6B Flashcards
The Resting Membrane Potential
In a neurones resting state (when its not being stimulated) the outside of the neurone is more positively charged compared to the inside. This means the membrane is polarised - Theres a different in charge (Potential Difference/Voltage) across it. Its resting potential is around 70 mV.
Movement of Sodium and Potassium Ions
- The Resting Potential is created and maintained by Sodium-Potassium pumps and Potassium ion channels in the neurones membrane
- Sodium-Potassium pumps use Active Transport to move 3 sodium ions out of the neurone for every 2 Potassium ions in.
- Potassium ion channels allow facilitated diffusion of Potassium ions out of the neurone, down their conc. grad.
- The Sodium-Potassium pumps move sodium ions out of the neurone, but the membrane isn’t permeable to sodium ions, so they can’t diffuse back in. This creates a sodium ion Electrochemical Gradient.
- When the cells at rest, most potassium ion channels are open. This means that the membrane is permeable to potassium ions, so some diffuse back out through Potassium ion channels
Action Potentials - What happens to cause one?
When a neurone is stimulated, Sodium ion channels, in the cell membrane, open. If the stimulus is big enough, it will trigger a rapid change in Potential Difference. This causes the cell membrane to become Depolarised.
The Sequence of events for an Action Potential - Stimulus
This excites the Neurone cell membrane, causing Sodium ion channels to open. The membrane becomes more permeable to Sodium, so Sodium ions diffuse into the neurone down the Sodium-Electrochemical gradient. This makes the inside of the neurone less negative.
The Sequence of events for an Action Potential - Repolarisation
At a potential difference of around +30mV occurs, the Sodium ion channels close and the Potassium ion channels open. The membrane is more permeable to Potassium so Potassium ions diffuse out of the neurone down the Potassium ion Concentration Gradient. This starts to get the membrane back to its Resting Potential
The Sequence of events for an Action Potential - Depolarisation
If the Potential Difference reaches the Threshold (Around - 55mV) more Sodium ion channels open. More Sodium ions diffuse into the neurone
The Sequence of events for an Action Potential - Hyperpolarisation
Potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many Potassium ions diffuse out of the neurone. The Potential Difference becomes more negative than the Resting Potential
The Sequence of events for an Action Potential - Resting Potential
The ion channels are reset by the Sodium-Potassium pump pumping 3 Sodium ions out for every 2 Potassium ions in the neurone.
What is the Refractory period?
After an Action Potential, the neurone cell membrane can’t be excited again straight away. This is because the Ion channels are recovering, so they can’t be made to open - The Sodium ion channels are closed during Repolarisation and Potassium ion channels are closed during Hyperpolarisation. The Refractory period can act as a time delay to prevent Action Potentials overlapping. It also means theres a limit to the frequency at which the nerve impulses can be transmitted, and ensures the Action Potentials are unidirectional.
Order of which An Action Potential is made
- Stimulus
- Threshold
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Resting Potential
The Waves of Depolarisation
When an Action Potential happens, some of the Sodium ions that enter the neurone diffuse sideways. This causes the Sodium ion channels in the next region of the neurone to open, and the Sodium ions diffuse into that part. This causes a Wave of Depolarisation to travel along the neurone. The wave moves from parts of the membrane in the Refractory period because these parts can’t fire Action Potentials
All or Nothing Principle
Once the Threshold is reached an Action potential will always fire with the same charge in voltage, no matter how big the stimulus is. If the stimulus is strong, a higher frequency of Action Potentials will be fired. If the threshold isn’t reached, an Action Potential won’t fire.
Speed of Conduction - Myelination
Some neurones, including Motor Neurones, are Myelinated; They have a Myelin Sheath. It is an electrical insulator. In the Peripheral Nervous System it is made from a type of cell called a Schwann cell. Between the Schwann cells, there is bare membrane, called Nodes of Ranvier - The Sodium ion channels are concentrated here.
What is Saltatory Conduction?
In Myelinated neurones, depolarisation only happens at the Nodes of Ranvier. The Neurones cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse ‘jumps’ from node to node. In Non-Myelinated neurones, the impulse has to travel along the whole length of the neurone, so therefore it is slower than Saltatory Conduction, although still pretty quick.
Factors affecting Impulse conduction
- Axon Diameter - The bigger the Diameter of an Axon, the quicker an Action Potential is conducted, as there’s less resistance to the flow of ions than in the cytoplasm of a smaller Axon. With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker
- Temperature - the higher the temperature, the higher the rate of conduction as ions can diffuse faster as well as an increased ATP supply for Active Transport from the increased
Respiration Rate is. The temperature must only increase up to 40 degrees, as any higher an the proteins will denature, and the channels won’t be able to produce an Action Potential
Draw and Label a Myelinated Neurone
Cell Body, Dendrons and Dendrites, Axon, Myelin Sheath - Schwann Cells, Axon Terminals and nodes of Ranvier
Synapses and Neurotransmitters
A synapse is the junction between a neurone and another neurone, or between a neurone and an effector cell. The tiny gap between the cells at a synapse is called the Synpatic Cleft. The Presynaptic neurone has a swelling called the presynaptic knob. This contains vesicles willed with the neurotransmitters
The Effect of an Action Potential on Synpase’s
When an action potential reaches the end of a neurone, it causes the neurotransmitters to be released into the synoptic cleft. They diffuse across to the postsynaptic membrane, and bind to specific receptors. When they bind to the receptors they might trigger an AP, causing muscle contraction or a hormone to be secreted from a gland. The neurotransmitters are then removed once the stimulus stops to ensure the response doesn’t keep happening
How do we know synapses ensure the impulses are Unidirectional?
The Presynaptic knob doesn’t have receptors, and only releases the neurotransmitters, whilst the Postsynaptic membrane inly has receptors which the neurotransmitters bind too
What is Acetylcholine?
it is a neurotransmitter which binds to Cholinergic receptors. Synpases that use Acetylcholine are called Cholinergic Synapses
What happens at a Cholinergic Synapse? 1. The Arrival of an Action Potential
When an AP arrived at the presynaptic knob, it depolarises the cell membrane, stimulating the Voltage-Gated Calcium ion channels to open. The Calcium ions diffuse into the synoptic knob.
What happens at a Cholinergic Synapse? 2. Fusion of the vesicles
The influx of Calcium ions into the synoptic knob causes the vesicles, containing the neurotransmitter to fuse with the presynaptic membrane. The vesicles release the neurotransmitter (Acetylcholine - ACh) into the synaptic cleft via exocytosis.
What happens at a Cholinergic Synapse? 3. Diffusion of ACh
ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane. This causes the Sodium ion channels in the postsynaptic membrane to open. An influx of Sodium ions into the postsynaptic membrane causes depolarisation. If the Generator Potential reaches its threshold, it causes an Action Potential. Each is removed from the synaptic cleft so the response doesn’t keep happening. It is broken down into Choline and Ethanoic Acid catalysed by an enzyme called Acetylecholinesterase (AChE) and the products are re-absorbed by the presynaptic neurone and used to make more ACh
Excitatory and Inhibitory Neurotransmitters
Neurotransmitters can be excitatory, inhibitory, or both. Excitatory neurotransmitters depolarise the postsynaptic membrane, making it fire an AP if the threshold is reached, Whilst inhibitory neurotransmitters hyperpolarise the postsynaptic membrane by causing protein channels carrying chloride ions to open, making the potential difference more negative, preventing it from firing an AP. Acetylcholine is both Excitatory (In Cholinergic synapses in the CNS and at neuromuscular junctions) and Inhibitory (In Cholinergic synapses in the heart)
Another Inhibitory neurotransmitter is GABA, Which binds to receptors and causes Potassium Ion channels to open.
