Action Potential of a Neuron

1. Resting Membrane Potential:

  • A neuron at rest has a negative charge inside relative to the outside, sitting at around -70 mV. This is known as the resting membrane potential.

  • The key ions involved in this process are sodium (Na⁺), which is more concentrated outside the neuron, and potassium (K⁺), which is more concentrated inside.

  • The imbalance in these ions (more positive ions outside than inside) causes the inside of the neuron to be negative.

  • The sodium-potassium pump is responsible for maintaining this balance by pumping 3 sodium ions out for every 2 potassium ions it pumps in.

2. Ion Channels:

  • Ion channels allow ions to pass through the membrane when they open.

  • There are different types of channels:

    • Voltage-gated channels open when the membrane potential reaches a certain threshold (around -55 mV).

    • Ligand-gated channels open in response to specific chemicals (neurotransmitters like serotonin).

    • Mechanically-gated channels open when the membrane is physically stretched.

3. Graded Potential vs. Action Potential:

  • Graded potentials occur when small changes in ion flow cause slight shifts in the membrane potential in a localized area. These are not enough to trigger a full action potential.

  • Action potentials are large, rapid changes in membrane potential that travel along the axon, triggering other neurons or muscles.

4. Action Potential:

  • Depolarization: When the membrane potential reaches the threshold (about -55 mV), voltage-gated sodium channels open, allowing sodium ions to rush in. This causes the neuron to become positive inside (about +40 mV).

  • Repolarization: After depolarization, potassium channels open, allowing potassium ions to flow out, bringing the membrane potential back down.

  • Hyperpolarization: Sometimes, the membrane potential goes slightly below the resting potential (around -75 mV) before returning to its normal resting state.

5. Refractory Period:

  • After an action potential, a neuron enters the refractory period, where it cannot fire another action potential immediately. This ensures that the signal only travels in one direction along the axon and prevents backtracking.

6. Frequency of Action Potentials:

  • The strength of a stimulus is not encoded in the size of the action potential (it's always the same), but in the frequency of the action potentials. A stronger stimulus results in a higher frequency of action potentials, signaling more intense stimuli.

7. Speed of Action Potentials:

  • The speed of an action potential varies depending on factors like axon diameter and whether the axon is myelinated.

  • Myelin sheaths (insulating layers around axons) allow for faster transmission by enabling saltatory conduction, where the signal "jumps" from one gap (Node of Ranvier) to the next, rather than traveling along the entire length of the axon.

In summary:

Neurons work by maintaining a resting potential of about -70 mV, which is polarized (negative inside). When a stimulus reaches a threshold, it triggers a depolarization (the inside becomes positive), followed by repolarization and a potential hyperpolarization. The signal travels down the axon via action potentials, and the frequency of these signals encodes the intensity of the stimulus. Neurons transmit these signals faster when their axons are myelinated, a process called saltatory conduction.

This process is the foundation of how the nervous system works, enabling us to perceive and react to the world around us, think, and feel!

GRADED POTENTIALS

Graded potentials are changes in the membrane potential of a neuron (or other excitable cell) that vary in size, depending on the strength of the stimulus, as opposed to action potentials, which are all-or-nothing responses. They can be either depolarizing or hyperpolarizing and occur in the dendrites and cell body of a neuron. Here’s what graded potentials do:

  1. Signal Transmission over Short Distances: Graded potentials help transmit signals across short distances along the neuron, especially in the dendrites and cell body. They are often generated by the binding of neurotransmitters to receptors on the postsynaptic membrane.

  2. Amplitude Depends on Stimulus Strength: The size (amplitude) of the graded potential depends on the intensity of the stimulus. A stronger stimulus will cause a larger change in membrane potential, while a weaker stimulus will cause a smaller change.

  3. Decay with Distance: Graded potentials lose strength as they travel away from the site of initiation, meaning they don't travel far like action potentials. This is due to the leakage of ions through the membrane and the resistance of the cytoplasm.

  4. Trigger Action Potentials: If a graded potential is large enough (i.e., it reaches the threshold potential), it can trigger an action potential at the axon hillock. This is critical in the process of converting local, graded changes in membrane potential into long-distance signals.

  5. Types of Graded Potentials:

    • Depolarizing Graded Potentials: These make the inside of the cell less negative (closer to zero), which may bring the neuron closer to the threshold for firing an action potential.

    • Hyperpolarizing Graded Potentials: These make the inside of the cell more negative, moving the membrane potential further away from the threshold, and thus making it less likely for an action potential to occur.

In summary, graded potentials play an important role in initiating and modulating neural responses by providing localized, variable signals that can influence whether a neuron will generate an action potential.


IPSPs (Inhibitory Postsynaptic Potentials) and EPSPs (Excitatory Postsynaptic Potentials) are types of graded potentials that occur in the postsynaptic membrane of a neuron, typically in response to neurotransmitter binding at synapses. These potentials influence whether a neuron will fire an action potential or not. They are integral to synaptic transmission and neural communication.

EPSPs (Excitatory Postsynaptic Potentials):

  • Definition: EPSPs are small, local depolarizations of the postsynaptic membrane, which bring the membrane potential closer to the threshold for firing an action potential.

  • Cause: They are usually caused by the influx of positive ions (like Na⁺) into the postsynaptic neuron when excitatory neurotransmitters (e.g., glutamate, acetylcholine) bind to their receptors.

  • Effect: This depolarization increases the likelihood that the neuron will reach the threshold for an action potential and fire. If the membrane potential reaches the threshold at the axon hillock, an action potential is generated.

IPSPs (Inhibitory Postsynaptic Potentials):

  • Definition: IPSPs are small, local hyperpolarizations of the postsynaptic membrane, which move the membrane potential further away from the threshold, making it less likely for the neuron to fire an action potential.

  • Cause: They are typically caused by the influx of negative ions (like Cl⁻) or the efflux of positive ions (like K⁺) from the postsynaptic neuron when inhibitory neurotransmitters (e.g., GABA, glycine) bind to their receptors.

  • Effect: This hyperpolarization decreases the likelihood of the neuron firing an action potential by making the membrane potential more negative and farther from the threshold.


Types of Summation:

The effects of EPSPs and IPSPs on the postsynaptic membrane are subject to summation, which determines whether or not an action potential will be triggered. There are two main types of summation:

  1. Spatial Summation:

    • Definition: Spatial summation occurs when multiple presynaptic neurons release neurotransmitters at different locations on the postsynaptic neuron at the same time.

    • How it works: If several EPSPs (or IPSPs) are generated simultaneously at different synapses on the postsynaptic neuron, their effects can add together.

      • If multiple EPSPs arrive at different locations and add up, they can combine to depolarize the neuron to threshold and trigger an action potential.

      • Similarly, multiple IPSPs can combine and further hyperpolarize the membrane, preventing the neuron from firing.

    • Example: If two presynaptic neurons both fire at the same time and release neurotransmitters, their EPSPs may combine at different locations on the postsynaptic cell to increase the depolarization.

  2. Temporal Summation:

    • Definition: Temporal summation occurs when a single presynaptic neuron fires action potentials in rapid succession, releasing neurotransmitters multiple times in quick succession.

    • How it works: If one presynaptic neuron fires frequently enough, the EPSPs (or IPSPs) generated by each successive action potential will add together.

      • If the frequency of EPSPs is high enough, the membrane potential may reach threshold, leading to the generation of an action potential.

      • For IPSPs, frequent firing of the presynaptic neuron will result in cumulative hyperpolarization, preventing the neuron from reaching threshold and thus inhibiting firing.

    • Example: A single presynaptic neuron firing multiple times in rapid succession can create a stronger or more prolonged depolarization or hyperpolarization at the postsynaptic membrane.


Summation of EPSPs and IPSPs:

  • When EPSPs and IPSPs combine: If EPSPs and IPSPs are both occurring at the same time, their effects can cancel each other out or counteract each other to some extent.

    • If an EPSP and an IPSP occur at the same time and at the same location, the postsynaptic potential may not change significantly, and the neuron may not reach the threshold.

    • However, if EPSPs dominate (i.e., more or stronger EPSPs than IPSPs), the neuron will be more likely to fire an action potential.

    • Conversely, if IPSPs dominate, the neuron will be less likely to fire an action potential.

Threshold for Action Potential:

  • The threshold is the membrane potential at which voltage-gated sodium channels open, triggering an action potential. Whether a neuron reaches this threshold depends on the balance between excitatory (EPSP) and inhibitory (IPSP) signals, as well as the type of summation occurring (spatial or temporal).

In summary:

  • EPSPs make the neuron more likely to fire an action potential by depolarizing the membrane.

  • IPSPs make the neuron less likely to fire an action potential by hyperpolarizing the membrane.

  • Spatial summation involves combining inputs from multiple sources at different locations.

  • Temporal summation involves the rapid successive firing of a single presynaptic neuron.

Sequence of Events:

  1. A stimulus (such as neurotransmitter release) triggers graded potentials (e.g., EPSPs or IPSPs).

  2. These graded potentials spread across the cell membrane, and if they depolarize the membrane enough at the axon hillock, they reach threshold.

  3. Action potential is generated at the axon hillock, traveling down the axon.

When the action potential reaches the synaptic bulb and releases neurotransmitters, this is where NMJ occurs.