunction characterized by recurrent seizures
• Many seizures do not involve convulsions, and may involve subtle changes in thought, mood, or behavior
• Diagnosis of epilepsy relies heavily on evidence from EEG, with associated with high EEG spikes
• Some epileptics experience peculiar psychological changes just before a convulsion, known as epileptic auras
• Partial seizures are a type of seizure that does not involve the entire brain, with specific behavioral symptoms depending on the location of the epileptic focus

Parkinson's Disease

• Is a movement disorder of middle and old age, affecting 1-2% of the elderly population
• More prevalent in males, with initial symptoms that are mild but increase in severity with advancing years
• Common symptoms include tremor, muscular rigidity, difficulty initiating movement, slowness of movement, and a masklike face
• Seems to have no single cause, with faulty genes, brain infections, strokes, tumors, traumatic brain injury, and neurotoxins all implicated
• Associated with widespread degeneration, particularly in the substantia nigra, with a loss of dopamine in the substantia nigra and striatum
• Symptoms can be alleviated by injections of L-dopa, but the treatment is not permanent and has side effects

Huntington's Disease

• Is a progressive motor disorder of middle and old age, with a strong genetic basis and associated with severe dementia
• The first sign is often increased fidgetiness, with rapid, complex, jerky movements of entire limbs as the disorder develops
• Eventually, motor and intellectual deterioration become so severe that sufferers are incapable of feeding themselves, controlling their bowels, or recognizing their own children
• There is no cure, and death typically occurs about 15 years after the appearance of the first symptoms

Multiple Sclerosis

• Is a progressive disease that attacks the myelin of axons in the CNS
• Typically attacks young people, with microscopic areas of degeneration on myelin sheaths, eventually leading to damage to the associated axons
• Is an autoimmune disorder, with the body's immune system attacking part of the body
• Diagnosing MS is difficult, with the nature and severity of the disorder depending on a variety of factors, including the number, size, and position of the sclerotic lesions
• Common symptoms include visual disturbances, muscular weakness, numbness, tremor, and ataxia, with cognitive deficits and emotional changes in some patients

Alzheimer's Disease

• Is the most common cause of dementia, with a progressive decline in cognitive function
• Can affect people as young as 40, with early stages often characterized by a selective decline in memory, deficits in attention, and other cognitive changes
• The incidence of Alzheimer's is highest in regions far from the equator, with a possible link to vitamin D levels.