BIOLOGY AND CRIMINAL BEHAVIOR Part II

What’s a Neuron? A Neuron is a specialized nerve cell that receives, processes, and transmits information to other cells in the body. While for many years it was believed that we have a fixed number of neurons, new research has shown that adults do grow new neurons, particularly in the hippocampus. About 10,000 neurons die everyday, but since we start out with between ten and 100 billion (Hooper & Teresi, 1987), we only lose about 2% over our lifetime.

What are Neurotransmitters? Neurotransmitters are chemicals that transmit signals across synapses (specialized junctions) to target neurons. They are easily synthesized from amino acids and other readily- available precursors that are found in the human diet.

What do they do? Neurotransmitters influence the actions of neurons by acting as chemical messengers. Neurotransmitters are released from one neuron at the presynaptic nerve terminal, and then they cross the synapse where they may be accepted by the next neuron at a specialized site called a receptor. A neurotransmitter is released at the presynaptic terminal in response to an action potential. An action potential occurs when a neuron sends information down an axon, away from the cell body. The action potential is an explosion of electrical activity that is created by a depolarizing current.

What happens next? The released neurotransmitter can then move across the synapse to be detected by and bind with receptors in the postsynaptic neuron. Binding of neurotransmitters can influence the postsynaptic neuron in either an inhibitory or excitatory way. If it acts in an excitatory way, it makes the postsynaptic neuron more likely to fire an action potential. If it acts in an inhibitory way, it makes the postsynaptic neuron less likely to fire an action potential. Neurotransmitters match up with receptors like a key in a lock. A neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event.

Why does any of this matter for purposes of this class? Neurotransmitters have a huge impact on our behavior! As we are about to learn, too much or too little of a specific neurotransmitter can lead to neurological disorders and other problems.

Dopamine and Behavior Dopamine is a neurotransmitter that helps regulate movement and emotional responses. Dopamine helps control thoughts and decision-making. Dopamine acts as a sort of a gatekeeper for which impulses and thoughts become conscious and can be acted upon. A 2005 article in the Journal of Computational Neurology explains that dopamine is “the driving force behind the generation of ideas and the creative drive.”

More about Dopamine and Behavior Having too much dopamine can cause euphoria, aggression, intense sexual feelings, and even psychosis. Parkinson's disease, a degenerative condition causing tremor and motor impairment, is caused by loss of dopamine-secreting neurons in the midbrain. People with low dopamine might be more prone to addiction. There is evidence that schizophrenia involves the hyperactivity of dopamine receptors in the prefrontal cortex. Cocaine and methamphetamine strongly inhibit the reuptake of dopamine.

Serotonin and Behavior Serotonin is a neurotransmitter that helps us regulate our mood, appetite, sleep, memory, and learning. There is a correlation between decreased serotonin levels and depression. Some antidepressants, Selective Serotonin Reuptake Inhibitors, prevent the reuptake of serotonin in order to alleviate the symptoms of depression. Serotonin has also been linked to antisocial personality disorder and aggression.

Endorphins Endorphins help reduce pain and stress by binding to opioid receptors in our brains. Exercise releases endorphins, which is why people talk about a “runner’s high.” Endorphins help control fight or flight responses. Endorphins bind with opioid receptors in our hippocampi; low levels of endorphins are correlated with obsessive-compulsive disorder. Problems with endorphin production or the binding process can lead to rapid mood swings or clinical depression.

Acetylcholine Linked to motor coordination, sleep, and memory. There is a correlation between low levels of acetylcholine and Alzheimer’s disease.

“Warrior Gene” MAOA gene, aka “warrior gene:” Studies have shown a correlation between low activity of the enzyme that breaks down serotonin and dopamine and extremely aggressive behavior. One study showed that people with the “warrior gene” were more likely to punish others than were people who didn’t have the gene. There is dispute about the warrior gene in the scientific community.