THE NEURON :D“the basic working unit of the brain”
Neuroscience Society
Neurons
Transmit information to other nerve cells, muscle, or gland cells
between 1 billion and 100 billion neurons, depending on the species (humans = 100 billion)
Parts of a Neuron
Cell body Cytoplasm Nucleus
Dendrites Receive messages
Axon Transmit messages End at nerve terminals
Synapse: contact points
Myelin Sheath
Axons are covered with myelin sheath
Speed transmissions of electrical signals along axons
Two types of cells: Oligodendrocytes: brain Schwann: peripheral nervous system
Action Potential Ion channels: selectively permeable, water-filled
molecular tunnels that pass through the cell membrane and allow ions — electrically charged atoms — or small molecules to enter or leave the cell.
Ability to generate electrical impulse: charge b/e inside and outside of cell
Nerve impulses dramatic reversal in electrical potential (internal negative to positive)
Action potentials pass along axon at speeds up to several hundred miles per hour
Neurotransmitters Brain’s “chemical messengers” Voltage changes trigger release Released at nerve terminals diffuse across
intrasynaptic space bind to receptors Neurotransmitter : receptor :: key : lock triggers a response, such as the generation
of an action potential, contraction of a muscle, stimulationof enzyme activity, or inhibition of neurotransmitter release from the target cell
Acetylcholine First neurotransmitter. Identified 75 yrs
ago Released by neurons connected to
voluntary muscles & muscles that control heartbeat
Formed at axon terminals Action potential calcium ion rushes in
ACh is released into synapse attach to ACh receptors broken down by acetylcholinesterase
Acetylcholine
Myasthenia gravis Caused by antibodies that block ACh
receptors Fatigue and muscle weakness
Alzheimer’s ACh-releasing neurons die Drugs that inhibit acetylcholinesterase
Amino Acids Building blocks of proteins INHIBITORS:
Glycine Gama-aminobutyric acid (GABA)▪ Activity increased by benzodiazephines (eg. Valium) &
anticonvulsunt drugs▪ Huntington’s Disease: hereditary disorder (midlife).
GABA-producing neurons in brain regions that coordinate movement degenerate uncontrollable movements
ACTIVATORS: Glutamate Aspartate Activate N-methyl-d-aspartate (NMDA) receptors
Catacholamines Dopamine and Norepinephrine in brain and
peripheral nervous system Dopamine’s three circuits:
1) control movement 2) psychiatric symptoms (psychosis) 3) regulate hormonal responses
Norepinephrine: Defincies occur in patients with: Alzheimer’s,
Parkinson’s, and Korsakoff’s Syndrome (cognitive disorder associated with chronic alcoholism)
Acute stress increases release from sympathetic nerves and the adrenal medulla
Catacholamines 1) Dopamine deficits Parkinson’s Disease
muscle tremors, rigidity, and difficulty in moving Levodopa: a substance from which dopamine is
synthesized 2) Abnormalities Schizophrenia
drugs that block certain dopamine receptors in the brain are helpful in diminishing psychotic symptoms
3) regulate endocrine system dopamine directs the hypothalamus to manufacture
hormones and hold them in the pituitary gland for release into the bloodstream or to trigger the release of hormones held within cells in the pituitary
Serotonin
Present particularly in blood platelets and the lining of the digestive tract
Controls switches affecting various emotional states
Fluoxetine and other drugs that alter seretonin’s actions relieve symptoms of depression and OCD
Peptides
Chains of amino acids linked together Enkephalin: opiate produced by brain
and resembles morphine (opium derivative used to kill pain). Means: “in the head”
Endorphins: Act like opium or morphine to kill pain or cause sleepiness. Name comes from endogenous morphine
Opiod peptides may explain why injuries received during stress combat
Peptides
Activated by painful or tissue-damaging stimuli
Signals transmitted to central nervous system by special sensory neurons, small myelinated fibers, and tiny unmyelinated C fibers Some C fibers contain peptide called
substance P which causes sensation of burning pain
Active component of chili peppers (capsaicin) causes release of substance P
Trophic Factors
Small proteins in the brain Necessary for the development,
function, and survival of specific groups of neurons
Information about these proteins will be useful to design new therapies for degenerative diseases including Alzheimer’s and Parkinson’s
Hormones Endocrine system = major communication
system SOURCES OF HORMONES: pancreas,
kidneys, heart, adrenal glands, gonads, thyroid, parathyroid, thymus, and pituitary gland Pituitary secretes hormones into blood Endorphins are released from pituitary so they
can be hormones too System important for: basic behavioral
activities and regulation of body functions
Hormones
Brain contains receptors for thryoid hormones and six classes of steroid hormones: androgens, estrogens, progestins, glucocorticoids, mineralocorticoids, and vitamin D
Metabolic hormones: insulin (regulates glucose), insulinlike growth factor, ghrelin (stimulates hunger), and leptin (regulates energy intake)
Hormones In response to stress and changes in our biological
clocks, such as day and night cycles and jet lag, hormones enter the blood and travel to the brain and other organs
hormones alter production of gene products that participate in synaptic neurotransmission & the structure of brain cells
Circuitry of the brain can be altered in the course of a couple of hours to a day
Hormones are important for protection and adaptation
Stress hormones (Glucocorticoid cortisol) alter brain functions which is why SEVERE & PROLONGED STRESS CAN CAUSE PERMANENT BRAIN DAMAGE
Sex Hormones Neurons in hypothalamus produce gonadotropin-
releasing hormone (GnRH): acts on cells in the pituitary
GnRH causes two hormones in men & women to be released into bloodstream: Follicle-stimulating hormone (FSH) Luteinizing hormone (LH)
Men: FSH & LH bind to receptors in testes and release testosterone
Female: FSH & LH act on ovaries and release estrogen and progesterone
Increased levels of these decreased levels of FSH & LH
Sex Differences Size and shape of brain structures in the
hypothalamus Arrangement of neurons in the cortex and
hippocampus Actually more similar than they are different Anatomical differences have also been
reported between the brains of heterosexual and homosexual men
hormones and genes act early in life to shape the brain in terms of sex-related differences in structure and function
Gases Nitric oxide and Carbon monoxide Can be stored in any structure, not in synaptic
storage structures Made by enzymes when needed and released from
neurons by diffusion Don’t need receptors. They act on chemical targets
(enzymes?) Carbon monoxide: exact function is not known Nitric Oxide
Erection Intestine: Relaxation and normal movements of digestion Brain: Major regulator of intracellular messenger molecule
– Cyclic GMP Neuronal damage after excess glutamate release (Stroke)
Second Messengers Trigger biochemical communication after
neurotransmitters at their receptor Relay message of neurotransmitter from cell
membrane to brain’s internal biochemical machinery
May endure from few milliseconds to up to several minutes
Before second messengers: ATP! Norepinephrine activated receptor binds G
protein on inside of membrane causes enzyme adenylyl cyclase to convert ATP to cyclic adenosine monophosophate (cAMP)