6.2 Nervous Coordination Flashcards
Where do sensory neurones carry impulses from and to?
From receptors to CNS/relay neurone.
Where do motor neurones carry impulses from and to?
From CNS/relay neurone to effectors.
Where do relay neurones carry impulses from and to?
From sensory neurones to motor neurones.
What carries nervous impulses towards a cell body?
Dendrites
What carries nervous impulses away from the cell body?
Axon
Where is the nucleus located in a nerve cell?
Cell body
What cells form the myelin sheath?
Schwann cells
What is the purpose of the myelin sheath and where is it?
Around the axon and it insulates.
What are gaps between adjacent Schwann cells called?
Nodes of Ranvier
What is the resting potential of a neurone?
It is when the inside of the membrane is more negatively charged than the outside. Resting potential is around -70mV.
How is resting potential maintained?
The resting potential is maintained by sodium-potassium pumps in the neurone membrane.
Three Na+ ions are actively transported out of the neurone by the pumps for every two K+ ions that are transported in.
This leads to a build-up of positive ions outside the cell.
There are potassium ion channels in the neurone membrane. This means it is permeable to K+ ions.
When K+ ions are transported into neurones, they can diffuse back out.
The neurone membrane is also impermeable to Na+ ions so the ions cannot diffuse back into the cell after they have been transported out.
The neurone is said to be polarised.
Describe depolarisation of the neurone cell membrane
Na+ ion channels in the cell membrane open when a neurone is stimulated.
Na+ ions flood into the neurone.
The potential difference across the membrane changes to become more positive inside the neurone.
If the potential difference increases above the threshold value (about −60mV) then the membrane will become depolarised.
More sodium channels open and there is a sharp increase in potential difference to about +30mV.
What is the threshold value for depolarisation of the neurone cell membrane
-60mV
Explain the concept of all-or-nothing
If the potential difference reaches the threshold, depolarisation will always take place and the change in potential difference will always be the same (all). If it doesn’t reach the threshold, depolarisation will not happen and an action potential will not be produced (nothing).
Describe the process of repolarisation
After the neurone membrane has depolarised to +30mV, the sodium ion channels close and potassium ion channels open.
K+ ions are transported back out of the neurone and the potential difference becomes more negative.
This is called repolarisation.
Describe the process of hyperpolarisation
There is a short period after repolarisation of a neurone where the potential difference becomes slightly more negative than the resting potential.
This is called hyperpolarisation.
Hyperpolarisation prevents the neurone from being restimulated instantly. This is called the refractory period. It is also to ensure that the wave of depolarisation travels in one direction.
What is an action potential?
The process where a neurone is depolarised and returns to resting potential (repolarised) is called an action potential.
What is the period where hyperpolarisation happens?
The refractory period
How do sodium ions move along the neurone?
Diffuse sideways
Describe the wave of depolarisation along the neurone
The presence of Na+ ions creates a change in potential difference further along the neurone membrane.
If this reaches the threshold value, sodium ion channels at this part of the membrane open.
Na+ ions diffuse into the neurone.
This part of the neurone now becomes depolarised.
Na+ diffuse all along the neurone in this way.
Depolarisation takes place along the neurone membrane. This creates a wave of depolarisation.
What does hyperpolarisation ensure?
The wave of depolarisation travels in one direction
What are the three factors that speed up the transmission of nerve impulses?
- Myelination - acts as an electrical insulator
- Temperature - the higher the temperature, the greater the kinetic energy of the ions
- Axon diameter - the greater the axon the greater the surface area for the movement of ions across the cell membrane
What is the gap between two neurones at a synapse called?
Synaptic cleft
What is the neurone before the synapse called?
Presynaptic neurone
What is the neurone after the synapse called?
Postsynaptic neurone
What is the end of the axon of the presynaptic neurone called?
Synaptic knob
What are there lots of in the synaptic knob and why are there lots of them?
Mitochondria because lots of energy is required to synthesise neurotransmitters
What do synaptic vesicles contain and where are they released from?
Contain neurotransmitters and released from synaptic knob on presynaptic neurone
Explain how an action potential is transferred across a synapse
The synaptic vesicles fuse with the presynaptic membrane to release neurotransmitters into the synaptic cleft.
When neurotransmitters are released from synaptic vesicles into the synaptic cleft, they diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane.
Receptors on the postsynaptic membrane have a complementary shape to the neurotransmitters released from the synaptic knob.
When neurotransmitters bind to their receptors, the action potential continues.
Why are there only neurotransmitter receptors on the postsynaptic neurone?
To ensure the nerve impulses only travel in one direction
What are the two types of neurotransmitter?
Excitatory and inhibitory
What is the difference between excitatory and inhibitory neurotransmitters?
Excitatory neurotransmitters generate an action potential in the postsynaptic cell.
When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is depolarised.
Inhibitory neurotransmitters prevent an action potential from being generated in the postsynaptic cell.
When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is hyperpolarised.
What is spatial summation?
Spatial summation takes place when multiple presynaptic neurones form a junction with a single neurone.
Each presynaptic neurone releases neurotransmitters. Overall there are many neurotransmitters that bind to the receptors on one postsynaptic membrane.
Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.
What is temporal summation?
Temporal summation takes place when multiple nerve impulses arrive at the same synaptic knob within a short period of time.
More neurotransmitter is released into the synaptic cleft, so more neurotransmitter is available to bind to receptors on the postsynaptic membrane.
Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.
What are the two types of synapses you need to know?
- Neuromuscular junction
- Cholinergic synapses
What is a neuromuscular junction between?
Between a motor neurone and a muscle cell
What are the steps involved in transmission across a neuromuscular junction?
The action potential depolarises the membrane of the synaptic knob. This causes voltage-gated calcium (Ca2+) ion channels to open.
Ca2+ ions diffuse into the synaptic knob.
This causes the synaptic vesicles to move and fuse with the presynaptic membrane.
Acetylcholine, the neurotransmitter inside the vesicles, is released into the synaptic cleft.
This process is called exocytosis.
Acetylcholine binds to specific receptors on the postsynaptic membrane called nicotinic cholinergic receptors which opens sodium ion channels in the postsynaptic muscle cell.
The membrane becomes depolarised.
If the potential difference reaches the threshold value, an action potential is generated and flows along the motor cell.
An enzyme called acetylcholinesterase (AChE) breaks down acetylcholine in the synaptic cleft.
The products of the break down are reabsorbed by the presynaptic neurone and reused to synthesise more acetylcholine.
How is acetylcholine broken down in a neuromuscular junction and why is it important?
An enzyme called acetylcholinesterase (AChE) breaks down acetylcholine in the synaptic cleft.
The products of the break down are reabsorbed by the presynaptic neurone and reused to synthesise more acetylcholine.
It is important that the acetylcholine is removed from the receptors. This stops action potentials from being continuously generated in the postsynaptic cell.
What are cholinergic synapses?
Synapses that use acetylcholine as a neurotransmitter
What are the key differences between neuromuscular junctions and cholinergic synapses?
- Cholinergic synapses are between two neurones. Neuromuscular junctions are between a motor neurone and a muscle cell.
- There are less receptors in the postsynaptic membrane at a cholinergic synapse than at a neuromuscular junction.
- A cholinergic synapse can trigger an inhibitory or excitatory response in the postsynaptic membrane. An action potential at a neuromuscular junction always triggers an excitatory response in the muscle cell.
- In a cholinergic synapse, depolarisation of the postsynaptic membrane results in an action potential. At a neuromuscular junction, depolarisation of the postsynaptic membrane results in muscle contraction.
- In cholinergic synapses, acetylcholinesterase is located in the synaptic cleft. At a neuromuscular junction, the enzyme is stored in clefts in the postsynaptic membrane.
What are the three effects excitatory drugs can have at the synapse?
- Mimic neurotransmitters - causes action potentials
- Inhibit enzymes - prevents breakdown of neurotransmitters
- Cause presynaptic neurone to release more neurotransmitters
What are the two effects inhibitory drugs can have at the synapse?
- Block calcium ion channels preventing release of neurotransmitters
- Block receptors on postsynaptic membrane preventing neurotransmitters from binding
