Unformatted text preview: Introduction to Neurobiology
Neurobiology is an interdisciplinary field, devoted to gaining an understanding of the nervous system using techniques from many different fields including medicine, biology, psychology, physics, chemistry, and mathematics. What does the nervous system do? Page 1-1 (1) To receive and interpret information about the internal and external environment - Sensory system (2) To organize and carry out action - Motor system (3) To make decisions about information - Integrating system Some fundamental questions that neurobiologists study: (1) How do genes regulate specific types of proteins present in nerve cells, and how does the presence of these proteins influence what a nerve cell may do? (2) How do nerve cells convey information to one another? (3) How are complex movements planned and coordinated? (4) How is information about sensory stimuli encoded and interpreted by the brain? (5) What are learning and memory, and how do they occur? (6) How does the intricate and highly specific organization of the nervous system come about during embryonic development? (7) What parts of the brain are used in human cognitive tasks such as thinking, and how do these parts carry out their functions? (8) And many others.... Two main cell types in the nervous system: Neurons: 100,000,000,000 (100 billion) neurons in human brain Glial cells: ~ 1,000,000,000,000 As a comparison, in C elegans, there are only 302 neurons and 56 glial cells (total: ~1000 cells) Functions of the two main cell types: Neurons: to carry information Glial cells: to insulate, support and nourish neurons Page 1-2 Neuron Theory:
Santiago Ramn y Cajal Camillo Golgi Newborn infant cerebral cortex Golgi staining Techniques for identifying neurons and tracing their connections? Neuron: Neurons come in many different shapes and sizes depending on where they are in the nervous system and what their function is. Neurons could be classified by function into sensory neurons, motor neurons or interneurons. In general, however, they all have the same overall structure. Each neuron has a cell body (also called a soma). This is the spherical central part of the neuron. inside the soma are found the same organelles that are seen in all animal cells. Page 1-3 Extending off of the cell body is the axon. This structure is specialized for the transfer of information over long distances (up to more than a meter in the human). The axon begins at the axon hillock (also called the spike initiating zone), where the all-or-none electrical impulse called an action potential begins. The ends of an axon are called the axon terminals, at which are found swellings called terminal boutons. The terminal is where information is passed on from one neuron to the next at a contact point called a synapse. Synapses: Page 1-4 Most synapses are between the axon of one neuron and the dendrite of another. These synapses are called axo-dendritic synapses. Although axo-dendritic synapses are the most common, axo-axonal, axo-somatic and dendrodendritc synapses also occur. There are two major classes of synapses, electrical synapses where electrical signals are passed directly between two cells, and chemical synapses, where electrical signals in the presynaptic cell cause the release of chemicals called neurotransmitters, which diffuse across the synaptic cleft and act on receptor molecules in the membrane of the postsynaptic neuron. The receptors then directly or indirectly cause channels in the postsynaptic membrane to open or close, resulting in the flow of ions across the membrane. This ionic current causes a graded electrical response in the postsynaptic cell called a postsynaptic potential (PSP). Postsynaptic potentials can be excitatory (causing the postsynaptic cell to be more likely to reach action potential firing threshold) or inhibitory (causing the postsynaptic cell to be less likely to reach threshold). Morphologically, chemical synapse is a complex of a presynaptic element, a cleft and a postsynaptic element. The presynaptic element is identified by the presence of synaptic vesicles and a presynaptic thickening at the active zone. The cleft is a space with a defined width. The postsynaptic element is marked by a postsynaptic thickening. Symmetric synapses both sides are equal thick inhibitory synapse. Asymmetric synapse the postsynaptic thickening is greater excitatory synapse. AT: Axon Terminal SP: Spine DS: Dendritic Shaft A Golgi-stained pyramidal cell in the parietal cortex of a rat Dendritic spines: The cell body and the dendrites are the two main areas of the neuron to receive inputs. There is a characteristic shape for each dendritic morphology which can be used for classification of neurons. An important specialization of the dendrites for many neurons is the presence of dendritic spines, which are small protrusions on dendritic shafts and are the sites of synaptic contacts, usually excitatory. Recent studies have demonstrated that spines are dynamic structures that can regulate synaptic transmission and modulate synaptic. Spines are also known to undergo pathological changes and have a reduced density in a number of experimental conditions such as deprivation of presynaptic input, and in many developmental, neurological and psychiatric disorders such as mental retardation, chronic alcoholism, schizophrenia, and autism.
Spine Abnormalities in Retarded Children Page 1-5 Normal 6mo-old infant 10-mo-old retarded child Some Properties of neurons: (1) Neurons produce electrical potentials across their cell membranes. (2) Neurons can transmit electrical pulses quickly. (3) Neurons have receptors which convert chemical messages to electrical. (4) Neurons are differentiated cells; they are developmentally mature, and do not divide (non-mitotic) (5) Neurons are specialized for integration and transmission of information. (6) Neurons form circuits; neuronal circuits constitute the structural basis for brain function. Functions of neurons supported by: (1) biophysical properties, including electrical properties and channels (2) structure, including a neuron's shape (3) how neurons are connected Page 1-6 Spread of information and Action Potential: If this postsynaptic potential is excitatory, and is large enough, it will cause the initiation of an action potential in the postsynaptic cell, allowing information to continue to propagate. Glial cells: Rudolf Virchow called non-neuronal cells in the brain: neuroglia (or nerve glue) Several types of glial cells are found in the nervous system. Functions of glial cells: (1) Act as a scaffolding for neuronal migration and axon outgrowth (2) Participate in the uptake and metabolism of the neurotransmitters (3) Take up and buffer ions from the extracellular environment (4) Act as scavengers to remove debris produced by dying neurons (5) Segregate groups of neurons one from another, and act as electrical insulators between neurons (6) Provide structural support for neurons (7) Play a nurturing role, supplying metabolic components and proteins necessary for neuronal function (8) Participate in intercellular signaling, and thereby play a role in information handling and memory storage (9) And more.... Page 1-7 Glial cells: Page 1-8 (1) The most numerous glia in the brain are astrocytes (so called because they are star shaped). Astrocytes fill the space between neurons. Their most important function is to regulate the chemical content of the extracellular space. There are two main forms of astrocytes: protoplasmic and fibrous, mostly in gray and white matter, respectively. Embryonically, astrocytes develop from radial glial cells, which serve as scaffolding for early neuronal migration in the developing brain. As the brain matures, radial glial cells retract their processes and become the progenitors of astrocytes. Some specialized astrocytes still maintain the morphology of a radial glial cell in the adult, such as Bergmann glial cell in the cerebellum and Muller cell in the retina. Radial glial cells perform support and guidance functions for migrating neurons. In early development, the radial glial cells span the thickness of the expanding brain parenchyma. Defined layers of the neural tube from the ventricular to the outer surface. VZ, ventricular zone; IZ, intermediate zone; CP, cortical plate; MZ, marginal zone. The radial process of the glial cell is indicated in blue, and a single attached migrating neuron is depicted at the right. Astrocytes in the adult cerebellar cortex. The Bergmann glial cells are in red; the protoplasmic astrocytes are in green; and the fibrous astrocytes are in blue. (2) Another main type of glial cells are myelinating glia (oligodendrocytes in the central nervous system, Schwann cells in the peripheral nervous system). These cells wrap many times around the axons of neurons to provide insulating layers of membrane called myelin. Gaps in this insulation are called nodes of Ranvier. We will see that the myelin sheath and the nodes of Ranvier are very important for allowing electrical signals to pass quickly and reliably down the length of an axon. Page 1-9 (3) The final type of glial cells is the microglia. These are mobile cells which can move to a site of a brain injury and consume cellular debris.
Activation of microglial cells in human brain. (A) Resting microglia in normal brain. (B) Activated microglia in diseased brain have thicker processes and larger cell bodies. Arrow indicates rod cell. (C) In regions of frank pathology microglia transform into phagocytic macrophages, which can also develop from circulating monocytes that enter the brain. Sections stained with antibody to ferritin. ...
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- Summer '08