Essay 3: action potentials

Action Potentials Essay

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• 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