Nerve cells and neural connectivity

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Nerve cells and neural connectivity
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Some Comparisons between Endocrine and Neural
Communication Endocrine secrete chemical at
one site uses circulation to act at another
site Relatively slow (0.1 m/sec) Acts on many
cells Can sustain action for long periods of
time Neural nerve cell to nerve cell or
nerve cell to effector cell (e.g.,
muscle) Extremely fast can travel 50-80
m/sec Very distinct effects Connection often
acts on one cell
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An Overview of the Vertebrate Nervous System
1. How does the nerve cell carry information? 2.
How does the information go from nerve to
nerve? 3. How does the effector cell respond?
central nervous system
peripheral nervous system
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Terminology NEURONS and NERVES
Dendrites
Anatomy of a neuron
Cell body
Nucleus
Synapse
Signal direction
Axon
Synapticterminals
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
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Principal functions of nerves
EXCITATION Getting the cell activated
CONDUCTION Moving the stimulus along
TRANSMISSION Communication between neurons
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The RESTING MEMBRANE POTENTIAL
How does this battery come about? How do we use
it for communication?
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The basis for the membrane potential
Three major proteins in the plasma membrane
Neuron
Na channel
K channel
Dendrites
protein
protein
Na/K pump
Cell Body
protein
Axon
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Physical basis for a membrane potential
Na K in mM 150 5
outside
15 150
inside (cytoplasm)
Kin Kout
NERNST equation
RT zF
EK (in mV) - ln
Kin Kout
RT zF
Nain Naout
EK - 60 log
ENa (in mV) - ln
150 mM 5 mM
15 150
EK - 60 log
ENa (in mV) - 60 log
EK - 60 x log 30 - 60 x 1.48
ENa 60 mV
EK - 89 mV
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Physical basis for a membrane potential
Lets say we have high K permeability (i.e.,
have a K channel) and no Na permeability
Na K in mM 150 5
Given this concentration gradient and exclusive
permeability to K we can predict Em EK -89 mV
outside
inside (cytoplasm)
15 150
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Now, lets say we have high Na permeability
(i.e., have a Na channel) and no K permeability
Na K in mM 150 5
outside
inside (cytoplasm)
15 150
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Soooo, the membrane has completely flipped its
polarity.. from high K permeability,
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outside
inside (cytoplasm)
K 150
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So, what are these things little membrane
cylinders?? They are Channel Proteins
Structure of an ion channel in the membrane
A ribbon diagram showing a side view of the
"inverted teepee" shape of a KcsA potassium
channel. The channel is shown embedded in a cell
membrane with the extracellular face above and
the cytoplasmic face below.
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The action potential - activating the neuron
-70
also called Spike
The larger the stimulus, the larger the
depolarization.
The larger the stimulus, the larger the
hyperpolarization.
The size of the spike is independent of the
stimulus size
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The molecular basis for the action potential
Role of Na K channels
Refractory period


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What is the Role of the SODIUM PUMP?
virtually no direct role ATP driven Na for K
exchange restores ion gradients
Na
ATP
Na
P
K
K
ADP
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CONDUCTION How does an action potential move
along a neuron?
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CONDUCTION How an action potential moves along
a neuron?
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What have we learned from a cephalopod, the squid?
Mouse (mammalian)
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A couple of points about speed of nerve
conduction
1. Bigger nerves carry impulses faster than
smaller ones
2. Myelinated nerves carry impulses much faster
than unmyelinated ones
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SALTATORY CONDUCTION How does myelination help
an action potential move along a neuron?
node of Ranvier
axon plasma membrane
myelin sheath
axon
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Principal functions of nerves -- a review
EXCITATION Getting the cell activated Action
potential - due to (1) ion gradients across
membrane (2) selective perm. changes in Na
and K channels
CONDUCTION Moving the stimulus along Local
depolarization event stimulates subsequent
region Speed depends on (1) size of
axon (2) myelination
TRANSMISSION Communication between
neurons Connection between nerve/nerve,
nerve/muscle nerve/gland is almost always via
chemical transmission and not electrical
Connection is called a SYNAPSE
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TRANSMISSION How does an action potential move
neuron to neuron? ...or from neuron to effector
cell?
Structure of a SYNAPSE
Presynaptic membrane
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Sequence of pre-synaptic events
1. Action potential travels down axon 2.
Voltage-sensitive Ca2 channels open at axon
terminal 3. Ca2 entry promotes vesicle docking
4. Vesicles release their transmitters
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For Nerve-Muscle Synapse Acetylcholine (ACh) is
the Usual Neurotransmitter
Sequence of post-synaptic events
ACh acetylcholine
pre-synaptic membrane
post-synaptic membrane
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Transmission Multiple synaptic inputs occur on
each neuron
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Transmission Excitatory and Inhibitory synaptic
inputs increase the degree of neuronal regulation
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Many different neurotransmitters operate in the
nervous system
Neurotransmitter Function
Location
Acetylcholine Excitatory in
skel musc. Nerve/musc junctions
CNS ANS Biogenic Amines
Norepinephrine
Ex. or Inhib.
CNS ANS Dopamine
Ex. (sometime Inhib.)
CNS ANS
Serotonin
generally Inhib.
CNS Amino Acids GABA
Inhib.
CNS invert NM j
glycine
Inhib.
CNS glutamate
Excit.
CNS invert NM j aspartate
Excit.

CNS Neuropeptides e.g., endorphins
Inhib.
CNS