16.5: Neurons: The Axon
Axons are long, cytoplasmic processes of nerve cells capable of propagating electrical impulses known as action potentials. The cytoplasm or axoplasm of an axon contains neurofibrils, neurotubules, small vesicles, lysosomes, mitochondria, and various enzymes, all encased within the axolemma, the plasma membrane of the axon.
The axon attaches to the cell body at a cone-shaped elevation called the axon hillock. The initial part of the axon, closest to the hillock, is known as the initial segment. Nerve impulses often originate at the junction of the axon hillock and the initial segment, an area referred to as the trigger zone.
Axons can branch out, creating side branches or collaterals, allowing one neuron to communicate with multiple other cells. These branches end in fine extensions called telodendria or terminal branches, culminating at synaptic terminals for communication with other cells. This site of communication is known as a synapse.
Two transport systems facilitate the movement of substances between the cell body and the axon: slow axonal transport and fast axonal transport. Slow axonal transport moves materials from the cell body towards the axon terminals at a pace of 1–5 mm per day, providing new axoplasm for developing, regenerating, or mature axons.
On the other hand, fast axonal transport can move materials at a speed of 200–400 mm per day in both directions. It uses protein motors to move materials along the microtubules of the neuron's cytoskeleton. Anterograde (forward) fast axonal transport carries organelles and synaptic vesicles from the cell body to the axon terminals. Conversely, retrograde (backward) fast axonal transport brings materials from the axon terminals to the cell body for degradation or recycling.
