Transcript Transmission of Nerve Impulses

Honors Anatomy & Physiology
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neurons are electrically excitable
2 types of electrical signals:
1. graded potentials
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only for short distance communication
2. action potentials
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allow communication over both short & long
distances
both depend on 2 basic features of plasma
membrane in excitable cells:
1. existence of a resting membrane potential
2. presence of specific ion channels
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electrical voltage difference across the membrane
flow of ions thru ion channels => flow of current
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when open: allow specific ions to move across
plasma membrane down their electrochemical
gradient by diffusion
◦ ions move down their concentration gradient & (+)
charged cations move toward (-) charged anions
4 types:
1. leakage channels
2. voltage-gated channels
3. ligand-gated channels
4. mechanically gated channels
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channels randomly alternate (open/close)
typically are more K+ leakage channels than Na+
& K+ channels are leakier than Na+
◦ (so membranes more permeable to K+ than Na+)
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open in response to change in membrane potential
important in generation & conduction of action
potentials
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1.
2.
open/ close in response to a specific chemical
stimulus
ligands include:
◦ neurotransmitters
◦ hormones
◦ particular ions
work in 1 of 2 ways:
directly: ligand molecule itself opens/closes gate
indirectly: ligand activates another molecule which in
turn opens/closes gate
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gates open/close in response to mechanical
stimulation in form of:
◦ vibration (sound waves)
◦ pressure (touch)
◦ tissue stretching
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exists because there is small build-up of (-) charge
just inside plasma membrane  small build-up of
(+) charge just outside membrane
separation of charge source of PE
greater the difference in charge the larger the
membrane potential (voltage)
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neurons: membrane potentials range from
 -90 mv to -40 mv
(-) indicates the inside of cell is (-) relative to the
outside
cells with membrane potential are said to be
polarized
most body cells are polarized with membrane
potentials vary from +5 mv to -100 mv
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http://www.sumanasinc.com/webcontent/ani
mations/content/electricalsignaling.html
2 phases: (lasts ~ 1msec)
1. Depolarizing Phase
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(-) membrane becomes less (-)
Repolarizing Phase
2.
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normal membrane potential restored
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2 types of voltage-gated channels open & then
close:
◦ present mainly in axon plasma membrane & axon
terminals
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1st to open: Na+
◦ Na+ rushes into cell
◦ causes depolarization
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then K+ channels open
◦ K+ flows out of cell
◦ produces repolarizing phase
1.
2.
“all-or-none” principle
threshold: (~ -55mv) must be reached for AP to
occur

http://highered.mcgrawhill.com/sites/0072495855/student_view0/c
hapter14/animation__the_nerve_impulse.html
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period of time after an AP begins during which an
excitable cell cannot generate another action
potential
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AP must travel from trigger zone  axon terminal
this mode of travel called propagation or
conduction
◦ uses (+) feedback
◦ when Na+ flows into cell it causes voltage-gated Na+
channels in adjacent segments to open
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substances that produce their poisonous effects by
acting on nervous system
Japanese pufferfish: produces very lethal
neurotoxin TTX (tetrodotoxin) which blocks AP by
inserting itself into voltage-gated Na+ channels so
they cannot open
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drugs that block pain & other somatic sensations
◦ cold applied to area also produces anesthetic effect:
axons propagate slower
◦ ice partially blocks axon propagation of pain
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examples:
◦ Lidocaine, Novacaine
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act by blocking the opening of voltage-gated Na+
channels:
◦ sensory nerve impulses cannot travel past obstructed
region so pain signals never make it to the brain
2 types:
1. Continuous Conduction
2. Saltatory Conduction
3. http://www.blackwellpublishing.com/matth
ews/actionp.html
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4.
http://www.siumed.edu/~dking2/ssb/saltc
on.htm
larger diameter axons propagate impulses faster
than smaller diameter ones due to larger surface
area
 Classified:
1. A-fibers: (5-20 μm) travel 12-130 m/s
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2.
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3.
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used by somatic sensory neurons & motor neurons
B-fibers: (2-3 μm) travel 15 m/s
found in sensory neurons going from viscera  brain
C-fibers: ( .5-1.5 μm) travel 0.5 – 2 m/s
all unmyelinated, found in some sensory for pain
from skin and viscera & in autonomic motor fibers
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presynaptic neuron: neuron sending the AP
postsynaptic neuron: neuron receiving the AP
synapse: space between the 2
synapses can be:
1. electrical
2. chemical
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common in visceral smooth muscle, cardiac
muscle, & in developing embryo
 2 advantages to electrical synapses
1. faster communication
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AP passes directly from presynaptic neuron 
postsynaptic neuron
synchronization
2.
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electrical synapses can coordinate the activity of a
group of neurons or muscle fibers
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synaptic cleft: space between filled with interstitial
fluid
in response to AP axon terminal of presynaptic
neuron releases neurotransmitters that diffuse
across the synaptic cleft bind to receptors in
plasma membrane of postsynaptic neuron 
producing a postsynaptic potential
electrical signal (AP)  chemical signal
(neurotransmitters)  electrical signal (AP)

http://www.mind.ilstu.edu/flash/synapse_1.s
wf
occurs in 3 ways:
1. diffusion
2. enzymatic degradation
3. uptake by cells
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2 classes:
1.
small-molecule neurotransmitters
2. neuropeptides
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nervous system exhibits plasticity: capability to
change based on experience
@ level of individual neurons:
◦ sprouting new dendrites
◦ synthesis of new proteins
◦ changes in synaptic contacts with other neurons
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limited capacity to replicate or repair itself
until recently: thought was little or no repair done
to damage in CNS: have found neuroglial cells
more capable of this than previously appreciated
PNS: as long as cell body intact & Schwann cell
active new axon can be regenerated
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Multiple Sclerosis (MS)
progressive destruction of myelin sheaths of
neurons in the CNS
autoimmune disorder
afflicts ~ 350,000 in USA (estimated) with:
◦ >2x female/male
◦ white > other races
◦ diagnosis difficult but most diagnosed ages 20 - 50
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unknown but do see:
genetic susceptibility: having 1st degree relative
with it increases your chances several-fold
possible association to living farther away from
equator (?) so maybe something to do with vitamin
D
possible association with history of herpes-6,
Epstein Barr virus, chlamydia
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characterized by short, recurrent attacks of motor,
sensory, or psychological malfunction
afflict ~1% of world’s population
http://www.dailymotion.com/video/xaigxc_b
rain-animation-of-epilepticseizu_creation#.UaPsb0B-8TY
brain damage
1.
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metabolic disturbances
2.
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encephalitis or meningitis
toxins
4.
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6.
hypoglycemia, hypocalcemia, uremia, hypoxia
infections
3.
5.
most frequently from birth trauma
alcohol, tranquilizers, hallucinogens
head injuries
tumors & abcesses of the brain
1.
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2.
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3.
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Guillain-Barre Syndrome (GBS)
acute demyelinating disorder in which macrophages
strip axons of myelin in PNS
may be response of immune system to bacterial
infection
Neuroblastoma
most common tumor in infants
immature neurons (neuroblasts) grow in abdomen or
adrenals
Neuropathy
any disorder that affects the nervous system but
particularly a disorder of a cranial or spinal nerve
example: Bell’s palsy