Questions and Answers
What is the typical range for resting membrane potential in neurons?
Resting membrane potential exists only in neurons.
False
What role do sodium-potassium ATPases play in maintaining resting membrane potential?
They pump three sodium ions out and two potassium ions into the cell.
The resting membrane potential is typically around _____ millivolts.
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Match the following components with their functions in maintaining resting membrane potential:
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What is the main effect of leaky potassium channels on resting membrane potential?
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The concentration of sodium is higher inside the cell than outside.
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What is the Nernst potential equation for potassium?
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The typical concentration of potassium outside the cell is _____ mM.
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Why does the permeability of the membrane to potassium contribute more to resting potential than that to sodium?
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What is the threshold voltage for action potential stimulation?
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Potassium has a greater impact on resting potential stabilization than sodium.
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What is the typical resting membrane potential of a neuron?
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During depolarization, the membrane potential moves towards ________ millivolts.
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Match the following ionic movements with their effects on the membrane potential:
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What happens during the absolute refractory period?
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Graded potentials only move the membrane potential further from the action potential threshold.
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What role do voltage-gated calcium channels play during an action potential?
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The process of exocytosis in neurotransmitter release is facilitated by ________ binding to SNARE proteins.
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Which of the following describes hyperpolarization?
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What is the main consequence of a cell reaching a membrane potential of negative 55 millivolts?
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Graded potentials always lead to action potentials.
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What effect do excitatory postsynaptic potentials (EPSPs) have on the membrane potential?
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During depolarization, the cell's internal charge becomes more ________ as sodium ions enter.
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Match the following voltages with their corresponding actions or states:
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Which ion influx is primarily responsible for initiating the action potential?
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Hyperpolarization increases the electrical charge of the inside of the cell above the resting potential.
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What role do voltage-gated potassium channels play after an action potential?
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The absolute refractory period occurs after the ________ of an action potential.
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Which mechanism allows neurotransmitter release into the synapse?
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What primarily establishes the negative charge inside the cell?
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The sodium-potassium ATPase pump moves more sodium ions into the cell than potassium ions it moves out.
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State the typical resting membrane potential in millivolts.
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The _____ concentration of sodium is higher outside the cell.
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Match the ion with its primary concentration location.
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Which of the following ions primarily dictates the resting membrane potential?
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Leaky sodium channels have a higher permeability compared to leaky potassium channels.
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What is the role of leaky potassium channels in neurons?
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The equilibrium potential for potassium based on typical concentrations is approximately _____ millivolts.
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Which mechanism helps maintain the ion concentration gradients essential for resting membrane potential?
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What function do astrocytes perform regarding potassium ions during action potentials?
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Astrocytes are only found in the central nervous system.
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What are the three layers of the blood-brain barrier?
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The area postrema samples blood for _____ and triggers a vomiting response.
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Match the following brain regions with their function regarding the blood-brain barrier:
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Which substance can easily cross the blood-brain barrier?
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Astrocytes increase tight junction formation within the blood-brain barrier by secreting growth factors.
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What neurotransmitter do astrocytes help regulate by converting it to glutamine?
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Astrocytes can redistribute excess potassium through _____ between each other.
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What is one of the primary functions of the blood-brain barrier?
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Which of the following cell types is responsible for myelinating axons in the central nervous system?
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Glutamine is converted back into glutamate in astrocytes before being transported to neurons.
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What is the role of microglia in the central nervous system?
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Astrocytes regulate glucose levels for neurons by breaking down glycogen into ________.
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Match the types of cells with their corresponding roles in the nervous system:
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Which type of fiber is characterized by having a high conduction velocity due to high myelination?
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Schwann cells are unable to regenerate after damage.
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What is the composition of myelin?
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The gaps between myelin sheaths on axons are known as ________.
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What happens to conduction velocity as axon diameter increases?
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What type of cells are found only in the central nervous system (CNS)?
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The blood-brain barrier allows for the easy passage of charged ions and larger molecules without transport proteins.
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What is the primary function of astrocytes in relation to potassium ions during action potentials?
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The blood-brain barrier consists of three layers: endothelial cells, basal lamina, and ________ processes from astrocytes.
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Match the following regions of the brain with their function related to the blood-brain barrier:
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Which substance can easily cross the blood-brain barrier?
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Astrocytes enhance the formation of tight junctions within the blood-brain barrier by secreting growth factors.
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What enzyme do astrocytes use to convert excess glutamate to glutamine?
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Astrocytes play a role in regulating _______, which is crucial for neurotransmission in the brain.
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What is one primary function of the blood-brain barrier?
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What is the primary function of astrocytes in glucose metabolism for neurons?
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Oligodendrocytes can regenerate after injury in the central nervous system.
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What is the main consequence of demyelination in the peripheral nervous system?
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Astrocytes assist in the synthesis of GABA from glutamate via the enzyme ________.
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Match the following types of cells with their primary functions:
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Which type of transporter is involved in glucose uptake in astrocytes?
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Nodes of Ranvier are concentrated areas of potassium channels on myelinated axons.
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What is the role of the neurolemma in Schwann cells?
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Microglia respond to pathogens by releasing ________ substances.
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Which type of neuron fibers is associated with the fastest conduction velocity?
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What is the role of dendrites in a neuron?
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The cell body of a neuron is mainly involved in conducting electrical impulses.
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What process involves the transcription of DNA into mRNA in neurons?
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The region where the axon begins is known as the __________.
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Match the following neuronal components with their functions:
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Which motor protein facilitates anterograde transport in neurons?
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Action potentials are mainly propagated through the axon by the movement of potassium and sodium ions.
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Which part of the neuron contains the nucleus?
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Neurotransmitters are packaged into vesicles by the __________ for delivery along the axon.
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What type of potential do EPSPs represent?
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Which transport mechanism is used by certain viruses like polio and rabies to move toward the cell body?
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Nerve growth factors are transported retrogradely to stimulate the cell body for neuronal repair.
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What is the primary role of the axon terminal?
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The _____ concentration of potassium is higher inside the neuron compared to outside.
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Match the following types of neurons with their functions:
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What is the role of calcium ions during neurotransmitter release?
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SSRIs work by promoting the enzymatic degradation of serotonin in the synapse.
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What outcome occurs when the varicella zoster virus reactivates?
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Interneurons are essential for mediating _____ actions within the spinal cord.
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During which phase of action potentials does sodium ion influx occur?
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Study Notes
Resting Membrane Potential
- Defined as the voltage difference across the cell membrane when a neuron is at rest.
- Exists in all cell types, not just neurons, highlighting its universal biological significance.
- Typical range for resting membrane potential in neurons is between negative 70 to negative 90 millivolts, with negative 70 millivolts being the average.
Mechanisms Contributing to Resting Membrane Potential
-
Sodium-Potassium ATPases
- Pumps three sodium ions out of the cell and two potassium ions into the cell.
- Establishes a slight negative charge inside the cell and maintains ion concentration gradients: higher sodium outside and higher potassium inside.
-
Leaky Potassium Channels
- Always open, allowing potassium ions to flow in and out of the cell passively.
- Higher potassium concentration inside drives potassium ions to exit, leading to a more negative internal environment.
- Loss of potassium creates unoccupied negative charges (anions) inside, increasing negativity up to around negative 90 millivolts.
-
Leaky Sodium Channels
- Also allow sodium ions to flow, but the permeability of the membrane to potassium is much higher than that to sodium.
- Sodium influx contributes less to resting membrane potential adjustment, as much of its concentration remains outside the cell.
Ion Concentration Gradients
- Sodium concentration is higher outside, while potassium concentration is higher inside due to the action of pumps.
- The movement of ions occurs based on their respective concentration gradients, contributing to the overall membrane potential.
Nernst Potential
- Calculated for various ions (e.g., potassium and sodium) to determine their equilibrium potential across the membrane.
- For potassium:
- Equation used: E(K) = 61.5 / z × log([K outside] / [K inside])
- Typical concentrations: 5 mM outside, 150 mM inside yield approximately negative 90 millivolts.
- For sodium:
- Equation: E(Na) = 61.5 / z × log([Na outside] / [Na inside])
- Typical concentrations: 140 mM outside, 10 mM inside yield approximately positive 70 millivolts.
Overall Impact
- The resting membrane potential is determined primarily by the equilibrium potentials of potassium and sodium.
- Significant shifts in membrane potential are primarily due to changes in permeability to potassium, rather than sodium, emphasizing the greater impact of potassium on resting potential stabilization.### Membrane Potentials
- Resting membrane potential is typically around negative 70 millivolts.
- The equilibrium potential for potassium influences the cell voltage when potassium ions move out.
- A cell with 90% permeability to potassium and 10% to sodium would average near negative 70 millivolts.
Graded Potentials
- Graded potentials adjust the resting membrane potential towards the threshold for action potentials.
- Threshold voltage for action potential stimulation is approximately negative 55 millivolts.
- Depolarization moves the membrane potential closer to threshold; hyperpolarization makes it more negative.
- Excitatory postsynaptic potentials (EPSPs) bring the membrane potential closer to threshold.
- Inhibitory postsynaptic potentials (IPSPs) move the potential further from threshold.
Neuronal Communication
- Presynaptic neurons release neurotransmitters (e.g., glutamate for EPSPs, GABA for IPSPs) at the synapse to influence the postsynaptic neuron.
- Ligand-gated ion channels open in response to neurotransmitter binding, allowing ion flow.
- Sodium (Na+) and calcium (Ca2+) influx increases the cell's positive charge.
- Chloride (Cl-) influx or potassium (K+) efflux makes the cell interior more negative, inducing hyperpolarization.
Summation of Potentials
- EPSPs and IPSPs compete to influence membrane potential, with the aim to have more EPSPs to trigger action potentials.
- Temporal summation involves rapid repeated firing from a single presynaptic neuron, cumulatively raising potential.
- Spatial summation involves simultaneous firing from multiple presynaptic neurons, increasing total EPSP effect.
Threshold and Action Potential
- Action potentials are triggered when the threshold of negative 55 millivolts is reached at the axon hillock.
- Voltage-gated sodium channels, concentrated at the axon hillock, open rapidly upon reaching threshold.
- Sodium influx during action potential can elevate membrane potential to positive 30 millivolts, reversing the polarity.
- Once the threshold is reached, voltage-gated channels create an all-or-nothing response, triggering the action potential.### Action Potential and Ion Channels
- Resting Membrane Potential: Neurons have a resting membrane potential around -70 millivolts.
- Threshold Potential: An action potential is initiated when the membrane potential reaches approximately -55 millivolts.
-
Voltage-Gated Sodium Channels:
- Activation gate opens at -55 millivolts, allowing sodium (Na+) ions to rush in.
- Positive feedback leads to rapid depolarization, peaking around +30 millivolts.
- Inactivation gate closes at +30 millivolts, preventing further Na+ entry.
Depolarization and Action Potential Propagation
- Depolarization: Movement from a negative to a positive internal charge, specifically when Na+ enters the cell.
- Propagation: The depolarizing wave moves along the axon towards the terminal bulb.
- Voltage-Gated Calcium Channels: Activated at +30 millivolts, allowing calcium (Ca2+) to enter the axon terminal necessary for neurotransmitter release.
Neurotransmitter Release Mechanism
- Synaptic Vesicle Fusion: Calcium binds to SNARE proteins, facilitating the fusion of vesicles with the cell membrane.
- Exocytosis: Release of neurotransmitters into the synaptic space to interact with adjacent neurons.
Repolarization and Hyperpolarization
- Repolarization: Returning the membrane potential back to resting state (-70 millivolts).
- Voltage-Gated Potassium Channels: Open at +30 millivolts, allowing potassium (K+) to exit and move the potential towards -90 millivolts.
- Hyperpolarization: Occurs when the potential goes below -70 millivolts due to excess K+ leaving before returning to resting potential.
Key Definitions
- Depolarization: Making the inside of the cell more positive.
- Repolarization: Returning from a positive back to a negative resting potential.
- Hyperpolarization: Increasing negativity beyond the resting potential.
Recovery to Resting Membrane Potential
- Restoration Mechanisms: Involvement of sodium-potassium ATPase and leak channels help stabilize the membrane potential back to -70 millivolts.
-
Refractory Periods:
- Absolute Refractory Period: Follows peak depolarization where no stimulation can initiate another action potential, regardless of stimulus strength.
- Relative Refractory Period: Occurs after the absolute phase when a stronger-than-normal stimulus is needed to initiate a new action potential.
Summary of Voltage-Gated Channel States
- At Rest: Activation gate closed, inactivation gate open.
- During Depolarization: Activation gate opens, inactivation gate begins to close.
- At Peak Depolarization: Activation gate open, inactivation gate closed.
- During Repolarization: Activation gate begins to close, K+ channels open.
- Post Action Potential: Recovery to resting state configuration upon reaching -70 millivolts.
Resting Membrane Potential
- Voltage difference across the cell membrane in a resting neuron, typically around -70 to -90 millivolts.
- Universal phenomenon observed in all cell types, not just neurons.
Mechanisms Contributing to Resting Membrane Potential
- Sodium-Potassium ATPases: Pumps 3 Na+ ions out and 2 K+ ions in, creating a slight negative interior and maintaining concentration gradients.
- Leaky Potassium Channels: Always open, allowing K+ to exit, enhancing negative charge inside the cell.
- Leaky Sodium Channels: Less impact on resting potential due to lower permeability; primarily sodium remains outside.
Ion Concentration Gradients
- Sodium concentration is higher outside (140 mM) and potassium is higher inside (150 mM).
- Ion movement driven by these gradients influences membrane voltage.
Nernst Potential
- Equilibrium potential calculated to determine ion movement across membranes.
- Potassium: E(K) calculation shows potential around -90 mV based on typical ion concentrations.
- Sodium: E(Na) is approximately +70 mV based on differing concentrations inside and outside.
Overall Impact
- Resting membrane potential highly influenced by potassium equilibrium potential and its permeability.
- Shifts in membrane potential primarily occur due to changes in potassium permeability rather than sodium.
Membrane Potentials
- Average resting membrane potential is about -70 mV.
- Higher potassium permeability results in a more stable resting potential around this value.
Graded Potentials
- Facilitate adjustments to resting potential, moving towards action potential threshold of approximately -55 mV.
- EPSPs bring the potential closer to threshold while IPSPs push it further away.
Neuronal Communication
- Neurotransmitters (e.g., glutamate for EPSPs, GABA for IPSPs) released by presynaptic neurons impact the postsynaptic neuron.
- Ligand-gated ion channels open upon binding of neurotransmitters, allowing ion flow that alters charge inside the neuron.
Summation of Potentials
- EPSPs and IPSPs balance to affect membrane potential; more EPSPs needed to trigger action potentials.
- Temporal summation combines rapid sequential firing from a single neuron.
- Spatial summation accumulates effects from concurrent firings of multiple neurons.
Threshold and Action Potential
- Action potential triggered at -55 mV at axon hillock, where voltage-gated sodium channels open.
- Rapid depolarization occurs as Na+ influx raises potential to +30 mV, reversing polarity.
Action Potential and Ion Channels
- Resting membrane potential around -70 mV; action potential initiation requires reaching the threshold.
- Voltage-gated sodium channels rapidly open at threshold, allowing Na+ entry and causing depolarization.
Depolarization and Action Potential Propagation
- Depolarization occurs when Na+ floods into cells, creating a positive charge.
- Propagation of depolarization signals travels along the axon toward the terminal.
- Voltage-gated calcium channels open at +30 mV, critical for neurotransmitter release.
Neurotransmitter Release Mechanism
- Calcium binds to SNARE proteins aiding vesicle fusion with the membrane for exocytosis of neurotransmitters.
Repolarization and Hyperpolarization
- Repolarization brings the membrane back to about -70 mV.
- Voltage-gated potassium channels open at +30 mV, causing K+ to exit and potential to drop towards -90 mV.
- Hyperpolarization occurs when the potential falls below -70 mV due to excess K+ outflow.
Key Definitions
- Depolarization: Inside of cell becomes more positive.
- Repolarization: Return from a positive back to negative resting potential.
- Hyperpolarization: Increased negativity beyond resting potential.
Recovery to Resting Membrane Potential
- Sodium-potassium ATPase and leak channels help stabilize the membrane back to -70 mV.
-
Refractory Periods:
- Absolute Refractory Period: No further action potentials can be initiated post-depolarization.
- Relative Refractory Period: Follows absolute period; requires a stronger stimulus to generate a new action potential.
Summary of Voltage-Gated Channel States
- At Rest: Activation gates closed, inactivation gate open.
- During Depolarization: Activation gate opens; inactivation gate starts closing.
- At Peak: Both activation gates are open; inactivation gate closes to stop Na+ entry.
- During Repolarization: K+ channels open, returning the cell to a negative state.
- Post Action Potential: Channels recover to resting state configuration as the potential returns to -70 mV.
Resting Membrane Potential
- Voltage difference across the cell membrane in a resting neuron, typically around -70 to -90 millivolts.
- Universal phenomenon observed in all cell types, not just neurons.
Mechanisms Contributing to Resting Membrane Potential
- Sodium-Potassium ATPases: Pumps 3 Na+ ions out and 2 K+ ions in, creating a slight negative interior and maintaining concentration gradients.
- Leaky Potassium Channels: Always open, allowing K+ to exit, enhancing negative charge inside the cell.
- Leaky Sodium Channels: Less impact on resting potential due to lower permeability; primarily sodium remains outside.
Ion Concentration Gradients
- Sodium concentration is higher outside (140 mM) and potassium is higher inside (150 mM).
- Ion movement driven by these gradients influences membrane voltage.
Nernst Potential
- Equilibrium potential calculated to determine ion movement across membranes.
- Potassium: E(K) calculation shows potential around -90 mV based on typical ion concentrations.
- Sodium: E(Na) is approximately +70 mV based on differing concentrations inside and outside.
Overall Impact
- Resting membrane potential highly influenced by potassium equilibrium potential and its permeability.
- Shifts in membrane potential primarily occur due to changes in potassium permeability rather than sodium.
Membrane Potentials
- Average resting membrane potential is about -70 mV.
- Higher potassium permeability results in a more stable resting potential around this value.
Graded Potentials
- Facilitate adjustments to resting potential, moving towards action potential threshold of approximately -55 mV.
- EPSPs bring the potential closer to threshold while IPSPs push it further away.
Neuronal Communication
- Neurotransmitters (e.g., glutamate for EPSPs, GABA for IPSPs) released by presynaptic neurons impact the postsynaptic neuron.
- Ligand-gated ion channels open upon binding of neurotransmitters, allowing ion flow that alters charge inside the neuron.
Summation of Potentials
- EPSPs and IPSPs balance to affect membrane potential; more EPSPs needed to trigger action potentials.
- Temporal summation combines rapid sequential firing from a single neuron.
- Spatial summation accumulates effects from concurrent firings of multiple neurons.
Threshold and Action Potential
- Action potential triggered at -55 mV at axon hillock, where voltage-gated sodium channels open.
- Rapid depolarization occurs as Na+ influx raises potential to +30 mV, reversing polarity.
Action Potential and Ion Channels
- Resting membrane potential around -70 mV; action potential initiation requires reaching the threshold.
- Voltage-gated sodium channels rapidly open at threshold, allowing Na+ entry and causing depolarization.
Depolarization and Action Potential Propagation
- Depolarization occurs when Na+ floods into cells, creating a positive charge.
- Propagation of depolarization signals travels along the axon toward the terminal.
- Voltage-gated calcium channels open at +30 mV, critical for neurotransmitter release.
Neurotransmitter Release Mechanism
- Calcium binds to SNARE proteins aiding vesicle fusion with the membrane for exocytosis of neurotransmitters.
Repolarization and Hyperpolarization
- Repolarization brings the membrane back to about -70 mV.
- Voltage-gated potassium channels open at +30 mV, causing K+ to exit and potential to drop towards -90 mV.
- Hyperpolarization occurs when the potential falls below -70 mV due to excess K+ outflow.
Key Definitions
- Depolarization: Inside of cell becomes more positive.
- Repolarization: Return from a positive back to negative resting potential.
- Hyperpolarization: Increased negativity beyond resting potential.
Recovery to Resting Membrane Potential
- Sodium-potassium ATPase and leak channels help stabilize the membrane back to -70 mV.
-
Refractory Periods:
- Absolute Refractory Period: No further action potentials can be initiated post-depolarization.
- Relative Refractory Period: Follows absolute period; requires a stronger stimulus to generate a new action potential.
Summary of Voltage-Gated Channel States
- At Rest: Activation gates closed, inactivation gate open.
- During Depolarization: Activation gate opens; inactivation gate starts closing.
- At Peak: Both activation gates are open; inactivation gate closes to stop Na+ entry.
- During Repolarization: K+ channels open, returning the cell to a negative state.
- Post Action Potential: Channels recover to resting state configuration as the potential returns to -70 mV.
Glial Cells Overview
- Nervous tissue is composed of neurons and glial cells.
- Glial cells are present in both the central nervous system (CNS) and peripheral nervous system (PNS).
Astrocytes
- Found exclusively in the CNS, astrocytes perform various functions, including maintaining the blood-brain barrier (BBB).
Blood-Brain Barrier Structure
- The BBB comprises three layers:
- Endothelial cells with tight junctions.
- Basal lamina, a connective tissue layer.
- Foot processes from astrocytes that help in forming the barrier.
Blood-Brain Barrier Function
- Regulates the transport of molecules between blood and nervous tissue.
- Lipid-soluble substances and gases (e.g., CO2, O2) cross the BBB easily.
- Charged ions and larger molecules require specialized transport proteins to pass through.
- Astrocytes secrete growth factors to strengthen tight junctions, enhancing the barrier's selectivity.
Areas with Compromised Blood-Brain Barrier
- Certain brain regions permit molecular exchange due to a compromised BBB:
- Area postrema: Detects blood toxins and triggers vomiting.
- Osmoreceptors near the hypothalamus: Monitor blood salts and glucose for fluid balance regulation.
- Hypothalamic-pituitary axis: Facilitates hormone signaling between hypothalamus and pituitary gland.
Potassium Buffering
- Astrocytes absorb excess potassium ions during neuronal action potentials to maintain neuronal excitability.
- They redistribute potassium among themselves via gap junctions, preventing dangerous extracellular potassium increases.
Neurotransmitter Regulation
- Astrocytes manage glutamate and GABA levels:
- Uptake excess glutamate from synapses and convert it to glutamine through glutamine synthetase.
- Glutamine is transported back to neurons, where it is reconverted to glutamate.
- Synthesize GABA from glutamate using glutamate decarboxylase.
Glycogen and Glucose Metabolism
- Astrocytes store glycogen and manage glucose levels for neurons.
- They sense low ATP levels in neurons, breaking down glycogen to release glucose.
- Glucose is transformed into pyruvate and lactate, which is sent to neurons for ATP production.
- Glucose enters astrocytes through GLUT1, while neurons use GLUT3.
Glut Transporters
- The BBB contains one type of glut transporter.
- Neurons have three distinct types of glut transporters.
Astrocytes and Synaptic Connections
- Astrocytes enhance synaptic connections between neurons, though the underlying mechanisms remain unclear.
Satellite Cells
- Satellite cells serve as the PNS counterpart to astrocytes.
- They regulate nutrient metabolism, neurotransmitter levels, and potassium homeostasis without involvement in the BBB.
Locations of Satellite Cells
- Present in the dorsal root ganglia and autonomic ganglia:
- Dorsal root ganglia: Contain cell bodies outside the spinal cord, surrounded by satellite cells.
- Autonomic ganglia: Include sympathetic (pre-vertebral and para-vertebral) and parasympathetic ganglia (near target organs).
Oligodendrocytes vs. Schwann Cells
- Oligodendrocytes myelinate axons in the CNS, covering multiple axons (30-60 per cell).
- Schwann cells myelinate axons in the PNS, covering distinct segments of individual axons.
Myelination in the Nervous System
- Damaged oligodendrocytes result in irreversible demyelination with no regeneration capacity.
- Schwann cells have regenerative potential, facilitating remyelination in damage scenarios like Guillain-Barre Syndrome.
Demyelination Diseases
- Multiple sclerosis affects demyelination in the CNS.
- Guillain-Barre Syndrome refers to demyelination in the PNS.
Structure and Function of Schwann Cells
- Schwann cells envelop axons with "arms" forming concentric layers, creating the myelin sheath.
- The neurolemma, the outer layer of Schwann cells, is crucial for post-injury regeneration.
Myelin Composition and Function
- Myelin primarily consists of lipids and proteins, serving as an electrical insulator to enhance action potential propagation.
- Myelinated axons facilitate saltatory conduction, leading to faster signal transmission compared to non-myelinated axons, which transmit signals continuously.
Nodes of Ranvier
- Gaps between the myelin sheath on axons are called nodes of Ranvier, densely packed with voltage-gated sodium channels.
- Action potentials leap from node to node, boosting conduction speed.
Factors Affecting Conduction Velocity
- Myelination and axon diameter both increase conduction velocity; larger diameters reduce resistance to signal flow.
- Neuron types vary:
- Type A fibers (alpha, beta, gamma, delta) have the fastest conduction due to high myelination.
- Type B fibers have moderate myelination and conduction speed.
- Type C fibers are unmyelinated or minimally myelinated, resulting in low conduction speeds.
Ependymal Cells
- Ependymal cells line brain ventricles and create a blood-cerebrospinal fluid barrier.
- They regulate molecule movement (water, glucose, ions) and produce cerebrospinal fluid (CSF) via ciliary movement.
Microglia
- Microglia are immune cells in the CNS, derived from bone marrow monocytes.
- They react to pathogens and injury by releasing inflammatory substances, which can harm healthy tissues.
- Capable of phagocytosing pathogens and presenting antigens to T cells, thus amplifying the immune response.
- Overactive microglia can lead to demyelination and contribute to conditions such as encephalitis, particularly in diseases such as HIV.
Glial Cells Overview
- Nervous tissue is composed of neurons and glial cells.
- Glial cells are present in both the central nervous system (CNS) and peripheral nervous system (PNS).
Astrocytes
- Found exclusively in the CNS, astrocytes perform various functions, including maintaining the blood-brain barrier (BBB).
Blood-Brain Barrier Structure
- The BBB comprises three layers:
- Endothelial cells with tight junctions.
- Basal lamina, a connective tissue layer.
- Foot processes from astrocytes that help in forming the barrier.
Blood-Brain Barrier Function
- Regulates the transport of molecules between blood and nervous tissue.
- Lipid-soluble substances and gases (e.g., CO2, O2) cross the BBB easily.
- Charged ions and larger molecules require specialized transport proteins to pass through.
- Astrocytes secrete growth factors to strengthen tight junctions, enhancing the barrier's selectivity.
Areas with Compromised Blood-Brain Barrier
- Certain brain regions permit molecular exchange due to a compromised BBB:
- Area postrema: Detects blood toxins and triggers vomiting.
- Osmoreceptors near the hypothalamus: Monitor blood salts and glucose for fluid balance regulation.
- Hypothalamic-pituitary axis: Facilitates hormone signaling between hypothalamus and pituitary gland.
Potassium Buffering
- Astrocytes absorb excess potassium ions during neuronal action potentials to maintain neuronal excitability.
- They redistribute potassium among themselves via gap junctions, preventing dangerous extracellular potassium increases.
Neurotransmitter Regulation
- Astrocytes manage glutamate and GABA levels:
- Uptake excess glutamate from synapses and convert it to glutamine through glutamine synthetase.
- Glutamine is transported back to neurons, where it is reconverted to glutamate.
- Synthesize GABA from glutamate using glutamate decarboxylase.
Glycogen and Glucose Metabolism
- Astrocytes store glycogen and manage glucose levels for neurons.
- They sense low ATP levels in neurons, breaking down glycogen to release glucose.
- Glucose is transformed into pyruvate and lactate, which is sent to neurons for ATP production.
- Glucose enters astrocytes through GLUT1, while neurons use GLUT3.
Glut Transporters
- The BBB contains one type of glut transporter.
- Neurons have three distinct types of glut transporters.
Astrocytes and Synaptic Connections
- Astrocytes enhance synaptic connections between neurons, though the underlying mechanisms remain unclear.
Satellite Cells
- Satellite cells serve as the PNS counterpart to astrocytes.
- They regulate nutrient metabolism, neurotransmitter levels, and potassium homeostasis without involvement in the BBB.
Locations of Satellite Cells
- Present in the dorsal root ganglia and autonomic ganglia:
- Dorsal root ganglia: Contain cell bodies outside the spinal cord, surrounded by satellite cells.
- Autonomic ganglia: Include sympathetic (pre-vertebral and para-vertebral) and parasympathetic ganglia (near target organs).
Oligodendrocytes vs. Schwann Cells
- Oligodendrocytes myelinate axons in the CNS, covering multiple axons (30-60 per cell).
- Schwann cells myelinate axons in the PNS, covering distinct segments of individual axons.
Myelination in the Nervous System
- Damaged oligodendrocytes result in irreversible demyelination with no regeneration capacity.
- Schwann cells have regenerative potential, facilitating remyelination in damage scenarios like Guillain-Barre Syndrome.
Demyelination Diseases
- Multiple sclerosis affects demyelination in the CNS.
- Guillain-Barre Syndrome refers to demyelination in the PNS.
Structure and Function of Schwann Cells
- Schwann cells envelop axons with "arms" forming concentric layers, creating the myelin sheath.
- The neurolemma, the outer layer of Schwann cells, is crucial for post-injury regeneration.
Myelin Composition and Function
- Myelin primarily consists of lipids and proteins, serving as an electrical insulator to enhance action potential propagation.
- Myelinated axons facilitate saltatory conduction, leading to faster signal transmission compared to non-myelinated axons, which transmit signals continuously.
Nodes of Ranvier
- Gaps between the myelin sheath on axons are called nodes of Ranvier, densely packed with voltage-gated sodium channels.
- Action potentials leap from node to node, boosting conduction speed.
Factors Affecting Conduction Velocity
- Myelination and axon diameter both increase conduction velocity; larger diameters reduce resistance to signal flow.
- Neuron types vary:
- Type A fibers (alpha, beta, gamma, delta) have the fastest conduction due to high myelination.
- Type B fibers have moderate myelination and conduction speed.
- Type C fibers are unmyelinated or minimally myelinated, resulting in low conduction speeds.
Ependymal Cells
- Ependymal cells line brain ventricles and create a blood-cerebrospinal fluid barrier.
- They regulate molecule movement (water, glucose, ions) and produce cerebrospinal fluid (CSF) via ciliary movement.
Microglia
- Microglia are immune cells in the CNS, derived from bone marrow monocytes.
- They react to pathogens and injury by releasing inflammatory substances, which can harm healthy tissues.
- Capable of phagocytosing pathogens and presenting antigens to T cells, thus amplifying the immune response.
- Overactive microglia can lead to demyelination and contribute to conditions such as encephalitis, particularly in diseases such as HIV.
Structure of Neurons
- Neurons consist of key components: dendrites, cell body (soma), axon, axon hillock, and axon terminals.
- Dendrites extend from the neuron, acting as the primary receptive zone for incoming signals.
- The cell body houses the nucleus and organelles essential for neuron function.
- Axons conduct electrical impulses away from the soma to the axon terminal.
- The axon hillock is the starting point of the axon, notable for a high density of voltage-gated sodium channels.
Functions of Neurons
- Dendrites receive neurotransmitters through ligand-gated ion channels, leading to excitatory (EPSP) or inhibitory (IPSP) graded potentials.
- EPSPs promote action potentials; conversely, IPSPs diminish the likelihood of action potentials.
- The soma engages in graded potentials and is vital for synthesizing proteins, including neurotransmitters and membrane components.
Protein Synthesis Process
- Protein synthesis begins with transcription, where DNA is converted into mRNA, which travels to the rough endoplasmic reticulum (Nissl bodies) for translation into proteins.
- Post-synthesis, proteins are packaged into vesicles by the Golgi apparatus and transported along the axon.
Axon Functions
- The axon transmits action potentials via a depolarization followed by repolarization phase.
- Action potentials are initiated at the axon hillock and propagated down the axon using voltage-gated sodium and potassium channels.
Axonal Transport Mechanisms
- Kinesin is the motor protein responsible for anterograde transport, delivering materials from the soma to the axon terminal.
- Dynein carries out retrograde transport, moving materials from the axon terminal back to the soma.
- Anterograde transport includes neurotransmitters, membrane proteins, and organelles like mitochondria.
- Retrograde transport can recycle organelles and transmit nerve growth factors, crucial for signaling damage.
Clinical Relevance
- Some viruses, like polio and rabies, exploit axonal transport mechanisms, utilizing dynein to travel from nerve terminals to the soma for replication and neuronal damage.
Summary of Action Potentials
- Action potentials consist of a depolarization phase (sodium influx) followed by a repolarization phase (potassium efflux).
- Voltage-gated sodium channels facilitate the depolarization process, while voltage-gated potassium channels restore the negative membrane potential during repolarization.
Additional Notes
- Retrogradely transported nerve growth factors can stimulate the soma to enhance protein production for neuronal repair and growth in response to injury.
Shingles and Viral Mechanism
- Shingles originates from the varicella zoster virus, which can remain inactive post-infection and can be reactivated under stress or immunosuppression.
- The virus utilizes kinesin for transport back down the axon, which can affect skin tissues.
Axon Terminal Functions
- The axon terminal is critical for the release and reuptake of neurotransmitters.
- Action potentials trigger depolarization at the axon terminal via voltage-gated sodium channels, leading to calcium influx through voltage-gated calcium channels, which initiates neurotransmitter release.
Neurotransmitter Release Process
- Neurotransmitter vesicles contain v-SNAREs, while the axon terminal features t-SNAREs, with calcium enabling vesicle fusion to the plasma membrane.
- Released neurotransmitters interact with receptors on adjacent neurons or muscles, exerting their effects.
Neurotransmitter Termination
- Neurotransmitter action is terminated by reuptake or enzymatic degradation.
- Specific proteins mediate the reuptake of neurotransmitters for recycling in the axon terminal, with SSRIs like Prozac inhibiting reuptake to enhance serotonin availability in synapses, beneficial in treating depression.
Structural Classification of Neurons
- Neurons are categorized as multipolar, bipolar, or pseudo-unipolar based on their structures.
- Multipolar neurons feature multiple dendrites and are commonly located in areas like the motor cortex.
- Bipolar neurons, possessing a single dendritic extension, are mainly found in sensory organs like the retina.
- Pseudo-unipolar neurons have one process that forks into peripheral and central branches, often found in dorsal root ganglia.
Functional Classification of Neurons
- Neurons are classified as sensory (afferent), motor (efferent), or interneurons.
- Sensory neurons convey signals from receptors to the CNS and include various specialized types.
- Motor neurons transmit signals from the CNS to effectors, categorized into visceral and somatic types.
- Interneurons serve as relays between sensory and motor pathways and constitute a significant portion of the CNS.
Importance of Interneurons
- Interneurons facilitate immediate reflex actions by connecting sensory inputs to motor outputs in the spinal cord.
- They are essential for processing information in the brain and spinal cord, supporting a variety of neural pathways.
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Explore the fundamental concept of Resting Membrane Potential in neurons and other cell types. Understand the roles of Sodium-Potassium ATPases and Leaky Potassium Channels in establishing and maintaining this essential biological state.