5.1.3 - Neuronal communication ( Part 2 ) Flashcards
(38 cards)
What is the refractory period ?
- A period of time after the action potential when both sodium ion channels are closed (during repolarisation) and potassium ion channels are closed (during hyperpolarisation)
- When this occurs, this section of the axon membrane is in a period of recovery and is unresponsive
What is the significance of the refractory period ?
- It ensures that action potentials are discrete events, stopping them from merging into one another
- It ensures that ‘new’ action potentials are generated ahead (ie. further along the axon), rather than behind the original action potential, as the region behind is ‘recovering’ from the action potential that has just occurred
- Impulses can only travel in one direction
- There is a minimum time between action potentials occurring at any one place along a neurone
- Length of the refractory period is key in determining the maximum frequency at which impulses can be transmitted along neurones
Define the term ‘Synapse’ ?
Synapse - The junction between two neurones
Define the term ‘Neurotransmitter’ ?
Neurotransmitter - A chemical involved in
communication across a snapse between
adjacent neurones or a neurone and muscle cell.
What are the key features of a synapse ?
- Presynaptic knob
- Synaptic cleft
- Postsynaptic membrane
Explain transmission of electrical impulses across a synapse ?
- When an electrical impulse arrives at the end of the axon on the presynaptic neurone /synaptic knob, chemical messengers called neurotransmitters are released from vesicles at the presynaptic membrane
- The neurotransmitters diffuse across the synaptic cleft and temporarily bind with receptor molecules on the postsynaptic membrane
- This stimulates the postsynaptic neurone to generate an electrical impulse that then travels down the axon of the postsynaptic neurone
- The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone, which could cause repeated impulses to be sent
What is a cholinergic synapse ?
Synapses that use acetylcholine (ACh) as a neurotransmitter
Explain transmission of electrical impulses across a cholinergic synapse ? ( part 1 )
- The arrival of an action potential at the presynaptic membrane causes depolarisation of the membrane
- This stimulates voltage-gated calcium ion channel proteins to open
- Ca2+ diffuse down an electrochemical gradient from the tissue fluid surrounding the synapse (high concentration of Ca2+) into the synaptic knob (low concentration of Ca2+ )
- This stimulates ACh-containing vesicles to fuse with the presynaptic membrane, releasing ACh molecules into the synaptic cleft by exocytosis
- The ACh molecules diffuse across the synaptic cleft and temporarily bind to cholinergic receptors in the postsynaptic membrane
Explain transmission of electrical impulses across a cholinergic synapse ? ( part 2 )
- This causes sodium ion channels to open
Sodium ions to diffuse down an electrochemical gradient into the cytoplasm of the postsynaptic neurone - The sodium ions cause depolarisation of the postsynaptic membrane, re-starting the electrical impulse once the threshold is reached
- The ACh molecules are broken down and recycled
- The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives
What is the significance of ACh molecules being broken down and recycled ?
This prevents odium ion channels staying permanently open and stops permanent depolarisation of the postsynaptic membrane
What is responsible for the breakdown of ACh ?
The enzyme acetylcholinesterase catalyses the hydrolysis of the ACh molecules into acetate and choline
What are the key features of synapses ?
- Unidirectionality
- Summation
Explain how unidirectionality is a key features of synapses ?
- Synapses ensure the one-way transmission of impulses
- This is because neurotransmitter is released on one side and its receptors are on the other – chemical transmission cannot occur in the opposite direction
Why may a single impulse that arrives at a synaptic knob be insufficient to generate an action poetical in the post-synaptic neurone ?
- Only a small amount of acetylcholine is released into the synaptic cleft
- This means only a small number of the gated ion channels are opened in the axon membrane
- So an insufficient number of sodium ions pass through the membrane
- So the threshold potential is not reached
- The small amount of acetylcholine attached to receptors is broken down rapidly by acetylcholinesterase
What is summation ?
The process by which multiple impulses added together
What are the benefits of summation ?
- It allows for the effect of a stimulus to be magnified
- A combination of different stimuli can trigger a response
- It avoids the nervous system being overwhelmed by impulses
What are the two types of summation ?
- Temporal summation
- Spatial summation
What is temporal summation ?
When the effect of multiple impulses that arrive within quick succession is added together to generate and action potential
What is the effect of temporal summation ?
- A large amount of acetylcholine is released into the synaptic cleft
- Therefore a large number of the gated ion channels open
- So a sufficient number of sodium ions pass through the membrane
What is spatial summation ?
When the effect of multiple impulses that drive simultaneously at different synaptic knobs stimulating the cell body are added together to generate and action potential
What is the effect of spatial summation ?
The multiple impulses result in a large amount of acetylcholine being released into the synaptic cleft which results in the generation of an action potential
What are the two types of neurotransmitters ?
- Excitatory neurotransmitters
- Inhibitory neurotransmitters
What is the effect of excitatory neurotransmitters ?
Excitatory neurotransmitters can stimulate the generation of an action potential in a postsynaptic neurone
How is the effect of excitatory neurotransmitters achieved ?
This is done by opening sodium ion channels in the postsynaptic membrane which causes depolarisation if a threshold is reached
