What is the 'all-or-none' principle in nerve impulse transmission?

The 'all-or-none' principle states that once a stimulus reaches the threshold potential, a neuron will fire a full-strength action potential, regardless of the strength of the stimulus beyond that threshold. If the stimulus is below the threshold, no action potential will be generated at all. It's like firing a gun: once you pull the trigger past a certain point, the bullet fires with its full force, and pulling the trigger harder won't make the bullet go faster or hit harder. This ensures consistent signal strength throughout the nervous system.

How does the Na\textsuperscript{+}/K\textsuperscript{+} pump contribute to the resting membrane potential?

The Na\textsuperscript{+}/K\textsuperscript{+} pump is an active transport mechanism that uses ATP to move ions against their concentration gradients. It actively pumps three sodium ions (Na\textsuperscript{+}) out of the neuron for every two potassium ions (K\textsuperscript{+}) it pumps into the neuron. This unequal exchange of positive charges creates a net loss of positive charge from inside the cell, contributing significantly to the negative resting membrane potential. It also maintains the crucial concentration gradients for Na\textsuperscript{+} and K\textsuperscript{+} that are essential for action potential generation.

What is the difference between an EPSP and an IPSP?

An EPSP (Excitatory Postsynaptic Potential) is a temporary depolarization of the postsynaptic membrane, making it less negative and thus bringing it closer to the threshold for firing an action potential. This typically occurs due to the influx of positive ions like Na\textsuperscript{+}. An IPSP (Inhibitory Postsynaptic Potential), conversely, is a temporary hyperpolarization or stabilization of the postsynaptic membrane, making it more negative or harder to depolarize, thus moving it further away from the threshold and making it less likely to fire an action potential. This often involves the influx of Cl\textsuperscript{-} or efflux of K\textsuperscript{+}.

Why is saltatory conduction faster than continuous conduction?

Saltatory conduction occurs in myelinated axons, where the myelin sheath acts as an electrical insulator, preventing ion flow across the membrane except at the Nodes of Ranvier. The action potential 'jumps' from one node to the next, regenerating only at these unmyelinated gaps. This significantly speeds up conduction because the impulse doesn't have to be regenerated along every millimeter of the axon. In continuous conduction (unmyelinated axons), the impulse must be regenerated sequentially along the entire membrane, which is a much slower process, requiring more ion channel opening and closing events.

What role does calcium play in synaptic transmission?

Calcium ions (Ca\textsuperscript{2+}) play a critical role in the presynaptic terminal during chemical synaptic transmission. When an action potential arrives at the presynaptic terminal, it depolarizes the membrane, opening voltage-gated Ca\textsuperscript{2+} channels. The influx of Ca\textsuperscript{2+} into the terminal acts as a crucial signal. This rise in intracellular Ca\textsuperscript{2+} concentration triggers the fusion of synaptic vesicles, which contain neurotransmitters, with the presynaptic membrane. This fusion leads to the release of neurotransmitters into the synaptic cleft via exocytosis, initiating the chemical signal to the postsynaptic neuron.

Transmission of Nerve Impulse

Biology

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Version 1Updated 22 Mar 2026

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The transmission of a nerve impulse, also known as an action potential, is the fundamental mechanism by which information is conveyed throughout the nervous system. This intricate process involves a rapid, transient change in the electrical potential across the neuronal membrane, propagating from one end of the neuron to the other and then across a synapse to another neuron or effector cell. It is…

Quick Summary

The transmission of a nerve impulse is an electrochemical process that allows neurons to communicate. It begins with the maintenance of a negative resting membrane potential (around -70mV) inside the neuron, primarily due to the Na\textsuperscript{+}/K\textsuperscript{+} pump and differential membrane permeability to ions, especially K\textsuperscript{+}.

When a sufficient stimulus reaches the threshold potential, voltage-gated Na\textsuperscript{+} channels open, causing rapid Na\textsuperscript{+} influx and depolarization (rising phase of action potential).

This is followed by repolarization, where Na\textsuperscript{+} channels inactivate and voltage-gated K\textsuperscript{+} channels open, leading to K\textsuperscript{+} efflux. A brief hyperpolarization may occur before the resting potential is restored.

This action potential propagates along the axon, either continuously in unmyelinated fibers or via faster saltatory conduction in myelinated fibers (jumping between Nodes of Ranvier). At the axon terminal, the electrical signal is converted into a chemical signal: Ca\textsuperscript{2+} influx triggers the release of neurotransmitters into the synaptic cleft.

These neurotransmitters bind to receptors on the postsynaptic membrane, causing either an excitatory (EPSP) or inhibitory (IPSP) potential, which, if summated to threshold, can generate a new action potential in the postsynaptic neuron.

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Key Concepts

Na\textsuperscript{+}/K\textsuperscript{+} Pump and RMP

The sodium-potassium pump is a crucial active transport protein that maintains the concentration gradients of…

Voltage-Gated Ion Channels and Action Potential Phases

Action potentials are entirely dependent on the sequential opening and closing of voltage-gated ion channels.…

Synaptic Integration: Summation of PSPs

A single excitatory postsynaptic potential (EPSP) is usually insufficient to depolarize the postsynaptic…

RMP: —

-70\,\text{mV}

, maintained by Na\textsuperscript{+}/K\textsuperscript{+} pump (3 Na\textsuperscript{+} out, 2 K\textsuperscript{+} in) and K\textsuperscript{+} leak channels.

Action Potential Phases:

- Threshold: $-55\,\text{mV}$ (all-or-none). - Depolarization: Voltage-gated Na\textsuperscript{+} channels open, Na\textsuperscript{+} influx (inside becomes positive). - Repolarization: Voltage-gated Na\textsuperscript{+} channels inactivate, voltage-gated K\textsuperscript{+} channels open, K\textsuperscript{+} efflux (inside becomes negative). - Hyperpolarization: Slow K\textsuperscript{+} channel closure, membrane briefly more negative than RMP.

Conduction:

- Continuous: Unmyelinated, slower. - Saltatory: Myelinated, faster (jumps between Nodes of Ranvier), energy efficient.

Synaptic Transmission (Chemical):

- AP arrives at presynaptic terminal $\rightarrow$ Voltage-gated Ca\textsuperscript{2+} channels open $\rightarrow$ Ca\textsuperscript{2+} influx $\rightarrow$ Neurotransmitter release (exocytosis). - Neurotransmitter binds to postsynaptic receptors $\rightarrow$ EPSP (depolarization, e.g., Na\textsuperscript{+} influx) or IPSP (hyperpolarization, e.g., Cl\textsuperscript{-} influx or K\textsuperscript{+} efflux). - Neurotransmitter removal: Degradation, reuptake, diffusion.

Summation: — Spatial (multiple inputs) and Temporal (rapid successive inputs).

For the sequence of ions in action potential phases: Naughty Kids Rush Home.

Naughty: Na\textsuperscript{+} influx (Depolarization)

Kids: K\textsuperscript{+} efflux (Repolarization)

Rush: Resting potential restoration (Na\textsuperscript{+}/K\textsuperscript{+} pump)

Home: Hyperpolarization (brief undershoot)