Nervous Tissue

Nervous Tissue

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   Nervous tissueis the term for groups of organized cells in the nervous system, which  controls the body’s movements,  sends and carries signals to and from the different parts of the body,  has a role in controlling bodily functions such as digestion. Nervous tissue is grouped into two main categories: neurons and neuroglia. Neurons, or nerves, transmit electrical impulses, while neuroglia do not; neuroglia have many other functions including supporting and protecting neurons. DEFINITION
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  NERVOUS SYSTEM Nervous tissuemakes up the nervous system. The nervous system is subdivided in several overlapping ways. The central nervous system (CNS) is composed of the brain and spinal cord, which coordinates information from all areas of the body and sends nerve impulses that control all bodily movements. The peripheral nervous system (PNS) consists of peripheral nerves that branch all throughout the body. It connects the CNS to the rest of the body and is directly responsible for controlling movements of specific parts of the body. Example: just before arm movement the CNS sends nerve impulses to the PNS nerves in the arm, which causes the arm to move.
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   Another subdivisionof the nervous system is into the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS).  The SNS activates in order to stimulate a fight-or-flight response in an organism when that organism encounters a threat and must decide whether to fight or flee from it. The nerves of the SNS have diverse effects on different parts of the body.  Activation of the SNS causes the pupils of the eyes to dilate, inhibits digestion, increases sweat secretion, and increases the heart rate.  Conversely, the PSNS is activated during moments of “rest and digest”, when an organism is not facing an immediate threat. The enteric nervous system (ENS) controls the gastrointestinal tract (digestive tract).  Nerves of the PSNS work to stimulate activities that can occur at rest such as digestion, waste excretion, and sexual arousal, and they also decrease the heart rate.
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  This division ofthe nervous system, along with the SNS and PSNS, are collectively referred to as the autonomic nervous system (ANS).  The ANS regulates activities that are performed unconsciously; we don’t have to think about digesting food for it to occur, for example. By contrast, the somatic nervous system (SoNS) controls voluntary body movements. It is made up of afferent and efferent nerves that send signals to and from the CNS, causing voluntary muscle contraction to occur. The cell bodies of other PNS neurons, such as the sensory neurons that provide information about touch, position, pain, and temperature, are located outside of the CNS, where they are found in clusters known as ganglia. The axons of peripheral neurons that travel a common route are bundled together to form nerves.
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  If you thinkabout the roles of the three classes of neurons, you can make the generalization that all neurons have three basic functions. These are to: Receive signals (or information). Integrate incoming signals (to determine whether or not the information should be passed along). Communicate signals to target cells (other neurons or muscles or glands). These neuronal functions are reflected in the anatomy of the neuron. THE BASIC FUNCTION OF NERVOUS TISSUE
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  LOCATION Technically they’re present allover, with motor neurons, different types of sensory neurons, and inter neurons connecting them and connecting them to the brain, which is composed of inter neurons itself. The brain probably has the highest neuron-density of the body due to being literally made of neurons.
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  CLASSIFICATION OF NERVOUS TISSUE NervousTissue Neurons Neuroglia
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  Neurons are cellsthat can transmit signals called nerve impulses, or action potentials. An action potential is a quick rise and fall in the electrical membrane potential of the neuron, which transmits signals from one neuron to the next. NEURON
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  The Cell Body Themain portion of the neuron is called the soma, or cell body.  In the center of the soma is the nucleus of the cell, which is where the chromosomes that contain all of the genetic material are stored. This is also the part of the cell that creates mRNA for cell replication. Various processes (appendages or protrusions) extend from the cell body. These include many short, branching processes, known as dendrites, and a separate process that is typically longer than the dendrites, known as the axon. TYPICAL STRUCTURE OF A NEURON
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  TYPICAL STRUCTURE OFA NEURON
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  Dendrites The first twoneuronal functions, receiving and processing incoming information, generally take place in the dendrites and cell body. Incoming signals can be either excitatory – which means they tend to make the neuron fire (generate an electrical impulse) – or inhibitory – which means that they tend to keep the neuron from firing. Most neurons receive many input signals throughout their dendritic trees. A single neuron may have more than one set of dendrites, and may receive many thousands of input signals. Whether or not a neuron is excited into firing an impulse depends on the sum of all of the excitatory and inhibitory signals it receives. If the neuron does end up firing, the nerve impulse, or action potential, is conducted down the axon.
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  Axons The dendrites tendto taper and are often covered with little bumps called spines. In contrast, the axon tends to stay the same diameter for most of its length and doesn't have spines. The axon arises from the cell body at a specialized area called the axon hillock. Finally, many axons are covered with a special insulating substance called myelin, which helps them convey the nerve impulse rapidly. Myelin is never found on dendrites. Towards its end, the axon splits up into many branches and develops bulbous swellings known as axon terminals (or nerve terminals). These axon terminals make connections on target cells. Synapse The synapse is the chemical junction between the axon terminals of one neuron and the dendrites of the next. It is a gap where specialized chemical interactions can occur, rather than an actual structure.
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  The myelin sheathis the location of a number of diseases that cause degeneration of the myelin sheath, also called demyelinating, such as: Multiple Sclerosis Optic Neuritisuillain-Barré syndrome Transverse Myelitis Central Pontine Myelinolysis Vitamin B-12 Deficiency Chronic Inflammatory Demyelinating Polyneuropathy The degeneration of the myelin sheath causes degradation of the neural impulses that are transmitted along an axon. The systems affected by this degradation depend on the location of the degenerating myelin. For example, multiple sclerosis (MS) affects the neurons of the spinal cord as well as the brain leading to degradation of both motor and cognitive functioning. DISORDERS DUE TO THE DEMYELINATION IN MYELIN SHEATH
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  CLASSIFICATION OF NEURONSBASED ON THEIR ROLES Based on their roles, the neurons found in the human nervous system can be divided into three classes: sensory neurons, motor neurons, and interneurons. Sensory neurons Sensory neurons get information about what's going on inside and outside of the body and bring that information into the CNS so it can be processed. For instance, if you picked up a hot coal, sensory neurons with endings in your fingertips would convey the information to your CNS that it was really hot. Motor neurons Motor neurons get information from other neurons and convey commands to your muscles, organs and glands. For instance, if you picked up a hot coal, it motor neurons innervating the muscles in your fingers would cause your hand to let go.
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  Interneurons Interneurons, which arefound only in the CNS, connect one neuron to another. They receive information from other neurons (either sensory neurons or interneurons) and transmit information to other neurons (either motor neurons or interneurons). For instance, if you picked up a hot coal, the signal from the sensory neurons in your fingertips would travel to interneurons in your spinal cord. Some of these interneurons would signal to the motor neurons controlling your finger muscles (causing you to let go), while others would transmit the signal up the spinal cord to neurons in the brain, where it would be perceived as pain. Interneurons are the most numerous class of neurons and are involved in processing information, both in simple reflex circuits (like those triggered by hot objects) and in more complex circuits in the brain. It would be combinations of interneurons in your brain that would allow you to draw the conclusion that things that looked like hot coals weren't good to pick up, and, hopefully, retain that information for future reference.
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  Glial cells playa supporting role thus glial cells are essential to nervous system function. There are many more glial cells in the brain than there are neurons. There are four main types of glial cells in the adult vertebrate nervous system. Three of these, astrocytes, oligodendrocytes, and microglia, are found only in the central nervous system (CNS). The fourth, the Schwann cells, are found only in the peripheral nervous system (PNS). GLIAL CELLS
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  CLASSIFICATION OF GLIALCELLS
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  Astrocytes These are themost numerous type of glial cell. In fact, they are the most numerous cells in the brain! Astrocytes come in different types and have a variety of functions. They help regulate blood flow in the brain, maintain the composition of the fluid that surrounds neurons, regulate communication between neurons at the synapse. During development, astrocytes help neurons find their way to their destinations and contribute to the formation of the blood-brain barrier, which helps isolate the brain from potentially toxic substances in the blood. Microglia These are related to the macrophages of the immune system and act as scavengers to remove dead cells and other debris.
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  Oligodendrocytes and Schwanncells These cells of of the CNS and the PNS share a similar function. Both of these types of glial cells produce myelin, the insulating substance that forms a sheath around the axons of many neurons. Myelin dramatically increases the speed with which an action potential travels down the axon, and it plays a crucial role in nervous system function.
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  Other types ofglia in addition to the four main types the glial cells also include satellite glial cells and ependymal cells. Satellite glial cells •These cover the cell bodies of neurons in PNS ganglia. • Satellite glial cells are thought to support the function of the neurons and might act as a protective barrier. Ependymal cells •These cells line the ventricles of the brain and the central canal of the spinal cord •They have hair-like cilia that beat to promote circulation of the cerebrospinal fluid found inside the ventricles and spinal canal.