Signal Transduction I

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  Signal Transduction I Electrical Signal in
Nerve Cells

• Neurons
• Cell body
• Dendrites
• Axon
• Neuroglial cells - supporting cells
• Exp., Shwann cells

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Electrical Activity in Neurons All animal
bodies have bio-electricity, which is related
with cell membrane potential - charge difference
between the two sides of the cell
membrane. Membrane potential is due to the
uneven distribution of ions. Potential
energy of position.
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• Resting Potential membrane potential at resting
  status
• Outside Inside
• Na 145 mM 10 mM
• K 5 mM 140 mM
• Inside of the cell membrane there are also PO4
  and
• SO4, which cannot cross the membrane freely
  (Fig. 9-5).
• Net result outside is more positively charged
  than
• inside - the membrane is polarized resting
  potential,
• cytoplasm is slightly negative, extracellular
  environment is
• slightly positive.

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Membrane potential is largely determined by the
membrane permeability. The resting membrane is
more permeable to K, K constantly leaks out
K inside the cell is not sufficient to
neutralize the negative charge of the
anions).  
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When the cell is not stimulated, the membrane
potential is called resting potential, and the
cell membrane is polarized.   When resting
potential is reduced - outside becomes less
positive and inside less negative
depolarization.
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All cells have a membrane potential, but only a
few types of cells can change membrane potential
in response to stimulation. These are called
excitable cells (muscle and nerve cells).
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• Electrical excitability
• Electrically excitable cells can respond to
  electrical
• and chemical stimulation and produce action
  potential.
• Change of membrane potential is due to the
  change of
• membrane permeability ion channels play the key
  role.
• Some ion channels can be opened by membrane
• depolarization - voltage gated channels
• Some channels are sensitive to chemicals such as
  
• neurotransmitters - ligand gated channels.

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• Action Potential
• After stimulation, cell membrane depolarizes
• When depolarization reaches to a threshold
  potential, Na channels open ? Na flood in ?
  potential reverses it changes from -70 mV to 30
  mV. (The Na channel is voltage regulated.)
• K channels open ? K flood out ? charge
  difference is restored, cytoplasm becomes
  negative again - repolarization - cell membrane
  to return to
• resting potential (-70 mV).

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Na influx
Kefflux
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• An action potential is a brief, but large
• electrical change caused by influx of Na
• and efflux of K.
• Action potential is all or none, the gates
• are either completely open or closed - once
• depolarization reaches the threshold, the
• membrane potential shoots to the limit, 30
• mV
• The magnitude and length is independent of
• the strength of the stimulation.

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From -70 mV to 30 mV to -70 mV lasts 3 msec.
The amplitude of action potential is about the
same in all neurons at all times.
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Refractory periods During the time when a region
of membrane is producing action potential, it is
not able to respond to another stimulus. The
cell membrane is said to be refractory.
Cardiac muscle cells have long refractory
period, therefore, two contractions will not
overlap.
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Nerve Impulse - signal transduction inside of a
neuron Depolarization of one region of a nerve
fiber will create a charge difference between
two adjacent regions.

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The local current will stimulate the adjacent
membrane to have action potential, which will
trigger another action potential in the next
area A wave of action potentials will move down
the nerve fiber to form an electrical current -
nerve impulse.
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• Conduction of impulses
• In a myolinated axon (Nerve fiber rapped with
• myoline sheath the insulation material)
• Ion channels only exist in the gaps between
• Schwann cells - nodes of Ranvier
• Action potential is therefore only produced in
  the
• nodes of Ranvier.
• Axon acts as a cable to allow impulses leap from
  node
• to node.

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Conduction Between Cells - Synaptic
transmission (connection between neurons or
between a neuron and an effector cell)
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A. Gap junctions electrical synapse Tight
connection between the two adjacent cells. Two
membrane are fused in certain small Areas.
Channels run through the two attached membrane
allowing ions and molecules to flow through.
Gap junctions in cardiac muscle ensure
coordinated contraction.
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• B. Chemical Synapse
• More often, the conduction of impulses between
• cells is chemical.
• Chemical synapse can also exist between
• neurons or between a neuron and other cells.
• For exp., a neuromuscular junction is a synapse
• between a motor neuron and a muscle cell.
•  

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• Neurotransmitters are
• stored in synaptic vesicles
• in the terminal bulb.
• When an action potential
• reaches to the end of the
• presynaptic fiber, Ca ?
• vesicles fuse to the membrane
• and neurotransmitters are
• released through exocytosis.
• Neurotransmitters diffuse
• across the cleft and bind to
• their receptors on the
• postsynaptic membrane.

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A chemical synapse contains a presynaptic
membrane, a postsynaptic membrane and synaptic
cleft. Neurotransmitters chemical messengers
released by the presynaptic cell, cross the
synaptic cleft and reach the postsynaptic
membrane.

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The binding between neurotransmitter and
receptor causes the membrane permeability of
postsynaptic membrane to change... Chemical
regulated channels can open or close in response
to neurotransmitters.
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The opening or closing of chemical regulated ion
channels may lead to two different consequences,
excitatory or inhibitory actions Some may
produce depolarization - excitatory postsynaptic
potential. Some may cause hyperpolarization (the
inside becoming more negative) inhibitory
postsynaptic potential.
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• EPSP will stimulate the postsynaptic cell
• to produce action potential and cause
• excitatory effect.
• IPSP will prevent action potential and
• cause relaxation effect.

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• Neurotransmitters can be excitatory or
• inhibitory
• Catecholamines (excitatory) epinephrine
• (adrenaline), norepinephrine, and dopamine.
• Serotonin (inhibitory).
•  
• Aetylcholine (ACh) the neurotransmitter
• released by a motor neuron to excite skeletal
• muscles, can induce either effects.

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Neurons that secrete adrenaline are called
adrenergic neurons those that secrete
acetyl- choline are called cholinergic neurons.
Postsynaptic receptors of adrenaline are called
adrenergic receptors those of acetylcholine
are called cholinergic receptors.
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Actylcholine (ACh)   The postsynaptic effect of
ACh can be both excitatory and inhibitory. In
skeletal muscle, ACh's effect is excitatory, but
in CNS it can be either excitatory or
inhibitory.  
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Acetylcholinesterase (AChE) After the action,
some neurotransmitters are decomposed by enzymes
that exist in synaptic cleft. Some are
recycled back to the nerve endings and will be
reused for the next action.
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Shortly after being released, the free ACh is
degraded by acetylcholinesterase that exists in
the synaptic cleft, so that muscle contraction
will seas. This enzyme can be inhibited by
nerve gas or tetanus toxin, which leads to
skeletal muscle spasm - spastic paralysis.
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