Interrogating the Nervous System [2]

Question 1

Review the differences between an EPSP, IPSP, and an action potential

Answer 1

• Excitatory postsynaptic potential: depolarization of the postsynaptic neuron; increases the probability of an action potential being fired
• Inhibitory postsynaptic potential: hyperpolarization of the postsynaptic neuron; decreases the probability of an action potential being fired
• Action potential: rapid conduction of an electric signal down the length of the cell as a result of depolarization of that cell to threshold (recall that threshold potential depends of the potential required to open voltage-gated sodium channels)

Question 2

Understand the “coupling” between electrophysiologic activity in the nervous system and CNS hemodynamics

Answer 2

Electrophysiologic activity is coupled to CNS hemodynamics via astrocytes. In areas of the brain with increased neuronal activity there is an excess of glutamate in the neuronal synapses. Astrocytes take up this excess glutamate.

Excess glutamate&raquo_space; high intracellular calcium levels» release of arachidonic acid» converted to EET by a P450 enzyme&raquo_space; EET released» hyperpolarizes vessel smooth muscle walls causing a decrease in vascular tone&raquo_space; increased blood flow.

So, increased electrophysiologic activity is “coupled” to an increase in blood flow in the same area of the brain

Question 3

Describe those techniques for evaluating “brain activity” that measure the ELECTROMAGNETIC properties of the nervous system

Answer 3

The electromagnetic properties of the nervous system can be measured clinically with an electroencephalogram (EEG), a magnetoencephalogram (MEG) or event related potentials (ERPs).

EEG: measures fluctuations in electric potentials at the scalp surface

MEG: measures magnetic fields produced by electric potentials to assess neuronal activity

ERP: EEGs recorded while a patient performs a specific task, creating characteristic patterns or positive and negative peaks during a task

Question 4

Describe those techniques for evaluating “brain activity” that measure the HEMODYNAMIC properties of the nervous system

Answer 4

Brain activity is measured hemodynamically with H215O PET scans, SPECTs and fMRIs.

PET: assesses blood flow, H215O is a radionucleotide tracer

SPECT: inject gamma-emitting radionucleotide. Measures blood flow

fMRIs: Changes in the ratio of deoxyhemoglobin to oxyhemoglobin produce changes in the MR signal that is reflected in the images produced

Question 5

Understand at a basic level the physiologic basis for the signal recorded in the EEG, the MEG, the fMRI, and the PET scan

Answer 5

Both EEG and MEG record electric signals. The electric signals are the result of the summation of electric potentials generated by groups of neurons within the cerebral cortex that run parallel to one another.

fMRIs and PET scans depict blood flow, which correlates with neuronal activity. fMRIs image blood flow based on the relative concentrations of deoxyhemoglobin and oxyhemoglobin within the blood (active brain area = higher oxyhemoglobin). PET scans use an isotope that is distributed into the blood. Increase in blood flow = increase in isotope picked up by PET