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The Nervous SystemIncludes the brain, spinal cord and the nerves
What does it mean to be “brain-dead?”
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Neurons = masses of nerve cells that transmit information (functional unit of the system)
1. Cell Body - contains the nucleus and other cell organelles2. Dendrites – shorter, more numerous, receive information3. Axons - single long fibers, conducts information away from the cell
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Two Divisions of the Nervous System
Central Nervous System (CNS)
Brain and spinal cord
Peripheral Nervous System (PNS)
Nerves throughout the body 31 pairs of spinal nerves 12 pairs of cranial nerves
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Overall function -
coordinate the body’s
systems by receiving and
sending information;
maintaining homeostasis
● Sensory - gathers info from receptors in the body
● Integrative - determines where information is sent
● Motor - responds to signals, homeostasis
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Motor Functions
Somatic Nervous System - skeletal; voluntary
Autonomic Nervous System
- smooth muscles, glands; involuntary
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Autonomic Nervous System (2 branches)
Parasympathetic (rest and digest)
Sympathetic (fight or flight)
*autonomic conflict
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Chromatophilic substance (rough ER) - transport system
Myelin -insulation surrounding axons
Nodes of Ranvier - gaps in the insulationNeurofibrils - fibers within the axon
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Neuroglial Cells - Support for neurons
node of ranvier
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1. Microglial Cells - Immune function; digest debris, kills bacteria
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2. Oligodendrocytes - make myelin sheath that provides insulation around the axons
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3. Astrocytes - connect blood vessels to neurons
I connect to blood vessels
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4. Ependymal Cells - forms membranes around tissue
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5. Schwann cells: form the insulating myelin sheath around the neurons in the PNS
(same function as oligodendrocytes, which are found in the CNS)
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Myelin Sheaths - insulate axons
Schwann cells supply the myelin for peripheral neurons.
Oligodendrocytes myelinate the axons of the central nervous system.
Gaps in the sheath are called:NODES OF RANVIER
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Which cell….
1. Creates myelin sheath
2. Has an immune function
3. Forms a membrane
4. Connects to a blood supply
Practice with neuroglia coloring!
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Myelinated (white matter) – myelinated axonsUnmyelinated (grey matter) - unmyelinated
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Functional: Sensory / Motor / Interneurons
Structural - Bipolar / Unipolar / Multipolar
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Fun Facts about Neurons
-Longevity – can live and function for a lifetime-Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception-High metabolic rate – require abundant oxygen and glucose
-The nerve fibers of newborns are unmyelinated - this causes their responses to stimuli to be coarse and sometimes involve the whole body. Try surprising a baby!
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Cell Membrane PotentialResting Potential / Threshold Potential / Action Potential
Nerve Impulse = weak electric current, like a wave
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1. Neuron membrane
maintains resting potential
2. Threshold stimulus is
received
3. Sodium channels open
4. Sodium ions diffuse inward,
depolarizing the membrane
5. Potassium channels open
6. Potassium ions diffuse
outward, repolarizing the
membrane
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7. The resulting action potential causes a local bioelectric current that stimulates the membrane.
8. Wave of action potentials travel the length of the axon as a nerve impulse
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Ions in the cell and outside the cell create a positive and negative side, which produces an electric current.
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Depolarization:
loss of the difference in charge between the inside and outside of the plasma membrane of a muscle or nerve cell due to a change in permeability and migration of sodium ions to the interior
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Nerve Impulse
Speed is proportional to the size of the axon
(greater diameter = faster impulse)
Myelinated axons conduct impulses faster than unmyelinated one
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The SynapseSynapse = junction between two communicating neurons
Nerve pathway - nerve impulse travels from neuron to neuron
Dendrite → cell body → along axon -> synapse (gap) → dendrite
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To complete the signal, a NEUROTRANSMITTER is released at the gap to signal the next neuron.
Receptors on the dendrite receive the chemical message
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Anatomy of the SynapseA: Neuron (axon)B: Neuron (dendrite)
1. Mitochondria2. Vesicle 3. Receptor4. Synapse5. Receptor6. Calcium Channel7. Releases neurotransmitter8. Re-uptake
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Types of Neurotransmitters
Excitatory - increase membrane permeability, increases chance for threshold to be achieved
Inhibitory - decrease membrane permeability, decrease chance for threshold to be achieved
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Examples of Neurotransmitters
Acetylcholine - stimulates muscle contraction
Dopamine - mood, happiness
Serotonin = sleepiness and mood
Endorphins = pain reduction, mood
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GABA
Reduces the activity of the neurons to which it binds (inhibitor).
● Most common type of receptor● 40% of all synapses work with
GABA● GABA has a tranquilising effect● Low levels of GABA associated
with anxiety and phobia
Sedatives bind to GABA receptors
(gamma-aminobutyric acid)
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Agonist = molecule that has the same effect on the postsynaptic neuron as the neurotransmitter itself does.
Antagonist = molecule that blocks the effect that the neurotransmitter normally has on the postsynaptic neuron.
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Antidepressants
Zoloft is part of a class of drugs called selective serotonin reuptake inhibitors
it inhibits the uptake of serotonin back into the cell, keeping mood elevated for a longer time.
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Drugs and Poisons that Affect Nerves
Curare - poison made from frog skin and causes paralysis by blocking Ach receptors at the neuromuscular junction.
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Strychnine poisoning can be fatal to humans and animals and can occur by inhalation, swallowingIt prevents the proper operation of the chemical that controls nerve signals to the muscles. The chemical controlling nerve signals works like the body's “off switch” for muscles. When this “off switch” does not work correctly, muscles throughout the body have severe, painful spasms.
Read about Strychnine Poisoning
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CocaineDopamine binds to receptors and is eventually recycled back into the neuron.
If cocaine is present, it attaches to the dopamine transporter and blocks the normal recycling process, resulting in a buildup of dopamine in the synapse, which contributes to the pleasurable effects of cocaine.
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Your neurotransmitters on cocaine
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Ecstasy (MDMA)The neurotransmitter serotonin is vital in regulating many of our basic functions. Serotonin is, among other things, the feel good neurotransmitter and helps to regulate body temp.
Our brain cells are recycle serotonin back into the cells and out of the synapse using serotonin reuptake transporters. Ecstasy essentially takes these upkeep transporters and reverses their roles. This causes a massive flood of serotonin from the brain cells into the synapse.
The most common cause of Ecstasy-related death is overheating (hyperthermia).
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Heroin
Activates opiate receptorsBlocks release of GABAMore dopamine is released
DOPAMINE
Dopamine Receptors
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Amphetamines
Mimic dopamine - binding to receptors
Dopamine does not re-enter the cell, depleting the cell’s supply
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What Is PRIALT?PRIALT is a calcium channel blocker that could be used to inhibit pain transmission from the spinal cord
PRIALT does not bind to opioid receptors
*Why might this be an important chemical for combating heroin addiction?
http://www.hhmi.org/biointeractive/prialt-blocks-pain-signaling-mice
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Check out Mouse Party!
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Impulse ProcessingNeuronal pool - groups of neurons that make hundreds of synaptic connections and work together to perform a common function
These "pools" help us remember sequential tasks, like tying a shoe or riding a bike.
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Nerve Pathways
Reflex arc = simple path, only includes a few neurons ( involuntary, instant)
Knee-jerk reflex = maintains uprightness
Withdrawal reflex = avoidance of painful stimuli
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Disorders Related to Neurons
ALS - (Lou Gehrig’s Disease) Amyotrophic Lateral Sclerosis
Progressive degeneration of nerve cells in the spinal cord and brain
NEURODEGENERATIVE
- this means symptoms will get worse over time.
- most patients die within 5 years of diagnosis
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EpilepsyEpileptic seizures are caused by excessive electrical activity within networks of neurons in the brain.
A fine balance between excitation and inhibition must be maintained in order for the brain to function normally.
If there is too much glutamate, neurons can become hyperexcitable and a seizure may result. Too little GABA (inhibitory) can also result in a seizure.
2 Minute Neuroscience - Epilepsy
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Myasthenia Gravis
autoimmune disorder in which antibodies destroy neuromuscular connections
We did a case study on this during the muscle chapter, do you remember it?
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Multiple Sclerosis (MS)Myelin around the nerve fibers is lost (autoimmune) and forms scar tissue called sclerosis. Nerves cannot conduct impulses to and from the brain.
Affects ability to walk, talk, swallow, and can affect vision.