13.5 Synapses Flashcards
What is the structure of a synapse?
What is the synaptic cleft?
The gap which separates the axon of one neurone from the dendrite of the next neurone. approx 20-30 nm
Definition of a synapse
The junction between two neurones or between a neurone and an effector cell
How are impulses transmitted across a synapse?
By neurotransmitters
What is the synaptic knob?
The swollen end of the presynaptic neurone, contains many mitochondria and endoplasmic reticulum for the manufacture of neurotransmitters
What are synaptic vesicles?
Vesicles containing neurotransmitters which 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 are synapses that use Acetylcholine called?
Cholinergic synapses
What are excitatory neurotransmitters?
Neurotransmitters depolarise the postsynaptic neurone, making it fire an action potential if the threshold is reached
What is an example of an excitatory neurotransmitter?
Acetylcholine
What are inhibitory neurotransmitters?
They bind to receptors on the postsynaptic membrane, they cause hyperpolarisation preventing an action potential from being fired
What is an example of an inhibitory neurotransmitter?
GABA
How does GABA act as an inhibitory neurotransmitter?
It binds to receptors on chlorine ion channels on the postsynaptic membrane, when open Cl- ions flood into the post synaptic membrane causing hyperpolarisation, preventing the neurone from reaching threshold potential and ultimately preventing an action potential
What are the steps for the transmission across a cholinergic synapse?
- An action potential arrives at the synaptic knob of the presynaptic neurone, this stimulates calcium ion channels to open and calcium ions to diffuse into the synaptic knob by facilitated diffusion down their electrochemical gradient
- The influx of calcium ions causes synaptic vesicles to move to the pre-synaptic membrane. They then fuse with the presynaptic membrane and release their contents Acetylcholine into the synaptic cleft by exocytosis, this sets up a concentration gradient across the synaptic cleft
- Acetylcholine diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane, causing sodium ion channels to open. The influx of sodium ions into the post synaptic membrane causes depolarisation, and an action potential is generated if the threshold potential is reached (-55 mV)
- Acetylcholinesterase an hydrologic enzyme hydrolyses Acetylcholine into choline and ethanoic acid (acetyl) which diffuse back across the synaptic cleft into the presynaptic neurone. This also prevents a continuous action potential being generated and recycles the neurotransmitters
- ATP is released from the mitochondria to recombine choline and ethanoic acid (acetyl) into acetylcholine. This is stored in vesicles for future use. Sodium ion channels also close in the absence of the acetylcholine on receptor sites
What is the role of synapses?
-Ensure impulses are unidirectional, as neurotransmitter receptors are only present on the postsynaptic membrane
-Allow impulses from one neurone to be transmitted to a number of neurones at multiple synapses, results in a single stimulus creating a number of simultaneous responses (synaptic divergence)
-Allows a number of neurones to feed into the same synapse with a single postsynaptic neurone, results in stimuli from different receptors interacting to produce a single result (syntactic convergence)
Definition of summation
The build up of neurotransmitter in a synapse to sufficient levels to trigger an action potential
What is spatial summation?
A number of presynaptic neurones connect too one postsynaptic neurone. Each releases neurotransmitter which builds up to reach a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone
What is temporal summation?
When a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period. This builds up in the synapse until the quantity is sufficient enough to trigger an action potential
