Chapter Text
The Gallifreyan nervous system begins in a structure known as the krif, or the brain. This structure is comprised of trillions of microscopic neurons and can weigh up to two kilograms. It is divided into three lobes: the zerif, or forebrain, the tilif, or hindbrain, and the verif, or accessory brain. All three lobes are connected by thin strands of neurons which allows them to communicate information and transfer signals to the rest of the body.
The verif maintains base vital functions within the body, including heartsbeat, digestion, nerve impulse transmission, processing of waste, and exchange of internal and external gases. It is often referred to as the 'tertiary brain stem' due to its location parallel to the stem of the tilif lobe, and can easily be confused with the spinal cord of a lesser species by an untrained examiner. It runs along the anterior face of the thoracic spinal column until the small of the back, where it inserts into and fully joins the spinal cord, where it facilitates heartsbeat and the storage and release of oxygen stored in the zokrih, or pulmonary bypass organs. Damage to the verif can have severe consequences though its responsibilities can be overtaken by the tilif, as all actions performed by the tilif must be done consciously, leading the affected individual to manually control their own basal bodily functions; a few Gallifreyans throughout history have elected to intentionally sever the verif, with the belief that they could increase their own bodily performance if given the ability to do so consciously. Injury to the verif is often incurred alongside injury to the spinal cord, meaning their symptoms are often confused for one another - paralysis, while common alongside verif injury, is not a symptom.
The tilif facilitates lower-grade conscious activities, such as muscular contraction, respiration, retinal dilation, sensory input, and basic repetitive tasks. It can be used to place the body on 'auto-pilot', allowing the zerif to rest while activity continues; nomadic Gallifreyans would often travel while resting their forebrains, with a single fully-awake member at the front as a lookout and guide. It can also be used to perform basic higher-function tasks, such as speaking, listening, reading, or basic arithmetic, but this comes at the cost of limited efficacy of the tilif's intended functions. The tilif comprises the lower, smaller lobe of the main krif, descending into the secondary brain stem and branching out into the body's nerves; these nerves branch out into the rest of the body where they allow for physical sensation and communication of nervous signals. Damage to the tilif alone is rare due to its location, but symptoms include limited or absent pain response, dysfunctional or compromised sensory comprehension, muscle dysfunction ranging from mild weakness to complete paralysis, difficulty breathing, and limited or absent pupillary response. Prognosis if the tilif is severely damaged or absent is poor.
The zerif is the only lobe of the krif that is not connected to any organs other than the tilif and verif and comprises nearly 60% of the krif's volume, located around the top and sides of the tilif. It is responsible for fully conscious higher thought, and is what allows for Gallifreyans to perform complex abstract thinking. It can be divided into a near-infinite amount of fully distinct sections, each capable of advanced thought on par with most lesser species; highly experienced and ancient Gallifreyans have been known to perform all the necessary calculations for space-time travel mentally instead of, or even faster than, a timeship's supercomputer, as well as communicate with other Gallifreyans telepathically. This level of advanced thought, however, takes an incredible amount of energy which was once often unavailable in Gallifreya's barren environment. The zerif is entirely disconnected from the rest of the body, meaning that it is unable to take over the responsibilties of the tilif or verif, as it cannot communicate with the rest of the nervous system; the use of zerif to mean 'primary brain stem' is a misnomer, as the base of the zerif is embedded within the tilif rather than outside of the krif, whereas the primary brain stem is the spinal cord, connected jointly to the tilif and verif. Because it is not needed for basic bodily function, it can be damaged or entirely removed without causing harm, though it will lead to cognitive deficits and an incapacity for advanced thought; these injuries are the most common due to the zerif's placement within the cranium.
The chyikadefetujeem, or spinal cord, originates from both the tilif and verif, inserting into the centre of the fetujeem, or spine. It is used by the verif to communicate directly with the smooth muscle of the abdominal internal organs, and by the tilif to perform the lobe's function when the individual is unconscious. It is significantly less vital to Gallifreyans than most lesser species, though damage can result in digestive dysfunction, sleep apnea, sensory deficits, or paralysis.
The peripheral nervous system consists of the secondary brain stem, which splits into millions of nerves which insert directly into skeletal muscle and the chyalka, or primary respiratory system. These transmit nerve impulses to the muscles and transmit sensations back to the tilif. There also exist nerves which originate from the lower spinal cord and insert into the abdominal organs to allow for unconscious control. Nerve damage is common in the event of injury, and has similar effects to those of lesser species: reduced or absent sensation, motor dysfunction, or pain.