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acetylcholinesterase (AChE): the enzyme that destroys acetylcholine soon after it is liberated by the terminal buttons, thus terminating the postsynaptic potential. action potential: a brief depolarization that conducts information down both a neuron's axon and dendrites (back-propagation). An action potential is triggered when integration of IPSPs and EPSPs at the axon hillock produces +20 mV shift in resting internal voltage. In specialized circuits, dendrites can also generate action potentials. An action potential is made possible by transient changes in membrane permeability to sodium and potassium ions. action potential firing rate: a neuron’s firing rate depends on the intensity of its stimulation by presynaptic neurons. This principle is summarized by the rate law. adenosine triphosphate (ATP): a molecule of prime importance to cellular energy metabolism; its breakdown liberates energy. afferent neuron: a neuron innervating a specific structure. Neurons afferent to the CNS are sensory neurons. all-or-none law: the principle that once an action potential is triggered in an axon, it is propagated, without decrement, to the end of the fiber. An action potential's size and shape are independent of the intensity of the triggering stimulus. anterograde: in a direction along an axon from the cell body toward the terminal buttons. area postrema: a region of the medulla where the blood-brain barrier is weak; poisons can be detected there and can initiate vomiting. astrocyte: CNS glial cell that supports neurons in the central nervous system, provides nutrients and other substances, and regulates the chemical composition of the extracellular fluid. Helps form scar tissue following injury to the brain and spinal cord. autoreceptor: presynaptic neuron receptor linked to a G-protein. Autoreceptors bind circulating drugs, neurochemicals released extracellularly by the neuron itself, or neurochemicals released by other neurons. Autoreceptors help neurons adjust the synthesis and release of neurochemicals through the second-messenger system. Their effect is generally inhibitory. axoaxonic synapse: synapse between two terminal buttons. These synapses do not directly affect integration, but change transmitter release by the postsynaptic neuron. Axoaxonic synapses can produce presynaptic facilitation and presynaptic inhibition. axoplasmic transport: an active process by which substances are propelled along microtubules that run from the soma to the terminal button. Anterograde movement transports substances from the soma to the terminal buttons using kinesin (500 mm/day). Retrograde movement transports substances from the terminal buttons to the soma using dynein (250 mm/day). axon: spiderthread-thin, elongated process that transmits action potentials toward a neuron's terminal buttons and transports molecules between the soma and terminal buttons. axon hillock: region of the soma from which the axon originates. The hillock is the primary site where postsynaptic potentials are integrated to trigger action potentials. binding site: the location on a receptor protein to which a ligand binds. bipolar neuron: a neuron with one axon and one dendrite attached to its soma. blood-brain barrier: a semipermeable barrier produced by astrocytes and the endothelial cells in the walls of the capillaries in the brain. The blood-brain barrier protects the nervous system from substances transported via the blood. This semipermeable barrier maintains the ion balance of the extracellular fluid (necessary for signal transmission) and excludes most toxins. cable properties: the passive conduction of electrical current, in a decremental fashion, down the length of an axon. central nervous system (CNS): the central nervous system consists of the brain, spinal cord, and retina. cerebral cortex: outer 2 mm (two thin coins) of the brain's cerebral hemispheres arranged in a more than six-layered structure. The cortex is needed for episodic memory, complex associations, and producing new movement programs. chromosome: a strand of DNA, with associated proteins, found in the nucleus; carries genetic information. cisterna: a part of the Golgi apparatus; through the process of pinocytosis, it receives portions of the presynaptic membrane and recycles them into synaptic vesicles. connexon: membrane channel bridging the space between two adjacent cells as in a gap junction. cytoplasm: the viscous, semiliquid substance contained in the interior of a cell. cytoskeleton: framework formed of microtubules, neurofilaments, and microfilaments, linked to each other and forming a cohesive mass that gives a cell its shape. dendrite: a treelike process attached to the soma of a neuron that receives messages from other neurons and back-propagated action potentials from the axon hillock, and transmits electrical potentials to other dendrites. dendritic spine: small buds on a dendrite's surface where terminal buttons from other neurons synapse or where other spines can form gap junctions. deoxyribonucleic acid (DNA): a long, complex macromolecule consisting of two interconnected helical strands; along with associated proteins, strands of DNA constitute the chromosomes. depolarization: reduction (toward zero) of the membrane potential of a cell from its normal resting potential. diffusion: movement of molecules from regions of high concentration to regions of low concentration. efferent neuron: neuron originating in and coursing away from a specific structure. Efferent axons of the CNS control glands, immune cells, muscles, and neurons. electrode: a conductive medium that can be used to apply electrical stimulation or to record electrical potentials. electrolyte: an aqueous solution of a material that ionizes--namely, a soluble acid, base, or salt. electrostatic pressure: the attractive force between atomic particles charged with opposite signs, or the repulsive force between atomic particles charged with the same sign. endocrine gland: a gland that liberates its secretions into the extracellular fluid around capillaries and hence into the bloodstream. endoplasmic reticulum: parallel layers of membrane found within the cytoplasm of a cell that is divided into rough and smooth endoplasmic reticulum. Rough endoplasmic reticulum contains ribosomes and is involved with production of proteins that are secreted by the cell. In contrast, the smooth endoplasmic reticulum is the site of synthesis of lipids and provide channels for the segregation of molecules involved in various cellular processes. enzymatic deactivation: the destruction of a transmitter substance by an enzyme after its release — for example, the destruction of acetylcholine (ACh) by acetylcholinesterase (AChE) or dopamine by MAO. enzyme: a molecule that controls a chemical reaction, combining two substances or breaking a substance into two parts. excitatory postsynaptic potential (EPSP): graded depolarization of a postsynaptic neuron which increases the probability that an action potential will be initiated. exocytosis: the secretion of a substance by a cell through means of vesicles; the process by which neurotransmitters are secreted. extracellular fluid: body fluids located outside of cells. G protein: protein coupled to a metabotropic receptor that binds with guanosine disphosphate and guanosine triphosphate. When the receptor is activated, a G protein modifies the action of specific enzymes to convey messages within the cell. gap junction: 2 nm region of contact between membranes of two cells, spanned by protein channels called connexons, which allows ion transfer. gene: the functional unit of the chromosome, which directs synthesis of one or more proteins. glial cells: nervous system cells including astroglia, microglia, oligodendroglia, and Schwann cells. The classical view of these cells is that they perform housekeeping and support functions. A controversial view is that they participate in information processing. Golgi apparatus: complex of parallel membranes located in the cytoplasm that wraps the products of a secretory cell and produces lysosomes. hyperpolarization: an increase in the membrane potential of a cell, relative to the normal resting potential. inhibitory postsynaptic potential (IPSP): graded hyperpolarization of a postsynaptic neuron which decreases the probability that an action potential will be initiated. ion: atom or small free-floating molecule with a net electrical charge. Cations are positively charged and anions are negatively charged. ion balance in resting potential: there is a high concentration of K+ and protein (A-) inside of the neuron, and Cl- and Na+ outside. ion channel: membrane-spanning protein that forms a pore that allows specific ions to enter or leave cells. ionotropic receptor: a receptor that contains a binding site for a neurotransmitter and an ion channel that opens when a molecule of the neurotransmitter attaches to the binding site. integration: summation of EPSPs and IPSPs at the axon hillock to determine whether to initiate an action potential. If the net effect reaches the threshold of excitation (nominally –50 mV), Na+ channels in the axon hillock membrane are opened and an action potential is propagated down the axon. interneuron: a neuron located entirely within the central nervous system. intracellular fluid: the fluid contained within cells. ligand: a chemical that binds with the binding site of a receptor. lysosome: an membrane-covered organelle that contains enzymes designed to break down waste products. Tay-Sachs, for example, is a lysosomal storage disorder. membrane: a structure consisting principally of lipid molecules that defines the outer boundaries of a cell and also constitutes many of the cell organelles, such as the Golgi apparatus. Membranes determine which substances enter or leave the cell. membrane potential: the electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell. messenger ribonucleic acid (mRNA): a macromolecule that delivers genetic information concerning the synthesis of a protein from a portion of a chromosome to a ribosome. metabotropic receptor: a receptor that contains a binding site for a neurotransmitter; activates an enzyme that begins a series of events that opens an ion channel elsewhere in the membrane of the cell when a molecule of the neurotransmitter attaches to the binding site. microelectrode: a very fine electrode, generally used to record activity of individual neurons. microfilament: the thinnest of the fibers of the cytoskeleton; forms a meshwork just inside the membrane that holds membrane-bound proteins in place. microglia: the smallest of glial cells; act as phagocytes and protect the brain from invading microorganisms. microtubule: a long strand of bundles of protein filaments arranged around a hollow core; part of the cytoskeleton and involved in transporting substances from place to place within the cell. mitochondria: organelles responsible for extracting energy from nutrients. motor neuron: neuron that when stimulated contracts muscle fibers. multipolar neuron: a neuron with one axon and many dendrites. myelin: flat layers of a fatlike substance that insulate myelinated axons like a bandage roll. Myelin is produced by oligodendrocytes in the CNS and Schwann cells in the PNS. Myelination increases conduction velocity a hundred-fold and reduces the energy required to operate sodium-potassium transporters. myelin sheath: a sheath that surrounds axons and insulates them, preventing messages from spreading between adjacent axons. neural integration: the process by which inhibitory and excitatory postsynaptic potentials summate and control the rate of firing of a neuron. neurites: a neuron's branches including an axon and dendrites. neurofilament: long protein fiber that helps comprise the cytoskeleton, located beneath a neuron's membrane, that gives a neuron shape and controls the location of embedded membrane proteins. neuromodulator: a naturally-secreted substance that acts like a neurotransmitter except that it is not restricted to the synaptic cleft but diffuses through the extracellular fluid. neurotransmitter-dependent ion channel: an ion channel that opens when a molecule of a neurotransmitter binds with a postsynaptic receptor. node of Ranvier: region of an axon not wrapped in myelin, located between myelin segments, where ion exchange occurs. nucleolus: a structure within the nucleus of a cell that produces the ribosomes. nucleus: a structure in the central region of a cell, containing the nucleolus and chromosomes. oligodendrocyte: a type of glial cell in the central nervous system that forms myelin sheaths. oscilloscope: a laboratory instrument capable of displaying a graph of voltage as a function of time on the face of a cathode ray tube. peptide: a chain of amino acids joined together by peptide bonds. peripheral nervous system (PNS): the peripheral nervous system consists that part of the nervous system outside the brain and spinal cord, including the cranial and spinal nerves, and peripheral ganglia. phagocytes: cells that engulf and digest other cells or debris caused by cellular degeneration. phagocytosis: the process by which cells engulf and digest other cells or debris caused by cellular degeneration. pinocytosis: the pinching off of a bud of cell membrane, which travels to the interior of the cell. postsynaptic membrane: the cell membrane opposite the terminal button in a synapse; the membrane of the cell that receives the message. postsynaptic potential: alterations in the membrane potential of a postsynaptic neuron, produced by liberation of transmitter substance at the synapse. postsynaptic receptor: a receptor molecule in the postsynaptic membrane of a synapse that contains a binding site for a neurotransmitter. presynaptic facilitation: the action of a presynaptic terminal button in an axoaxonic synapse; increases the amount of neurotransmitter released by the postsynaptic terminal button. presynaptic inhibition: the action of a presynaptic terminal button in an axoaxonic synapse; reduces the amount of neurotransmitter released by the postsynaptic terminal button. presynaptic membrane: the membrane of a terminal button that lies adjacent to the postsynaptic membrane. rate law: the principle that variations in the intensity of a stimulus or other information being transmitted in an axon are represented by variations in the rate at which that axon fires. release zone: a region of the interior of the presynaptic membrane of a synapse to which synaptic vesicles attach and release their neurotransmitter into the synaptic cleft. resting potential: the membrane of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials; approximately -70 mv in the giant squid axon. retrograde transmission: neuron signaling back to the presynaptic neuron. For example, nitric oxide is released into the extracellular fluid and travels back to the presynaptic neuron during long-term potentiation. reuptake: the reentry of a transmitter substance just liberated by a terminal button back through its membrane, thus terminating the postsynaptic potential. This is the most common method for terminating EPSPs and IPSPs. ribosome: a cytoplasmic structure, made of protein, that serves as the site of production of proteins translated from mRNA. saltatory conduction: conduction of action potentials by myelinated axons. The action potential "jumps" from one node of Ranvier to the next since ions can only enter or leave a myelinated axon at the node of Ranvier. Schwann cells: support PNS axons, produce myelin sheaths, digest dead and dying axons, and form cylinders to guide axon growth. second messenger: a chemical produced when a G protein activates an enzyme; carries a signal that results in the opening of the ion channel or causes other events to occur in the cell. second messenger system: metabotropic receptors contain a binding site and activate an enzyme when the neurotransmitter attaches to this site. The enzyme creates a second messenger, like cyclic AMP, which can travel throughout the cell producing chemical changes (opening ion channels or manufacturing proteins). These receptors expend energy and produce slower, longer-lasting, and more diverse changes. sensory neuron: neuron that detects internal or external environmental changes and reports this information to the central nervous system. sodium-potassium pump: a protein found in the membrane of all cells (fueled by ATP) that removes intracellular sodium from the neuron in exchange for extracellular potassium ions. soma: the cell body of a neuron which contains the nucleus. stages of an action potential: (1) threshold of excitation is reached and Na+ channels open (2) K+ channels open (3) Na+ channels close (4) K+ continues to leave the neuron (5) K+ channels close (6) K+ outside the neuron diffuses away. steps in transmitter release: (1) an action potential reaches and depolarizes the terminal button (2) calcium ions enter the terminal button from the extracellular fluid (3) calcium binds with clusters of protein molecules that join the vesicles with the presynaptic membrane (4) the clusters move apart, forming a hole through both membranes called a fusion pore (5) neurotransmitter leaves the terminal button for the synaptic cleft or extracellular fluid (exocytosis). steroid: a chemical of low molecular weight, derived from cholesterol. Steroid hormones affect their target cells by attaching to receptors found within the cell. synapse: junction between an axon terminal button and the membrane of another neuron. synaptic cleft: the space between the presynaptic membrane and the postsynaptic membrane. synaptic release: the destruction of a transmitter substance by an enzyme after its release — for example, the destruction of acetylcholine (ACh) by acetylcholinesterase (AChE) or dopamine by MAO. synaptic vesicle: a small, hollow, beadlike structure found in terminal buttons; contains molecules of a neurotransmitter. target cell: the type of cell that is directly affected by a hormone or nerve fiber. terminal button: rounded swelling at the end of an axon that synapses upon another cell or releases neurochemicals by volume transmission. threshold of excitation: the value of the membrane potential that must be reached in order to produce an action potential. Typically, this value is 20 mv more positive than the resting potential. transmitter substance: a chemical that is released by a terminal button (axon varicosity or dendritic spine) that has an excitatory or inhibitory effect on another neuron. unipolar neuron: a neuron with one axon attached to its soma; the axon divides, with one branch receiving sensory information and the other sending the information into the central nervous system. voltage-dependent ion channel: an ion channel that opens or closes according to the value of the membrane potential. volume transmission: chemical transmission outside the synapse via release into the extracellular fluid. Neurochemicals are released by axon varicosities (swellings) and from the upper neck of terminal buttons. Neuromodulators like the monoamines (serotonin) are released by this method and target metabotropic receptors to produce long-lasting changes.
In the 1960s, the passive cable model was challenged by findings of action potentials in Purkinje cells (in the cerebellum) and pyramidal neurons. The active dendrite model developed slowly because dendrites have been difficult to visualize at less than 5 mm in diameter. This model proposes that action potentials can be initiated in specialized dendrites (this is normally prevented by potassium channels). Second, spikes generated at the axon hillock travel backwards to the dendrites. New research techniques (immunocytochemical, microelectrode, optical, and patch clamp) have identified calcium, sodium, and recently, potassium channels, in mammalian dendrites (Yuste, Nature, June 26, 1997). Recent evidence reviewed by Sejnowski (Science, January 10, 1997) and Yuste (Nature, June 26, 1997) reveals that action potentials (called spikes) generated at the axon hillock also travel backwards to dendrites. This action potential is made possible by dendritic sodium channel. As the spike reaches the fine dendritic branches, calcium channels open so that calcium ions enter dendritic shafts and spines. Back-propagating action potentials transmit information about action potential timing and influence the strengths of synapses with dendrites in the hippocampus and cortex. Data suggest that back-propagation can affect both long-term potentiation and depression, and adjust neuron sensitivity to even subtle changes in the firing of its afferents. Hoffman and colleagues (Nature, June 26, 1997) recently
discovered potassium channels in hippocampal pyramidal neurons. This is the first
discovery of potassium channels in mammalian
dendrites and raises the possibility that these channels serve as "shock
absorbers." glial cells Astrocytes illustrate the "smart" functions of glial cells. Astrocytes control potassium ion levels outside neurons and supply lactate to neurons for energy. Five lines of
evidence suggest astrocytes may be important in neuronal communication. Second, astrocytes can cover or uncover receptor sites at synapses, which can depress or enhance synaptic transmission and synapse formation, and may influence learning. Third, glial cells, like neurons, respond to changes in electrical potential across their membranes and chemicals like neurotransmitters and growth factors. This allows bi-directional communication: from neuron to glial cell and from glial cell to neuron. For example, astrocytes have receptors for neurochemicals like glutamate. When glutamate binds to receptors on astrocytes, they communicate with each other by releasing waves of calcium ions. They also send calcium ions into adjacent neurons across unidirectional gap junctions. This could allow astrocytes to adjust neuron firing and metabolism (Delcomyn, Foundations of neurobiology, 1998; Nedergard, Science, March 25, 1994). Fourth, Venance and colleagues reported (Nature, August 17, 1995) that anandamide inhibits gap junction transmission in striatal astrocytes (caudate nucleus and putamen of the basal ganglia). This implies that anandamide may control both intercellular communication between astrocytes and signaling between astrocytes and neurons. Finally, Pfreiger and Barres (Science, September 12, 1997), showed that neurons raised in culture required astrocytes or oligodendrocytes to develop functional synapses. The presence these glial cells increased the frequency and amplitude of spontaneous synaptic currents generated by retinal ganglion cells, reduced transmission failure, and increased transmitter release. |
