Question

For a neuron to fire an action potential, its membrane must reach________. a. hyperpolarization b. the threshold of excitation c. the refractory period d. inhibitory postsynaptic potential

Short answer

Answer: The threshold of excitation.

Step-by-step solution

Understanding action potentials in neurons

An action potential is the rapid electrical signal that travels along the membrane of a neuron when it is stimulated. It is the neuron's way of transmitting information. The action potential is elicited once the membrane potential reaches a specific value called the threshold of excitation. This threshold is the point at which the net inward current becomes greater than the net outward current, causing depolarization of the membrane and initiation of the action potential.

Analyzing each option

a. Hyperpolarization: This refers to the process in which the membrane potential becomes more negative than the resting potential. Hyperpolarization makes it more difficult for the neuron to reach the threshold of excitation and fire an action potential. Hence, this option is incorrect. b. The threshold of excitation: This is the membrane potential value that must be reached for the neuron to generate an action potential. As mentioned previously, reaching this threshold triggers depolarization of the membrane and the propagation of the action potential. This option aligns with our understanding of action potentials, and it is the correct answer. c. The refractory period: This is the period following an action potential during which a neuron cannot initiate another action potential. The refractory period ensures that action potentials only travel in one direction and allows the neuron to recover from the previous action potential. This option is related to action potentials, but it is not the value that the membrane must reach for an action potential to be generated. Therefore, this option is incorrect. d. Inhibitory postsynaptic potential (IPSP): IPSPs are neurotransmitter-induced changes in the membrane potential that decrease the likelihood of an action potential. They are usually caused by the opening of ion channels that allow the passage of negatively charged ions into the neuron, leading to a hyperpolarization. This option is the opposite of what we're looking for, so it is also incorrect.

Selecting the correct option

Based on our analysis of each option, the correct answer is: b. The threshold of excitation

Key concepts

Threshold of excitation

The threshold of excitation is a crucial concept in understanding how neurons communicate. Imagine it as a tipping point that a neuron must reach to "decide" to send out an electrical signal called the action potential. When a neuron is stimulated, its membrane potential shifts. If this shift leads to a net inward current that surpasses the outward current, the threshold is reached.

Once this threshold is surpassed, the neuron becomes depolarized, meaning its membrane potential becomes more positive. This change initiates an action potential, allowing the neuron to transmit information along its axon. This threshold ensures that neurons do not fire off random signals, maintaining efficient communication within the nervous system.

Neuron membrane potential

The neuron membrane potential refers to the electrical potential difference across a neuron's cell membrane. At rest, this potential is usually termed the resting membrane potential, typically around -70 mV. This value indicates more negative charges inside the neuron compared to the outside. The membrane potential is critical because it sets the stage for changes that lead to neuronal signaling.

Various factors influence the membrane potential:

• The concentration of ions inside and outside the neuron, including potassium (K+) and sodium (Na+).
• The permeability of the neuron's membrane to these ions, regulated by ion channels.

Changes in the membrane potential, such as depolarization and hyperpolarization, are central to the neuron's ability to generate and transmit signals.

Refractory period

The refractory period is a time during and after an action potential when a neuron cannot fire another action potential immediately. It consists of two phases:

• Absolute refractory period: During this time, no amount of stimulation can cause another action potential. This ensures that each action potential is a distinct electrical impulse.
• Relative refractory period: Here, a stronger-than-usual stimulus is necessary to trigger an action potential. This phase helps control the frequency of neuronal firing.

The refractory period is essential for the orderly propagation of action potentials along neurons, ensuring signals only move in one direction and the neuron has time to reset for the next signal.

Inhibitory postsynaptic potential

An inhibitory postsynaptic potential (IPSP) helps modulate neuronal activity by making it less likely for a neuron to reach the threshold of excitation. It occurs when neurotransmitters bind to receptors on a postsynaptic neuron, causing specific ion channels to open. These channels typically allow negatively charged ions, like chloride (Cl-), to enter, hyperpolarizing the neuron.

This hyperpolarization moves the membrane potential further from the threshold, decreasing the chance of an action potential. IPSPs play a critical role in the nervous system by providing balance and preventing excessive neuronal firing. They help ensure that only important and relevant signals are propagated, contributing to the precise control of neurological processes.