The postsynaptic density (PSD) is a protein dense specialization attached to the postsynaptic membrane. PSDs were originally identified by electron microscopy as an electron-dense region at the membrane of a postsynaptic neuron. The PSD is in close apposition to the presynaptic active zone

Active zone

The active zone is a term first used by Couteaux and Pecot-Dechavassinein in 1970 and is defined in the neuron as the site of neurotransmitter release. Neurons contain structures called synapses that allow for the communication from one neuron to another...

 and ensures that receptors are in close proximity to presynaptic neurotransmitter release sites. PSDs vary in size and composition among brain regions and have been studied in great detail at glutamatergic synapses. Hundreds of proteins have been identified in the postsynaptic density including glutamate receptors, scaffold proteins

Scaffold protein

In biology, scaffold proteins are crucial regulators of many key signaling pathways. Although scaffolds are not strictly defined in function, they are known to interact and/or bind with multiple members of a signaling pathway, tethering them into complexes...

, and many signaling molecules.

Function

The PSD has been proposed to concentrate and organize neurotransmitter receptors in the synaptic cleft. The PSD also serves as a signaling apparatus. For instance kinases and phosphatases in the PSD are activated and released from the PSD to change to activity of proteins located in the spine or are transported to the nucleus to affect protein synthesis. Some of the features of the PSD are similar to the neuromuscular junction

Neuromuscular junction

A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...

 and other cellular junctions

Adherens junction

Adherens junctions are protein complexes that occur at cell–cell junctions in epithelial tissues, usually more basal than tight junctions...

, as the PSD has been modeled as a specialized cellular junction that allows for rapid, asymmetrical signaling.

Structure

The structure and composition of the PSD have been the focus of numerous molecular studies of synaptic plasticity

Synaptic plasticity

In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...

, a cellular model of learning and memory. PSDs are sized on the order of 250 to 500 nanometres in diameter and 25 to 50 nanometres in thickness, depending on the activity state of the synapse.

Composition

Many proteins in the PSD are involved in the regulation of synaptic function. Key among these, are postsynaptic density-95 (PSD95), neuroligin

Neuroligin

300px|thumb|right|alt = Colored dice with checkered background|Tertiary structure of Neuroligin 4.Neuroligin , a type I membrane protein, is a protein on the postsynaptic membrane that mediates synapse formation between neurons. Neuroligins mediate signaling across the synapse and affect the...

 (a cellular adhesion molecule), NMDA receptor

NMDA receptor

The NMDA receptor , a glutamate receptor, is the predominant molecular device for controlling synaptic plasticity and memory function....

s, AMPA receptor

AMPA receptor

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor is a non-NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system . Its name is derived from its ability to be activated by the artificial glutamate analog AMPA...

s, calcium/calmodulin-dependent protein kinase II and actin. As protein detection technologies have increased in sensitivity, such as with improvements in mass spectrometry

Mass spectrometry

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

 techniques, more numerous proteins have been attributed to the PSD. Current estimates are greater than several hundred proteins are found at PSDs among brain regions and during different states of development and synaptic activity. PSDs also contain cell adhesion molecule

Cell adhesion molecule

Cell Adhesion Molecules are proteins located on the cell surface involved with the binding with other cells or with the extracellular matrix in the process called cell adhesion....

s and a diverse set of other signaling proteins. Many of the PSD proteins contain PDZ domains.

External links

• Postsynaptic Density- Cell Centered Database
• The postsynaptic density
• Molecular Composition of the Postsynaptic Density