Central Nervous System (CNS)
Peripheral Nervous System (PNS)
Sensory input: monitors internal and external
environments
 Integration: processes & interprets sensory
information
 Motor Output: Coordinates voluntary and
involuntary responses of effector organs
 2 subdivisions:

 CNS – brain and spinal cord (dorsal body cavity)
▪ Integration, Intelligence, memory, emotion
 PNS – all other neural tissue
▪ Cranial nerves and Spinal nerves
▪ sensory, motor
Include:
 sensory input
 integration
 motor output
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Receptors – receive sensory info
Afferent division – carries info from receptors to the CNS
(somatic & visceral)
Efferent division – carries info from CNS to PNS effectors
(muscles, glands, adipose)
 Somatic Nervous System (SNS)
▪ Controls skeletal muscles (voluntary)
 Autonomic Nervous System (ANS)
▪ Controls involuntary actions
▪ Sympathetic Division (increase heart rate)
▪ Parasympathetic Division (decreases heart rate)
1.
2.
3.
4.
Somatic division
Sympathetic
division
Afferent division
Efferent division
47%
35%
12%
6%
1
2
3
4
1.
2.
3.
4.
Somatic division
Sympathetic
division
Afferent division
Efferent division
71%
24%
6%
0%
1
2
3
4

Communicate w/other neurons

Large Complex Cells:
 Soma -cell body
 Dendrites -receive info
 Axon -sends signal to synaptic terminals
as nerve impulse
 Synapse – site of neural
communication (gap)

Special characteristics:
 Extreme longevity (100 years +)
 Amitotic – lose ability to divide (G0)
 High metabolic rate – O2 & glucose
Biosynthetic center
Outgrowth of neuron
processes during embryonic
development
 Lacks centrioles
 Nissil bodies – Rough ER
stains darkly
 Nuclei - Clusters of cell bodies
in CNS
 Ganglia - Clusters of cell
bodies in PNS


Armlike processes - extend from cell body
Tracts - Bundles of neuron processes in CNS
Nerves - Bundles of neuron processes in CNS
Dendrites
 Convey graded potentials towards cell body
 Short and branching receptive regions
 Dendritic spines -bulbous ends that form synapses
 Axon (single)
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Generates and transmits nerve impulse away from cell body
Axon hillock – cone shaped area where axon extends from soma
Nerve fiber – long axon (as long as 4 feet!)
Axon collaterals – occasional 900 branch
1,000 - 10,000+ Terminal branches w/ Axon terminals (synaptic knobs)
Myelin Sheath




Protein-lipid electrical insulation on axons
Increases speed of transmission
Neurilemma – exposed plasma membrane of Schwann cell
Nodes of Ranvier – gaps in the myelin sheath (widely spaced in CNS)

Multipolar
 multiple dendrites & single axon
 motor neurons
 most common in humans

Bipolar
 2 processes: one dendrite and one axon
 cell body between them
 Rare: special senses (retina & olfactory)

Unipolar
 1 continuous dendrites & axon
 cell body lies to side
 sensory neurons (ganglia of PNS)

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Sensory – afferent division
 info about surrounding environment
 position/movement skeletal muscles
 digestive, resp, cardiovasc, urinary, reprod, taste, and pain
 Mostly unipolar (some bipolar in special senses)
Motor – efferent division (response)
 skeletal muscles
 cardiac and smooth muscle, glands, adipose tissue
 Mostly multipolar
Interneurons
 Integration
 Brain and spinal cord - memory, planning, and learning
 Mostly multipolar
Regulate environment around neurons, smaller & outnumber
neurons
 2 Types in PNS:
 Satellite Cells
 Surround neuron cell bodies of NS
 Function unknown
 Schwann Cells
 Surround nerve fibers of PNS
 Secrete myelin sheath
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4 types inCNS:
Astrocytes (most common in CNS)

Radiating processes connect to
capillaries
 Control chemical environment

Microglia

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Ovoid shape w thorny processes
Moniter nueron health
Can turn into macrophages
Ependymal

Range shape from squamour to
columnar, usually ciliated
 Circulate CSF

Oligodendrocytes

Wrap around nueron fibers & produce
myelin
1.
2.
3.
multipolar
bipolar
unipolar
100%
0%
1
2
0%
3
1.
2.
3.
4.
Dendrites
soma
axon
Myelin sheath
94%
6%
1
2
0%
3
0%
4
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
Basic Electrical Principles
Voltage
 measure of electrical charge (mV = 1/1000 V)
 potential difference measure between two points
 Current – flow of electrical charge from one point
to the next, used to do work
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Membrane proteins that allow specific type of ion(s) to pass
Electrochemical gradient: ions move with concentration
gradient and along electrical gradients (towards opposite
charge)
Chemically (Ligand) gated channels
 Open when appropriate chemical (neurotransmitter) binds
Voltage gated channels
 Open and close in response to changes in membrane
potential
Mechanically gated channels
 Open in response to physical deformation
Non-gated (leakage) channels
 Always open
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-70mV (inside of cell is negatively charged in comparison to the
outside of the cell)
Is said to be polarized due to difference of ionic concentrations of
intracellular and extracellular fluids
Cytosol has low concentrations of Na+, and high conc of K+
K+ ions diffuse out of leak channels causing the cell to be neg inside
(more than Na+ leak in)
Na+/K+ pumps stabilizes the resting membrane potential
Incoming signals over short distance
Decrease in magnitude with distance
 Magnitude dependent upon stimulus
 Stimulus causes gated channel to open



Receptor potential – heat, light, or other form of energy
 Post-synaptic potential – neurotransmitter
 Current carried by ions thru fluid in/out of cells
Positive ions move towards neg areas and vice versa
K+ ions move away from depolarized area and accumulate in neighboring
membrane areas neutralizing neg ions
 Meanwhile positive ions move towards depolarized regions being momentarily
replaced by neg ions (Cl- or HCO3 -), then causing the neighboring membrane to
depolarize
 The plasma membrane is “leaky” and charge is quickly lost and dissipates quickly
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Long distance signals of axons (do not decrease)
Only cells w/excitable membranes (neurons & muscle)
Transition from graded potential to action potential at the axon
hillock
Brief reversal of membrane potential (-70mV  +30mV)
Depolarization
 reduction in membrane potential (less negative)
Hyperpolarization
 Increase in membrane potential (more negative)
Resting State – all voltage gated Na+ and K+ gated channels closed
Depolarizing phase – Na+ channels open (increasing + charge…opening
more Na+ channels)
 Critical Threshold reached at -60 to -50mV and becomes selfgenerating (+ feedback)
 Until all Na+ channels open and membrane potential reaches +30mV
 Repolarizing phase – internal negativity restored
 Na+ channels close, Na+ stops entering cell
 Potassium channels open, K+ leaves cell w/electrochemical gradient
 Hyperpolarization
 K+ channels remain open temporarily
 Na+ channels reset to their original position
 Note: electrical conditions restores not ionic conditions, ionic distribution
is restored by 1,000’s of Na+/K+ pumps in axon membrane
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Action potential propagates (is transmitted)
away from its point of origin towards the
axon terminals
Threshold – unstable equilibrium state
 Weak stimuli – generate subthreshold
depolarizations that do not generate AP
 AP is an ALL or NONE Phenomenon

Once AP is generated all alike
Refractory period - When neuron membrane is generating AP
and Na+ channels are open, neuron can NOT respond to any
other stimulus
 Conduction velocity – rate of propagation depend on
 Axon diameter – the bigger the faster
 Degree of myelination (insulation – preventing leakage)

▪ Continues conduction - unmyelinated conduction is relatively slow
▪ Saltatory conduction – AP triggered only at nodes where Na+
channels are located (30x faster!)

Nerve Fiber Classification
 Group A – somatic sensory & motor (300mph)
 Group B & C – viscera sensory, ANS fibers to viscera, and skin
sensory (40mph – 2mph)
1.
2.
3.
4.
Increases electrical
impulse
Causes the release
of more
neurotransmitters
Is released in a
synaptic cleft
All of the above
25%
1
25%
25%
2
3
25%
4
1.
2.
3.
4.
0mV
30mV
-60mV
-70mV
25%
1
25%
25%
2
3
25%
4
1.
2.
3.
depolarization
repolarization
hyperpolarization
33%
1
33%
2
33%
3
1.
2.
3.
4.
More Na+ rushing
into the cell
K+ leaving the cell
Neurotransmitters
binding to dendrite
Vesicles release
neurotransmitters
25%
1
25%
25%
2
3
25%
4
1.
2.
3.
4.
0mV
30mV
-60mV
-70mV
25%
1
25%
25%
2
3
25%
4
1.
2.
3.
4.
The resting potential is
restored
K+ diffuse out of cell
The cell membrane
becomes negatively
charged again
All of the above
25%
1
25%
25%
2
3
25%
4
25%
1.
2.
3.
4.
25%
25%
2
3
25%
Na+ ions
Neurotransmitters
K+ ions
All of the above
1
4
33%
1.
2.
3.
more
less
No effect at all
33%
33%
17%
1.
2.
3.
4.
5.
6.
17%
17%
17%
3
4
17%
17%
Increase electrical stimulus
Decrease electrical stimulus
Increase neurotransmitters
released
decreased neurotransmitters
released
1&3
2&4
0 of 25
1
2
5
6


You spray your house with insecticide.
Shortly afterwards, you observe roaches
lying on the ground with legs and wings
twitching uncontrollably. What might the
insecticide have done to the bug’s nervous
system to cause this reaction?
Multiple Sclerosis is a disease in which the
nerve fibers in the CNS lose their myelin.
Why would this affect the person’s ability to
control their skeletal muscles?


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Most insecticides affect the nervous system
by disrupting the Acetylcholine Esterase
enzyme that regulates the neurotransmitter
acetylcholine
ACh accumulates in the synapse repetitively
stimulating receptors
Organophosphate pesticides were also used
in World War II as nerve agents due to similar
effects on humans
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Symptoms: visual disturbance, weakness,
clumsiness, paralysis, speech disturbance
Autoimmune disease
Myelin sheaths in CNS gradually destroyed
leaving lesions (scleroses)
Causes “short circuiting”, AP slows until
ceases
Axons not damaged and more Na+ channels
can appear


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Synapse – junction that mediates info transfer from neuron to neuron (or
effector)
 Presynaptic neuron – conducts impulse towards synapse
 Postsynaptic neuron-conducts impulse away from synapse
Electrical synapse (uncommon)
 Gap junctions between adjacent cells that allow for direct flow of ions
and small molecules
 Rapid transmission for synchronized activity (eye movements,
hippocampus, and embryonic nervous tissue)
Chemical synapse – release/receive neurotransmitters
 Axon terminal of presynaptic neuron w/synaptic vesicles filled w/thousands of
neurotransmitters
 Synaptic cleft – fluid filled space in between
 Neurotransmitter receptor on dendrite membrane
1.
2.
3.
4.
5.
Ca2+ channels open in presynaptic axon terminal
 When nerve impulse reaches axon terminal Ca2+ gated channels
also open w/Na+ channels, Ca2+ rushes in causing
Neurotransmitters are released
 Synaptic vesicles fuse w/membrane
 Ca2+pumped out, or taken in by mitochondria
Neurotransmitter binds to postsynaptic receptor
Ion channels open in the postsynaptic membrane
 Receptor changes shape, causing ion channels to open generating
graded potential
Neurotransmitter effects are terminated
 Degradation by enzymes
 Reuptake by astrocytes or presynaptic terminal
 Diffusion away from synapse
*Note: Synaptic delay – rate determining step b/s slower than AP



50+ have been indentified
Most neurons make 2 or more
Chemical Classifications
 Ach
 Amines
 Purines
 Amino Acids
 Peptides
 Dissolved Gasses

Functional Classifications
 Effects
▪ Excitatory – cause depolarization
▪ Inhibitory – cause hyperpolarization
▪ Both – dependent on receptor type
 Action Mechanism
▪ Direct - bind to ion channels
▪ Indirect – long lasting
▪ Intracellular 20 messenger


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Neurons function in groups
Neuronal pools – integrate incoming info in CNS
Circuits – patterns of neuronal pools
 Diverging circuits
▪ Amplify (1 triggers many, which each trigger many more)
▪ Sensory & motor
 Converging circuits
▪ Funnel or concentrating effect
▪ Different sensory can have same effect
 Oscillating (reverberating) circuits
▪ Chain of neurons w/colateral synapses (+) feedback
▪ Sleep-wake cycle, breathing, arm swing w/walk
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Reflex – involuntary response to
stimulus w/o requiring the brain
Particular stimulus always causes the
same response
Reflex arc- receptor  sensory
neuron Interneuron motor neuron
 effector
Ex. Knee jerk reflex
Babinski reflex (infants only)
 Stroke sole of foot  toes fan out
Plantar reflex (adults only)
 Stroke sole of foot toes curl
Signals sent to brain by interneurons
allow for control
 Ex. Toilet training, gag, blink