Graded potential

Graded potentials

signal molecules bind to receptors, ion channels open, ions move down concentration gradients

• for ex: Na+ influxes through the plasma membrane and causes a small depolarization that dissipates over space and time

→ more Na+ ions at the point of entry;less further away

• More signal molecules received → more binding to receptors → more channels open → more ions move → greater graded potential in target cell

When a neuron binds chemical signal molecules at the dendrites or cell body, it experiences one of two types of graded potential

• Excitatory post-synaptic potentials (EPSP)

• Inhibitory post-synaptic potential (IPSP)

Excitatory post-synaptic potential

• depolarization occurs in the dendrites or cell body

• These are caused by neurons binding excitatory neurotransmitters

• Movement of Na+ and Ca++ would cause this

Inhibitory post-synaptic potential

• hyperpolarization occurs in the dendrites or cell body

• these are caused by neurons binding inhibitory neurotransmitters

• movement of Cl- and K+ would cause this

If the neuron’s “trigger zone detects sufficient depolarization from the sum of all EPSPs and IPSPs in a short period of time, an action potential will be created

• the trigger zone is the axon hillock in most neurons

• The sum of all EPSPs and IPSPs must reach a threshold voltage (typically -55mV in neurons) to generate an action potential

→ an all-or-nothing electrical (depolarization) signal

→ action potentials are not large or small - they are the same every time

Graded potentials dissipate over time and space

Subthreshold and suprathreshold graded potentials in a neuron

• Sub = below; no action potential is generated at the axon hillock

• Supra = above; an action potential will be initiated at the axon hillock

→ voltage gated channels are opened at the axon hillock when the threshold is reached (~55mV)

Integration of multiple graded potentials

• Neurons receive MANY inputs from other neurons (perhaps hundreds or even thousands)

• Neurons must integrate many excitatory and inhibitory stimuli

• Integration in the summation of these excitatory and inhibitory signals received by a postsynaptic neuron

Integration in the summation of these excitatory and inhibitory signals received by a postsynaptic neuron

this integration occurs through spatial and temporal summation of multiple stimuli

What is spatial summation?

• multiple excitatory inputs

• when more than one pre-synaptic neuron stimulates the post-synaptic cell simultaneously

→ all the EPSPs and IPSPs add up across the surface area of the neuron to determine if threshold is reached or not

• All stimuli are excitatory in this example

Special summation continued (a mix of excitatory and inhibitory inputs)

• simultaneous excitatory AND inhibitory signals can co-occur across the surface of a neuron

• Sum of all determines activity state of this neuron

What is temporal summation?

• Temporal = relating to time

• a kind of integration influenced by the frequency or timing of incoming signals

• In this example, no temporal summation occurs, because EPSPs are too far apart in time (10 milliseconds)

• If there is an increase in the rate of the incoming signals, then temporal summation occurs

Temporal summation occurring

• increase frequency or rate of EPSP

→ they will have an additive effect

• as seen in this figure, the neuron receives a second signal before it has returned to resting state

• These two EPSPs, very close in time (separated by only a few milliseconds), add together and the neuron reaches its threshold voltage

→ therefore, an action potential fires

Summary of electrical signals in neurons

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