VPB 121: Veterinary Physiology - II (2+1)

ELECTRO ENCEPHALO GRAM

• It is the graphic record of electrical activity of the brain recorded from the surface.
• Technique is known as Electro Encephalography
• It is the recording of the potential difference as rhythm (frequency) and magnitude.
• Rhythm of the EEG are designated as alpha, beta, theta and delta rhythm

Alpha rhythm

Frequency	Amplitude	Characteristics
8-13 cycles/sec	50-100 mv	Seen in subject when awake relaxed state and closed eyes.

• Replaced by faster wave <---- alpha blocking <----> when respond to stimuli and wave disappear (Not responding to any stimuli)
• Alpha rhythm represents state of relaxed wakefulness of reflex the optimal state of brain excitability namely synchronized rhythm.

Beta rhythm

Frequency	Amplitude	Characteristics
More than 20-30 cycles/sec	25-50mv	Seen in alert cortex wakefulness and responding to a stimulus knowns as desynchronized rhythm.

Theta rhythm (slow rhythm)

Frequency	Amplitude	Characteristics
37 cycles/sec	50-100 mv	dominant in wakefuness is young subjects. seen in early stages of slow wave sleep from hippocampus.

Delta rhythm

Frequency	Amplitude	Characteristics
Less than 3 cycles/sec	High amplitude < 100 mv	stages of slow wave sleep

• Lamda wave - have high amplitude (from fronto-temporal regions)
• Mu waves - from motor cortical areas
• Spike and wave pattern: indicates abnormal electrical activty of neurons seen in epileptic cases.

Mechanism of EEG

• Wave pattern is due to the Oscillation of alternating excitatory and inhibitory post synaptic potentials produced by impulses of non-specific thalamic nuclei.
• Frequency of the rhythm is by the activity of non-specific thalamic nuclei.
• At low intrinsic frequencies (8-13 cycles/sec) the EPSP’s are summated & give rise to low frequency synchronised EEG rhythm (alpha).
• When thalamic neurons are driven by the reticular activating system, the frequency becomes too high for summation and becomes desynchronized to become fast EEG rhythm – beta rhythm results.
• Normal ECG
  • 8-13 cycles/sec - alpha wave
  • 20-30 cycles/sec - beta wave
  • 3-7cycles/sec - theta wave
  • Less than 3 cycles/sec - delta wave

EEG and sleep stages

• EEG studies help in understanding the different stages of sleep Each stage of sleep manifest different wave pattern in EEG. 4 stages of slow wave sleep can be distinguished with different wave pattern, frequency and amplitude.

Drowsiness

• In the relaxed awake state alpha waves predominant 1^st stage of slow wave sleep is characterised by fluctuations between alpha wave and low amplitude wave (3-7 cycles/sec) .
• As sleep deepens into stage 2 wave pattern changes.
• In stage 3 and 4 EEG shows gamma waves (less than 3 cycles/sec) .
• In REM sleep low voltage fast waves are seen.