6.2 Applications and skills
6.2.5 The synapse
- The junction between two neurons, as well as at the junction between a neuron and a sensor/effector, is called a synapse.
- The functional unit of a synapse between two neurons consists of an axon terminal (of the presynaptic neuron), a synaptic cleft, and a dendrite (of the postsynaptic neuron).
- When the axon terminal of a presynaptic neuron is depolarized, calcium ions, Ca2+, diffuse through the membrane into the cell.
- Calcium signals vesicles of neurotransmitters to fuse with the membrane.
- Molecules of neurotransmitters are released into the gap between two neurons. At the postsynaptic neuron, they bind to protein receptors.
- Binding of neurotransmitters changes the permeability of the postsynaptic cell to sodium ions. Sodium ions from the extracellular fluid rush in to depolarize the postsynaptic neuron.
- The synapse is a chemical signal as opposed to an electrical impulse. Click Figure 6.2.5a to review how the synapse works.
Figure 6.2.5a – The synapse
Neurotransmitters are released at the synapse when presynaptic neurons are depolarized.
Application: Secretion and reabsorption of acetylcholine
- Acetylcholine, ACh, is a neurotransmitter that operates at neuromuscular junctions and other cholinergic synapses.
- Figure 6.2.5b shows a neuromuscular synapse. Describe what is happening at each step.
Figure 6.2.5b – A cholinergic neuromuscular synapse
- Acetylcholine is bound to receptors at the postsynaptic cell for a short time. It is quickly broken down into acetic acid and choline by an enzyme called acetylcholinesterase, AChE.
- AChE prevents excessive firing of nerves by ensuring that only one action potential is propagated through the synapse.
- Choline molecules are reabsorbed into the presynaptic neuron and are used to form new molecules of ACh.
Figure 6.2.5c – Reabsorption
Acetylcholine is rapidly broken down by acetylcholinesterase.
Application: Neonicitinoid pesticides block acetylcholine receptors
- Neonicitinoids are organic pesticides that bind permanently to some types of ACh receptors, blocking the attachment of the neurotransmitter at the postsynaptic cell.
- Once the pesticide is bound, sodium channels open and remain open. This results in constant firing of the synapse and eventually paralysis and death to the affected insect.
- Neonicitinoids are less toxic to birds and mammals than older pesticides such as DDT and organophosphates. They were developed during the 1990s and are the most widely used class of pesticides today.
- There is growing evidence that neonicitinoids are at least partially responsible for declining honeybee and bird populations. The future of these pesticides remains in question.
Skill: Analysing oscilloscope traces showing resting and action potentials
Figure 6.2.5d shows the typical shape of an oscilloscope trace during a nervous impulse.
Figure 6.2.5d – Oscilloscope trace
- State the values of the resting potential, threshold potential, action potential and the potential when the neuron is hyperpolarized.
- State the duration, in ms, of the action potential and refractory period.
- Describe the action of different membrane proteins at each of the labelled stages.
- Explain why the neuron is hyperpolarized after an action potential.
- ‘Cholinergic’ (adjective) refers to any process that uses or produces acetylcholine (noun), e.g. cholinergic receptor, cholinergic synapse.
- ‘Acetate’ is another word for acetic acid.
- ‘Neonicitinoids’ are so named because they are molecules that chemically resemble nicotine. An example is shown in the following diagram:
Science and Science Responsibility (Aim 8)
- Many psychoactive drugs affect the functioning of a neurotransmitter called dopamine. Discuss some of the social effects of drug abuse.
- Botox is a neurotoxin that inhibits the release of acetylcholine at neuromuscular junctions, effectively paralysing muscles. Should there be limits on its use for cosmetic purposes?
How do different governing bodies deal with ambiguous evidence? In 2013, the European Union announced a two-year ban on neonicitinoids. This was to allow for evaluation of scientific evidence related to honeybee populations and other environmental effects. A similar proposal was made in the United States in 2013, but it did not leave Congress.