5.1.3 Neuronal communication Flashcards
what is a sensory receptor?
A cell that detects a specific stimulus and converts the stimulus energy into the electrical energy of a generator potential
what is a Photoreceptors?
sensory receptors which are sensitive to light – found in the retina of the eye
what is a Pressure receptors?
sensory receptors which are sensitive to pressure – found in the skin
what is a Thermoreceptors?
sensory receptors which aresensitive to temperature – found in the skin and within the hypothalamus
what is a Chemoreceptors?
sensory receptors which are sensitive to the concentration of a specific chemical stimulus – found on the tongue, in the nose and in the wall of the carotid artery
what is a Baroreceptors?
sensory receptors which are sensitive to blood pressure – found in the walls of certain arteries
what is a Osmoreceptors?
sensory receptors which are sensitive to plasma salt ion concentration or water potential – found within the hypothalamus
what will a generator potential trigger in a sensory neurone?
An action potential
how would you describe Photoreceptors, thermoreceptors, Chemoreceptors, etc?
highly specific
Give an example of a sensory receptor
The Pacinian corpuscle
what is the Pacinian corpuscle?
sensory receptors present in the skin which are sensitive to pressure
what are the pacinian corpsucle reffered to as?
mechanoreceptor
what is a mechanoreceptor
the general term of any receptor sensitive to
movement or pressure changes
how does the pacinian corpscle work?
Pacinian corpuscles convert the stimulus energy (pressure) into the electrical energy of a generator potential, which may trigger an action potential in a sensory neurone
what is the pacinain corpsucel made up of?
a sensory neurone ending at its core, surrounded by layers of tissue that have gel between them.
what is inside the plamsa membrane of the sensory neurone of the pacinian corpuslce?
stretch‐mediated sodium ion channels
how do stretch‐mediated sodium ion channels work?
When the membrane is deformed (pushed into a different shape), theseretch‐mediated sodium ion channels change 3D shape into an open state. Hence the membrane becomes more permeable to sodium ions in response to the stimulus of pressure.
What is this? Can you label it?
The pacinian Corpuscle
how does the pacinian corpscle work? (mechanism) 7 steps
- In the resting state (which occurs when the stimulus is absent):
o The stretch‐mediated sodium ion channels in the plasma membrane of the sensory neurone (within the Pacinian corpuscle) are closed, i.e. impermeable to sodium ions;
o There is however a resting potential of ‐70mV across the plasma membrane,such that the sensory neurone is negative inside – this has been established by the ongoing activity of sodium/potassium pumps; - Pressure (the stimulus) is applied to the layers of tissue that make up most of the corpuscle’s structure, deforming the shape of the corpuscle;
- This causes deformation of the membrane of the sensory neurone within the corpuscle;
- The consequence is that the stretch‐mediated sodium ion channels in the membrane of the sensory neurone undergo a change in 3D shape, such that they open.
- The membrane is now more permeable to sodium ions, hence sodium ions now move by facilitated diffusion into the neurone, down their electrochemical gradient;
- This depolarises the membrane, causing the membrane potential to become less negative: this is the generator potential;
- If the extent of depolarisation of the generator potential exceeds a threshold, an action potential is triggered, which is transmitted by the dendron (and later the axon) of the sensory neurone, towards the central nervous system
what is a neurone?
an individual specialised cell within the nervous system, capable of transmitting action potential
what is a nerve?
a bundle of thousands of parallel neurones, surrounded by a protective sheath of connective tissue
what is the conective tissue in a nerve called?
the perineurium
what is a sensory neurone needed for?
Sensory neurones are needed to pass action potentials from receptors to the CNS; a sensory nerve is a nerve containing thousands of parallel sensory neurones
what is a Motor neurones needed for?
Motor neurones pass action potentials from the CNS to effectors (i.e. to muscles or glands); a motor nerve is a nerve containing thousands of parallel motor neurones.
what is a mixed nerve?
A mixed nerve is a nerve containing both sensory and motor neurones, which are transmitting action potentials in opposite directions.
what is the neuronal pathway?
A sequence of neurones
what is the neuronal pathway from receptor to effector
Receptor
–
Sensory neurone
- (synapse)-
Relay neurone
-(synapse)-
motor neurone
–
effector
what is the cell body of a neurone? 3 marks
contains the nucelus and majority of the cytoplasm
cytoplsma has lots of mitochondria - (producing ATP for synthesis of neurotransmitter molecules and for the sodium‐potassium pumps that maintain resting potential)
many ribosomes - (synthesising enzymes, ion channels and carrier proteins [sodium‐potassium pumps]
what is the dendron of a neurone?
extensions of the plasma membrane and cytoplasm
transmitting action potentials TOWARDS the cell body
end of a dendron may branch into even thinner dendrites - (allows multiple receptors,
sensory neurones or relay neurones can form synapses with this neurone)
what is a dendrite?
small branches coming from the Dendron of a neurone - (allows multiple receptors,
sensory neurones or relay neurones can form synapses with this neurone)
what is the axon of a neurone?
single cylindrical extension of the plasma membrane and cytoplasm
transmitting action potentials AWAY from the cell body
how to remember the position of the axon on a neurone?
Axon - Away
What is the role of a sensory neurone?
To transmit action potentials from sensory receptors to the CNS, where the sensory neurone will synapsewith a relay neurone (or sometimes directly with a motor neurone).
what is this? Label it.
sensory neurone
what do these arrows represent?
the movement of an action potential along a sensory neurone
what are the key features of a sensory neurone?
a single, long dendron
A cell body that is off to the side (in the middle)
a single, short axon
may be myelinated
what is the role of a relay neurone?
To transmit impulses from sensory neurones to motor
neurones, or between other relay neurones in a neural network.
where are relay neurones contained?
Only within the Cnetral Nervous System (Brain and Spinal cord)
What is this? Label it.
Relay neurone
what do these arrows represent?
the movement of an action potential along a relay neurone
what are the key features of a relay neurone?
multiple, short dendrons
cell body located centrally - dendrons and axons radiating from it
multiple, short axons
usually not (fully) myelinated
what is the role of a motor neurone?
To transmit impulses from a relay (or sensory) neurone to an
effector (muscle or gland), stimulating the effector to produce a response.
what is this? label it
motor neurone
what are the key features of a motor neurone?
multiple, short dendrites - connected to cell body (unmyelinated)
a cell body at one end
single, very long axon (myelinated) to a Neuromuscular junction
what does it mean if a neurone is myelinated?
they are surrounded by a myelin sheath
what is myelin? What is it made of?
myelin is made up of multiple layers of plasma membrane, forming when a Schwann cell wraps around the axon many times as it grows.
what is between the schwann cells?
Nodes of Ranvier - narrow unmyelinated regions, where the axon is exposed
what is the function of the nodes of ranvier?
During transmission of an action potential along the axon, only the nodes of Ranvier can depolarise, leading to saltatory (‘jumping’) conduction
what is this? label it.
A meylinated axon
What are the 3 main benefits of using a myelinated axon?
electrical insulation – the multiple layers of membrane (containing phospholipids) are an effective electrical insulator, decreasing the chance that action potentials will erroneously be transmitted directly from one axon to other axons that may lie in parallel with it within the same nerve;
Increased speed of transmission of action potentials, due to saltatory conduction – the depolarisation of the action potential ‘jumps’ from node of Ranvier to node of Ranvier, rather than being transmitted continuously as a wave of depolarisation; saltatory conduction is around 100x faster than continuous transmission, allowing more rapid responses to stimuli and hence increased chance of survival;
Less energy needed for transmission of action potentials – only the nodes of Ranvier needed to undergo repolarisation (involving active transport of Na+ and K+ ions) after the action potential has passed, so less ATP is needed.
what must occur for an action potential to be submitted?
neurone must first be in a suitable resting state, more specifically there must be a resting potential across its plasma membrane
what must happen to the resting potential after an action potential is transmitted?
the resting potential must be restored, so that the neurone can transmit another action potential when stimulated to do so.
what is resting potential?
The resting potential is the potential difference (voltage) across the plasma membrane of a neurone, when it is not in the process of transmitting an action potential.
what is the value of the resting potential? How can it be interpreted?
‐70mV
The minus sign here is indicating that the inside (cytoplasm) of the cell has an excess of
negative charge, whereas the outside (tissue fluid) has an excess of positive charge.
how is resting potential formed? (basics)
Potential difference means there is an imbalance of charge across the membrane, i.e. an unequal distribution of charged particles (ions) between the cytoplasm on one side of the membrane and the tissue fluid on the other side.
what are the two key mechanisms that contribute to establishing and maintaining the resting
potential of a neurone?
- The sodium/potassium (Na+/K+) pump
- Higher membrane permeability to K+ than to Na+
describe the The sodium/potassium (Na+/K+) pump
Carrier proteins embedded in the plasma membrane
Active transport of ions using energy from ATP hydrolysis
3 Na+ ions out of the cell
2 K+ ions in to the cell.
net movement of positive charge out of the cell (3 cations out, 2 cations in)
inside of the cell now negative compared to the outside, i.e. the membrane is polarised.
(Both the Na+ ions and the K+ ions are being moved against their own concentration gradients, i.e. active transport from a region of lower to a region of higher concentration)
What do the The sodium/potassium (Na+/K+) pump establish after active transport?
very steep concentration gradients for both ions are established:
the concentration of Na+ will now be much higher outside the cell than inside;
the concentration of K+ will now be much higher inside the cells than outside.
Describe the Higher membrane permeability to K+ than to Na+
The phospholipid bilayer, has a hydrophobic core of fatty acid tails and is impermeable to ions
ion channels embedded in the bilayer
provide permeability to specific ions by allowing facilitated diffusion of ions down their concentration (or electrochemical) gradient [Only if the appropriate channels are present in an open state]
Some ion channels may be ‘open’, but many ion channels may be ‘gated’
Na+ ion channels:
Due to Na+/K+ pump, there is a higher conc of Na+ out the cell, ∴ Na+ move into the cell down its concentration gradient
HOWEVER, voltage‐gated Na+ ion channels are likely to be closed, ∴ plasma membrane has very low permeability to Na+
∴very little positive charge can actually move into the cell (despite the steep concentration gradient for Na+), preventing the dissipation (loss) of the inside‐negative resting potential.
K+ ion channels
due to Na+/K+ pump, there is a higher conc of K+ in the cell, ∴ K+ move out of the cell down its concentration gradient
Many K+ ion channels are open at this time, meaning the plasma membrane has
much higher permeability to K+ than it does to Na+
There will be a significant net loss of positive charge from the cell, as more
K+ ions can move out of the cell than Na+ can move in.
This results in net loss of positive charge from the neurone and hence an insidenegative
resting potential of ‐70mV.
what does ‘gated’ mean in terms of ion channels?
The ion channels can either be in an open state or a closed state (with a ‘gate’ blocking the pore).
what causes ‘gated’ channels to open and close?
several different triggers
for example:
voltage‐gated ion channels - only open if the membrane potential reaches a certain threshold
ligand‐gated ion channels - only open if a certain molecule binds to a complementary receptor site on the channel
Draw the cell membrane to show how the resting potential is maintained
what is depolarisation?
Depolarisation is the loss and eventual reversal of the potential difference across the membrane of a neurone
how is an action potential generated?
when the extent of depolarisation of a membrane exceeds a threshold potential (‐50mV)
what is the value of the threshold potential?
(‐50mV)
what is the value of the maximum extent of depolarisation during an action potential?
+40mV
how does the transmission or propagation of an action potential along a dendron or axon work?
there is a spreading of a wave of depolarisation down the plasma membrane of the neurone.
how long is cell depolarisation?
very short‐lived: as soon as the maximum depolarisation of +40mV is reached, there is immediately a repolarisation process to reverse the balance of charge and restore the resting potential of ‐70mV once more.
Draw a graph showing the changes in potential differences during an action potential
Describe the mechanism for the generation of an action potential in a sensory neurone
- There is an (inside‐negative) resting potential across the plasma membrane of the neurone of ‐70mV – this has been established by the action of the Na+/K+ pump and the higher membrane permeability to K+ than to Na+; there are many open K+ channels at this point but voltage‐gated Na+ channels are closed
- The stimulus energy triggers the opening of some Na+ channels, increasing the permeability of the membrane to Na+: so, Na+ ions begin to move by facilitated diffusion into the neurone, down their electrochemical gradient – the process of depolarisation has begun and the membrane potential starts to become less negative, due to the net influx of cations
- If the extent of depolarisation is sufficient (which corresponds to the original stimulus being strong enough to open enough Na+ channels), the threshold potential of ‐50mV will be exceeded: this triggers the opening of the voltage‐gated Na+ channels which were previously closed – the membrane is now even more permeable to Na+, so more Na+ ions move by facilitated diffusion into the neurone, causing further depolarisation; since the entry of some Na+ ions has triggered the entry of further Na+ ions, this step is an example of positive feedback
- Due to the entry of so many Na+ ions, the depolarisation reaches its maximum extent, +40mV (inside‐positive) – at this point, the voltage‐gated Na+ channels close, but voltagegated K+ channels now open; this means that the plasma membrane of the neurone now has much higher permeability to K+ than to Na+;
- K+ ions move out of the neurone by facilitated diffusion, down their electrochemical gradient, through the open voltage‐gated K+ channels – this represents a loss of positive charge from the cell and causes repolarisation of the membrane, back towards the insidenegative resting potential
- So many K+ ions leave the cell that there is an overshoot into a membrane potential that is even more negative (e.g. ‐80mV) than the resting potential – this is called hyperpolarisation, which is short‐lived as the voltage‐gated K+ channels now close; however, during hyperpolarisation it is not possible for a new action potential to be initiated
- Resting potential is now restored: the Na+/K+ pump restores the concentration gradients of Na+ and K+ so that it becomes possible for a new action potential to be generated if there is an appropriate stimulus.
what must occur for the action potential to be transmitted down an axon?
the wave of depolarisation has to spread along the plasma membrane.
Once one region of the plasma membrane is depolarised (in response to a stimulus), this depolarisation itself acts as a stimulus for depolarisation of the next region of the membrane (as described in more detail below).
what occurs during hyperpolarisation?
during the hyperpolarisation phase and for a short time afterwards, transmission of a new action potential is not yet possible – this is called the refractory period. During the refractory period, the voltage‐gated Na+ channels are not yet able to reopen; this means it will not be possible to achieve the extent of depolarisation (due to Na+ entry into the neurone) that needs to occur in an action potential.
what is the refactory period?
during the hyperpolarisation phase and for a short time afterwards, transmission of a new action potential is not yet possible. The voltage-gated Na+ channels are not able to open yet
why is the refactory period significant?
prevents action potentials spreading backwards into regions of the membrane that have already undergone recent depolarisation;
prevents action potentials overlapping and merging with one another, making sure each action potential is a discrete signal
describe the mechanism for the continuous propagation of an action potential down an axon for an UNMYELIANTED AXON
- In the region of the membrane that is already depolarising, there is entry of Na+ ions into the cytoplasm of the neurone through open Na+ channels;
- Due to the high Na+ ion concentration building up in the cytoplasm at this point, some of these Na+ ions diffuse through the cytoplasm into a region further along the neurone where the Na+ concentration is lower;
- Meanwhile, outside the neurone (in the surrounding tissue fluid), there is a decreasing concentration of Na+ ions in the region where depolarisation is already occurring, since Na+ ions are moving from the tissue fluid into the neurone;
- Hence, outside the neurone, Na+ ions from a region further along will diffuse in the tissue fluid towards the region of lower Na+ concentration (where depolarisation is already occurring);
- These movements of Na+ ions in the cytoplasm and tissue fluid (in opposite directions) set up a ‘local circuit’ and cause depolarisation of the next region of membrane;
- Once the extent of depolarisation is sufficient here to exceed the threshold potential of ‐50mV, voltage‐gated Na+ channels open;
- This is a positive feedback step that allows facilitated diffusion of Na+ ions into the neurone in this region, increasing the depolarisation to reach its maximum of +40mV;
- In this way, the action potential is propagated along the axon of the neurone, with the above sequence of events repeating continuously.
what is this? label this diagram
A diagram showing how local circuits cause propagation of an action potential
what is an action potential described as?
An action potential is an ‘all‐or‐nothing’ event
what is an ‘all‐or‐nothing’ event?
the event (the action potential) either occurs (with a fixed amplitude and predictable profile of changes in potential difference), or it does not occur at all.
why is action potential described as an all or nothing event?
Due to threshold potential
If a stimulus is strong enough, it will trigger an action potential – if not, it will not.
An even stronger stimulus cannot trigger a higher amplitude action potential though, because all action potentials are the same amplitude.
what causes the action potential to be an all or nothing event?
only if the extent of depolarisation exceed this will the positive feedback phenomenon of voltage‐gated Na+channels opening occur.
how does an action potential transmit infomation abouit a stronger stimulus?
the frequency of action potentials will be higher if the stimulus is stronger, i.e. more action potentials are generated and transmitted in given time
what does this graph represent?
what is Saltatory conduction?
The movement of an action potential across a meylinated neurone, jumping from one node of Ranvier to another.
which area in a myelinated axon can depolarise?
only the unmyelinated regions - NODES OF RANVIER
what does depolarisation actually mean?
a change in potential difference from negative to postive across the membrane of a neurone
What are the pro’s of salatatory conduction?
- rate of transmission is much faster than in unmyelianted neurones. - action potential therefore appears to jump from node to node, giving more rapid transmission of the action potential (of around 100m s‐1) compared to the continuous propagation (at around 1m s‐1) [increasing adaptive vaule]
- more energy efficient - only the nodes of Ranvier depolarise, only these regions need to be repolarised using Na+/K+ pumps, so overall there will be less ATP needed.
describe the mechansism of an Action potential transmission in a myelinated neurone by saltatory conduction
- There is depolarisation of the plasma membrane of the axon at an unmyelinated region (node of Ranvier);
- A local circuit is established that reaches the next node of Ranvier, due the diffusion of Na+ ions away from the next node in the tissue fluid and towards the next node in the cytoplasm;
- The next node begins to depolarise due to this Na+ ion movement and, once the threshold potential of ‐50mV has been reached, voltage‐gated Na+ channels open at this node;
- Na+ ions enter the neurone causing further depolarisation of the node, corresponding to an action potential;
- The process repeats, with Na+ ion movement in the tissue fluid and cytoplasm causing the next node to start depolarising;
- The local circuits involved in depolarisation of each subsequent node are much longer that the local circuits that bring about in continuous transmission: the action potential jumps from node to node – because there only needs to be voltage‐gated Na+ channels opening at nodes and only Na+ ion entry into the neurone at nodes, the overall speed of transmission is much faster.
what are the three main mechanisms in increasing the speed of an action potential?
- saltatory conduction due to myelination (see previous flashcards)
- increased axon diameter - wider axon = resistance to diffusion of Na+ ions in the cytoplasm
- increased temperature - Na+ and K+ = more kinetic energy = move more quickly by (facilitated) diffusion. [Only up to 40°C, after transmission slows due due to ion channel and Na+/K+ pump denaturation]
what is a synapse?
the junction between two neurones
what is the small gap between the pre and post synaptic neurones called?
Synaptic cleft
what type of transmission occurs in the synaptic cleft?
chemical transmission
why are synapses important?
- Allos for transmission of impulses in one direction only
- impulse from one neurone to pass to many other neurones, resulting in many simultaneous responses to a single stimulus
- Multiple pre‐synaptic neurones may synapse with a single post‐synaptic neurone, allowing summation of impulses from multiple receptors to produce a single response
how wide is the synaptic cleft?
20‐30nm wide
what is the most common type of synapse in our nervouse system, and what is the name of the neurotransmitter which is used?
cholinergic synapse
acetylcholine molecules as the neurotransmitter
Read and remember this 16 step mechanism for the cholinergic synapse LMAO
- The arrival of an action potential at the synaptic knob of the pre‐synaptic neurone results in depolarisation of the plasma membrane of this neurone;
- Voltage‐gated calcium ion channels in the plasma membrane open;
- Calcium ions move by facilitated diffusion into the pre‐synaptic neurone through these open channels;
- The calcium ions bind to synaptic vesicles in the cytoplasm, which contain the acetylcholine (ACh) neurotransmitter molecules;
- This triggers the vesicles to move towards the pre‐synaptic membrane – the vesicles attach to microtubules (via motor proteins), which act as tracks directing the movement, using energy from ATP hydrolysis;
- The vesicle membranes fuse with the pre‐synaptic membrane, resulting in the release of ACh molecules into the synaptic cleft by exocytosis;
- The ACh molecules diffuse across the cleft;
- The ACh molecules bind to glycoprotein receptors with binding sites of complementaryshape, located in the post‐synaptic membrane;
- The binding of ACh to a receptor triggers a change in the 3D shape of the receptor, resulting in the opening of a sodium ion channel (called a neurotransmitter‐gated sodium ion channel) that is part of the same glycoprotein;
- Sodium ions move by facilitated diffusion into the post‐synaptic neurone through these open channels, depolarising the post‐synaptic membrane11. If the threshold potential is exceeded (due to sufficient movement of positivelycharged sodium ions into the cell), more sodium ion channels open: these are voltagegated sodium ion channels;
- Now, more sodium ions can enter the post‐synaptic neurone, as the membrane is now more permeable to sodium ions – this is a positive feedback step (as the entry of some sodium ions has triggered the entry of even more sodium ions);
- The post‐synaptic membrane is further depolarised and a full‐strength action potential has been triggered;
- The action potential is now transmitted via the membrane of the post‐synaptic neurone as far as the next synapse (or to a neuromuscular junction);
- To prevent continuous inappropriate stimulation of the post‐synaptic neurone, there is an enzyme called acetylcholinesterase (ACE) that breaks down the acetylcholine that is attached to the receptors (resulting in the sodium ion channels returning to their closed state).
- The breakdown products of ACh are choline and ethanoic acid: these are reabsorbed by the pre‐synaptic neurone and are recycled to produce new ACh molecules (in a process that requires energy from ATP).
what is going on in each step of this diagram?
what is Acetylcholine (Ach) an example of?
an excitatory neurotransmitter - triggers opening of sodium ion channels
This results in positively‐charged sodium ions entering the post‐synaptic neurone, leading to depolarisation of the membrane; an action potential will be the consequence if the threshold potential is exceeded
what does an excitatory neurotransmitter do?
triggers opening of sodium ion channels
give an example of an inhibitory neurotransmitter
GABA
what does an inhibitory neurotransmitter do?
These cause chloride ion channels to open in the post‐synaptic membrane
The result is that negatively‐charged chloride ions enter the post‐synaptic neurone, leading to the hyperpolarisation of the membrane; an action potential is now very unlikely to occur because the membrane potential has become even more negative (further from the threshold).
