NSCI200 TA tutorial 2

Suvrathan Tutorial 2 L3-6

Overview of neural circuits and their assembly.

Neural Circuit Assembly

Question: How are neural circuits assembled?Discussion on developmental biology and connections between neurons.

Axon Targeting Mechanisms

Target FindingKey question: "How do axons find their targets?"The role of various molecular cues in guiding axons to their destinations in the nervous system, similar to using Google Maps where axons receive cues at each decision point, directing their path step by step.


Sperry's Eye Rotation ExperimentsOverview of the experiments conducted by Sperry which examined axon guidance using rotated eyes.Important components of the visual system:

  • Retina: receiving visual information.

  • Tectum: processing and integrating visual inputs.

  • Key findings:Displacement of axons in visual pathways demonstrated the specificity needed for proper neural connection.Following the chemoaffinity hypothesis proposed by Sperry, differences among neurons arise from biochemical tags appearing early in development, guiding them to their specific targets.His experiments used the frog visual system, which is beneficial due to its regenerative capabilities, making behavioral tests straightforward.




Growth Cones: Crucial structures in axon targeting:

  • Growth cones are the tips of axons that guide neurons towards their target cells.They detect attractive and repulsive cues, dynamically changing structure based on environmental conditions. This includes broad exploration in search of cues and simplified structure upon reaching targets.

  • Mechanisms of Growth Cones: Growth cones utilize various signaling molecules, such as netrins and semaphorins, to navigate the complex extracellular environment. These molecules interact with specific receptors on the growth cone surface, leading to changes in cytoskeletal dynamics that facilitate movement and direction. In addition, the role of cell adhesion molecules is vital, as they help stabilize the growth cone's position and promote interactions with the surrounding extracellular matrix. Furthermore, the interplay between these signaling pathways ensures that growth cones can adapt their behavior in response to the presence of guidance cues, allowing for precise navigation during neural development.

Attractive and Repulsive Signals in Axon Targeting

  • Attractive Cues: These signals promote axon growth towards specific directions. They are vital for guiding axons to their intended targets. Key examples include:

    • Netrins: A class of molecules that attract axons during development, helping them navigate towards the correct pathway.

    • Neurotrophic Factors: Such as NGF (Nerve Growth Factor), which support the survival and growth of neurons by providing attractive signals that encourage axons to extend.

  • Repulsive Cues: These signals prevent axons from growing in unwanted directions, ensuring that they do not stray from their designated pathways. Examples include:

    • Semaphorins: A family of proteins known to provide repulsive guidance cues to axons, effectively steering them away from certain areas.

    • Ephrins: These molecules are involved in repulsion as well, particularly influencing the topographic distribution of axon projections. Ephrins interact with Eph receptors, leading to repulsive signaling that helps delineate areas within the neural circuits.

  • Mechanisms of Action:

    • Chemotaxis: Movement of axons based on chemical gradients produced by attractive and repulsive cues, allowing them to follow the right paths through the nervous system.

    • Contact Guidance: This involves the direct interaction between cell adhesion molecules and the axons, guiding their growth based on the physical structure of the environment.

    • Dynamic Reactions: Growth cones, the tips of the extending axons, can rapidly alter their structure in response to the presence of these cues, broadening their explorative behavior when searching for attractive signals or retracting in response to repulsive signals.

Ligand-Receptor Systems

Importance of Various Molecular Interactions in Axon Guidance

In the development and functioning of neural circuits, ligand-receptor systems play a crucial role in guiding axon growth and establishing synaptic connections. Here are key components of these molecular interactions:

  1. Extracellular Matrix (ECM) Molecules:

    • ECM molecules provide both structural support and biochemical signals that facilitate axon growth and guidance. The ECM not only creates a scaffold for neurons but also releases growth factors that influence cell migration and differentiation. They help in maintaining the structural integrity of the developing nervous system while also playing an active role in signaling pathways that direct axon pathfinding.

  2. Cadherins and Cell Adhesion Molecules (CAMs):

    • Cadherins are a class of type-1 transmembrane proteins that mediate cell-cell adhesion. They play vital roles in the formation of adherens junctions to bind cells together, thereby influencing the morphology of growing axons.

    • CAMs, which include various homophilic and heterophilic adhesion molecules, are also essential for establishing connections between neurons. They facilitate axon fasciculation, where bundling of axons occurs, ensuring that axons adhere to one another while navigating through the complex extracellular environment. Together, cadherins and CAMs are fundamental in neuron recognition and targeting.

  3. Ephrins:

    • Ephrins are a family of proteins that interact with Eph receptors, playing a significant role in axon guidance and the establishment of topographic maps within the nervous system. Ephrins are classified into ephrin-A and ephrin-B subclasses, each interacting with specific Eph receptor families. The interactions between ephrins and Eph receptors provide crucial repulsive signals that prevent axons from growing into inappropriate areas, which is critical for refining neural circuits during development.

    • Additionally, ephrins guide the formation of boundaries between different neuron populations, enabling the precise organization of the brain hemispheres and cortical areas. Their signaling also contributes to cellular processes such as migration and positioning of synapses, thereby influencing the functional architecture of neural networks.

These molecular interactions in ligand-receptor systems are vital for the proper navigation and connectivity of axons, ultimately contributing to the functional integrity of the nervous system.

Specificity of Synaptic Connections

Factors Influencing Synaptic FormationQuestions arise regarding how synaptic connections are established and maintained.Molecular factors include:

  • DSCAMs: involved in homophilic interactions.

  • Protocadherins: establish specific synaptic partners.

  • Neurofascin: maintains axon initial segment structure/function.

Neuromuscular Junction (NMJ) Development

Anecdotes important for discussing synaptic specificity include the role of Agrin, which is critical for clustering acetylcholine receptors at NMJ formation, and the function of Schwann cells in developing neural connections.

Experience-Dependent Plasticity

Critical Periods: Experience plays a fundamental role in the development and refinement of neural circuits within the brain. During critical periods, specific experiences can lead to significant changes in synaptic strength and connectivity in response to sensory inputs. Notably, visual stimuli during early life significantly influence ocular dominance columns in the visual cortex. These columns are organized into bands based on the input from the left and right eyes, and the dominance of one eye over the other can be shaped by visual experiences received during these critical periods.

Research shows that there are sensitive windows when the neural circuitry is particularly receptive to environmental input. For instance, if one eye is deprived of visual stimuli (a condition known as monocular deprivation), this can lead to lasting changes in ocular dominance, often leading to conditions such as amblyopia, where the vision in one eye is compromised.

The development of ocular dominance columns is underpinned by experience-dependent plasticity, where the strength and number of synaptic connections are adjusted based on experiences. This adaptability is mediated by various molecular mechanisms, including the signaling of neurotransmitters like glutamate, which drives synaptic changes during these critical periods. Activities such as visual experience, motor activities, and even social interactions can lead to significant rewiring of these connections, demonstrating the brain's remarkable capacity for adaptation and change as it encounters new stimuli.

Example Questions

A range of questions test understanding of key concepts pertaining to:

  • Molecular signaling in axon growth.

  • Specificity of synaptic connections.

  • Implications of various molecules in guiding neural development.

Enhancements from Presentation Transcript

  • Comparison of axon guidance to Google Maps, highlighting decision points and environmental cues.

  • Discussion on the historical context of Sperry's experiments and the efficacy of the chemoaffinity hypothesis, notably in the frog visual system.

  • Insight into the dynamics of growth cones, crucial for guiding axons using various attractive and repulsive cues.

  • Inclusion of more detailed signaling mechanisms related to ligand-receptor interactions and their roles in cellular adhesion and axon guidance processes.

  • Emphasis on the importance of experience in shaping synaptic connections, with various examples ranging from visual to unitary signaling paths in the CNS.