Nerve Impulses

The Over all StoryKatie Burke

Summary

Receptors

• A stimulus is received by the relevant receptors• The main types of receptor are:– Chemoreceptors• This is for chemicals: for taste, smell and chemical

concentration in the blood

– Mechanoreceptors• For balance, touch and hearing

– Photoreceptors• Light: for sight

– Thermoreceptors • For temperature control and awareness of surroundings

1. Sodium-potassium ion pump creates concentration gradients across the membrane

2. Potassium ions diffuse out of the cell down the potassium ion concentration gradient, making the outside of the membrane positive and the inside negative

3. The electrical gradient will pull potassium ions back into the cell

4. At -70mV potential difference, the two gradients counteract each other and there is no net movement of potassium ions

Resting Potential

Action Potential

What causes action potential?• Action potential is caused by

changes in the permeability of the cell surface membrane to Na+ and K+ channels

• At the resting potential, these channels are blocked by gates preventing the flow of ions through them

• Changes in the voltage across the membrane cause the gates to open, and so they are referred to as voltage-dependent gated channels

Depolarisation• 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 channels

• As 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 membrane

• Action potentials are either there or they are not (all-or-nothing)

Action Potential

Repolarisation• The voltage-dependent Na+ channels

spontaneously close and Na+ permeability of the membrane returns to its usual very low level

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

• 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

• This is the falling phase of the oscilloscope trace

Restoring the resting potential

• The 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.

At the Synapse1. An action potential arrives2. The membrane depolarises. Calcium ion

channels open. Calcium ions enter the neurone

3. Calcium ions cause synaptic vesicles containing neurotransmitter to fuse with the presynaptic membrane

4. Neurotransmitter is released into the synaptic cleft

5. Neurotransmitter binds with receptors on the postsynaptic membrane. Cation channels open. Sodium ions flow through the channels

6. The membrane depolarises and initiates an action potential

7. When released the neurotransmitter will be taken up across the presynaptic membrane (whole or after being broken down), or it can diffuse away and be broken down

Neurotransmitter

Summary • Neurotransmitters include

acetylcholine and glutamate• There are three stages

leading to the nerve impulse passing along the postsynaptic neurone:– Neurotransmitter release– Stimulation of the

postsynaptic membrane– Inactivation of the

neurotransmitter

Neurotransmitter release• When the presynaptic

membrane is depolarised, channels open, increasing the permeability to calcium ions

• Calcium ions diffuse into the cytoplasm

• This causes synaptic vesicles with the neurotransmitter to diffuse with the membrane

• They release the neurotransmitter by exocytosis

NeurotransmitterStimulation of the postsynaptic membrane• Specific receptor proteins in the

postsynaptic membrane bind to the neurotransmitter molecule

• When it binds it changes shape of the protein, opening cation channels

• The membrane is permeable to sodium ions, causing depolarisation

• The extent of depolarisation is caused by the amount of neurotransmitter which depends on the frequency of impulses

Inactivation of the neurotransmitter

• It can be actively taken up by the presynaptic membrane and reused

• They can rapidly diffuse from the synaptic cleft or taken up by other cells

• An enzyme will break it down which can be reabsorbed and reused