Problem 3 - College Physics

Question

Transmission of nerve impulses. Nerve cells transmit electric signals through their long tubular axons. These signals propagate due to a sudden rush of \(\mathrm{Na}^{+}\) ions, each with charge \(+e,\) into the axon. Measurements have revealed that typically about \(5.6 \times 10^{11} \mathrm{Na}^{+}\) ions enter each meter of the axon during a time of 10 \(\mathrm{ms}\) . What is the current during this inflow of charge in a meter of axon?

Step-by-Step Solution

Answer

The current is \(8.96 \times 10^{-6}\) A.

Step 1: Determine Total Charge Inflow

The number of \(\mathrm{Na}^{+}\) ions entering the axon in one meter is given as \(5.6 \times 10^{11}\). Each \(\mathrm{Na}^{+}\) ion has a charge \(+e\), where \(e\) is the elementary charge \(1.6 \times 10^{-19}\) C. Therefore, the total charge \(Q\) is calculated using: \[ Q = (5.6 \times 10^{11}) \times (1.6 \times 10^{-19}) \]. This results in \( Q = 8.96 \times 10^{-8} \) C.

Step 2: Convert Time to Seconds

The duration for which the ions flow is given as 10 milliseconds. We need time in seconds for the current calculation. Thus, \(10\) ms is equivalent to \(0.01\) seconds.

Step 3: Calculate the Current

Current \(I\) is defined as the rate of flow of charge, expressed as \[ I = \frac{Q}{t} \], where \(Q\) is the total charge and \(t\) is the time in seconds. Substituting the values, \[ I = \frac{8.96 \times 10^{-8}}{0.01} \]. This gives \( I = 8.96 \times 10^{-6} \) A.

Key Concepts

Nerve Impulse Transmission

Nerve impulse transmission is a fascinating process where nerve cells, otherwise known as neurons, communicate by transmitting electrical signals. These signals travel along the axon's length, which is a part of the neuron designed specifically for this purpose. The axon is like a long tubular highway and plays a crucial role in ensuring messages are sent quickly and efficiently to other parts of the body.

A nerve impulse, also known as an action potential, is triggered when there is a change in electrical charge across the neuron's membrane. This shift in charge occurs due to the movement of ions—atoms with a net electric charge—across the membrane. The process starts with the opening of ion channels, prompting the influx or outflow of ions like sodium and potassium. As these charged particles move, they create an electric current, thus propagating the impulse down the axon.

This rapid exchange of ions generates the electric signals needed for neurons to communicate swiftly. Understanding this transmission is essential for comprehending how our brains process information, control movements, and interpret sensory experiences.

Sodium Ion Flow

Sodium ion flow is a critical component in the transmission of nerve impulses. It involves the movement of sodium ions (\(\mathrm{Na}^{+}\) from outside the nerve cell into the cell through sodium channels. These channels are specialized proteins in the cell membrane that open to allow ions to flow in and out, triggering electrical signals.

When a nerve impulse begins, sodium channels open, and there is a rapid influx of sodium ions into the axon. This inflow causes the inside of the cell to become more positively charged compared to the outside. This change in charge is what drives the nerve impulse along the axon.

The flow of sodium ions is typically measured in terms of concentration and charge. In nerve cells during an impulse, this flow is quantified to estimate how much change it induces in the membrane potential, and it's crucial for quickly relaying signals in the nervous system. Without proper sodium ion flow, the transmission of nerve signals would be impaired, affecting overall neurological function.

Quantitative Problem Solving

Quantitative problem solving involves using mathematical methods and calculations to understand and solve problems. When analyzing nerve impulse transmission, applying quantitative skills can involve determining levels of ion flow, current flow, or the charge transferred during an impulse.

In the example exercise, we calculate the current generated when sodium ions enter a meter of axon during a nerve impulse. We do this by first determining the total charge that the sodium ions bring into the axon using the formula: \[ Q = (\text{number of ions}) \times (\text{elementary charge}) \].

After finding the total charge, the next step is calculating the current over a specific time frame. This is done using the formula: \[ I = \frac{Q}{t} \], where \(Q\) represents total charge, and \(t\) is time. Such calculations enable us to quantify the biological activity within nerve cells, leading to better understanding of their functioning and behavior.

Elementary Charge

The concept of elementary charge is pivotal in both physics and biology. It refers to the smallest unit of electric charge that is possible in isolated systems, which is a fundamental constant in nature. The elementary charge is denoted by \(e\) and has a value of approximately \(1.6 \times 10^{-19}\) coulombs.

In biological systems, this concept becomes pertinent when considering the charge of ions like sodium \(\mathrm{Na}^{+}\), which carry exactly one elementary charge. When discussing ion movement across cell membranes during nerve impulses, calculating the total charge involves multiplying the number of ions by \(e\).

Understanding elementary charge helps in making precise measurements of ion-related activities in cells. This can be especially useful in neuroscience where electric charge can impact neuron functioning. Knowing how elementary charge works enables scientists and researchers to conduct accurate bioelectric experiments and develop medical technologies targeting neural activity.