Module 3 | Source: A Brain-Mind Odyssey (UC Berkeley), Chapter 10
Tags: neural development, neuroplasticity, human genome, transcription factors, stem cells, cell differentiation, neural progenitor cells, neural tube, growth cone, cytoskeleton, Roger Sperry, chemoaffinity hypothesis, neurotrophins, synaptic pruning, neurogenesis, hippocampus
Chapter 10 traces how the nervous system builds itself, from the genome through stem cell differentiation, neural tube formation, axon guidance, and synaptogenesis. It introduces Sperry's chemoaffinity hypothesis, the role of neurotrophins and growth cones, and the concept of neuroplasticity, including both synaptic pruning and adult neurogenesis in the hippocampus.
Human Genome
46 chromosomes total. The 23 chromosomes of the haploid genome contain approximately 3 billion (3 x 10^9) nucleotide base pairs. The entire genome has been sequenced. Current estimates place the number of protein-coding genes at approximately 21,000.
Transcription Factors
Proteins that bind to regions of the DNA, regulate gene readout, and control cell differentiation.
Stem Cells
After conception, the fertilised egg divides to form embryonic stem cells, which have the capacity to continue dividing and to differentiate into any cell type in the body.
Cell Differentiation
The process by which stem cells become specialised cell types, guided by transcription factors.
Neural Progenitor Cells
Embryonic stem cells in the developing nervous system differentiate into neural progenitor cells, which are committed to becoming various types of neurons or glia. Formation of nerve cells from these progenitors is called neurogenesis; formation of glial cells is called gliogenesis.
Neural Tube
Structure formed within three weeks of conception. Elongates to form forebrain, midbrain, and hindbrain regions; the remainder forms the spinal cord. The entire CNS develops from this folded piece of tissue as it grows and differentiates.
Growth Cone
Term first used by Santiago Ramon y Cajal. The terminal club at the end of a growing axon. Progresses via extension of fingerlike structures called filopodia. The site of axon elongation.
Cytoskeleton
Composed of ordered arrays of protein polymers: microfilaments and microtubules.
Microfilaments
Made of actin proteins. Form long strands involved in growth and movement of cell processes (axons, dendrites, dendritic spines) and intracellular transport.
Microtubules
Made of tubulin proteins. Perform similar functions to microfilaments: structural support, process growth, and transport of materials within the cell.
Roger Sperry (1913–1994)
Studied how axons know where to grow and how synapses decide to form. Used frogs and salamanders (amphibians capable of neural regeneration). Key experiments:
Cut frog's optic nerve; within weeks, it regenerated and normal vision was restored.
Rotated frog's eyeball 180 degrees; frog saw the world upside down and backward.
Combined both procedures (rotation plus nerve cut); after regeneration, the frog still saw the world upside down and backward, demonstrating that regenerating axons reconnect to their original brain targets based on chemical identity, not functional feedback.
Chemoaffinity Hypothesis
Formulated by Roger Sperry. Proposes that nerve cells use specific chemical signals to guide their wiring during development and during neural regeneration.
Neurotrophins
The first nerve growth factor to be discovered was simply named "nerve growth factor." Neurotrophins are proteins that promote growth or survival of neurons.
Nerve Growth Factors
Proteins found to promote the growth or survival of neurons.
Nerve Guidance Factors
Proteins involved in axon and dendrite guidance and other developmental processes. Some involve direct contact between proteins anchored to adjacent cells.
Synaptic Pruning
The elimination of synapses that form before and after birth but are not used. An aspect of neuroplasticity.
Neuroplasticity
The brain's ability to modify, change, and adapt both structure and function throughout life and in response to experience. Encompasses both synaptic pruning and the stabilisation and strengthening of synapses.
Embryonic Neurogenesis
Mostly takes place in the womb but continues after birth. More neuronal connections form during the first year of life than will ultimately be retained. Robust rates continue through childhood. Myelination of axons continues past twenty years of age.
Adult Neurogenesis
Approximately 1,400 new neurons are added each day within the two hippocampi of the human brain.
Synaptogenesis
The process by which maturing neurons wire together and form synapses.
Hippocampus
Bilateral structure located beneath the surface of the temporal lobe. Plays a pivotal role in formation and stabilisation of memories. Site of ongoing adult neurogenesis.
The human genome's approximately 21,000 protein-coding genes provide the blueprint, but transcription factors control which genes are read out in which cells, driving differentiation.
Embryonic stem cells differentiate into neural progenitor cells, which then commit to becoming specific neuron types or glia.
Neurogenesis (neuron formation) and gliogenesis (glial cell formation) proceed in parallel with cell migration, as differentiating cells move to occupy their specific locations in the developing nervous system.
The neural tube forms within three weeks of conception and is the structural precursor of the entire CNS.
It elongates and differentiates into three primary brain regions (forebrain, midbrain, hindbrain) plus the spinal cord.
Growth cones at the tips of elongating axons navigate using filopodia, extending and retracting to probe the environment.
The cytoskeleton (actin microfilaments and tubulin microtubules) provides the structural machinery for axon and dendrite growth, process movement, and intracellular transport.
Sperry's chemoaffinity hypothesis: axons find their correct targets using specific chemical signals, as demonstrated by his rotated-eye experiments in frogs.
Neurotrophins and nerve guidance factors provide the molecular cues, some acting at a distance and others requiring direct cell-to-cell contact.
Neuronal development involves four concurrent processes: neurogenesis, gliogenesis, cell migration, and synaptogenesis.
After birth, more synaptic connections form than will be retained. Unused synapses are eliminated through synaptic pruning.
Synapses that are used are stabilised and strengthened. Both pruning and strengthening are forms of neuroplasticity.
Myelination continues past age twenty, meaning the brain is still structurally maturing well into early adulthood.
Adult neurogenesis occurs in the hippocampus, with roughly 1,400 new neurons added daily across the two hippocampi.
At glutamatergic synapses, released glutamate can interact with glutamate receptors on the presynaptic axon terminal itself.
This opens Na+ or Ca++ ion channels on the presynaptic side, prolonging depolarisation in the axon terminal and thereby strengthening the synapse.
⚠️ Know the four concurrent processes of neuronal development: neurogenesis, gliogenesis, migration, and synaptogenesis.
⚠️ Sperry's rotated-eye experiment is a classic. The key result: regenerating axons reconnect to original targets (chemical identity), not to functionally appropriate targets.
⚠️ Chemoaffinity hypothesis: chemical signals guide axon wiring. This is the conceptual foundation for understanding nerve guidance factors and neurotrophins.
⚠️ Synaptic pruning eliminates unused synapses; strengthening retains used ones. Both are neuroplasticity. Do not confuse pruning with neurodegeneration.
⚠️ Adult neurogenesis in the hippocampus (approximately 1,400 new neurons per day) is a commonly tested modern finding.
⚠️ The neural tube forms within three weeks of conception and gives rise to the entire CNS.
⚠️ Microfilaments are made of actin; microtubules are made of tubulin. A simple naming detail that examiners test frequently.
Q: Approximately how many protein-coding genes does the human genome contain, and across how many base pairs?
A: Approximately 21,000 protein-coding genes, distributed across roughly 3 billion (3 x 10^9) nucleotide base pairs on 23 chromosomes (haploid).
Q: What is the neural tube, and what structures does it give rise to?
A: A folded structure formed within three weeks of conception. It elongates to form the forebrain, midbrain, and hindbrain, with the remainder forming the spinal cord. The entire CNS develops from it.
Q: Describe Roger Sperry's combined eye-rotation and optic-nerve-cut experiment and its result.
A: Sperry rotated a frog's eyeball 180 degrees and cut the optic nerve. After the nerve regenerated, the frog still saw the world upside down and backward, demonstrating that regenerating axons reconnect to their original chemical targets in the brain rather than rewiring for correct function.
Q: What is the chemoaffinity hypothesis?
A: Proposed by Roger Sperry, it states that nerve cells use specific chemical signals to guide their wiring during both development and neural regeneration.
Q: Distinguish between microfilaments and microtubules.
A: Microfilaments are made of actin proteins; microtubules are made of tubulin proteins. Both form long strands that support cell process growth (axons, dendrites, spines) and intracellular transport.
Q: What is synaptic pruning, and how does it relate to neuroplasticity?
A: Synaptic pruning is the elimination of synapses that are not used. Along with the stabilisation and strengthening of active synapses, it is one of the two key aspects of neuroplasticity.
Q: Where does adult neurogenesis occur, and at what approximate rate?
A: In the hippocampus. Approximately 1,400 new neurons are added each day across the two hippocampi.
neural development, neurodevelopment, neuroplasticity, human genome, chromosomes, nucleotide base pairs, protein-coding genes, transcription factors, cell differentiation, stem cells, embryonic stem cells, neural progenitor cells, neurogenesis, gliogenesis, neural tube, forebrain, midbrain, hindbrain, spinal cord, growth cone, Santiago Ramon y Cajal, filopodia, axon elongation, cytoskeleton, microfilaments, actin, microtubules, tubulin, Roger Sperry, optic nerve regeneration, chemoaffinity hypothesis, chemical guidance, neurotrophins, nerve growth factor, nerve guidance factors, cell migration, synaptogenesis, synaptic pruning, use-dependent plasticity, embryonic neurogenesis, adult neurogenesis, hippocampus, temporal lobe, memory formation, myelination, presynaptic strengthening, glutamatergic synapse, glutamate receptors