P1 Neuronal Communication Flashcards
What are neurones, and what types are there?
- Specialised animal cells that pass on nerve impulses.
- The 3 types of neurone are sensory, motor and relay neurone.
Describe the structure and function of a sensory neurone.
- Pass impulses from receptors to the CNS.
- They are myelinated, the cell body is in the middle, dendrites at one end and an axon terminal at the other end.
Describe the structure and function of a motor neurone.
- Pass impulses from the CNS to muscles/glands (effectors).
- They are myelinated, cell body at the end and is surrounded by dendrites, with an axon terminal at the opposite end.
Describe the structure and function of a relay neurone.
- Pass impulses between sensory and motor neurones (found in the CNS).
- They are non myelinated, have their cell body at the end and contain lots of dendrites.
What direction does charge move in an axon?
- From the receptor, towards the effector.
- The movement of charge is unidirectional.
Which part of the neurone does the charge move through, and what is the charge called?
The charge is called a nerve impulse and it moves through the cytoplasm of neurones.
What is the distribution of charge across an axon at rest, and how does a nerve impulse travel down an axon?
- At rest, there is a more positive charge on the outside of the axon compared to the inside, due to an overall greater concentration of ions on the outside.
- Sodium ions have a higher concentration on the outside of the axon, and potassium ions have a higher concentration on the inside of the axon.
- Sodium ions diffuse through a sodium ion channel (always open), down the electrochemical gradient, to the inside of the axon, while potassium ions diffuse through a potassium ion channel, down the electrochemical gradient, to the outside of the axon.
- The overall charge inside the axon becomes more positive than the outside. As like charges repel, potassium ions are repelled out of the axon down the electrochemical gradient and the charge inside the axon becomes less positive.
What is the role of the sodium-potassium ion pump?
- The axon membrane contains a sodium-potassium ion pump, which is a co-transport carrier protein that can actively transport sodium and potassium ions.
- 3 sodium ions and a phosphate ion (from the hydrolysis of ATP) bind, triggering the carrier protein to change shape and force the 3 sodium ions across the membrane.
- This change in shape allow 2 potassium ions to bind, and then the phosphate ion is released, causing the protein to revert to it’s original shape, releasing 2 potassium ions across the membrane.
- This allows a neurone to maintain a higher concentration of ions outside the axon, and a lower concentration inside the axon.
What is the resting potential?
- When there is no stimulus, the neurone maintains a less positive charge on the inside of the axon. It is maintained by the sodium-potassium ion pump.
- This means it is able to response to a nervous impulse that reaches the axon.
What is the role of voltage-gated ion channels?
- At resting potential, voltage-gated ion channels are closed.
- When a stimulus triggers a nervous impulse, the inside of the axon becomes more positively charged than the outside of the axon (in one specific place, as a nerve impulse is a movement of positive charge).
- This nerve impulse causes the nearby axon to become slightly more positively charged, which causes voltage-gated sodium ion channels to open (to create an action potential).
- A reduce in the charge in the axon, causes voltage-gated potassium ion channels to open (to return to resting potential).
- Voltage-gated ion channels open/close by changing their tertiary structure.
How does an action potential establish?
- A nerve impulse arrives at one end of an axon, causing the nearby section of axon to become slightly more positively charged.
- Therefore some voltage-gated sodium ion channels open, allowing some sodium ions to diffuse into the axon, so the overall charge inside the axon becomes more positive, so even more voltage-gated sodium ion channels open (more sodium ions diffuse into the axon).
- Once the overall charge inside the axon has reached it’s maximum positive charge, it has reached it’s action potential (an example of positive feedback).
- An action potential is very localised, it happens at one point of the axon and then travels as a wave of depolarisation.
What is the all-or-nothing principle?
- An action potential is only triggered if enough sodium ions enter the axon (this is called the threshold for an action potential).
- If not enough sodium ions enter the axon, the threshold is not met and no action potential is triggered.
How is a resting potential reestablished?
- In response to an action potential being triggered, voltage-gated sodium ion channels close (reducing the number of sodium ions entering the axon), and voltage-gated potassium ion channels begin to open, allowing some potassium ions to diffuse out of the axon.
- This makes the overall charge inside the axon less positive, so more voltage-gated ion channels open, and potassium ions diffuse out of the axon until it returns to it’s original level of charge.
- Voltage-gated potassium ion channels take a while to close, so even more potassium ions diffuse out, resulting in the overall charge in the axon becoming temporarily less positive than usual (hyperpolarisation).
- Voltage-gated potassium ion channels close, and sodium ions diffuse through sodium ion channels (down the electrochemical gradient), making the axon more positive.
- To reestablish a resting potential the sodium-potassium ion pump actively transports sodium ions out of the axon and potassium ions into the axon.
What is depolarisation?
- When an action potential is being established, and the potential difference changes to a positive value (inside the axon becomes more positive than outside due to voltage-gated sodium ion channels opening).
- If enough sodium ions enter the axon, depolarisation results in an action potential.
What is repolarisation?
- After the action potential, voltage-gated sodium ion channels close, and voltage-gated potassium ion channels begin to open, resulting in a change in potential difference from it’s maximum positive value, to a slightly negative value.
How does an action potential move through an unmyelinated neurone?
- The influx of sodium ions into the axon (a positive potential difference inside the axon), triggers some sodium ions inside the axon to diffuse down an electrochemical gradient, along the axon. At the same time sodium ions outside of the axon diffuse in the opposite direction, down the electrochemical gradient.
- This movement of ions causes voltage-gated sodium ion channels further along the axon to open, meaning this part of the axon is depolarised and eventually causes an action potential.
- As the action potential moves down the neurone, the area behind resets to a resting potential. Voltage-gated sodium ion channels close and voltage-gated potassium ion channels open, so potassium ions diffuse out, it is depolarised (then hyper polarised), before returning to a resting potential.
How does axon diameter affect the speed of conductance?
- The greater the axon diameter, the greater the speed of conductance:
1. Axons with larger diameters have a lower resistance, so sodium ions can diffuse through at a faster rate.
2. When axons have larger diameters, fewer potassium ions make contact with potassium ion channels, so fewer potassium ions diffuse out of the axon. When more potassium ions diffuse out of the axon, it becomes less positive and more difficult to maintain resting potential. Whereas when resting potentials are maintained, action potentials are triggered more quickly which increases speed of conductance.
How does temperature affect speed of conductance?
- The higher the temperature, the greater the speed of conductance in the axon.
- At a higher temperature, there is a greater rate of diffusion, so sodium ions diffuse more quickly, resulting in a greater speed of conductance.
- However, if temperature is too high enzymes (eg. enzymes in respiration) will denature, meaning no ATP is generated, which affects the ability of the sodium-potassium ion pump to actively transport ions. Without the sodium-potassium ion pump, resting potential cannot be maintained and nerve impulses cannot be conducted.
How does an action potential travel down a myelinated neurone?
- In a myelinated neurone, only Nodes of Ranvier contain voltage-gated sodium ion channels, therefore action potentials can only be triggered at the Nodes of Ranvier.
- Once an action potential is reached, sodium ions inside the axon diffuse across to the second Node of Ranvier (as there are no ion channels at the myelin sheath, so sodium ions can keep moving), depolarising the second Node of Ranvier, creating an action potential.
- Nodes of Ranvier also contain voltage-gated potassium ion channels, so the first node of Ranvier is repolarised, then hyper polarised and returns to resting potential.
How does the myelin sheath affect the speed of conductance in myelinated neurones?
- The myelin sheath is made up of Schwann cells, which have a lipid cell membrane (phospholipid bilayer). Schwann cells are repeatedly wrapped to form the myelin sheath, so lipids are stacked, forming layers.
- Lipids don’t allow ions to pass through them, so ions cannot leave the axon via the myelin sheath. Therefore the myelin sheath acts like an electrical insulator.
- Therefore myelinated neurones only need to trigger action potentials at the nodes of Ranvier (as opposed to the whole axon), reducing the number of action potentials that need to be triggered, so myelinated neurones have a greater speed of conductance.
What is the refractory period, and what is it’s function?
- The refractory period is when all voltage-gated sodium ion channels are closed, meaning an action potential cannot be triggered (hyperpolarisation causes the refractory period).
- The refractory period prevents the action potential from moving backwards, ensuring the nerve impulse is UNIDIRECTIONAL.
- The refractory period also ensures that the axons have a break between action potentials, meaning different impulses travelling down the same axon cannot merge together (DISCRETE impulses).
How can strong stimuli and weak stimuli be differentiated (as they both reach action potential)?
- A weak stimuli triggers a small number of action potentials within a given period of time, whereas a strong stimuli triggers a large number of action potentials in the same period (a stronger stimuli has a greater frequency of action potentials).
- This occurs because stronger stimuli cause shorter refractory periods.
What is a synapse?
- A junction that transfers an action potential between two neurones, or a neurone and a muscle fibre.
What is a chemical synapse, and what structure does it have?
- Chemical synapses carry out the transfer of an action potential using neurotransmitters.
- It is made up of a presynaptic and a postsynaptic neurone, separated by a synaptic cleft.
- The synaptic knob (end of the presynaptic neurone) is filled with synaptic vesicles that store neurotransmitters, and has a large number of mitochondria.
- The pre-synaptic membrane contains voltage-gated sodium ion channels, and voltage-gated calcium ion channels.
- The post-synaptic membrane contains sodium ion channels with receptor sites that are complementary to the neurotransmitter.