1.Types of ion channels found in nerve cell membranes:

• Voltage-gated ion channels
• Ligand-gated ion channels

 

2. 3 Differences between Voltage-gated and  Ligand-gated ion channels:

Voltage-gated ion channels open and close in response to a change in membrane potential to control ion flow (do not bind to neurotransmitters). Whereas, ligand-gated channels open and close in response to the binding of neurotransmitter and are weakly sensitive to changes in membrane potentials.

Voltage-gated ion channels are usually selective with regards to allowing only certain ions to permeate into the cell, they also do not bind to neurotransmitters directly. They also have channels that act much slower time scale. Conversely, ligand-gated channels rely on the binding of neurotransmitter to receptors and the activation of these channels is very short and are responsible for fast synaptic transmission.

Voltage-gated ion channels are very concentrated on the initial segment of the axon in the nerve cells and are also found on the cell body and dendrites. These channels allow the movement of sodium, potassium and calcium. In contrast, ligand-gated ion channels are membrane proteins that contain a pore for ion flow. These types of ion channels respond to the binding to gamma aminobutyric acid (GABA), acetylcholine, glutamate and serotonin.

           

3. Comparison of Ionotropic receptors and Metabotropic receptors:

Ionotropic receptors	Metabotropic receptors
Consists of multiple subunits per receptor.	Consists of 7 transmembrane G-protein-coupled receptor.
Binding of neurotransmitter to receptor directly opens ion channels.	Binding of neurotransmitter does not directly open channel, but rather activates production of second messengers that conveys signal intracellularly to open channel.
Channels are insensitive to membrane potential and activation is quick.	Activation of these receptors lead to regulation of voltage-gated ion channels.
Effects of receptors are short.	Effects of receptor activation can last up to several minutes.
Transmission follows hierarchical pathways in the central nervous system.	These receptors follow the diffuse neuronal system in the central nervous system.

 

4. CNS receptors classification as Ionotropic or Metabotropic and transduction mechanisms:

Ionotropic receptors:

• GABAᴀ receptors with gamma aminobutyric acid as a transmitter – It has an Inhibiting Post-Synaptic Potential (IPSP), it increases Clˉ conduction.
• Nicotinic receptors with acetylcholine as a transmitter – It has an Excitatory Post-Synaptic Potential (EPSP), it increases cation conduction (Na⁺).
• EAA receptors with glutamate as a transmitter – It has an EPSP, it increases cation conduction, especially Ca²⁺.
• 5-HT₃ receptors with serotonin as a transmitter – It has an EPSP, it increases cation conduction.

Metabotropic receptors:

• 7-Transmembrane G-protein-coupled receptors that activate the production of second messengers when neurotransmitters bind to the receptor on the G-protein. It follows one of two transduction systems, namely:
  • Adenylyl cyclase system that makes use of adenylyl cyclase to convert ATP to c-AMP, which uses PDE to convert to AMP(c-AMP activates kinase enzymes which phosphorylate enzymes and effect ion channels).
  • Phospholipase C system that makes use of phospholipase c to convert PIP₂ to IP₃ and DAG (which are both second messengers).

 

5. Difference between EPSP and IPSP, with examples:

EPSP – excitatory post synaptic potential. Change in charge that allows it to become more positive than the resting membrane potential and become closer to the threshold of excitation. Also known as depolarization and causes an increase in cation permeability. For example, the sudden inward flow of Na⁺ into the cell, causing the cell to become more positive and generate an action potential.

IPSP – inhibitory post synaptic potential. Change in charge that makes the current become more negative compared to the resting membrane potential and move further away from the threshold of excitation. Also known as hyperpolarization. For example, the opening of chloride channels allowing Clˉ to flow into the cell, making it more negative inside the cell. An EPSP that initiated an action potential under normal conditions is now inhibited and fails to initiate an action potential.

 

6. Role of Calcium in the development of synaptic potential:

As an action potential propagates down the axon to the presynaptic terminal, it activates the opening of voltage-gated calcium channels. The calcium rushes into the terminal and then causes the fusion of the synaptic vesicles containing neurotransmitters with the membrane of the pre-synapse. The vesicles release the neurotransmitters into the synaptic cleft and they diffuse to the receptors found on the post-synaptic membrane.