13.5 Synapses

What needs to happen for the impulse to reach the CNS or an effector?

the impulse often needs to be passed between several neurones

Synapse?

The junction between two neurones (or a neurone and effector)

How are impulses transmitted across the synapse?

using chemicals called neurotransmitters

Synapse structure: 1?

myelin sheath

Synapse structure: 2?

axon of presynaptic neurone

Synapse structure: presynaptic neurone?

neurone along which the impulse has arrived

Synapse structure: 3?

membrane of presynaptic neurone

Synapse structure: 4?

smooth endoplasmic reticulum

Synapse structure: 5?

Ca²⁺

Synapse structure: 6?

calcium ion channel

Synapse structure: 7?

mitochondrion

Synapse structure: synaptic knob?

the swollen end of the presynaptic neurone. It contains many mitochondria and large amounts of endoplasmic reticulum to enable it to manufacture neurotransmitters (in most cases)

Synapse structure: 8?

synaptic vesicle (containing acetylcholin)

Synaptic vesicles?

vesicles containing neurotransmitters - The vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft

Synapse structure: 9?

membrane of postsynaptic neurone

Synapse structure: postsynaptic neurone?

neurone that receives the neurotransmitter

Synapse structure: 10?

sodium ion channels

Synapse structure: 11?

synaptic cleft

Synapse structure: synaptic cleft?

the gap which separates the axon of one neurone from the dendrite of the next neurone

Synapse structure: size of synaptic cleft?

It is approximately 20-30 nm across

Synapse structure: 12?

neurotransmitters (acetylcholine molecule)

Synapse structure: 13?

empty receptor site

Synapse structure: Neurotransmitter receptors?

receptor molecules which the neurotransmitter binds to in the postsynaptic membrane

Synapse structure: 14?

One of five protein sub-units that make up the sodium ion channel

Synapse structure: 15?

acetylcholine molecule in receptor site

Why is this synapse known as a cholinergic synapse?

the neurotransmitter which passes across the synapse is acetylcholine

The two types of neurotransmitter?

Excitatory and inhibitory

Excitatory neurotransmitters: what do they do?

These neurotransmitters excite the post-synaptic neurone

Excitatory neurotransmitters: example?

Acetylcholine

Inhibitory neurotransmitters: what do they result in?

hyperpolarisation in the post-synaptic neurone

Inhibitory neurotransmitters: what does hyperpolarisation in the post-synaptic neurone prevent?

an action potential being triggered

Inhibitory neurotransmitters: example (that can be found in some synapses in the brain)?

Gamma-aminobutyric acid (GABA)

Transmission across the synapse​ - 1: where does the action potential reach?

the end of the presynaptic neurone

Transmission across the synapse​ - 2: what does Depolarisation of the presynaptic membrane cause?

calcium ion channels open

Transmission across the synapse​ - 3: where do calcium ions diffuse?

into the presynaptic knob

Transmission across the synapse​ - 4: what does this cause synaptic vesicles containing neurotransmitters to do?

to fuse with the presynaptic membrane

Transmission across the synapse - 4.2​: how is the Neurotransmitter released into the synaptic cleft?

by exocytosis

Transmission across the synapse​ - 5: what happens to the neurotransmitter?

diffuses across the synaptic cleft and binds with its specific receptor molecule on the postsynaptic membrane

Transmission across the synapse​ - 6: what does this cause to open?

sodium ion channels

Transmission across the synapse​ - 7: what does this trigger in the post-synaptic neurone?

(Sodium ions diffuse into the postsynaptic neurone) which triggers an action potential and the impulse is propagated along the postsynaptic neurone

Transmission across the synapse​ - 8: what do enzymes break down?

any leftover acetyl choline, and also remove it from the receptor molecules

Once a neurotransmitter has triggered an action potential in the postsynaptic neurone, why is it important that it is removed?

so the stimulus is not maintained, and so another stimulus can arrive at and affect the synapse

Therefore, what do enzymes do to any neurotransmitter left in the synaptic cleft (e.g. Acetycholine)?

breaks it down and removes it

Where are the products taken after being released from the receptors on the postsynaptic membrane?

taken back into the presynaptic knob

Why is Removing the neurotransmitter from the synaptic cleft important (2)?

prevents the response from happening again and allows the neurotransmitter to be recycled

What neurotransmitter do Cholinergic synapses use?

the neurotransmitter acetyl choline

Where are Cholinergic synapses common?

in the CNS of vertebrates and at neuromuscular junctions

Neuromuscular junctions?

where a motor neurone and a muscle cell (an effector) meet.

What does Acetyl Choline trigger (If the neurotransmitter reaches the receptors on a muscle cell)?

it will cause the muscle to contract

What is Acetylcholine released from?

the vesicles in the presynaptic knob

Acetylcholine then diffuses across the synaptic cleft where it binds with specific receptors in the postsynaptic membrane - what does this trigger?

an action potential in the postsynaptic neurone or muscle cell

Once an action potential has been triggered what happens to acetylcholine?

it is hydrolysed

What is acetyl choline hydrolysed into?

choline + ethanoic acid

What hydrolyses acetyl choline?

a specific enzyme - acetylcholinesterase

Where is acetylcholinesterase situated?

on the postsynaptic membrane

How many molecules of acetylcholine can acetylcholinesterase break down per minute?

around 25 000 molecules

Where are the breakdown products taken?

back into the presynaptic knob to be reformed into acetylcholine (and the postsynaptic membrane is ready to receive another impulse)

Cholinergic synapse transmission - 1: where does an action potential arrive?

at the end Of the presynaptic neurone

Cholinergic synapse transmission - 1.2: what does this arrival of an action potential cause to open?

calcium ion channels to and calcium ions (Ca²⁺) enter the synaptic knob

Cholinergic synapse transmission - 2: what is released into the synaptic cleft (due to the influx of calcium ions into the presynaptic neurone causing synaptic vesicles to fuse with the presynaptic membrane)?

causes the release of the neurotransmitter acetylcholine

Cholinergic synapse transmission - 3.1: what do Acetylcholine molecules fuse with?

receptor sites on the sodium ion channel in the membrane of the postsynaptic neurone

Cholinergic synapse transmission - 3.2: what does acetylcholine molecules fusing with receptor sites on the sodium ion channel in the membrane of the postsynaptic neurone cause?

the sodium ion channels to allowing sodium ions (Na⁺) to diffuse in rapidly along a concentration gradient

Cholinergic synapse transmission - 4: what does this influx of sodium ions initiate?

an action potential in the postsynaptic neurone

Cholinergic synapse transmission - 5: what does Acetylcholinesterase hydrolyses acetylcholine into?

choline and ethanoic acid (acetyl)

Cholinergic synapse transmission - 5.2: how is the choline and ethanoic recycled?

diffuse back across the synaptic cleft into the presynaptic neurone

Cholinergic synapse transmission - 5.3: what does the breakdown of acetylcholine also prevent?

prevents it from continuously generating a new action potential in the postsynaptic neurone

Cholinergic synapse transmission - 6: how are choline and ethanoic acid recombined into acetycholine?

ATP released by mitochondria is used

Cholinergic synapse transmission - 6.2: where is this acetylcholine stored for future use?

in synaptic vesicles

Cholinergic synapse transmission - 6.3: why do sodium ion channels close?

in the absence Of acetylcholine in the receptor sites

A specialized cholinergic synapse?

A neuromuscular junction - the postsynaptic cell is a skeletal muscle fibre instead of another neurone

Transmission across a neuromuscular junction - 1: where does an action potential arrive?

at the end of a motor neurone, at the neuromuscular junction

Transmission across a neuromuscular junction - 2: what does this action potential cause the release of?

the neurotransmitter acetylcholine

Transmission across a neuromuscular junction - 3: what does the release of the neurotransmitter acetylcholine initiate?

an action potential in the muscle cell membrane (Sarcolemma)

Transmission across a neuromuscular junction - 4: what is this action potential carried quickly into the large muscle cell by?

invaginations in the cell membrane called T-tubules

Transmission across a neuromuscular junction - 5: what does the action potential cause to release its store of calcium ions into the myofibrils?

the sarcoplasmic reticulum

Transmission across a neuromuscular junction - 6: what does Ca²⁺ cause to be displaced?

causes tropomoysin to be displaced

Transmission across a neuromuscular junction - 7: what does tropomoysin being displaced uncover?

myosin binding sites on actin

Transmission across a neuromuscular junction - 8: what can now attach and take place?

Myosin cross bridges can now attach and the cross bridge cycle can take place

Transmission across a neuromuscular junction: what is the reverse of these steps?

relaxation

Role of synapses: what do they ensure about impulses?

that they are unidirectional

Role of synapses: why is it good they ensure impulses are unidirectional?

the neurotransmitter receptors are only present on the postsynaptic membrane, so impulses can only travel from the presynaptic neurone to the postsynaptic neurone

Role of synapses: what can they allow?

an impulse from one neurone to be transmitted to a number of neurones at multiple synapses

Role of synapses: what does allowing an impulse from one neurone to be transmitted to a number of neurones at multiple synapses result in?

a single stimulus creating a number of simultaneous responses

Role of synapses: what may a number of neurones feed into?

the same synapse with a single postsynaptic neurone

Role of synapses: what does a number of neurones feeding in to the same synapse with a single postsynaptic neurone result in?

stimuli from different receptors interacting to produce a single result

Summation?

if the amount of neurotransmitter builds up sufficiently to reach the threshold then this will trigger an action potential

Two types of summation?

Spatial summation and Temporal summation

Spatial summation: when does it occur?

when a number of presynaptic neurones Connect to one postsynaptic neurone

Spatial summation: how does it occur?

each presynaptic neurone releases neurotransmitters which builds up to a high enough level in the synapse to trigger an action potential in the single postsynaptic neurone

Temporal summation: when does it occur?

when a single presynaptic neurone releases neurotransmitter as a result of an action potential several times over a short period

Temporal summation: how does it happen?

the neurotransmitters build up in the synapse until the quantity is sufficient to trigger an action potential