Synaptic signalling

Question 1

What is the (average) resting membrane potential of a neuron?

Answer 1

Around 70 millivolts (mV)

Question 2

Describe how the resting membrane potential is partially maintained through active transport of ions across the cell membrane

Answer 2

Na+/K+ ATPase (the sodium / potassium pump) expends energy to pump 3 molecules of Na+ [sodium] into the cell, and 2 molecules of K+ [potassium] into the cell. This maintains a greater amount of potassium inside the cell.

Question 3

What is the phospholipid bilayer

Answer 3

The phospholipid bilayer is a two-layered arrangement of phosphate and lipid molecules that forms a cell membrane. The hydrophilic (water-attracting) phosphate heads face outward, and the hydrophobic (water-repelling) lipid tails face inward, creating a barrier that regulates the entry and exit of substances.

Question 4

Describe the fluid mosaic model

Answer 4

The fluid mosaic model is a way to describe the structure of cell membranes. It suggests that the membrane is not solid, but rather fluid, with various proteins and other molecules embedded in or attached to a flexible bilayer of phospholipids. This mosaic of components can move laterally within the layer, giving the membrane its fluid character.

Question 5

What type of passive channels (aka leak channels) exist in the neuronal cell membrane, and what do they do?

Answer 5

Passive channels, also known as leak channels, exist in the neuronal cell membrane and are primarily responsible for the movement of ions down their concentration gradient without the use of energy. These channels contribute to the resting membrane potential and can be specific for different ions, such as potassium or sodium ions.

Question 6

What type of passive channels (aka leak channels) exist in the neuronal cell membrane, and what do they do?

Answer 6

In the neuronal cell membrane, passive or leak channels predominantly include potassium (K+) and chloride (Cl-) channels. They allow ions to move down their concentration gradients without the use of ATP, contributing to the resting membrane potential. Potassium leak channels, in particular, are more numerous and allow K+ ions to exit the neuron, making the inside of the cell negatively charged compared to the outside.

Question 7

How do leak channels contribute to the maintenance of the resting potential

Answer 7

Leak channels contribute to the maintenance of the resting potential by allowing ions to move passively across the neuronal membrane. Specifically, potassium (K+) leak channels are more numerous and are critically important in setting the resting membrane potential.

Question 8

What laws govern the membrane potential?

Answer 8

The membrane potential is governed by the Nernst equation and the Goldman-Hodgkin-Katz voltage equation. The Nernst equation predicts the equilibrium potential for a single ion type based on its concentration gradient across the membrane. The Goldman-Hodgkin-Katz equation extends this to multiple ion types, considering their relative permeability and concentration gradients, to predict the overall membrane potential.

Question 9

What is an anion, what is a cation? Give examples of the major anions and cations found in and around cells?

Answer 9

An anion is a negatively charged ion, while a cation is a positively charged ion. Major anions involved in synaptic signalling include chloride (Cl-) and bicarbonate (HCO3-), as well as the negative charges found on the many amino acids that form proteins.

Major cations include sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium (Mg2+).

Question 10

Name the three main mechanisms that generate the resting membrane potential.

Answer 10

1) Differential permeability of the neuronal membrane to various ions, especially potassium.
2) The action of the sodium-potassium pump which actively transports Na+ out and K+ in.
3) The presence of negatively charged proteins and other molecules within the cell.

Question 11

Describe the graded potential.

Answer 11

A graded potential is a local change in membrane potential that varies in size depending on the strength of the stimulus. It can depolarize or hyperpolarize the membrane and diminishes as it spreads away from the site of stimulation.

Question 12

What is an EPSP and what can cause one?

Answer 12

An EPSP, or excitatory postsynaptic potential, is a postsynaptic potential that makes the neuron more likely to fire an action potential. It is usually caused by the influx of Na+ or Ca2+ through ligand-gated channels.

Question 13

What is an IPSP and what can cause one?

Answer 13

An IPSP, or inhibitory postsynaptic potential, is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential. It can be caused by the outflow of K+ or the inflow of Cl-.

Question 14

What is a ligand-gated ion channel?

Answer 14

A ligand-gated ion channel is a type of ion channel that opens or closes in response to the binding of a chemical messenger (ligand), such as a neurotransmitter.

Question 15

What is the action potential firing threshold?

Answer 15

The action potential firing threshold is the critical level of depolarization that must be reached for an action potential to be initiated. It is typically around -50 to -55 mV for many neurons.

Question 16

What happens when the membrane potential reaches the action potential firing threshold?

Answer 16

When the membrane potential reaches the action potential firing threshold, voltage-gated Na+ channels open rapidly, causing a rapid influx of Na+ ions, which further depolarizes the membrane and initiates the action potential.

Question 17

What is hyperpolarization?

Answer 17

Hyperpolarization is an increase in the membrane potential of a cell, making the inside more negative compared to the outside. This usually moves the membrane potential away from the action potential threshold, and can occur when the postsynaptic membrane is subject to an inhibitory post-synaptic firing potential (IPSP).

Question 18

Describe the 5 main steps of an action potential

Answer 18

1) Resting State: The neuron is at rest with a potential of around -70 mV.

2) Depolarization: A stimulus causes the membrane potential to become less negative.

3) Rising Phase: The membrane potential reaches the threshold, and voltage-gated Na+ channels open, causing rapid depolarization.

4) Falling Phase: Na+ channels close, and voltage-gated K+ channels open, allowing K+ to exit the cell, beginning repolarization.

5) Undershoot: Membrane potential temporarily becomes more negative than the resting potential, also known as the hyperpolarization phase, before returning to the resting state.

Question 19

What is homeostasis in the context of membrane potentials?

Answer 19

Homeostasis in the context of membrane potentials refers to the neuron’s ability to maintain a stable resting membrane potential, which is critical for the proper function of nerve cells. This involves a balance between the ionic gradients and electrical forces across the cell membrane, ensuring that the neuron is ready to respond to stimuli and conduct action potentials.

Question 20

Why is it important for a neuron to maintain a negative resting membrane potential?

Answer 20

It’s important for a neuron to maintain a negative resting membrane potential because this electrical charge difference across the membrane sets the stage for the possible generation of action potentials. This polarity allows for the rapid response to a stimulatory signal, leading to the propagation of nerve impulses necessary for communication throughout the nervous system.

Question 21

Describe how spatial summation can influence the membrane potential.

Answer 21

Spatial summation occurs when multiple presynaptic neurons release neurotransmitters at various locations onto the postsynaptic neuron at the same time. The combined effect of these simultaneous inputs can either depolarize or hyperpolarize the postsynaptic neuron to a greater extent than one input alone, influencing the likelihood of an action potential being generated.

Question 22

Describe how temporal summation can influence the membrane potential.

Answer 22

Temporal summation occurs when one presynaptic neuron releases neurotransmitters in quick succession. If these consecutive signals occur close enough in time, they can collectively depolarize or hyperpolarize the postsynaptic neuron more effectively than a single stimulus, potentially leading to an action potential.

Question 23

Describe how astrocytes can influence spatial and temporal summation.

Answer 23

Astrocytes can modulate synaptic activity by taking up neurotransmitters from the synaptic cleft, affecting their availability. By modulating neurotransmitter levels, astrocytes can indirectly affect the degree of spatial and temporal summation at the synapse. They can also release gliotransmitters that influence synaptic function.

Question 24

Describe the role of sodium ions in an action potential.

Answer 24

Sodium ions (Na+) play a critical role in the initiation and propagation of an action potential. When voltage-gated Na+ channels open, Na+ rushes into the neuron, causing depolarization. This rapid influx of positive charge is what drives the rising phase of the action potential.

Question 25

Describe the role of potassium ions in an action potential.

Answer 25

Potassium ions (K+) are essential for repolarizing the neuron after the peak of an action potential. When voltage-gated K+ channels open, K+ exits the neuron, causing the membrane potential to become more negative again, moving towards and beyond the resting potential in the repolarization and hyperpolarization phases.

Question 26

Describe the role of chloride ions in an action potential.

Answer 26

Chloride ions (Cl-) typically do not have a direct role in the generation of an action potential. However, their movement can affect the neuronal membrane potential. For example, when Cl- enters the neuron, it can cause hyperpolarization, making it less likely for an action potential to occur.

Question 27

Describe the role of calcium in an action potential.

Answer 27

Calcium ions (Ca2+) are involved in the latter part of the action potential at the synapse. When the action potential reaches the axon terminal, voltage-gated Ca2+ channels open, allowing Ca2+ to enter, which triggers the release of neurotransmitters into the synaptic cleft.

Question 28

What is an ion?

Answer 28

An ion is an atom or molecule that has gained or lost one or more electrons, giving it a net electrical charge. Ions are critical for various biological functions, including generating electrical signals in neurons, muscle contraction, and maintaining fluid balance.

Question 29

What is a neurotransmitter?

Answer 29

A neurotransmitter is a chemical substance released by neurons at the synapse to transmit a signal to another neuron or effector cell (like a muscle or gland). Neurotransmitters bind to specific receptors on the postsynaptic cell, which can either stimulate or inhibit electrical impulses within the receiving cell, thereby facilitating, modulating, or inhibiting signaling throughout the nervous system. Common examples include glutamate, acetylcholine, dopamine, serotonin, and gamma-aminobutyric acid (GABA).

Question 30

What happens when the action potential reaches the axon terminal?

Answer 30

Voltage-gated calcium channels open, allowing calcium ions (Ca2+) to enter the axon terminal.

The Ca2+ causes synaptic vesicles to bind to the presynaptic membrane and release neurotransmitters into the synaptic cleft through exocytosis.

Neurotransmitters cross the synaptic cleft and bind to transmitter receptors on the postsynaptic cell, which can generate an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).

Question 31

What role do autoreceptors play in neurotransmission?

Answer 31

Autoreceptors, located on the presynaptic neuron, regulate neurotransmitter release by detecting the amount of transmitter in the synaptic cleft and adjusting release accordingly.

Question 32

How are neurotransmitters cleared from the synaptic cleft?

Answer 32

Neurotransmitters can be cleared from the synaptic cleft by reuptake into the presynaptic neuron through transporters, enzymatic degradation, or diffusion away from the synaptic site.

Question 33

Describe the ‘all or none law’.

Answer 33

he ‘all or none law’ states that once a neuron’s membrane potential reaches the threshold to trigger an action potential, the neuron will fire at full amplitude. If the threshold is not reached, the neuron will not fire at all. This principle applies to the action potential; it either occurs completely or does not occur at all, regardless of the stimulus’s strength, as long as it is above the threshold.

Question 34

Describe the ‘integrate and fire’ principle.

Answer 34

The ‘integrate and fire’ principle refers to how neurons integrate all incoming excitatory and inhibitory signals, both spatially and temporally, to determine whether or not to fire an action potential. If the cumulative sum of these signals reaches a certain threshold, the neuron will ‘fire,’ generating an action potential. This principle is foundational to how neurons process and transmit information.