1. Nerve cell membrane
Electrochemical message is created by the movement of ions across the nerve cell membrane
The resting nerve membrane has a electrical potential difference (potential) of -70 mV due to an unequal concentration of positive ions across the membrane
More positive ions outside of the membrane
Resting potential = -70 mV
When the nerve is excited there is a rapid reversal in the potential of the membrane
Becomes +40 mV
Called the action potential
The movement of the action potential through an axon conducts the neural impulse (message)

2. Resting Potential High concentration of K+ ions inside cell
High concentration of Na+ ions outside cell
K+ ions diffuse in and Na+ diffuse out
But cell more permeable to K+
The net result is that relatively more +ve ions end up outside the neuron
so the outside is more positive with respect to the inside
This establishes electrochemical gradient for the ions across the membrane
this charge difference is responsible for the resting potential.
At this point the neuron is said to be polarized.

3. K+ diffuses out faster than Na+ diffuses in; membrane is said to be polarized

4. The action potential
[1] Resting potential: neuron is polarized at -70 mV.
[2] Upon excitation Na+ gated channel proteins open (due to a change in shape of the protein itself: makes the membrane more permeable to Na+ ions now) which allows Na+ ions to diffuse into the neuron down the electrochemical diffusion gradient.
[3] This causes the inside of the neuron to become increasingly more positive: this is depolarization and neuron has become depolarized

5. The action potential
[4] Na+ channels close and K+ gated channels now open
[5] K+ ions diffuse out of the neurone down the electrochemical diffusion gradient, so making the inside of the neuron less positive (= more negative) again: this is repolarization and the neuron has become repolarized
[6] The neurone has its resting potential restored. During this time the Na+ and K+ ions which have diffused in/out of the cell are redistributed by active transport (sodium-potassium pump)

6. Na-K pump
ion channel protein

9. Refractory Period
Nerves conducting an impulse cannot be activated until resting membrane is restored
Must happen before next action potential can be conducted
Time required for repolarization to happen is call the refractory period
Last between 1 to 10 ms

10. Movement of Action Potential

13. Threshold Levels & the All-or-None Response
A potential stimulus must be above a critical value (threshold level) to produce a response.

14. Threshold Levels & the All-or-None Response
Increasing the intensity of the stimuli above threshold will not produce an increased response.
Intensity of impulse & speed of transmission remain the same.
Known as the all-or-none response.
Neurons either fire maximally or not at all.

15. Threshold Levels & the All-or-None Response
Differentiating Between Warm & Hot
The more intense the stimulus, the greater the frequency of impulses.
Intense stimuli excite more neurons.
Different neurons will have different threshold levels.
This affects the number of impulses reaching the brain.

16. Synaptic Transmission
Spaces between two neurons or a neuron & an effector is called a synapse.
Synaptic vesicles containing neurotransmitters (NTs) found in the end plates of axons.
Impulse down axon  NTs released from presynaptic neuron  NTs diffuse across synaptic cleft  depolarizes postsynaptic neuron.

17. Synaptic Transmission

18. Motor end plate
Synapses between motor nerve and muscle

19. Note: this is not a physical junction, there is actually a small gap of approx 20 nm between the cells so there is no membrane continuity so nerve impulses cannot cross directly.
synaptic vesicles
pre-synaptic membrane
post-synaptic membrane

20. Types of synapse Excitory synapses
Binding of neurotransmitter to postsynaptic neurone
opens Na+ gated channels
 Na+ diffuses IN
depolarisation
 action potentials
so nerve impulses can continue around the nerve circuit.

21. Types of synapse Inhibitory synapses
Binding of neurotransmitter to postsynaptic neurone opens K+ gated channels
 K+ diffuses OUT
 inside of neurone becomes even more – ve and so impossible to depolarize
 no action potentials
so nerve impulses cannot continue around the nerve circuit.

22. Neurotransmitters
Acetylcholine acts as an excitatory NT by opening Na+ channels on postsynaptic neuron, causing depolarization.
Cholinesterase (from postsynaptic neuron) destroys acetylcholine preventing a constant state of depolarization.
Inhibitory NTs make the postsynaptic membrane more permeable to K+.
Neuron becomes hyperpolarized.
More Na channels must be opened to depolarize and get an action potential

24. Neurones are connected together (normally via axons and dendrites) at synapses

26. Summation
Effect produced by the accumulation of NTs from two or more neurons.