Synaptic Transmission Flashcards
What is a synapse?
Junction between 2 neurones or between neurone and effector cell
What is a synaptic cleft?
Tiny gap between cells at synapse
What is the synaptic knob?
The presynaptic neurone that has a swelling
Synaptic knob contains synaptic vesicles that are filled with ______
neurotransmitters
Explain how synapses make sure impulses are unidirectional
-(Vesicles containing) neurotransmitter only in presynaptic membrane/neurone
-Receptors are only on postsynaptic membranes
Describe what happens briefly when an action potential reaches the end of a neurone
-Causes neurotransmitters to be released into synaptic cleft
-Diffuse across to postsynaptic membrane & bind to specific receptors
-Can trigger action potential & cause muscle contraction or hormone secretion
Describe what happens in the synapse to stop a response from keep happening
-Neurotransmitters are removed from cleft
-Taken back into presynaptic neurone or broken down by enzymes
Neurotransmitters are removed from cleft
Taken back into presynaptic neurone or broken down by enzymes
Synapses that use acetylcholine (neurotransmitter - ACh)
Describe how a nerve impulse is transmitted across a cholinergic synapse
-Arrival of action potential (at synaptic knob) causes calcium ion channels to open and Ca2+ to enter synaptic knob
-Influx of Ca2+ causes synaptic vesicles to fuse with presynaptic membrane
-Releases ACh from vesicles into synaptic cleft (exocytosis)
-ACh diffuse across synaptic cleft and bind to specific cholinergic receptors on postsynaptic membrane
-Na+ channels in postsynaptic neurone to open
-Influx of Na+ causes depolarisation & generates action potential in postsynaptic neurone (if threshold reached)
-AChE (acetylcholinesterase) hydrolyses ACh into choline and ethanoic acid (acetyl) which diffuses back into presynaptic neurone
-Removed so that its response doesn’t keep happening
Sodium channels close
-ATP is released by mitochondria to recombine choline and ethanoic acid to form ACh
-Stored in synaptic vesicles for future use
Neurotransmitters can be both _____ and ______
Neurotransmitters can be both excitatory and inhibitory
What are excitatory neurotransmitters?
-Neurotransmitters that depolarise postsynaptic membrane
-Making it fire action potential if threshold is reached
State where is acetylcholine is excitatory
At cholinergic synapse in CNS
State what (excitatory) acetylcholine does at the cholinergic synapses in CNS
-Binds to cholinergic receptors = causes action potential in postsynaptic membrane and neuromuscular junctions
What are inhibitory neurotransmitters?
-Hyperpolarise postsynaptic membrane (make pd more negative)
-Preventing it from firing action potential
State where is acetylcholine is inhibitory
At cholinergic synapse in heart
State what (inhibitory) acetylcholine does at the cholinergic synapses in the heart
Binds to receptors = causes potassium ion channels to open on postsynaptic membrane = hyperpolarising it
What is summation?
The effect of neurotransmitter released from many neurones is added together
What does summation allow?
Allows synapses to accurately process info, finely turning the response
Name 2 types of summation
-Temporal summation
-Spatial summation
What is temporal summation?
Where 2 or more nerve impulses arrive in quick succession from same presynaptic neurone
Why does temporal summation make an action potential more likely?
∵ more neurotransmitters is released into synaptic cleft
Describe how spatial summation results in an action potential
-Sometimes many neurones connect to 1 neurone
-Small amount of neurotransmitter released from each neurone = enough altogether to reach threshold in postsynaptic neurone & trigger action potential
What is a neuromuscular junction?
Is synapse between motor neurone and muscle cell
What neurotransmitter is used in a neuromuscular junction?
-ACh
-Binds to cholinergic receptors (called nicotinic cholinergic receptors)
-Work same way as cholinergic synapse
Name 3 differences between neuromuscular junctions and cholinergic synapses
-Postsynaptic membrane has lots of folds that form clefts
-Clefts stores acetylcholinesterase (AChE)
-Postsynaptic membrane has more receptors than other synapses
-ACh is always excitatory at neuromuscular junction
-When motor neurone fires action potential = triggers response in muscle cell
-Isn’t the case for synapse between 2 neurones
An action potential is generated at the cell body of the motor neurone. Explain how this action potential passes along the motor neurone to the neuromuscular junction. (3)
Depolarisation of axon membrane/influx of Na+ establishes local currents
Change permeability to Na+/open Na+ gates of adjoining region
Adjoining region depolarises/influx of Na+
In an investigation, the higher the concentration of sucrose in a rat’s mouth, the higher the frequency of nerve impulses from each taste receptor to the brain. If rats are given very high concentrations of sucrose solution to drink, the refractory period makes it impossible for information about the differences in concentration to reach the brain. Explain why. (2)
(Refractory period) leads to separate nerve impulses
OR (Refractory period) limits frequency of nerve impulses
When maximum frequency reached, no further increase in information/all (higher) concentrations of sucrose seem the same
The rate of ATP consumption in a non-myelinated neurone is greater than that of a myelinated neurone when conducting electrical impulses at the same frequency. Explain why. (2)
Greater entry of sodium ions
Active transport
