synaptic transmission

synapses

• a point when one neurone communicates with another / an effector

• transmit info in the form of neurotransmitters

neurotransmitters

- chemical messengers that allow impulses to be sent from one neurone to the next
- pass along a synaptic cleft (gap between neurones) from the pre-synaptic neurone
- a new AP will be triggered in the post-synaptic neurone (if the neurotransmitter is excitatory)
- can either be excitatory or inhibitory
- electrical → chemical → electrical

examples of neurotransmitters

• norepinephrine (excitatory, fight or flight)

• dopamine (reward mechanisms)

• serotonin (inhibitory, emotion/mood/sleep)

• acetylcholine (excitatory, muscle contraction)

cholinergic synapse

• neurotransmitter = acetylcholine

• found in CNS and at neuromuscular junctions

• excitatory at the neuromuscular junction in skeletal muscle, causing the muscle to contract

• common vertebrates

transmission across a cholinergic synapse

1. an AP arriving at the end of presynaptic neurone causes Ca2+ to diffuse in

2. synaptic vesicles containing acetylcholine fuse with pre-synaptic membrane and release acetyl choline into the synaptic cleft by exocytosis

3. acetyl choline diffuses to the post synaptic neurone and binds to its receptors opening these sodium ion channels

4. sodium ions diffuse into postsynaptic neurone causing depolarisation and if threshold is exceeded, AP generated

5. acetyl choline is broken down by acetylcholinesterase into choline and ethanoic acid

6. products diffuse back to pre-synaptic neurone and are reabsorbed

7. they are reformed into acetylcholine using the energy from ATP hydrolysis

synapse features

1. unidirectionality

2. summation

3. inhibition

unidirectionality

APs can only pass across a synapse from a pre- to post-synaptic neurone
- neurotransmitter only released from the end of the pre-synaptic neurone
- receptors only located on post-synaptic neurone membrane

summation

- low frequency AP can lead to insufficient neurotransmitter release, and therefore no AP in the postsynaptic neurone
- threshold more likely to be reached in the postsynaptic neurone, this enables an AP to be generated and neurone depolarises
- spatial summation + temporal summation

spatial summation

neurotransmitter accumulates due to the convergence of two or more neurones, this makes it likely that threshold can be exceeded in the post synaptic neurone

temporal summation

neurotransmitter accumulates due to several APs arriving in quick succession from one neurone, this makes it likely that threshold can be exceeded in the post synaptic neurone

convergence

• nerve impulses from different receptors reacting to different stimuli

• can contribute to a single response

divergence

one creates a number of different simultaneous responses

inhibition

some synapses make it less likely that a new AP will be made on the postsynaptic neurone

transmission across synapse (?)

1. presynaptic neurone releases neurotransmitter that binds to chloride ion channel on the postsynaptic membrane → chloride ion channels open, Cl- diffuses into postsynaptic neurone
2. binding of neurotransmitter causes opening of nearby K+ channels
3. K+ diffuse out of postsynaptic neurone into synapse → inside of postsynaptic neurone becomes more negative
4. membrane potential decreases to -80mV (hyperpolarisaiton) → this makes it less likely that a new AP will be created, as a larger influx of Na+ is needed to produce one

effects of drugs on synapses

drugs interfere with neurotransmission:

1. increasing number of impulses

2. release NT from vesicles with or without impulses

3. block reuptake or block receptors

4. produce more or less NT

5. prevent vesicles from releasing NT

drugs that affect the autonomic nervous system

• agonists

• antagonists

agonists

substances that stimulate receptors

antagonists

substances that block a receptor

agonist - drugs stimulate nervous system

create more APs in postsynaptic neurone
- can stimulate release of more neurotransmitter
- or inhibits the enzyme that breaks down the neurotransmitter

examples of agonists

nicotine, cocaine, amphetamine, prozac

prozac

• serotonin is a NT involved in sleep regulation and causes emotional states

• reduced activity of neurones releasing serotonin is thought to be a cause of clinical depression

• prozac is an anti-depressant

drugs inhibit nervous system

create fewer APs in the postsynaptic neurones
- inhibits the release of the neurotransmitter
- or blocks receptors on the ion channels on the postsynaptic membrane

examples of drugs that inhibit the nervous system

snake venom (blocks acetylcholine receptors), THC, alcohol

inhibitory synapse

- inhibitory neurotransmitter binding causes: Cl- enter and K+ leave
- causes hyperpolarisation of neurone less likely that a new AP will be created because it will need more Na+ entering to depolarise the neurone

hyperpolarisation

becomes more negative

acetyl choline

acts as an excitatory neurotransmitter at synapses and at neuromuscular junctions

neuromuscular junctions

connecting motor neurones with skeletal muscles

excitatory synapse - e.g. cholinergic synapses

• acetylcholine binds to postsynaptic membranes and stimulates the production of nerve impulses

• depolarising the neurone

• opens Na+ ion channels

inhibitory synapse - GABA is another neurotransmitter

• binds to postsynaptic membranes and causes hyperpolarisation of neurone (it becomes more negative)

• need more Na+ entering, so neurone less likely to depolarise