Central nervous system
Also known as: CNS, neuraxis
The brain and spinal cord: the integrating centre of the nervous system, enclosed in the skull and vertebral canal and cushioned by meninges and cerebrospinal fluid.
Overview
The CNS receives sensory input, integrates it with stored information and generates motor, autonomic and endocrine output. It consists of the brain (cerebrum, diencephalon, brainstem, cerebellum) and the spinal cord, which continues the brainstem below the foramen magnum. All of it develops from the neural tube, and its lumen persists as the ventricles and central canal.
CNS tissue is organised into grey matter (neuronal cell bodies, dendrites, unmyelinated axons and synapses) and white matter (myelinated axon tracts, whose lipid-rich myelin looks white). In the cerebrum and cerebellum the grey matter forms an outer cortex over deep white matter with embedded nuclei; in the spinal cord the arrangement is inverted, with central grey and peripheral white. A cluster of neuronal cell bodies inside the CNS is a nucleus; a bundle of axons with a common origin and destination is a tract (also called fasciculus, lemniscus or peduncle).
The CNS is protected by bone, the three meninges, a cushion of CSF that reduces the brain's effective weight from ≈1400 g to ≈25–50 g, and the blood–brain barrier. Its glia are astrocytes, oligodendrocytes, microglia and ependymal cells. Unlike peripheral axons, CNS axons regenerate very poorly because of oligodendrocyte-derived inhibitors (Nogo-A, MAG), the astrocytic glial scar and low intrinsic growth capacity.
- Location
Cranial cavity (brain) and vertebral canal from the foramen magnum to ≈L1–L2 (spinal cord).
Parts & subdivisions
- Cerebrum, diencephalon, brainstem and cerebellum; ≈1.2–1.4 kg.
- ≈42–45 cm cylinder from the foramen magnum to the conus medullaris; 31 segments.
- Grey matterCell bodies, dendrites, synapses. Cortex (cerebrum, cerebellum), deep nuclei, and the central H-shaped column of the cord.
- White matterMyelinated tracts: association (within a hemisphere), commissural (between hemispheres) and projection (to and from lower centres) fibres in the brain; ascending and descending tracts in the cord.
- Skull and vertebrae; dura, arachnoid and pia mater; CSF in the subarachnoid space; blood–brain barrier.
Anatomy
- Histology
Neurons with CNS glia (astrocytes, oligodendrocytes, microglia, ependymal cells) in a sparse extracellular space (≈20% of volume) with little collagen. Blood vessels are invested by astrocyte end-feet (glia limitans) and lined by tight-junction endothelium (blood–brain barrier).
- Development
From the neural tube (neuroectoderm). The rostral part forms the brain vesicles; the caudal part forms the spinal cord. The lumen becomes the ventricular system.
Function
- Integrates sensory information and generates perception
- Plans and initiates voluntary movement; coordinates posture and balance
- Controls autonomic and endocrine homeostasis
- Generates cognition, language, emotion, memory and consciousness
- Mediates spinal and brainstem reflexes
Clinical significance
Failure of CNS regeneration
After spinal cord or brain injury, transected CNS axons sprout but rarely reconnect. They are blocked by myelin-associated inhibitors (Nogo-A, MAG, OMgp), chondroitin sulphate proteoglycans in the astrocytic glial scar and poor intrinsic growth programmes. Peripheral axons, guided by Schwann cells, regrow ≈1–3 mm/day. Recovery after CNS injury relies mainly on plasticity: reorganisation of surviving circuits, helped by rehabilitation.
Test yourself
Which cell forms myelin in the central nervous system?
- Schwann cell
- Astrocyte
- Oligodendrocyte
- Satellite cell
Answer: Oligodendrocyte
One oligodendrocyte myelinates internodes on many CNS axons (up to ≈50). A Schwann cell myelinates a single internode of one peripheral axon. This is why MS (oligodendrocytes, CNS) and GBS (Schwann cells, PNS) affect different parts of the nervous system.
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.