13.4 Nervous Transmission Flashcards
Definition of resting potential
The difference in change between the inside and outside of a membrane at rest
What is the resting potential of a membrane
-70 mV
What is a transducer?
Something which coverts mechanical energy into electrical energy
How is resting potential maintained across a membrane?
-Resting potential is created by the sodium-potassium pump and potassium ion channels
-Sodium ions are actively transported out of the axon, potassium ions are actively transported into the axon by intrinsic protein called sodium-potassium pump and the use of ATP into ADP + Pi. But the movement is not equal, for every 3 Na+ ions pumped out, 3 K+ ions are pumped in
-The membrane isn’t permeable to sodium ions, voltage gated sodium ion channels are closed so they cannot diffuse back in creating a sodium ion electrochemical gradient
-Potassium ion channels are open, allowing potassium ions to diffuse back out of the axon by facilitated diffusion, down their concentration gradient
-This makes the outside more positively charged than the inside of the axon, creating a resting potential across the membranes of the -70 mV
-There are also permanent negative ions inside the axon (in the cytoplasm) which stay inside, making the inside more negative
What is depolarisation?
A change in potential difference from negative to positive across the membrane of a neurone
What is repolarisation?
A change in potential difference from positive to negative across the membrane of a neurone
What is the maximum potential difference of a neurone?
+40 mv
Definition of action potential
The change in potential difference across a neurone membrane of an axon when stimulated
When does an action potential occur?
When protein channels in the axon membrane change shape as as result of a change of voltage (potential difference) across a membrane
Explain the graph for the changes in potential difference which occurs across a neurone membrane during an action potential?
- The neurone has a resting potential of -70 mV, no impulse is being transmitted, potassium ion channels are open, voltage-gated sodium channels are closed
- The energy of the stimulus excites the neurone cell membrane, sodium ion channels are opened, the membrane becomes more permeable to sodium ions, therefore sodium ions diffuse back into the axon down their concentration sodium ion electrochemical gradient, making the inside of the neurone less negative
- Depolarisation - If the potential difference reaches a threshold of -55 mV, the change in charge causes more sodium ions channels to open, more sodium ions diffuse into the neurone. This is positive feedback
- When the potential difference reaches its maximum of +40 mV the sodium ion channels close and the voltage-gated potassium ion channels open. Sodium can no longer enter the axon, but the membrane is now more permeable to potassium
- Repolarisation - Potassium ions diffuse out of the axon down their electrochemical gradient, reducing the charge of the membrane back to its resting potential, this is negative feedback
- Hyperpolarisation - Potassium ions diffuse channels are slow to close so there is an ‘overshoot’ where too many potassium ions diffuse out of the neurone, the potential difference becomes more negative then resting potential (less than -70 mV). The sodium-potassium pump causes more sodium ions to move out of the cell and potassium ions to move back in. The axon is now repolarised and returns to its resting potential
What is the refractory period?
The period of recovery after an action potential, where the ion channels are recovering and the axon cannot be excited again. The voltage-gated sodium ion channels remain closed, preventing the movement of sodium ions into the axon
What are the stages of propagation of an action potential along a non-myelinated neurone?
- At resting potential the concentration of sodium ions outside the axon is greater than inside the axon, whereas the concentration of potassium ions is greater inside the axon. The overall concentration is greater on the outside, making it positive compared with the inside.
- A stimulus causes a sudden influx of sodium ions and hence a reversal of charge, the membrane becomes depolarised
- The localised electrical circuits established by the influx of sodium ions cause the opening of voltage-gated sodium ion channels further down the axon, resulting in a influx of sodium ions in this region causing it to be depolarised. Behind this regions the voltage-gated sodium ion channels closes and potassium ions open, and potassium leaves the axon down their electrochemical gradient
- The action potential (depolarisation) is propagated in the same way further down the axon. The outward movement of potassium ions has continued to the extent that the axon membrane behind the action potential has returned to its resting potential by repolarisation
- Following repolarisation the axon membrane returns to its resting potential in readiness for a new stimulus if it comes
Why is the refractory period important?
Makes sure action potentials are unidirectional, that they do not overlap, and occur as discrete impulses
How does saltatory conduction benefit a myelinated neurone?
In myelinated neurones depolarisation only occurs at the nodes of Ranvier. The electrical charge jumps from one node to the next causing a faster action potential transmission
What are the factors affecting the speed of action potentials?
-If the neurone is myelinated, the action potential can jump from node to node, creating a faster speed
-If the axon diameter is bigger, impulse can travel faster as there is less resistance to the flow of ions into the cytoplasm compared with those in a small axon
-If the temperature is higher, the nerve impulse can travel faster because ions can diffuse at a faster rate. But this only occurs up toll 40°C as higher temperatures can cause the membrane to become denatured
