Neuroscience - Synaptic Transmission & Neural Circuits Flashcards

Question 1

What is the Resting Membrane Potential (RMP)?

Accepted Answer

The electrical potential difference across the plasma membrane of a neuron at rest, typically around $-70 mV$. It's maintained by the differential distribution of ions ($Na^+$, $K^+$, $Cl^-$) and the action of the $Na^+/K^+$ pump.

Question 2

Explain the role of the Sodium-Potassium ($Na^+/K^+$) Pump in maintaining RMP.

Accepted Answer

The $Na^+/K^+$ pump actively transports $3 Na^+$ ions out of the cell and $2 K^+$ ions into the cell for every ATP molecule hydrolyzed. This creates and maintains the concentration gradients essential for the RMP and action potentials.

Question 3

Define Depolarization in the context of a neuron.

Accepted Answer

Depolarization is a decrease in the absolute value of a cell's membrane potential, making the inside of the cell less negative (or more positive). It typically occurs due to the influx of positive ions, such as $Na^+$, into the cell.

Question 4

What is the Threshold Potential and its significance?

Accepted Answer

The threshold potential is the critical level of depolarization (typically around $-55 mV$) that must be reached for an action potential to be initiated. Once reached, voltage-gated $Na^+$ channels open rapidly, leading to a regenerative depolarization.

Question 5

Describe the 'Rising Phase' of an Action Potential.

Accepted Answer

The rising phase is characterized by rapid depolarization. Upon reaching threshold, voltage-gated $Na^+$ channels open, causing a massive influx of $Na^+$ ions into the cell, driving the membrane potential from negative to positive (e.g., up to $+30 mV$).

Question 6

Explain Repolarization during an Action Potential.

Accepted Answer

Repolarization is the phase where the membrane potential returns to a negative value. It occurs due to the inactivation of voltage-gated $Na^+$ channels and the opening of voltage-gated $K^+$ channels, leading to an efflux of $K^+$ ions out of the cell.

Question 7

What is Hyperpolarization (Undershoot) and why does it occur?

Accepted Answer

Hyperpolarization, or the undershoot, is a brief period where the membrane potential becomes even more negative than the resting potential. It occurs because voltage-gated $K^+$ channels close relatively slowly, allowing excess $K^+$ efflux before returning to the RMP.

Question 8

What is the 'All-or-None' Principle of Action Potentials?

Accepted Answer

The 'all-or-none' principle states that if a stimulus reaches the threshold potential, an action potential will fire with its full, characteristic amplitude. If the threshold is not reached, no action potential will occur. The strength of the stimulus does not affect the amplitude of the action potential, only its frequency.

Question 9

Differentiate between the Absolute and Relative Refractory Periods.

Accepted Answer

The **Absolute Refractory Period** is when a second action potential cannot be initiated, regardless of stimulus strength, because voltage-gated $Na^+$ channels are inactivated. The **Relative Refractory Period** is when a second action potential can be initiated, but only by a stronger-than-normal stimulus, because some $K^+$ channels are still open and $Na^+$ channels are recovering from inactivation.

Question 10

Provide an overview of Synaptic Transmission.

Accepted Answer

Synaptic transmission is the process by which neurons communicate. It involves the release of neurotransmitters from a presynaptic neuron, their diffusion across the synaptic cleft, and binding to receptors on a postsynaptic neuron, leading to an electrical response.

Question 11

How are Neurotransmitters synthesized and stored?

Accepted Answer

Neurotransmitters are synthesized either in the neuronal cell body (peptides) or in the presynaptic terminal (small-molecule NTs) from precursor molecules. They are then packaged and stored in synaptic vesicles within the presynaptic terminal.

Question 12

Describe the process of Neurotransmitter Release.

Accepted Answer

When an action potential reaches the presynaptic terminal, voltage-gated $Ca^{2+}$ channels open, causing $Ca^{2+}$ influx. This $Ca^{2+}$ triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft via exocytosis.

Question 13

Distinguish between Ionotropic and Metabotropic Receptors.

Accepted Answer

**Ionotropic receptors** are ligand-gated ion channels that open directly upon neurotransmitter binding, causing rapid ion flow and a quick postsynaptic response. **Metabotropic receptors** are G-protein coupled receptors that, upon binding, initiate a slower, longer-lasting intracellular signaling cascade that can indirectly modulate ion channels or other cellular functions.

Question 14

What is an Excitatory Postsynaptic Potential (EPSP)?

Accepted Answer

An EPSP is a transient depolarization of the postsynaptic membrane caused by the influx of positive ions (e.g., $Na^+$ or $Ca^{2+}$) when an excitatory neurotransmitter binds to its receptor. EPSPs increase the likelihood of the postsynaptic neuron firing an action potential.

Question 15

What is an Inhibitory Postsynaptic Potential (IPSP)?

Accepted Answer

An IPSP is a transient hyperpolarization or stabilization of the postsynaptic membrane caused by the influx of negative ions (e.g., $Cl^-$) or efflux of positive ions (e.g., $K^+$) when an inhibitory neurotransmitter binds. IPSPs decrease the likelihood of the postsynaptic neuron firing an action potential.

Question 16

How are Neurotransmitters inactivated or removed from the synaptic cleft?

Accepted Answer

Neurotransmitters are removed from the synaptic cleft by several mechanisms: 1) **Enzymatic degradation** (e.g., Acetylcholinesterase breaking down ACh), 2) **Reuptake** into the presynaptic terminal or glial cells via specific transporter proteins, and 3) **Diffusion** away from the cleft.

Question 17

Explain Spatial Summation.

Accepted Answer

Spatial summation is the process by which multiple presynaptic neurons simultaneously release neurotransmitters onto a single postsynaptic neuron, and their combined EPSPs (or IPSPs) summate to reach (or prevent reaching) the threshold potential at the axon hillock.

Question 18

Explain Temporal Summation.

Accepted Answer

Temporal summation is the process by which a single presynaptic neuron fires rapidly in succession, releasing neurotransmitters multiple times. The successive EPSPs (or IPSPs) overlap in time and summate to reach (or prevent reaching) the threshold potential at the axon hillock.

Question 19

What is the primary role of Acetylcholine (ACh) in the nervous system?

Accepted Answer

ACh is a key neurotransmitter at the neuromuscular junction, mediating muscle contraction. In the CNS, it plays roles in learning, memory, attention, and arousal. It can be excitatory or inhibitory depending on the receptor type (nicotinic or muscarinic).

Question 20

What is the primary excitatory neurotransmitter in the CNS?

Accepted Answer

Glutamate is the primary excitatory neurotransmitter in the CNS. It is crucial for learning, memory formation (e.g., long-term potentiation), and synaptic plasticity. Excessive glutamate can be neurotoxic.

Question 21

What is the primary inhibitory neurotransmitter in the CNS?

Accepted Answer

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the CNS. It reduces neuronal excitability by opening $Cl^-$ channels, leading to IPSPs, and is vital for regulating anxiety, sleep, and muscle tone.

Question 22

Name three common monoamine neurotransmitters and their general functions.

Accepted Answer

1. **Dopamine:** Involved in reward, motivation, motor control, and pleasure. 
2. **Norepinephrine (Noradrenaline):** Plays roles in arousal, attention, stress response, and mood. 
3. **Serotonin (5-HT):** Regulates mood, sleep, appetite, learning, and memory.

Neuroscience - Synaptic Transmission & Neural Circuits

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