12.4: Electrophysiology of Neurons

Neuron Doctrine

The nervous pathway was not a continual “wire” or tube, but a series of separate cells separated by synapses

Nerve Growth Factor (NGF)

Protein secreted by a gland, muscle, or glial cells and picked up by the axon terminals of neurons; prevents apoptosis in growing neurons; enables growing neurons to make contact with targets (important for nerve regeneration)

Electrophysiology

The study of cellular mechanisms for producing electrical signals; basis for neural communication and muscle contractions

Electrical Potential

A difference in the concentration of charged particles between one point and another; potential energy measured in volts; ability to cause a current

Current

Electrical flow of charged particles from one point to the other; created in the body by the flow of ions (ex: Na+/K+ pumps through channels in the plasma membrane (gated, so we can “turn current on and off”))

Polarized

Has electrical potential; living cells

Resting Membrane Potential

The charge difference across the plasma membrane;( 70 mV “resting” neuron) (Negative = more negative charged particles on the inside of the membrane than on the outside); diffusion of ions down their concentration gradient through the plasma membrane; selective permeability of plasma membrane; electrical attraction of cations and anions to each other

Na++/K+ Pump influence in RMP

K+ greatest influence on RMP; high permeability; (plasma membrane); diffuses down concentrate gradient (with other inclusions/anions unable to exit cell) towards ECF > ICF increase negative > increase attraction of positive ions (K+) > K+ comes back into cell > equilibrium (eq. rates of K+ going out of cell and coming back into cell) > net diffusion stops. (K+ 40 times more conc. in ICF than ECF); ~ 90 ml/; Na++ is 12 times more conc. in ECF than ICF which helps to balance disparity; Na+ diffuses down concentration gradient (less permeable than K+) into cell (attributed by negative charge –ICF) and K+ leaks out (Na+/K+ pump) (3NA+:2K+); ATP consumed energy time Na/K+ pump used, all the time, even at “rest” (getting ready for action); ~70T of ATP in body is used for this purpose; high glucose and oxygen demand too

Depolarization

Shift in the voltage across the membrane to a less negative value

Hyperpolarization

Shift in the voltage across the membrane to a more negative value

Local Potential

Change in membrane potential at and nearby point of stimulation; stimulated by chemicals, light, heat or mechanical distortion of plasma membrane; general MOA: starts at dendrite then spreads throughout soma, into the axon then finally into synaptic knob; special MOA: stimulate binds to receptors or neuron (ex: ligand regulated Na+ gates), gates opens and Na+ comes into cell, neutralizing negative charge and mV comes toward zero (depolarization when voltage shifts to a less negative value), Na+ produces a current right under plasma membrane, traveling from stimulant point to cell’s trigger zone (short range change in voltage is local potential)

Graded Property

They vary in magnitude (voltage) according to the strength of the stimulus; increase intense or prolonged stimulus = more gates open (ex: increase Na+ into cell and Increase voltage change)

Decremental Property

They get weaker as they spread from the point of stimulation; the farther the stimulant (positive “voltage” current) has to travel, the weaker the effect or the faster the effect is back to resting state (because the faster it is reversed Na+/K+ pump)

Reversible Property

If stimulation ceases, membrane voltage quickly returns to normal resting potential (K+ diffuses out of the cell quickly returns the membrane voltage to its resting potential)

Excitability or Inhibitory Property

(E) Depolarize a cell and make a neuron more likely to produce an action potential; ex: acetylcholine (neurotransmitter).

(I) hyperpolarize a cell and make a neuron more negative, the neuron is less sensitive and less likely to produce an action potential; ex: Glycine (neurotransmitter)

Action Potential

A more dramatic change (vs. local potential) produced by voltage regulated ion gates in the plasma membrane; need high density of voltage regulated gates; rapid up and down shift in membrane potential; (depolarize) Na+ into axon hillock until threshold (-55 mv) is met, neuron “fires” and action potential (spike) is produced, positive feed back produced with Na+ gated channels; more and more Na+ gated channels open and let Na+ in (K+ gated channels are slowly opening), OmV is reached (via Na+ gated channels being opened), K+ gated channels continue to open and Na+ gated channels start to close, +35 mV is (the avenue) reached by the time all Na+ gated channels now are fully opened (repolarize), until RMP is reached (-70 mV) or possible overshoot (hyperpolarize)

All or None Law

A neuron either produce an action potential of maximum strength if it is depolarized too above threshold, or produce no action potential at all if the stimulant is not strong enough to reach the threshold; there are no action potential if intermediate strength (not graded/proportional to strength)

Nondecremental

Does not get weaker with distance

Irreversible

Action potential goes to completion, if threshold is met

Refractory Period

period of resistance to restimulation; during an action potential and a few milliseconds after; only referring to a small patch of membrane where an action potential has already begun, not to the entire neuron

Absolute Refractory Period

No stimulus of any strength will trigger a new action potential; caused by inactivation of voltage gated Na+ channels

Relative Refractory Period

A stronger stimulus is needed to trigger a new action potential; during hyper polarization, a larger depolarization is needed

Unmyelinated Fiber Signal Conduction

(Continuous) Axon has voltage gated channels along the entire length; action potential at trigger zone causes Na+ to enter axon and diffuse into adjacent regions; depolarization opens the channels; opening the channels results in a new action potential which allows Na+ diffusion to excite membrane immediately distal to that; chain-like continues down the axon (wave of falling dominoes)

Myelinated Fiber Signal Conduction

(Saltatory) Nodes of Ranvier are the only spot with Na+/K+ pumps; electrical signals jump from node to node; signal moves faster through insulated (myelinated) segments and slows down in bare sections; ions are heavily concentrated at the nodes and myelin covered areas contain very few; electrical signal must spread passively between nodes and once it reaches a node, it is still strong enough to depolarize the membrane to threshold, causing new voltage gated Na+ channel to open and new full strength action potential to occur