Neurons may lack dendrites or have no axon. Metabotropic dopamine receptors are present both on spiny neurons and on cortical axon terminals. A neuron consists of a cell body, axon, and dendrites. Cells called guidepost cells assist in the guidance of neuronal axon growth. Myelin prevents ions from entering or leaving the axon along myelinated segments. Some types of neurons have no axon and transmit signals from their dendrites. The axon and dendrites are filaments that extrude from it. Action potentials cannot propagate through the membrane in myelinated segments of the axon. This projection can be either an axon or a dendrite. Presynaptic neurotoxins disrupt neurotransmitter release from the axon terminal. Persistent compression results in local demyelination with no loss of axon potential continuity. This is due to the light or non-myelination of the axon. Signals including glutamate from axon branches may increase branch additions. This is reflected by the diameter of the nerve axon. The nucleus of the damaged axon undergoes chromatolysis in preparation for axon regeneration.