Stratum lucidum of hippocampus
|Brain: Stratum lucidum of hippocampus|
|Latin||stratum lucidum hippocampi|
- For the layer of the skin, see stratum lucidum.
The stratum lucidum of the hippocampus is also a layer of the hippocampus between the stratum pyramidale and the stratum radiatum. It is the tract of the mossy fiber projections, both inhibitory and excitatory from the granule cells of the dentate gyrus. One mossy fiber may make up to 37 connections to a single pyramid cell, and innervate around 12 pyramidal cells on top of that. Any given pyramidal cell in the stratum lucidum may get input from as many as 50 granule cells.
The stratum lucidum is located within the CA3 region of the hippocampus distally to the dentate gyrus and proximally to the CA2 region. It is composed of a densely packed bundle of mossy fibers (unmylinated) and spiny and aspiny interneurons that lie immediately above the CA3 pyramidal cell layer in the hippocampus, and immediately below the stratum radiatum. Most mossy fiber axons are perpendicular to the CA3 pyramidal region where they project and synapse to either the CA3 pyramidal cells or the stratum oriens below the pyramidal region. The interneurons of the stratum lucidum are generally found to be local circuit neurons remaining within the CA3 region. A majority of the interneuron axons remain within the statum lucidum but some also extend to the stratum radiatum and stratum lacunosum-molecular above the radiatum as well as to the CA1 and hilur regions. Diagram of the hippocampus showing the location of the stratum lucidum.
What/where is the stratum pyramidale
In hippocampus anatomy, the Stratum Pyramidale is one of seven layers, or stratums, that make up the entire neural structure. The Stratum Pyramidale is the third deepest hippocampal layer, and in relation to the stratum lucidum, is located underneath it. The stratum Pyramidale houses cell bodies of the pyramidal neurons, which are the foundational excitatory neurons of the hippocampus. In the CA3 region of the hippocampus, the stratum pryamidale connects with the statum lucidum by mossy fibers that run though both subfields.
What/where is the stratum radiatum
The layer above the stratum lucidum
What are mossy fibers
Mossy fibers  are one of two types of axons that enter the cerebellum. The axonal inputs of these fibers are integral to neuronal function, as the information that these axons carry will be delivered, indirectly, to Purkinje cells. As the only cell axon to leave the brain, the Purkinje cells act to “turn off” or inhibit deep nucleus neurons in the brain, making it is essential that mossy fibers in the CA3 region of the stratum lucidum play their part to connect synaptically with the Purkinje cells. An “excitatory transmitter, glutamate”, is released by mossy fibers to transmit information to their destination Purkinje cell. Each mossy fiber branches extensively into white matter of the brain, and each of these branches themselves has many “swellings” which “contain round vesicles and synaptic thickenings”, called a rosette. Each rosette is a mossy fiber synapse which connects with a dendrite of a graunle cell. Mossy fibers fire “spontaneously and rapidly (50-100 times per second)”, and, due to their connection with granule cells dendrites, cause Purkinje cells to fire at the “same frequency”. The Purkinje cells, as a result of their stimulation by mossy fibers, inhibit unwanted activity in regions of brain nuclei.
Interneurons in the stratum lucidum
The types of neurons found in the Stradum Lucidum are called interneurons, neurons which form a connection between other neurons in a different location. This situation is described in the mossy fiber axon connection in the CA3 Statum Lucidum region of the hippocampus as is in relation to Pukinje cells. The interneurons found in the Stradum Lucidum are of two classes, spiny and aspiny. Spiny neurons are a “special type of inhibitory cell”, characterized by spiny projections on the dendrites of the cell. The axons of these neurons in the hippocampus terminate primarily in the stratum lucidum and statum radiatum of CA3. Spiny neurons are of importance because of their “pivotal role in motor control, habit formation, and motivated behavior”. They receive synaptic inputs from mossy fibers boutons, as well as multiple other synaptic terminals. The majority of axon collaterals of these neurons remain in the stratum lucidum and CA3 region, though in some cases axon collaterals of these neurons are observed migrating into CA1 region of hippocampus. Aspiny neurons, the second class of neurons found in the Stadium Lucidum, are another type of inhibitory cell similar to spiny neurons, though lacking dendrite projections. They make up the majority of the neuron composition in comparison to spiny neurons, about 63 percent. The somata of aspiny neurons are for the most part bipolar, generating 2-5 primary dendrites “that to a varying extent displayed varicose swellings in their course”. Similar to spiny neurons, aspiny neuron dendrites “branch extensively in statum lucidum and statum radiatum of CA3”, in contrast to spiny neurons, however, some dendrites “traversed statum pyramidale and entered statum oriens”, the second deepest layer of the hippocampus. Additionally, what distinguishes aspiny neurons form spiny neurons is their higher maximal firing rates and narrower action potential half-widths than their spiny counterparts.
Function within the nervous system
In the hippocampus, the stratum lucidum contains many neurons that act locally in local pathways. The spiny neurons of the stratum lucidum act in primary motor control as interneurons that relay to other neurons. The spiny and aspiny neurons act in both inhibitory and excitatory circuits.