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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
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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
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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
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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.
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