5.1.3 - Neuronal communication Flashcards
What is a stimulus ?
Changes in the internal and external environment
What does the body need to do once a stimulus is detected ?
Needs to process the information and produce an appropriate response
What are neurones ?
Specialised nerve cells
What is the role of a neurone ?
To transmit electrical impulses rapidly around the body so that the organism can respond to changes in its internal and external environment
What do the neurones work together to do ?
Carry information detected by sensory receptors to the effector, which in turn carries out the appropriate response
What are the different neurones found in the body ?
- Sensory
- Relay
- Motor
What is the structure of a neurone like ?
- Cell body
- Axons
- Dendrons
What does the cell body contain ?
Nucleus surrounded by cytoplasm
What is found in the cytoplasm of the cell body ?
- Large amounts of mitochondria and endoplasmic reticulum
- They are involved in the production of neurotransmitters
What are neurotransmitters ?
These are chemicals which are used to pass signals from one neurone to the next
What are dendrons ?
These are short extensions which come from the cell body
What do dendrons divide into ?
They divide into smaller branches called dendrites
What is the role of dendrons ?
Responsible for transmitting electrical impulses towards the cell body
What are axons ?
These are singular, elongated nerve fibres that transmit impulses away from the cell body
What is the structure of an axon like ?
Cylindrical in shape consisting of a very narrow region of cytoplasm surrounded by a plasma membrane
What is the role of a sensory neurone ?
They transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain
What is the role of a sensory neurone ?
What is the structure of the sensory neurone like ?They transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain
What is the structure of the sensory neurone like ?
- They have one dendron, carries the impulse to the cell body
- They have one axon, carries the impulse away from the cell body
What is the role of a relay neurone ?
They transmit impulses between neurones
What is the structure of the relay neurone like ?
They have many short axons and dendrons
What is the role of a motor neurone ?
They transmit impulses from a relay or sensory neurone to an effector
What is the structure of the motor neurone like ?
They have one long axon and many short dendrites
What is the normal nervous response pathway ?
Receptor → sensory neurone → relay neurone → motor neurone → effector cell
What covers the axon of some neurones ?
Myelin sheath
What is the myelin sheath made of ?
Plasma membrane layers
What are schwann cells ?
- They produce the layers of membrane making up the myelin sheath
- They do this by growing around the axon
What happens when each schwann cell grows ?
It lays down a double layer of phospholipid bilayer
What does the myelin sheath act as ?
It acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulse at a much faster speed than non myelinated neurones
How quickly can myelinated neurones conduct an electrical impulse ?
100 times faster than non myelinated neurones
What is present in between each schwann cell ?
Node of ranvier
What do the nodes of ranvier create ?
They create gaps in the myelin sheath
What does the node of ranvier allow for the impulse ?
- It allows for the impulse to jump from one node to the next as it travels along the neurone
- This allows the impulse to be transmitted much faster
How does the impulse travel in non myelinated neurones ?
- The impulse does not jump
- It transmits continuously along the nerve fibre
- This is much slower
What is the role of the sensory receptors ?
Detect changes in the environment
Where are the sensory receptors located ?
Sense organs e.g. ear and eye
What do sensory receptors do ?
They convert the stimulus they detect into a nerve impulse
Where do nerve impulses travel to ?
Passed through the nervous system and on into the CNS - normally to the brain
What does the brain do with information from a nerve impulse ?
- Coordinates the response and sends an impulse to an effector
- The effector results in the desired response
What are the two main features of sensory receptors ?
- Specific to a single type of stimulus
- Act as transducers
How do sensory receptors act as transducers ?
They convert a stimulus into a nerve impulse
What are the four main types of sensory receptors ?
- Mechanoreceptor
- Chemoreceptor
- Thermoreceptor
- Photoreceptor
What is the stimulus for a mechanoreceptor ?
Pressure and movement
What is the stimulus for a chemoreceptor ?
Chemicals
What is the stimulus for a thermoreceptor ?
Heat
What is the stimulus for a photoreceptor ?
Light
What is a generator potential ?
It is the nervous impulse that is formed by the receptor converting the stimulus
What is a Pacinan Corpuscle ?
They are specific sensory receptors that detect mechanical pressure
Where are Pacinian Corpuscle’s located ?
- Deep within your skin
- Most abundant in the fingers and the soles of the feet
What is found in the centre of the Pacinian Corpuscle ?
The end of the sensory neurone
What is the neurone surrounded by in the Pacinian Corpuscle ?
Layers of connective tissue
What separates each layer of tissue in the Pacinian Corpuscle ?
A layer of gel
What is present in the membrane of the neurone ?
Sodium ion channels
What are the sodium ion channels responsible for ?
Transporting sodium ions across the membrane
What type of sodium ion channel is present in the neurone ending in a Pacinian Corpuscle ?
Stretch-mediated sodium channels
What happens to stretch-mediated sodium channels when they change shape ?
Their permeability to sodium changes
What are the stretch-mediated sodium ion channels like at rest ?
They are too narrow to allow sodium ions to pass through them
What is the neurone of the Pacinian Corpuscle said to have at rest ?
A resting potential
What happens when pressure is applied to the Pacinian Corpuscle ?
- The corpuscle changes shape
- The membranes surrounding the neurone stretch
What happens when the membrane in the Pacinian Corpuscle stretches ?
- The sodium ion channels present widen
- Sodium ions can now diffuse into the neurone
What does an influx of sodium ions cause in the membrane of the Pacinian Corpuscle ?
The membrane becomes depolarised
What does the depolarisation of the membrane in the Pacinian Corpuscle result in ?
Results in a generator potential
What does the generator potential create ?
An action potential
What happens to the action potential once it has arisen ?
It is transmitted along neurones to the CNS
What are the two potentials that the axon membrane can go between ?
- Resting potential
- Action potential
When does a neurone have a resting potential ?
When the neurone is not transmitting an impulse
What is the charge like when the neurone is at its resting potential ?
- The outside of the membrane is more positively charged than the inside of the axon
- Membrane is said to be polarised
What is the potential difference of the membrane when it is resting (polarised) ?
- 70 mv
Why does the resting potential occur ?
It occurs as a result of the movement of the sodium and potassium ions moving across the axon membrane
What acts as a barrier to Na+ and K+ ions crossing into the membrane ?
The phospholipid bilayer
What are the two types of protein channels in the axon ?
- Voltage gated channel proteins
- Channel proteins that remain open
Where are sodium ions transported to ?
Out of the axon
Where are potassium ions transported to ?
Into the axon
What transports sodium and potassium ions ?
Na+/K+ pump
What is the movement of ions like (Na+/K+ pump) ?
- Movement is not equal
- 3 sodium out and 2 potassium in
What does the action of the Na+/K+ pump lead to ?
More sodium ions outside the membrane and more potassium ions in the membrane
What does more sodium ions outside of the axon lead to ?
Sodium ions diffusing back into the axon
What gradient do the sodium ions use to move back into the axon ?
Electrochemical gradient
What are most of the gated sodium ion channels like ?
They are closed and therefore prevent the movement of sodium ions
Why is there a large movement of potassium ions out of the axon ?
Because there are more potassium ion channels that are open
What does the outflow of potassium ions from the axon lead to ?
- Leads to a more positive charge outside the axon
- This creates the resting potential across the membrane of -70 mV
What happens to the charge on the membrane when a stimulus is detected ?
The charge is temporarily reversed
What happens to the potential difference across the membrane when the charge is reversed?
- The potential difference across the membrane changes and becomes positively charged
- It is now +40 mV
What is depolarisation ?
A change in potential difference from negative to positive
What is repolarisation ?
A change in potential difference from positive to negative
When does an action potential occur ?
When protein channels ion the axon membrane change shape as a result of the change of voltage across its membrane
Which channels open and close as a result of a change in action potential ?
Voltage gated ion channels
What is the activity of the proteins like when the neurone has a resting potential ?
Some potassium channels are open but sodium voltage gated ion channels are closed
What does the energy of a stimulus trigger in a neurone ?
Triggers some sodium voltage gated ion channels to open, making the membrane more permeable to sodium ions
What happens when the membrane becomes more permeable to sodium ions ?
- Sodium ions diffuse into the axon down their electrochemical gradient
- This makes the inside of the neurone less negative
What does a change in charge across the membrane lead to ?
It causes more sodium ion channels to open, allowing more sodium ions to diffuse into the axon
What happens when the potential difference reaches approximately +40 mV ?
- The voltage gated sodium ion channels close and voltage gated potassium ion channels open
- Sodium ions can no longer enter the axon but the membrane is more permeable to potassium ions
How is the charge reduced when the pd is +40 mv ?
Potassium ions diffuse out of the axon down their electrochemical gradient
What does potassium ions moving out of the axon lead to ?
Reduces the charge, resulting in the inside of the axon becoming more negative than the outside
What is hyperpolarisation ?
When the axon becomes more negative (relative to the outside) than in its normal resting state
What causes hyperpolarisation ?
Potassium ions diffusing out of the axon
What happens once the membrane is hyperpolarised ?
- Voltage gated potassium ion channels close
- Sodium potassium pump causes sodium ions to move out and potassium ions to move in
- Resting potential reinstated
How does a nerve impulse move along a neurone ?
It is propagated along the axon from one end of the neurone to the other
What does the depolarisation of the first region of the axon act as ?
- It acts as a stimulus for the depolarisation of the next region of the membrane
- The process continues along the length of the axon, forming a wave of depolarisation
What happens once sodium ions are inside the axon ?
They are attracted by the negative charge ahead and the concentration gradient to diffuse further along the axon
What does the movement of sodium ions along the membrane cause ?
They trigger the depolarisation of the next section
What is the refractory period ?
A short period of time when the axon cannot be excited again
What happens during the refractory period ?
The voltage gated sodium ion channels remain closed, preventing the movement of sodium ions into the axon
Why is the refractory period important ?
It prevents the propagation of an action potential backwards along the axon as well as forwards
What does the refractory period ensure ?
- It ensures the action potentials do not overlap
- It makes sure action potentials are unidirectional
Why are myelinated axons faster than non myelinated axons at transferring electrical impulses ?
This is because depolarisation of the axon membrane can only occur at the nodes of ranvier where no myelin is present
What happens at the node of ranvier ?
The sodium ions can pass through the protein channels in the membrane
What does the action potential do between nodes ?
It jumps from one node to another
What is saltatory conduction ?
The action potential jumping from one node to another
Is saltatory conduction more efficient than repolarisation ?
Yes it is more efficient than repolarisation
Why is saltatory conduction more efficient than repolarisation ?
Repolarisation uses ATP, so by reducing the amount of repolarisation needed, saltatory conduction makes the conduction of impulses more efficient
What are the two factors that affect the speed at which an action potential travels ?
- Temperature
- Axon diameter
How does axon diameter change the speed at which an action potential travels ?
- The bigger the axon diameter, the faster the impulse is transmitted
- This is because there is less resistance to the flow of ions in the cytoplasm
How does the temperature change the speed at which an action potential travels ?
- The higher the temperature, the faster the nerve impulse
- This is because ions diffuse faster at higher temperatures
What does a stimulus have to reach for an action potential to be triggered ?
A certain threshold value that will trigger a response
What happens if the threshold is not reached ?
No action potential will be triggered
What does the size of the stimulus affect ?
Affects the number of action potentials that are generated in a given time
What happens as the stimulus gets larger ?
Action potentials are generated more frequently
What is a synapse ?
The junction between two neurones
What is a neurotransmitter ?
Chemicals that transmit impulses across the synapse
What is the synaptic cleft ?
The gap which separates the axon of one neurone from the dendrite of the next neurone
What is the presynaptic neurone ?
Neurone along which the impulse has arrived
What is the postsynaptic neurone ?
Neurone that receives the neurotransmitter
What is the synaptic knob ?
- The swollen end of the presynaptic neurone
- It contains many mitochondria and large amounts of ER to enable it to manufacture neurotransmitters
What are synaptic vesicles ?
- Vesicles containing neurotransmitters
- The vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft
What are neurotransmitter receptors ?
Receptor molecules which the neurotransmitter binds to in the postsynaptic membrane
What are the two types of neurotransmitters ?
- Excitatory
- Inhibitory
What is an excitatory neurotransmitter ?
- These neurotransmitters result in the depolarisation of the postsynaptic neurone
- If the threshold is reached in the postsynaptic membrane, an action potential is triggered
Give an example of an excitatory neurotransmitter
Acetylcholine
What is an inhibitory neurotransmitter ?
- These neurotransmitters result in the hyperpolarisation of the postsynaptic membrane
- This prevents an action potential being triggered
Give an example of an inhibitory neurotransmitter
GABA, found in some synapses in the brain
How does synaptic transmission occur ?
- The action potential reaches the end of the presynaptic neurone
- Depolarisation of the presynaptic membrane causes calcium ion channels to open
- Calcium ions diffuse into the presynaptic knob
- This causes synaptic vesicles containing the neurotransmitters to fuse with the presynaptic membrane
- Neurotransmitter is released into the synaptic cleft by exocytosis
- Neurotransmitter diffuses across the synaptic cleft and binds with its specific receptor molecule on the postsynaptic membrane
- This causes sodium ion channels to open
- Sodium ions diffuse into the postsynaptic neurone
- This triggers an action potential and the impulse is propagated along the postsynaptic neurone
Why do action potentials have to be removed from a postsynaptic neurone ?
- So the stimulus is not maintained
- So another stimulus can arrive and affect the synapse
What happens to neurotransmitters left in the cleft ?
They are removed
What happens to the breakdown products of the neurotransmitter ?
- They are taken back into the presynaptic knob
- This acts as a way of recycling
Which neurotransmitter do cholinergic synapses use ?
Acetylcholine
Where are cholinergic synapses usually found ?
In the CNS of vertebrates and at neuromuscular junctions
What is a neuromuscular junction ?
Where a motor neurone and a muscle cell (effector) meet
Describe the mechanism of transmission across a cholinergic synapse
- Arrival of an action potential at the end of the presynaptic neurone causes calcium ion channels to open and calcium to enter the synaptic knob
- The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, and so releases acetylcholine into the synaptic cleft
- Acetylcholine molecules fuse with receptor sites on the sodium ion channel in the membrane of the postsynaptic neurone
- This causes the sodium ion channels to open allowing sodium ions to diffuse in
- The influx of sodium ions generates a new action potential in the postsynaptic neurone
- Acetylcholinesterase hydrolyses acetylcholine into choline and acetyl, which diffuse into the presynaptic neurone
- ATP released by mitochondria is used to recombine choline and acetyl into acetylcholine.
- This is stored in the synaptic vesicle for future use
What are the main roles of synapses ?
- Ensure impulses are unidirectional
- Allow an impulse from one neurone to be transmitted to a number of neurones at multiple synapses
- Alternatively, a number of neurones may feed into the same synapse with a single postsynaptic neurone
How do synapses ensure impulses are unidirectional ?
As the neurotransmitter receptors are only present on the postsynaptic membrane, impulses can only travel from the presynaptic neurone to the postsynaptic neurone
What does each stimulus from a presynaptic neurone cause ?
- Causes the release of the same amount of neurotransmitter into the synapse, sufficient neurotransmitter impulse will generate an action potential
- However, in some synapses the amount of neurotransmitter from a single impulse is not enough to trigger an action potential
What is summation ?
The triggering of an action potential if there is enough build up of neurotransmitter
What is spacial summation ?
- This occurs when a number of presynaptic neurones connect to one postsynaptic neurone
- Each releases neurotransmitters which build up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone
What is temporal summation ?
- This occurs when a single presynaptic neurone releases neurotransmitters as a result of an action potential several times over a short period
- This builds up in the synapse until the quantity is sufficient to trigger an action potential
