[qdeck ” bold_text=”false”]
[h] IB DP Biology HL C2.2 Neural signaling Flashcards
[q] What forms the body of a neuron?
[a] Cytoplasm and Nucleus
[q] Axon
[a] long elongated nerve fibres responsible for transmitting signals away from the cell body
[q] Dendrites
[a] short branched nerve fibres that receives signals from other neurons and transmits them toward the cell body
[q] 2 body systems for internal communication
[a] Nervous system
Endocrine system
[q] Membrane potential
[a] the difference in electric potential (voltage) between the interior and the exterior of a cell
[q] Resting membrane potential
[a] The voltage across the neuron’s membrane when it is not transmitting signals (at rest)
[q] Because there is a potential difference across the cell membrane, the membrane is said to be
[a] polarized
[q] Pumps
[a] integral membrane proteins that use active transport
[q] Sodium-potassium pump found in many cell membranes,
[a] powered by ATP & moves NA+ and K+ ions in opposite directions against their concentration gradient (low to high)
[q] The sodium-potassium pump
[a] has binding sites for 3 Na+ ions, 2 K+ ions and an inorganic phosphate group (from ATP)
[q] nerve impulse is electrical because
[a] it involves movement of positively charged ions
[q] Threshold potential
[a] level to which a membrane potential must be depolarized to initiate an action potential.
[q] Action Potential
[a] also called a nerve impulse
[q] When a signal reaches the dendrite, the neuron will
[a] fire by sending an electrical impulse called an action potential down the length of its axon
[q] Action potential involves 3 stages:
[a] 1. Depolarization
2. Repolarization
3. Refractory period (hyperpolarization)
[q] Depolarization
[a] -Reverse the charge
-An electrical change within a neuron from a (relatively) negative charge to a positive charge
[q] Repolarization
[a] -Restore the charge
-An electrical change within a neuron from a (relatively) positive charge to a negative charge
[q] Refractory Period (Hyperpolarization)
[a] -Restore the charge
-Period in which neuron is unresponsive to stimulation (ion distribution is being restored)
[q] Nerve Fibre Structure
[a] -Circular in cross-section with a plasma membrane enclosing cytoplasm
-Diameter is ~1µm
-Nerve impulses are about 1 m/s on a nerve fibre
[q] Nerve Fibre Structure
Diameter
[a] Increasing the diameter reduces resistance → impulses transmit faster
[q] Nerve Fibre Structure Myelin Sheath
[a] -insulating layer, or sheath that forms around nerves
Increases the speed of nerve impulses
-Schwann cell: cell that surrounds neurons
-Nodes of Ranvier: gaps between the myelin sheath
-Allows nerve impulses to jump from one node to the next → speeds of transmission (by as much as 100 m/s!)
[q] Reasons resting potential is negative
[a] -Pums are pumping more sodium out than potassium in
[q] Synapse
[a] -a junction between 2 cells in the nervous system
-Signals can only pass in one direction of a synapse
[q] Synaptic transmission ( Release of neurotransmitters from a presynaptic membrane)
[a] -Depolarization of the presynaptic membrane cause an uptake of calcium
(signaling chemical reaction inside a neuron)
[q] Releasing neurotransmitters
[a] -Neurotransmitters molecules diffuse across the synaptic cleft
-Neurotransmitters binds to receptors in the postsynaptic membrane
[q] ex of type of neurotransmitter
[a] Acetylcholine
-Exists in many types of synapses including neuromuscular junctions
[q] Neuron
[a] Individual cells in the nervous system that receive, integrate, and transmit information.
[q] Structure of a neuron
[a]
[q] Structure of sensory neuron
[a]
[q] Structure of interneuron
[a]
[q] Structure of motor neuron
[a]
[q] Resting potential
[a] When the neuron is not stimulated (about −70 millivolts (mV).
[q] Establish and maintain concentration gradients of sodium and potassium ions
[a] The sodium-potassium pump in the cell membrane helps to transport the Na+ ions out and K+ ions inside the cell via active transport.
The Na+/K+ pump utilizes energy from ATP to perform this function.
The ATPase enzyme breaks down ATP into ADP and each time this happens, three Na+ ions are pumped out in exchange for two K+ ions that enter into the cell against the concentration gradient.
The extracellular fluid has a higher concentration of positive charges compared with the intracellular fluid.
As a result of this, a negative charge is developed inside the neuron and a positive charge outside.
In this condition, the neuron is polarized.
This helps to stabilize the membrane potential.
[q] Nerve impulses
[a] A nerve impulse is electrical because it involves movement of positively charged ions.
[q] Speed of nerve impulse
[a] The speed of transmission depends on the following factors:
Amount of myelination
Diameter of the axon
Temperature
[q] Myelinated neurons
[a] Myelination is insulation by the wrapping of Schwann cells around the axon.
Myelination is discontinuous and leaves some exposed axon called the nodes of Ranvier.
The actional potential has to jump from node to node and in doing so, it covers a larger distance in a short time.
[q] Unmyelinated neurons
[a] In unmyelinated axons, the depolarisation takes place throughout the length of the axon and the action potential thus has to travel the entire length.
This takes up more time than the ‘jumping signals’ in the myelinated axon.
[q] Diameter of axon
[a] In humans, the average diameter of an axon is 1 μm, which can conduct nerve impulses at a speed of 100 m/s.
Less leakage of ions from wider diameter axons results in faster generation of action potential.
In axons with a smaller diameter, the ions face a lot of resistance from other molecules such as proteins and they get delayed in transmitting the impulse
[q] Temperature
[a] The cooler the temperature, the slower the transmission of nerve impulse.
[q] Squids & axons
[a] Interestingly, squids have giant axons which can measure more than 500 μm in diameter.
Although their axons are unmyelinated, they can achieve quick transmission of nerve impulses due to the large diameter
[q] Synapses
[a] A junction between two nerve cells, consisting of a minute gap across which impulses pass by diffusion of a neurotransmitter.
[q] Synapses as junctions
[a] They can occur between two neurons and between a neuron and an effector cell.
A signal can only pass in one direction across a typical synapse.
[q] Release of neurotransmitters from a presynaptic membrane
[a] An action potential travels down the axon of the presynaptic neuron and reaches the presynaptic terminal.
The terminals are bulbs with neurotransmitters stored in the vesicles.
The action potential causes the voltage-gated calcium ion channels to open and calcium ions flow into the presynaptic neuron from high to low concentration.
[q] Generation of an excitatory postsynaptic potential
[a] The neurotransmitters diffuse into the synaptic cleft from high concentration to low concentration.
There are several receptors present on the postsynaptic membrane which readily receive the neurotransmitters.
This binding allows sodium ion channels to open and there is an inflow of Na+ into the postsynaptic neuron.
This causes an imbalance of charge and the resting membrane potential is disturbed, causing depolarisation.
[q] Acetylcholine
[a] Acetylcholine (ACh) is the most common neurotransmitter found in our nervous systems.
It exists in many types of synapse including neuromuscular junctions.
[q] Example of acetylcholine
[a] When the motor neuron releases the neurotransmitter acetylcholine, it binds to receptors on the plasma membrane of the muscle fibre (sarcoplasm), initiating the muscle contraction.
This is achieved by depolarisation of the muscle membrane (sarcolemma) and release of calcium ions by the sarcoplasmic reticulum.
[q] C2.2.1—Neurons as cells within the nervous system that carry electrical impulses
[a] Students should understand that cytoplasm and a nucleus form the cell body of a neuron, with elongated nerve fibres of varying length projecting from it.
An axon is a long single fibre.
Dendrites are multiple shorter fibres.
Electrical impulses are conducted along these fibres.
[q] C2.2.2—Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions
[a] Students should understand how energy from ATP drives the pumping of sodium and potassium ions in opposite directions across the plasma membrane of neurons.
They should understand the concept of a membrane polarization and a membrane potential and also reasons that the resting potential is negative.
[q] C2.2.3—Nerve impulses as action potentials that are propagated along nerve fibres
[a] Students should appreciate that a nerve impulse is electrical because it involves movement of positively charged ions.
[q] C2.2.4—Variation in the speed of nerve impulses
[a] Compare the speed of transmission in giant axons of squid and smaller non-myelinated nerve fibres.
Also compare the speed in myelinated and non-myelinated fibres.
Application of skills: Students should be able to describe negative and positive correlations and apply correlation coefficients as a mathematical tool to determine the strength of these correlations.
Students should also be able to apply the coefficient of determination (R2 ) to evaluate the degree to which variation in the independent variable explains the variation in the dependent variable.
For example, conduction speed of nerve impulses is negatively correlated with animal size, but positively correlated with axon diameter.
[q] C2.2.5—Synapses as junctions between neurons and between neurons and effector cells
[a] Limit to chemical synapses, not electrical, and these can simply be referred to as synapses.
Students should understand that a signal can only pass in one direction across a typical synapse.
[q] C2.2.6—Release of neurotransmitters from a presynaptic membrane
[a] Include uptake of calcium in response to depolarization of a presynaptic membrane and its action as a signaling chemical inside a neuron.
[q] C2.2.7—Generation of an excitatory postsynaptic potential
[a] Include diffusion of neurotransmitters across the synaptic cleft and binding to transmembrane receptors.
Use acetylcholine as an example.
Students should appreciate that this neurotransmitter exists in many types of synapse including neuromuscular junctions.
[q] C2.2.8—Depolarization and repolarization during action potentials
[a] Include the action of voltage-gated sodium and potassium channels and the need for a threshold potential to be reached for sodium channels to open.
[q] C2.2.9—Propagation of an action potential along a nerve fibre/axon as a result of local currents
[a] Students should understand how diffusion of sodium ions both inside and outside an axon can cause the threshold potential to be reached.
[q] C2.2.10—Oscilloscope traces showing resting potentials and action potentials
[a] Application of skills: Students should interpret the oscilloscope trace in relation to cellular events.
The number of impulses per second can be measured.
[q] C2.2.11—Saltatory conduction in myelinated fibres to achieve faster impulses
[a] Students should understand that ion pumps and channels are clustered at nodes of Ranvier and that an action potential is propagated from node to node.
[q] C2.2.12—Effects of exogenous chemicals on synaptic transmission
[a] Use neonicotinoids as an example of a pesticide that blocks synaptic transmission, and cocaine as an example of a drug that blocks reuptake of the neurotransmitter.
[q] C2.2.13—Inhibitory neurotransmitters and generation of inhibitory postsynaptic potentials
[a] Students should know that the postsynaptic membrane becomes hyperpolarized.
[q] C2.2.14—Summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron
[a] Multiple presynaptic neurons interact with all-or-nothing consequences in terms of postsynaptic depolarization.
[q] C2.2.15—Perception of pain by neurons with free nerve endings in the skin
[a] Students should know that these nerve endings have channels for positively charged ions, which open in response to a stimulus such as high temperature, acid, or certain chemicals such as capsaicin in chili peppers.
Entry of positively charged ions causes the threshold potential to be reached and nerve impulses then pass through the neurons to the brain, where pain is perceived.
[q] C2.2.16—Consciousness as a property that emerges from the interaction of individual neurons in the brain
[a] Emergent properties such as consciousness are another example of the consequences of interaction.
[q] C2.2.1—Neurons as cells within the nervous system that carry electrical impulses
Students should understand that cytoplasm and a nucleus form the cell body of a neuron, with elongated nerve fibres of varying length projecting from it.
An axon is a long single fibre. Dendrites are multiple shorter fibres. Electrical impulses are conducted along these fibres.
[a] Nervous tissue makes up the nervous system and is composed of neurons and glial cells.
Neurons are considered to be the functional cells of the nervous system as they are responsible for electrical signalling and communication of information within the body.
Glial cells (‘glia’ meaning ‘glue’ in Greek) are supporting cells that help neurons carry out their functions.
The main part of a neuron is the soma (‘soma’ meaning ‘body’), also known as the cell body, which contains the nucleus and major organelles within the cytoplasm.
What makes neuron cells special are fibers, which are protrusions or appendages of the plasma membrane that extend from the soma.
These fibers can be dendrites or axons.
Dendrites are highly branched and short extensions of the plasma membrane (fibers) that receive signals from other neurons.
They tend to taper towards the end and are covered with spines (tiny bumps) to increase surface area for receiving input from other neurons.
Axons are long singular fibres that emerge from the cell body and propagate the action potential (neural signal) from the soma unidirectionally towards the axon terminal, in which several small branches extend towards the neighboring neuron (or target cell) to transmit the signal via synapses (small junctions between neurons that allow for the transmission of signals).
The initial segment of the axon is called the axon hillock in which the cytoplasm changes to a solution of limited organelles and components called the axoplasm.
[q] C2.2.2—Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions
Students should understand how energy from ATP drives the pumping of sodium and potassium ions in opposite directions across the plasma membrane of neurons.
They should understand the concept of a membrane polarization and a membrane potential and also reasons that the resting potential is negative.
[a] Electricity is the phenomena associated with stationary or moving charged particles (positive and/or negative).
Thus, the movement of charged ions across the plasma membrane of neurons is what creates the electrical signals within the nervous system.
The electrical state of a cell depends on its membrane potential, which is a measure of the distribution of positive and negative ions across the cell membrane and is measured in millivolts (mV).
The standard method of representing membrane potential is to denote the intracellular membrane charge based on the extracellular side being zero, relatively speaking.
When a neuron is at rest (not transmitting a signal), its resting membrane potential is around -70mV, meaning that the cytosol is more negatively charged than the extracellular fluid.
Generally, potassium ions are concentrated intracellularly whereas sodium ions are concentrated in the extracellular matrix.
This distribution of ions within and around the cell is critical for neural function.
The cell takes advantage of this ion distribution to maintain a negative charge relative to the outside through the following mechanisms:
• The sodium-potassium pump moves 3 sodium ions out and 2 potassium ions in by active transport (since both are cations, the net total is a loss of 1 cation from the cytoplasm per pump, making it more negative)
• Leakage channels that open randomly to allow Na+ and K+ ions to travel down their concentration gradients.
However, K+ leakage channels are around 50 times more ‘leaky’ than Na+ channels which ensures that the cell loses more cations than it gains (thus will be more negative than the outside)
• The existence of more organic anions inside the cell than outside, contributing to a negative charge
[q] C2.2.2—Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions
Students should understand how energy from ATP drives the pumping of sodium and potassium ions in opposite directions across the plasma membrane of neurons.
They should understand the concept of a membrane polarization and a membrane potential and also reasons that the resting potential is negative.
[a] The sodium-potassium pump functions as follows:
1. To begin, the pump is open to the inside of the cell.
In this form, the pump really likes to bind (has a high affinity for) sodium ions, and will bind to three of them.
2. When the sodium ions bind, ATP hydrolysis is triggered and the pump is phosphorylated, releasing ADP as a by-product
3. Phosphorylation makes the pump change shape, re-orienting itself so it opens towards the extracellular space.
In this conformation, the pump loses its affinity to sodium ions, releasing them outside of the cell.
4. In its outward-facing form, the pump switches allegiances and now really likes to bind to (has a high affinity for) potassium ions.
It will bind two of them, causing the pump to be dephosphorylated and returning it to its original in-ward conformation.
5. Affinity for potassium ions becomes low so they are released into the cytoplasm and the cycles repeats
[q] C2.2.3—Nerve impulses as action potentials that are propagated along nerve fibres
Students should appreciate that a nerve impulse is electrical because it involves movement of positively charged ions.
[a] Action potentials (nerve impulses) are simply very rapid changes in the membrane potential of a cell caused by ions moving in and out of the cell. They occur in 3 main stages:
1. Initiation: triggering an action potential requires receiving signals from other cells through neurotransmitters.
This signal is received by the target neuron’s dendrites, and can either be excitatory or inhibitory.
The combined action of all neurotransmitters on the target neuron determines whether an action potential will take place or not.
2. Depolarization: If an action potential is triggered, voltage-gated sodium ion channels open to allow for the flow of Na+ ions into the cell, causing the membrane to become depolarized (a decrease in the voltage difference between the inside and outside of the cell) – reaching a final membrane potential of around +40mV. Potassium ion channels are still closed at this point.
3. Repolarization: this stage restores the membrane potential back to its resting state of -70mV immediately after complete depolarization (at the action potential peak) by closing sodium ion channels and opening potassium ion channels.
For a short moment in time the neural membrane is hyperpolarized at around -80mV, at which point the potassium ion channels close and the membrane potential is slowly brought back to -70mV by the sodium-potassium pump.
Until resting potential is reached, the cell enters a refractory period in which another action potential cannot be produced until resting potential is restored.
This also prevents the action potential from travelling backwards, maintaining its unidirectionality as it is propagated along nerve fibers.
[q] C2.2.4—Variation in the speed of nerve impulses
Compare the speed of transmission in giant axons of squid and smaller non-myelinated nerve fibres.
Also compare the speed in myelinated and non-myelinated fibres.
Application of skills: Students should be able to describe negative and positive correlations and apply correlation coefficients as a mathematical tool to determine the strength of these correlations.
Students should also be able to apply the coefficient of determination (R2 ) to evaluate the degree to which variation in the independent variable explains the variation in the dependent variable.
For example, conduction speed of nerve impulses is negatively correlated with animal size, but positively correlated with axon diameter.
[a] Some types of glial cells – oligodendrocytes in the CNS and Schwann cells in the PNS – produce a lipidrich substance that surrounds the axon called the myelin sheath.
This sheath acts as a barrier (electrical insulator) between the axon membrane and the extracellular matrix, preventing ions from entering or exiting, essentially blocking depolarization.
This reduces the surface area of the axon membrane that needs to depolarize, resulting in a faster transmission of neural signals along nerve fibers.
Hence, myelinated axons only depolarize at Nodes of Ranvier, which are small sections of the axon that are not covered with myelin.
This is useful for many organisms.
For example, squids have giant axons with a diameter of 500 µm and a conduction rate of 25 m/s to allow them to escape from danger through their jet-propulsion response.
[q] C2.2.4—Variation in the speed of nerve impulses
Compare the speed of transmission in giant axons of squid and smaller non-myelinated nerve fibres.
Also compare the speed in myelinated and non-myelinated fibres.
Application of skills: Students should be able to describe negative and positive correlations and apply correlation coefficients as a mathematical tool to determine the strength of these correlations.
Students should also be able to apply the coefficient of determination (R2 ) to evaluate the degree to which variation in the independent variable explains the variation in the dependent variable.
For example, conduction speed of nerve impulses is negatively correlated with animal size, but positively correlated with axon diameter.
[a] Factors affecting the velocity of action potentials include:
• Myelination: for example, myelinated axons can conduct neural signals at velocities up to 150m/s whereas unmyelinated axon velocities of conduction range from around 0.5 to 10m/s.
• Axon diameter: the larger the diameter of the axon the faster the transmission of the signal as the ions have more space to travel in and thus face less resistance.
• Ion channel density: higher density of sodium ion channels at Nodes of Ranvier allow for faster depolarization and thus transmittance of signals across the axon.
Determining whether a certain independent variable will affect action potential velocity can be done statistically through the Coefficient of Determination, R2.
This method evaluates the degree to which variation in the independent variable explains the variation in the dependent variable.
This relationship between the two variables is represented by a number between 0 and 1 as found in the table below.
For example, while the independent variable of axon diameter is positively correlated with conduction speed, the size of an animal is negatively correlated with it.
[q] C2.2.4—Variation in the speed of nerve impulses
Compare the speed of transmission in giant axons of squid and smaller non-myelinated nerve fibres. Also compare the speed in myelinated and non-myelinated fibres.
Application of skills: Students should be able to describe negative and positive correlations and apply correlation coefficients as a mathematical tool to determine the strength of these correlations.
Students should also be able to apply the coefficient of determination (R2 ) to evaluate the degree to which variation in the independent variable explains the variation in the dependent variable.
For example, conduction speed of nerve impulses is negatively correlated with animal size, but positively correlated with axon diameter.
[a]
[q] C2.2.5—Synapses as junctions between neurons and between neurons and effector cells
Limit to chemical synapses, not electrical, and these can simply be referred to as synapses.
Students should understand that a signal can only pass in one direction across a typical synapse.
[a] Synapses are junctions between neurons (sensory, interneural, motor) and between neurons and effector cells (muscles, glands).
The synapse is usually only 20nm wide to allow for fast and unidirectional signal transmission.
Chemical synaptic transmission involves the release of a neurotransmitter from the presynaptic membrane, transmitting it through the synaptic cleft (gap), receiving the neurotransmitter on the postsynaptic membrane, and depolarization of the postsynaptic neuron/effector cell.
[q] C2.2.6—Release of neurotransmitters from a presynaptic membrane
Include uptake of calcium in response to depolarization of a presynaptic membrane and its action as a signalling chemical inside a neuron.
[a]
[q] C2.2.6—Release of neurotransmitters from a presynaptic membrane
Include uptake of calcium in response to depolarization of a presynaptic membrane and its action as a signalling chemical inside a neuron.
[a] Synaptic transmission occurs through the following steps:
1. The action potential reaches the axon terminal, causing the presynaptic membrane to depolarize
2. Depolarization causes voltage-gated calcium channels to open, allowing Ca2+ to diffuse in
3. Ca2+ ions act as second messengers by initiating a signalling cascade that causes synaptic vesicles containing neurotransmitters to release these neurotransmitters into the synaptic cleft via exocytosis
[q] C2.2.7—Generation of an excitatory postsynaptic potential
Include diffusion of neurotransmitters across the synaptic cleft and binding to transmembrane receptors.
Use acetylcholine as an example.
Students should appreciate that this neurotransmitter exists in many types of synapse including neuromuscular junctions.
[a] 4. The neurotransmitter diffuses down its concentration gradient across the synaptic cleft (the extracellular space between the presynaptic and postsynaptic membranes) and binds to ligandgated sodium ion channels on the postsynaptic membrane
5. Depolarization of the local postsynaptic membrane occurs if the neurotransmitter is an excitatory molecule, and if the threshold potential is reached an action potential is triggered and propagated away from the synapse and across the postsynaptic neuron’s axon
6. To reset the postsynaptic neuron so it can be ready to receive another signal, the neurotransmitter must be removed from the synaptic cleft.
This can be achieved in three ways (DRD):
• Degradation through enzymes in the synaptic cleft
• Recycling (reuptake) by the presynaptic neuron
• Diffusion of neurotransmitter away from synaptic cleft
For example, acetylcholine is an excitatory neurotransmitter used between neurons and muscle cells (neuromuscular junctions) to stimulate muscle contraction.
Once they bind to their receptors on the postsynaptic membrane and cause depolarization, they are degraded by the enzyme acetylcholinesterase and the monomers are recycled by the presynaptic neuron.
[q] C2.2.8—Depolarization and repolarization during action potentials
Include the action of voltage-gated sodium and potassium channels and the need for a threshold potential to be reached for sodium channels to open.
[a]
The threshold potential is a certain critical membrane potential value (around -50mV) that needs to be reached in order for voltage-gated sodium ion channels to depolarize/open.
Thus, a neuron only fires if the signal from the neurotransmitter is strong enough to reach the threshold potential.
Ligand-gated sodium ion channels respond to neurotransmitters whereas voltage-gated sodium ion channels respond to changes in the membrane potential.
Thus, ligand-gated ones are found within the postsynaptic membrane and voltage-gated ones are found across the axon.
Voltage-gated sodium ion channels are made up of two subunits:
1. Alpha (α) subunit, which is the porous channel that is selectively permeable to Na+ ions only
2. Beta (β) subunit, which is responsible for gating the alpha subunit and consists of two gates:
• Gate m (the activation gate), which is normally closed and opens during depolarization
• Gate h (the ‘ball’ or deactivation gate) which is normally open and closes (swings shut) at the action potential peak
[q] C2.2.8—Depolarization and repolarization during action potentials
Include the action of voltage-gated sodium and potassium channels and the need for a threshold potential to be reached for sodium channels to open.
[a] Voltage-gated sodium ion channels exist in three primary states:
1. Deactivated (Closed) – the cell is at resting potential; gate m is closed and gate h is swinging open.
2. Activated (Open) – stimulated by a nerve impulse, both gates of the β subunit are open to allow Na+ ions to flow through the α subunit
3. Inactivated – at the action potential peak, gate h swings shut while gate m remains open until the refractory period ends
Voltage-gated potassium ion channels also have a similar structure to sodium ion channels.
Even though both sodium and potassium ions are small and positively charged, their channels are able to distinguish them from each other by accounting for the difference in ionic radius.
This selectivity is accomplished through the primary, secondary, and tertiary structures of the voltage-gated channels that allows them to only attract their respective ions through their pores.
[q] C2.2.9—Propagation of an action potential along a nerve fibre/axon as a result of local currents
Students should understand how diffusion of sodium ions both inside and outside an axon can cause the threshold potential to be reached.
[a] When sodium ions diffuse through their channels during depolarization, that specific section of the membrane which has its sodium ion channels open becomes positively charged at around +40mV due to the large numbers of sodium ions that entered it.
This establishes an electrochemical gradient between nearby areas of the membrane that are at resting potential, since the depolarized membrane is more positive and contains more sodium ions.
Thus, sodium ions begin to diffuse into the nearby areas of the membrane, causing the threshold potential to be reached and depolarization to occur at the neighboring membrane area.
Out of these two nearby areas, only one will depolarize, since the other side is at its refractory period.
[q] C2.2.10—Oscilloscope traces showing resting potentials and action potentials
Application of skills: Students should interpret the oscilloscope trace in relation to cellular events.
The number of impulses per second can be measured.
[a] Oscilloscopes measure membrane potential across time; the horizontal lines represent the resting potential whereas the spikes depict the action potential.
[q] C2.2.11—Saltatory conduction in myelinated fibres to achieve faster impulses
Students should understand that ion pumps and channels are clustered at nodes of Ranvier and that an action potential is propagated from node to node.
[a]
Nodes of Ranvier are gaps in myelin coverage along an axon and about one micrometer long.
They provide insulation for the axon and increase the speed of action potential conduction by decreasing the membrane surface area needed to depolarize and propagate the neural signal.
This allows the action potential to ‘jump’ or ‘skip’ the membrane area wrapped in myelin and cause depolarization only at nodes of
Ranvier, which is called saltatory conduction.
This type of conduction also saves energy for the neuron since the ion channels only need to be present at nodes of Ranvier and not along the whole axon.
[q] C2.2.12—Effects of exogenous chemicals on synaptic transmission.
Use neonicotinoids as an example of a pesticide that blocks synaptic transmission, and cocaine as an example of a drug that blocks reuptake of the neurotransmitter.
[a] Exogenous chemicals are substances within an organism’s body that have entered from an external source.
Some of these chemicals can affect synaptic transmission by either promoting it or inhibiting it.
Nicotinoids are insecticides that bind to acetylcholine receptors in the CNS’s cholinergic synapses (synapses that use acetylcholine as a neurotransmitter), preventing synaptic transmission.
This leads to paralysis and eventual death. Nicotinoids are not very toxic to mammals and humans but very lethal to insects as they have more cholinergic synapses in their CNS, making them effective pesticides.
However, their effects on non-insect species and the environment has sparked concern.
Cocaine is a stimulant psychoactive drug that prevents dopamine reuptake into the presynaptic neuron, causing it to accumulate in the synaptic cleft and leading to continuous excitation of the postsynaptic neuron.
This gives false feelings of euphoria and pleasure.
[q] C2.2.13—Inhibitory neurotransmitters and generation of inhibitory postsynaptic potentials
Students should know that the postsynaptic membrane becomes hyperpolarized.
[a] Neurotransmitters that inhibit depolarization bind to the postsynaptic membrane and cause the opening of chloride ion channels.
This leads to the influx of Cl- ions into the cytoplasm, causing the membrane potential to become even more negative (hyperpolarization).
As a result, this increases the amount of sodium ions needed to make the membrane potential positive enough for the threshold potential to be reached, making the neuron less likely to fire.
[q] C2.2.14—Summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron
Multiple presynaptic neurons interact with all-or-nothing consequences in terms of postsynaptic depolarization.
[a]
Neurons receive input signals from multiple other neurons, which can be either excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs).
EPSPs cause depolarization whereas IPSPs cause hyperpolarization, and they both cancel out each other’s effects (for example, if an EPSP caused the membrane potential to increase by 10mV and an IPSP caused it to decrease by 10mV, net change in membrane potential is 0).
These inputs are added together at the axon hillock in a process called summation.
If the EPSPs are strong enough to overcome IPSPs and reach threshold potential at the axon hillock, an action potential is propagated along the axon.
If the IPSPs are stronger, hyperpolarization occurs and no action potential is triggered.
Hence, multiple presynaptic neurons interact with all-or-nothing consequences in terms of postsynaptic depolarization.
[q] C2.2.15—Perception of pain by neurons with free nerve endings in the skin
Students should know that these nerve endings have channels for positively charged ions, which open in response to a stimulus such as high temperature, acid, or certain chemicals such as capsaicin in chilli peppers.
Entry of positively charged ions causes the threshold potential to be reached and nerve impulses then pass through the neurons to the brain, where pain is perceived.
[a] Perception of sensations like pain, taste, and smell occur through receptors in skin nerve endings.
These receptors detect the specific sensation (for example, pain receptors detect the chemical within hot spices) and open in response to the stimulus, allowing positively charged ions to flow into the neuron.
This causes the threshold potential to be reached and nerve impulses then pass through the neurons to the brain, where pain is perceived.
[q] C2.2.16—Consciousness as a property that emerges from the interaction of individual neurons in the brain
Emergent properties such as consciousness are another example of the consequences of interaction.
[a] Consciousness is a property of the human body that emerges from the interaction of all individual neurons in the brain.
Like other emergent properties, it only occurs due to interaction and interdependence of billions of cells in the body.
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