Inferior Olivary Neurons
From NeuronBankWiki
Inferior Olivary Neurons (ION) are the neurons associated with the Inferior Olivary Nucleus or the Inferior Olivary Complex (IOC). The IOC is a part of the paired Olivary Bodies which are located in the Medulla Oblongata. They play are integral role in the function of the Cerebellum, helping to initiate and regulate motor functions and is also involved in somatosensory functions as well as cerebellar learning.
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Anatomy
Embryologically, the IOC is formed laterally from the Precerebellar Neuroepithelium of the Neural Tube. Once formed, the Inferior Olive is composed of three subunits: the Medial Accessory Olive (MAO), Dorsal Accessory Olive (DAO), and the Principle Olive (PO). Each region is then divided into many sheets form further subdivisions known as Lamella. The presence of lamella is consistent through different species of vertebrates. The variations between species occur in the proportional difference in the sizes of the three subnuclei. A study of the Inferior Olive in cats presented evidence that all three subnuclei are responsive to both cutaneous and proprioceptive stimulation. These somatosensory responses varied in amount, with the neurons of DAO area being completely responsive, followed by close to 90% in the MAO and 74% in the PO. ION's receive input from throughout the body that is then relayed to the Cerebellar Cortex and thus plays a major role in motor functions of vertebrates. The cell body of an Inferior Olive Neuron is ventrally located in the Caudal Medulla. The ION’s axons, known as Climbing Fibers, extend into the Cerebellar Cortex and synapse on the dendrites of Pukinje neurons (PC). During development, PCs interact with multiple Climbing Fibers of IONs. However, after maturation each PC receives input from only one climbing fiber, which causes a large EPSP that results in activation.
Physiology
The membrane properties of olivary neurones provide well-defined sites at which serotonin may act to alter olivary activity (4).
Synaptic Connections
Neurons of the Inferior Olive receive inhibitory input from Deep Cerebellar Nuclei (DCN). These GABAnergic inputs inhibit the ION’s from propagating an action potential through the Climbing Fibers onto the PCs of the Cerebellar Cortex. When the neurons of the Inferior Olive are not inhibited, they send action potentials down their climbing fibers that synapse on the dendrites of Purkinje Cells. Each PC receives input from only one climbing fiber, while an ION axon can act on a varied number of PCs.
Spiking properties
When observing the synapse of the DCN to the IONs, it is seen that the release of GABA is asynchronous. This varied release occurs because the release of GABA is dependent on the frequency of the stimulation more than the stimulation itself. When frequency is above 10 Hz, the DCN cause a consistent inhibition of the IONs. At frequencies lower than 10Hz, the stimulations ability to maintain constant firing can vary. Frequencies as low as 5Hz are able to sustain consistent firing.
References
1. A. Best, W. Regehr (2009) Inhibitory Regulation of Electrically Coupled Neurons in the Inferior Olive Is Mediated by Asynchronous Release of GABA Neuron, Volume 62, Issue 4, Pages 555-565
2. B. Connors, M. Bear, M. Paradiso (2007) Neuroscience/ Exploring The Brain. Pages 772-773
3. S. Ausim Azizi (2007) ...And the Olive Said to the Cerebellum: Organization and Functional Significance of the Olivo-Cerebellar System. Neuroscientist 2007; 13; 616
4. D. Placantonakis, C. Schwarz and J. Welsh (2000) Serotonin suppresses subthreshold and suprathreshold oscillatory activity of rat inferior olivary neurones in vitro J Physiol Volume 524, Number 3, 833-851
5. R. Gellman, A. R. Gibson and J. C. Houk (1985) Inferior olivary neurons in the awake cat: detection of contact and passive body displacement. Journal of Neurophysiology, Vol 54, Issue 1 40-60
6. Larouche M, Hawkes R. (2006). From clusters to stripes: the developmental origins of adult cerebellar compartmentation. Cerebellum 5: 77–88.
