Central nervous system

The central nervous system, or CNS, is primarily made up of the brain and spinal cord, though the retina is also considered part of it. It gets its name because the brain acts as the main integrator, taking in information and coordinating the activity of all body parts in bilaterally symmetric, triploblastic animals—essentially all multicellular creatures except sponges and diploblasts. This system is a structure of nervous tissue that runs along the body’s rostral-to-caudal axis, often with an enlarged section at the rostral end that forms a brain. Only arthropods, cephalopods, and vertebrates have a true brain, though simpler precursors exist in onychophorans, gastropods, and lancelets. The rest of this entry focuses solely on the vertebrate central nervous system, which is fundamentally different from that of other animals.

In vertebrates, the brain and spinal cord are wrapped in the meninges, which create a barrier against chemicals in the blood, shielding the brain from most common food-borne neurotoxins. Inside the meninges, both the brain and spinal cord are surrounded by cerebrospinal fluid, which replaces the body fluid found outside cells in all bilateral animals. The vertebrate CNS sits within the dorsal body cavity: the brain is housed in the cranial cavity inside the skull, and the spinal cord runs through the spinal canal within the vertebrae. The spaces between neurons in the CNS are filled with many supporting non-nervous cells called neuroglia, or glia (from the Greek for "glue"). The vertebrate CNS also includes the retina, the optic nerve (cranial nerve II), and the olfactory nerves and epithelium. These connect directly to brain neurons without intermediate ganglia. The olfactory epithelium is the only central nervous tissue outside the meninges that has direct contact with the environment, providing a route for therapeutic agents that otherwise cannot cross the meningeal barrier.

The CNS has two major structures: the brain and spinal cord. The brain is encased in the skull and protected by the cranium. The spinal cord is continuous with the brain, lying caudally to it, and is protected by the vertebrae. It extends from the base of the skull, starting below the foramen magnum, and ends roughly at the level of the first or second lumbar vertebra, occupying the upper part of the vertebral canal.

Microscopically, CNS neurons and tissue differ from those of the peripheral nervous system (PNS). The CNS is made up of white and gray matter, which can also be seen macroscopically on brain tissue. White matter contains axons and oligodendrocytes, while gray matter consists of neurons and unmyelinated fibers. Both tissues include glial cells (though white matter has more), often called supporting cells of the CNS. Different glial cells have different roles: some, like Bergmann glia, act as scaffolding for neuroblasts during neurogenesis; others, such as microglia, are specialized macrophages involved in the brain’s immune system and clearing metabolites. Astrocytes may help clear metabolites and transport fuel and beneficial substances from brain capillaries to neurons. After a CNS injury, astrocytes multiply, causing gliosis—a form of neuronal scar tissue lacking functional neurons. The brain (including the cerebrum, midbrain, and hindbrain) has a cortex of neuron bodies forming gray matter, with white matter internally forming tracts and commissures. There is also subcortical gray matter that makes up many different nuclei.

From the spinal cord, projections of the PNS form spinal nerves (sometimes called segmental nerves). These nerves connect the spinal cord to skin, joints, muscles, and so on, allowing for the transmission of efferent motor signals and afferent sensory signals. This enables voluntary and involuntary muscle movements and the perception of senses. There are 31 spinal nerves projecting from the brain stem, some forming plexuses as they branch out, like the brachial and sacral plexuses. Each spinal nerve carries both sensory and motor signals, but they synapse at different regions of the spinal cord—either from the periphery to sensory relay neurons that send information to the CNS, or from the CNS to motor neurons that relay information outward. The spinal cord relays information up to the brain via spinal tracts, following the final common pathway to the thalamus and ultimately the cortex.

Besides the spinal cord, there are also peripheral nerves of the PNS that synapse through intermediaries or ganglia directly on the CNS. These 12 nerves, located in the head and neck, are called cranial nerves. They bring information to and from the face and certain muscles, such as the trapezius muscle, which is innervated by accessory nerves and some cervical spinal nerves. Two pairs of cranial nerves—the olfactory nerves and the optic nerves—are often considered CNS structures because they synapse directly on CNS neurons without first going through peripheral ganglia. The olfactory epithelium is notable because it consists of CNS tissue in direct contact with the environment, allowing certain pharmaceuticals and drugs to be administered through it.

At the anterior end of the spinal cord lies the brain.