The Human Brain : From Neurone to Nervous System

SYNAPSES: SYNAPTIC STRENGTH and LONG TERM POTENTIATION


  1. In some synapses, such as dendritic spine synapses, or synapses concerned with nociceptive transmission in the dorsal horn, the size of the EPSP increases following bursts of activity (long-term potentiation, LTP), or decreases after a period of inactivity (long-term depression, LTD).

  2. In the CNS there are a number of ways in which the size of the EPSP generated at a synapse can be changed. Here are some :
    • changing the amount of transmitter released by a synaptic bouton, e.g. by pre-synaptic inhibition
    • altering the number of post-synaptic receptors, e.g. by trafficking of receptors between the membrane and internal stores
    • recruiting more powerful types of post-synaptic receptor, e.g. by depolarisation to a threshold level that allows NMDA receptors to carry larger currents into the neurone
    • activating signalling cascades within the post-synaptic cell that produce longer-term changes in synaptic strength e.g by activating enzymes or G-proteins that regulate the expression of receptors.

  3. In the CNS, there are also specialised synapses
    • between axon terminals (axo-axonic synapses), and
    • axonal terminals on the dendrites of cortical pyramidal cells, cerebellar Purkinje cells, and striatal medium spiny cells, which are the site of dendritic spine synapses, that allow local regulation of synaptic strength.

  4. Dendritic Spine Synapses can alter the size of the EPSP by changing the number of glutamatergic receptors in the post-synaptic membrane; this is achieved using two ionotropic receptors and two groups of metabotropic receptors. Synaptic strength is increased following repetitive activity of the synapse, and decreased by the absence of activity. The glutamatergic ionotropic and metabotropic receptors will be described separately.

  5. Key Words: Long-term potentiation, Long-term Depression, Pre-synaptic Inhibition; Trafficking of receptors; Glutamatergic ionotropic and metabotropic receptors.




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