Learning Objectives:
Differentiate ionic basis and
characteristics of grade potentials
and action potentials
Explain how neurons integrate
graded potentials through
temporal and spatial summation
QUIZ/TEST REVIEW NOTES
SECTION 1 NEUROPHYSIOLOGY
[GRADED AND ACTION POTENTIALS]
1. TWO TYPES OF ELECTRICAL SIGNALS
a. Overview
1. Neurons integrate many incoming stimuli = GRADED potentials
2. Neurons transmit information over distances to larger for diffusion = ACTION potentials
2. GRADED POTENTIALS AND CURRENT FLOW
a. Graded Potentials Definition:
- Are depolarization’s or hyperpolarizations that occur in the dendrites/cell body
- Graded = Size/amplitude is directly proportional to strength of triggering event;
Ex: Large stimulus = Strong graded potential/MORE charge
Small stimulus = Weak graded potential/LESS charge
- Travel over short distances and LOSE Strength as they travel through cell
- If depolarizing graded potentials is strong enough at integration region it will initiate
action potential
[Local Current Flow = the depolarized wave that moves through
the cell created by positively charged Cation; waves ripples that
spread outward from points of entry/AXON HILLOCK]
(Current = in biological systems is the net movement of
positive electrical charge)
[Reasons graded potentials lose strength as move through cytoplasm
[1] Current leak = positive charges leak across membrane/neuron cell
body is not good insulator and has open leak channels allowing positive
charge to flow out into E.C.F
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[2] Cytoplasmic Resistance = cytoplasm provides resistance to flow of
electricity just was water creates resistance that diminishes waves from
a stone thrown
b. Integration (trigger Zone)
(1) DEFINITION
- Known as the trigger zone, is integrating center of neuron and contains
high concentration of voltage-gated Na channels
- Triggering zone locations differ from efferent/inter-neurons to sensory neurons
Efferent/Inter Neurons
Trigger Zone
Axon Hillock/Initial Segment of Neuron
Sensory Neurons
Adjacent to receptor/where
dendrites joins the axon
- Trigger Zones depolarized = Action Potential EXCITATORY
- Trigger Zones hyperpolarized = No Action Potential INHIBITORY
NOTE: For
excitatory/inhibitory
graded potentials go
to page 3
(2) SPATIAL SUMMATION
[POSTSYNAPTIC MODULATION]
- Initiation of an action potential from several simultaneous graded potentials
- Multiple EPSP that fire simultaneously to go suprathreshold, creating A.P.
- Spatial = graded potentials originate at different locations on neuron
- Convergent Pathway information integration transfer = presynaptic neurons
converge to influence small number of postsynaptic neurons
(Requires input from more then one presynaptic neuron)
2
Figure 27 b : Postsynaptic Inhibition: Presynaptic neuron releases inhibitory neurotransmitter onto postsynaptic
cell; Totaling TWO excitatory and ONE inhibitory onto postsynaptic cell; the one IPSP lowers the graded potential
sum at trigger zone creating a subthreshol signal (another postsynaptic inhibition on 8-29 b)
(3) TEMPORAL SUMMATION
[POSTSYNAPTIC MODULATION]
- Two subthreshold potentials arriving in trigger zone in rapid succession
- Summation that occurs from graded potentials overlapping in time
- Two subthresold graded potentials from SAME PRESYNAPTIC neuron can
be summed if they arrive at trigger zone close enough in time
KEY: IMPORTANT FOR ALL SUMMATION: Postsynaptic Integration allows neuron to evaluate strength and
duration of multiple signals
c. Excitatory graded potentials (EPSPs = Excitatory Postsynaptic Potential) reaching threshold at
trigger zone initiate Action Potentials
- Depolarized graded potentials flow through cytoplasm increasing chance of A.P
- If total/sum excitatory graded potentials reach suprathreshold at Axon Hillock/Trigger
zone, an action potential occurs
d. Inhibitory graded potentials (IPSPs = Inhibitory Postsynaptic Potential) hyperpolarize the cell
- Hyperpolarized graded potentials flow through cytoplasm decreasing chance of A.P.
- If total/sum of inhibitory graded potentials doesn’t reach suprathreshold of Axon
Hillock, action potential will not occur
3
Note: Presynaptic
Modulation starts on
page 279
e. Presynaptic Modulation
[PRESYNAPTIC MODULATION]
- Provides more precise means of control than postsynaptic modulation
- Modulators can alter synaptic activity by changing identity, affinity, and number of
neurotransmitter receptors to alter responsiveness of target cell to neurotransmitters
[Neuromodulators]
(a) PRESYNAPTIC INHIBITION
- Presynaptic inhibition on a divergent neuron allows selective
modulation of collaterals and their target cells (8-29 A)
(b) POSTSYNAPTIC INHIBITION
- If the responsiveness of a neuron is altered at the dendrites and cell
body, all target cells of the neuron will be affected equally (8-29 B)
Examples:
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