ch13 - synapses

What is a synapse

the junction between two neurones where a nerve impulse is transmitted from pre to post synaptic neurone by neurotransmitters 

What is another name for the synapse

synaptic cleft 

What are the 3 reasons why the synapse is important

• ensures unidirectional transmission

• multiple stimuli can cause one response (spatial summation)

• one stimulus can cause multiple responses (neural divergence)

What are the two effects that a neurotransmitter can have on a synapse

excitatory OR inhibitory 

What is an excitatory neurotransmitters and name an example

neurotransmitter increases positive charge of post synaptic neurone - will trigger a new AP if threshold is reached 

e.g. ACh 

What is an inhibitory neurotransmitters and name an example 

neurotransmitter which increases negative charge of post synaptic neurone - prevents AP occurring 

e.g. GABA

What is the role of the presynaptic neurone

to release neurotransmitters into synaptic cleft 

What is the role of the post synaptic neuron

to receive neurotransmitter and generate new AP 

What is a synaptic vesicle

sacs in the presynaptic knob containing neurotransmitters  

What are the 6 steps in synaptic transmission

1. AP arrives at presynaptic knob causing membrane to become depolarised 

2. VG calcium ion (Ca2+) channels open and Ca2+ diffuses in down electrochemical gradient 

3. Ca2+ causes synaptic vesicles to move and fuse with presynaptic cell surface membrane releasing NT into synaptic cleft by exocytosis 

4. NT diffuse across to postsynaptic and bind to specific receptors on ligand-gated (Na+) channels causing them to open 

5. Na+ diffuses into postsynaptic neurone BIG Na+ INFLUX, down electrochemical gradient, causing it to depolarise

6. If depolarisation reaches threshold level an AP will be triggered in postsynaptic neurone 

what are ligand-gated Na+ channels

proteins with specific receptors on the surface that open when NT binds allowing Na+ to flow into cell to depolarise membrane

What is a cholinergic synapse

a synapse that uses acetylcholine (ACh) as its neurotransmitter 

Why do neurotransmitters like ACh need to be removed from synaptic cleft

to prevent continuous stimulation as it could lead to seizures

What are the 3 steps in which ACh is removed

1.enzyme acetylcholinesterase (AChE) breaks down acetylcholine into choline and acetate causing ligand-gated Na+ channels to close preventing new AP being generated

2.Acetate and choline are reabsorbed into presynaptic neurone via active transport 

3.Mitochondria in the presynaptic neurone releases ATP used to recombine acetate and choline back into ACh 

Why is this process known as recycling

because none of the products are lost, they are always recycled back into ACh

What is summation

the buildup of neurotransmitters in the synaptic cleft to increase likelihood of new AP being generated in postsynaptic neurone 

Name the 2 types of summation

spatial summation and temporal summation 

What is spatial summation and how does it operate 

multiple presynaptic neurones converging onto one postsynaptic neurone, combined input increases NT being released 

> hence higher change of new AP being triggered in postsynaptic neurone

What is temporal summation and how does it operate

multiple high frequency AP reaching one presynaptic neurone increases concentration of NT being released 

> hence higher change of new AP being triggered in postsynaptic neurone

what is neural divergence

when a single presynaptic diverges/ branches off to multiple postsynaptic neurones

» enables a single impulse to coordinate responses simultaneously in different parts of the body

e.g. reflex arc - withdraw reflex = allows effector to respond (withdraw a limb) and simultaneously send signals to the brain (to register pain)