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Synapses and Drugs The Importance Of Synapses ► In order to understand how drugs work on the brain, we must first have some understanding of how the brain is constructed. The basic unit: Neurons ► The brain is a very complicated collection of cells known as neurons or (more informally) nerves. ► Neurons receive information through branchlike structures called dendrites. As neurons grow, their dendrites reach out and make contact with the axons of adjacent neurons. ► There is no physical contact between the axon terminal of one neuron, and the dendrites of another. Rather, between the axon and the dendrites is a space or gap, which is called the 'synapse'. *Review the Synapse ► What is a synapse? ► A synapse is the “gap” between the axon of one nerve and the dendrite of the next one. ► The average neuron has 1,000 synapses with other neurons. Synaptic Transmission ► When the action potential signal reaches the terminal, the vesicles containing neurotransmitters (NT) moves to the end and the NT are released into the synapse. ► The NT floats across the synapse and connects in lock-and-key fashion with protein 'receptors' embedded in the dendrites of the receiving neurons. ► It is the presence of the NT 'keys' opening the receptor 'locks' on the surface of the dendrites of the post-synaptic neurons (and not any electrical signal that jumps the synapse) that excites or inhibits the postsynaptic neurons into activating or not. Figure 48.12 A chemical synapse *In more detail… 1. 2. 3. Impulse from the action potential opens ion channels for Ca++ The increased Ca++ concentration in the axon terminal initiates the release of the neurotransmitter (NT) NT is released from its vesicle and crosses the “gap” or synaptic cleft and attaches to a protein receptor on the dendrite (continued) 4. 5. Interaction of NT and protein receptor open post-synaptic membrane ion channel for Na+ After transmission the NT is either degraded by an enzyme or taken back into the pre-synaptic membrane by a transporter or reuptake pump “Re-uptake” ► After a short while in the synapse, the neurotransmitters that have been released are recalled back into the axon terminal in a process called 'reuptake' so that they are available should the neuron need to fire again. The Neurotransmitters ► There are many different chemicals in the brain that function as neurotransmitters, but a small handful do most of the work. Acetylcholine Serotonin EPI Endorphins GABA Neurotransmitters ► There are dozens of different neurotransmitters (NT) in the neurons of the body. ► NTs can be either excitatory, which causes the next neuron to fire, or inhibitory, which stops the next neuron from firing. ► Each neuron generally synthesizes and releases a single type of neurotransmitter Major Neurotransmitters in the Body ► GABA – Inhibitory - helps induce relaxation and sleep. It balances the brain by inhibiting over-excitation. (Marijuana, depressant drugs) ► Glutamate – Excitatory -is a major excitatory neurotransmitter that is associated with learning and memory. ► Dopamine - Excitatory - involved in controlling movement and posture. It also modulates mood and plays a central role in positive reinforcement and dependency. (cocain, PCP, Heroin, opiates, marijuana, amphetamines). ► Serotonin –Excitatory - contributes to various functions, such as regulating body temperature, sleep, mood, appetite, and pain. Low serotonin levels have been linked to depression, suicide, impulsive behavior, and aggressiveness. (alcohol, hallucinogens, marijuana, stimulants). ► Endorphins – Excitatory and inhibitory - interact with the opiate receptors in the brain to reduce our perception of pain. Secretion of endorphins leads to feelings of euphoria, modulation of appetite, release of sex hormones, and enhancement of the immune response. (runner’s high) (opiates, depressants) Drugs Interfere with Neurotransmission ► Drugs can affect synapses at a variety of sites and in a variety of ways (agonize or antagonize), including: 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 ►Most of the drugs that get abused are agonists (enhancers) of various neurotransmitters - they work to enhance the natural effect of neurotransmitters. Three Drugs (of many) which affect Neurotransmission Methamphetamine Nicotine seattlepi.nwsource.com/ methamphetamines/ Alcohol science.howstuffworks.com/ alcohol.htm