E.4.1. Some Pre-synaptic neurons excite postsynaptic transmission and others inhibit postsynaptic transmission
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. • Nerve signals are processed and integrated through the adding together of small changes in membrane potential caused by the binding of neurotransmitters to the receptors. Neurone
Post-Synaptic Potentials (PSPs) can be Excitatory (EPSP)(+) or Inhibitory (IPSP) (-) - + Neurone - +
The EPSP must build up in the postsynaptic neuron to the threshold level to allow the formation of an action potential. - + Neurone + + For example this Neuron needs a 2 more “+” than “-” before it will fire.
Neurons can integrate signals from other neurons 1) through spatial summation (a number synaptic inputs combine to cause a depolarization event). - + Neurone + +
2) through temporal summation where a number of Excitory Post Synaptic Potentials occur quickly without enough recovery time between the Excitory Post Synaptic Potentials to cause a depolarization event. - + + + + + + Neurone - +
Neurons can integrate both excitatory and inhibitory signals from other neurons. • Thus the adding together of the inputs from synapses leads to whether or not an action potential is formed in the postsynaptic neuron.
E.4.3 Explain how psychoactive drugs affect the brain and personality by either increasing or decreasing postsynaptic transmission.
E.4.4 List three examples of excitatory and three examples of inhibitory psychoactive drugs. • Excitatory drugs: • nicotine, • cocaine • amphetamines • Inhibitory drugs: • benzodiazepines, • alcohol • tetrahydrocannabinol (THC).
E.4.5 Explain the effects of THC and Cocaine in terms of their action at synapses in the brain. Marijuana is the buds and leaves of the Cannabis sativa plant. This plant contains more than 400 chemicals, including tetrahydrocannabinol (THC), the plant's main psychoactive chemical.
THC is known to affect our brain’s short-term memory. Additionally, marijuana affects motor coordination, increases your heart rate and raises levels of anxiety. Studies also show that marijuana contains cancer-causing chemicals typically associated with cigarettes. An intravenous dose of only 1 milligram can produce serious mental and psychological effects. Once in your bloodstream, THC typically reaches the brain within seconds after it is inhaled.
Neurotransmitters fill the gap, or synapse, between two neurons and bind to protein receptors, which enable various functions and allow the brain and body to be turned on and off. • Some neurons have thousands of receptors that are specific to particular neurotransmitters. • Foreign chemicals, like THC, can mimic or block actions of neurotransmitters and interfere with normal functions.
In your brain, there are groups of cannabinoid receptors concentrated in several different places. These cannabinoid receptors have an effect on several mental and physical activities, including: • Short-term memory • Coordination • Learning • Problem solving Cannabinoid receptors are activated by a neurotransmitter called anandamide. Anandamide belongs to a group of chemicals called cannabinoids. THC is also a cannabinoid chemical. THC mimics the actions of anandamide, meaning that THC binds with cannabinoid receptors and activates neurons, which causes adverse effects on the mind and body.
High concentrations of cannabinoid receptors exist in the hippocampus, cerebellum and basal ganglia. • The hippocampus is located within the temporal lobe and is important for short-term memory. • When the THC binds with the cannabinoid receptors inside the hippocampus, it interferes with the recollection of recent events. • THC also affects coordination, which is controlled by the cerebellum. • The basal ganglia controls unconscious muscle movements, which is another reason why motor coordination is impaired when under the influence of marijuana
Cocaine (Crack) Cocaine acts upon a part of the brain called the ventral tegmental area (VTA). It interferes with a chemical messenger in the brain called dopamine, which is involved in the body's pleasure response. Dopamine is released by cells of the nervous system during pleasurable activities such as eating or having sex. Once released, dopamine travels across a synapse, and binds to a receptor on a post-synaptic neurone.
This sends a signal to that nerve cell, which produces a good feeling. Under normal conditions, once the dopamine sends that signal it is reabsorbed by the neuron that released it. This reabsorption happens with the help of a protein called the dopamine transporter. Crack interrupts this cycle. It attaches to the dopamine transporter, preventing the normal reabsorption process. As dopamine builds up in the synapse, it continues to stimulate the receptor, creating a lingering feeling of exhilaration or euphoria in the user.
Addictive drugs chemically alter a part of the brain called the reward system. Using Cocaine as an example of addiction: The drug traps the chemical dopamine in the spaces between nerve cells. Dopamine creates the feelings of pleasure we get from enjoyable activities such as eating and having sex. But in cocaine users, dopamine keeps stimulating those cells, creating a "high" -- a euphoric feeling that lasts anywhere from five to 15 minutes. But then the drug begins to wear off, leaving the person feeling let-down and depressed, resulting in a desire to smoke more crack in order to feel good again. The brain responds to the dopamine overload of the crack high by either destroying some of it, making less of it or shutting down its receptors. The result is that, after taking the drug for a while, crack users become less sensitive to it and find that they must take more and more of it to achieve the desired effect. Eventually, they cannot stop taking the drug because their brains have been "rewired" -- they actually need it in order to function.
Sources • Slide 2 http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-13/1322.jpg • Slides 11 to 18 adapted from: • http://health.howstuffworks.com/marijuana.htm • http://health.howstuffworks.com/crack3.htm