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64 Cards in this Set
- Front
- Back
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How does brain development occur?
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Over the course of development, the brain transforms from a single undifferentiated cell into billions of highly interconnected neurons. This happens in a progression of stages using various genetic and molecular markers to trigger development. Even after development, mechanisms of plasticity remain that help the brain learn, remember, and adapt to changes.
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When does anatomical development begin?
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Anatomical development begins during embryogenesis (when the embryo is forming) and continues into early adulthood. This development uses molecular signals that will be covered later.
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Where does brain tissue come from?
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Brain tissue comes from the ectoderm, the outer layer of tissue in the developing embryo.
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What is the first step in brain development?
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First a groove forms. The groove folds up into a neural tube, which is brought into the developing embryo.
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What starts to develop from the neural tube?
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Different areas of the brain start to develop from the tube. The three embryonic brain areas are the prosencephalon, mesencephalon, and rhombencephalon. The retina of the eye is part of the prosencephalon.
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What do neural inducer genes cause?
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The development of these different areas is triggered by neural inducer genes. These cause different types of growth in different areas of the brain.
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What signaling proteins do the neural inducer genes express?
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These inducers express signaling proteins such as cordin, noggin, and SHH (sonic hedgehog).
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How big is the brain of an infant at birth and at one year old?
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At birth, the infant brain is roughly ¼ of the size of an adult brain. It nearly triples in size in the first year of life.
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How is the infant brain?
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The infant’s brain is unmyelinated (no white matter). White matter continues to grow into adulthood.
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What is the last area of the brain that matures?
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The prefrontal cortex is the last area of the brain to mature. This can take upwards of 20 years to happen.
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How many development mechanisms are there?
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There are six major processes in play during development. Some of these last into adulthood, while others are largely restricted to embryogenesis or childhood.
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Proliferation
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Proliferation: stem cells start dividing during embryogenesis. All neurons are formed by proliferation since they cannot divide once mature.
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Migration
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Newly-formed neurons travel up the “spokes” of radial glia cells. They start at the subventricular zone and move toward the outside of the brain.
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Differentiation:
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There are many different types of neurons. Differentiation is when neurons “decide” what type of cell to become based on many factors, such as what chemicals are being released by nearby cells.
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Myelination:
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Myelin forms around the axons of some neurons. All neurons start out ummyelinated. Without myelin, there could not be effective long-range communication within the brain.
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Synaptogenesis:
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The formation of new synapses. Axons follow chemical signals (neurotrophins) out to locations where they can synapse.
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Pruning:
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Neurons that don’t form enough successful synapses wither and may die. This is called pruning. Most of your brain cells never make it because they’re unsuccessful and get pruned.
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What is plasticity and why is it useful?
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Plasticity is the brain’s ability to change. This helps fine-tune our brain’s systems during development and is important for recovery after brain injury. The change in synapse structure as a result of learning and experience is a form of plasticity.
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When is plasticity greatest?
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Plasticity is greatest early in development. Having a complex, interactive (“enriched”) environment during development leads to more complex neurons.
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What happens to underused areas of the cortex?
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Underused areas of the cortex can be “invaded” by nearby areas. This is why deaf or blind individuals often have sound and vision areas of the cortex activated by different things.
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What does plasticity often involve?
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Plasticity often involves the growth or degeneration of axons (both are important).
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What causes plasticity?
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Plasticity is often caused by the activation of cells, such as by a puff of glutamate.
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What happens if a neuron is damaged?
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If a neuron is damaged, nearby neurons can take its place by following neurotrophic factors to the cell that the damaged neuron used to synapse with.
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What can result in damage to the brain?
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Damage to the brain can result in a profound impairment in function. Some of this function can be recovered through mechanisms of plasticity. Neurogenesis, the growth of new neurons in adults, is important in several aspects of behavior.
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Brain Damage
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Physical trauma to the skull or brain can result in brain damage (such as during a car accident). Additionally, strokes are a major cause of brain damage. This is when a blood clot blocks flow to part of the brain (ischemia) or when a blood vessel bursts in the brain, resulting in a brain bleed (hemorrhage).
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Bleeding in the brain kills cells in several ways:
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-Lack of oxygenated blood starves cells.
-Pressure in the head can squish the cells to death. -The iron in the blood is toxic to neurons and can kill them. -Dying cells can cause nearby cells to die. |
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The brain areas adjacent to the area of damage are called:
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The penumbra.
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What do dying cells release and what can occur?
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Dying cells release cell death factors and also fire erratically. Rapid firing can “fry” cells in the penumbra. Death factors can trigger apoptosis.
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Can cells in the penumbra be saved? How?
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Cells in the penumbra can be saved by restoring blood flow or cooling the brain. Neuroprotective drugs can also help. This can block apoptosis.
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What is apoptosis and what triggers it?
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Apoptosis is the orderly death of a cell (as opposed to necrosis). It is often triggered by cell death signals or by overexcitation (called excitotoxicity). The cell shrinks and sends off parts of itself for recycling. It sends signals for microglia to “eat” the remaining cell.
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What causes apoptosis?
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Apoptosis is caused by a chemical “cascade” in the cell. If the cascade is stopped early enough, the cell can be saved. Once the cell reaches a point of no return, it will die.
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Recovery from Brain Damage:
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There is often some degree of recovery after an individual suffers from brain damage. This is due to the various mechanisms of plasticity present in the brain. Young people have more plasticity than the elderly, but plasticity is present to some extent in all individuals.
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What happens when brain area is damaged?
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When a brain area is damaged, axons to that area retract. They may rewire to other brain areas by following neurotrophic signals. They branch off to find functioning neurons.
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What happens if a cell does not find a new neuron to contact?
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Cells that don’t find a new neuron to contact may die. This is called diaschisis – when a brain area degenerates due to damage in a distant structure that normally activates that area.
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When do new axons grow?
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New axons grow when filopodia (growth cones) follow molecular signals to form a synapse. Sometimes they follow the wrong signal and re-wire things a bit wrong.
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What is phantom limb syndrome?
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Phantom limb syndrome is a symptom of plasticity after the brain has stopped receiving signals from a part of the body that’s been removed.
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Neurogenesis:
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Neurogenesis is the growth of new neurons after embryogenesis. It used to be thought that no new neurons grow in adults, but we now know that this is incorrect. New neurons can be detected via BrDU staining
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What happens to the precursor cells in the subventricular zone?
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They divide. The new neurons then migrate out to their destination.
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What is neurogenesis important for?
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Neurogenesis is important for mood. Depressed individuals often have low neurogenesis in the hippocampus. If you suppress neurogenesis, this causes depression-like symptoms. Antidepressants restore neurogenesis to the hippocampus.
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What also forms new neurons?
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The olfactory bulb also forms new neurons.
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What aids in brain plasticity in the young?
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Part of the greater brain plasticity in the young is due to neurogenesis. An enriched environment can enhance neurogenesis in animals.
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Biological Rhythms
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All organisms engage in certain biological rhythms. These are usually governed by feedback systems, such as circadian, circannual, and hormonal rhythms. Various sensory, chemical, and hormonal feedback signals result in the oscillation of biological rhythms. The brain’s response to these signals often has a strong genetic component that is rooted in the way neurons respond to different stimuli.
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Circadian Rhythms
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Circadian rhythms happen about once a day (hence their name) and are entrained to light. There is an intrinsic circadian oscillator called the SCN (suprachiasmatic nucleus) within the brain, but its oscillations aren’t exactly 24 hours and need lighting cues in order to oscillate in a manner that is appropriate to the day/night cycle. Even comparatively organisms (like Drosophila fruit flies) engage in some form of circadian behavior.
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Many physiological variables such as body temperature, blood pressure, digestive function, and numerous blood hormones fluctuate throughout the day.
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• This fluctuation is entrained to the Earth’s day-night cycle.
• If you fly to the other side of the globe, it takes about 8 days to adjust. • Newborns have poor circadian rhythms which then normalize in the first few months of life. |
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There is an internal circadian clock, but light resets the circadian rhythms.
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• Light activates a special set of photoreceptors in the retina that respond slowly to light.
• These cells respond slowly to light and eventually send signals to the SCN (this is the circadian oscillator) in the hypothalamus. • Some animals that are otherwise blind still entrain to light through this pathway. |
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The SCN is active during the day and when does it turn off?
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It turns off at night.
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Destroying the SCN removes circadian rhythms.
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• Transplanted brain tissue can restore the circadian rhythms.
• If you transplant a mutant SCN into a normal hamster, it will start expressing the mutant oscillation. |
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The rhythm in the SCN is controlled by clock genes.
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• These genes encode proteins that change neuron activity.
• Once the protein level gets too high, they shut their own production off through a negative feedback loop. • Exposure to light sends signals to the SCN that resets the clock genes to the starting point. |
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Many of the clock genes were first identified in Drosophila fruit flies. Similar genes are at work in humans.
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• Mutations in these genes is what causes abnormal circadian rhythms.
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Functions of sleep
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Sleep has several putative functions. Observations of various animal species indicate that sleep plays an important role in conserving energy and keeping animals hidden from predators. Sleep is also essential for maintaining the balance of various neurotransmitters and energy molecules. The consolidation of recent memories also seems to occur during sleep. In some animal species, sleep deprivation can be deadly.
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Different organisms sleep for different amounts of time:
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• Carnivores (and especially certain insectivores like bats) sleep for most of the day.
• Grazing herbivores only sleep a few hours a day. • Animals that must be constantly alert for predators get very little sleep. • This still doesn’t explain sleep too well since you could just be resting (not asleep) and still accomplish the above tasks. |
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Sleep is restorative.
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• Monoamine neurotransmitters (such as serotonin and dopamine) are depleted during the day. These are essential for alertness and concentration.
• Structural and energy proteins are depleted during the day. • Growth hormone (GH) levels are low during the day. • All of these things are restored during sleep. Increased GH leads to faster healing. |
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Sleep is important for memory formation.
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- Memories are consolidated during sleep.
- If an animal is sleep deprived, this introduces memory deficits. |
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Sleep stages are characterized by the patterns they produce on an EEG (electroencephalogram).
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• This pattern represents the EPSPs and IPSPs (excitatory and inhibitory postsynaptic potentials) that contribute to brain activity.
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There are three main stages of sleep:
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1. REM (rapid eye movement) or “Paradoxical” Sleep
2: NREM (non-REM) Stage 1 and 2 Sleep (light sleep) 3: NREM Stage 3 Sleep (slow wave or deep sleep). |
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REM (rapid eye movement) or “Paradoxical” Sleep
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-Characterized by fast brain waves similar to those seem while awake.
- There is low muscle tone, except for the eyes, which shift back and forth. - Dreams happen during this stage. |
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NREM (non-REM) Stage 1 and 2 Sleep (light sleep).
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- Characterized by relatively fast repeating “alpha” and “theta” waves.
- It is easy to wake someone up from this type of sleep. - People in Stage 1 are often unaware that they were sleeping. |
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NREM Stage 3 Sleep (slow wave or deep sleep).
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- Characterized by big, slow “delta” waves.
- It is very difficult to wake someone up from this type of sleep. - This type of sleep is very restorative. |
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There is a discrete sleep cycle across the night.
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• You gradually go through stage 1 and 2 sleep and enter a period of deep NREM sleep.
• You quickly come out of deep sleep and enter a stage of REM sleep. • You go back into NREM sleep, but it takes longer and the sleep isn’t quite as deep. • This pattern repeats until you don’t go back into sleep and wake up. |
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Sleep Disorders
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Given the important role of sleep in maintaining various brain functions, it is unsurprising that problems with the sleep process can result in functional deficits. Sleep problems are particularly common among the elderly. Other sleep disorders are associated with problems such as obesity, substance abuse, and psychiatric disorders.
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Why do children need a lot of sleep?
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Children need lots of sleep because their brains are still developing.
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What is atonia?
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- it is when normally the motor (movement) system turns off at night
- If the motor system doesn’t turn off all the way, this can result in “restless legs” or sleepwalking. - If the motor system doesn’t turn back on as you come out of sleep, this can result in “sleep paralysis”. |
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What is insomnia?
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Insomnia is the inability to get to sleep or stay asleep. It has numerous causes.
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Insomnia/sleep deprivation results in numerous cognitive deficits:
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• This can result in death in some animal species.
• Humans can enter “microsleeps” while awake. This might be why sleep deprivation isn’t deadly in us. • Symptoms include extreme sleepiness, poor attention span, irritability and mood swings, poor reaction speed, and dreamlike hallucinations. Memory formation and recall is very poor. |
