- 1 week ago
大脑并不是一下子长成的，而是在整个生命周期中持续重塑、完善和改变。这里用清晰的神经科学讲解，带你从神经管形成、脑泡分化、神经发生、细胞迁移与分化，一路看到突触生成、细胞凋亡、突触修剪，再到青春期和老年期的大脑变化。
内容重点解释了胚胎期神经板如何折叠成神经管，神经嵴细胞如何参与周围神经系统的形成，以及端脑、间脑、中脑、后脑和延脑如何发展为成熟脑结构。随后还会看到为什么大脑皮层会折叠成脑回和脑沟，以及为什么大多数神经元都在出生前生成。青春期部分则聚焦于髓鞘化、前额叶皮层成熟、冲动控制、计划与风险行为之间的关系；最后讨论老化如何影响灰质、白质、信息处理速度、工作记忆和记忆能力。
这是一段适合神经科学入门、脑科学学习、教育类播客式观看和课堂补充理解的内容，也适合想了解 brain development, adolescent brain, aging brain, neurogenesis, synaptogenesis 和 myelination 的观众。无论你是在搜索大脑发育全过程、青春期大脑变化，还是想找一段关于神经系统发育与认知老化的清晰讲解，这里都能提供有用的概览。
内容重点解释了胚胎期神经板如何折叠成神经管，神经嵴细胞如何参与周围神经系统的形成，以及端脑、间脑、中脑、后脑和延脑如何发展为成熟脑结构。随后还会看到为什么大脑皮层会折叠成脑回和脑沟，以及为什么大多数神经元都在出生前生成。青春期部分则聚焦于髓鞘化、前额叶皮层成熟、冲动控制、计划与风险行为之间的关系；最后讨论老化如何影响灰质、白质、信息处理速度、工作记忆和记忆能力。
这是一段适合神经科学入门、脑科学学习、教育类播客式观看和课堂补充理解的内容，也适合想了解 brain development, adolescent brain, aging brain, neurogenesis, synaptogenesis 和 myelination 的观众。无论你是在搜索大脑发育全过程、青春期大脑变化，还是想找一段关于神经系统发育与认知老化的清晰讲解，这里都能提供有用的概览。
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LearningTranscript
00:00Hi and welcome to 10 Minute Neuroscience.
00:02In this installment I'll be covering how the brain changes across the lifespan, from
00:07early neural development to older adulthood.
00:10Of course there is a lot of variation in neural development among individuals, so I'll be
00:14describing broad developmental patterns that show up consistently, not a rigid sequence
00:18that unfolds in exactly the same way for every person.
00:22The development of the nervous system starts at around 3 weeks after conception, when part
00:26of the ectoderm, which is one of the major cell layers in the embryo, thickens to form
00:31what is called the neural plate.
00:33The neural plate is the first visible structure that will eventually give rise to the nervous
00:37system, and as development continues, this neural plate begins to change shape.
00:41A groove forms down the middle, called the neural groove, and the edges of the plate rise up
00:47on either side to form what are called neural folds.
00:50As these folds continue to move toward one another, they eventually fuse, transforming
00:55the flat neural plate into a hollow structure called the neural tube.
00:59This process is known as neuralation, and it's one of the most important events in early
01:04neural development because the neural tube is what ultimately becomes the central nervous
01:08system, or the brain and spinal cord.
01:11Most of this process is referred to as primary neuralation, which is the folding and closure
01:15of the neural plate to form the majority of the neural tube.
01:18There is also a process, however, called secondary neuralation, which contributes to the formation
01:23of the caudal, or tail end, portion of the spinal cord through a somewhat different mechanism.
01:29But a key point is that the central nervous system begins as a tube, and that tube forms
01:34very early in embryonic development.
01:36At around the same time, another important population of cells appears at the border of the
01:41neural plate.
01:42These are called neural crest cells.
01:44Neural crest cells migrate away from the developing neural tube, and give rise to a wide variety of
01:49structures throughout the body.
01:51In terms of the nervous system, neural crest cells contribute to much of the peripheral
01:55nervous system, including many sensory neurons and autonomic neurons.
01:59So the neural tube gives rise to the central nervous system, while the neural crest cells contribute
02:04substantially to the peripheral nervous system.
02:08Once the neural tube is formed, it does not remain a simple, uniform tube.
02:12Instead, different regions become specialized for different functions.
02:15For example, the dorsal part of the neural tube, which is situated toward the back, becomes
02:19specialized for motor functions.
02:26This basic organization is an important feature of nervous system development, and is reflected
02:30in structures like the spinal cord and brainstem.
02:33As development continues, the rostral, or front end of the neural tube, begins to expand
02:38into a series of bulges known as brain vesicles.
02:41Initially there are three primary vesicles.
02:44The prosencephalon, or forebrain, the mesencephalon, or midbrain, and the rhombencephalon, or hindbrain.
02:53These primary vesicles later subdivide into five secondary vesicles.
02:57The prosencephalon divides into the telencephalon and diencephalon.
03:03The telencephalon will eventually form the cerebral hemispheres, while the diencephalon gives rise
03:08to structures like the thalamus and hypothalamus.
03:11The mesencephalon remains as the midbrain.
03:14The rhombencephalon divides into the metencephalon and myelencephalon.
03:20The metencephalon develops into structures such as the pons and cerebellum, while the myelencephalon
03:25becomes the medulla oblongata.
03:28Over the rest of gestation, the neural tube will continue to transform and shape until
03:33it starts to resemble a brain and spinal cord.
03:36One of the most dramatic aspects of brain development is how much the telencephalon expands
03:41relative to the rest of the central nervous system.
03:43Early in development, the brain is much more dominated by structures that will become the
03:47brainstem and other lower regions.
03:49But over time, the telencephalon grows at a disproportionate rate, especially in humans.
03:53This growth leads to the formation of large cerebral hemispheres, which come to dominate the adult
03:59brain.
04:00Additionally, the cerebral cortex grows dramatically, and because the amount of cortical surface area
04:05increases faster than the available space in the skull, the cortex begins to fold.
04:10These folds form the ridges and grooves, or gyri and sulci, of the mature brain.
04:16Early in gestation, the cortical surface is relatively smooth, but as development progresses, folding
04:21becomes increasingly pronounced.
04:23This allows a large amount of cortical tissue to fit into a limited volume, which is one
04:27of the reasons the human brain can support such complex functions.
04:32So far, I've been focusing mostly on visible features of development.
04:36But a great deal of neural development involves events happening at the cellular level.
04:40One of the first of these is neurogenesis, which is the production of new neurons.
04:45During prenatal development, enormous numbers of neurons are generated, and the vast majority
04:50of our neurons overall are produced before birth.
04:53Some research suggests that neurogenesis may continue after birth in a limited way in certain
04:58brain regions, but there's ongoing debate about how much of this occurs in adult humans,
05:02and what functional significance it has.
05:04So the overall contribution of postnatal neurogenesis appears to be much smaller than the massive
05:10wave of neuron production that takes place during prenatal development.
05:14After neurons are generated, many of them have to travel to the right place.
05:18This is referred to as cell migration.
05:20In the developing brain, neurons are often born near ventricles, and then migrate outward
05:25toward the cortical surface.
05:26Many of them do that by moving along glial cells called radial glia, which act like scaffolding.
05:34This migration is essential because a neuron's location strongly influences what kind of role
05:39it will play in a circuit.
05:41Once neurons reach their destinations, they begin to differentiate.
05:45Differentiation means they become specialized.
05:47They develop distinct shapes, express particular neurotransmitters and receptor proteins, and
05:53begin to take on the functional identity that will characterize them in the mature nervous
05:57system.
05:58For example, some will become excitatory projection neurons, and others will become inhibitory interneurons,
06:03and so on.
06:05They also begin to form axons and dendrites, which are necessary for communication with other
06:10cells.
06:11So, differentiation is the process by which a newly generated neuron becomes a particular kind
06:15of neuron with a specific role to play.
06:19After differentiation comes synaptogenesis, which is the formation of synapses between neurons.
06:25During development, the brain forms huge numbers of synaptic connections.
06:28In fact, it often forms more connections than it will retain long term.
06:33This overproduction is a normal part of development.
06:35It gives the brain a great deal of flexibility, allowing activity and experience to help shape which
06:42connections are preserved and which are eliminated.
06:46So, rather than constructing a perfectly optimized network from the beginning, the brain initially
06:51creates a very rich network of possible connections, and then refines that network over time.
06:57That refinement involves another key developmental process, which is neuronal cell death.
07:02Although it may sound like a bad thing, the death of many neurons during development is
07:07a normal and necessary part of nervous system formation.
07:11The developing brain typically produces more neurons than it ultimately needs, and neurons
07:16that fail to establish appropriate connections or receive sufficient survival signals may undergo
07:21programmed cell death, also known as apoptosis.
07:26This helps sculpt neural circuits so that they properly match their targets and don't contain
07:31cells that aren't contributing productively to the system.
07:35Synapses also undergo major rearrangement during development.
07:40Some synapses are strengthened and stabilized because they are repeatedly used, while others
07:45are weakened and eventually eliminated.
07:47This is often referred to as synaptic pruning.
07:50This process is influenced strongly by neural activity and experience.
07:54So, development is not just a story of growth, it's also a story of selective removal.
08:00The brain becomes more efficient, not simply by adding more and more connections, but by
08:05refining those connections and reducing redundancy.
08:09Taken together, neurogenesis, cell migration, differentiation, synaptogenesis, neuronal cell death, and synaptic
08:16rearrangement are some of the major stages of neural development.
08:19They overlap considerably, and in many cases, one process influences another, but collectively,
08:24they help explain how the nervous system goes from a relatively simple embryonic structure
08:29to a highly organized and functional brain.
08:33Although a large amount of neural development occurs before birth and in early childhood,
08:37brain maturation continues for decades.
08:40Adolescence for example is a particularly important period of continued development.
08:44During the adolescent and teen years, the brain is still undergoing significant remodeling.
08:48Synaptic pruning continues in a number of regions, and patterns of connectivity continue
08:54to be refined.
08:56Myelination, the process of wrapping axons in an insulating layer called myelin, also
09:01continues through adolescence and into early adulthood, and in some brain regions even extends
09:05into the third decade of life.
09:08Myelin helps neural signals travel more quickly and efficiently, so increases in myelination
09:13improve the speed and reliability of communication between brain regions.
09:16As a result, cognitive processes like attention, working memory, and decision making generally
09:22become more efficient over time.
09:24At the same time, increasing myelination can make neural circuits more stable and less easily
09:29changed, which may contribute to a gradual shift from the flexible, highly plastic brain
09:34of childhood to a more specialized and efficient adult brain.
09:39Importantly, different brain regions mature on different timelines.
09:43In general, systems involved in emotion, reward sensitivity, and motivation can become highly
09:48active during adolescence, while parts of the prefrontal cortex involved in impulse control,
09:54planning, and long-term decision making continue to mature later.
09:58Because myelination is still ongoing in these prefrontal regions, the connections that support
10:03top-down control are not yet operating at full efficiency in adolescence.
10:08This mismatch is one factor that may contribute to some stereotypical aspects of teen behavior,
10:13like increased novelty seeking, heightened sensitivity to peers, and a greater tendency toward risk-taking
10:20in some situations.
10:21As we move to older adulthood, the brain continues to change, but the pattern of change is different
10:27from what we see earlier in life.
10:29Aging is commonly associated with reductions in gray matter volume in some regions of the
10:33brain, and decreases in dendritic arborization, meaning less extensive branching of dendrites
10:39in certain neurons.
10:41A reduction in the amount of white matter may also occur with age, which can affect how efficiently
10:46different brain regions communicate with one another.
10:49These structural changes are often linked to declines in certain aspects of cognition,
10:53particularly processing speed, working memory, and some other types of memory.
10:58At the same time, the effects of aging are not uniform across all types of cognition.
11:02Some abilities like vocabulary, general knowledge, and skills built through experience are often
11:07maintained relatively well and may even improve until later adulthood.
11:10There's also a great deal of individual variability in how the brain ages.
11:14So, things like physical health, cognitive engagement, social interaction, sleep, and
11:19exercise can all influence the extent to which age-related brain changes affect cognition.
11:25So, while aging is often associated with some decline, it doesn't affect all people or
11:29all mental abilities in the same way.
11:31So, although we often talk about development as something that happens before birth or in
11:35infancy or childhood, the reality is that the brain is changing across the entire lifespan.
11:40The nature of those changes differs depending on the stage of life, but the central idea remains
11:45the same.
11:46The nervous system is dynamic.
11:48It's built, refined, reorganized, and gradually altered over time in ways that reflect both
11:54biology and experience.
11:56And that's a brief overview of how the brain changes over the lifespan.
11:59So, if you have any questions, feel free to drop them in the comments below and I'll
12:03do my best to answer them.