BIOM2011 - Module 3 - Neurons

What are the ways in which neurons vary?

• Shapes and sizes

• Responses

• Functions

• Modifiability

What are the properties of neuron signalling?

• Fast - conducting signals at 0.5-120 m/s

• Inexpensive - only requires small amounts of signalling molecule

• Directional - signals can travel long distances to reach their specific targets.

• Selective - signals are small, discrete, and targeted.

What are the structures of a neuron?

• Dendrite

• Soma

• Axon hillock

• Axon

• Synapse

What are dendrites?

Thin branching extensions of soma.

Receives signals from other neurons.

They are variable in length and diameter - can taper (become thinner at ends further out).

Can be covered in small protuberances (spines) - makes them distinguishable from the axon. Excitable synapses attach here.

What is the soma?

Cell body that contains the nucleus.

What is the axon hillock?

The proximal end of an axon.

Initiates the signal / generates action potentials.

What is the axon?

A long process that is encased my myelin sheath.

Conducts the signal / carries signals to synapses.

Variable in length and diameter - no tapering, diameter remains constant.

What is the synapse?

Specialised ending of the axon.

Releases neurotransmitters onto other cells.

What are the functions of the synapse?

• Make and store neurotransmitters.

• Facilitate membrane fusion.

• Respond to axon action potential signal (to trigger membrane fusion).

What are synaptic vesicles?

Tiny globs of plasma membrane within the synapses’ cytoplasm that contain NTs.

What is the pre-/post-synaptic density?

‘Clouds’ of electron dense material.

What is the presynaptic density?

Synaptic vesicles are brought here to perform exocytosis with the plasma membrane to dump their NTs into the synapse.

What is the postsynaptic density?

Located in the target cell opposite the presynaptic density, accepts the NTs from the synapse.

What is the synaptic cleft?

The junction between the pre and postsynaptic densities.

It facilitates the diffusion of NTs through exocytosis from the presynaptic density.

The extracellular space is quite narrow - shorter distances are easier to cross.

How does the cell membrane act as an ionic barrier?

The biplanar lipid structure of the cell membrane is non-permeable to ions.

In order to generate or change membrane potential, ions must be able to move across the membrane.

Direction and rate of ion movement depends on:

• Charge of ion.

• Chemical gradient of ion across the membrane.

• Electrical charge distribution across the membrane.

What are ion channels? and what is their fuction?

Proteins that contain pores which allow ions to pass through, crossing the cell membrane.

Ion channels are necessary for the transport of ions through the cell membrane. Without them, ions would not be able to travel in and out of the cell as the cell membrane is impermeable to them (due to it being a poor conductor of electricity). Important ion channels in neurons include specialised ones that transport Na+, K+, and Cl-, respectively.

What is resting membrane potential? and what is it dependent on?

The voltage (electrical charge) difference across the cell membrane at rest.

It is dependent on the permeability of the ion channels to K+, Na+, and Cl-.

RMP shows that inside of the membrane is negative with respect to outside.

RMP values range from -90 mV to -45 mV.

RMP is ~ -70 mV.

What is the flow of ions through channels dependent on?

Ion transport through channels is dependent on concentration and electrical gradients across the membrane.

What is ion flow like at rest?

At resting membrane potential, ion channels are always open.

Each ion passes their selective pathway until equilibrium potential is reached - each equilibrium potential is different for each ion.

The equilibrium potentials for each ion come together to form resting membrane potential.

What is the process of measuring membrane potential?

MP is measured using an electrometer / a volt metre.

Electrometers contain 2 probes:

• One electrode is in the extracellular space - this serves as a reference value.

• The other electrode is stuck inside the neuron. It is filled with a high molarity KCl (a good conductor of electricity).

The two electrodes measure the difference in charge between the extracellular space and the neuron.

What are the concentrations of Na+, K+, and Cl- outside and inside the cell?

Na+

Extracellular - 145

Intracellular - 5-15

K+

Extracellular - 5

Intracellular - 140

Cl-

Extracellular - 110

Intracellular - 4-30

High conc of Na+ and Cl- in the EC and low conc in the IC - coincides with low relative permeability.

Low conc of K+ in the EC and high conc in the IC - coincides with high relative permeability.

What are the two types of ion channels?

1. Positive-selection ion channels - only allow the passing of positive ions (e.g. Na+, K+, Ca2+)

2. Negative-selection ion channels - only allow the passing of negative ions (e.g. Cl-)

Why is resting membrane potential largely dependent on K+?

Resting membrane potential is largely dependent on K+ because, at rest, lots of K+ is present inside the cell (due to high relative permeability).

Whereas, only a small amount of Na+ and Cl- are present in the cell (due to low relative permeability).

Due to the large difference in conc of K+ to Na+ and Cl-, this results in a negative value for RMP.

What is Ohm’s law?

V = IR

• V = voltage (potential difference)

• I = current (rate of movement of charged particle)

• R = resistance/conductance (the ease with which the charged particle can move)

What are ion exchanger pumps? What are the two types of exchangers, and what are some examples?

Membrane proteins that are involved in maintaining ion gradients across cell membranes.

They utilise energy to transport ions against their concentration gradient - active transport.

Two types:

1. Primary active exchangers - perform primary active transport (e.g. Na+/K+-ATPase)

2. Secondary active exchangers - perform secondary active transport (e.g. K/Cl)

They can be symporters (transport ions in same direction, e.g. SGLT) or antiporters (transport ions in opposite directions, e.g. Na+/K+-ATPase).

To change membrane potential, do we require movement of many ions or a few ions across the cell membrane?

Very large changes in membrane potential occur from very small changes in ion concentration.

Thus, only a couple of ions need to move across the membrane to result in significant changes in membrane potential.

What is an inward current?

Movement of positive ions into the cell.

• Positivity is moving into the cell.

Movement of negative ions out of the cell.

• Positivity is moving into the cell because negativity is moving out of the cell.

Overall, positive charge inside (relative to outside).

What is an outward current?

Movement of positive ions out of the cell.

• Positivity is moving into the cell.

Movement of negative ions into the cell.

• Positivity is moving out of the cell because negativity is moving into the cell.

Overall, negative charge inside (relative to outside).

Where do connections between neurons occur? and how do they occur?

At synapses

Neurons have to fire action potentials to communicate with other neurons.

What needs to happen for action potential to fire?

Depolarisation of the membrane potential in the axon hillock from resting (~ -70 mV) to threshold (~ -40 mV) potential.

What creates depolarisation? and what does it do?

Depolarisation is due to the integrated sum of responses to all active synaptic inputs to a neuron.

Depolarisation activates voltage-gated ion channels that are permeable to Na+ or K+.

What are the 4 phases of an action potential?

1. Rising

2. Overshoot

3. Falling

4. Undershoot

What is the rising phase of an action potential?

Depolarisation

Triggered when MP reaches threshold potential (~ -55 mV).

Voltage-gated Na+ channels open rapidly.

Na+ rushes into the cell.

Inside of the cell becomes less negative / more positive.

This strong depolarisation leads to more and more voltage-gated Na+ channels opening, creating a positive feedback loop.

What is the overshoot phase of an action potential?

Na+ continues entering the cell, briefly making membrane potential positive (+30 - +40 mV).

What is the falling phase of an action potential?

Repolarisation

Voltage-gated Na+ channels inactivate.

Voltage-gated K+ channels open, allowing K+ to flow out of the cell, down its conc gradient - restoring negative charge inside the cell.

Membrane potential falls back down toward resting level (~ -70 mV).

What is the undershoot phase of an action potential?

Hyperpolarisation

Voltage-gated K+ channels remain open a bit too long, allowing excess K+ to leave the cell.

The membrane becomes more negative than resting potential (~ -80 to -90 mV).

Eventually, K+ channels close and membrane potential returns to resting.