(CNS) The central nervous system is the integral part of the NERVOUS SYSTEM comprising the brain and the SPINAL CORD. The nerves which emerge from the brain and spinal cord and distribute throughout the body represent the PERIPHERAL NERVOUS SYSTEM. While the peripheral nervous system is primarily concerned with transmitting sensory information about the external and internal milieux to the CNS and conveying motor commands to the effectors such as muscles and glands, the CNS represents a central processing unit that receives such information from the periphery, integrates it, and issues commands for an action or response. The AUTONOMIC NERVOUS SYSTEM, concerned with homeostatic (see HOMEOSTASIS) and ENDOCRINE functions, is a separate functional entity, and is part central and part peripheral.
The brain and spinal cord are protected by cranial bones and spinal vertebra, as well as by MENINGES including the dura mater, arachnoid mater and pia mater. The space between the meninges is filled by the CEREBROSPINAL FLUID, a clear colourless liquid that contains glucose, proteins, lactic acid, urea and electolytes. The cerebrospinal fluid mechanically and chemically protects the brain and spinal cord, and serves as a vehicle for nutrients and waste products. The entire central nervous system is also protected by the BLOOD-BRAIN BARRIER (BBB) against blood-borne toxins.
The spinal cord is the lower part of the CNS, and consists of the GREY MATTER shaped like the letter H at the core, and the WHITE MATTER surrounding the grey matter. The grey matter is comprised of sensory relay neurons and INTERNEURONS in the DORSAL HORN, and MOTOR NEURONS and interneurons in the VENTRAL HORN. Autonomic neurons (see PREGANGLIONIC NERVE) are also present in the lateral horn at all thoracic as well as upper lumbar segments. The white matter represents the columns (or funiculi) which are bundles (tracts) of sensory and motor fibres coursing up and down in the spinal cord. SPINAL NERVES emerge at all levels of the spinal cord. A major function of the spinal cord is to connect the peripheral nervous system with the brain and vice versa, but the spinal cord can also function independently as in reflexes (see REFLEX) which occur in response to specific sensory input from the periphery.
The brain, the upper part of the CNS, can be divided into four parts: the BRAINSTEM, CEREBELLUM, DIENCEPHALON and CEREBRAL HEMISPHERES. The brainstem comprises the MEDULLA, PONS and MIDBRAIN, which are derived from the MYELENCEPHALON, METENCEPHALON, and MESENCEPHALON, respectively. The cerebellum is also derived from the metencephalon. The metencephalon and myelencephalon are collectively referred to as the HINDBRAIN or RHOMBENCEPHALON. The brainstem is concerned with autonomic and homeostatic regulation as well as relay of sensory and motor information. Ten of the twelve CRANIAL NERVES emerge from the brainstem. The cerebellum is attached to the dorsal aspect of the pons via three fibre bundles, and is involved in balance and motor coordination. The diencephalon consists of the THALAMUS and HYPOTHALAMUS. The thalamus is a mass of relay nuclei for all sensory systems except OLFACTION, and the hypothalamus is the master structure for autonomic, homeostatic and endocrine functions. As part of the LIMBIC SYSTEM, the hypothalamus is also involved in species-specific behaviours such as FEEDING, DRINKING and SEXUAL BEHAVIOUR and parental behaviours. The most rostral structures of the brain are the cerebral hemispheres which represent the TELENCEPHALON. The cerebral hemispheres are paired structures that are separated by the longitudinal fissure in the midline, but are also interconnected via commissural fibres (see COMMISSURE) through the CORPUS CALLOSUM. Deep in the cerebral hemispheres are the BASAL GANGLIA and limbic system. The basal ganglia are involved in coordination of movements, whereas the limbic system is involved in emotional and motivational behaviours (see EMOTION; MOTIVATION).
The superficial part of cerebral hemispheres is the CEREBRAL CORTEX, critically important for higher functions including LEARNING and MEMORY and higher sensory functions, such as OBJECT RECOGNITION. These functions reach their highest level of complexity in humans. By comparing brains across different species, what is most striking is the larger proportion of the cerebral cortex in higher mammals. This is in contrast to relatively smooth and thin cortex in lower mammals, or the cortex that is almost non-existent in reptiles or amphibians. Clearly, the process of ENCEPHALIZATION reached its peak in the human brain. Consistent with this, although the human brain weighs about 1400g and comprises only about 2% of total body weight, it consumes about 20% of oxygen in a resting adult. Local blood flow in different parts of the brain varies depending on the nature of mental activity and can be studied by using FUNCTIONAL NEUROIMAGING techniques. These techniques provide an exciting avenue for studying the neurobiological basis of higher psychological functions such as planning, creativity and artistic appreciation that are uniquely human.
KAZUE SEMBA
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