Synapses and neurotransmitters

Question 1

Define excitability, and list the excitable cells in human body

Answer 1

• Excitability refers to the ability of cells to be electrically excited resulting in the formation of an action potential
• Neurons
• Muscles (smooth, skeletal and cardiac)
Endocrine cells

Question 2

Define membrane potential, and describe the basis of the resting membrane potential.

Answer 2

• Membrane potential refers to the electrical potential difference between the inside of the cell and the surrounding extracellular fluid
• Resting membrane potential is -70mV. It is said to be polarized. The inside of the cell is negative compared to the surrounding extracellular fluid
• The cell membrane has different permeability to ions (charged particles)
• This forms a concentration gradient between the inside and outside of the cell
• If potassium channels in the membrane open,K + will begin to move down its concentration gradient and out of the cell
• Every time aK+ion leaves the cell, the cell’s interior loses a positive charge
• Because of this, a slight excess of positive charge builds up on the outside of the cell membrane, and a slight excess of negative charge builds up on the inside
• This sets up a difference in electrical potential across the membrane
• Like magnets, in ions like charges repel each other and unlike charges attract
• So, the establishment of the electrical potential difference across the membrane makes it harder for the remainingK+ ions to leave the cell
• Positively chargedK+ions will be attracted to the free negative charges on the inside of the cell membrane and repelled by the positive charges on the outside, opposing their movement down the concentration gradient
• Eventually, the electrical potential difference across the cell membrane builds up to a high enough level that the electrical force drivingK+back into the cell is equal to the chemical force drivingit out= equilibrium potential for K+
* Factors such as the 3Na/ 2K pump help maintain this membrane potential

Question 3

Define ligand- gated and voltage- gated channels in relation to action potentials

Answer 3

• In relation to action potentials, aligand-gated channelopens because a ligand, or more specifically a neurotransmitter, binds to the extracellular region of the channel, opening it and enabling ions to cross the membrane and change its charge
* Avoltage-gated channelis a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage, so when that voltage becomes less negative, the channel begins to allow ions to cross the membrane

Question 4

Explain the steps that lead to the generation of an action potential

Answer 4

1. Depolarization:
   • This starts with a channel opening for Na+in the membrane
   • Because the concentration of Na+is higher outside the cell than inside, ions will rush into the cell
   • Sodium is a positively charged ion = makes inside of cell to become less negative =depolarization
   • The electrical gradient also plays a role, as negative proteins below the membrane attracts sodium ions
   • The membrane potential will reach +30 mV by the time sodium has entered the cell
2. Repolarisation
   • As the membrane potential reaches +30 mV, other voltage-gated channels are opening in the membrane (due to high positivity within the cell, they open)
   • These channels are specific for the potassium ions
   • As K+starts to leave the cell, taking a positive charge with it, the membrane potential begins to move back toward its resting voltage
   • Inactivation gates of Na+ ions are activated
3. Hyperpolarisation
   • Repolarization momentarily goes below the -70mV value of resting membrane potential
   • Potassium ions reach equilibrium when the membrane voltage is below -70 mV, so a period of hyperpolarization occurs while the K+channels are open
   * Those K+channels are slightly delayed in closing, accounting for this short overshoot

Question 5

Differentiate the prorogation of an action potential in unmyelinated and myelinated

Answer 5

For both types of neurons:
• It starts at the axon hillock, or at the beginning of the axon
• This a continuous process
* It is propagated in one direction along the axon

Unmyelinated neurons
• The positive charge associated with the first action potential at the axon hillock begins to spread to areas ahead of it
• As a result, sodium voltage gated channels open, enabling more sodium to rush into the cell. A threshold is reached, and an action potential is generated
* The region where the first action potential was fired at axon hillock enters a repolarization stage while the region ahead of it experiences depolarization

Myelinated neurons
• Action potentials cannot be generated within Myelin sheaths because they are an insulating layer
• Action potentials can only be generated in the unmyelinated areas called nodes of Ranvier
• Thus, the action potentials have to “jump” from one node to another
* Hence, neurotransmission is faster within myelinated sheaths than non myelinated sheaths

Question 6

Define synapse and neurotransmitter

Answer 6

• In the central nervous system a synapse is a small gap at the end of a neuron that allows a signal to pass from one neuron to the next
• Neurotransmitters are the molecules used by the nervous system to transmit messages betweenneurons, or from neurons to muscles

Question 7

From a physiological point of view, explain how anaesthetics work

Answer 7

• Local anaesthetic – A drug which reversibly prevents transmission of the nerve impulse in the region to which it is applied, without affecting consciousness
* Lignocaine binds to voltage-gated Na+ channels in the peripheral nerve cell membrane and blocks the influx of Na+

Question 8

From a physiological point of view, explain tooth pain

Answer 8

A fibre: Myelinated, bigger diameter

• Fast transmission in information
• sharp pain, well localised

C Fibre: Unmyelinated, thinner diameter

• Slow pain, burning sensation, lingering pain
• chronic pain, poorly localised

Question 9

What are the effects of nerve diameter and myelination on conductance?

Answer 9

Nerve diameter:
• Larger diameter axons have a higher conduction velocity= send signals faster
• This is because there is less resistance facing the ion flow
• We have a lot of ions flooding into the axon, so the more space they have to travel, the more likely they will be able to keep going in the right direction
• Axons are full of cytoplasmic proteins, vesicles, etc. The larger the diameter of the axon, the less likely the incoming ions will run into something that could bounce them back

Myelination
• The second way to speed up a signal in an axon is to insulate it with myelin, a fatty substance
• In the PNS= myelin and in the CNS = oligodendrocytes
• These cells wrap around the axon, creating several layers insulation
• The presence of myelin makes ions escaping via crossing the membrane impossible
• Plus, depolarisation can only occur at the nodes of Ranvier
• Since the action potential jumps from node to node (this is called saltatory conduction), the action potential travels a greater distance for a shorter period of time
• In an unmyelinated axon, every singlesection of the membrane will have to be depolarised for the impulseto conduct along the axon, hence taking more time.