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A dendritic spine is a small membranous protrusion from a neuron's dendrite that typically receives input from a single axon at the synapse. Dendritic spines serve as a storage site for synaptic strength and help transmit electrical signals to the neuron's cell body. Most spines have a bulbous head, and a thin neck that connects the head of the spine to the shaft of the dendrite. The dendrites of a single neuron can contain hundreds to thousands of spines. In addition to spines providing an anatomical substrate for memory storage and synaptic transmission, they may also serve to increase the number of possible contacts between neurons. It has also been suggested that changes in the activity of neurons have a positive effect on spine morphology.
In neuroscience, long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neurons. The opposite of LTP is long-term depression, which produces a long-lasting decrease in synaptic strength.
An inhibitory postsynaptic potential (IPSP) is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential. IPSPs were first investigated in motorneurons by David P. C. Lloyd, John Eccles and Rodolfo Llinás in the 1950s and 1960s. The opposite of an inhibitory postsynaptic potential is an excitatory postsynaptic potential (EPSP), which is a synaptic potential that makes a postsynaptic neuron more likely to generate an action potential. IPSPs can take place at all chemical synapses, which use the secretion of neurotransmitters to create cell to cell signalling. Inhibitory presynaptic neurons release neurotransmitters that then bind to the postsynaptic receptors; this induces a change in the permeability of the postsynaptic neuronal membrane to particular ions. An electric current that changes the postsynaptic membrane potential to create a more negative postsynaptic potential is generated, i.e. the postsynaptic membrane potential becomes more negative than the resting membrane potential, and this is called hyperpolarisation. To generate an action potential, the postsynaptic membrane must depolarize—the membrane potential must reach a voltage threshold more positive than the resting membrane potential. Therefore, hyperpolarisation of the postsynaptic membrane makes it less likely for depolarisation to sufficiently occur to generate an action potential in the postsynaptic neurone.
Pyramidal cells, or pyramidal neurons, are a type of multipolar neuron found in areas of the brain including the cerebral cortex, the hippocampus, and the amygdala. Pyramidal cells are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. Pyramidal neurons are also one of two cell types where the characteristic sign, Negri bodies, are found in post-mortem rabies infection. Pyramidal neurons were first discovered and studied by Santiago Ramón y Cajal. Since then, studies on pyramidal neurons have focused on topics ranging from neuroplasticity to cognition.
Schaffer collaterals are axon collaterals given off by CA3 pyramidal cells in the hippocampus. These collaterals project to area CA1 of the hippocampus and are an integral part of memory formation and the emotional network of the Papez circuit, and of the hippocampal trisynaptic loop. It is one of the most studied synapses in the world and named after the Hungarian anatomist-neurologist Károly Schaffer.
An apical dendrite is a dendrite that emerges from the apex of a pyramidal cell. Apical dendrites are one of two primary categories of dendrites, and they distinguish the pyramidal cells from spiny stellate cells in the cortices. Pyramidal cells are found in the prefrontal cortex, the hippocampus, the entorhinal cortex, the olfactory cortex, and other areas. Dendrite arbors formed by apical dendrites are the means by which synaptic inputs into a cell are integrated. The apical dendrites in these regions contribute significantly to memory, learning, and sensory associations by modulating the excitatory and inhibitory signals received by the pyramidal cells.
A basal dendrite is a dendrite that emerges from the base of a pyramidal cell that receives information from nearby neurons and passes it to the soma, or cell body. Due to their direct attachment to the cell body itself, basal dendrites are able to deliver strong depolarizing currents and therefore have a strong effect on action potential output in neurons. The physical characteristics of basal dendrites vary based on their location and species that they are found in. For example, the basal dendrites of humans are overall found to be the most intricate and spine-dense, as compared to other species such as Macaques. It is also observed that basal dendrites of the prefrontal cortex are larger and more complex in comparison to the smaller and simpler dendrites that can be seen within the visual cortex. Basal dendrites are capable of vast amounts of analog computing, which is responsible for many of the different nonlinear responses of modulating information in the neocortex. Basal dendrites additionally exist in dentate granule cells for a limited time before removal via regulatory factors. This removal usually occurs before the cell reaches adulthood, and is thought to be regulated through both intracellular and extracellular signals. Basal dendrites are part of the more overarching dendritic tree present on pyramidal neurons. They, along with apical dendrites, make up the part of the neuron that receives most of the electrical signaling. Basal dendrites have been found to be involved mostly in neocortical information processing.
The superior olivary complex (SOC) or superior olive is a collection of brainstem nuclei that functions in multiple aspects of hearing and is an important component of the ascending and descending auditory pathways of the auditory system. The SOC is intimately related to the trapezoid body: most of the cell groups of the SOC are dorsal to this axon bundle while a number of cell groups are embedded in the trapezoid body. Overall, the SOC displays a significant interspecies variation, being largest in bats and rodents and smaller in primates.
Bursting, or burst firing, is an extremely diverse general phenomenon of the activation patterns of neurons in the central nervous system and spinal cord where periods of rapid action potential spiking are followed by quiescent periods much longer than typical inter-spike intervals. Bursting is thought to be important in the operation of robust central pattern generators, the transmission of neural codes, and some neuropathologies such as epilepsy. The study of bursting both directly and in how it takes part in other neural phenomena has been very popular since the beginnings of cellular neuroscience and is closely tied to the fields of neural synchronization, neural coding, plasticity, and attention.
Neural backpropagation is the phenomenon in which, after the action potential of a neuron creates a voltage spike down the axon, another impulse is generated from the soma and propagates towards the apical portions of the dendritic arbor or dendrites. In addition to active backpropagation of the action potential, there is also passive electrotonic spread. While there is ample evidence to prove the existence of backpropagating action potentials, the function of such action potentials and the extent to which they invade the most distal dendrites remain highly controversial.
Subthreshold membrane potential oscillations are membrane oscillations that do not directly trigger an action potential since they do not reach the necessary threshold for firing. However, they may facilitate sensory signal processing.
Low-threshold spikes (LTS) refer to membrane depolarizations by the T-type calcium channel. LTS occur at low, negative, membrane depolarizations. They often follow a membrane hyperpolarization, which can be the result of decreased excitability or increased inhibition. LTS result in the neuron reaching the threshold for an action potential. LTS is a large depolarization due to an increase in Ca2+ conductance, so LTS is mediated by calcium (Ca2+) conductance. The spike is typically crowned by a burst of two to seven action potentials, which is known as a low-threshold burst. LTS are voltage dependent and are inactivated if the cell's resting membrane potential is more depolarized than −60mV. LTS are deinactivated, or recover from inactivation, if the cell is hyperpolarized and can be activated by depolarizing inputs, such as excitatory postsynaptic potentials (EPSP). LTS were discovered by Rodolfo Llinás and coworkers in the 1980s.
Voltage-sensitive dyes, also known as potentiometric dyes, are dyes which change their spectral properties in response to voltage changes. They are able to provide linear measurements of firing activity of single neurons, large neuronal populations or activity of myocytes. Many physiological processes are accompanied by changes in cell membrane potential which can be detected with voltage sensitive dyes. Measurements may indicate the site of action potential origin, and measurements of action potential velocity and direction may be obtained.
In neurophysiology, a dendritic spike refers to an action potential generated in the dendrite of a neuron. Dendrites are branched extensions of a neuron. They receive electrical signals emitted from projecting neurons and transfer these signals to the cell body, or soma. Dendritic signaling has traditionally been viewed as a passive mode of electrical signaling. Unlike its axon counterpart which can generate signals through action potentials, dendrites were believed to only have the ability to propagate electrical signals by physical means: changes in conductance, length, cross sectional area, etc. However, the existence of dendritic spikes was proposed and demonstrated by W. Alden Spencer, Eric Kandel, Rodolfo Llinás and coworkers in the 1960s and a large body of evidence now makes it clear that dendrites are active neuronal structures. Dendrites contain voltage-gated ion channels giving them the ability to generate action potentials. Dendritic spikes have been recorded in numerous types of neurons in the brain and are thought to have great implications in neuronal communication, memory, and learning. They are one of the major factors in long-term potentiation.
Nonsynaptic plasticity is a form of neuroplasticity that involves modification of ion channel function in the axon, dendrites, and cell body that results in specific changes in the integration of excitatory postsynaptic potentials and inhibitory postsynaptic potentials. Nonsynaptic plasticity is a modification of the intrinsic excitability of the neuron. It interacts with synaptic plasticity, but it is considered a separate entity from synaptic plasticity. Intrinsic modification of the electrical properties of neurons plays a role in many aspects of plasticity from homeostatic plasticity to learning and memory itself. Nonsynaptic plasticity affects synaptic integration, subthreshold propagation, spike generation, and other fundamental mechanisms of neurons at the cellular level. These individual neuronal alterations can result in changes in higher brain function, especially learning and memory. However, as an emerging field in neuroscience, much of the knowledge about nonsynaptic plasticity is uncertain and still requires further investigation to better define its role in brain function and behavior.
The name granule cell has been used for a number of different types of neurons whose only common feature is that they all have very small cell bodies. Granule cells are found within the granular layer of the cerebellum, the dentate gyrus of the hippocampus, the superficial layer of the dorsal cochlear nucleus, the olfactory bulb, and the cerebral cortex.
An autapse is a chemical or electrical synapse from a neuron onto itself. It can also be described as a synapse formed by the axon of a neuron on its own dendrites, in vivo or in vitro.
Dendrodendritic synapses are connections between the dendrites of two different neurons. This is in contrast to the more common axodendritic synapse (chemical synapse) where the axon sends signals and the dendrite receives them. Dendrodendritic synapses are activated in a similar fashion to axodendritic synapses in respects to using a chemical synapse. An incoming action potential permits the release of neurotransmitters to propagate the signal to the post synaptic cell. There is evidence that these synapses are bi-directional, in that either dendrite can signal at that synapse. Ordinarily, one of the dendrites will display inhibitory effects while the other will display excitatory effects. The actual signaling mechanism utilizes Na+ and Ca2+ pumps in a similar manner to those found in axodendritic synapses.
Rishikesh Narayanan is an Indian neuroscientist, computer engineer and a professor at the Molecular Biophysics Unit (MBU) of the Indian Institute of Science. He is the principal investigator at the Cellular Neurophysiology Laboratory of MBU where his team is engaged in researches on experimental and theoretical aspects of information processing in single neurons and their networks. The Council of Scientific and Industrial Research, the apex agency of the Government of India for scientific research, awarded him the Shanti Swarup Bhatnagar Prize for Science and Technology, one of the highest Indian science awards, in 2016, for his contributions to biological sciences.
An axo-axonic synapse is a type of synapse, formed by one neuron projecting its axon terminals onto another neuron's axon.
References
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- ↑ Masurkar, AV; Chen, WR (29 September 2011). "Potassium currents of olfactory bulb juxtaglomerular cells: characterization, simulation, and implications for plateau potential firing". Neuroscience. 192: 247–62. doi:10.1016/j.neuroscience.2011.06.012. PMC 3170655 . PMID 21704678.
- ↑ Zhou, Z; Xiong, W; Masurkar, AV; Chen, WR; Shepherd, GM (November 2006). "Dendritic calcium plateau potentials modulate input-output properties of juxtaglomerular cells in the rat olfactory bulb". Journal of Neurophysiology. 96 (5): 2354–63. doi:10.1152/jn.00003.2006. PMID 16855116. S2CID 11512553.
- ↑ Major, G; Polsky, A; Denk, W; Schiller, J; Tank, DW (May 2008). "Spatiotemporally graded NMDA spike/plateau potentials in basal dendrites of neocortical pyramidal neurons". Journal of Neurophysiology. 99 (5): 2584–601. doi:10.1152/jn.00011.2008. hdl: 11858/00-001M-0000-002C-07A3-4 . PMID 18337370.
- ↑ Milojkovic, BA; Radojicic, MS; Antic, SD (13 April 2005). "A strict correlation between dendritic and somatic plateau depolarizations in the rat prefrontal cortex pyramidal neurons". The Journal of Neuroscience. 25 (15): 3940–51. doi:10.1523/JNEUROSCI.5314-04.2005. PMC 5643048 . PMID 15829646.
- ↑ Suzuki, T; Kodama, S; Hoshino, C; Izumi, T; Miyakawa, H (August 2008). "A plateau potential mediated by the activation of extrasynaptic NMDA receptors in rat hippocampal CA1 pyramidal neurons". The European Journal of Neuroscience. 28 (3): 521–34. doi:10.1111/j.1460-9568.2008.06324.x. PMID 18702724.
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