Histology of the nervous system
The nervous system is a network of neurons whose main feature is to generate, modulate and transmit information between all the different parts of the human body. This property enables many important functions of the nervous system, such as regulation of vital body functions (heartbeat, breathing, digestion), sensation and body movements.
Here, we will describe the microscopic appearance of the various parts of the brain and the spinal cord.
The cerebral cortex comprises the outer layer of the cerebrum and is mainly made up of gray matter. This region of the brain is responsible for numerous higher cognitive functions, including memory, attention, perception and consciousness. The cerebral cortex consists of the evolutionary older allocortex and the younger isocortex, which represents the majority of the cortex. The allocortex consists of 3 - 4 layers and the isocortex consists of 6 layers.
Each of the six layers (horizontal laminae) of the isocortex has a unique cell population and function. The layers are, from superficial to deep, the molecular layer, the external granular layer, the external pyramidal layer, the internal granular layer, the internal pyramidal layer and the multiform layer. Neurons are the dominant cells in the cortex and can be divided into two main types, pyramidal cells and nonpyramidal cells (interneurons).
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The hippocampus is part of the limbic system and plays a crucial role in memory and spatial information formation as well as retrieval. It is located in the medial temporal lobe and is characterized by its unique twisting structure that resembles a seahorse, hence its name. The hippocampal region is composed of three parts: the hippocampus (also known as hippocampus proper or Ammon’s horn), the dentate gyrus and the subiculum.
Based on histological criteria, the hippocampus is part of the archicortex. It has four layers in the cornu ammonis (CA) and three layers in the dentate gyrus. The four layers of the hippocampus proper are divided from superficial to deep into CA1, CA2, CA3 and CA4. The three layers of the dentate gyrus are the molecular layer, the intermediate granular layer and the multiform layer.
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The cerebellum is part of the central nervous system and plays an important role in coordinating movements and maintaining balance. It consists of gray matter, which forms the cerebellar cortex and nuclei, and white matter, which includes the medulla and cerebellar peduncles.
Histologically, the cortex is characterized by three defined layers: the molecular layer, the Purkinje cell layer and the granular layer. The outer molecular layer contains few neurons, mainly stellate cells and basket cells, and is rich in parallel fibers and climbing fibers. In the middle Purkinje cell layer are the large, characteristic Purkinje cells, whose axonal processes represent the only efferents (inhibitory) of the cerebellar cortex. The inner granular layer is densely populated with small granule cells that pass on sensory information to the Purkinje cells.
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The medulla oblongata represents the lowest section of the brainstem and lies between the pons and spinal cord. It is responsible for numerous vital functions, including controlling breathing, heart rate and blood pressure. In addition, the medulla oblongata houses various core areas that are relevant for processing sensory and motor information.
The medulla oblongata consists of gray and white matter. Gray matter contains nuclei that are responsible for specific reflex functions such as coughing, sneezing and swallowing. The important core areas include, among others, the spinal trigeminal nucleus, the inferior olive nucleus complex and the dorsal column nuclei (cuneate and gracile nuclei), which process sensorimotor information. The white matter, on the other hand, contains ascending and descending fiber tracts that transmit information between the brain and the spinal cord.
The medulla oblongata is also the decussation point for the sensory fibers from the spinal cord and contains the medulla–spinal cord junction, where the fibers of the corticospinal tract cross almost entirely to the opposite side.
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The spinal cord extends within the spinal canal from the foramen magnum of the skull to the lumbar region. It is protected by three meninges, the dura mater, arachnoid mater and pia mater. It is responsible for both relaying sensory information to the brain and transmitting motor commands from the brain to the muscles.
The spinal cord is divided into a central, gray matter and a peripheral, white matter. The gray matter is the butterfly-shaped central part of the spinal cord and is comprised of neurons and synapses. It is divided into anterior, lateral and posterior horns. The anterior horn houses the motor neurons, while the posterior horn receives sensory information. White matter consists primarily of myelinated nerve fibers and is organized into ascending (sensory) and descending (motor) tracts. The fiber tracts are organized into an anterior funiculus, a lateral funiculus and a posterior funiculus.
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