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Neurons:nerve cells found in the brain, spinal cord, and nervous system that specialize in communication. There are 100 billion neurons present from birth (actually from 24 weeks’ gestation). Commonly thought that once neurons die, they’re lost forever, but some scientists are challenging this assumption, saying that neurons can be regenerated. It’s currently unknown whether neurons can be regenerated. 3 parts: 1. Cell body (nucleus) 2. Dendrites (bring information into the cell body) 3. Axons (take information away from the cell body to other neurons 3 types of neurons: 1. Interneurons: communicate only with other neurons; found in brain and spinal cord 2. Sensory neurons: relay information occurring outside the nervous system; send messages to the brain and spinal cord through interneurons 3. Motor neurons: send message from the nervous system to all the different kinds of muscles (including heart, small intestine, arteries, internal organs, skeletal muscles) Myelin sheath: fatty protein tissue that insulates axons and speeds impulses. Multiple sclerosis: deterioration of the myelin sheath. Myelinization depends on dietary fat during 3rd trimester and first 2-3 years of life. The thicker the myelin sheath, the better the conduction. How neurons communicate: A neuron fires an impulse when it receives signals from sense receptors stimulated by pressure, heat, light, or messages from neighboring neurons. The impulse is called the action potential and is a brief electrical charge that travels down the axon. Some signals are excitatory (“Fire”) and others are inhibitory (stop!). Neuron does a brief calculation to determine whether to send the signal to fire. If excitatory signals outweigh the inhibitory ones, an action potential is triggered. **The stronger the stimulus, the more neurons that fire, and the more often they fire. The impulse is sent down the axon and ends at the axon terminals, which contain neurotransmitters (chemical messengers). The neurotransmitters are released into the synapse, which is less than a millionth of an inch wide. The dendrites of the neighboring neuron pick up the neurotransmitters (which act as a key unlocking the lock), and the message has been 1 transmitted. Excess neurotransmitters are taken back into the sending neuron in a process called reuptake. Neurotransmitters Acetylcholine: muscle contractions, memory 1. Curare (inhibits Ach, thus paralyzing you) 2. Botulism/botox (inhibits Ach) 3. Black widow spider (flood of Ach; muscle spasms) 4. Marijuana (disrupts Ach, thus impairing memory). Dopamine: smooth muscle movements, pleasure/reward 1. Parkinson’s: not enough dopamine/jerky movements 2. Schizophrenia: too much dopamine/hallucinations, bizarre behaviors 3. Depression: probably not enough dopamine 4. New research from 2010 suggests that dopamine is released when we fall in love. Norepinepherine: arousal, mood, hunger, sleep 1. Depression: low levels of norepinepherine/no energy 2. Induces eating in rats infused with norepinepherine Serotonin: sleep, mood, inhibits aggressiveness & pain sensitivity 1. Depression: low levels of serotonin 2. Probably plays a role in chronic pain (too low levels) Endorphins: pain reduction 1. Exercise—releases endorphins 2. Opiates (heroin, pain pills)—produce artificial endorphins 3. Acupuncture—may work because it stimulates endorphins Glutamate: main excitatory neurotransmitter 1. Keeps the central nervous system aroused 2. Plays a role in learning and memory GABA: (gamma-aminobutyric acid) main inhibitory neurotransmitter 1. Calming and relaxing effect on the nervous system 2 2. Implicated in anxiety and sleep disorders (lower levels of GABA= increases in these disorders) 3. Anti-anxiety and insomnia medications (e.g., Lunesta, Ambien) bind to GABA receptors and increase the amount of GABA in the brain. Drugs and Neurotransmitters: 1. Agonists: mimic the basic effects of the neurotransmitter (pain pills, sleep medications that bind to GABA) 2. Antagonists: block the release of the neurotransmitter (antihistamines) 3. For drugs to work, they must be able to cross the blood-brain barrier (the fatty coating that wraps around tiny blood vessels going to the brain). The blood-brain barrier protects the brain against bacterial infection. Nervous System: Central nervous system: consists of brain and spinal cord. **Two main functions of the spinal cord: 1) carries sensory information to and from brain 2) reflexes Peripheral nervous system: consists of neural pathways that bring information to and from brain. Two branches: 1. Somatic: transmits information regarding sensation; VOLUNTARY movement of the body 2. Autonomic: carries information related to survival; INVOLUNTARY control a. Sympathetic division: mobilizes the body in the face of threat (goosebumps; pupil dilation; BP increase; heart rate increases; digestion stops) b. Parasympathetic division: calms the body back down (digestion increases; heart rate and BP decrease, pupils constrict Endocrine System: consists of a number of glands that release chemicals called hormones directly into the bloodstream. Hormones originate in one tissue, travel through the bloodstream, and affect other tissues, including the brain. They exert profound effects on a wide range of bodily functions and also impact emotions. When they act on the brain, they influence our interest in sex, food, and aggression. A special type of hormone called 3 a neurohormone is especially of interest because it influences neural activity (just like a neurotransmitter). They’re different from neurotransmitters, though, because they’re released into the bloodstream rather than into the synapse. They can travel greater distances throughout the body and have longer effects than neurotransmitters. They take a little longer time to exert their effects, but they can affect cells and organs distant from the source of the hormone’s production. Some neurotransmitters also function as neurohormones. Examples: Dopamine is released both by the axons and the hypothalamus. Norepinepherine is released both by the axons and by the adrenal glands, which sit on top of the kidneys. Stress response: The stress response involves both neurotransmitters and neurohormones. First, in Selye’s GAS model, we enter the alarm stage. Neurotransmitters react first and help mobilize us for the stressful situation. The sympathetic nervous system comes into play and mobilizes us for the “fight or flight” response. Usually, the source of stress is removed, and the parasympathetic system demobilizes us. If the stress continues, though, we go into the resistance stage of Selye’s General Adaptation Syndrome (GAS) model. Here, hormones start coming into play to prepare us for a long siege of stress. Hormones like cortisol (produced by the adrenal glands) increase blood sugar levels to sustain energy and raise blood pressure. Overuse leads the sufferer into fatigue, concentration lapses, irritability, and insomnia. Finally, in the third stage, exhaustion, we enter into “adrenal exhaustion.” The body has run out of energy and immunity, and mental, physical, and emotional effects occur. Blood sugar levels decrease as the adrenals become depleted, leading to decrease stress tolerance, depression, illness, and collapse. High cortisol levels suppress the immune system by increasing production of interleukin-6, an immune-system messenger. Cancer may result. Furthermore, because stress increases BP, the arteries get clogged with fat and cholesterol that are released by the body during prolonged stress, leading to heart attacks or stroke. 4