Neurophysiology. I. Background. A. Cell Types 1. Neurons 2. Glia. B. Subtypes 1. Differ based on their structure, chemistry and function

Transcription

1 Neurophysiology I. Background A. Cell Types 1. Neurons 2. Glia B. Subtypes 1. Differ based on their structure, chemistry and function C. Relative distribution billion neurons (give or take 100 million) times as many glia as neurons D. Functional significance 1. Neurons confer the unique functions of the nervous system II. Cellular Structure of Neurons A. Neurons contain the same basic structures as most other cells B. Structure of animal cells

2 1. Cell body (soma) a. 20 um in diameter b. Surrounded by a membrane that separates the inside of the cell from the outside i. 5 nm thick 2. Cellular contents a. Everything within the cell membrane other than the nucleus is the considered cytoplasm 3. Nucleus a. Contain the chromosomes that confer the heritable material DNA b. Gene expression i. DNA to Protein DNA (transcription) mrna (translation) Protein 3. Ribosomes a. Site where protein is made 4. Endoplasmic reticulum a. Rough i. Have ribosomes b. Smooth i. Transport completed protein to other cellular sites 5. Mitochondria a. Site where metabolic functions are performed 6. Golgi apparatus a. Post-translational modification of proteins 7. Neuronal membrane a. Cannot understand the function of the brain without understanding the structure and function of the membrane and its associated proteins C. Unique features of neurons 1. Morphological regions a. Cell body (soma or perikaryon) b. Neurites 2. Types of neurites a. Axons

3 b. Dendrites 3. Axons a. Cell body usually gives rise to a single axon i. Conducts nerve impulse from one neuron to the next Up to 1 meter in length i Speed of the nerve impulse is a function of the diameter of the axon 4. Dendrites a. Small i. Rarely more than 2mm b. Organized symmetrically i. Antennae c. Dendritic tree i. Collective term for all neurites of a given neuron D. Neural signals 1. Efferent a. Away from the cell body 2. Afferent b. Towards the cell body E. Synapse

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5 1. Site of neurotransduction a. Electrical to chemical signal 2. Structural elements a. Axon terminal i. Site where axon comes in contact with another neuron b. Presynaptic terminal c. Postsynaptic terminal i. Usually found on dendrite d. Cleft i. Space between the two sides of a synapse 3. Synaptic transmission a. Process by which information is transferred from one side of the synapse to the other b. Most adult vertebrate synapses are chemical c. Electrical impulse that travels down the axon is converted to a chemical message 4. Neurotransmitter a. Chemical signal b. Different neurons use different types of neurotransmitters 5. Receptor a. Specialized proteins responsible for detecting neurotransmitters b. Involved in transduction of signal III. Non-Neuronal Cells A. Glia 1. Support neuronal function 2. Types a. Astrocytes i. Regulate extracellular space Remove neurotransmitters, restrict movement of neurotransmitter from synapse, etc. b. Oligodendrocytes (Schwann Cells) i. Myelinating glia Wrap around the axons i Insulation iv. Myelin sheath (what holds a sword) v. Node of Ranvier: where the myelin sheath is interrupted IV. Functional Activity of Neurons A. Electrical current created by the movement of ions 1. Properties of ions differ from those of electrons a. Free electrons and more nearly at the speed of light b. Electrons are good conductors and the air surrounding a wire is not c. Ions in the cytosol of the nerve cell are less conductive than electrons d. Fluid around neurons is also a conductor 2. Membranes are leaky a. Current moving down an axon leaves passively i. Like water in a leaky hose 3. Active process is needed to overcome passive current flow from neuron a. Action potential

6 B. Properties of action potentials 1. Do not diminish 2. Fixed in size and duration (independent of the amount of current that evokes it) 3. All or nothing C. Action potentials occur because of the properties of the neuronal membrane 1. Neuronal membrane is excitable D. Functional states of a neuron 1. Rest a. Neurons do not fire continuously b. When not generating action potentials, neurons are at rest c. Cytosol along the inside of the membrane has a negative charge relative to the outside 2. Resting membrane potential a. Difference in the electrical charge across the membrane i. Difference is always negative Can be measured using an intracellular microelectrode 3. Action potential a. Brief reversal of the resting membrane potential b. Electrical signal created during action potential generation is the basic information unit of the nervous system i. Binary code (actually analogue) c. Result from the flow of current across the membrane i. Current is supplied by other neurons 4. Current plot a. Potential x time i. Hyperpolarization Depolarization

7 i Threshold V. Properties that Make Action Potentials Possible A. Three questions 1. How does the neuronal membrane at rest separate electrical charge? 2. How is this charge rapidly redistributed across the membrane during an action potential? 3. How does the impulse (action potential) travel reliably down the axon? (Properties that make it possible for a neuron to separate charge when at rest are the same factors that allow action potentials to occur and for that impulse to be propagated along the axon.) B. Resting membrane potential 1. Important considerations a. Nature of the fluids on the two sides of the membrane b. Structure of the neuronal membrane c. Proteins that span the membrane C. Cytosol and extracellular fluid 1. Fluids are aqueous a. Water distribute charges unevenly i. Oxygen attracts more negative charge than hydrogen b. Water is held together by polar covalent bonds i. An effective solvent for charged molecules 2. Ions a. An atom or molecule with a net electrical charge b. Types i. Cation (+) Anion (-) 3. Ionic bond a. Molecule held together by the electrical attraction of oppositely charged atoms 4. Charged portion of water has a greater attraction for the ions than they have for each other a. Ionic bond is broken D. Phospholipid membrane 1. Terms a. Hydrophilic: water loving i. Polar compounds and ions 2. Hydrophobic: water fearing a. Nonpolar covalent bonds i. Do not interact with water 3. Lipids a. Water insoluble biological molecule 4. Phospholipid bilayer a. Tail i. Long chain of carbons Nonpolar b. Head i. Polar end Comprised of P plus 3 O's

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9 5. Functional consequence a. Tails arrange themselves in a bilayer i. Tails do not like water Tails are inside i Heads are outside E. Proteins associated with the membrane 1. Background a. Proteins are the product of gene expression b. Type and distribution of protein molecule distinguish neurons from other cells c. Resting and action potentials depend on the special proteins that span the lipid bilayer d. Protein chemistry i. Primary structure: aa chain Secondary structure: certain types of organizations such as helices and sheets result when certain aa's are combined in the primary structure i Tertiary structure: individual protein molecules can fold and form a more highly organized structure (e.g. globule) iv. Quaternary structure: when different polypeptides combine to for a larger molecule 2. Ion channels

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11 a. Number of individual protein molecules organized to create a pore in the membrane i. Membrane spanning protein b. Diameter of the pore limits what can pass through the channel c. Selectivity is also conferred by the nature of the amino acids that line the inside of the pore i. Positively charged amino acids will attract negatively charged ions Negatively charged amino acids will attract positively charged ions d. Gating i. Unique micro-environmental conditions that alter the selectivity of an ion channel changes (e.g., when voltage changes) Only when the membrane is within a particular voltage range does the channel open e. Function i. Permit and control movement of charged molecules across the neural membrane Movement is selective: size, charge and environmental condition F. Diffusion (One of two primary forces that create resting membrane potentials) 1. Net movement of ions from a higher concentration to a lower concentration 2. Ions will not pass through the membrane a. Can diffuse through ion channels selective for that particular ion 3. Concentration gradient a. Difference in concentration between one side and the other b. Solute will move down its concentration gradient 4. Factors necessary for diffusion of ions across the neuronal membrane a. Ion channel for that ion b. Concentration gradient 5. Ions will flow down a concentration gradient G. Electric field (One of two primary forces that create resting membrane potentials) 1. Ions can also move as a result of an electric field 2. Background a. Opposite charges attract and like charges repel. (Na+ moves towards negative field and Cl- moves towards positive field) b. Anode i. Positive pole of a battery Negative flow to here c. Cathode i. Negative pole of a battery Negative flow away d. Electric current i. Movement of charges e. Electrical potential (voltage) i. Difference in charge between the anode and the cathode Reflects the force exerted on a charged particle f. Electrical conductance i. Ease with which a charged particle can move g. Resistance i. Difficulty with which a charged particle can move 3. Factors necessary for charged particles to move across the neuronal membrane a. Ion channel for that ion

12 b. A potential difference across the membrane Example: K+ of differing concentration separated by a semi-permeable membrane. This difference generates an electrical potential. The side with the higher concentration is negative. If the ions were allowed to freely move, the movement will stop at some point, but not when the concentrations are equal. As positive charges accumulate on one side, the positivity makes it less attractive to positive ions-the potential charge across the membrane offsets the concentration gradient. The point at which this occurs is known as electrochemical equilibrium. This relationship is described by the Nernst equation. In biological systems, there multiple ions involved, each governed by a separate permeability factor. This relationship is described by the Goldman equation. H. Equilibrium state 1. Diffusional and electrical forces are equal and opposite 2. For neurons, when these forces are balanced, the resting membrane potential is negative (see below) Overview: 1. Neuronal membrane acts as a barrier to charges a. Permits generation of concentration gradients b. Permits generation of electric fields 2. Membrane has ion channels that are selective for ions of different ions a. Specific ions can move under particular condition I. Sodium-potassium pump 1. Necessary for the inside of the neuron to become negative relative to the outside of the neuron

13 2. Membrane associated protein a. Transfers ions across the membrane at the expense of metabolic energy i. 70% of all brain energy is consumed by this pump 3. Net movement of ions a. 3 Na+'s from the inside to the outside b. K+'s are moved into the neuron 4. Result a. Both electrical and a concentration gradients are created b. Na+ is greater outside c. K+ is greater inside d. More positive ions outside than inside i. Inside of the neuron is negative relative to the outside J. Control of ionic movement 1. K+ a. K+ wants to move out based on the difference in concentration b. K+ is attracted to the relative negative charge inside the neuron c. Balance of these forces creates the resting potential 2. Na+ a. Na+ wants to move in based on the difference in concentration b. Na+ is attracted to the relative negative charge inside the neuron c. Tightly gated Na+ channels prevent the movement of Na+ d. Channels will not open unless a certain voltage range exists i. Threshold (see below) VI. Action Potentials A. Definition 1. Rapid reversal of the resting potential a. For an instant the inside of the neuron becomes positive relative to the outside B. Voltage versus time plot 1. Terms a. Rising Phase b. Overshoot c. Falling Phase d. Undershoot e. Depolarization i. Less negative f. Threshold i. Critical level of depolarization needed for an AP g. Hyperpolarization i. More negative C. Permeability changes underlie the action potential

14 1. Selective increase in Na+ conductance coincident to the rising phase a. Na+ is responsible to AP initiation b. Positive feedback loop causes increased Na+ conductance c. Na+ conductance slowly activates K+ conductance d. Na+ conductance inactivates (see below) 2. Selective increase in K+ conductance coincident to the falling phase D. Refractory periods 1. Absolute refractory period a. Time period during which it is not possible to generate an AP 2. Relative refractory period a. Time period during which additional depolarizing current is necessary to generate an AP 3. Absolute and relative refractory periods are dependant on the properties of the ion channels that are involved in the AP (see below) E. Initiation of an action potential 1. At rest: a. Na+ channels are closed b. A concentration gradient and an electrical potential exist because of the Na+/K+ pump c. K+ channels are closed but leaky i. Diffusional and electrical forces in balance (K+ wants to stay and leave at the same time) 2. Effect of opening Na+ channels a. Na+ would move down its concentration gradient and towards the negative potential b. Inside of the neuron becomes positive relative to the outside c. Na+ influx accounts for the rising phase of the action potential

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16 F. Falling phase of the action potential 1. Leaky K+ channels open a. K+ leaves by flowing down its concentration gradient, away from the now positive (inside) side of the membrane towards the more negative side of the membrane G. Voltage gated Na+ channels 1. Highly selective for Na+ 2. Opened and closed by changes in the electrical potential of the membrane a. When the resting potential is changes from -65mV to -45mV i. Channels opens b. Channels inactivate (close) spontaneously after approximately 1msec (inactivate) c. Cannot de-inactivate until the neuron returns to its resting membrane potential i. Responsible for the absolute refractory period H. Voltage gated K+ channels 1. Opening is delayed a. Coincides with the closing of the Na+ channels 2. K+ channels do not inactive 3. K+ continues to flow out of the neuron until it reaches its ionic equilibrium 4. Voltage inside the neuron will briefly be hyperpolarized a. Less negative than the resting potential b. Relative resting membrane potential i. Additional current (more depolarizing current) would be required to reach threshold Neurotransmission I. Background A. Sequence of events 1. Action potential generation 2. Propagation of action potential along axon 3. Intra-neuron communication II. Propagation of Action Potential

17 A. Active process is required 1. Current not sufficient to generate an action potential is passively conducted 2. Current leaks across the axonal membrane a. Magnitude of the voltage change decays i. Exponential decay Decay increasing distance from the site that the current was introduced 3. Leakiness of the axonal membrane prevents effective passive transmission B. Action potential occurs without decrement along the entire length of the axon 1. Action potential propagation is not passive 2. Action potentials have conduction velocity a. Occurrence time differs as a function of distance from stimulation site C. Mechanism involves the passive spread of current 1. Current created by inward movement of Na+ associated with action potential 2. Depolarizing stimuli (see below) locally depolarize the axon a. Open voltage-gated Na+ channels b. Cause the influx of Na+ 3. Current flows passively down the axon a. Depolarizes adjacent areas of the axon i. Opens Na+ channels in those areas D. Action potential can only propagate away from the source of the depolarizing current 1. Na+ channels inactivate 2. Do not "deinactivate" until the membrane returns to resting membrane potentials E. Process 1. Na+ channels open in response to stimulus a. Action potential at that site 2. Depolarizing current passively flows down the axon 3. Local depolarization causes adjacent Na+ channels to open and generate an action potential a. Upstream Na+ channels inactivate b. K+ channels open i. Membrane repolarizes Membrane is refractory 4. Process is repeated in neighboring segment a. Impulse is propagated F. Site of action potential in vivo 1. Axon 2. Axon hillock a. Small part of the soma where the axon originates 3. Function of the density of Na+ and K+ channels Fuse Example: 1. Strike a match and light the fuse, like reaching threshold 2. As a fuse burns, it ignites the combustible material just ahead 3. It burns only in one direction

18 III. Conductance Velocity A. Factors affecting velocity 1. Axon diameter a. Direct relationship i. Increase diameter, increase velocity b. Physiologically limiting 2. Saltatory conduction B. Saltatory conduction 1. Myelin a. Function as insulation i. Promotes movement of current down the axon Equivalent to increasing the thickness of the axonal membrane 100 fold i Reduces membrane capacitance iv. Rate of passive spread is inversely proportionate to membrane capacitance v. Distance that the current spreads down the inside of the axon and causes an AP is enhanced by myelin b. Produced by glia i. Schwann cells in the periphery Oligodendrocytes in the CNS c. Nodes of Ranvier i. Intermittent breaks in the myelin Site of action potential regeneration 2. Time for an action potential to occur is rate limiting a. Eliminate action potentials b. Impulse travels faster

19 IV. Events at Synapse A. Background 1. Types of synapses a. Electrical i. Rare in adult mammalian NS Gap junction i Current flows directly through a specialized protein molecule connexon iv. Distance between the two sides of the membrane is very small (5nM) b. Chemical i. Predominant type 2. Terminology a. Neurotransmitter i. Chemical used to communicate with the postsynaptic membrane b. Active zone i. Site of neurotransmitter release c. Postsynaptic density i. Contain neurotransmitter receptors Intercellular chemical messages converted into intracellular signal i Occurs in the postsynaptic cell d. Neurotransmitter receptor i. Specialized protein molecules that bind the chemical signal Transduces chemical signal into an intracellular message i Nature of response depends on receptor type e. Synaptic vesicle i. Membrane spheres containing neurotransmitter B. Events at chemical synapse

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21 1. Neurotransmitters are synthesized and stored in synaptic vesicles a. Takes place in the golgi apparatus b. Transported via secretory granules 2. Action Potential arrives at the axon terminal a. Opens a voltage gated Ca 2+ channel 3. Intracellular Ca 2+ concentrations signals the neurotransmitter to be released a. Exocytosis i. Process by which vesicles release their contents b. Vesicles fuse with the active zone c. Not known how Ca 2+ acts as the signal 4. Neurotransmitter diffuses across the synaptic cleft a. Binds to its receptor on the postsynaptic membrane b. Postsynaptic action depends on the nature of the receptor i. Events are summed over time and space (see below) 5. Neurotransmitter inactivation a. Information in the brain is based primarily on the frequency of the signal (#/sec) b. The magnitude of the postsynaptic response needs to be in proportion to the presynaptic signal i. Preserves the integrity of the message Chemical message must be controlled c. NT must be inactivated i. Degradation Reuptake i Diffusion iv. Bioconversion V. Neurochemistry A. Background 1. Neurons in the human brain communicate primarily by the release of small quantities of chemical messenger a. Neurotransmitters i. Interact with receptors on neuronal surfaces Alter the electrical properties of neurons B. Information transfer occurs at synapses 1. Most synapses use chemical messages released from presynaptic axonic terminals a. Released in response to depolarization of the terminal b. Messages diffuse across the synaptic cleft c. Bind with specialized receptors that span the postsynaptic membrane d. Receptor binding of the chemical messages alters neuronal function i. Electrical Biochemical i Genetic C. Chemical communication in the human brain depends on: 1. Nature of the presynaptically released chemical message 2. Type of postsynaptic receptor to which it binds 3. Mechanism that couples receptors to effector systems in the target cell

22 D. Nature of chemical messages 1. Criteria for classification as a neurotransmitter a. Molecule must be synthesized and stored in the presynaptic neuron b. Molecule must be released by the presynaptic neuron upon stimulation c. Application of the neurotransmitter directly to the target cell must be shown to produce the same effects as the response produced by the release of the neurotransmitter from the presynaptic neuron 2. Few chemical substances meet these criteria E. Classification 1. Size a. Neuropeptides: 3-30 aa's (e.g., met-enkephalin) b. Small molecule neurotransmitters i. Individual amino acids (glutamate, aspartate, GABA, glycine, acetycholine) Biogenic amines (dopamine, norepinephrine, epinephrine, serotonin) 2. Neurons that use particular neurotransmitters a. Cholinergic b. Catecholinergic c. Serotonergic d. Amino acidergic e. Other (neuropeptides, NO, etc.) 3. Many neurons release more than a single neurotransmitter a. Dufferential release is base on the conditions that exist F. Cholinergic neurons 1. Utilize acetylcholine (Ach-"vagus substance") as their neurotransmitter. a. ACh is the neurotransmitter for: i. Neuromuscular junction Preganglionic neurons of the sympathetic and parasympathetic PNS i Postganglionic neuron of the parasympathetic PNS iv. Basal forebrain and brain stem complexes b. ACh is synthesized from acetyl coenzyme A and choline i. Reaction is catalyzed by CAT-choline acetyl transferase c. ACh is degraded in the synaptic cleft by acetylcholinesterase G. Catecholaminergic neurons 1. Types a. Dopamine b. Norepinephrine c. Epinephrine 2. Synthesized from the amino acid tyrosine a. Each has a catechol group 3. Inactivated by reuptake a. Substances that block their reuptake, prolong their activity i. Cocaine Amphetamine H. Serotonergic (serotonin) neurons 1. 5-hydroxytryptamine, commonly referred to as 5-HT 2. Inactivated by reuptake 3. 5-HTergic neurons appear to play a role in the brain systems that regulate mood, emotional behavior, and sleep

23 a. Compounds like Prosak (SSRI) i. Block reuptake Prolong activity in the synapse I. Diffuse neuromodulatory system 1. Catecholaminergic and serotonergic neurons 2. Modulate large numbers of neurons a. Spread diffusely throughout the nervous system 3. Use similar effector systems (see below) 4. Commonalities a. Cell bodies for these neurons are localized to small populations of cell in the brain stem b. Each neuron can influence many others i. Each axon makes a 100,000 or more synapses widely spread across the brain c. Synapses are designed to release the neurotransmitter into the extracellular fluid i. Allows the NT to spread and affect many neurons 5. Sites of dopamine action a. System I i. Cells bodies in the substantia nigra Regulates movement b. System II i. Cell bodies in the ventral tegmental area (VTA) Involved in reinforcement 6. Sites of serotonin action a. Cell bodies are in the raphe nuclei b. Involved in sleep, mood and emotional behavior 7. Sites of norepinephrine action a. Cell bodies are in the locus coeruleus b. Makes the most diffuse contacts of any neurons in the CNS i. A single neuron can make 250,000 synaptic contacts in the cerebrum Have a second axon making another 250,000 contacts in the cerebellum c. Contacts are non-specific i. General regulation of brain activity Activity is coincident to state of CNS 8. Site of acetylcholine action a. Basal forebrain b. Neuromuscular junction (PNS)

24 J. Amino acidergic neurons 1. Types a. Glutamate (Glu) b. Glycine (Gly) c. Gamma-aminobutyric acid (GABA) 2. Serve as the neurotransmitters at most CNS synapses a. Glutamate is the primary excitatory neurotransmitter b. GABA is the principle inhibitory neurotransmitter VI. Transduction of Chemical Signals A. Background 1. Chemical messengers are released from the presynaptic terminal in response to an impulse traveling down the axon a. Impulse is a unit of information 2. Information needs to be transferred to the postsynaptic neuron 3. Process of transferring information to the postsynaptic neuron is transduction a. Neurotransmitter binds with a specific receptor protein in the postsynaptic membrane that uniquely identifies the NT 4. A limited number of chemicals that serve as NT's a. NT's have multiple receptors (sub-types) that bind them (The binding of the NT by the receptor is like inserting a key into a lock; if it is the correct key, it will cause conformational changes in the protein.) B. Two major classes of receptors 1. Ligand-Gated Ion Channels (Ionotropic) 2. G-Protein-Coupled Receptors (Metabotropic) C. Classification based on speed of chemical synaptic transmission 1. Types a. Fast signal transduction b. Slow signal transduction 2. Factors affecting speed a. Diffusion of the chemical message across the synaptic cleft and bind with the receptor b. The time it takes for the receptor to transduce the chemical signal into a functional change in the postsynaptic neuron 3. Fast neurotransmission

25 a. Postsynaptic receptor is a transmitter-gated ion channel i. Ion channels function much more rapidly than G-proteins b. Rate limiting step is time of diffusion i. Therefore rapid (2-5 msec) 4. Slow neurotransmission b. Postsynaptic receptor is a G-protein-coupled receptor i. Rate limiting step is time for G-proteins to elicit their effect 100 s of msec to days D. Ligand-gated ion channels 1. Membrane spanning proteins that form a pore a. Pore is closed b. NT binds to the receptor i. Receptor undergoes a conformational change Pores open i Ions can now pass through (i.e., generate current) 2. Postsynaptic potentials a. Excitatory postsynaptic potentials (EPSPs) i. Bring the membrane potential toward threshold (depolarize) Cations in or anions out b. Inhibitory postsynaptic potentials (IPSPs) i. Move membrane potential away from threshold (hyperpolarize) Anions in or cations out 3. Effects are transient a. EPSP's and IPSP's can be summed temporally and/or spatially i. Effects be additive or subtractive b. When enough EPSP's are summed: i. Threshold is reached Action potential results 4. NT has a direct effect on receptor a. Binding of the NT opens an ion channel b. Causes a direct change in the membrane potential E. G-Protein-coupled receptors 1. Process a. NT is bound to a postsynaptic receptor b. Receptor proteins activate small protein molecules, called G-Proteins i. Found inside the postsynaptic neuron c. G-Protein activates an "effector" molecule 2. Types of effector proteins a. Ion channels in the membrane b. Enzymes that synthesize second messengers i. 2nd messengers can activate other enzymes in the cytosol Enzymatic action regulates ion channel function and alter cellular metabolism 3. Receptors linked to G-Proteins are referred to as Metabotropic Receptors a. Can have widespread metabolic effects