BIO 203 Lecture Notes - Nervous Tissue PDF

BIO 203 Lecture Notes Chapter 12, Nervous Tissue Overview of the nervous system: central and peripheral nervous systems; ganglion is a group of neuron cell bodies in the peripheral nervous system; nucleus is a group of cell bodies in the central nervous system Divis...

BIO 203 Lecture Notes Chapter 12, Nervous Tissue Overview of the nervous system: central and peripheral nervous systems; ganglion is a group of neuron cell bodies in the peripheral nervous system; nucleus is a group of cell bodies in the central nervous system Divisions of the nervous system: CNS: brain and spinal cord PNS: sensory division -visceral and somatic motor division -somatic -visceral -sympathetic and parasympathetic Universal properties of neurons -excitability -conductivity -secretion Functional classification of neurons: sensory (afferent), motor (efferent), interneuron Parts of a typical neuron Neuroglia cells: CNS -oligodendrites, ependymal, microglia, astrocytes PNS -schwann cells, satellite cells Myelin sheath: Schwann and oligodendrites cells surround axons with layers of cell membrane and fatty molecules to supply insulation; space between Schwann cells called nodes of Ranvier; even unmyelinated axons are in association with Schwann cells Conduction velocity: the presence of myelin allowed greatly accelerated conduction velocities of nerve impulses,.5 to 2 m/sec in unmyelinated fibers and 3 to 15 m/sec in myelinated fibers of the same size, larger fibers have faster conduction velocities regardless of whether or not they have myelin, large myelinated fibers can conduct impulses up to 120 m/sec Nerve regeneration: in peripheral nerves as long as the myelin sheath is still intact the axon can regenerate and reinnervate its target, not possible with oligodendrites in CNS because the sheath exists as extentions of the oligodendrite that are withdrawn after the axon is severed and so the sheath disappears  2 Electrophysiology: potentials, current, polarization Resting Membrane Potential: bioelectricity carried by ions; K+ has the greatest effect because it is the most permeable; equilibrium potential – the potential at which the force exerted by concentration gradient and electrical gradient across a membrane are equal so that there is no net movement of ions across the membrane, potassium equilibrium potential is -90 mv; RMP is usually at -70 mv because it is a composite of the equilibrium potentials for all the ions dissolved in the intra and extracellular fluids; leakage currents occur and they must be addressed by the Na+/K+ pump, 70% of energy used by the nervous system is for the operation of the pump Local potentials: neurons can be stimulated by light, chemicals, mechanically, heat, or voltage changes; Ligand gated channels – ion channels that have a gate on them that is closed except in the presence of a particular molecule or condition; when the membrane potential inside a nerve cell becomes less negative (more positive) it is depolarized, if it becomes more negative (less positive) it is hyperpolarized, generally depolarization means bringing a nerve cell closer to excitation, and hyperpolarization brings a nerve cell farther away from excitation Characteristics of local potentials -graded – they very in magnitude -decremental – they decrease in size from their source -reversible – membrane potential can quickly return to normal -excitatory or inhibitory – they can be depolarizations or hyperpolarizations Action Potentials: very rapid, large change in the membrane potential caused by the activation of voltage-gated ion channels -local potential reaches axon hillock -local potential depolarizes axon hillock above threshold -this opens both Na+ and K+ channels, but Na+ open more quickly and the more sodium comes in the more channels are opened causing a very rapid depolarization towards the sodium equilibrium potential -as the membrane potential depolarizes the sodium gates begin to close and at about +35 mv they are all deactivated -at this point the K+ gates are completely opened and they allow K+ to leave the cell causing the membrane potential to repolarize -the potassium gates are slow to close and so they don’t completely close until the cell is slightly hyperpolarized -once they are completely closed the membrane potential goes back to normal Properties of AP -all-or-none: the AP happens completely or not at all -non-decremental: the size of the AP does not change as it is propagated -irreversible: once an AP starts it cannot be stopped  3 Refractory periods: -absolute: when the Na+ gates are fully open or deactivated another AP cannot be generated -relative: the Na+ gates are reactivated, but the slow closing of the K+ gates means that the cell is hyperpolarized so that a larger than normal stimulation will be required to bring the cell to threshold again Synapses -electrical and chemical -directional: presynaptic neuron to postsynaptic neuron -types: axodendritic, axosomatic, axoaxonic -anatomy of a chemical synapse: presynaptic membrane, vesicles, synaptic clefts, receptors on postsynaptic membrane -neurotransmitters: small molecules that are synthesized by the presynaptic cell, released into the synaptic cleft by the presynaptic cell, bind to receptors on the postsynaptic cell, and alter the physiology of the postsynaptic cell -acetylcholine; amino acids; monoamines; neuropeptides -neuromodulators – act as nervous system paracrines -gases: nitric oxide -effect of a neurotransmitter on a postsynaptic cell is determined by the type of receptor that it binds to on the posynaptic membrane Examples of three types of chemical synaptic transmission -excitatory cholinergic: AP in synaptic bulb leads to opening of calcium channels, increased calcium leads to the merging of synaptic vesicles with the presynaptic membrane, Ach diffuses across the synaptic cleft, Ach binds to receptors on postsynaptic membrane, sodium channels are opened, excitatory postsynaptic potential or EPSP is produced, Ach is destroyed by acetylcholinesterase -inhibitory GABA-ergic: same as excitatory cholinergic, but neurotransmitter is GABA, and the receptors are associated with potassium ion channels, so the postsynaptic membrane is hyperpolarized, this is an inhibitory postsynaptic potential or IPSP -excitatory adrenergic: same process happens in presynaptic membrane, but neurotransmitter is norepinephine, and receptor is associated with a G protein, the binding of the neurotransmitter to the receptor causes the dissociation with the G protein, the G protein activates adenylate cyclase, which produces cyclic AMP from ATP, the cyclic AMP is a second messenger that can have several different effects, it can attached to ligand-gated ion channels and open them, it can activate enzymes that can conduct reactions, or it induce transcription of genes; advantage is enzyme amplification -synaptic delay: the time between arrival of the AP at the presynaptic bulb and the response of the postsynaptic cell  4 Cessation of a chemical synaptic signal -diffusion of the neurotransmitter away from the synaptic cleft -reuptake of the neurotransmitter into the presynaptic cell -enzymatic degradation of the neurotransmitter in the synaptic cleft Neural Integration: ability of neurons to process information, store it, recall it, and make decisions -postsynaptic potentials are always going on in dendrites and somas, these are added together or integrated at the axon hillock and if the EPSPS outweigh the IPSPs then the cell will fire, if not the cell will remain quiescent -postsynaptic potentials can summate or add together -temporal: one presynaptic neuron that cause rapid multiple PSPs in postsynaptic neuron -spatial: more than one presynaptic neuron firing at the same time -facilitation is when one neuron enhances the effect of another neuron -presynaptic inhibition: an axoaxonic synapse can fire an IPSP onto a synaptic bulb of another neuron and blocks its calcium channels, so when the AP reaches the synaptic bulb no calcium flows in and so no neurotransmitter is released Neural Coding: the way in which the nervous system converts information into a meaningful pattern of action potentials -interpretation of sensory modality or type through labeled lines, nerves carry information of a specific type and it goes only to a specific place -interpretation of intensity uses two mechanisms -recruitment – a weak stimulus will only stimulate a few neurons, a stronger stimulus will stimulate a greater number -frequency – a weak stimulus will generate a small number of APs per second while a stronger one will generate a larger number of APs per second, or a higher frequency Neural codes and circuits: ensembles of neurons that have a specific function -input neurons to discharge zones and facilitated zones -four circuits -diverging – command neuron -converging – integration for decision making -reverberating – for repetitive tasks and short-term memory -parallel after-discharge – brief stimulation leads to longer lasting effect Memory and synaptic plasticity: a memory is a pathway through the brain called a memory trace or engram; synapses in brain are constantly being remodeled, some being made stronger, some weaker, and some new ones are made; the process of making synaptic transmission easier is called synaptic potentiation -types of memory -immediate – holding something in your mind for only a few seconds -short-term – holding something for a few seconds to a few hours; working memory is a type that allows us to remember something just long enough perform a function; tetanic stimulation is the facilitation of a synapse by rapid stimulation so that some of the  5 calcium that enters because of one stimulation is still in the cell when the next stimulation occurs; sometimes tetanic stimulation can lead to posttetanic potentiation, and that is when the calcium remains high in the cell for hours and so a further stimulation will still result in the release of more neurotransmitter than normal -long-term – two types -declarative – retention of events, facts that you can put into words -procedural – retention of motor skills -can be the result of the remodeling of synapses and the formation of new ones, dendritic spines can occur on cells that increase the surface area for synapses -long-term potentiation – involves NMDA receptors that bind glutamic acid, if a cell is under tetanic stimulation and stimulation of NMDA receptors then calcium channels are opened in the postsynaptic membrane that will lead to more NMDA receptors, remodeling of synapses, release of nitric oxide that increase glutamate release; these thing will lead to a long-term change in the postsynaptic cells so that synapses will have a larger effect on the postsynaptic cells than before, that is, the synapses will have been facilitated for a long period of time