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Ch 8: Neurons: Cellular and
Network Properties
Objectives:
Describe the Cells of the NS
Explain the creation and propagation of
an electrical signal in a nerve cell
Outline the chemical communication and
signal transduction at the synapse
Organization
of NS
Afferent division
New 3rd division:
Enteric NS
Compare
to Fig 8-1
Efferent division
Cells of NS
• Nerve cell = Neuron
• Support cells =
Fig 8-2
Neuron: functional unit of nervous system
– excitable
 can generate & carry electrical signals
• Neuron classification either
structural or functional (?)
Fig 8-3
Axonal Transport
What is it? Why is it necessary?
Slow axonal transport (.2 - 2.5 mm/day)
Carries enzymes etc. that are not quickly
consumed – Utilizes axoplasmic flow
Fast axonal transport (up to 400 mm/day)
Utilizes kinesins, dyneins and microtubules
Actively walks vesicles up or down axon
Fig 8-4
Neuroglia cells
In CNS:
1.
2.
3.
4.
Oligodendrocytes
Astrocytes
Microglia (modified macrophages)
(Ependymal cells)
In PNS:
5. Schwann cells
6. Satellite cells
See Fig 8-5
Electrical Signals in Neurons
Changes in membrane potential are the basis for
electrical signaling
Only nerve and muscle cells are excitable (= able
to propagate electrical signals)
Review “resting membrane potential” (Ch 5)
Factors influencing membrane potential
1.
2.
Membrane Potential Review
Nernst equation describes equilibrium
potential for single ions
Goldman-Hodgkin-Katz (GHK) equation
considers contribution of all permeable ions
to membrane potential
Resting membrane potential of cell is
based on combined contributions of
conc. gradients and membrane
permeability for Na+, K+ (and Cl-)
Depolarization / Hyperpolarization
• Due to net movement of ions across cell
membrane
• Very few ions need to move for big change
in membrane potential  membrane
potential changes but ion conc. stays the
“same”
• Gated ion channels control membrane
permeability
– Mechanically gated channels
– Chemically gated channels
– Voltage-gated channels
Two Types of Electrical Signals
Graded potentials
variable strength
travel over short distances only
Action potentials
constant strength
travel rapidly over longer distances
initiated by strong graded potential
Start using 10 SYSTEM SUITE
Four Basic Components of Signal Movement
Through Neuron
1. Input signal (Graded Potential)
2. Integration of input signal at trigger zone
3. Conduction signal to distal part of neuron (=
Action Potential)
4. Output signal (usually neurotransmitter)
Input Signal: Graded Potentials
Location?
- How created?
Strength (= amplitude) reflects strength of
triggering event
Travel over short distances to trigger zone
Diminish in strength as they travel
May be depolarizing (excitatory) or hyperpolarizing
(inhibitory)
Figs 8-7/8
Graded Potentials
Diminished strength due
to
1. Current leak
2. Cytoplasmic
resistance
Fig 8-7
Subthreshold potential vs. Suprathreshold potential
Fig 8-8
Graded
potential
starts here
Trigger
zone
AP
Conduction Signals:
Action Potentials (AP)
Location ?
Travel over long distances
Do not loose strength as
travel
they
Compare to Fig 8-9
Are all identical
(all-or-none principle): 100mV amplitude
Represent movement of Na+ and K+ across
membrane
Ion Movement across Cell Membrane During AP
Sudden increase in Na+ permeability
Na+ enters cell down electrochemical
gradient (+ feedback loop for ~ .5 msec)
Influx causes depolarization of membrane
potential = electrical signal
What stops + feedback loop?
Na+ Channels in Axon Have 2 Gates
Activation gate and Inactivation gate
Na+ entry based on pos. feedback loop
 needs intervention to stop
Inactivation gates close in delayed response
to depolarization
Fig 8-10
 stops escalating pos. feedback loop
AP-Graph
has 3 phases
– Rising (Na+ permeability )
– Falling (K + permeability )
– “Undershoot” or ________
Absolute & Relative Refractory Periods
No movement of Na+ possible
Na+ channels
reset to resting
state; K+ channels
still open  > normal
stimulus necessary
Purpose of Refractory Periods
1. Limit signal transmission rate (no
summation!)
2. Assure one way transmission!
Forward current excites, backward
current does NOT re-excite !
Fig 8-15
Other Characteristics of APs
• Stimulus intensity encoded by AP frequency
• One graded potential triggers burst of APs
• Amount of NT released at axon terminal is
 to AP frequency
• One AP does not change ion conc. gradients
Conduction speed depends on . . . .
Axon diameter
Size constraints on axons become problem
with increasing organismal complexity
Fig 8-17
Membrane resistance
High resistance of myelin prevents current flow
between axon and ECF  saltatory
conduction from node to node
Fig 8-18
Alteration of Electrical Activity
• Various chemical factors responsible, e.g.:
Procaine blocks Na+ channels
• Altered potassium levels will change resting
membrane potential
– Hyperkalemia  ?
– Hypokalemia  ?
– Gatorade and other sports drinks
Output Signal: Cell to Cell Communication at
Synapses
Synapse = point where neuron meets
target cell (e.g. ?)
2 types
chemical
electrical
3 components of chemical synapse
presynaptic cell
synaptic cleft
postsynaptic cell
Chemical Synapses
= Majority of synapses
Neurotransmitters carry info from cell to cell
Axon terminals have mitochondria & synaptic
vesicles containing neurotransmitter
Fig 8-20
Events at the Synapse
AP reaches axon terminal
Voltage-gated Ca2+ channels open
Ca2+ entry
Ca2+ is signal for
neurotransmitter
release
Exocytosis of neurotransmitter containing
Fig 8-21
vesicles
Fig 8-21
3 Classes of Neurotransmitters (of 7)
1. Acetyl Choline
– Synthesized in axon terminal from acetyl
CoA and choline
– Quickly degraded by ACh-esterase
– Cholinergic neurons
– Different receptor types:
– nicotinc
– muscarinic
Fig 8-22
2. Amines
– Serotonin (tryptophane) and Histamine
(histidine)
– Dopamine and Norepinephrine (tyrosine)
– Widely used in brain, role in emotional
behavior (NE used in ANS)
– (Nor)adrenergic neurons
– Different receptor types ( and β)
3. Gases
– NO (nitric oxide) and CO
Postsynaptic Responses
Can lead to either EPSP or IPSP
Any one synapse can only be either excitatory or
inhibitory
Fast synaptic potentials
Opening of chemically gated ion channel
Rapid & of short duration
Slow synaptic potentials
Involve G-proteins and 2nd messengers
Can open or close channels or change protein
composition of neuron
Fig 8-23
Fig 8-23
Integration of Neural
Information Transfer
Convergence and divergence
Fig 8-26
Multiple graded potentials are integrated at
axon hillock to evaluate necessity of AP
Spatial Summation: stimuli from different
locations are added up
Temporal Summation: sequential stimuli added
up
Synapse: Most Vulnerable Step in
Signal Propagation
Many disorders of synaptic transmission,
e.g.:
•
•
•
•
Myastenia gravis (PNS)
Parkinson’s (CNS)
Schizophrenia (CNS)
Depression (CNS)
Mysterious paralysis