Central nervous system

The central nervous system (CNS) comprises the brain, spinal cord, and retina, forming the primary coordinating and integrative network of the vertebrate nervous system. It processes incoming sensory information, generates motor responses, and governs higher cognitive functions. Positioned along the rostral–caudal axis and enclosed within protective skeletal structures, the CNS is distinguished from the peripheral nervous system by its anatomy, cellular composition, and functional specialisation.

General Organisation and Protective Structures

In vertebrates, the CNS is housed within the dorsal body cavity. The brain occupies the cranial cavity of the skull, whereas the spinal cord lies within the vertebral canal. Both are enveloped by the meninges—three connective tissue layers that provide mechanical protection and form part of the blood–brain barrier. Between the meningeal layers, the cerebrospinal fluid circulates, cushioning the CNS and maintaining a controlled extracellular environment distinct from that of other tissues.
The retina, optic nerve, and olfactory nerves are embryologically derived from neural tissue and are therefore considered components of the CNS. Uniquely, the olfactory epithelium contains central nervous tissue directly exposed to the external environment, creating a route for therapeutic agents incapable of traversing the meningeal barrier.

Cellular Composition: White and Grey Matter

The CNS contains two major tissue types: grey matter, composed primarily of neuronal cell bodies and unmyelinated fibres, and white matter, consisting mainly of myelinated axons and oligodendrocytes. These distinctions are visible both microscopically and macroscopically.
Glial cells, including astrocytes, oligodendrocytes, microglia, and specialised forms such as Bergmann glia, support neuronal function in diverse ways. Astrocytes regulate the extracellular environment and help form scar tissue following injury, while microglia act as immune sentinels, clearing debris and mediating inflammatory responses. Oligodendrocytes form the myelin sheaths that increase the speed of electrical conduction within white matter tracts.

Brain: Structure and Major Divisions

The brain constitutes the largest portion of the CNS and is responsible for complex sensory integration, motor coordination, and higher mental functions.
BrainstemThe brainstem includes the medulla oblongata, pons, and midbrain. It serves as a conduit for ascending sensory and descending motor tracts connecting the spinal cord with higher brain regions. The medulla regulates essential autonomic processes such as breathing and blood pressure. The pons contains nuclei involved in relaying signals between the cerebellum and cerebral cortex and contributes to functions including breathing, sleep, and taste. The midbrain integrates components of the motor system and contains nuclei linked to visual and auditory functions, including the control of automatic eye movements. The brainstem also houses the reticular formation, which is vital for arousal and alertness, and provides exit and entry points for many cranial nerves involved in motor and autonomic control.
CerebellumLocated posterior to the pons, the cerebellum is organised into lobes and fissures and plays a fundamental role in maintaining posture, coordinating voluntary movements, and refining motor skills learned through practice. Modern neuroimaging demonstrates that cerebellar circuits also interact with cortical regions associated with language and cognition, indicating a broader integrative role than previously understood.

Spinal Cord: Pathways and Function

The spinal cord is continuous with the brainstem and extends to the level of the first or second lumbar vertebra. It contains both ascending tracts carrying sensory information to the brain and descending tracts transmitting motor commands to the periphery. Spinal nerves arise in 31 pairs, each containing both sensory and motor fibres that branch into networks such as the brachial and sacral plexuses.
Within the spinal cord, sensory inputs from the periphery synapse on relay neurons, which transmit information to higher CNS regions, while motor neurons project outward to stimulate muscles. Although capable of mediating reflexes and certain patterned movements such as locomotor rhythms, the spinal cord primarily serves as the conduit to the brain, where higher processing occurs.

Cranial Nerves and Sensory Integration

Twelve pairs of cranial nerves connect the brain to structures of the head and neck. Most cranial nerves belong to the peripheral nervous system, synapsing in ganglia before reaching the CNS. However, the optic and olfactory nerves are considered central nervous structures due to their direct connection to brain neurons without intermediate ganglia.
These nerves support functions ranging from facial sensation and eye movement to taste, hearing, balance, and visceral control. The vagus nerve, in particular, provides extensive autonomic regulation of thoracic and abdominal organs.

Functional Integration and Autonomic Control

The CNS coordinates voluntary actions, such as movement and speech, and involuntary functions, including heart rate, digestion, and pupil response. Autonomic control is mediated through pathways in the brainstem and spinal cord that influence smooth muscle, cardiac muscle, and glands. The reticular formation regulates consciousness and sleep–wake cycles, while the cerebral cortex underpins perception, reasoning, and decision making.

Subcortical Structures and Cortical Organisation

The brain consists of both cortical and subcortical grey matter. The cerebral cortex contains neuron-rich layers responsible for processing sensory input, planning movement, and facilitating cognition. Beneath the cortex lie numerous nuclei with specialised roles, including the basal ganglia, thalamus, hypothalamus, and limbic structures that contribute to motor control, emotional regulation, memory, and homeostasis.
White matter tracts, including commissural fibres and long projection pathways, interconnect these regions, enabling rapid communication across the CNS.