Synaptic Transmission (DP IB Biology)

Effects of Exogenous Chemicals

Neonicotinoids

• Neonicotinoids are synthetic compounds similar to nicotine that are commonly found in pesticides

• Neonicotinoids can block synaptic transmission at cholinergic synapses in insects by binding to acetylcholine receptors

  • This binding is irreversible, as acetylcholinesterase cannot break down neonicotinoids

  • As the acetylcholine receptors are blocked, acetylcholine is unable to bind, which stops impulses from being transmitted across synapses

  • This leads to paralysis and death in insects

• Neonicotinoids are considered to be especially suitable as pesticides because they're not toxic to humans and other mammals

  • A much larger proportion of synapses in insects are cholinergic compared to mammals

  • Neonicotinoids bind much more strongly to acetylcholine receptors in insects

• There is a great deal of controversy over the use of neonicotinoid pesticides because of the impact that they are thought to have on essential pollinators such as bees

Cocaine

• Cocaine is a drug which blocks the reuptake of neurotransmitters into the presynaptic knob

• Primarily cocaine affects reuptake of dopamine as it binds to the dopamine transporter protein

• This prevents dopamine from binding to the transporter so it is not able to move through the membrane back into the presynaptic neurone

• As a result dopamine builds up in the synapses which can lead to feelings of pleasure

• Cocaine also blocks the neurotransmitters serotonin and norepinephrine which enhances feelings of confidence and energy

• In regular users of cocaine, the brain responds by increases numbers of dopamine receptors to respond to the high levels of dopamine

  • Once levels return to normal, more dopamine receptors results in increased sensitivity and depression

Inhibitory Postsynaptic Potentials

• Some neurotransmitters result in the generation of an action potential in a postsynaptic neurone

• Other neurotransmitters can prevent the generation of an action potential in a postsynaptic neurone

  • This is inhibition - the impulse stops at the synapse

• One way in which a neurotransmitter can inhibit an impulse is by opening the gated potassium ion channels in the membrane so that potassium ions are able to diffuse out of the cell body

  • The result is that the postsynaptic neurone becomes even more negatively charged, or hyperpolarised

  • If the neurone is hyperpolarised, the threshold will not be reached when the neurone is stimulated and an action potential cannot be triggered

• If the cell body of a motor neurone is subject to both excitatory and inhibitory synapses at the same time the following happens:

  • Sodium ions enter the cell body following stimulation by the excitatory synapse

  • The stimulation of the inhibitory synapse causes potassium ions to diffuse out of the cell body

  • This cancels out the effect of the sodium ions entering

  • The threshold potential is not reached so no action potential is generated

Inhibitory Synapse Diagram

The inhibitory synapse (Y) causes the membrane potential to decrease, cancelling out the effect of the excitatory synapse (X) so that the threshold is not reached and no action potential is generated

• Inhibitory synapses play a vital role in the nervous circuit

• They prevent random impulses from being sent around the body

• They allow for specific pathways to be stimulated

  • For example, reflex actions should be rapid but specific

  • If an individual grabs a plank of wood that has a nail sticking out they need their arm muscles to pull their hand away

  • It would be unhelpful if their leg muscles contracted and moved their foot away

• Inhibitory pathways can develop over time

• These pathways are very important for skills such as painting and drawing

  • Children initially struggle with these skills as their inhibitory pathways have not yet developed to refine their uncontrolled movements

Summation of Neurotransmitter Effects in Postsynaptic Neuron

• When an impulse arrives at a synapse it does not always cause impulses to be generated in the next neurone

• In some cases, a single impulse that arrives at a synaptic knob is insufficient to generate an action potential in the post-synaptic neurone, for instance

  • Only a small amount of acetylcholine is released into the synaptic cleft

  • A small number of the gated ion channels are opened in the axon membrane

  • An insufficient number of sodium ions pass through the membrane

  • The threshold potential is not reached

  • The small amount of acetylcholine attached to receptors is broken down rapidly by acetylcholinesterase

• The effect of multiple impulses can be added together to overcome this in a process known as summation

• There are two types of summation:

  • Temporal

  • Spatial

• There are several benefits of summation

  • It allows for the effect of a stimulus to be magnified

  • A combination of different stimuli can trigger a response

  • It avoids the nervous system being overwhelmed by impulses

    • Synapses act as a barrier and slow down the rate of transmission of a nerve impulse that has to travel along two or more neurones

    • They only allow the impulses to pass on if there has been input from other neurones and receptors

Temporal summation

• If multiple impulses arrive within quick succession the effect of the impulses can be added together to generate an action potential

  • A large amount of acetylcholine is released into the synaptic cleft

  • A large number of the gated ion channels open

  • A sufficient number of sodium ions pass through the membrane

Spatial summation

• Multiple impulses arriving simultaneously at different synaptic knobs stimulating the same cell body can also generate an action potential through spatial summation

• The multiple impulses result in a large amount of acetylcholine being released into the synaptic cleft which results in the generation of an action potential

Temporal and Spatial Summation Diagram

Temporal summation involves only one synaptic knob whereas spatial summation involves multiple synaptic knobs. The different types of summation produce different shaped graphs.