Lissauer’s tract, also known as the posterolateral tract of Lissauer, is a white matter tract that carries somatosensory fibers within the spinal cord. The sensory fibers enter the posterior funiculus of the spinal cord via the dorsal root. The fibers that carry pain and temperature sensations are small in diameter and unmyelinated. As soon as these fibers enter the spinal cord, they synapse with the gray matter in the dorsal horn known as substantia gelatinosa, specifically laminae I and V.

The posterior spinal arteries branch off the posterior inferior cerebellar artery in approximately 75% of people, and from the vertebral artery in the remaining 25%. They perfuse the posterior one-third of the spinal cord, specifically the dorsal columns, which provide vibration and special sense information from the periphery to the brain.

The central sulcus is a furrow separating the precentral gyrus from the postcentral gyrus (two mounds). Each hemisphere (half-rounds) contains a central sulcus.

The anterior horn of the spinal cord, also known as the ventral horn, is an area of gray matter positioned anterolateral to the central canal of the spinal cord. It contains the alpha motor neurons, which give rise to motor axons via the ventral root for the mixed peripheral nerves. These cells are located at every spinal level, though are more highly concentrated in the cervical and lumbar regions, given their projection to the extremities through the brachial and lumbar plexi, respectively.

The third ventricle is one of the four cerebrospinal fluid-filled ventricles and is the first midline ventricle into which cerebrospinal fluid flows from the lateral ventricles, and from which cerebrospinal fluid flows to the fourth ventricle. Though all four ventricles have a choroid plexus that help create cerebrospinal fluid, the third ventricle is the only one whose posterior wall is comprised of the pineal gland (named for its resemblance to the pine cone). The lateral ventricles drain into the third ventricle via the foramen of Monro. The third ventricle, in turn, drains via the cerebral aqueduct of Sylvius into the fourth ventricle. The thalamus and hypothalamus form the walls of the third ventricle.

The T1 nerve root is the most superior root in the second region of the spine, the thoracic region. The T1 root joins the C8 root to form the lower trunk of the brachial plexus.

The primary tool for analysing groups acting on trees is Bass--Serre Theory. It is comprised of two parts: a decomposition result, in which an action is decomposed via a graph of groups, and a construction result, in which graphs of groups are used to build examples of groups acting on trees. The usefulness of the latter for constructing new examples of `large (e.g.~nondiscrete) groups acting on trees is severely limited. There is a pressing need for new examples of such groups as they play an important role in the theory of locally compact groups. An alternative `local-to-global approach to the study of groups acting on trees has recently emerged, inspired by a paper of Marc Burger and Shahar Mozes, based on groups that are `universal with respect to some specified `local action. In recent work, the authors of this survey article have developed a general theory of universal groups of local actions, that behaves, in many respects, like Bass--Serre Theory. We call this the theory of local action diagrams. The theory is powerful enough to completely describe all closed groups of automorphisms of trees that enjoy Tits Independence Property $\propP{}$. This article is an introductory survey of the local-to-global behaviour of groups acting on trees and the theory of local action diagrams. The article contains many ideas for future research projects.

The foramen magnum is the bottom-most foramen in the skull and the largest. Through it passes the spinal accessary nerve (CN XI) as well as five arteries: the two vertebral arteries, the posterior spinal arteries, and the anterior spinal artery. The inferior medulla also passes through this opening to join the cervical spinal cord.

Cranial nerve one, the olfactory nerve, is one of the only two nerves (CN I and II) that do not have a nucleus in the brainstem, also known as lizard brain. Near its endpoint, fibers from the olfactory nerve travel through the cribriform plate, which is a region of ethmoid bone. It is unique because it is the only nerve that is exposed to the outside environment.

The vestibulocochlear nerve exits the pons as one nerve; however, the vestibular components come from nuclei in the pons and medulla while the cochlear components come from nuclei in the inferior cerebellar peduncle, thus the nerve enters the pons (derived from the Latin word for “bridge”) as two components but exits the pons as one combined nerve with both vestibular and cochlear components. After merging and traveling through the internal acoustic meatus, the two components split on exit, with the vestibular component/vestibular nerve heading to the otolith organs and semicircular canals (that look like a snail) and the cochlear components/cochlear nerve going to the cochlea (shell). The primary functions of these nerves include regulating balance (vestibular nerve) and hearing (cochlear nerve).

The medial longitudinal fasciculus (MLF) is a bilateral paramedian white matter tract located in the midbrain tegmentum and the dorsal pons, which helps to coordinate conjugate eye movements. Its fibers connect the abducens nucleus to the nuclei of the oculomotor and trochlear nerves, allowing for the coordinated adduction and abduction of separate eyes conjugately. It is not only a necessary tract for the functioning of volitional horizontal and vertical gaze but also vital for the vestibulo-ocular reflex.