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.