Nervous tissue histology
Nervous tissue constitutes the primary building block of the nervous system. Comprising of neurons and glial cells, nervous tissue is designed for the rapid transmission of electrical impulses. This intricate network facilitates the processing and communication of information, enabling essential functions such as sensory perception, motor coordination, and cognitive processes throughout the body, with peripheral nerves and ganglia serving as vital conduits for these signals.
Nerve cells (neurons) form the structural and functional units of the central and peripheral nervous systems. Their function is to receive stimuli and transmit nerve impulses to the target organ. Nerve cells consist of a cell body (soma), dendrites, an axon and a specialized cytoskeleton. Dendrites receive and integrate signals from other neurons or from sensory stimuli and their number can vary. However, each neuron only has one axon that transmits the electrical impulses. To increase conduction velocity, axons are insulated with myelin sheaths. In the central nervous system (CNS), this function is performed by oligodendrocytes and in the peripheral nervous system (PNS) by Schwann cells, respectively.
There is a great variety in the types of neurons of the nervous system, which are differentiated according to the shape and size of their cell bodies as well as the number, length and type of processes. Based on this morphological classification, neurons are categorized as multipolar, bipolar, unipolar and pseudounipolar neurons. Multipolar neurons, which have one axon and multiple dendrites, are the most common neurons in the human nervous system.
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The nervous system is divided into the CNS consisting of the brain and spinal cord and the PNS. The peripheral nervous system lies outside the skull and spinal canal and includes all neural tissue outside the CNS. Peripheral nerves carry impulses from the CNS to the organs and from the organs back to the CNS.
Peripheral nerves are composed of axons and Schwann cells, which collectively form bundles of nerve fibers encased in connective tissue. Each nerve fiber is individually wrapped by a delicate layer of connective tissue called the endoneurium. The endoneurium, along with the basement membrane of the Schwann cell, constitutes the endoneural sheath. Groups of these nerve fiber bundles are enclosed by a protective sheath called the perineurium, and several such bundles together comprise the entire nerve, which is, in turn, enveloped by the epineurium.
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Ganglia of the nervous system
Ganglia are clusters of nerve cell bodies in the peripheral nervous system. Ganglia can be broadly categorized into two groups, that is, sensory ganglia (relating to the somatic nervous system (SNS)), and autonomic ganglia (relating to the autonomic nervous system (ANS)). The function of ganglia is not yet fully understood, but we know that they are able to process signals to a certain extent, independently of the CNS.
Spinal ganglia, a subtype of sensory ganglia, consist of neurons of the somatic nervous system and contain the nerve cell bodies of pseudounipolar neurons. These ganglia are located in the dorsal root of the spinal nerve and transmit signals from the environment to the CNS.
Autonomic ganglia consist of nerve cells of the autonomic nervous system, which can be further categorized into sympathetic, parasympathetic and enteric ganglia. The nerve cells of the autonomic ganglia consist of multipolar neurons.
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