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Nervous Tissue Overview of the Nervous System: The nervous system is part of the body’s 11 systems and though small, it’s extremely complex. The nervous system consists of 2 types of cells, neurons and neuroglia that work together to form an extremely intricate network. It is made up of the body’s most important structures the brain, spinal cord, cranial, spinal, and peripheral nerves, and their motor and sensory endings. Organization of the Nervous System To better understand the nervous system, it is often divided anatomically into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS is made up of the brain and the spinal cord, which together processes the body’s sensory information. The PNS encompasses all of the nervous system outside of the CNS. It includes the cranial nerves, spinal nerves, and sensory receptors. The peripheral nervous system can be further divided into 3 subdivisions: the somatic nervous system, the autonomic nervous system, and the enteric nervous system. The somatic nervous system is made up of somatic sensory neurons that provide information from sensory receptors to the CNS. It plays a fundamental role in the input information that the CNS processes. It also includes the somatic motor neurons that provide information from the CNS to the skeletal muscles. It’s part of the output of information that the CNS is involved in. The autonomic nervous system of the PNS also has sensory and motor neurons. However, these neurons are autonomic in that it is done involuntarily. The motor neurons of the ANS are again divided into sympathetic and parasympathetic divisions. The sympathetic division is involved in the body’s “fight-or-flight” responses and the parasympathetic in the “rest and digest” responses of the body. A third division of the PNS is the enteric nervous system (ENS), which refers to the neurons in the gastrointestinal tract. It has sensory and motor neurons that monitor the gastrointestinal tract. Neurons vs Neuroglia Nervous tissue consists of neurons and neuroglia, which are intermixed in the body in complex networks. Neurons are highly specialized cells that provide the backbone for body’s unique functions of thinking, remembering, and regulating. Because of this, they are unable to undergo mitotic division. Neuroglia on the other hand, are support structures to the neurons, there to nourish and protect the neurons. Unlike they neurons, they continually divide through a lifetime. Neurons’ Structures A neuron consists of a cell body where the nucleus, mitochondria, and other cell structures can be found. At one end of the neuron are the dendrites, multiples tree-like structures that acts as the receiving portion of the neuron. The other end is the axon, where the nerve impulse travels through to the next neuron. It is typically an long and thin projection. Between neurons is the synapse, the site of communication between two axons. Here, a neurotransmitter is usually released where it produces an action potential. Neuroglia Neuroglia are all the other cells other than the neurons and they play a supportive role in the nervous system. There are 6 types of neuroglia, 4 that are found in the CNS and the other 2 found in the PNS. The four found in the CNS are the astrocytes, oligodendrocytes, microglia, and ependymal cells. They all perform a vital role in the nervous system and are essential to its functioning. Neuroglia in the nervous system far outnumbers neurons. With a 10 to 1 ratio, neurons form the minority of structures making up the nervous system. They are easily distinguishable from the neurons as they lack axons and contain only one type of processes. They don’t form synapses; instead they are supportive structures to the neurons. And unlike neurons, they are able to divide through their lifetime. Astrocytes are star-shaped cells with many processes. They are the largest and most numerous of the neuroglia. Their primary functions are to support the neurons, create a blood-brain barrier, and secret chemicals that regulates the neurons. The blood-brain barrier is extremely important in the nervous system in that it prevents toxins and pathogens from reaching the neurons. It’s a buffer that restricts the movement of harmful organisms. Oligodendrocytes are similar to astrocytes in shape, but small and with fewer processes. Their main purpose is to create a myelin sheath around the axons of the neurons in the CNS. This sheath increases the speed of the nerve impulse of conduction. Microglia cells are fundamentally phagocytes, responsible for removing waste. It is essentially an immune cell, there to protect the neurons. The ependymal cells lines the ventricles of the brain and it produces and helps in the movement of cerebrospinal fluid. The Neuroglia found in the PNS are the Shwann cells and the satellite cells. Schwann cells are almost identical to the oligodendrocytes of the CNS. It creates a myelin sheath that surrounds the axons of the neurons. However, unlike the oligodendrocytes, it can regenerate. Satellite cells are flat cells that monitor the exchange of materials from the interstitial fluid and the neurons. Myelination Myelin, an insulating sheath, is found on the axon of most neurons. It is created by the Schwann cells in the Peripheral Nervous System and oligodendrocytes in the Central Nervous System. These cells are wrapped around the axons of a neuron, 15 to 20 times, creating a phospholipid bilayer. They are interrupted along the axons by gaps called the nodes of Ranvier. The purpose of myelination and which cells are myelinated and which cells aren’t is not yet fully understood. However, it is believed that myelin sheaths conduct nerve impulses faster along the axons with impulses jumping from node to node.