This document summarizes the process of nerve impulses in the human body. It describes how stimuli are received by receptors and converted into electrochemical signals called action potentials. Action potentials travel along neurons when voltage-gated sodium channels open, causing the neuron to depolarize. At the synapse, an action potential causes neurotransmitters to be released, which then bind to receptors on the next neuron and may trigger another action potential. Neurotransmitters are then inactivated by reuptake or enzymatic breakdown to end synaptic transmission and allow the process to repeat.

1. Nerve ImpulsesThe Over all StoryKatie Burke

2. Summary

3. ReceptorsA stimulus is received by the relevant receptorsThe main types of receptor are:ChemoreceptorsThis is for chemicals: for taste, smell and chemical concentration in the bloodMechanoreceptorsFor balance, touch and hearingPhotoreceptorsLight: for sightThermoreceptorsFor temperature control and awareness of surroundings

4. Sodium-potassium ion pump creates concentration gradients across the membranePotassium ions diffuse out of the cell down the potassium ion concentration gradient, making the outside of the membrane positive and the inside negativeThe electrical gradient will pull potassium ions back into the cellAt -70mV potential difference, the two gradients counteract each other and there is no net movement of potassium ionsResting Potential

5. Action PotentialWhat causes action potential?The change in the potential difference across the membrane causes a change in the shape of the Na+ gate, opening some of the voltage-dependent sodium ion channelsAs the sodium ions flow in, depolarisation increases, triggering more gates to open once a certain potential difference threshold is reached, thus increasing depolarisation (positive feedback)There is no way of controlling the degree of depolarisation of the membraneAction potentials are either there or they are not (all-or-nothing)Action potential is caused by changes in the permeability of the cell surface membrane to Na+ and K+ channelsAt the resting potential, these channels are blocked by gates preventing the flow of ions through themChanges in the voltage across the membrane cause the gates to open, and so they are referred to as voltage-dependent gated channelsDepolarisation

6. Action PotentialRepolarisationThe voltage-dependent Na+ channels spontaneously close and Na+permeability of the membrane returns to its usual very low level

7. Voltage-dependent K+ channels open due to the depolarisation of the membrane

8. Potassium ions move out of the axon, down the electrochemical gradient, and the inside of the cell once again becomes more negative than the outside

9. This is the falling phase of the oscilloscope traceRestoring the resting potentialThe membrane is now highly permeable to potassium ions, and more ions move out than occurs at resting potential, making the potential difference more negative than the normal resting potential (hyperpolarisation)The resting potential is re-established by closing of the voltage-dependent K+ channels and potassium ion diffusion into the axon.