HappyMutations:

Nervous systems – General info:

Sponges have no nervous system.
Cnidarians (phylum that includes almost exclusively marine animals like jellyfish, sea anemones and corals) have a diffuse nerve net.
All the rest of the multicellular animals are bilaterians and are thought to have descended from a common ancestor that appeared around 485-540 million years ago. This hypothetical ancestor called the Urbilaterian would possibly have the shape of a simple tubeworm with a hollow gut cavity running from the mouth to the anus. Its body would be segmented and with a nerve cord running through it with a ganglion at each body segment and an especially large ganglion (its “brain”) at the front; see picture below:

Bilatereans then divide into invertebrates and vertebrates.

Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others), and cephalopods (octopuses, squids, and others).

The brain of arthropods is their supraesophageal ganglion – it has three divisions and large optical lobes behind each eye for visual processing. Cephalopods such as the octopus and squid have the largest brains of any invertebrates. In the earliest forms of vertebrates, the brain appears as three swellings at the front end of the neural tube; these swellings eventually become the forebrain, midbrain, and hindbrain:

In many classes of vertebrates, such as fish and amphibians, the three parts remain similar in size into adulthood, but in mammals the forebrain becomes much larger than the other parts, and the midbrain becomes very small. Vertebrate brains are surrounded by a system of connective tissue membranes called meninges that separate the skull from the brain. The surface of the brain (below the meninges) looks like this (photo by Robert Ludlow—of the Institute of Neurology at UCL):

Here you see the blood vessels that feed the neuronal cells. The cells in the blood vessel walls are joined tightly to one another, forming the blood–brain barrier, which restricts the passage of toxins and pathogens while allowing the diffusion of hydrophobic molecules O₂, CO₂, hormones, and small polar molecules such as glucose.

Before going through the brain regions let’s also see the hollow regions between them – the ventricular system. It consists of four ventricles; the right and the left lateral ventricles, the third ventricle and the fourth ventricle:

The ventricles contain a region of choroid plexus, which is a network of ependymal cells involved in the production of the cerebrospinal fluid (CSF). The CSF CSF acts as a cushion or buffer for the brain, providing basic mechanical protection, regulation of the distribution of substances between brain cells, and the removal of metabolic waste products by passing them into the bloodstream. CSF is derived from blood plasma but is largely protein-free and has different electrolyte levels. The fourth ventricle is continuous with the central canal of the spinal cord, allowing for the flow of CSF to circulate throughout the CNS:

The brains of all species are composed primarily of two classes of cells: neurons and glial cells.

Glial cells (aka glia or neuroglia) come in several types, and perform various functions, inc. structural support, metabolic support, insulation, and guidance of development.

Neurons (neuronal cells) send signals to specific target cells.

Visually the brain and spinal cord separate into grey matter and white matter.

Gray matter consists of neuronal cell bodies, neuropil (dendrites and myelinated as well as unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. In relation to white matter it contains numerous cell bodies, and only few myelinated axons.

White matter contains relatively few cell bodies and is composed chiefly of long myelinated axons (aka tracts). Myelin gives them their white colour.