Week 11

Chapter 16: Electroencephalogram and Sensory-Evoked Potentials

Overview of Electrophysiological Techniques

  • Detection of Electrical Activity

    • Electrical behavior of excitable cells can be detected by macroelectrodes on the body.

    • Techniques rely on Volume Conduction Theory:

    • Describes ionic current spread in extracellular fluid from a group of neurons/muscle cells to distant points (e.g., skin).

    • Ionic currents produce waveforms characteristic of their tissue origin.

  • Common Electrophysiological Recordings:

    • Electrocardiogram (ECG) from cardiac muscle (Chapter 20).

    • Electromyogram (EMG) from skeletal muscle (Chapter 6).

    • Electroretinogram from the retina (Chapter 14).

  • Electrophysiological Tools Introduced:

    • Electroencephalogram (EEG)

    • Reflects spontaneous brain activity.

    • Sensory-Evoked Potentials

    • Measure potentials evoked by stimulation of tissue or sensory receptors.

Histology and Electrophysiology of Cerebral Cortex

  • Cortical Functionality:

    • Different regions have unique functions:

    • Motor Cortices: Initiate skilled, learned, voluntary movements.

    • Occipital Cortex: Processes visual information from retina.

    • Temporal Cortex: Processes auditory information from the ear.

  • Common Histological Features:

    • Despite functional differences, all cortical areas share similar histological features.

    • Cortical Synaptic Processing: Shares common characteristics across regions.

    • Functional differences mainly arise from input signal origins and output destinations.

  • Cerebral Cortex Neuron Types:

    • Pyramidal Cells:

    • Pyramid-shaped cell bodies.

    • Dendrites project towards pial surface, reach and branch within Layer I.

    • Projection neurons; excitatory at axonal synapses.

    • Stellate Cells:

    • Star-shaped appearance; local-circuit interneurons.

    • Can be both excitatory or inhibitory.

  • Layered Arrangement:

    • Six layers (I-VI) in the cortex serve distinct roles in processing.

    • Majority of thalamic input arrives primarily at Layer IV.

  • Glial Cells:

    • More abundant than neurons in the cortex.

    • Types: Astrocytes, Oligodendrocytes, Microglia.

    • Functions:

    • Do not generate action potentials but modulate neuronal activity.

    • Clear excess potassium, neurotransmitters, and toxins.

    • Support synapse formation and stabilize neuronal positions.

Electroencephalogram (EEG)

  • Definition and Measurement:

    • First known recordings since the 1930s.

    • Fluctuating electrical voltage from brain activity recorded via scalp macroelectrodes.

  • EEG Waveform Characteristics:

    • Frequency inversely related to amplitude:

    • Alert state: High-frequency, low-amplitude.

    • Relaxed state: Lower frequency, higher amplitude.

    • Sleep state: Slow-wave, high-amplitude.

  • EEG Frequency Ranges:

    • Alpha: 8-13 Hz

    • Beta: 13-30 Hz

    • Theta: 4-7 Hz

    • Delta: 0.5-4 Hz

  • Clinical Applications:

    • Used for diagnosing brain diseases, epilepsy classification, localizing lesions, and defining brain death.

Collective Behavior of Cortical Neurons

  • Voltage Changes as EEG:

    • Voltage recorded represents postsynaptic potential changes in neurons below the electrode.

    • Excitatory Postsynaptic Potentials (EPSPs): Positive ions enter cells, creating a negative extracellular environment beneath the electrode, recording a positive voltage.

    • Inhibitory Postsynaptic Potentials (IPSPs): Reverse this polarity relationship.

  • Synchrony and Desynchrony in EEG:

    • Synchronized EEG: High-amplitude, slow-frequency; due to many neurons firing in unison.

    • Desynchronized EEG: Low-amplitude, high-frequency; results from random firing of neurons.

  • Regulation of EEG Frequency:

    • Modulated by the Reticular Activating System (RAS):

    • Ascending projections from the reticular formation play a crucial role in consciousness, arousal, and attention.

Sensory-Evoked Potentials

  • Recording Techniques:

    • Can record large brain and spinal cord areas not reflected in EEG through averaging techniques to extract sensory responses from the EEG.

    • Sensory-Evoked Potentials: Changes observed after stimulating sensory pathways; termed far-field potentials.

  • Brainstem Auditory-Evoked Responses (BAER):

    • Measures electrical events from the auditory pathway post-stimulus (click sound) in the brainstem.

    • Typically seven waves recorded, linked to specific auditory pathway structures (e.g., cochlea, cochlear nuclei).

    • Utilized for assessing brainstem and auditory function.

Clinical Correlations

  • Brain Tumor Case Study:

    • Symptoms in a 13-year-old Boston Terrier, including seizures and altered awareness.

    • EEG findings indicate slower dominant frequency with increased amplitude over affected cortical areas, suggesting a focal lesion (potential tumor).

    • Diagnostic imaging (CT/MRI) recommended to evaluate tumor presence.

Practice Questions

  1. Which of the following regarding EEG is false?

    • a. Its measurement relies on volume conduction.

    • b. It predominantly measures postsynaptic potentials in the cerebral cortex.

    • c. It is commonly used to measure the activity of a small number of neurons.

    • d. It is a measure of spontaneous activity of brain tissue.

    • e. Both a and d.

  2. A lesion in which structure would least likely affect EEG?

    • a. Cerebral cortex

    • b. Thalamus

    • c. Hypothalamus

    • d. Cerebellum

    • e. Reticular activating system

  3. Which statement is true?

    • a. A slow-frequency, high-amplitude EEG is said to be “desynchronized.”

    • b. The EEG alone confirms brain death.

    • c. There are periods of high-frequency, low-amplitude during sleep.

    • d. The EEG is typically measured in response to sensory receptor stimulation.

  4. The BAER requires averaging out random background EEG activity. True or False?

  5. A brain tumor may cause focal slowing of the EEG from surrounding brain tissue. True or False?

Bibliography

  • Bagley RS: Fundamentals of veterinary clinical neurology, Ames, Iowa, 2005, Blackwell Publishing.

  • Bear MF, Connors BW, Paradiso MA: Neuroscience: exploring the brain, ed 3, Philadelphia, 2007, Lippincott, Williams & Wilkins.

  • Ducote JM, Dewey CW: Neurodiagnostics. In Dewey CW, ed: A practical guide to canine and feline neurology, Ames, Iowa, 2003, Iowa State Press.

  • Kandel ER, Schwartz JH, Jessell TM, eds: Principles of neural science, ed 4, New York, 2000, McGraw-Hill.