Difference Between Graded And Action Potential

Let's explore the fundamental differences between graded potentials and action potentials, two crucial electrical signals in the nervous system. Understanding these differences is key to grasping how neurons communicate and process information.

Graded Potentials vs. Action Potentials: A Comprehensive Comparison

The nervous system relies on electrical signals to rapidly transmit information throughout the body. These signals arise from changes in the electrical potential across the neuron's membrane. Two key types of electrical signals are graded potentials and action potentials. While both involve changes in membrane potential, they differ significantly in their characteristics, function, and how they contribute to neuronal communication. Let's delve into a detailed comparison of these two essential concepts.

What are Graded Potentials?

Graded potentials are localized changes in the membrane potential that vary in magnitude, or "grade," with the strength of the stimulus. They occur primarily in the dendrites and cell body (soma) of a neuron. Think of them as small ripples in a pond – their size depends on the size of the pebble dropped (the stimulus), and they fade as they travel outward.

Key Characteristics of Graded Potentials:

• Variable Amplitude: The size (amplitude) of a graded potential is directly proportional to the strength of the stimulus. A stronger stimulus leads to a larger graded potential.
• Localized: Graded potentials are confined to a small area of the cell membrane. The change in potential decreases with distance from the site of the stimulus.
• Decremental Conduction: As graded potentials spread away from the site of origin, their amplitude decreases due to leakage of charge across the membrane and cytoplasmic resistance. This fading is called decremental conduction.
• Summation: Graded potentials can summate, meaning that multiple graded potentials occurring close in time or space can add together. This summation can be temporal (multiple stimuli arriving at the same location in rapid succession) or spatial (stimuli arriving at different locations on the neuron simultaneously).
• Can be Depolarizing or Hyperpolarizing: Graded potentials can either depolarize the membrane (make the inside of the cell less negative, bringing it closer to the threshold for firing an action potential) or hyperpolarize the membrane (make the inside of the cell more negative, moving it further away from the threshold).
• Ligand-gated or Mechanically-gated Ion Channels: Graded potentials are typically generated by the opening of ligand-gated ion channels (channels that open in response to the binding of a neurotransmitter) or mechanically-gated ion channels (channels that open in response to physical distortion of the membrane).
• No Refractory Period: A refractory period is a period of time during which it is difficult or impossible to trigger another action potential. Graded potentials don't exhibit a refractory period, meaning they can occur in rapid succession.

Examples of Graded Potentials:

• Postsynaptic Potentials (PSPs): These occur at synapses, the junctions between neurons. Neurotransmitters released by the presynaptic neuron bind to receptors on the postsynaptic neuron, causing ion channels to open and creating graded potentials. Excitatory postsynaptic potentials (EPSPs) are depolarizing, while inhibitory postsynaptic potentials (IPSPs) are hyperpolarizing.
• Receptor Potentials: These occur in sensory receptors in response to a stimulus. For example, in the skin, pressure can open mechanically-gated ion channels, leading to a receptor potential.

What are Action Potentials?

Action potentials are rapid, large, and stereotyped changes in the membrane potential that travel long distances along the axon of a neuron. They are the primary mechanism for long-distance communication in the nervous system. Think of them as a burning fuse – once lit (threshold is reached), the flame (action potential) travels down the entire fuse (axon) without diminishing.

Key Characteristics of Action Potentials:

• All-or-None Principle: Action potentials follow the all-or-none principle. This means that if the depolarization at the axon hillock reaches a certain threshold (typically around -55 mV), an action potential will fire with its full amplitude. If the threshold is not reached, no action potential will occur. The strength of the stimulus does not affect the amplitude of the action potential.
• Constant Amplitude: Unlike graded potentials, the amplitude of an action potential is constant. Regardless of the strength of the stimulus (provided it exceeds the threshold), the action potential will always reach the same peak voltage (typically around +30 mV).
• Long-Distance Propagation: Action potentials are capable of traveling long distances along the axon without decreasing in amplitude. This is because they are regenerated at each point along the axon.
• Voltage-gated Ion Channels: Action potentials are generated by the opening and closing of voltage-gated ion channels, specifically voltage-gated sodium (Na+) and potassium (K+) channels. These channels open and close in response to changes in membrane potential.
• Refractory Period: Following an action potential, there is a brief period called the refractory period during which it is difficult or impossible to trigger another action potential. This period is divided into two phases: the absolute refractory period (during which another action potential cannot be generated, no matter how strong the stimulus) and the relative refractory period (during which a stronger-than-normal stimulus is required to trigger an action potential).
• Depolarization and Repolarization: Action potentials involve a rapid depolarization of the membrane (influx of Na+ ions) followed by a rapid repolarization (efflux of K+ ions).
• Axon Hillock Initiation: Action potentials are typically initiated at the axon hillock, a specialized region of the neuron where the axon originates from the cell body. The axon hillock has a high density of voltage-gated sodium channels, making it the most excitable part of the neuron.

Steps of an Action Potential:

1. Resting Membrane Potential: The neuron is at its resting membrane potential (typically around -70 mV). Voltage-gated Na+ and K+ channels are closed.
2. Depolarization to Threshold: A graded potential depolarizes the membrane at the axon hillock to the threshold potential (around -55 mV).
3. Activation of Voltage-gated Na+ Channels: At the threshold, voltage-gated Na+ channels open rapidly, causing a massive influx of Na+ into the cell. This influx of positive charge depolarizes the membrane further, driving the membrane potential towards the positive end.
4. Inactivation of Voltage-gated Na+ Channels and Activation of Voltage-gated K+ Channels: At the peak of the action potential (around +30 mV), voltage-gated Na+ channels begin to inactivate, halting the influx of Na+. At the same time, voltage-gated K+ channels open, allowing K+ to flow out of the cell.
5. Repolarization: The efflux of K+ repolarizes the membrane, bringing the membrane potential back towards its resting value.
6. Hyperpolarization: Because the K+ channels remain open for a longer period than the Na+ channels, the membrane potential briefly becomes more negative than the resting potential. This is called hyperpolarization or after-hyperpolarization.
7. Return to Resting Membrane Potential: The voltage-gated K+ channels eventually close, and the membrane potential returns to its resting value, maintained by the Na+/K+ pump and other ion channels.

Key Differences Summarized

To clearly distinguish between graded potentials and action potentials, consider the following table:

The Importance of Graded and Action Potentials in Neuronal Communication

Graded potentials and action potentials work together to enable neurons to receive, process, and transmit information.

1. Receiving and Integrating Information (Graded Potentials): Neurons receive information from other neurons through synapses. These synapses generate graded potentials (EPSPs and IPSPs) in the dendrites and cell body. These graded potentials represent the initial response of the neuron to incoming signals. The neuron then integrates these graded potentials through spatial and temporal summation. This integration determines whether the neuron will fire an action potential.

2. Decision Making at the Axon Hillock (Graded Potentials): The summation of graded potentials occurs at the axon hillock. If the sum of the EPSPs is strong enough to depolarize the membrane at the axon hillock to the threshold potential, an action potential is triggered. The axon hillock acts as a decision-making point, determining whether the neuron will "fire" and transmit the information to other neurons.

3. Long-Distance Transmission (Action Potentials): Once an action potential is initiated at the axon hillock, it travels down the axon to the axon terminals. Because action potentials are non-decremental, they can transmit signals over long distances without losing strength. This is crucial for communication between different parts of the nervous system, such as between the brain and the muscles.

4. Releasing Neurotransmitters (Action Potentials): When the action potential reaches the axon terminals, it triggers the opening of voltage-gated calcium (Ca2+) channels. The influx of Ca2+ into the axon terminals causes the release of neurotransmitters into the synaptic cleft. These neurotransmitters then bind to receptors on the postsynaptic neuron, generating graded potentials and continuing the cycle of neuronal communication.

Factors Affecting Action Potential Propagation

The speed at which an action potential propagates down the axon is critical for rapid communication in the nervous system. Several factors can influence the speed of action potential propagation:

• Axon Diameter: Larger diameter axons have lower internal resistance, allowing for faster propagation of action potentials.
• Myelination: Myelination is the process by which axons are coated with a fatty substance called myelin. Myelin is formed by glial cells (Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system). Myelin acts as an insulator, preventing the leakage of ions across the membrane. Action potentials can only occur at the Nodes of Ranvier, the gaps in the myelin sheath where voltage-gated ion channels are concentrated. This "jumping" of the action potential from node to node is called saltatory conduction and significantly increases the speed of propagation.
• Temperature: Higher temperatures generally increase the speed of ion channel kinetics and therefore increase the speed of action potential propagation. However, excessively high temperatures can damage the neuron.

Clinical Relevance

Understanding the differences between graded potentials and action potentials is crucial for understanding various neurological disorders and the mechanisms of action of many drugs.

• Multiple Sclerosis (MS): MS is an autoimmune disease in which the myelin sheath is damaged. This demyelination slows down or blocks the propagation of action potentials, leading to a variety of neurological symptoms, such as muscle weakness, fatigue, and vision problems.
• Local Anesthetics: Local anesthetics, such as lidocaine, block voltage-gated sodium channels, preventing the generation of action potentials. This blocks the transmission of pain signals from the site of injury to the brain.
• Neurotoxins: Many neurotoxins, such as tetrodotoxin (TTX) from pufferfish, also block voltage-gated sodium channels, leading to paralysis and death.
• Epilepsy: Epilepsy is a neurological disorder characterized by recurrent seizures. Seizures are caused by abnormal, excessive electrical activity in the brain. Some anti-epileptic drugs work by enhancing inhibitory neurotransmission (e.g., by increasing GABA activity) or by blocking excitatory neurotransmission (e.g., by blocking glutamate receptors), which reduces the likelihood of neurons firing action potentials.

Graded Potentials and Action Potentials: A Symbiotic Relationship

It's vital to remember that graded potentials and action potentials don't operate in isolation. They are intrinsically linked in the process of neuronal signaling. Graded potentials serve as the initiators, the local whispers of information that, when summed together, determine whether a neuron will "speak" in the form of an action potential. The action potential, in turn, is the neuron's voice, a robust signal capable of traversing long distances to communicate with other neurons, muscles, or glands. This intricate interplay allows for complex information processing and coordinated responses throughout the body. Understanding their individual characteristics and their collaborative function is essential to unraveling the mysteries of the nervous system.

Conclusion

In summary, graded potentials and action potentials are two distinct but interconnected types of electrical signals that play essential roles in neuronal communication. Graded potentials are localized, variable changes in membrane potential that are important for receiving and integrating information. Action potentials are all-or-none, long-distance signals that are essential for transmitting information throughout the nervous system. Understanding the differences between these two types of signals is crucial for understanding how the nervous system works and how it can be affected by disease or injury. The dance between these electrical signals is the foundation upon which all our thoughts, feelings, and actions are built.