E-Book, Englisch, 200 Seiten
E-Book, Englisch, 200 Seiten
ISBN: 978-0-12-803063-9
Verlag: Elsevier Reference Monographs
Format: PDF
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
Provides a resource on the anatomy of the PNS nerves in the limbs, including key facts and summary tables that are essential to clinical practiceReports on typical injuries to the nerves of the PNS, as well as clinical findings on examination and treatmentsPresents a succinct, yet comprehensive, format with quick and easy access facts for quick reference Includes comprehensive chapters on nerves of the upper and lower limbs, discussing origin, course, distribution, and relevant pathologies
Paul Rea graduated in Medicine from the University of Glasgow and then went into clinical training in a wide range of hospital specialties. He was then appointed as an Associate Lecturer in Anatomy at the University of Glasgow teaching medical, dental and science students.
During his time as an Associate Lecturer in Anatomy, he was awarded a scholarship to undertake a part time MSc in craniofacial anatomy alongside his teaching duties. His research won the Scottish Royal Medico-Chirurgical Society of Glasgow prize. He returned to clinical practice in pathology receiving training in all areas of histopathology, including post-mortem analysis.
He then returned to the University of Glasgow and was appointed as a University Teacher and subsequently Senior University Teacher. He is also one of the Licensed Teachers of Anatomy, appointed by St. Andrew's House, Edinburgh. He teaches across the medical, dental and science programmes. He sits on many university committees and is a member of the Dental School Liaison Group. Paul has published in numerous journals and presented his work at many international conferences. He is also involved with public engagement with the Glasgow Science Centre as a Meet the Expert, and was key to the anatomical input to the international exhibition BodyWorks, and was a member of its Advisory Committee. He is also a STEM ambassador.
His research involves a successful strategic partnership with the Digital Design Studio, Glasgow School of Art. This has led to multi-million pound investment in creating world leading 3D digital datasets to be used in undergraduate and postgraduate teaching to enhance learning and assessment. This successful collaboration has resulted in the creation of the world's first taught MSc in Medical Visualisation and Human Anatomy combining anatomy and digital technologies with internationally recognised leading digital experts. Paul is the joint Programme Coordinator for this programme.
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Chapter 1 Overview of the Nervous System
Abstract
Broadly speaking, the nervous system can be divided into two major areas – central and peripheral. The central nervous system comprises the brain and the spinal cord. The peripheral nervous system comprises all of the nerves – cranial, spinal, and peripheral, including the sensory and motor nerve endings of these nerves. Keywords
central nervous system spinal nerves cranial nerves forebrain midbrain hindbrain hypothalamus thalamus peripheral nervous system spinal cord 1.1. Divisions of the nervous system
1.1.1. Central Nervous System
The CNS is comprised of the brain and the spinal cord. The role of the CNS is to integrate all the body functions, from the information it receives. Within the peripheral nervous system (PNS), there are abundant nerves (group of many nerve fibers together), however, the CNS does not contain nerves. Within the CNS, a group of nerve fibers traveling together is called a pathway or tract. If it links the left- and right-hand sides, it is referred to as a commissure. 1.1.1.1. Neurons Within the CNS, there are many, many millions of nerve cells called neurons. Neurons are cells that are electrically excitable and can transmit information from one neuron to another by chemical and electrical signals. There are three very broad classifications of neurons – sensory (which process information on light, touch, and sound to name some sensory modalities), motor (supplying muscles), and interneurons (which interconnect neurons via a network). Typically, a neuron comprises a few simple features, though there are a variety of specializations that some have depending on the location they are found within the nervous system. In general, a neuron has a cell body. Here, the nucleus, or the powerhouse of the neuron, lies within its cytoplasm. At this point, numerous fine fibers enter called dendrites. These processes receive information from adjacent neurons keeping it up-to-date with the surrounding environment. Through these dendrites, the amount of information that a single neuron receives is significantly increased. From a neuron, there is a long single process of variable length called an axon. This conducts information away from the neuron, or the cell body. Some neurons however have no axons and the dendrites will conduct information to and from the neuron. In addition to this, a lipoprotein layer called the myelin sheath can surround the axon of a principal cell. This is not a continuous layer along the full length of the axon. Rather, there are interruptions called nodes of Ranvier. It is at this point where the voltage-gated channels occur, and it is here that conduction occurs. Therefore, the purpose of the myelin sheath is to enable almost immediate conduction between one node of Ranvier and the next, thus ensuring quick communication between neurons, and keeping those nerves up to date with body processes around them. The size of neurons however varies considerably. The smallest of our neurons can be as small as 5 µm, and the largest, for example, motor neurons, can be as large as 135 µm. In addition, axonal length can vary considerably too. The shortest of these can be 100 µm, whereas a motor axon supplying the lower limb, for example, the toes, can be as long as 1 m. In the PNS, neurons are found in ganglia, or in laminae (layers) or nuclei in the central nervous system (CNS). Neurons communicate with each other at a point called a synapse. Most of these junctional points are chemical synapses where there is the release of a neurotransmitter that diffuses across the space between the two neurons. The other type of synapse is called an electrical synapse. This form is generally more common in the invertebrates, where there is close apposition of one cell membrane and the next, that is, at the pre- and postsynaptic membranes. Linking these two cells is a collection of tubules called connexons. The transmission of impulses occurs in both directions and rapidly. This is because there is no delay in the neurotransmitter having to be activated and released across the synapse. Instead, the flow of communication depends on the membrane potentials of the adjacent cells. 1.1.1.2. Neuroglia Neuroglia, or glia, are the supportive cells for neurons. Their main purpose is not in relation to the transmission of nerve impulses. Rather, they are involved in provision of nutrients, maintenance of a stable homeostatic environment, and the production of the myelin sheath. There are two broad classifications of neuroglia – microglia and macroglia. Microglia have a defensive role and are known as phagocytic cells. They are found throughout the brain and spinal cord, and can alter their shape, especially when they engulf particulate material. Therefore, they function in a protective role for the nervous system. Macroglia are subdivided into seven different types, again with each having a special role. 1. Astrocytes: These cells fill the spaces between neurons and provide for structural integrity. They also have processes that join to the capillary blood vessels. These are known as perivascular end feet. Therefore, with their close apposition to the vasculature, they are also thought to be responsible for metabolite exchange between the neurons and the blood vessels. They are found in the CNS. 2. Ependymal cells: There are three types of ependymal cells – ependymocytes, tanycytes, and choroidal epithelial cells. The ependymocytes allow for the free movement of molecules between the cerebrospinal fluid (CSF) and the neurons. Tanycytes are generally found in the third ventricle of the brain and can be involved in responding to changing hormonal levels of the blood-derived hormones in the CSF. Choroidal epithelial cells are the cells that control the chemical composition of the CSF. They are found in the CNS. 3. Oligodendrocytes: These cells are responsible for the production of myelin sheaths. They are found in the CNS. 4. Schwann cells: Like oligodendrocytes, Schwann cells are responsible for the production of the myelin sheath, but in the PNS. They also have an additional role in phagocytosis of any debris; therefore help to clean the surrounding environment. 5. Satellite cells: These cells surround those neurons of the autonomic system and also the sensory system. They maintain a stable chemical balance of the surrounding environment to the neurons. Therefore they are found in the PNS. 6. Radial glia: Radial glial cells act as a means of scaffolding onto which new neurons migrate. They are found in the CNS. 7. Enteric glia: These cells are found within the gastrointestinal tract and aid digestion and maintenance of homeostasis. They are, by their very nature, found in the PNS. In the CNS, several different types of neuron are found. The most common type of neuron found in the CNS are called multipolar neurons, because of having many types of dendrites, as well as the single axon. The summary of the main types of neurons is as follows. • Multipolar neurons: These have at least two dendrites that extend from the neuronal soma. Multipolar neurons are also classified as Golgi Type 1 neurons, and Golgi Type 2 neurons. Golgi Type 1 neurons have long axons that originate in the gray matter of the spinal cord. Golgi Type 1 neurons are typically found in the ventral gray horn of the spinal cord. They are also typical of pyramidal neurons of the cerebral cortex or Purkinje neurons of the cerebellum. On the other hand, Golgi Type 2 neurons either do not have an axon at all, or if they do, the axon does not exit from the gray matter of the CNS. Golgi Type 2 neurons are found in the granular layer of the cerebellum and hippocampus. These neurons are called granule cells, typically found throughout the cerebral cortex, cerebellum, hippocampus, olfactory bulb, and the dorsal cochlear nucleus. • Bipolar neurons: These neurons are less typical and have an axon and dendrite (or extension of the axon) at opposite sides of the neuronal cell body. These neurons are typically involved in transmission of information related to the special senses. They are found in the retina (transmission of visual information), olfactory epithelium (transmission of information related to smell), and the vestibulocochlear nerve (transmitting information related to sound and balance). • Unipolar neurons: These are also referred to as pseudounipolar neurons. They have a single axon that extends both centrally and peripherally. The central portion of this neuron extends into the spinal cord and the peripheral portion will extend into the periphery, terminating perhaps in the skin, muscle, or joints. Pseudounipolar neurons do not have dendrites and are typically...
