Nerve and synapses Flashcards
What are the 3 types of fibers present in the PNS? Describe what they are.
Afferent (sensory): brings signals in and towards the CNS
Efferent (motor): sends signals from CNS to skeletal muscles
Autonomic: sends signals from CNS to visceral muscles
The smallest unit in the nervous system is the […]
neurons
What are synapses?
They are specialized sites where communication between neurons takes place.
Describe the standard structure of a neuron.
All neurons have a cell body, several branching dendrites and a single axon that can vary in length that ends in a presynaptic terminal
Describe the soma of the neuron.
This is where the nucleus and organelles, such as the mitochondria, are. It is also where chemical reactions and processes take place. It could survive if you removed all other parts of the cell.
Describe the dendrites of the neuron.
They allow neurons to receive inputs from other cells and are highly branched to allow more synapses from which they can receive signal
Describe the axon of the neuron.
A neuron only has one axon, which allows it to send information to other neurons using an electrical impulse that propagates to the presynaptic terminal.
Describe the flow of information in the neuron.
It comes in through the dendrites, travels into the soma, then travels down the axon towards the dendrites of another neuron.
What is the resting membrane potential of the neuron? What is the cause?
Around -70 mV compared to the outside. This is due to a small excess of negatively charged ions inside the cell.
Explain the two major causes of the voltage gradient in the neuron.
- There is a concentration gradient of Na+ (a lot on the outside, little on the inside), K+ (a lot on the inside, little on the outside), Cl- (a lot on the outside, little on the inside), and other anions (a lot on the inside, little on the outside).
- At rest, the neuronal membrane is only highly permeable to K+, which wants to flow out of the cell. So K+ will naturally leak out of the cell from leak channels down their concentration gradient, leaving behind unpaired negative ions that pull K+ back in.
Explain why the resting potential is a bit more positive than the Equilibrium of K.
While the K ions are trying to reach their equilibrium, there is a minuscule amount of Na that leaks into the cell (the membrane is not perfectly permeable) Because neither the Na nor K. ions reach their concentration gradients, if it was left to itself, the leaking would continue until there was no more concentration gradient. That’s why the Na/K pump is useful.
What does the K/Na pump do?
At rest, because there is a constant leaking of K and Na ions, the pump will be there to create and maintain the concentration gradient. It becomes the source of energy for the ions to keep moving.
When the neuron is at rest, the dominant ion determining the membrane potential is […]
K+
What are action potentials?
They are brief electrical impulses that axons propagate from one region of the nervous system to another.
Action potentials travel from the […] to the […] of a neuron
Initial segment, presynaptic terminals
What is the difference between depolarization and hyperpolarization?
Depolarization is when the membrane gets more positive
Hyperpolarization is when the membrane gets more negative
The action potential involves a [depolarization/hyperpolarization] from […] mV to […] mV
depolarization, -70 mV, the required
Explain the threshold potential and how it relates to the action potential.
electrical impulses are often sent but don’t always produce an action potential. It only produces an action potential if it’s above the threshold potential. If not, the membrane relaxes back to -70 mV without sending a signal.
The depolarizing phase of the action potential is caused by […]. Explain their critical 3 properties.
Sodium ions flowing into the cell through voltage-gated sodium channels. Sodium channels are (1) closed at resting membrane potential but open when the membrane depolarizes, (2) selective for Na+, (3) able to rapidly inactivate after opening, stopping the flow of Na+ ions.
Explain the process by which reaching the threshold of depolarization releases the action potential.
When the membrane depolarizes to it’s action potential threshold, a small number of Na+ channels will open and allow Na+ to flow in, further depolarizing the membrane potential. This will cause more Na+ channels to open, allowing more Na+ in and further depolarizing the membrane potential. This leads to a positive feedback loop at results in the membrane potential reaching +30 mV, closer to the Na equilibrium.
Why do action potentials top out at +30 mV?
- The Na+ channels that open due to membrane depolarization quickly deactivate.
- Some Na+ can still leak out. When the channels deactivate, there’s no more Na+ permeability, and the membrane potential returns to -70 mV.
Explain why Na+ is able to become the dominant ion when the membrane is depolarized.
The density of voltage-gated sodium channels in the axon membrane is much higher than the density of leak potassium channels, so at the peak action potential, Na+ permeability becomes much higher than the resting permeability for K+.
Explain the role of voltage-gated potassium channels in the action potential. What are they activated by?
Voltage-gated K+ ion channels are slowly opened when the cell membrane depolarizes. However, It is only really effective once the cell has started repolarizing. When those ion channels are open, it lets K+ out of the cell more than the leaking channels. This is to get the cell back to its resting potential faster [get more info to go through after]
What is the advantage of a short action potential?
Neurons send different messages with frequency and patterns. By having a short action potential, more information can be sent in a given period.
Explain how action potential propagation down the axon occurs.
The Na+ coming into the cell at the initial segment is going to be “attracted” by the other negative ions down the axon, and so the process of propagation is going to continuously regenerate all the way to the presynaptic terminal.
Why does propagation occur in one direction down the axon?
The Na+ voltage-gated channels are inactivated and is in a period of absolute refraction meaning that nothing will be able to open them in that period.
What are the absolute and relative refractory periods?
Absolute: The sodium channels are INACTIVATED, and so the membrane is completely unexcitable
Relative: The sodium channels are CLOSED while he voltage-gated potassium channels are open. The membrane potential is more negative than it’s resting potential meaning that the axon is less excitable than normal (you would need a bigger signal to activate it again)
How do neurons transmit specific information using action potentials?
with different frequencies and patterns
Neurotoxins are usually target […] in victims, because […]
sodium channels
They are important for all functions of the body
Puffer fish make the neurotoxin […]. Describe the effect that it has.
tetrodotoxin
It inhibits the opening of sodium channels. This means that the functions will be reduced if not everything is touched but could eventually lead to death (diaphragm paralysis)
Phyllobates frogs secrete the neurotoxin […]. Describe the effect that it has.
Batrachotoxin
It activates the sodium channels. This means that they always stay open and this big storm of action potentials causes seizures.
What types of drugs can block sodium channels?
Local anesthetics and antiepileptic
Give 4 examples of local anaesthetics that can block sodium channels.
Lidocaine
Benzocaine
Tetracaine
Cocaine
Give 2 examples of antiepileptics that can block sodium channels.
Phenytoin
Carbamazepine
What is the purpose of drugs that block sodium channels?
Local anesthetics will block any pain signal from getting to the brain (getting processed), while antiepileptic drugs will block the excessive brain activity (electrical storm) that could cause an epilepsy attack
How do the axons of squids differ from those of mammals? Explain the effect that this has on their function.
They have a big diameter, which allows for more channels to be present and have faster depolarizations
How do mammals make up for the small diameter of the axons of the neurons?
They have myelinated axons which propagates the electrical signal faster
Describe the structure and makeup of the myelin that surrounds vertebrate axons.
Myelin acts as an electrical insulator and is interrupted by periodic nodes of Ranvier
What is the function of the myelin and nodes of Ranvier?
Myelin makes the electrical signal propagate faster because less regeneration is needed whereas the Ranvier Nodes allow for the regeneration to happen periodically before the signal fades out.
