Nervous System

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Nervous System

• Brain, spinal cord, efferent and afferent neurons
• Pattern of information flow

Receptor Afferent path Integration
Efferent Path Effect
Central Nervous System (CNS)

• Main cell types are neurons and glial cells

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Typical Arrangement of Neural Connections

• Neurons communicate via electrical signaling
• They are excitable
• Structurally the soma (cell body) has an
  extensive ER and prominent nucleoli
• Long appendages or processes
• Dendrites (receive info)
• Axons (deliver info) some are covered by myelin

A collection of axons is called a NERVE
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Types of glial cells CNS oligodendrocytes,
astrocytes, microglia, ependymal cells PNS
Schwann cells, satellite cells
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Myelin acts as an insulator and inhibits ion
movement in the axonal membrane that is surrounds.
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Neurons as Excitable Tissue

• Excited by altering the resting membrane
  potential (-90 mV)
• Depolarize
• Hyperpolarize
• Most changes in membrane potential occur through
  the opening or closing of certain ion channels
  (they are voltage-gated).

Ion Intracellular (mM) Extracellular (mM)
Na 2 140
Cl- 10 105
Ca2 10-8 2.5
K 150 5
Proteins (-) 65 2
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What will happen to the resting membrane
potential if the activation gate is opened? How
could a cell open this activation gate?
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• Gates can be chemically opened by
  neurotransmitters
• Gates can be opened via signal transduction
  mechanisms linked to neurotransmitter binding to
  receptor
• Gates can be opened by stretch, pressure, etc.

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Stimulus anything that can cause the opening or
closing of gated channels in a neuronal membrane
What happens to the resting membrane potential of
the membrane adjacent to the site of Na
entry? How about here?
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The axon hillock (trigger zone) is sensitive to
changes in ion concentration and is the site at
which an action potential is initiated. An action
potential is a self-propagating depolarization of
the axonal membrane that initiates at the hillock
and runs to the axon terminus without diminishing
in strength.
What determines whether an action potential will
occur or not?
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If the graded potential doesnt change the
resting membrane potential enough, the signal
from the stimulus will die out and the neuron
will not respond with an action potential. The
amount of change in membrane potential necessary
to generate an action potential is called a
threshold stimulus.
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Action Potential depolarization along the axon
1
3
2
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If the trigger area of the axon reaches
threshold, the influx of Na and the generation
of the action potential will be repeated over and
over again in one direction, at each segment of
membrane, down the axon.
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What will happen at this area of membrane?
What will happen at this area of membrane?
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One portion of the membrane has just been
depolarized and is relatively insensitive to
changes in cation concentration. It is said to
be refractory to stimulus. Downstream membrane
is at resting potential, and can be influenced by
cation influx.
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Saltatory conduction in myelinated neurons
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Action potentials cause the release of
neurotransmitter from the presynaptic axon
terminus
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Strength of stimulus determines neuronal response
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EPSP
mV
time
time
mV
time
IPSP
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Neurotransmitter activity is stopped by
diffusion away from the synapse, transport into
cells (glial or back into presynaptic neuron), or
degradation by specific enzymes.
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What is the response in the post-synaptic neuron?
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What will determine whether this postsynaptic
neuron will respond?
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Red neuron is releasing serotonin which causes an
IPSP. The neuron is firing at 70 APs/sec Neuron
A is releasing dopamine, causing and EPSP. The
neuron is firing at 40 APs/sec Neuron B is
releasing acetylcholine to create an EPSP. It is
firing at 20 APs/sec. What will the outcome be in
the postsynaptic cell?
A
B
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Neurotrans. Types of receptors Mode of action Result in postsynaptic cell Target
Acetylcholine Nicotinic Muscarinic Opens ion channels EPSP CNS neurons skeletal muscle
Serotonin To main classes multiple subclasses G-protein coupled receptors both AC and IP3/DAG Depends on receptor type Platelet aggregation, smooth muscle contraction, satiety, vomiting
GABA GABA-A GABA-B Receptor Cl- channel G-linked K channel IPSP in all cases Throughout CNS and in retina
Norepinephrine Receptor b receptor G-protein linked to cAMP G-protein linked to cAMP IPSP EPSP Relaxes smooth muscles of gut, bronchial tree, and vessels to skel. muscle Increases rate and strength of cardiac contraction excites smooth muscle in vessels
Dopamine D1, D2, D3, D4, and D5 G-protein linked to cAMP, direct channel opening, cAMP to K channel opening EPSP and IPSP D1-3 are located in the striatum of the CNS, and the basal ganglia D3-5 play a role in mood, psychosis and neuroprotection