What physiological system motivates these activities? Behaviors such as these activate dopamine receptors, and the increased dopamine produces a desire to perform the behavior.
By contrast, a lack of dopamine may be involved in problems with movement. Severe loss of dopamine is connected to Parkinson’s disease. First identified by the physician James Parkinson in 1917, Parkinson’s is a degenerative and fatal neurological disorder. It affects about 1 in every 200 older adults and occurs in all known cultures. Most people with Parkinson’s do not experience symptoms until after age 50, but the disease can occur earlier in life. For example, the actor Michael J. Fox was diagnosed with Parkinson’s at age 30.
With Parkinson’s disease, the dopamine-producing neurons in the midbrain (see Figure 2.12) slowly die off. The resulting lack of dopamine causes distur- bances in motor function: rigid muscles, tremors, and difficulty initiating volun- tary action. You can see these symptoms in the shuffling walk of a person with the disease. In the later stages of the disorder, people experience severe cogni- tive and mood disturbances. Injections of one of the chief chemical building blocks of dopamine, l-DOPA, help the surviving neurons produce more dopa- mine. When l-DOPA is used to treat Parkinson’s disease, patients often have a remarkable, though temporary, recovery.
Gaba and GLutaMate The main inhibitory neurotransmitter is GABA (gamma-aminobutyric acid). It is more widely distributed throughout the brain than most other neurotransmitters. Without the inhibitory effect of GABA, the excitation of neurons might get out of control and spread through the brain chaotically. In fact, epileptic seizures may be caused by low levels of GABA (Chung et al., 2015).
Drugs that are GABA agonists (e.g., Valium) are widely used to treat anxi- ety disorders. The increased inhibitory effect provided by these drugs helps calm anxious people. Alcohol has similar effects on GABA receptors. As a result, people typically experience alcohol as relaxing. GABA reception may also be the primary mechanism that causes alcohol to interfere with motor coordination.
In contrast, glutamate is the main excitatory neurotransmitter. It is involved in fast-acting neural transmission throughout the brain. Glutamate receptors aid learning and memory by strengthening synaptic connections.
endOrphIns You’ve no doubt heard about, or perhaps experienced, “runner’s high.” This psychological state results from a release of endorphins. Endorphins are a class of neurotransmitters involved in reward, such as runner’s high, as well as in natural pain reduction. Pain is useful because it signals that we are hurt or in danger. That signal should then prompt us to try to escape or withdraw. If you didn’t experience pain when you touched a hot stove, you wouldn’t know that you should pull your hand away to avoid being injured.
Pain can interfere with adaptive functioning, however. If pain prevents us from eating, competing, or mating, then people will fail to pass along their genes. Endorphins’ painkilling, or analgesic, effects help us perform these behaviors even when we are in pain. In humans, drugs that bind with endor- phin receptors (e.g., morphine) reduce the subjective experience of pain. Apparently, morphine does not block the nerves that transmit pain signals. Instead, it alters the way pain is experienced. In other words, people still feel pain, but they report a sense of detachment that lets them not care about the pain.