5.3 - Neuronal Communication Flashcards
Pacinian corpuscle def
A pressure sensor that detects changes in pressure on the skin
Sensory receptors def
Cells/sensory nerve endings that respond to a stimulus in the internal or external environment of an organism and can create action potentials
Transducer def
A cell that converts one form of energy into another - in this case into an electrical impulse
Stimulus def
A change in energy levels in the environment
Simple mechanism for a nervous impulse pathway
- Stimulus
- Receptor
- Cell signalling
- CNS
- Cell signalling
- Response form effector
How does the Pacinian corpuscle work?
3 points
- Pressure on the skin causes the connective tissue to deform
- This causes sodium ion channels to distort and open
- Sodium ions diffuse into the axon and produce an action potential
Function of sensory neurone and info
- Carries action potential from a sensory receptor to the CNS
- Long dendron, short axon
Relay neurone info and function
- Connects sensory and motor neurones in the CNS
- Short dendrites, no dendron, short axon
Creation of nerve impulses mechanism
- More general mechanism
(13 points)
- The permeability of the nerve cell to sodium ions is altered
- This is achieved by opening the Na+ ion channels
- As Na+ ion channels open, the membrane permeability is increased,
- This allows Na+ ions to move across the membrane down their conc. gradient into the cell (diffusion)
- Movement of ions across the membrane creates a change in the potential difference across the membrane
- Inside of cell becomes less negative (compared with the outside) than usual
- This is called depolarisation
- The change in potential difference across a receptor membrane is called a generator potential
- If stimulus detected is small, only a few Na+ channels will open.
- The larger the stimulus, the more gated channels will open
- If enough gates are opened, and enough Na+ ions enter the cell
- Potential difference across cell membrane changes significantly
- This will initiate an action potential
What two channels do cells associated with the nervous system have?
- Sodium channels
- Potassium channels
Function of motor neurones
Carry an action potential from the CNS to an effector (muscle or gland)
Function of sensory neurones
Carry action potential from sensory receptor to CNS
Function of relay neurones
Connect sensory and motor neurone
- Transmit action potential from sensory to motor neurone
Features and structures of neurones
- adaptations of neurones
- Many neurones are very long
- Many gated channels on neurone plasma membrane
- Sodium/potassium ion pumps use ATP
- Neurones maintain a potential difference across their cell surface membrane
- A cell body contains the nucleus, mitochondria and ribosomes
- Numerous dendrites connect to other neurones - they carry impulses towards the cell body
- An axon carries impulses away from cell body
- Neurones surrounded by a fatty layer that insulates cell from other electrical activities nearby
- This is composed of of Schwann cells closely associated with the neurone
How do listed adaptations of neurones aid their function?
Look at previous flashcard
List differences between different types of neurone
- Motor neurones have cell body in the CNS
- They have a long axon that carries the action potential out to the effector - Sensory neurones have a longe dendron - carrying the action potential from a sensory receptor to the cell body
- They have a short axon carrying the action potential to the CNS - Relay neurones connect the sensory and motor neurones together.
- Many short dendrites and a long axon
- Conduct impulses in coordinated pathways
Myelinated neurones
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Non myelinated neurones info
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Advantages of myelination and non-myelination
- Transmit action potentials much faster than non-myelinated neurones
- This means they can transmit action potentials across much wider distances, as they can transmit signals much faster
- Non-myelinated neurones tend to be shorter and carry action potentials over a short(er) distance
- They are often used in coordinating body functions such as breathing, and digestive system action.
- Therefore the increased speed of transmission is not so important
Resting potential of neurones mechanism
- When neurone is not transmitting action potentials, it is at rest
- It is actively pumping ions in and out of the plasma membrane
- Sodium/potassium ion pumps use ATP to pump three Na+ ions out of the cell for every two potassium ions pumped in
- Gated Na+ ion channels are kept closed
- Some K+ ions are open
- So plasma membrane is more permeable to K+ ions than Na+ ions
- K+ ions tend to diffuse out of cell
- Interior of cell is maintained at a negative potential compared with the outside
- The cell membrane is said to be polarised
- potential diff is about -70mV
- This is called the resting potential
- In myelinated neurones, ion exchanges occur only at the nodes of Ranvier
Depolarisation of neurone mechanism
- When axon stimulated, voltage-gated sodium channels in membrane open
- Sodium ions flood into axon
- Causes inside of membrane to become more positive
- Potential difference reaches +25mV
- This change then causes neighbouring channels to open
- So action potential moves along the axon
Repolarisation of neurone mechanism
- After about 0.5ms
- The sodium voltage-gated channels close
- Potassium voltage-gated channels open
- Potassium ions flood out of the axon
- Inside of the axon becomes negative again compared to the outside
Hyper-polarisation of neurone mechanism
- Potassium channels remain open
- So inside temporarily becomes too negative
- Until resting potential is stored
Describe how a statocyst acts as a transducer (in squid).
3 Marks
- Kinetic energy converted to electrical potential energy
- Movement of statolith moves sensory hairs
- Membrane of sensory hairs depolarises
Outline the ways in which the structures of a sensory neurone and a motor neurone are similar.
(4 Marks)
- Both contain dendrites
- Both contain an axon
- Both contain a cell body, with a nucleus
- Both contain a myelin sheath/Schwann cell/node of Ranvier
- Both contain voltage-gated channels/ sodium-potassium ion pumps
Describe the charges inside and outside the neurone membrane at rest
- At rest the inside of the membrane is negatively charged
- Compared to the outside which is positively charged
Movement of ions at resting state of neurones
- 3Na+ taken into cell
- 2K+ released outside cell
- This makes the charge more negative
Depolarisation mechanism - better
- Stimulus arrives
- Causes Na+ ion channels to open
- Membrane is permeable to Na+
- Influx of Na+ ions diffusing into neurone cell
- Depolarisation occurs
- If it reaches -55mV - the threshold value - the threshold is met
- This is called the all-or-nothing law
- either the threshold value is reached an action potential is generated or: - If the threshold isn’t reached, a generator potential is created, so an action potential isn’t quite produced
- When the threshold value is reached, voltage-gated Na+ channels are opened
- only open at threshold value, -55mV - This is an example of positive feedback
Exact value of threshold value
-55 mV
Repolarisation mechanism - better
- Voltage-gated Na+ channels close
- Membrane is impermeable to Na+ ions
- Voltage-gated K+ channels open
- K+ diffuses out of neurone cell
- The neuronal membrane repolarises
- The membrane will also start to hyperpolarise
- This is because the K+ ion channels are slow to close
- The refractory period is where you cannot have another action potential immediately after the first one
Describe and explain how the resting potential is established and how it is maintained in a sensory neurone.
(4 Marks)
- Sodium-potassium ion pump uses ATP
- Uses ATP to actively pump out Na+ ions out of the cell and move K+ ions into the cell
- K+ diffuses back out of cell
- Membrane is less permeable to Na+, as there are fewer Na+ channels
- This causes the membrane to become polarised in relation to the outside of the cell
Synaptic transmission mechanism
- Nervous impulse arrives along the axon
- Voltage gated-channel opens
- Calcium enters the synaptic knob/bouton
- Exocytosis occurs of vesicles containing neurotransmitters
- Neurotransmitter diffuses across synaptic cleft
- Neurotransmitter binds to receptors upon the post-synaptic membrane
- Receptors open and allow sodium Na+ to enter the post -synaptic neurone
- Causes a generator potential
- If the generator potential reached Threshold value
- Action potential occurs
A*
- Acetylcholine is hydrolysed by acetycholinesterase
- Within synaptic cleft
- Breakdown products diffuse back into the presynaptic neurone
- Use active transport to remove the neurotransmitter
Resting potential mechanism - better
- 3Na+ out, 2K+ in via Sodium/potassium ion pumps
- Actively transports these ions, requires ATP
- The membrane is also slightly permeable to K+
- This causes some back leakage out of the neuronal membrane
- Charge imbalance and back leakage cause interior of cell to become negative
- approx -65mV - resting potential
(A*) Anti-cholinesterases uses
- Used to treat myasthenia Gravis
- an autoimmune disease of ACh receptors
- Can act as nerve agents like Novichok, in larger amounts
Temporal Summation mechanism
- This is where the post-synaptic membrane normally doesn’t fire
- As not enough neurotransmitter is released when the pre-synaptic neurone fires
- However, if pre-synaptic neurone fires more frequently
- More neurotransmitter will be released
- More sodium channels open
- And the generator potential can reach threshold - becomes an action potential
Spatial summation
- To reach threshold potential
- We need more neurotransmitter
- If multiple pre-synaptic neurones fire simultaneously upon the same post-synaptic neurone
- Then accumulation of neurotransmitter will be enough to cause threshold potential
- To be reached in the post-synaptic neurone
Transmission of action potential along a neurone mechanism
- Stimulus causes Na+ channels to open
- Na+ diffuse ion to neurone
- Causes and action potential to be generated
- Disrupts the resting potential ion balance
- There is a high conc. of Na+ ions where they entered the neurone
- And a lower conc of Na+ to the side
- This causes them to diffuse sideways towards the resting (negative) region
- This depolarises the region
- Movement of the ions is a local current
- This will continue along the neurone
As Na+ ions diffuse and open voltage-gated sodium ion channels, what is this mechanism an example of?
A positive feedback mechansim
Refractory period mechanism
• follows an action potential in an area along the neurone
• in the absolute refractory period no impulse can be generated
• in the relative refractory period an impulse can only be generated if the
stimulus is more intense than the normal threshold value
• the sodium v‐g channels close to stop another impulse being generated
• this happens because the resting potential needs to be restored
(redistribute sodium and potassium ions)
• it also ensures impulses are separated and that they pass in one direction
only along the axon
Myelin Sheath info
- Myelinated neurones are insulated by the myelin sheath
- The sheath is a series of Schwann cells
- Each Schwann cell is wrapped around the neurone a few times
- Creates several layers of phospholipid bilayer and cytoplasm
- The gaps invert when the Schwann cells are the Nodes of Ranvier
Function and adaptations of myelin sheath
- Myelin sheath acts as an electrical insulator
- Impermeable to Na+ and K+ ions
- As there are no ion channels in the myelinated regions
- Movement of ions that cause action potentials (depolarisation) can only occur at Nodes of Ranvier
- This makes local currents longer
- As Na+ ions diffuse from one Node of Ranvier to another
- Called saltatory conduction
- Speeds up transmission
Non-myelinated neurones info
- More than one neurone wrapped loosely in one Schwann cell
- Action potential moves along neurone as a wave
- Instead of jumping from node to node (no nodes)
Compare and contrast the structure and function of myelinated and non-myelinated neurones.
Uninhibited
Describe how information about the strength and intensity of a stimulus is communicated to the brain.
- Strength of stimulus can be represented by the frequency of action potentials that are produced
- High frequency of action potentials shows a strong stimulus
Synapses are an integral part of the nervous system.
Outline the roles of synapses in the nervous system.
(3 Marks)
- Allows neurones to communicate - via cell signalling
- Ensure transmission between neurones in one direction only
- Allows convergence - impulses from more than one neurone can be passed to a single neurone
- allows divergence - impulses from a single neurone can be passed to more than one neurone
- Ensures that only stimulation that is strong enough will be passed on (threshold value etc.)
- Prevents fatigue/over-stimulation
- Allows many low-level stimuli to be amplified
- Inhibitory and stimulatory synapses allow impulses to follow specific path
- Permits memory, learning, decision-making etc.
Suggest the part of the neurone where the plasma membrane has receptors. Explain your answer.
(2 Marks)
- Pre-synaptic knob
- Prevents release of (named) neurotransmitter
- Prevents influx of (named) ions being released
Synaptic cleft def
The small gap in between 2 neurones [action potentials can’t get through this gap]
Cholinergic synapse def
Uses acetylcholine (ACh) as the neurotransmitter
Synaptic knob def
Swelling at the end of the pre‐synaptic neurone
Info about excitatory neurotransmitters
- Result in depolarisation of the post synaptic neurone
- If threshold value is reached in post-synaptic neurones
- Action potential is triggered
Example = acetylcholine
Info about excitatory neurotransmitters
- Result in hyper-polarisation of post-synaptic membrane
- Prevents an action potential being triggered
- Example = GABA
What needs to happen after a neurotransmitter has triggered an action potential?
The neurotransmitter needs to be removed from the post-synaptic neurone
- It needs to be removed
- E.g. acetylcholinesterase breaks down acetylcholine by hydrolysis action
Acetylcholinesterase def
Enzyme that hydrolysed acetylcholine into acetic acid (ethanoic acid)and choline in the synaptic cleft
Reasons for hydrolysis of acetylcholine by acetylcholinesterase
- Stops continuous production of action potentials in the post-synaptic neurone
- Enable repolarisation of post-synaptic membrane by unblocking receptors
- Stops Na+ channels from staying open
- Recycles the acetylcholine
What happens to products of hydrolysis of acetylcholine?
- Acetic acid and choline diffuse back into presynaptic knob
- Combined back into acetylcholine using ATP
- Stored in vesicles
Outline the events following the arrival of an action potential at the synaptic knob until the acetylcholine has been released into the synapse.
(4 Marks)
- Action potential arrives at the axon of the pre-synaptic neurone
- This causes Ca2+ ion channels to open
- This causes vesicles containing ACh to move towards the pre-synaptic neurone plasma membrane
- Vesicles fuse with the plasma membrane
- The acetylcholine is released into the synaptic cleft by exocytosis - requires ATP
Botulinum toxin is a protease that is produced by the bacterium, Clostridium botulinum. If this toxin is present in the body, for example as a result of eating contaminated food,
the toxin enters neurones.
With reference to Fig. 1.2, suggest, with reasons, the effects that botulinum toxin may have once it has entered a neurone.
(2 Marks)
- Protease enzyme can hydrolyse the VAMP protein/peptide bonds
- VAMP proteins cannot bonds to SNARE complex
- Microtubules broken down so vesicle containing ACh cannot move towards membrane
- Vesicle cannot fuse with plasma membrane, and ACh is not secreted by exocytosis
The heart rate is controlled by both nervous and hormonal mechanisms.
(i) Name one hormone which will increase the heart rate
(ii) State one way in which the nervous system decreases the heart rate.
(2 Marks)
(i) Noradrenaline/adrenaline
(ii) less nervous impulses along vagus nerve
- causes SAN to fire waves of excitation less frequently
Action potential generation mechanism - better
- Nerves are at resting potential
- Generator potentials occur at the end of the neurone where there is a transducer or synapse
- If the generator potential reaches threshold then that section of the neurone -
- Depolarises due to opening of v-g Sodium ion channels
- Na+ diffuses into the cell membrane
- Na+ that has entered the neurone diffuses sideways
- Causes the next section of the neurone to reach threshold
- Causes depolarisation to occur in the section
- Process of depolarisation continues in a cascade effect across length of neurone
- Neurone becomes more positive to (30 - 40mV) (30mV)
- At which point v-g Na+ ion channels close
- V-g K+ ion channels open
- K+ diffuses out of the neurone/through the membrane
- The neurone becomes more negative compared to the outside of the neurone
- Neurone is now back to resting potential
- But Na+ and K+ are in the wrong places
- This must be restored by the Na+/K+ pump
- So the cycle reoccurs to restore normal conditions
- This requires a period of time
- This is called the refractory period
How does the refractory period occur?
- After depolarisation, resting potential is restored
- But Na+ and K+ are in the wrong places - after cascade of Na+ diffusing in during depolarisation
- This must be restored by the Na+/K+ pump
- So the cycle reoccurs to restore normal conditions
- This requires a period of time
- This is called the refractory period
Why is the refractory period important?
Prevents the action potentials from travelling backwards
Saltatory conduction mechanism
- This occurs in myelinated neurones
- Depolarisation occurs at Node of Ranvier
- Na+ enters and diffuses sideways
- There are no Na+ channels in the myelinated section
- Na+ continues to move towards the next Node of Ranvier as an ionic current
- Next Node of Ranvier reaches threshold potential and depolarises
- Ionic currents move much more quickly through the nerve cytoplasm -
- Than depolarisations across the membrane
Multiple sclerosis mechanism
- An autoimmune disease
- Where the myelin sheath is attacked by the immune system
- Loss of insulation causes weakness or failure of saltatory conduction
- Can cause fatigue, weakness of muscle contractions,
Why do cats have faster conduction of nervous impulses than lizards?
- Lizards are Endotherms
- They don’t regulate their internal body temperature, or have homeostasis mehcansisms
Why do action potentials only travel in one direction?
- Vesicles with ACh only found in presynaptic knob
- Receptors for ACh only found on post-synaptic membrane
- Refractory period prevents action potentials moving backwards in neurone
Acclimatisation def
- Synapses fatigue and stop responding to stimuli
- Helps avoid overstimulation of effectors
- Which could cause damage
- By running out of neurotransmitter vesicles after repeated stimulation
Divergence of nervous pathways
- Presynaptic neurone diverges into several post-synaptic neurones
- Transmits to several parts of the nervous system
Function of synapses
- Transmit information between neurones
- Ensure one way transmission of impulses
List differences in structure between motor and sensory neurones.
(3 Marks)
