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Chapter 2

Chapter 2 - Neuroanatomy.doc

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University of Toronto Scarborough
Ted Petit

Chapter 2: Neuroanatomy Module 2.1: Cells of the Nervous System Every living thing made up of cells. What makes human a higher-functioning organism is fact that humans have aggregates of specialized cells that perform specialized functions. Brain composed of many parts, which have multiple functions, larger components of brain are made up of individual cells. Neurons and glia are specialized cells of nervous system; they are specialized in both structure and function. Glia provide support and functions, and neurons are communicators. Neurons react and respond to stimuli, they are basis of behaviors. Neurons also learn and store information about their external environment. Neurons and Glia: Structure and Function Gross Anatomy of the Neuron Most distinctive structural feature of neuron is its shape. Neurons shape is closely related to its function: to receive, conduct, and transmit signals to collect information and send it on (or not). Neurons consists of three main components: 1. Dentrites: Which receive incoming information from other neurons. 2. Soma or cell body: Which contain the genetic machinery and most of the metabolic machinery needed for common cellular functions. 3. Axon: Which sends neural information to other neurons. Information is passed from axon to dendrite across a gap called synapse. Basis of their positions relative to synapse, events that occur in the axon referred to as presynaptic and events that happened in dendrite referred as postsynaptic. Dendrites essentially increase surface area available for reception of signals from axons of other neurons. Extent of branching of dendrites gives indication of number of connections or synapses it makes w/ incoming axon. All of this information is sent to rest of neuron in form of electrical charge or action potential. Dendrites often covered w/ tiny spines, which grow and retract in response to experience. Spines themselves can form synapses w/ other neurons. Axon is commonly thought of as information sender. Neuron has only one axon, although axon can divide at its far end into many branches (increasing number of synapses it can form). Axon essentially long thin fibre or wire that can pass its message along many different cells simultaneously. Many axons in mammalian nervous system are covered w/ insulation, called myelin. It helps speed rate of information transfer and ensure message gets to end of axon. End of axon is terminal button. Information send from terminal button across synapse to dendrite. Information that passes from axon across synapse is in form of neurochemical message (by substances referred to as neurotransmitters), which may be transformed into electrical message w/in dendrite. Internal Anatomy of the Neuron Neuron is covered w/ a membrane. Nothing obvious sets neural cell membranes apart from other animal cell membranes. Plasma Membrane consists of bilayer of continuous sheets of phospholipids that separate two fluid environments inside (cytoplasm) and outside the cell. W/in membrane are proteins and channels that allow passage of materials into and out of neuron. Inside main cell body, small components of the cell (called organelles) form complex environment which organelles perform various genetic (nucleus), synthetic (ribosomes, endoplasmic reticulum), and metabolic (mitochondria) processes that keep neuron functioning. Nucleus packages and controls genetic information contained in DNA (Deoxyribonucleic acid). Nucleus processes genetic information needed to complete series of events that form path from the recipe that genetic information provides to form proteins that neuron needs. Nucleus also contains all genetic information needed to code proteins such as for eye or hair color, as well as those thought to underlie complex processes such as linguistic ability Structure and Functions of Neurons Neurons can be classified according to structure and function. In nervous system, structure and function are related. Some common neurons are labelled as unipolar, bipolar, and multipolar (most common). Unipolar neurons have only one process emanating from cell body; bipolar neurons have two processes; and multipolar have numerous processes extending from cell body. Neurons w/ no axons or only very short axons are called interneurons and they tend integrate information w/in a structure rather than sending information between structures. Functionally, neurons can be classified by type of signals they process. For example, signals motor neurons convey muscle contraction. Sensory neurons process information elicited from sensory-type stimuli, interneurons make connections between cells, enabling sort of convergence and combination of behavioural responses. Type of information that represented by neural activity related to function of neuron. Neurons can be classified as being afferent (bringing information to central nervous system or structure) or efferent (sending information from brain or away from structure). Neurons vary in size, shape, and function and that neuron can change shape as result of experience. Glia Glia performs an essential role in functioning of central nervous system. Glia performs support functions, different types of glia providing different types of support. Support cells outside of brain and spinal cord called satellite cells. At least three diff. types of glia: astrocytes, oligodendocrytes, and microglia. Astrocytes are largest glia cells and named astrocytes because tend to be star- shaped. Astrocytes involved in blood-brain barrier, protective system that keeps brain separate from rest of body. Astrocytes also perform nutritive and metabolic functions for neurons. Astrocytes also essential for regulation of chemical content of extracellular space because the envelope synapse and can regulate how far neurotransmitters and other substances released by terminal button can spread. Astrocytes important in storage of neurotransmitters. Clear we do not know all of the functions of astrocytes. Oligodendrocytes one very clear function: to make myelin. Oligodendrocytes wrap their processes around most axons in brain and spinal cord. These processes made of myelin, which fatty substance that acts to insulate axon. Axons outside brain and spinal cord frequently myelinated, w/ myelin provided by Schwann cells. Schwann cells provide only one segment of myelin to an axon, oligodendrocytes can contribute many segments to many axon. Microglia named w/ reference to their size smallest of the glia. Microglia is phagocytes remove debris from nervous system. Debris can accumulate in brain as result of injury, disease, infection or aging. Microglia very different from other cells of nervous system: made outside of brain and spinal cord by ma
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