QUESTION? Communication between neurons depends on the cell membrane Why is this so?? Consider the structure of the membrane. ECF ICF
Possible ANSWERS?? Membrane Ion Channels and Receptors: neuron membranes are peppered w/ diff. ion channels and receptors made of protein embedded in the cell membrane. *Ion channels regulate the movement of ions into & out of the cell. *Receptors on the membrane receive a chemical signal (NT).
Summary of ion channels found on the nerve cell membrane (NT s)
C. The Resting Membrane Potential: The Polarized Cell Resting membrane potential (RMP) the membrane voltage in a resting neuron; it is said to be polarized because the inside(icf) of a neuron is negatively charged w/ respect to the outside(ecf), i.e. the inside charge of 70 mv. Membrane potential (mv) is measured on the inside of the cell + + - + - + - + - - + + - - - + + - + - - - - + + + +
ECF Distribution of Charges on either side of the membrane, i.e. an electrochemical gradient ICF
Body fluid composition - ion conc. Na + K + Cl - Blood ECF ICF HPO 4 2- Proteins(-) Na + K + Cl - HPO 4 2- Proteins(-)
A transfer of potential energy to kinetic energy (work!)
Hydroelectric Dams have potential energy to do work - to make electricity
A polarized cell: * most body cells have a membrane potential which varies from - 40 mv to -90 mv depending on the cell * when ion channels open or close in the cell membrane, ions flow & this changes the membrane potential (mv) [Na + ] [Na + ] [K + ] [K + ] ICF - - - + + + ECF
Ion distribution
ECF + + + - - Axon in X-section @ RMP = -70mV - [amino acids] [PO 4 2- ] [K + ] ICF [Cl - ] [Na + ] RMP = -70mV
ECF + - ICF
ECF ICF
An action potential - one hump-de-dump
B. Nerve impulse - a flow of electrical energy; consists of many depolarizations & repolarizations. *Made of many continuous action potentials. * Lasts ~ 1 millisec. ( ~ 10 1,000 impulses/sec) * During an action potential, 2 voltage gated channels open and then close. 1st - Na + gated channels open & close 2nd - K + gated channels open & close
4 basic Components of an Action potential
Recording of an Action Potential
-55 mv
2. Depolarization - rapid opening of voltage gated Na + channels (-65 mv towards 0mV to +30mV); results in an action potential.
3. Repolarization - slower opening of voltage gated K + channels and closing of Na + channels. The membrane returns to a more negative value (0mV to -70mV).
4. Hyperpolarization - the outward flow of K + during repolarization causes the memb. potential to reach a more negative level ( -90 mv) than that of the RMP (-70 mv); an action potential does not result. -90
Local anesthetic drugs act mainly by inhibiting sodium influx through sodium ion channels in the neuron cell membrane. When the influx of sodium is interrupted, no depolarization, so an action potential cannot occur and signal conduction in pain nerve fibers is inhibited.
Many Action Potentials make up a nerve impulse
Absolute refractory period when a patch of neuron membrane is generating an action potential and it is depolarizing (Na + channels opening & closing), the neuron is incapable of responding to another stimulus; the Na + channels are inactive behind the action potential [for a few msecs.]. This forces one-way transmission of a nerve impulse down the neuron.
Relative refractory period follows the absolute refractory period (during a repolarization). The neuron can be re-stimulated at this time if the stimuli is strong enough to reach -55 mv.
Dendrites Axon terminal Cell body Node of Ranvier Nucleus Axon Hillock Myelin sheath Schwann cell
Point of stimulus Propagation of a Nerve Impulse down the Axon. So, why do nerve impulses travel in one direction?
Summary of ion channels found on the nerve cell membrane Axon hillock (NT s) Remember, and Cl - are found only on the cell body and dendrites, whereas Na + ion channels are found on the dendrites and axon.
Question? So, why do nerve impulses travel in one direction? Answer: Since the sodium ion channels that are behind the action potential are inactive (absolute refractory) they cannot open, thus the action potentials are forced to travel towards where there are sodium ion channels that are not in refractory, i.e. in front of the action potential.
E. Propagation of A Nerve Impulse - nerve impulses, (many depolarizations & repolarizations) self-propagate ; travel along the axonal membrane from cell body to the endbuttons of the neuron. A Domino Effect. (with magic dominos ) - dominos topple-over and then standing back up again in the same order in which they fell. Leading edge of action potential the toppling domino
(Axon hillock) aka. Trigger Zone Propagation of a Nerve Impulse