SEM II CC 3 2020
AXOPLASMIC FLOW: • Axon are specialized to convey chemical and electrical signals. Their cytoplasm is filled with many types of fibres and filaments, but lacks ribosomes and endoplasmic reticulum. For this reason, any proteins destined for the axon/axon terminal must be synthetized on the rough endoplasmic reticulum in the cell body (Soma). The proteins and polypeptides are then moved down the axoplasm along with other cellular subs and organelles to the axon terminal by the process of axoplasmic flow. The cell body maintains the functional and anatomic integrity of the axon (So if the axon is cut, the part distal to the cut degenerates – Wallerian degeneration). • Axoplasmic transport can be abolished by application of colchicine, dinitrophenol, azide, cyanide and prolonged anoxia. Colchicine disrupts the movement of microtubules, and others block the process of oxidative phosphorylation.
Types of Axoplasmic transport/flow: In the axoplasm, transport process can occur in both directions by different transport mechanisms. Accordingly, they are called anterograde, retrograde and transneuronal transport • Anterograde transport (centrifugal) – The transport of materials from the cell body to the axon terminals is known as anterograde transport, occurs along microtubules, with the involvement of molecular motors. For example, various neurotransmitters synthesized in the cell body are packaged in vesicles and get secreted at the nerve terminals through axoplasmic microtubules. This process is mapped by [3 H]-leucin. The rate of transport process may be: 1. Fast axoplasmic transport: occurs at about 400mm/day, which is accomplished by Kinesin, a microtubule associated protein that transports many organelles, vesicles and membrane glycoproteins. 2. Slow axoplasmic transport: occurs at 0.5-10mm/day. Various structural proteins like actin, neurofilaments, and microtubules are transported by this method. It has an important role in supplying the required materials for the regeneration of axons following nerve injury.
• Retrograde transport – in opposite direction, from axon terminal to the cell body, also occurs along microtubules at about 200mm/day, brought about by dynein, another microtubule associated protein, mapped by horse-radish peroxide. This mechanism keeps the soma informed about the synaptic environment.
The examples are- o Synaptic vesicles recycle in the membrane but some used vesicles are carried back to the cell body & deposited in lysosomes. o Reuptake of synaptic transmitters: Neurotransmitters like NE released at the nerve terminals are rapidly removed from the synaptic cleft by reuptake into the presynaptic neuron → repackaged into the vesicles or deaminated by mitochondrial monoamine oxidase → some of the vesicles may be transported back to the cell body → provide feedback signal for further synthesis of transmitters Choline is also taken up by the axon terminal and reused for new synthesis. o Some of the materials taken up at the endings by endocytosis, including nerve growth factor (E.g. Neurotrophins 3,4 & 5, brain derived neurotrophic factor (BDNF)) and transferred to soma. o Various viruses (chicken pox virus) transported to soma. o Transport of toxins (eg. Tetanus toxin) at motor neuron ending is also transported to soma this way.