Transmission of nerve impulses Flashcards
(18 cards)
Polarised membrane + resting potential value
In a neurone at rest, the inside of the membrane is negatively charged compared to the outside (polarised)
-The value of this charge difference is known as the resting potential
-The resting potential is usually around -70mv
Why are the concentrations of ions vastly different on either side of the membrane?
-Ions cannot pass through the phospholipid bilayer
-There is an integral membrane protein (sodium- potassium pump)
-This pump uses ATP to pump sodium ions outside of the membrane and potassium ions inside
-This means we have a higher concentration of sodium ions outside of the membrane than outside and a higher concentration of potassium ions inside than outside
Why is there a charge difference
-For every 3 sodium ions being transported out, only 2 potassium ions are transported in via the sodium-potassium pump
-Because of this, the number of positive ions on the outside of the membrane is greater than the number of ions on the inside
Ion Channels
- contains ion channels that span the membrane
-allow ions to move through via facilitated diffusion
-sodium ions channels are mainly closed, meaning there is less diffusion of sodium ions to the inside of the membrane
-However, the potassium ion channels are mainly open, meaning we have a high rate of diffusion of potassium ions to the outside of the membrane
-This outward diffusion of potassium ions makes the inside of the membrane more negative
summary of factors affecting resting potential
-For every two potassium ions being pumped to the inside of the membrane by the sodium-potassium pump, three sodium ions are being pumped tot he outside of the membrane
-Sodium ion channels are mostly closed, causing a low rate of inward diffusion
-However, Potassium ion channels are mostly open, leading to a high rate of outward diffusion
-These two factors make the inside of the membrane more negative as there are more positively charged ions on the outside of the membrane
Depolarisation
-Depolarisation is the reversal of the electrical potential difference across the membrane
-To transmit a nerve impulse in a neurone, the membrane needs to be depolarised
Action Potential definition + function +role in accordance with depolarisation
-An action potential is the potential electrical difference across the membrane when a neurone is stimulated, e.g, a receptor cell detects an environmental stimulus
-Action potentials reverse the resting potential from around -70 mV to around +30 mV
-Depolarisation of the membrane occurs when an action potential is reached
-Action potentials involve the rapid movement of sodium and potassium ions across the membrane
Voltage gated ion channels
-ion channels that open and close in response to changes in the potential electrical difference across the membrane
-Voltage gated ion channels are closed at resting potential
What happens when a neurone is stimulated
-When a neurone is stimulated, a small number of sodium ion channels open
-Sodium ions begin moving across the membrane to the inside of the axon
-This reduces the potential difference as the inside of the axon is less negative
-Once the potential difference reaches around -55 mV (threshold potential), more voltage-gated sodium ion channels start opening, leading to an influx of sodium ions crossing the membrane to the inside of the axon
-The action potential cannot be initiated if the threshold potential is not achieved
-Once the charge has been reversed from -70 mV to around +30 mV, the membrane is depolarised so a nerve impulse can be transmitted (action potential has been generated)
Repolarisation
-About one millisecond after the action potential is generated, all voltage gated sodium ion channels close
-Voltage gated potassium channels open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient
-This causes the inside of the axon to become negatively charged again, repolarising the membrane
Hyperpolarisation + Refractory Period
Hyperpolarisation is the short period of time in which the membrane potential is more negative than the resting potential
-The period of time in which the membrane is hyperpolarised is called the refractory period
-During the refractory period, an action potential cannot be generated, and the impulse can only move in one direction
What happens after hyperpolarisation
-Voltage-gated potassium ion channels close, and the sodium-potassium pump works to restore the resting potential
-Only once resting potential is achieved can the membrane be stimulated again
Transmission of an action potential
-Once an action potential has been generated, it can be transmitted along the length of an axon
-the depolarisation of the membrane at the site of the first action potential causes sodium ions to diffuse along the cytoplasm to the next section of the axon
-This depolarises the next section of the axon, and causes voltage-gated sodium ion channels open
-This triggers another action potential in this section of the axon membrane
-This process repeats itself along the length of the axon
All or nothing principle
-The all-or-nothing principle states that in order for an impulse to be transmitted, the initial stimulus must be enough to increase the membrane potential above the threshold potential
-If a stimulus is weak, then only a few sodium ion channels will open, meaning the membrane will not be sufficiently depolarised to reach the threshold potential
-Therefore an action potential will not be generated
How can stimulus size be measured by the brain?
-Stimulus size can be measured by the brain because as the intensity of the stimulus increases, the frequency of action potentials transmitted along the neurone also increases
Myelination
-In umyelinated neurones, the speed of conduction is much slower because depolarisation needs to occur along the whole length of the membrane
-However, by insulating the axon, myelin increases the speed at which action potentials travel along the neurone
how does myelination work to speed up an impulse
-In sections of the axon that are surrounded by the myelin sheath, depolarisation cannot occur
-This is because the myelin sheath prevents the diffusion of sodium and potassium ions
-Action potentials can only occur at the nodes of Ranvier (gaps in between the Schwann cells that make up the myelin sheath)
-Sodium ions diffuse along the neurone within the schwann cells
- Membranes at the Nodes of Ranvier depolarise when the sodium ions arrive
-The impulse then appears to “jump” between nodes (saltatory conduction)
-Saltatory conduction helps the impulse to travel much faster than in an unmyelinated axon
Medication that prevents the transmission of an impulse
-These drugs may work by binding to the sodium ion channel, preventing them from opening and therefore preventing an influx of sodium ions into the axon
-This prevents depolarisaiton of the membrane occuring, therefore the action potential cannot be generated
