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Neuroglia
nourish and protect neurons
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Central Nervous System (CNS)
- Spinal cord + brain
- intergrates and coordinates sensory and motor information
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Peripheral Nervous System (PNS)
All neural tissue outside CNS
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Afferent Division
carries sensory info from receptors toward CNS
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Efferent Division
Carries motor commands from CNS to effectors
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Somatic Nervous System (SNS)
controls skeletal muscle contractions
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Autonomic Nervous System (ANS)
Controls smooth/cardiac muscle contractions
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Sympathetic Division
activiated during times of danger
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Parasympathetic Division
Activated during times of NO danger
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Cell Body (soma)
Contains large nucleus, cytoplasm (perokaryon) and Nissl Bodies (RER [rough endoplasmic reticulum] + ribosomes)
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Dendrites
- Branched processes from the soma
- carry impulses toward soma
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Axon
- Long cytoplasmic processes with smaller branches (telodendria) ending in synaptic terminals
- May branch into collaterals
- Cytoplasm (axoplasm) and membrane (axolemma)
- Base is axon hillock
- Carry information away from soma
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Synapse Overview
- synapse is where pre and post synaptic cells communicate thru release of chemicals across spaces called synaptic clefts
- Neuron - Neuron (typical synapse)
- Neuron - Gland (neuroglandular synapse)
- Neuron - Muscle (neuromuscular synapse)
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Anaxonic Structure
- small, no defined axon/dendrites
- unknown function
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Bipolar Structure
- small, cell body in middle, two distinct processes
- found in eyes, ears
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Unipolar Structure
- cell body to side, axon/dendrites are continuous
- very long (~1m)
- found in PNS
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Multipolar Structure
- cell with axon, two or more dendrites
- very long (~1m)
- found in CNS
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Sensory (afferent neuron)
(functional classification)
- Unipolar
- Axons (afferent fibers) that extend between the receptor and CNS
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Motor (efferent) Neuron
(functional classification)
- Multipolar
- Axons (efferent fibers) extend between CNS and effectors
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Interneuron (Association Neuron)
(functional classifcation)
- Multipolar
- Distribute information between the sensory neruon and motor neuron
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Ependymal Cells (neuroglia of CNS)
- line central canal and vetricles of the brain
- secretes and moves CSF
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Astrocytes (neuroglia of CNS)
- large and numerous
- cell extensions surround capillaries
- maintain blood/brain barrier
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Microglia (neuroglia of CNS)
remove cell debris and pathogens
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Oligodendrocytes (Neuroglia of CNS)
- lipid-rich pads wrap axons to form a myelin sheath
- sheath insulates and speeds up action potential
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Satellite Cells (neuroglia of PNS)
surround cell bodies of PNS neurons
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Schwann Cells (neuroglia of PNS)
- lipid-rich pads wrap axons to form a myelin sheath
- lightly wrap unmyelinated axon
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Repair in PNS
Wallerian Degeneration
- Step 1: fragmentation of axon myelin occurs in distal stump
- Step 2: schwann cells form cord, grow in cut, and unite stumps, macrophages engulf disintergrating axon and myelin
- Step 3: Axon sends buds into network of Schwann cells and then starts growing along cord of schwann cells
- Step 4: Axon continues to grow into distal stump and is enclosed in schwann cells
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Repair in CNS
- limited due to numerous neurons and scarring
- some released chemicals (of the inflammatory response) block axon regrowth
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Potential Difference
when positive and negative charges are held apart
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Transmembrane Potential
transmembrane potential is a potential difference across the membrane
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Current
movement of charges to eliminate potential difference
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Resistance
- membrane restricts ion movement
- creates resistance to current
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Resting Potential
- potential difference at rest is -70mV
- inside has more K+ and negative proteins
- outside has more Na+ and Cl-
- K+ move through leak channels faster than Na+ moves in (negative proteins stay in cell) so inside is negative and outside is positive
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Chemical Gradient
- high concentration of K+ inside and high concentration of Na+ outside
- drives Na+ in and K+ out
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Electrical Gradient
- high negative charge inside and high positive charge on the outside
- drives sodium ions in and potassium ions in
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Electrochemical Gradient for Potassium Ions
- drives potassium ions out of cell
- less than chemical gradient alone due to opposing electrical gradient
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Electrochemical Gradient of Sodium Ions
- drives sodium ions into the cell
- greater than chemical gradient alone due to the aid of the electrical gradient
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Sodium/Potassium Pump
- uses ATP to pump 3 sodiums out and 2 potassiums in
- along with leak channels, the pump maintains resting potential of a cell
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Leak Channels
Always open
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Gated (active) Channels
open and close
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Chemically Regulated Channel
open/close in response to chemicals
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Voltage Regulated Channel
open/close in response to changes in the transmembrane potential
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Mechanically Regulated Channel
open/close in response to physical stimulation
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Graded Potentials
- changes in transmembrane potential that do not spread far from the point of stimulus
- occur at axon hillock, can lead to action potential
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Depolarization
shift in resting potential toward a more positive potential
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Repolarization
the process of restoring resting potential
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Hyperpolarization
increase in negative aspect of the resting potential
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Action Potential
- changes in the transmembrane potential that affects the entire excitable membrane
- occur at the axon
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All or None Principal
- need stimulus large enough to reach threshold (+10mV) which opens voltage regulated sodium ion channels
- actional potential properties are independent of the stimulus strength
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Generation of an Action Potential
- stimulus causes depolarization to threshold
- sodium ion channels open and sodium ions move in (depolarization)
- sodium ion channels inactivated
- potassium channels open and potassium ions move out (repolarization)
- more potassium ions move out (hyperpolarization)
- potassium ion channels close
- sodium/potassium ion pump and leak channels restore resting potential
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Refractory Period
time from beginning of action potential until the resting potential is stabilized
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Absolute Refractory Period
- from the time when sodium ion channels open until sodium ion channel inactivation ends
- membrane does not respond to second stimulus
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Relative Refractory Period
- from when sodium ion channels regain their resting condition until transmembrane potential stabilizes
- membrane responds to greater than normal second stimulus
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Propagation
repeated action potential along entire membrane (axon)
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Continuous Propagation
- action potential spreads along unmyelinated axon
- Step 1 - an action potential develops in the initial segment, transmembrane potential depolarizes
- Step 2 - local current depolarizes the adjacent portion
- Step 3 - an action potential develops at this location and the initial segment enters refractory period
- Step 4 - a local current depolarizes the adjacent portion of the membrane and the cycle is repeated
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Saltatory Propagation
- action potential spreads along myelinated axon (faster)
- the same steps happen but it happens node to node (skips over internode) causing it to be faster
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Axon Diameter
increased diameter decreased resistance
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Type A Fibers
- largest, myelinated, action potential = 140m/sec
- deliver very important information
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Type B Fibers
- smaller, myelinated, action potential = 18m/sec
- delivers less urgent info
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Type C Fibers
- smallest, unmyelinated, action potential = 1m/sec
- delivers least urgent info
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Nerve Impulse
- movement of the action potential along an axon
- moves from presynaptic cell to a postsynaptic cell
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Electrical Synapse
pre and postsynaptic cell have direct contact via gap junction
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Chemical Synapse
- pre and postsynaptic cell do not have direct contact (synaptic cleft inbetween)
- neurotransmitter released at synapse
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Excitatory Neurotransmitter
causes depolarization and action potential
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Inhibitory Neurotransmitter
causes hyperpolarization and prevents action potential
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Cholenergic Synapse
- acetylcholine released
- neuron-neuron synapses in CNS and PNS and neuromuscular junctions
- action potential arrives, synpatic knob depolarizes, calcium ion channels open, calcium enters
- entry of calcium ions causes acetylcholine release into synapse which binds receptors of postsynaptic membrane
- postsynaptic membrane depolarizes
- acetylcholinesterase digests acetylcholine
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Neurotransmitters
result in changes in postsynaptic cell membrane permeability
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Norepinephrine (Noradrenaline)
released at adrenergic synapses in CNS and ANS with excitatory effects
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Dopamine
released in CNS with inhibatory role that helps control precise movements
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Serotonin
released in CNS with effects on emotions and attention states
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Gamma-aminobutyric Acid (GABA)
released at CNS with effect of reducing anxiety
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Neuromodulators
- affect neurotransmitter release or postsynaptic cell response
- peptide opioids for pain control
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Information Processing by Neurons
integration process at axon hillock which determines action potential generation
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Postsynaptic Potentials
graded potential at postsynaptic membrane in response to neurotransmitters
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Excitatory Postsynaptic Potential (EPSP)
graded depolarization at postsynaptic membrane
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Inhibitory Postsynaptic Potential (IPSP)
graded hyperpolarization at postsynaptic membrane
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Summation
integrated effect of all graded potentials at membrane
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Temporal Summation
add stimuli in quick succession
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Spacial Summation
simultaneous stimuli having cumulative effects
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Facilitation
- bring neuron transmembrane potential closer to threshold
- dur to summation of EPSPs or drugs
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