Membrane Potential, Synapses, and Neurotransmitters

Question 1

What are excitable cells?

Answer 1

Cells that can be electrically excited resulting in the generation of action potentials

Examples = neurons, muscle cells (skeletal, cardiac, smooth), some endocrine

Question 2

What is action potential?

Answer 2

Brief reversal of electric polarisation of the cell membrane
The dynamic changes in the membrane potential in response to stimulus
Also called nerve impulse
After action potential, cell membrane returns to its resting state

Question 3

What are nerve impulses?

Answer 3

Nerve impulses are the commuincation between cells and the cause of action potential

Very brief electrical impulses that travel along the nerve fibre (axon), this electrical signal is converted to chemical signal via neurotransmitters

Question 4

What are the key advantages of nerve impulses?

Answer 4

Fastest process in the cell

Energy efficient by applying energy gradients that already exist

Dependable over long distances

Question 5

What is membrane potential and what is its function?

Answer 5

Electrical potential is the difference between the inside of the cells and the surrounding extracellular fluid

Present in all cells and especially important in nerve and msucle cells

Function = code and transmit sensory and motor information

Question 6

What is resting membrane potential?

Answer 6

When the cell is in its “rest state”, determined by the concentration gradient of ions across the membrane

Neuron = -70mV (polarised)

The inside of the cell is negative with respect to the surrounding extracellular fluid

Question 7

What is an ion?

Answer 7

An atom or molecule in which the total number of electrons is not equal to the total number of protons (charged)

Cation = positive charge
Anion = negative charge

Question 8

How is resting membrane potential achieved?

Answer 8

Higher levels of sodium (Na+) and chloride (Cl-) outside of cells
Higher levels of potassium (K+) and anions inside the cell

Inside neuron is more negatively charged than outside, resting membrane potential = -70mV
This is maintained through transport proteins such as 3Na / 2K + ATP-ase pumps and ion channels which allow K+ or Na+ to move fairly easily through the cell membrane
Many more K+ channels than Na+ channels

Question 9

How does potassium use ion channels to maintain resting membrane potential?

Answer 9

Less potassium outside the cell which creates concentration gradient encourgaing K+ from inside to cross the membrane

As K+ diffuse out of cell, electrical potential created by the remaining anions (remember that -ve charge) pull K+ back in

Resting membrane potential is achieved when the concentration gradient and electrical potential are equalise (-70mV)

Question 10

How are action potentials generated?

Answer 10

Ligand-gated channels open with stimulating allowing inflow of cations (Na+ and Ca+) which lowers the membrane potential

Lowered membrane potential then triggers voltage-gated Na+ channels to open making the cell ~ 600 times more permeable to Na+

This achieves local depolarisation of cell membrane (membrane potential mecomes less negative +30mV and spreads depolarisation down the axon)

This is action potential

Question 11

What is a voltage-gated Na+ channel?

Answer 11

Ion selective (Na+ only)

Gate controlled by a voltage sensor which responds to the level of membrane potential unlike ligand-gated channels which respond to neurotransmitter signals

Normally closed and open only when prompted by gating agent - voltage above the threshold

Question 12

What are the three phases of actoin potential?

Answer 12

Phase 1 = Depolarisation

Phase 2 = Repolarisation

Phase 3 = Hyper-polarisation and back to resting membrane potential

Question 13

How does repolarisation occur?

Answer 13

Inactivation of voltage-gated NA+ cahnnels after 0.5 - 1 msec

Opening of voltage-gated K+ channels and K+ efflux starts to re-polarise the cell

Question 14

How does hyper-polarisation and achieving resting membrane potential happen?

Answer 14

Increased K+ permeability from voltage-gated K+ channels which has a delayed closure process causing an icnrease in polarisation

Voltage-gated Na+ cahnnels return to active but closed status

The Na+/K+ pump returns the membrane potential to resting state

Question 15

What is the purpose of hyper-polarisation?

Answer 15

When voltage-gated Na+ channels are in their inactive state, they cannot be opened again until they have a returned to a closed state

This ensures a refactory period when the membrane is hyper-polarised and not as excitable

Ensures one direction of current flow

Question 16

How does axon resistance impact the speed of action potential propagation?

Answer 16

Larger diameter = less resistance
Myelin provides additional isolation on the axon, maintains the initial stimulus
Action potentials are only produced in the exposed spaces of ranvier’s nodes
Current spreads under myelin sheath = appears to jump across
Therefore, unmyelinated nerve has a slower conduction

Question 17

What is synaptic signalling?

Answer 17

It’s a special case of paracrine signalling
Can only occur between cells linked with synapses between the originating cell and receiving cell
It’s the communication between neurons or between neurons and effectors

At synapses a chemical signal (transmitter) is released from one neuron and diffuses into another neuron or target cell to generate a signal

Question 18

What is a synapse?

Answer 18

A junction between two nerve cells consistinng of a tiny gap across whcih impulses pass by diffusion of a neurotransmitter

Question 19

What is a neurotransmitter?

Answer 19

Biological active chemicals that transmit signals from one neuron to another or to a target cell across a synapse

There are two types: Excitatory NTs and Inhibitory NTs

Question 20

What’s the difference between excitatory NTs and inhibitory NTs?

Answer 20

Excitatory NTs generates an action potential in the receiving neuron
(influx of NA+ > depolarisation > action potential)

Inhibatory NTs prevent action potentials
(influc of Cl- > hyper-polarisation > difficult to reach threshold)

Question 21

How do A-delta fibres and C fibres transmit tooth pain?

Answer 21

A-delta fibres are myelinated and thicker (fast transmitting information, sharp pain, well localised)

C fibres are unmyelinnated and thinner (slow pain, burning sensations and lingering pain, chronic pain that is poorly localised)

Question 22

How does LA impact pain transmission?

Answer 22

LA - a drug which reversibly prevents transmission of the nerve impulse in the region where it is applied, without affectingn consciousness

Lignocaine binds to voltage-gated Na+ channels in the peripheral nerve cell membrane and blocks the influx of Na+