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+)
Slide 3
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
Slide 4
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
Slide 5
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
Slide 6
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
Slide 7
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
Slide 8
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
Slide 9
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
Slide 10
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
Slide 11
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)
Slide 12
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
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