Neuroanatomy is a branch of anatomy and neuroscience that studies the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems. Their neuroanatomy is therefore better understood. In vertebrates, the nervous system is segregated into the central nervous system (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) comprising the connecting nerves between them. Much of what has informed neuroscientists has come from observing how lesions (damage) to specific brain areas affects behavior or other neural functions.
For information about the composition of non-human animal nervous systems, see nervous system. For information about the typical structure of the human nervous system, see human brain, and peripheral nervous system.
History
The first known written record of a study of the anatomy of the human brain is an ancient Egyptian document, the Edwin Smith Papyrus. In Ancient Greece, interest in the brain began with the work of Alcmaeon, who appeared to have dissected the eye and related the brain to vision. He also suggested that the brain, not the heart, was the organ that ruled the body (what Stoics would call the hegemonikon) and that the senses were dependent on the brain.
The debate regarding the hegemonikon persisted among ancient Greek philosophers and physicians for a very long time. Those who argued for the brain often contributed to the understanding of neuroanatomy as well. Herophilus and Erasistratus of Alexandria were perhaps the most influential with their studies involving dissecting human brains, affirming the distinction between the cerebrum and the cerebellum, and identifying the ventricles and the dura mater. The Greek physician and philosopher Galen, likewise, argued strongly for the brain as the organ responsible for sensation and voluntary motion, as evidenced by his research on the neuroanatomy of oxen, Barbary apes, and other animals.
The cultural taboo on human dissection continued for several hundred years afterward, which brought no major progress in the understanding of the anatomy of the brain or of the nervous system. However, Pope Sixtus IV effectively revitalized the study of neuroanatomy by altering the papal policy and allowing human dissection. This resulted in a flush of new activity by artists and scientists of the Renaissance, such as Mondino de Luzzi, Berengario da Carpi, and Jacques Dubois, and culminating in the work of Andreas Vesalius.
In 1664, Thomas Willis, a physician and professor at Oxford University, coined the term neurology when he published his text Cerebri Anatome which is considered the foundation of modern neuroanatomy. The subsequent three hundred and fifty some years has produced a great deal of documentation and study of the neural system.
Composition At the tissue level, the nervous system is composed of neurons, glial cells, and extracellular matrix. Both neurons and glial cells come in many types (see, for example, the nervous system section of the list of distinct cell types in the adult human body). Neurons are the information-processing cells of the nervous system: they sense our environment, communicate with each other via electrical signals and chemicals called neurotransmitters which generally act across synapses (close contacts between two neurons, or between a neuron and a muscle cell; note also extrasynaptic effects are possible, as well as release of neurotransmitters into the neural extracellular space), and produce our memories, thoughts, and movements. Glial cells maintain homeostasis, produce myelin (oligodendrocytes, Schwann cells), and provide support and protection for the brain's neurons. Some glial cells (astrocytes) can even propagate intercellular calcium waves over long distances in response to stimulation, and release gliotransmitters in response to changes in calcium concentration. Wound scars in the brain largely contain astrocytes. The extracellular matrix also provides support on the molecular level for the brain's cells, vehiculating substances to and from the blood vessels.
At the organ level, the nervous system is composed of brain regions, such as the hippocampus in mammals or the mushroom bodies of the fruit fly.Mushroom Bodies of the Fruit Fly These regions are often modular and serve a particular role within the general systemic pathways of the nervous system. For example, the hippocampus is critical for forming memories in connection with many other cerebral regions. The peripheral nervous system also contains afferent or efferent nerves, which are bundles of fibers that originate from the brain and spinal cord, or from sensory or motor sorts of peripheral ganglia, and branch repeatedly to innervate every part of the body. Nerves are made primarily of the axons or dendrites of neurons (axons in case of efferent motor fibres, and dendrites in case of afferent sensory fibres of the nerves), along with a variety of membranes that wrap around and segregate them into nerve fascicles.
The vertebrate nervous system is divided into the central and peripheral nervous systems. The central nervous system (CNS) consists of the brain, retina, and spinal cord, while the peripheral nervous system (PNS) is made up of all the nerves and ganglia (packets of peripheral neurons) outside of the CNS that connect it to the rest of the body. The PNS is further subdivided into the somatic and autonomic nervous systems. The somatic nervous system is made up of "afferent" neurons, which bring sensory information from the somatic (body) sense organs to the CNS, and "efferent" neurons, which carry motor instructions out to the voluntary muscles of the body. The autonomic nervous system can work with or without the control of the CNS (that's why it is called 'autonomous'), and also has two subdivisions, called sympathetic and parasympathetic, which are important for transmitting motor orders to the body's basic internal organs, thus controlling functions such as heartbeat, breathing, digestion, and salivation. Autonomic nerves, unlike somatic nerves, contain only efferent fibers. Sensory signals coming from the viscera course into the CNS through the somatic sensory nerves (e.g., visceral pain), or through some particular cranial nerves (e.g., chemosensitive or mechanic signals).
Orientation in neuroanatomy
In anatomy in general and neuroanatomy in particular, several sets of topographic terms are used to denote orientation and location, which are generally referred to the body or brain axis. The axis of the CNS is often wrongly assumed to be more or less straight, but it actually shows always two ventral flexures (cervical and cephalic flexures) and a dorsal flexure (pontine flexure), all due to differential growth during embryogenesis. The pairs of terms used most commonly in neuroanatomy are:
Dorsal and ventral: Dorsal refers more or less to the top or upper side of the brain, which is symbolized by the floor plate, and ventral to the bottom or lower side. These descriptors originally were used for dorsum and ventrum – back and belly – of the body; the belly of most animals is oriented towards the ground; the erect posture of humans places our ventral aspect anteriorly, and the dorsal aspect becomes posterior. The case of the head and the brain is peculiar, since the belly does not properly extend into the head, unless we assume that the mouth represents an extended belly element. Therefore, in common use, those brain parts that lie close to the base of the cranium, and through it to the mouth cavity, are called ventral – i.e., at its bottom or lower side, as defined above – whereas dorsal parts are closer to the enclosing cranial vault. Reference to the roof and floor plates of the brain is less prone to confusion, also allow us to keep an eye on the axial flexures mentioned above. Dorsal and ventral are thus relative terms in the brain, whose exact meaning depends on the specific location. Rostral and caudal: rostral refers in general anatomy to the front of the body (towards the nose, or rostrum in Latin), and caudal refers to the tail end of the body (towards the tail; cauda in Latin). The rostrocaudal dimension of the brain corresponds to its length axis, which runs across the cited flexures from the caudal tip of the spinal cord into a rostral end roughly at the optic chiasma. In the erect Man, the directional terms "superior" and "inferior" essentially refer to this rostrocaudal dimension, because our body and brain axes are roughly oriented vertically in the erect position. However, all vertebrates develop a very marked ventral kink in the neural tube that is still detectable in the adult central nervous system, known as the cephalic flexure. The latter bends the rostral part of the CNS at a 180-degree angle relative to the caudal part, at the transition between the forebrain (axis ending rostrally at the optic chiasma) and the brainstem and spinal cord (axis roughly vertical, but including additional minor kinks at the pontine and cervical flexures) These flexural changes in axial dimension are problematic when trying to describe relative position and sectioning planes in the brain. There is abundant literature that wrongly disregards the axial flexures and assumes a relatively straight brain axis. Medial and lateral: medial refers to being close, or relatively closer, to the midline (the descriptor median means a position precisely at the midline). Lateral is the opposite (a position more or less separated away from the midline). Note that such descriptors (dorsal/ventral, rostral/caudal; medial/lateral) are relative rather than absolute (e.g., a lateral structure may be said to lie medial to something else that lies even more laterally).
Commonly used terms for planes of orientation or planes of section in neuroanatomy are "sagittal", "transverse" or "coronal", and "axial" or "horizontal". Again in this case, the situation is different for swimming, creeping or quadrupedal (prone) animals than for Man, or other erect species, due to the changed position of the axis. Due to the axial brain flexures, no section plane ever achieves a complete section series in a selected plane, because some sections inevitably result cut oblique or even perpendicular to it, as they pass through the flexures. Experience allows to discern the portions that result cut as desired.
A mid-sagittal plane divides the body and brain into left and right halves; sagittal sections, in general, are parallel to this median plane, moving along the medial-lateral dimension (see the image above). The term sagittal refers etymologically to the median suture between the right and left parietal bones of the cranium, known classically as sagittal suture, because it looks roughly like an arrow by its confluence with other sutures (sagitta; arrow in Latin).