# 5.5 Nerve Impulse A nerve impulse is a signal transmitted along a nerve fiber. It consists of a wave of electrochemical changes that travel from one end of a neuron to the other, carrying information from receptors to the Central Nervous System (CNS) and from the CNS to effectors. ## Membrane Potential The electrical potential (a capacity to do electrical work) across the neuron's membrane. It exists in two primary states: - Resting Membrane Potential (RMP) - Action Potential (AP) ### Resting Membrane Potential (RMP) The RMP is the membrane potential of a neuron that is not currently sending an impulse. It is in a *polarized state*. - **Characteristics:** - The outer surface of the membrane is more positive than the inner surface. - The potential difference is approximately **-70 mV** (the inside is negative relative to the outside). - **Factors Maintaining RMP:** 1. **Sodium-Potassium (Na⁺/K⁺) Pump:** - Actively transports ions against their concentration gradients, using energy (ATP). - Pumps **3 Na⁺ ions out** for every **2 K⁺ ions pumped in**. - This results in a net positive charge moving out, making the inside more negative. - Maintains high [K⁺] inside and high [Na⁺] outside the neuron. 2. **Negative Organic Ions:** - Large, negatively charged molecules (proteins, amino acids, RNA) are trapped inside the neuron, contributing to the negative internal charge. 3. **Potassium (K⁺) Leak Channels:** - The membrane is more permeable to K⁺ than to Na⁺ at rest due to "leaky" K⁺ channels. - K⁺ ions continuously leak out of the cell down their concentration gradient, further increasing the positive charge on the outside. ### Action Potential (AP) An action potential is a rapid, temporary reversal of the membrane potential that occurs when a neuron is stimulated by a **threshold stimulus**. It is a wave of *depolarization* followed by *repolarization*. #### Phases of an Action Potential 1. **Stimulus and Depolarization:** - A **threshold stimulus** (a stimulus strong enough to trigger an AP) is received. - Voltage-gated **Na⁺ channels open** rapidly. - Na⁺ ions rush *into* the neuron, down their electrochemical gradient. - The membrane potential changes from -70 mV to a positive value (up to **+50 mV**). This reversal of charge is called **depolarization**. 2. **Repolarization:** - After a brief period (~1 ms), the Na⁺ channels close. - Voltage-gated **K⁺ channels open**. - K⁺ ions rush *out of* the neuron, down their electrochemical gradient. - The exit of positive K⁺ ions makes the inside of the membrane negative again, restoring the polarized state. 3. **Hyperpolarization:** - The K⁺ channels are slow to close, causing a brief "overshoot" where the membrane becomes *more negative* than the resting potential. 4. **Refractory Period:** - A brief recovery period after an AP during which the neuron cannot generate another impulse. - The **sodium-potassium pump** actively works to restore the original ion concentrations (high K⁺ in, high Na⁺ out). - This period ensures the one-way travel of the nerve impulse and limits the frequency of firing (to about 250 impulses/second). | Phase | Membrane State | Key Event | Ion Channel Status | Membrane Potential (mV) | | :--- | :--- | :--- | :--- | :--- | | **Resting** | Polarized | Na⁺/K⁺ pump active | Leaky K⁺ channels open | -70 mV | | **Depolarization** | Depolarized | Influx of Na⁺ ions | Voltage-gated Na⁺ channels open | Rises to +50 mV | | **Repolarization** | Repolarizing | Efflux of K⁺ ions | Na⁺ channels close, K⁺ channels open | Falls from +50 mV | | **Hyperpolarization** | Hyperpolarized | K⁺ channels slow to close | K⁺ channels closing | Dips below -70 mV | ### Velocity of Nerve Impulse The speed at which a nerve impulse travels is variable and depends on two main factors: 1. **Myelination:** - *Myelinated fibers* are covered by an insulating myelin sheath. The impulse "jumps" from one gap (Node of Ranvier) to the next in a process called **saltatory conduction**. This is extremely fast (up to **120 m/s**). - *Non-myelinated fibers* conduct the impulse continuously along the entire axon membrane, which is slower (**1-3 m/s**). Myelination also conserves energy. 2. **Axon Diameter:** - Thicker axons have less resistance to electrical current flow and therefore conduct impulses faster than thin axons. - **Synaptic Delay:** The transmission of a signal across a synapse takes approximately one millisecond, which is significantly slower than impulse travel along an axon. --- ## Possible Questions/Answers - **Q:** What is a nerve impulse? **A:** A wave of electrochemical changes (an action potential) that travels along the length of a neuron to transmit a signal. - **Q:** What is the difference between Resting Membrane Potential (RMP) and Action Potential (AP)? **A:** RMP is the stable, negative charge (-70 mV) of an unstimulated neuron (polarized state). AP is a rapid, temporary reversal of this charge (depolarization to +50 mV) that propagates along the axon when the neuron is stimulated. - **Q:** How is the Resting Membrane Potential maintained? **A:** It is maintained by three factors: the sodium-potassium pump (3 Na⁺ out, 2 K⁺ in), the presence of large negative organic ions inside the cell, and the outward leakage of K⁺ ions through leak channels. - **Q:** What causes depolarization during an action potential? **A:** The rapid opening of voltage-gated sodium (Na⁺) channels and the subsequent influx of Na⁺ ions into the neuron. - **Q:** What is saltatory conduction? **A:** It is the rapid transmission of a nerve impulse along a myelinated axon, where the action potential "jumps" from one Node of Ranvier to the next, which is much faster than continuous conduction in unmyelinated axons.