Option E.4 – Neurotransmitters and Synapses
Action potentials are passed between neurons across the synapse using chemical transmission in the form of neurotransmitters. These are stored at the end of the axon.
When the action potential arrives, the ion channels open to allow Ca2+ to enter, activating the enzymes that act on presynaptic proteins. These proteins cause the release of the neurotransmitters from the vesicles and across the membrane of the nerve ending. Neurotransmitters are constantly recycled.
It then diffuses across the synaptic cleft and interacts with the receptors of the next neuron. Glial cells around the synaptic cleft contain transporters to remove any remaining neurotransmitters.
E.4.1 – State that some presynaptic neurons excite postsynaptic transmission and others inhibit postsynaptic transmission
Each neurotransmitter must bind to its matching receptor, which then stimulates the opening of the ion channels, exciting the neuron. If it reaches the threshold, an action potential will form and be passed along the post-synaptic neuron. When the process occurs this way, it is called an excitatory synapse.
On the other hand, there are also inhibitory synapses. Instead of triggering the release of Ca2+, the neurotransmitters cause the ion channels to open to allow the entry of Cl- ion or the exit of K+. The gradient changes in the opposite direction, making the overall charge of the neuron more negative or hyperpolarised. An action potential cannot be initiated.
An action potential may be formed from a number of impulses reaching the synapse in quick
succession, or from the combined effected of impulses from different axons.
E.4.2 – Explain how decision-making in the CNS can result from the interaction between the activities of excitatory and inhibitory presynaptic neurons at synapses
The brain coordinates and controls our body’s functions, as well as storing memories. It can also initiate activity, and allows us to perform abstract reasoning.
Each area of the brain performs a different function, and they are all connected. Using fMRI, these areas can be identified.
Integration is the process of data being taken into the centres and compiled with the existing data, or memories. This is then used in decision making. Each neuron has many synaptic knobs, forming different types of connections.
Decision making is also dependent on the interaction between excitatory and inhibitory synapses and the different connection pathways between neurons to produce a different result.
Divergent connections are when information from one pathway branches out and is passed to a number of others, forming the basis of a variety of responses.
Convergent connections are when information from multiple pathways focuses onto fewer pathways. This leads to stronger excitation or inhibition, and can trigger a response to multiple stimuli.
Circular or Reverbatory connections are when the information returns to its source to reinforce a message or make it last longer
Parallel or After-Discharge connections are when the post synaptic neuron sends out a number of impulses without any feedback, resulting in a precise, strong response.
Psychoactive drugs affect the mind by altering the performance of synapses. Some drugs amplify their processes to increase post-synaptic transmission, such as nicotine and atropine. Some inhibit the process of synaptic transmission to decrease transmission, such as amphetamines and beta blocker drugs.
Synaptic transmission is significant in brain function. The brain in integral for the coordination of body functions, storing information, maintaining our memory bank, imagination, create, plan, calculate, predict and for abstract reasoning, but excluding reflexes.
Psychoactive drugs affect behaviour and possibly personality. Many of these drugs are used in medicine as tranquilisers and painkillers to make the brain neurons resistant to excitation. They are also used in horticulture as insecticides, to inhibit enzymic breakdown of transmitter substances after attachment to the post-synaptic membrane. It disrupts the nervous system to the point of death. Nerve gases can be used as weapons.
These are also used in a social and recreational context, but often have dangerous or tragic consequences.
E.4.4 – List three examples of excitatory and three examples of inhibitory psychoactive drugs
E.4.5 – Explain the effects of THC and cocaine in terms of their action at synapses in the brain
This is also called marijuana, pot, grass or weed and is usually smoked. The chief mind altering chemical in it is THC (delta-9-tetrahydrocannabinol), which is typically present in concentration of 1-4%.
It is a mild hallucinogen, showing similar disinhibiting properties to ethanol. It induces a sense of well-being and a dreamy state of relaxation. Also:
- Encourages fantasies
- User becomes suggestible and vulnerable
- Possible paranoia
Emotions peak after 10-30 minutes, and wear off after 2-3 hours.
Affects ability to absorb and retain information, maturity, leads to inappropriate lifestyle choices, gateway to cocaine. Also increases risk of car crashes, STI’s and unwanted pregnancy.
It is a highly addictive stimulant, and is usually ingested, snorted or injected. Crack cocaine is heated to evaporate the water and inhaled from a heated pipe for faster absorption. it affects neurons containing dopamine, which affects pleasure. It has euphoric effects, interfering the reabsorption of dopamine and prolonging stimulation for extended pleasure response.
It induces euphoria and hyperstimulation, reducing fatigue and increasing mental clarity. It causes intense pleasure beyond the normal range of human experience. Faster absorption leads to more intense sensations.
Use of cocaine is then followed by a crash, where the user become restless, irritable and anxious. Some people will binge, and suffer auditory hallucinations, paranoid psychosis and lose contact with reality
Users can quickly become addicted, and the cravings lead to repetitive and compulsive behaviours. It gives sensations of insects crawling under skin, severe depression, agitated delirium and paranoid psychosis. It has destructive social consequences, causing the user’s family to become alienated, with the user becoming isolated and suspicious. They will spend all their time and money obtaining more.
- Spend time and money obtaining more
- Often resort to crime
- Put aside loved ones
E.4.6 – Discuss the causes of addiction, including genetic predisposition, social factors and dopamine secretion
Addiction – State of taking a mood-altering drug habitually, and being unable to give it up without experience unpleasant side-effects. The user is unable to control or abandon their drug use.
Drug addiction is characterised by a pathological desire for drugs. It occupies their thoughts and they can only think of obtaining more. They will spend time seeking drugs, with use occurring at the expense of other activities such as study, work and social activity. They cannot control the frequency of use or stop altogether, even if they want to.
The initial motivation for taking the drug affects the likelihood of addiction. If it is taken for medical purposes, then it rarely leads to addiction. However, if it is taken for pleasure, then they are more likely to get addicted.
Some people are predisposed to addiction because their brain is changed in complex ways. This specifically happens in the regions including the reward system and areas involved in executive functions and judgement. Their decision-making ability is also affected.
Other personality types are more likely to become addicted, including those who are inclined to risk-taking or hedonistic.
They might also have a metabolic state that makes drugs more effective, such as the absence of an enzyme to dispose of the drug. These conditions may be inherited.
Environmental factors, such as stress, affect people’s response to the drug. Poor diet, high unemployment and limited access to education and training are also contributing factors to susceptibility. Little opportunity for personal fulfilment leads to a sense of hopelessness, so that the drugs become an escape.
Drug abuse can eventually alter the structure and chemical makeup of the brain. Since they affect the reward system, they can produce please, or remove stress and pain. This area is involved in learning, natural rewards, and causes the body to expect more or repeat the action.
Some drugs mimic dopamine, causing them to want to repeat use. However, they will eventually become less sensitive to or more tolerant of it, and they will need higher doses to get the same effect. Eventually, they may become dependent and feel like they can’t function without it. If they try to stop use, they will suffer from withdrawal symptoms.
Drugs interfere with the dopamine metabolism to produce a state of dependence. This often starts with gateway drugs, then later experimentation with more harmful ones, as more is required to produce the same effect. This drug use is habit-forming and users feel that they cannot live without it.