Nerve Impulses and Synaptic Transmission Flashcards
How is a resting potential established in nerve cells?
- The sodium-potassium pump uses energy released by ATP to actively transport 3 sodium ions out of the nerve cell and two potassium ions into the cell.
- The sodium ion channels in the membrane are closed and the potassium ion channels are open.
- This means that the potassium ions can diffuse out of the nerve cells down an electrochemical gradient and that the sodium ions can’t diffuse back into the cell.
- This means that overall there are fewer positive ions inside of the nerve cell compared to outside and therefore the charge inside the cells is much more negative ( - 70 mv)
What are the stages of the generation of an action potential and how do they come about?
- Depolirisation: The sodium ion channels open causing the sodium ions to diffuse into the nerve cells. This increases the charge inside the nerve cells to increase. If the cell potential increases to the threshold of -55mv, more sodium ion channels open, and this causes the cell potential to increase further. This means an action potential is triggered.
- Repolarisation: At +30 mv, the sodium ion channels close, and the potassium ion channels open. This causes potassium ions to diffuse out of the cell and decreases the cell’s potential.
- Hyperpolarization: The potassium ion channels are slow to close, meaning too many potassium ions diffuse out of the nerve cells, causing the cell potential to decrease below the resting potential.
- With the complete closing of the potassium ion channels and the sodium-potassium pump distributing ions, the resting potential is re-established.
What is the all-or-nothing principle for action potentials?
They are only generated if the threshold(-55 mv) is reached and a weak stimulus may not cause the threshold to be reached.
Why is the all or nothing principle important?
It makes sure that organisms only react to strong enough stimuli and not every change in the environment which would overwhelm them.
How are waves of depolirisation formed?
- The stimuli cause the sodium ion channels to open and the sodium ions diffuse into the cell. This increases the charge of that section of the nerve cell, and if it reaches the threshold, an action potential is triggered.
- They then diffuse sideways, which causes the sodium ion channels in the next section to open, and sodium ions diffuse into that section.
What is the refractory period?
After a nerve cell has had an action potential triggered inside it there is a period in which another action potential can’t be triggered as the ion channels are recovering and can’t be forced open.
What is the importance of the refractory period?
- It ensures that action potentials are discrete.
- It makes sure the action potentials are unidirectional.
- It limits the number of impulse transmissions, which decreases the chances of the senses being overwhelmed.
Draw and label the structure of a nerve cell.
How does axon diameter affect the speed of conduction?
The larger the axon the speed increases. This is because there is less resistance to the movement of sodium ions.
How does temperature affect the speed of conduction?
The higher the temperature, the higher the speed of conduction, as the ions have more kinetic energy, meaning they move faster.
How does the presence of a myelin death on the nerve cells affect the speed of conduction?
It increases the speed of conduction. The myelin sheath provides electrical insulation, meaning that the sodium ions can only diffuse into the cell at the nodes of Ranvier. This means that an action potential can only be produced at the nodes, and to travel along the axon, it jumps from node to node. This makes the speed of conduction much higher as the action potential doesn’t have to travel across the whole length of the axon.
What is the name for the process in which the action potentials jump from node to node?
Saltatory conduction.
What are synapses?
Junctions between neurones.
How is an electrical impulse transmitted across a synapse?
- The depolarisation of the presynaptic membrane causes the calcium ion channels in the membrane to open.
- The calcium ions move into the presynaptic know via facilitated diffusion and bind to the vesicles.
- This causes the vesicles to move using energy from ATP and to fuse to the presynaptic membrane and release acetylcholine into the synaptic cleft.
- The acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane.
- This causes the sodium ion channels to open and the sodium ions to diffuse into the nerve cell.
- This increases the cell potential, and if it reaches the threshold, an action potential will be triggered.
- The acetylcholine is broken down by acetylcholinesterase, and the product is reabsorbed into the presynaptic membrane to synthesise acetylcholine.
How do synapses ensure that the transmission of impulses is unidirectional?
The calcium ion channel vesicles are only present in the presynaptic membrane meaning the neurotransmitters can only be released from them. The specific receptors are only on the postsynaptic membrane.
Draw and label a synapse.
What is an exicutatory response?
The bonding of the neurotransmitters to the receptors causes the postsynaptic membrane to be depolarized.
What is an inhibitory response?
The bonding of the neurotransmitters to the receptors causes the postsynaptic membrane to be hyperpolarised.
How does an inhibitory response work?
More potassium ions diffuse out of the cell, and less sodium ions diffuse into the cell, meaning depolarisation is less likely.
What are neuromuscular junctions?
Junctions between motor neurone and muscle fibre cells.
How are neuromuscular junctions different to normal cholinergic synapses?
- They have folds to store more acetycholesterate. This makes sure the acetylcholine is broken down quickly and doesn’t continuously produce an action potential, which could lead to loss of muscle control.
- They have more receptors in the postsynaptic membrane.
- The acetylcholine is always excitatory.
What is spatial summation?
Multiple presynaptic neurones fire small amounts of neurotransmitters towards on postsynaptic neurone. The combined action of all the transmitters can increase the cell potential above the threshold and trigger an action potential.
What is temporal summation?
Multiple different action potentials arrive at the presynaptic membrane in succession. This causes more neurotransmitters to be released into the synaptic cleft. This then means that they are more likely to be able to increase the nerve cell potential above the threshold and trigger an action potential.
