12.4 The Action Potential
3 min read•june 18, 2024
Neurons communicate through electrical signals called action potentials. These rapid changes in membrane voltage allow information to zip along nerve cells. Understanding how neurons generate and transmit these signals is key to grasping how our nervous system functions.
Action potentials rely on the movement of ions across cell membranes. By exploring ion channels, resting potentials, and the steps of an action potential, we can see how neurons create and propagate these crucial electrical messages throughout the body.
Membrane Potential and Action Potentials
Ion channels and resting potential
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- is the difference in electrical charge across a neuron's membrane when not conducting an impulse, typically around -70 mV with the inside more negative than the outside
- Ion concentrations inside and outside the cell contribute to the resting potential, with high K+ concentration inside and high Na+ concentration outside
- Ion channels allow specific ions to move across the membrane
- K+ leak channels enable K+ to diffuse out of the cell along its concentration gradient
- Na+ leak channels permit some Na+ to enter the cell
- Na+/K+ pump actively transports ions to maintain concentration gradients, pumping 3 Na+ out and 2 K+ into the cell for each ATP consumed, helping maintain the resting potential
Sequence of action potential generation
- occurs when a stimulus causes the to become less negative, triggering an action potential if it reaches the (around -55 mV)
- Rising phase: Voltage- Na+ channels open, allowing rapid Na+ influx, causing the membrane potential to peak around +30 mV
- Falling phase: Voltage-gated Na+ channels inactivate and close, while voltage-gated K+ channels open, allowing K+ efflux and membrane potential to return towards resting level
- Afterhyperpolarization: Voltage-gated K+ channels remain open, briefly causing the membrane potential to become more negative than the resting potential
- Refractory periods:
- : Na+ channels are inactivated, preventing another action potential
- : Na+ channels have partially recovered, requiring a stronger stimulus to trigger an action potential
Continuous vs saltatory conduction
- in unmyelinated axons:
- Action potential propagates continuously along the membrane, with of one segment causing depolarization of the adjacent segment
- Slower conduction velocity compared to
- in myelinated axons:
- insulates the axon, preventing ion flow across the membrane
- Action potentials occur only at (gaps in )
- Depolarization at one node triggers depolarization at the next, causing the action potential to "jump" from node to node ()
- Faster conduction velocity due to reduced membrane capacitance and increased membrane
- Advantages of saltatory conduction:
- Faster velocity allows rapid transmission over long distances (spinal cord)
- Reduces energy requirements for action potential propagation
Synaptic Transmission and Neurotransmitter Release
- Action potentials arriving at the axon terminal trigger
- Voltage-gated Ca2+ channels open, allowing Ca2+ influx
- Increased intracellular Ca2+ causes synaptic vesicles to fuse with the presynaptic membrane
- Neurotransmitters are released into the synaptic cleft
- Neurotransmitters bind to receptors on the postsynaptic membrane, potentially initiating a new action potential
- of the postsynaptic neuron determines its response to neurotransmitter binding
Key Terms to Review (56)
Absolute refractory period: The absolute refractory period is the time during which a neuron is unable to fire another action potential, regardless of the strength of incoming stimuli. It ensures that each action potential is a separate, all-or-none event and aids in the unidirectional propagation of nerve impulses.
Acetylcholine: Acetylcholine is a neurotransmitter that plays a crucial role in the communication between neurons, the activation of muscle fibers, and the regulation of various physiological processes in the body. It is a key player in the functioning of the nervous system, muscle tissues, and the autonomic nervous system.
Acetylcholine (ACh): Acetylcholine is a neurotransmitter in the nervous system that plays a crucial role in stimulating muscle contractions and is involved in various brain functions including memory and learning. In the context of skeletal muscle, it is essential for transmitting nerve signals to muscle cells, leading to muscle movement.
Action Potential Propagation: Action potential propagation refers to the process by which an electrical signal, known as an action potential, travels along the length of a neuron or muscle fiber. This propagation allows for the rapid and coordinated transmission of information throughout the body's nervous and muscular systems.
Activation gate: An activation gate is a regulatory protein structure on a neuron's ion channel that opens or closes in response to electrical signals, allowing ions to flow across the cell membrane. This movement of ions initiates or propagates an action potential, a crucial step in nerve signal transmission.
All-or-None Principle: The all-or-none principle is a fundamental concept in physiology that describes how certain biological processes, such as the generation of action potentials in neurons and the contraction of muscle fibers, occur in an all-or-nothing manner. This principle states that these processes either occur fully or not at all, with no intermediate or partial responses.
Axon: The axon is a long, slender projection of a neuron that transmits electrical signals away from the cell body to other neurons, muscles, or glands. It is a critical component of the nervous system, responsible for the rapid and efficient communication between different parts of the body.
Axon segment: An axon segment is a distinct portion of an axon, the long, slender projection of a neuron that conducts electrical impulses away from the neuron's cell body. Each segment is delimited by nodes of Ranvier, which are gaps in the myelin sheath that facilitate rapid signal transmission.
Calcium: Calcium is a vital mineral that plays a crucial role in numerous physiological processes within the human body. It is essential for the formation and maintenance of strong bones and teeth, nerve function, muscle contraction, and the regulation of various metabolic activities.
Continuous conduction: Continuous conduction is the process by which an action potential moves step-by-step along an unmyelinated axon, depolarizing each successive segment of the axonal membrane. It is a slower propagation compared to saltatory conduction in myelinated axons due to the continuous involvement of every part of the axon membrane.
Dendrite: Dendrites are the branched extensions of a neuron that receive signals from other neurons and transmit them toward the cell body. They play a crucial role in integrating neural information received from multiple sources.
Dendrite: A dendrite is a branched projection of a neuron that receives signals from other neurons and transmits them to the cell body. Dendrites are a crucial component of the nervous system, playing a vital role in the perception and response to stimuli, the function of nervous tissue, and the generation of action potentials.
Depolarization: Depolarization is the process during which a neuron's normally negative resting membrane potential becomes less negative, reaching a positive value. This change is crucial for the initiation and propagation of action potentials along the neuron.
Depolarization: Depolarization is the process by which the electrical potential across a cell membrane, typically a neuron or cardiac muscle cell, becomes less negative. This change in membrane potential is a crucial step in the generation and propagation of electrical signals within the body's nervous and cardiovascular systems.
Diffusion: Diffusion is the spontaneous movement of particles from an area of higher concentration to an area of lower concentration, driven by the random thermal motion of the particles. This process is a fundamental mechanism for the transport of substances across cell membranes and in various physiological processes throughout the body.
Electrochemical exclusion: Electrochemical exclusion is a process during an action potential where specific ions are selectively allowed or denied passage through ion channels in the neuron's membrane. This selectivity is crucial for the generation and propagation of action potentials along neurons.
Excitable membrane: An excitable membrane is a cell membrane that has the ability to rapidly change its electrical state (voltage) in response to stimuli, allowing for the initiation and propagation of action potentials. This characteristic is fundamental for the function of nerve and muscle cells, enabling communication and contraction.
Facilitated diffusion: Facilitated diffusion is a type of passive transport in which molecules move across the cell membrane through protein channels or carriers without the expenditure of cellular energy. It allows substances that are not able to directly pass through the lipid bilayer to enter or exit the cell based on concentration gradients.
Gated: In the context of anatomy and physiology, specifically within the nervous system and nervous tissue, "gated" refers to ion channels in the cell membrane that can open or close in response to certain stimuli. These gated channels are crucial for initiating and propagating the action potential along neurons.
Glutamate: Glutamate is a key neurotransmitter in the central nervous system, playing a vital role in neural communication, perception, and response. It is the most abundant excitatory neurotransmitter, responsible for transmitting signals between neurons and enabling various neurological functions.
Hyperpolarization: Hyperpolarization is a process in which the membrane potential of a cell becomes more negative relative to the resting potential, making it more difficult for the cell to reach the threshold for generating an action potential. This term is particularly relevant in the context of understanding the action potential and communication between neurons.
Inactivation gate: An inactivation gate is a component of voltage-gated sodium channels in neurons that closes shortly after the channel opens during an action potential, temporarily preventing the flow of sodium ions into the neuron. This mechanism helps to restore the resting state of the neuron after depolarization.
Ionotropic receptor: An ionotropic receptor is a type of receptor found in the membrane of nerve cells that directly controls the opening of ion channels upon binding with a neurotransmitter. This action allows ions to flow into or out of the neuron, contributing to the generation or inhibition of an action potential.
Leakage channel: Leakage channels are specialized protein structures in neuronal membranes that allow ions to passively flow across the membrane, following their concentration gradient. These channels help maintain the resting membrane potential and contribute to the neuron's ability to generate action potentials.
Ligand-gated channel: A ligand-gated channel is a type of ion channel in the cell membrane that opens or closes in response to the binding of a specific chemical messenger (ligand), such as a neurotransmitter. This action allows ions to pass through the membrane, influencing the cell's electrical charge and signaling.
Mechanically gated channel: Mechanically gated channels are ion channels in cell membranes that open or close in response to mechanical stimulation such as stretch, pressure, or vibration. These channels play a crucial role in initiating electrical signals in sensory neurons and other cells within the nervous system.
Membrane Excitability: Membrane excitability refers to the ability of a cell's plasma membrane to generate and propagate electrical signals, known as action potentials. This property is crucial for the functioning of excitable cells, such as neurons and muscle cells, which rely on these electrical impulses to transmit information and coordinate physiological responses.
Membrane potential: Membrane potential is the electrical voltage difference across a cell's plasma membrane, crucial for the conduction of action potentials in neurons. It arises due to the differential distribution of ions (such as sodium and potassium) across the membrane.
Myelin sheath: The myelin sheath is a fatty layer that surrounds the axons of many nerve cells, serving as electrical insulation and increasing the speed at which nerve impulses are conducted. It is essential for the proper functioning of the nervous system by facilitating rapid signal transmission.
Myelin Sheath: The myelin sheath is a protective fatty layer that surrounds the axons of certain nerve cells, called myelinated neurons. It acts as an insulator, increasing the speed of electrical impulse transmission along the neuron.
Na+/K+-ATPase: The Na+/K+-ATPase, also known as the sodium-potassium pump, is an integral membrane protein that plays a crucial role in maintaining the electrochemical gradients across the cell membrane, which is essential for various physiological processes, including the generation of action potentials as described in the topic 12.4 The Action Potential.
Neurotransmitter Release: Neurotransmitter release is the process by which neurotransmitters are released from the presynaptic terminal of a neuron into the synaptic cleft, allowing for communication between neurons and the propagation of electrical signals throughout the nervous system. This term is crucial in understanding the function of nervous tissue, the action potential, and central processing.
Nodes of Ranvier: The nodes of Ranvier are regularly spaced gaps or constrictions along the length of a myelinated nerve fiber. They play a crucial role in the rapid and efficient transmission of electrical signals through the nervous system.
Nonspecific channel: A nonspecific channel is a type of protein channel in the membranes of neurons that allows ions to move across the membrane without selectivity for a particular type of ion. These channels play a critical role in changing the electrical charge of the neuron, facilitating the generation and propagation of action potentials.
Potassium: Potassium is an essential mineral that plays a crucial role in various physiological processes within the human body. It is involved in maintaining fluid and electrolyte balance, nerve impulse transmission, muscle contraction, and the regulation of heart function. Potassium is a key player in many of the topics covered in this course, including inorganic compounds, action potentials, neuron communication, nutrition, fluid balance, and acid-base regulation.
Potassium Channels: Potassium channels are specialized membrane proteins that allow the selective passage of potassium ions (K+) across the cell membrane. They play a crucial role in regulating the electrical activity of cells, particularly in the context of action potentials and cardiac muscle function.
Refractory period: The refractory period is a brief interval following an action potential during which a neuron is unable to fire another action potential. This period ensures that action potentials travel along neurons in one direction and allows the neuron to reset before firing again.
Refractory Period: The refractory period is a crucial concept in various physiological processes, including the function of nervous tissue, cardiac muscle, and the propagation of action potentials. It refers to the time interval during which a cell or tissue is unable to respond to a new stimulus or generate another action potential, even if the necessary stimulus is applied.
Relative refractory period: The relative refractory period is the phase following an action potential during which a neuron can be stimulated to initiate another action potential, but only by a stronger-than-usual stimulus. It occurs right after the absolute refractory period and represents a time of decreased sensitivity to new stimuli.
Repolarization: Repolarization is the process during an action potential when a neuron's membrane potential returns to its resting negative state after depolarization. It occurs due to the efflux of potassium ions (K+) through channels in the neuron's membrane.
Repolarization: Repolarization is the process by which the resting membrane potential of an excitable cell, such as a neuron or a cardiac muscle cell, is restored after an action potential has been generated. This crucial phase of the action potential cycle allows the cell to regain its ability to respond to subsequent stimuli.
Resistance: In the context of the nervous system and blood flow, resistance refers to the opposition to blood flow in the blood vessels. It is influenced by factors such as vessel diameter, blood viscosity, and overall vessel length.
Resting membrane potential: Resting membrane potential is the electrical charge difference across the neuronal membrane when the neuron is not actively transmitting a signal. It is typically negative, indicating that the inside of the neuron is more negatively charged compared to the outside.
Resting Membrane Potential: The resting membrane potential is the electrical charge difference across the cell membrane when the cell is not actively transmitting an electrical signal. It is a crucial concept in understanding the function of nervous tissue, the action potential, and the electrical activity of cardiac muscle.
Saltatory conduction: Saltatory conduction is the process by which nerve impulses jump between the nodes of Ranvier along myelinated axons, significantly speeding up electrical transmission. This method allows for faster communication between neurons without increasing the size of the axon.
Saltatory Conduction: Saltatory conduction is the rapid transmission of electrical impulses along the length of a nerve fiber by 'jumping' from one node of Ranvier to the next, rather than propagating continuously. This specialized form of signal transmission is a key feature of the nervous system and plays a crucial role in the function of nervous tissue.
Size exclusion: In the context of the nervous system and nervous tissue, size exclusion refers to the mechanism by which molecules are selectively allowed or denied passage through the neural membrane based on their size. This process is crucial for maintaining the ion concentration gradient essential for generating an action potential.
Sodium: Sodium is an essential mineral that plays a crucial role in various physiological processes within the human body. It is an electrolyte that helps maintain fluid balance, nerve function, and muscle contraction. Sodium is involved in several key topics in anatomy and physiology, including chemical bonds, inorganic compounds, the action potential, communication between neurons, tubular reabsorption, fluid volume and composition regulation, body fluid compartments, and electrolyte balance.
Sodium-potassium pump: The sodium-potassium pump is a protein complex found in cell membranes that moves sodium ions out of and potassium ions into the cell, using ATP for energy. It plays a crucial role in maintaining the cell's electrochemical gradient and volume.
Sodium-Potassium Pump: The sodium-potassium pump, also known as the Na+/K+ ATPase, is a crucial membrane-bound protein that actively transports sodium ions (Na+) out of cells and potassium ions (K+) into cells. This electrochemical gradient created by the pump is essential for a variety of physiological processes, including nerve impulse transmission, muscle contraction, and fluid and electrolyte balance.
Spontaneous depolarization: Spontaneous depolarization is the automatic and gradual change in membrane potential that occurs in certain cardiac muscle cells, leading them to reach the threshold potential and generate an action potential without external stimulation. It is crucial for initiating and regulating the heart's rhythm.
Summation: Summation refers to the process of combining or adding up individual electrical signals within a neuron to determine if the overall signal is strong enough to generate an action potential. It is a crucial concept in understanding the generation and propagation of nerve impulses in the body.
Synaptic Transmission: Synaptic transmission is the process by which an electrical or chemical signal is transmitted from one neuron to another across the synaptic cleft, the small gap between the axon terminal of the presynaptic neuron and the dendrite or cell body of the postsynaptic neuron. This process is essential for the communication and coordination of the nervous system.
Threshold Potential: The threshold potential is the minimum electrical charge required to trigger an action potential in a neuron. It represents the critical level of depolarization that must be reached in order to initiate the rapid influx of sodium ions that propagates the nerve impulse.
Voltage-gated channel: A voltage-gated channel is a type of protein channel in the cell membrane that opens or closes in response to changes in the electrical charge (voltage) across the membrane. These channels are crucial for initiating and conducting action potentials in neurons.
Voltage-Gated Sodium Channels: Voltage-gated sodium channels are specialized membrane proteins that selectively allow the passage of sodium ions across the cell membrane in response to changes in the electrical potential across the membrane. These channels play a crucial role in the generation and propagation of action potentials, which are the fundamental electrical signals that enable communication within the nervous system and between cells.