16.5:
Neurons: The Axon
The axon is a long, thin extended portion of the neuron that carries impulses from its cell body toward the effector cell.
Axoplasm, the axon's cytoplasm, is surrounded by the axolemma, or plasma membrane. The distal end of the axon branches into several slender processes called axon terminals or telodendria.
The axon emerges from a cone-shaped region of the cell body called the axon hillock, which is followed by the initial segment. In a stimulated neuron, action potentials are initiated at the trigger zone, the junction of the axon hillock and the initial segment, and advance along the length of the axon.
Axon terminals have bulb-shaped structures at their tips called synaptic end bulbs. These contain vesicles with neurotransmitters, which communicate signals to effector cells.
Neurons transport molecules, organelles, and cellular components through the axoplasm using a process called axonal transport.
Anterograde transport moves organelles and synaptic vesicles from the cell body to axon terminals, while retrograde transport carries membrane vesicles and cellular materials toward the cell body.
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.