* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Neuron Function
Multielectrode array wikipedia , lookup
Synaptic gating wikipedia , lookup
Nervous system network models wikipedia , lookup
Chemical synapse wikipedia , lookup
Signal transduction wikipedia , lookup
Patch clamp wikipedia , lookup
Neuropsychopharmacology wikipedia , lookup
Nonsynaptic plasticity wikipedia , lookup
Node of Ranvier wikipedia , lookup
Biological neuron model wikipedia , lookup
Single-unit recording wikipedia , lookup
Channelrhodopsin wikipedia , lookup
Action potential wikipedia , lookup
Electrophysiology wikipedia , lookup
G protein-gated ion channel wikipedia , lookup
Molecular neuroscience wikipedia , lookup
Stimulus (physiology) wikipedia , lookup
End-plate potential wikipedia , lookup
Neuron Function Electrical properties of Neurons Membrane potential is a fundamental property of essentially all cells There is inherently an excess of positive charge on one side of PM and excess negative charge on the other Cells are more negative inside and more positive outside Resting membrane potential The electrical potential that exists between inside and outside Describe as the cell having a negative resting potential Measured in millivolts (mV) Place an electrode into cell Place second electrode outside cell Electric Excitability Cells of the body that have this Nerve cells Muscle cells Islet cells of pancreas Certain types of stimuli trigger Rapid sequence of changes in membrane potential This rapid sequence is called action potential Action Potential changes Electrical changes Changes from negative values to positive values Changes back from positive to negative Time it takes Occurs in a little over millisecond Rapid change allows quick communication between cells located at times great distances from one another Source of Resting Potential Cytosol and extracellular fluid Contain different complement of cations and anions Are different in overall composition Extracellular fluids Watery solution of salts NaCl and KCl Cytosol [K+] over [Na+] Anions of macromolecules Proteins/RNA Basic Physical Principles Diffusion - all substances tend to diffuse from high concentration to area of lower concentration Electroneutrality - ions in solution are always present in pairs + with - (this is necessary to balance the charges Tendency of oppositely charge ions to flow back toward each other is called potential or voltage Basis of Concentration Gradients Sodium Potassium ATPase pump - is present in all eukaryotic cells Ratio or Stoichiometry for the pump 3 Na+ ions pumped out 2 K+ ions pumped in One ATP hydrolyzed Na+/K+ ATPase Pump Two subunits Alpha (a) subunits do the pumping Beta (b) subunits are glycoproteins anchoring the complex Next slide for mechanism of pumping Na+/K+ ATPase action 1 Several conformations are possible for asubunits As the shape changes the protein complex opens alternatively to inside / outside of cell. Affinities for Na+ and K+ vary also This mechanism is an ex. of antiport Na+/K+ATPase action 2 Na+/K+ ATPase sequence Open towards cytosol, a -subunits have high affinity for Na+ ions Once 3 Na+ bind ATP phosphorylates a subunits causing conformational change opening to the environmental face At same time a -subunits loose affinity for Na+ and it diffuses out to the environment Na+/K+ ATPase sequence2 K+ affinity is increased in Phosphorylated form 2 K+ bind and this causes an increased rate of hydrolysis of PO4-2 from a. Release of causes conformational shift of a -subunits re-opening them to cytosol and releasing 2 K+ Electrical Excitability The resting membrane potential is characteristic of all eukaryotic cells Electrical excitability is characteristic of only some cells This is due to the response of these cells to membrane depolarization These cells are electrically excitable because of the presence of particular types of ion channels Ion channels 1 These are integral membrane proteins that are capable of forming ion conductive channels through the lipid bilayer of PM Channels are generally classified by the kind of ion they conduct Sodium channels Potassium channels Chloride channels Ion Channels 2 Influence rate, but not the direction of ion flow Common structural motif a-helices pass through PM Hydrophilic residues toward interior of channel Hydrophobic residues toward lipids of PM Typical channel has six a-helical passes through PM Ion channels 3 Controlling the opening and closing is called gating Channels differ in the stimulus that causes them to open and how long they stay open Voltage gated channels - respond to specific voltage changes across the PM; imp in AP Ligand gated channels - open when particular molecules bind to the channel; imp in chemical communication between neurons across the synapse Structure and Function of voltage gated channels Voltage gated potassium channels Multimeric proteins- formed by the interaction of four separate protein subunits When joined in the membrane these form a pore for K+ ions Voltage gated sodium channels One large protein - with four separate domains Each domain similar to K+ gate subunits Common Features of Voltage Gates Both kinds of channels Domains are subunits are made up of six transmembrane a-helices One of the a -helices has charged amino acids important in acting as a voltage sensor Changes in voltage across the membrane cause these amino acids to shift The changes lead to opening The changes lead to closing Gated channels Are specific for a single ion Experience an all or none phenomenon Open channels conduct ions at maximum rate Closed channels do not conduct ions Channel inactivation Channel closes in a way that does not allow it to open again right away even if stimulated Action Potentials Electrical changes that occur when an action potential is generated are shown in The squid axon is the experimental model for early studies Human axon has slightly different potentials Graph of Action Potential Resting Neuron First the resting neuron has to be stimulated Depolarization causes the membrane potential to shift in a more positive direction Most stimuli that have excitatory potential cause leakage of Na+ ions from the extracellular space into the cell Threshold potential If the depolarization is small (less than 20 mV the resting membrane potential is reestablished If the depolarization is greater than about 20 mV the cell reaches the threshold potential At threshold potential the neuron commits to an action potential Action potential changes A rapid swing in membrane potential in the positive direction is observed to about 40 mV (35 mV in human). This is followed by another rather rapid swing in the negative direction to a hyperpolarized -75 mV (-80 mV in human) Then the resting potential is reestablished The ion movements of AP The stimulus is usually bound to the leakage of Na+ ions into the cell through ligand gated channels (best example neurotransmitters cause this to happen) Once this graded response reaches the threshold potential an AP is engaged The rapid phase of depolarization occurs when voltage gated Na+ channels open The ion movements of AP 2 Repolarization At the apex of the AP current changes Na+ voltage gates slam shut K + voltage gates swing open K + is powered out of the cell by two forces It runs down its concentration gradient It is repelled strongly by the excess of positive charge in the cytoplasm (remember Na+ ions just came screaming into this space) The K+ ions over do it a little and the membrane becomes hyperpolarized The ion movements of AP 3 Finally the Resting potential is reestablished by the action of the Na+/K+ ATPase pump Keep in mind this pump is running all the time so once the AP has passed it is the natural order for the resting potential to reform There is a refractory period after an AP has passed Graph of Action Potential Y axis is change mV X axis time ms D is Threshold P E is Resting P A Na+ voltage gates open (depolarization) B Na+ voltage gates close Graph of Action Potential 2 B K+ voltage gates open C - repolarization Dip in curve hyperpolarized Action Potentials final Non-myelinated axon AP moves along entire length of axon Myelinated axon (see figure in lab) Saltatory latin for dancing Saltatory AP moves from one Node of Ranvier to the next Speeds up transmission