6.5 Neurons & Synapses Flashcards
What are neurons?
Neurons are specialised cells that function to transmit electrical impulses within the nervous system
What is the role of the nervous system?
The nervous system converts sensory information into electrical impulses in order to rapidly detect and respond to stimuli
Why may neurons differ?
neurons may differ according to role (sensory, relay or motor)
What are the 3 basic components of all neurones?
dendrites
axon
soma
What are dendrites?
Short-branched fibres that convert chemical information from other neurons or receptor cells into electrical signals
What is an axon?
An elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors
What is a soma?
A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival
What may the axon be surrounded by?
In some neurons, the axon may be surrounded by an insulating layer known as a myelin sheath
What is the role of the myelin sheath?
The myelin sheath improves the conduction speed of electrical impulses along the axon, but require additional space and energy
How do neurons generate and conduct electrical signals?
Neurons generate and conduct electrical signals by pumping positively charged ions (Na+ and K+) across their membrane
What does the unequal distribution of ions cause?
The unequal distribution of ions on different sides of the membrane creates a charge difference called a membrane potential
What is a resting potential?
A resting potential is the difference in charge across the membrane when a neuron is not firing
In a typical resting potential, which part of the neurone is more negative?
In a typical resting potential, the inside of the neuron is more negative relative to the outside (approximately –70 mV)
What type of process is the maintenance of a resting potential?
The maintenance of a resting potential is an active process (i.e. ATP dependent) that is controlled by sodium-potassium pumps
What is a sodium-potassium pump and what is its role?
The sodium-potassium pump is a transmembrane protein that actively exchanges sodium and potassium ions (antiport)
What amount of ions are exchanged in a sodium-potassium pump?
It expels 3 Na+ ions for every 2 K+ ions admitted (additionally, some K+ ions will then leak back out of the cell)
What gradient does the sodium-potassium channel create?
This creates an electrochemical gradient
What does the electrochemical gradient create in terms of cell potential?
the cell interior is relatively negative compared to the extracellular environment (as there are more positively charged ions outside of the cell and more negatively charged ions inside the cell)
What does the exchange of sodium and potassium require?
The exchange of sodium and potassium ions requires the hydrolysis of ATP (it is an energy-dependent process)
What are action potentials?
Action potentials are the rapid changes in charge across the membrane that occur when a neuron is firing
What are the 3 stages of action potentials?
Action potentials occur in three main stages: depolarization, repolarization and a refractory period
What is depolarisation?
Depolarisation refers to a sudden change in membrane potential – usually from a (relatively) negative to positive internal charge
- What occurs in response to a signal initiated at a dendrite?
(depolarisation)
In response to a signal initiated at a dendrite, sodium channels open within the membrane of the axon
- What does the opening of sodium channels?
As Na+ ions are more concentrated outside of the neuron, the opening of sodium channels causes a passive influx of sodium
- How does the influx of sodium ions affect membrane potential?
The influx of sodium causes the membrane potential to become more positive (depolarisation)
What is repolarisation?
Repolarisation refers to the restoration of a membrane potential following depolarisation (i.e. restoring a negative internal charge)
What channels after the influx of sodium ions?
Following an influx of sodium, potassium channels open within the membrane of the axon
By what mechanism do potassium ions in/out the neuron?
As K+ ions are more concentrated inside the neuron, opening potassium channels causes a passive efflux of potassium
What does the efflux of potassium cause?
The efflux of potassium causes the membrane potential to return to a more negative internal differential (repolarisation)
What is the refractory period?
The refractory period refers to the period of time following a nerve impulse before the neuron is able to fire again
Where are sodium ions in a normal resting state?
In a normal resting state, sodium ions are predominantly outside the neuron and potassium ions mainly inside (resting potential)
What is reversed following depolarisation?
Following depolarisation (sodium influx) and repolarisation (potassium efflux), this ionic distribution is largely reversed
What restores the resting potential during the refractory period?
Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium-potassium pump
What are nerve impulses?
Nerve impulses are action potentials that move along the length of an axon as a wave of depolarisation
When does depolarisation occur?
Depolarisation occurs when ion channels open and cause a change in membrane potential
What can the channels along the axon be categorised as?
The ion channels that occupy the length of the axon are voltage-gated (open in response to changes in membrane potential)
What does the ion channels being voltage-gated mean for depolarisation?
Hence, depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon
Therefore how does depolarisation spread?
This causes depolarisation to spread along the length of the axon as a unidirectional ‘wave’
According to what principle are action potentials generated?
Action potentials are generated within the axon according to the all-or-none principle
What is needed for an action potential to be propagated?
An action potential of the same magnitude will always occur provided a minimum electrical stimulus is generated
What is the threshold potential?
This minimum stimulus – known as the threshold potential (–55 mV) – is the level required to open voltage-gated ion channels
What happens if the threshold potential is not reached?
If the threshold potential is not reached, an action potential cannot be generated and hence the neuron will not fire
When are threshold potentials triggered?
Threshold potentials are triggered when the combined stimulation from the dendrites exceeds a minimum level of depolarisation
What happens if the threshold potential is reached?
If the overall depolarisation from the dendrites is sufficient to activate voltage-gated ion channels in one section of the axon, the resulting displacement of ions should be sufficient to trigger the activation of voltage-gated ion channels in the next axon section
What are oscilloscopes?
Oscilloscopes are scientific instruments that are used to measure the membrane potential across a neuronal membrane
How is data from an oscilloscope presented?
Data is displayed as a graph, with time (in milliseconds) on the X axis and membrane potential (in millivolts) on the Y axis
For how long will a typical action potential last?
A typical action potential will last for roughly 3 – 5 milliseconds and contain 4 key stages
What 4 stages are shown on the graph?
resting potential
depolarisation
repolarisation
refractory period
What is the resting potential of an oscilloscope trace?
Before the action potential occurs, the neuron should be in a state of rest (approx. –70 mV)
What is depolarisation on an oscilloscope trace?
A rising spike corresponds to the depolarisation of the membrane via sodium influx (up to roughly +30 mV)
What is repolarisation on an oscilloscope trace?
A falling spike corresponds to repolarisation via potassium efflux (undershoots to approx. –80 mV)
What is refractory period on an oscilloscope trace?
The oscilloscope trace returns to the level of the resting potential (due to the action of the Na+/K+ pump)
When will an action potential occur?
An action potential will only occur if the initial depolarization exceeds a threshold potential of approximately –55 mV
Are all neurones covered with myelin?
NO
In certain neurons, the axon may be covered by a fatty white substance called myelin which functions as an insulating layer
What is myelin and what produces it?
Myelin is a mixture of protein and phospholipids that is produced by glial cells (Schwann cells in PNS; oligodendrocytes in CNS)
What is the main purpose of myelin?
The main purpose of the myelin sheath is to increase the speed of electrical transmissions via saltatory conduction
How is an action potential propagated along unmyelinated neurons?
Along unmyelinated neurons, action potentials propagate sequentially along the axon in a continuous wave of depolarisation
How is an action potential propagated along myelinated neurons?
In myelinated neurons, the action potentials ‘hop’ between the gaps in the myelin sheath called the nodes of Ranvier
By what factor does myelination increase the speed of electrical conduction?
This results in an increase in the speed of electrical conduction by a factor of up to 100-fold
What is the advantage of myelination?
The advantage of myelination is that it improves the speed of electrical transmission via saltatory conduction
What is the disadvantage of myelination?
The disadvantage of myelination is that it takes up significant space within an enclosed environment
What appears as white matter?
Regions of the nervous system composed of myelinated axon tracts appear as white matter, all other areas appear as grey matter
What appears as grey matter?
Grey matter consists of neuronal cell bodies and dendrites, as well as support cells (glial cells) and synapses
How do nerves transmit electrical impulses?
Nerves transmit electrical impulses by changing the ionic distribution across the neuronal membrane (membrane potential)
When are electrical signals unable to be propagated?
Therefore, electrical signals are not able to be conducted when a semi-permeable membrane is absent
What are synapses?
Synapses are the physical gaps that separate neurons from other cells (other neurons and receptor or effector cells)
How do neurons transmit information across synapses?
Neurons transmit information across synapses by converting the electrical signal into a chemical signal
- What happens when an action potential reaches the axon terminal?
synaptic transmission
When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels
- What is there an influx of?
synaptic transmission
Calcium ions (Ca2+) diffuse into the cell and promote the fusion of vesicles (containing neurotransmitter) with the cell membrane
- What is released into the synapse and by what process?
synaptic transmission
The neurotransmitters are released from the axon terminal by exocytosis and cross the synaptic cleft
- What do neurotransmitters bind to?
synaptic transmission
Neurotransmitters bind to specific receptors on the post-synaptic membrane and open ligand-gated ion channels
- What does the opening of ion channels cause?
synaptic transmission
The opening of ion channels generates an electrical impulse in the post-synaptic neuron, propagating the pre-synaptic signal
- What happens to the neurotransmitters?
synaptic transmission
The neurotransmitters released into the synapse are either recycled (by reuptake pumps) or degraded (by enzymatic activity)
What are neurotransmitters?
Neurotransmitters are chemical messengers released from neurons and function to transmit signals across the synaptic cleft
When are neurotransmitters released?
Neurotransmitters are released in response to the depolarisation of the axon terminal of a presynaptic neuron
Where do neurotransmitters bind?
Neurotransmitters bind to receptors on post-synaptic cells and can either trigger (excitatory) or prevent (inhibitory) a response
What 3 cells can neurotransmitters trigger?
neuron
glandular cell
muscle fibre
What is the response of a neuron when triggered by neurotransmitters?
stimulation or inhibition of an electrical signal (nerve impulse)
What is the response of a glandular cell when triggered by neurotransmitters?
stimulation or inhibition of secretion (exocrine or endocrine)
What is the response of muscle fibre when triggered by neurotransmitters?
stimulation or inhibition of muscular contraction/relaxation
What is an example of a neurotransmitter?
One example of a neurotransmitter used by both the central nervous system and peripheral nervous system is acetylcholine
Where can acetylcholine be released?
neuromuscular junctions
autonomic nervous system
What is the role of acetylcholine in neuromuscular junctions?
It is commonly released at neuromuscular junctions and binds to receptors on muscle fibres to trigger muscle contraction
What is the role of acetylcholine in the autonomic nervous system?
It is also commonly released within the autonomic nervous system to promote parasympathetic responses (‘rest and digest’)
Where is acetylcholine created and how?
Acetylcholine is created in the axon terminal by combining choline with an acetyl group (derived from mitochondrial Acetyl CoA)
Where is acetylcholine stored and when is it released?
Acetylcholine is stored in vesicles within the axon terminal until released via exocytosis in response to a nerve impulse
What does acetylcholine activate?
Acetylcholine activates a post-synaptic cell by binding to one of two classes of specific receptor (nicotinic or muscarinic)
What must be done to acetylcholine in the synapse, continuously?
Acetylcholine must be continually removed from the synapse, as overstimulation can lead to fatal convulsions and paralysis
What breaks down acetylcholine?
Acetylcholine is broken down into its two component parts by the synaptic enzyme acetylcholinesterase (AChE)
Where can acetylcholinesterase be found?
AChE is either released into the synapse from the presynaptic neuron or embedded on the membrane of the post-synaptic cell
What is done with the degraded components of acetylcholine?
The liberated choline is returned to the presynaptic neuron where it is coupled with another acetate to reform acetylcholine
What are neonicotinoid pesticides?
Neonicotinoid pesticides are able to irreversibly bind to nicotinic acetylcholine receptors and trigger a sustained response
Why do neonicotinoid pesticides persist?
Neonicotinoid pesticides cannot be broken down by acetylcholinesterase, resulting in permanent overstimulation of target cells
What does overstimulation of acetylcholine receptors produce?
While low activation of acetylcholine receptors promotes nerve signalling, overstimulation results in fatal convulsions and paralysis
Why are neonicotinoids more persistent in insects?
Insects have a different composition of acetylcholine receptors which bind to neonicotinoids much more strongly
Hence, neonicotinoids are significantly more toxic to insects than mammals, making them a highly effective pesticide
What are 3 disadvantages of neonicotinoids?
Neonicotinoid use has been linked to a reduction in honey bee populations (bees are important pollinators within ecosystems)
Neonicotinoid use has also been linked to a reduction in bird populations (due to the loss of insects as a food source)
Consequently, certain countries (including the European Union) have restricted the use of neonicotinoid pesticides
What is the role of neurotransmitters?
Neurotransmitters bind to neuroreceptors on the post-synaptic membrane of target cells and open ligand-gated ion channels
What is a graded potential?
The opening of these channels cause small changes in membrane potential known as graded potentials
When is a nerve impulse initiated?
A nerve impulse is only initiated if a threshold potential is reached, so as to open the voltage-gated ion channels within the axon
What do excitatory neurotransmitters cause?
Excitatory neurotransmitters (e.g. noradrenaline) cause depolarisation by opening ligand-gated sodium or calcium channels
What do inhibitory neurotransmitters cause?
Inhibitory neurotransmitters (e.g. GABA) cause hyperpolarisation by opening ligand-gated potassium or chlorine channels
What determines whether a threshold potential is reached?
The combined action of all neurotransmitters acting on a target neuron determines whether a threshold potential is reached
Considering depolarisation and hyperpolarisation, when will threshold potential be reached?
If overall there is more depolarisation than hyperpolarisation and a threshold potential is reached, the neuron will fire
Considering depolarisation and hyperpolarisation, when will threshold potential NOT be reached?
If overall there is more hyperpolarisation than depolarisation and a threshold potential is not reached, the neuron will not fire
What is the typical threshold potential?
For a typical neuron, the threshold potential (required to open voltage-gated ion channels) is approximately –55 mV
