A dendritic spine, or simply a spine, is a knob-shaped protrusion from the dendrite of a neuron. It looks like a bulb with a thin neck or stalk. The bulb is also called spine head. A dendritic spine is in close proximity to an axon. It receives signal inputs from the neighboring axons, and functions in memory storage and signal transmission.
Dendritic spines are found in most neurons in the central nervous system, including the pyramidal neurons in the cortex; spiny neurons of the putamen, caudate nucleus, and internal capsule; and Purkinje cells in the cerebellum. The dendritic spine density can reach up to 50 spines per 10-micrometer stretch of a neuron’s dendrite. Spines are denser in the cerebellar Purkinje cells than in the pyramidal and hippocampal neurons.
The appearance of a dendritic spine depends on the strength and duration of spine-synapse contacts. A spine head has a volume that ranges from 0.01 to 0.8 cubic micrometer. Some spine heads are described as mushroom-like, stubby, thin, or branched. Generally, the larger the spine head, the stronger and more mature the synaptic contact.
Nevertheless, the strength and maturity of synaptic contact depend on environmental factors. Dendritic spines change in volume, shape, and quantity depending on their exposure to these factors. This characteristic is known as plasticity.
Spines are plastic because they contain the protein actin. This is the same protein present in muscles for contraction and in cytoskeletons for cell division. Based on studies, it has been observed that the actin in spines has an average cycling time of 44 seconds. This dynamic property brought about by actin remodeling means that a dendritic spine could change its volume and shape in a few seconds or minutes. Additionally, spines can appear and disappear completely in a spontaneous manner.
It has been postulated that the plasticity of spines is the basis of memory. In particular, long-term memory is believed to be dependent on the formation of new dendritic spines or the growth of existing ones when reinforced by a learning environment. Among young people, there is a net loss or disappearance of dendritic spines, which is said to reflect their capacity for learning. In adults, most spines do not disappear and instead become more stable. This explains why memories become firmly established in adulthood.
Many scientists believe the apparent association between dendritic spines and memory. A causal relationship, however, is not yet established. Additionally, a theory has been proposed that the increase in the volume or size of the spine heads has greater contribution to memory retention than the formation of new spines.