A<unk>  is a small (sub-micrometre) membranous extrusion that protrudes from a dendrite and forms one half of a synapse. Typically<unk> s have a bulbous head (the<unk>  head) which is connected to the parent dendrite through a thin<unk>  neck.<unk> s are found on the dendrites of most principal neurons in the brain including cortical pyramidal neurons, medium<unk>  neurons of the striatum and Purkinje cells in the cerebellum. Hippocampal and cortical pyramidal neurons may receive tens of thousands of mostly excitatory inputs from other neurons onto their equally numerous<unk> s, whereas the number of<unk> s on Purkinje neuron dendrites is an order of magnitude larger.<unk> s come in a variety of shapes and have been categorized accordingly, e.g. mushroom<unk> s, thin<unk> s and stubby<unk> s. Electron microscopy studies have shown that there is a continuum of shapes between these categories. There is some evidence that differently shaped<unk> s reflect different developmental stages and also strengths of a synapse. Using two-photon laser scanning microscopy and confocal microscopy, it has been shown that the volume of<unk> s can change depending on the types of stimuli that are presented to a synapse. Also using the same technique, time-lapse studies in the brains of living animals have shown that<unk> s come and go, with the larger mushroom<unk> s being the most stable over time.
Dendritic spine
White matter
Ganglion
Dendrite
Basal ganglia
Plexus
Trigeminocerebellar fibers
Cranial nerve nucleus