16.5
Neuroni: l'assone
Gli assoni sono lunghi processi citoplasmatici delle cellule nervose in grado di propagare impulsi elettrici noti come potenziali d'azione. Il citopla…
L'assone è una porzione estesa lunga e sottile del neurone che trasporta gli impulsi dal suo corpo cellulare verso la cellula effettrice.
L'assoplasma, il citoplasma dell'assone, è circondato dall'assolemma, o membrana plasmatica. L'estremità distale dell'assone si ramifica in diversi processi sottili chiamati terminali dell'assone o telodendri.
L'assone emerge da una regione a forma di cono del corpo cellulare chiamata collinetta dell'assone, seguita dal segmento iniziale. In un neurone stimolato, i potenziali d'azione vengono avviati nella zona di innesco, nella giunzione della collinetta dell'assone e nel segmento iniziale e avanzano lungo la lunghezza dell'assone.
I terminali degli assoni hanno strutture a forma di bulbo sulle loro punte chiamate bulbi terminali sinaptici. Questi contengono vescicole con neurotrasmettitori, che comunicano segnali alle cellule effettrici.
I neuroni trasportano molecole, organelli e componenti cellulari attraverso l'assoplasma utilizzando un processo chiamato trasporto assonale.
Il trasporto anterogrado sposta gli organelli e le vescicole sinaptiche dal corpo cellulare ai terminali degli assoni, mentre il trasporto retrogrado trasporta le vescicole di membrana e i materiali cellulari verso il corpo cellulare.
View the full transcript and gain access to JoVE Core videos
Q1: What is the axon and what is its main function in a neuron?
The axon is a long, thin extended portion of the neuron that carries electrical impulses from the cell body toward effector cells. It contains axoplasm, cytoplasm surrounded by the axolemma plasma membrane. The axon's primary function is to propagate action potentials along its length, enabling communication between neurons and target cells.
Q2: Where do action potentials originate in the axon?
Action potentials are initiated at the trigger zone, the junction between the axon hillock and the initial segment. The axon hillock is a cone-shaped region where the axon emerges from the cell body. From this trigger zone, action potentials advance along the entire length of the axon to transmit neural signals.
Q3: What are synaptic end bulbs and what do they contain?
Synaptic end bulbs are bulb-shaped structures at the tips of axon terminals. They contain vesicles filled with neurotransmitters, chemical messengers that communicate signals to effector cells. These structures are essential for transmitting information across synapses to target neurons or muscle cells.
Q4: How do axons branch and communicate with multiple cells?
The distal end of the axon branches into several slender processes called axon terminals or telodendria. These terminal branches create side branches or collaterals, allowing one neuron to communicate with multiple other cells simultaneously. Each branch ends at synaptic terminals where communication occurs.
Q5: What is the difference between anterograde and retrograde axonal transport?
Anterograde transport moves organelles and synaptic vesicles from the cell body toward axon terminals, supplying materials for neurotransmission. Retrograde transport carries membrane vesicles and cellular materials back toward the cell body for degradation or recycling. Both processes use protein motors to move materials along microtubules.
Q6: What is the difference between slow and fast axonal transport?
Slow axonal transport moves materials from the cell body toward axon terminals at 1-5 mm per day, providing new axoplasm for developing or regenerating axons. Fast axonal transport moves materials at 200-400 mm per day in both directions using protein motors along microtubules, enabling rapid delivery of organelles and vesicles.
Q7: What cellular components are contained within the axoplasm?
Axoplasm contains neurofibrils, neurotubules, small vesicles, lysosomes, mitochondria, and various enzymes. These components are enclosed within the axolemma, the plasma membrane of the axon. Together, they support axonal function, including energy production, protein synthesis, and transport of materials throughout the axon.