Evolution and Embryonic Development Notes

Evolution and Embryonic Development

Part 1: Evolution and Neural Networks

Evolution of the Brain
  • Lancelet (Amphioxus):
    • Studying the lancelet helped neuroscientists understand the early stages of brain development.
    • The brain originated as a simple tube.
    • Early vertebrates developed a nerve cord with 3 distinct bulges, which evolved into the forebrain, midbrain, and hindbrain.
  • Notochord:
    • The notochord is considered an evolutionary precursor to the spinal cord.
  • Brain Expansion:
    • Over time, the forebrain and hindbrain bulges expanded, adding layers and folds to increase neuron numbers and processing power.
    • The forebrain expanded into cerebral hemispheres, enabling the processing of visual and auditory information and triggering behaviors like escape, feeding, and mating.
    • The hindbrain expanded to control escape movements and orient the organism in space, which is crucial for actively swimming fish compared to sedentary lancelets.
Neural Networks
  • Neural Tracts:
    • Chains of neurons allow information to move between brain regions and transmit signals over long distances.
    • A neural tract is a bundle of neurons.
    • Example: The corpus callosum connects the cerebral hemispheres.
  • Neural Networks:
    • Groups of nerve tracts form neural networks that connect brain regions.
    • Neural networks transmit signals through linear pathways and can analyze/organize information rapidly.
Example of a Mapped Neural Network (Visual Processing)
  • Process:
    1. Photoreceptors in the eye trigger electrical signals based on light wavelengths.
    2. Signals travel through the optic nerve to the optic tract and then to the thalamus.
    3. Thalamic neurons respond to shape, color, and movement and pass signals to the primary visual cortex (PVC) in the occipital lobe.
    4. The PVC integrates signals from each eye to form 3D representations of objects.
    5. The image is further refined by signals sent down two parallel processing streams: the dorsal and ventral streams.
  • Dorsal Stream:
    • Processes visual information related to spatial location and orientation of objects.
    • Visual information from the occipital lobe is sent dorsally to the parietal lobe.
  • Ventral Stream:
    • Processes visual information to recognize and identify familiar objects.
    • Visual information from the occipital lobe is sent ventrally to the temporal lobe.
Mapping Neural Networks
  • Technologies Used:
    • Scientists use technologies like functional Magnetic Resonance Imaging (fMRI), Computerized Tomography (CT scan), Positron Emission Tomography (PET scan), and Electroencephalography (EEG) to map neural networks by observing brain region activation during various functions.
Neuroimaging Techniques
  • CAT (or CT) Scanning (Computerized Axial Tomography):
    • Information Provided: Detailed structural images of the brain.
  • EEG (Electroencephalography):
    • Information Provided: Measures electrical activity in the brain.
  • MEG (Magnetoencephalography):
    • Information Provided: Measures magnetic fields produced by electrical activity in the brain.
  • MRI and fMRI (Magnetic Resonance Imaging and Functional Magnetic Resonance Imaging):
    • Information Provided: MRI provides detailed structural images; fMRI measures brain activity by detecting changes associated with blood flow.
  • PET (Positron Emission Tomography):
    • Information Provided: Measures metabolic activity in the brain using radioactive tracers.

Part 2: The Developing Brain (Brain Facts Chapter 6)

Stages of Brain Development
  • The development of the brain in utero is divided into six stages:
    1. Formation and Induction
    2. Proliferation
    3. Migration
    4. Synapse Formation
    5. Myelination
    6. Paring Back
1) Formation and Induction
  • Early Development:
    • Occurs early in development.
    • Involves the emergence of three germ layers during initial differentiation.
  • Three Germ Layers:
    • Ectoderm (outer-most layer)
    • Mesoderm (middle layer)
    • Endoderm (inner-most layer)
  • Gene Expression:
    • The process by which genes are turned on (transcribed into proteins) or turned off (not transcribed).
  • Neural Induction:
    • Mesoderm tissue signals ectoderm cells to turn specific genes on or off to become nerve tissue.
    • This process differentiates neural ectoderm into neurons or glia (support cells) and then into subclasses of each cell type (e.g., motor neurons, sensory neurons, astrocytes).
  • Sonic Hedgehog Signaling Molecule:
    • Secreted from mesoderm tissue beneath the spinal cord.
    • Cells close to high concentrations become glia, cells exposed to lower concentrations become motor neurons, and cells farthest away with the lowest concentrations become interneurons.
2) Proliferation
  • Mitosis:
    • Cell division, referred to as proliferation in this context.
  • Stages of Proliferation:
    • Symmetrical Division: Early stages forming two identical daughter cells.
    • Asymmetrical Division: Later stages where divisions produce a larger cell that continues proliferation and a smaller cell that progresses towards its fate as a neural or glial cell.
    • Fate is determined by induction, occurring simultaneously.
  • Disorders:
    • Microcephaly: Protein defects cause early switch to asymmetrical division, leading to reduced brain size.
    • Macrocephaly: Protein defects cause delayed switch, resulting in excessive symmetrical proliferation and an abnormally large brain.
3) Migration
  • Process:
    • Neurons travel from their formation point on the inner surface of the embryonic brain to their final location.
    • Begins 3-4 weeks after zygote formation.
  • Steps:
    1. The neural ectoderm (neural plate) grows and folds to form the neural tube and migratory neural crest.
    2. The neural tube thickens into three bulges (forebrain, midbrain, and hindbrain).
    3. Neurons travel through the neural tube's ventricular zone using radial glia (astrocytes) for guidance.
  • Factors Affecting Migration:
    • Exposure to alcohol, drugs, and radiation can disrupt neuron migration, leading to misplacement of cells and intellectual disabilities and epilepsy.
4) Synapse Formation
  • Process:
    • Neurons form connections with other neurons based on experiences.
    • Synapse forms when an axon of one neuron connects to the dendrites of another.
  • Axon Growth:
    • A developing axon grows via extension of its growth cone, guided by molecular cues.
    • Axons can extend from the lower spinal cord to the toes.
  • Neurotransmitters:
    • Molecules mediate axon growth towards a target location.
    • Axons release neurotransmitters like Glutamate and GABA, which dendrites receive and use to produce proteins that anchor the synapse.
  • Synapse Specialization:
    • Presynaptic side (axon): Terminal specializes in releasing neurotransmitters into the synaptic cleft.
    • Postsynaptic side (dendrite): Builds protein receptors for neurotransmitters on the membrane surface.
  • Astrocytes:
    • Play an important role in synaptic development.
    • Secrete molecules that regulate synaptic development.
  • Location:
    • Immature neurons can produce different neurotransmitters depending on their location (e.g., norepinephrine or acetylcholine).
  • Disorders:
    • Deficiencies in molecules promoting synapse formation can cause disorders like autism spectrum disorder (ASD).
    • Loss of these molecules can contribute to synapse degradation during aging.
5) Myelination
  • Myelin:
    • A lipid layer providing insulation for action potentials along axons.
  • Nodes of Ranvier:
    • Gaps in the myelin that allow for faster conduction of electrical signals.
  • Saltatory Conduction:
    • The “leaping” of electrical signals rapidly down an axon.
  • Myelin-Forming Cells:
    • Schwann cells in the peripheral nervous system.
    • Oligodendrocytes in the central nervous system.
  • Brain Matter:
    • Myelinated neurons make up the white matter.
    • Unmyelinated neurons make up the gray matter.
  • Lipid Composition:
    • Myelin is a lipid, so Schwann cells and Oligodendrocytes membranes are full of lipids.
6) Paring Back
  • Synaptic Pruning:
    • After initial growth, only about half of the neurons generated during development survive.
    • Entire populations of neurons are removed through apoptosis (programmed cell death).
  • Trophic Factors:
    • Life-sustaining chemicals that determine neuron survival.
    • Neurons not receiving enough trophic factors undergo apoptosis.
  • Nerve Growth Factor:
    • A trophic factor essential for sensory neuron survival.
  • Glia's role:
    • Astrocytes and other glia aid in the formation of connections and pruning synapses that do not receive enough signals.
  • Disorders:
    • Misactivation of apoptotic pathways can lead to neurodegenerative diseases (e.g., Alzheimer's and Parkinson's).