Excitable cells: Synaptic integration I

Question 1

Why is synaptic integration important?

Answer 1

Neurons receive multiple synaptic inputs
- from same and/or different neurons

Neurons provide multiple synaptic outputs
- to same and/or different neurons

• > synaptic integration enables information processing
• > integration of synaptic inputs determines nervous system function

Question 2

Parameters that affect synaptic integration?

Answer 2

Neuronal morphology and synaptic distribution

• Complexity of neuritic processes
• Distance of synapse to soma
• Relative position of synapses to each other

Synaptic properties

• amplitude of current flow at synapse
• multiple synaptic inputs are required to depolarise neuron sufficiently to trigger AP

Membrane properties

• length constant
• Time constant

Question 3

What is spatial synaptic integration?

Answer 3

Spatial summation - when multiple presynaptic neurons together release enough neurotransmitter to exceed the threshold of the postsynaptic neuron

e.g.
neurone A and neurone B may individually release insufficient neurotransmitter but when these quantities are combined, threshold may be exceeded and an action potential generated

Question 4

What is temporal synaptic integration?

Answer 4

Temporal summation occurs when one presynaptic neuron releases neurotransmitter many times over a period of time.

The total amount of neurotransmitter released may exceed the threshold value of the postsynaptic neuron.

The higher the frequency of the action potential the more quickly the threshold may be exceeded.

Question 5

WHat is the length constant? Why does length constant affect spatial integration?

Answer 5

Length constant used to quantify the distance that a graded electric potential will travel along a neurite via passive electrical conduction. The greater the value of the length constant, the farther the potential will travel

resistance of neuron membrane (rm) /
internal neuron resistance (ri)
length constant = sqrt(rm/ri)

Synaptic changes in membrane potential spread passively from site of synapse

• > Amplitude of synaptic potential change decays with distance from synapse
• > Decline/decay is determined by length constant

Question 6

Exponential function linked with amplitude decay?

Answer 6

Vm = VEPSPmax * e^(-x/(length constant))

length constant = lambda

Question 7

What is the effect of a long/short time constant on temporal summation?

Answer 7

Time constant = how quickly a neuron voltage level decays to resting state

Long time constant = temporal summation -> PSPs add up

Short time constant = no summation

Question 8

Time constant is dependent on which membrane properties?

Answer 8

Membrane resistance (Rm)

Membrane capacitance (Cm)

Time constant = Rm x Cm

• > Greater Rm = synaptic current doesnt leak as rapidly, PSP lasts longer
• > Greater CM = more charge resulting from synaptic current flow is stored and discharged after the synaptic current flow has stopped, PSP lasts longer