unit 1 , 2 marks 23rd June

Axoplasmic Flow

  • Definition: Axoplasmic flow is a dynamic, bidirectional active transport system within a nerve fiber's axoplasm. It is not a passive diffusion process but an active metabolic one requiring energy in the form of ATP.

  • Directional Types and Molecular Motors:

    • Anterograde Transport: This involves the movement of materials from the cell body (Soma) toward the axon terminal. It carries essential cellular components, including mitochondria, lipids, and synaptic vesicles. The primary molecular motor responsible for this movement is Kinesin.

    • Retrograde Transport: This involves the movement of materials from the axon terminal back to the Soma. It carries worn-out organelles and trophic signals intended for recycling or degradation. The primary molecular motor responsible for this movement is Dynein.

  • Physiological Significance:

    • Metabolic Necessity: Axons lack ribosomes and are therefore unable to synthesize structural proteins or enzymes independently. They depend entirely on axoplasmic flow for the delivery of these materials from the Soma for survival.

Co-transmitters in Nerve Physiology

  • Definition: Co-transmitters are distinct chemical messengers (typically neuropeptides, purines, or gases) that are stored and co-released alongside the primary classical neurotransmitters (such as Acetylcholine) from a single presynaptic nerve terminal.

  • Functional Difference: Unlike primary neurotransmitters that induce rapid, direct Excitatory Post-Synaptic Potentials (EPSPs) or Inhibitory Post-Synaptic Potentials (IPSPs) via ionotropic receptors, co-transmitters primarily bind to metabotropic (G-Protein Coupled Receptors) to slow, modulate, fine-tune, or prolong the post-synaptic cellular response.

  • Dale's Principle: This phenomenon updates Dale's Principle, which originally proposed that a single neuron synthesizes and releases only one type of neurotransmitter.

  • Mechanism of Storage and Frequency-Dependent Release:

    • Dual Storage Pools:

      • Small Clear Synaptic Vesicles (SSVs): These store the primary, low-molecular-weight neurotransmitters (e.g., Acetylcholine, Glutamate, GABA).

      • Large Dense-Core Vesicles (LDCVs): These store high-molecular-weight peptide co-transmitters (e.g., Vasoactive Intestinal Peptide (VIP), Neuropeptide Y, Substance P).

    • Differential Exocytosis:

      • Low-Frequency Stimulation: This evokes localized, low-level intracellular Calcium (Ca2+Ca^{2+}) microdomains strictly near the active zone, causing the exocytosis of only the SSVs and the release of primary neurotransmitters.

      • High-Frequency Burst Stimulation: This causes a global rise in presynaptic terminal intracellular Ca2+Ca^{2+} levels. This triggers the mobilization and exocytosis of LDCVs into the synaptic cleft alongside the corelease of primary neurotransmitters.

Classical Physiological Examples of Co-transmission

  • **