Chapter 4 - The growth cone and the synapse

Abstract

Each dendrite and axon has to wend its way through a jungle of cell bodies and other neuronal processes. Each nerve process has a growth cone at its end that determines the direction of growth. Every growth cone has a series of long, narrow protrusions called filopodia interspersed with ruffle-like folds called lamellipodia. When an attractive molecule binds to its receptor on one side of the growth cone; the activated receptor signals through release or synthesis of a second messenger. This signal produces neurite turning toward the attractive signal via an actin linked motile apparatus. Also new membrane is inserted at the growth cone tip. In contrast, when a repulsive signal is encountered, another second messenger is produced. This causes the cytoskeleton on the surface facing the repulsive signal to dissolve, turning the growth cone in a different direction. The presynaptic ending has small Glu or GABA containing synaptic vesicles. Also there are larger, dense core vesicles containing the neurotransmitters dopamine, norepinephrine, serotonin, or a neuropeptide. The active zone underlies the plasma membrane opposite the postsynaptic ending. Synaptic vesicles are anchored to the active zones by three integral membrane proteins called SNARES. When the presynaptic membrane is depolarized, Ca++ rushes in, triggering neurotransmitter release. The synaptic vesicle membranes are recycled to form new synaptic vesicles. The postsynaptic endings contain a high concentration of receptors specific for the released transmitter. The receptors are anchored to the plasma membrane by PDZ domain proteins. There are also electrical synapses formed between neurons and/or glia. These synapses are composed of repeating integral membrane proteins that fit together with identical proteins on the opposing membrane. These receptor-like proteins are hollow in the middle allowing fairly large molecules to pass from one cell to another.