Question

Hyperpolarization is caused by all of these neurotransmitters excepl a. glutamic acid in the CNS. b. \(A C h\) in the heart. c. glycine in the spinal cord. d. GABA in the brain.

Short answer

The neurotransmitter that does not cause hyperpolarization is (a) glutamic acid in the CNS. Glutamic acid is an excitatory neurotransmitter that helps in the transmission of action potentials by depolarizing the neuron's membrane potential.

Step-by-step solution

Review the effects of glutamic acid in the CNS

Glutamic acid is an excitatory neurotransmitter in the central nervous system (CNS). It facilitates the transmission of an action potential by helping to depolarize the neuron's membrane potential, thus making it easier for an action potential to occur. Therefore, glutamic acid does not cause hyperpolarization.

Review the effects of ACh in the heart

Acetylcholine (ACh) acts on the muscarinic receptors in the heart and causes hyperpolarization by increasing the permeability of the membrane to potassium ions, thus making the membrane potential more negative. Therefore, ACh in the heart does cause hyperpolarization.

Review the effects of glycine in the spinal cord

Glycine is an inhibitory neurotransmitter in the spinal cord. It opens chloride channels in the postsynaptic membrane, causing chloride ions to flow into the cell. This makes the membrane potential more negative, which inhibits the transmission of action potentials. As a result, glycine in the spinal cord does cause hyperpolarization.

Review the effects of GABA in the brain

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter in the brain. It also opens chloride channels in the postsynaptic membrane, leading to an increased flow of chloride ions into the cell. This results in hyperpolarization, as the membrane potential becomes more negative and inhibitory to action potentials. Therefore, GABA in the brain does cause hyperpolarization.

Identify the neurotransmitter that does not cause hyperpolarization

From the above analysis, we can conclude that: a. Glutamic acid in the CNS does not cause hyperpolarization. b. ACh in the heart does cause hyperpolarization. c. Glycine in the spinal cord does cause hyperpolarization. d. GABA in the brain does cause hyperpolarization. Hence, the correct answer is (a) glutamic acid in the CNS.

Key concepts

Hyperpolarization

Hyperpolarization occurs when a neuron's membrane potential becomes more negative than its resting potential. This process plays a crucial role in regulating nerve signal transmission. During hyperpolarization, it becomes more difficult for an action potential to be initiated, as the inside of the neuron becomes overly negative compared to the outside.
Neurons rely on the flow of ions such as potassium (K+) and chloride (Cl-) through channels in their membranes to create this electrical gradient.
Here's a quick look at what causes hyperpolarization:

• An increase in K+ ion flow out of the neuron.
• An increase in Cl- ion flow into the neuron.

Hyperpolarization is fundamentally important because it helps prevent excessive neuronal excitation, contributing to the proper balance and function of nervous systems.

Glutamic Acid

Glutamic acid, often just referred to as glutamate, is primarily known as an excitatory neurotransmitter in the central nervous system (CNS).
It helps propagate the signal between neurons by increasing the chances that the following neuron will fire its own action potential.
In essence, glutamic acid does the opposite of hyperpolarization; it facilitates depolarization.

• Glutamate binds to its receptors, leading to an opening of sodium (Na+) or calcium (Ca2+) ion channels.
• This influx of positive ions makes the interior of the neuron more positive, thus closer to triggering an action potential.

Therefore, in contrast to hyperpolarization, glutamic acid in the CNS is linked with excitatory actions.

GABA

Gamma-aminobutyric acid, or GABA, is a key inhibitory neurotransmitter in the brain.
It works by opening chloride channels in the neuron's cell membrane, causing an influx of Cl- ions, leading to hyperpolarization.
In this manner, GABA significantly reduces the likelihood of a neuron firing an action potential.

• By making the inside of the neuron more negative, GABA effectively calms neuron activity.
• This inhibitory function is essential for preventing overactivity and plays a role in conditions like anxiety when dysregulated.

Through its hyperpolarizing action, GABA is pivotal in maintaining the brain's balance of excitation and inhibition.

Acetylcholine

Acetylcholine (ACh) is a versatile neurotransmitter involved in several functions depending on where it's acting in the body.
When acetylcholine acts on muscarinic receptors in the heart, it leads to hyperpolarization.
This happens by increasing the heart cell's permeability to potassium ions, which exit the cell, making the membrane potential more negative.

• This action of ACh effectively slows down the heart rate, as it makes the membrane less likely to reach the threshold needed for action potential generation.
• Different from its excitatory role at neuromuscular junctions, ACh's inhibitory role in the heart helps to regulate cardiac function and maintain balance.

ACh's ability to cause hyperpolarization highlights its diverse functions and the specificity of neurotransmitter action based on target tissue.

Glycine

Glycine is another inhibitory neurotransmitter, but it primarily exerts its effects in the spinal cord and brainstem areas.
Glycine causes hyperpolarization by facilitating the influx of Cl- ions, much like GABA.
This lowers the resting membrane potential and makes it more difficult for an action potential to occur.

• Glycine opens chloride channels, increasing the passage of Cl- ions into neurons.
• This hyperpolarizing effect has a calming influence, reducing the possibility of excessive neuronal firing in these regions.

Due to its inhibitory role, glycine is essential for controlling motor and sensory pathways, ensuring smooth muscle movement and response.