Essay 3: action potentials

Ap3 (binary/octet-stream)

Action Potentials Essay

Caption: : Amelia Reece - UoL - Brain, behaviour and Cognition

Points from lecture slides

Some atoms have an electrical charge, when they move they carry electricity with them A nerve impulse is a sequence of small movement of these charged atoms into and out of the nerve cell - more precisely a nerve impulse is actually a sequence of small movements of atoms into and out of the axon of the neuron. each movement is an action potential  The first small movement triggers the second, the second triggers the third, etc. Sodium (Na+) Potassium (K+)  

Why Do They Move at All

Sodium has a positive electric charge and inside the axon is a strong negative charge  atoms normally have the same number of positively charged protons and negatively charged electrons  if it loses an electron it becomes overall more positive if it gains an electron it becomes overall more negative inside the axon is a strong negative charge ( opposites attract) there are two positive forces that make these electrically charged atoms (ions) move  electrostatic pressure - the negative charge inside the cell attracts all positively charged atoms into the cell and repels all negatively charged atoms into the cell pressure from the concentration gradient - atoms move from a location of high concentration to an area of low concentration  Despite the pressure, sodium does not automatically enter the cell because the channels may not be open this is the state of the neuron during the resting potential sodium channels are almost shut     

Potassium

Despite the electrostatic pull on the positively charged potassium atoms (Na+) to enter  the neuron, the concentration inside is so great that it overwhelms this  therefore the overall pressure on potassium atoms is to move out of the cell  this does not happen automatically because the potassium channel may also be shut  this is the state of the neuron during the resting potential channels are almost shut 

sodium-potassium pump

there is a great pressure on k+ atoms to exit the cell There is a great pressure on Na+ atoms to enter the cell They do this with difficulty because of the channels through the membrane are almost shut  Despite this a few passes through the sodium-potassium pump fixes this and keeps the concentrations constant  ​​​​​​​the sodium-potassium pump moves 3 sodium ions out and moves 2 potassium ions 

Chloride Cl

The pressure from the concentration gradient can be measured  the pressure from the electrostatic force can be measured  For chloride (Cl-) the two pressures are equal  This is the state of the neuron during the resting potential sodium channels are open

The Action Potential

The threshold of excitation (-70 microvolts to -50mv) sodium channels open sodium rushes in = +50mv potassium rushes out  sodium channel closes, potassium still comes in  potassium channel closes

What a Neuron Does

A neuron can be 'quiet' in a resting state. - the resting membrane potential   A Neuron can be active in an active state  - an action potential - firing   

The Resting Membrane Potential

using an electrode to measure the membrane potential of a neuron reads -70v  the inside of the neuron is more negative than the outside and vice versa the resting membrane potential has different concentrations of ions inside and outside the neuron it takes a lot of hard work to maintain the RMP bow and arrow analogy - at rest it is like a taut bow - ready to fire, resting is a misnomer there are more Na+ ions outside the cell  there are more k+ ions inside the cell  the inside of the cell has large -ve charged protein molecules. that's why it's negative  3 forces/pressures act to maintain the unequal distribution of ions in the resting membrane potential  electrochemical pressure  concentration gradient  sodium potassium pump  

Resting Membrane Potential Ionic Basis

salts in a solution exist as negative and positive ions - positive cations and negative anions  atoms normally have the same number of positively charged protons and negatively charged electrons  if it loses an electron it becomes overall positive and vice versa   

Resting membrane potential

Two passive forces 'work' on these ions electrostatic pressure - the negative charge inside the cell attracts cations - positive ions into the cell the negative charge inside the cell repels all negatively charged atoms in the cell  pressure from the concentration gradient  - ions move from a location of high concentration to an area of low concentration  One active force 'works' on these ions  Sodium-potassium pump. there is a great pressure on Na+ atoms to enter the cell. there is great pressure on k+ atoms to exit the cell. they do this with difficulty because of the channels through the membrane are almost shut. despite this, a few pass through. for every 3 Na+ ions that are moved out, 2 K+ ions are moved in. this takes a lot of energy. there is great pressure for Na+ ions to 'flood' into the neuron and for K+ ions to rush out (pulling the bow taut)  

The Action Potential

the threshold of excitation (-70 to -50mV), sodium channels open, Na+ rushes in = + 50mV, k+ ions rushes out, sodium channel closes k+ ions still comes in. potassium channel closes  polarized - the neuron in the resting membrane potential -70mV depolarized - in the action potential +50mV repolarized back to - 70 mV hyperpolarized drops below -70mV