# What Do We Know about the Use of EEG Monitoring during Equine Anesthesia: A Review
## 2 Physiology of Brain Activity--Why Is the EEG so Important in the Management of Anesthesia?  
The nervous system can be divided into so-called levels, described by [PERSON] as the triune model of the brain. The lowest level of response to stimuli consists of the spinal cord centers, which control reflexes (also muscle tone, reflex reactions of blood vessels). The next one is the lower brain level (subcortical) with reticular formation of the medulla oblongata, midbrain vestibular nucleus, hypothalamus, and limbic system. They are responsible for maintaining blood pressure, breathing pattern, and the position of the eye. The highest in the hierarchy is the higher (cortex) level with the cerebral cortex, coordinating all brain functions including the sight, hearing, and other complex activities, apart from reflex reactions controlled by lower centers [11; 12]. The sympathetic and parasympathetic systems are associated with all brain levels through afferent and efferent pathways in all organs, with a different predominance. They control organs by reflexes and through cortical and other central control centers--some of the functions controlled are heart rate, arterial diameter, regulation of the urinary bladder, and tracheal diameter, which are measured as vital signs during anesthesia [13].  
Electroencephalography signal is generated by rhythmic neural sources that are spontaneous per se but also act as regular oscillators. Their source is in the cerebral cortex, exactly in its layer of pyramid cells [1] partially modulated by glial cells [12]. The cerebral cortex is responsible for many key functions such as learning and cognitive processes [14]. In addition, it has been shown that the cortex is stimulated during the action of nociceptive stimuli [15], which is important in anesthetic monitoring. The impulses recorded during the EEG measurement are the sum of electrical signals originating from neurons within the cerebral cortex and as a result of postsynaptic potentials in the neurons located in the lower layers of the cortex [2]. The excitation and inhibitory potentials are distinguished, according to the effect of the type of neuroreceptor, that is stimulated [16].  
The frequency and amplitude of the EEG is modulated by the nuclei of the thalamus, which transduces peripherally received signals to the cortex and between its parts. It is generally assumed that the signals measured by the EEG are proportional to the level of consciousness and their frequency decreases with the deepening of the level of anesthesia [16]. The EEG frequency ranges that can be used to determine the level of general anesthesia are:  
* \(\delta\) (0-4 Hz)--with a large amplitude, characteristic for deep sleep or anesthesia
* \(\theta\) (4-8 Hz)--observed in light sleep or anesthesia
* \(\alpha\) (8-13 Hz)--in awake but relaxed state
* \(\beta\) (13-30 Hz)--rapid oscillations characteristic for awake and high activity of cortex [16; 17; 18].  
The alpha-waves are recorded from the parietal region in conscious and calm animals. Generally, general anesthesia is characterized by the increase of the amplitude and decrease of the frequency. However different anesthetic agents can change the neurologic response and EEG results. This can be used to evaluate the agent influence on cortical activity and patient consciousness [19; 20; 21].