DENDRITIC SPINE SYNAPSES : STRUCTURAL AND FUNCTIONAL PLASTICITY
The effectiveness of some synapses depends on the the previous level of activity. If the synapse has previously been active, the size of the EPSP is increased : this is called long-term potentiation (LTP). If the synapse has previously been inactive for a time, the size of the EPSP is decreased : this is long-term depression (LTD).
LTP and LTD have been described in various sites including (a) the dorsal horn of the spinal cord and (b) the cerebral cortex, where they appear to be involved in (a) modulation of nociceptive transmission and (b) in memory. In both sites, specialised synaptic structures are present: synaptic glomeruli in the sperficial dorsal horn, and dendritic spine synapses in the cortex, striatum and cerebellum.
The dendrites of neurones in the cerebral cortex, the cerebellar cortex and the basal ganglia have specialised structures called dendritic spines, which are the site of synapses which can modify their synaptic strength. They appear to be involved in learning and memory.
Many of these synapses are glutamatergic, and the changes in their effectiveness are attributable to pre-and post-synaptic mechanisms.
Dendritic spines are small protrusions of cell membrane and cytoplasm from the dendrites of neurones and are commonly found in pyramidal cells of the cerebral cortex, Purkinje cells of the cerebellum, medium spiny neurones of the corpus striatum.
The dendrites of these neurones are covered with spines, each of which is the site of a synaptic contact with an afferent neurone.
Their synaptic strength can adapt to repeated inputs, a process known as long-term potentiation (LTP), and the adaptation is believed to be a mechanism for information storage.
The superficial layers of the dorsal horn contain specialised synapses called glomeruli that are involved in modulation of nociceptive messages.
What are Dendritic Spines?
A branched dendrite with dendritic spines
Dendritic Spine Synapses
Many parts of the brain contain neurones whose dendrites have spines, the site of a synaptic contact with an afferent axon.
The synaptic strength can be altered in the longer term according to their previous activity. This is thought to be achieved by the expression of additional receptors within the post-synapitc membrane, and chemical feedback from the spine on to pre-synaptic boutons, possibly using nitric oxide, NO, as a neuromodulator.
Spine synapses are extremely common in the cerebral cortex, the basal ganglia, cerebellum and hippocampus.
Spine synapses are involved in learning and neuroplasticity.
How do spine synapses develop on dendrites?
Image source: Phil Trans Roy Soc
Model for activity-dependent stimulation of synaptogenesis and spine formation. Synaptic activity and increased glutamate transmission can lead to increased synapse formation and spine density. This occurs through insertion of glutamate-AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors into the postsynaptic membrane. The mechanisms underlying the regulation of synaptogenesis and spine formation have been studied using a cellular Model for activity-dependent stimulation of synaptogenesis and spine formation. Synaptic activity and increased glutamate transmission can lead to increased synapse formation and spine density. This occurs through insertion of glutamate-AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors into the postsynaptic membrane. The mechanisms underlying the regulation of synaptogenesis and spine formation have been studied using a cellular model of learning and memory, known as long-term potentiation (LTP). model of learning and memory, known as long-term potentiation (LTP). [quote from Phil Trans Roy Soc 2012]
Spine Synapses: Size and Shape
The size of dendritic spines can vary, and increased activity and synaptic strength results in the development of the spine, which increases in size because of the production of additional spine proteins (induced by second messengers). This is a dynamic phenomenon, and spines that are not used retract. Repetition promotes memory and learning!!
Image source: wikipedia
Spines come in different shapes and sizes; it may be that spines can grow, depending on the activity of the axonal contacts. Growth could be due to the expression of genes concerned with the structure of the spine.
How can the Synaptic Strength of Spine Synapses increase?
Image source: 7e.biopsychology.com
The role of NMDA receptors in dendritic spines. The diagram shows that the activation of NMDA receptors can lead to the insertion of extra AMPA receptors into the post-synaptic membrane, resulting in an increase in synaptic strength.
In the absence of depolarisation, glutamate does not open the NMDA channel because the channel is blocked by a magnesium ion; if the spine is sufficiently depolarised, the magnesium ion is displaced and glutamate opens the channel.
When the magnesium ion is displaced from the NMDA ion channel, glutamate can open the channel and allow calcium to move into the dendritic spine.
Glutamatergic Transmission at Spine Synapses
Glutamate is an important neurotransmitter at spine synapses.
Glutamatergic EPSPs are generated by AMPA receptors at these synapses, but when the membrane is depolarised to a certain threshold, glutamate also opens NMDA receptors, allowing calcium to enter the neurone.
These calcium ions depolarise the spine using strong currents, and also activates enzymes that cause sequestered AMPA receptors to be added to the post-synaptic membrane from internal reservoirs.
The addition of AMPA receptors to the post-synaptic membrane increases the strength of the synapse.
Increasing synaptic strength is seen as an important mechanism that allows spine synapses to be involved in learning and memory storage.