Central Nervous System
The central nervous system (CNS) is composed of the brain, brainstem, and spinal cord. It is responsible for receiving and interpreting signals from the peripheral nervous system (PNS). The brain integrates sensory information and coordinates body function, both consciously and unconsciously. Complex functions including thinking, feeling, and regulation of homeostasis take place in various parts of the brain. The spinal cord enables communication between the brain and the rest of the body and controls musculoskeletal reflexes.
Anatomy and Physiology
The basic unit of the CNS is the neuron, which is specialized for rapid nerve impulse conduction and for exchanging signals with other neurons. Billions of neurons allow different parts of the body to communicate with one another via the brain, brainstem, and the spinal cord. Glia (plural), which is a Greek word for “glue,” are supportive non-neuronal cells that surround the cell bodies, dendrites, and axons of neurons. Glial cells of the CNS include astrocytes, oligodendrocytes, microglia, and ependymal cells. Their main functions are to ensure neurons are “glued” together; nutrients and oxygen reach the neurons; neurons are insulated from one another; and pathogens are destroyed while removing dead neurons.
The cerebral cortex is the outermost part of the brain and consists of gyri and sulci, which give it a wormy appearance. It is the largest part of the brain and controls higher thought, language, consciousness, and memory. The cerebrum is divided into two hemispheres that are connected by the corpus callosum. Each hemisphere is responsible for controlled, voluntary limb movement of the contralateral side of the body.
The brain is divided into four lobes: frontal, parietal, temporal, and occipital. The frontal lobe contains dopamine-sensitive neurons that are associated with reward, attention, short-term memory, planning, and motivation. Executive functions of the frontal lobe include choosing between good and bad and recognition of consequences. In humans, the frontal lobe is not fully developed until the early 20s. The motor cortex is located in the frontal cortex and receives information from various parts of the brain to control movement.
The parietal lobe processes sensory information including pressure, touch, and pain via the somatosensory cortex. It integrates this sensory information and determines spatial sense and navigation. Attentiveness to the position of one’s body and one’s relationship to space is organized in this lobe. Additionally, language processing also takes place here.
The temporal lobe allows for interpretation of sounds and language and is the primary location of the auditory cortex. The hippocampus is also located in the temporal lobe and is important in the formation of memories. Damage to this area of the brain may lead to problems with memory, speech perception, and language skills.
The occipital lobe is associated with interpreting visual stimuli and information. The primary visual cortex is located in the occipital lobe and is responsible for receiving and interpreting information from the retina of the eye. These lobes are the smallest of the four.
The cerebellum, “little brain,” is a complex motor coordination structure and has gyri and sulci similar to the cerebral cortex. It is connected to the brainstem by motor fiber tracts called peduncles. Its function is to produce changes in muscle tone in relation to equilibrium, locomotion, and posture and to coordinate the timing of contraction of muscles being used for skilled movements. In general, the cerebellum can be thought of as the quality control center of the CNS.
The diencephalon contains two structures: the thalamus and hypothalamus. The thalamus serves as a relay station, taking in sensory information and passing it to the cerebral cortex. The hypothalamus regulates autonomic, endocrine, and visceral functions including hunger, thirst, emotion, body temperature, and circadian rhythm.
The brainstem consists of the medulla oblongata, pons, and midbrain. The midbrain is located at the most anterior portion of the brainstem and contains ascending and descending pathways, which play roles in the visual and auditory systems. Located below the midbrain, the pons contains nuclei that mediate several auditory and balance functions. The pons receives input from the vestibular system within the inner ear and communicates with the cerebellum for balance. Below the pons, the medulla oblongata consists of neural fiber tracts, called pyramids, which are large and pyramidal-shaped. The pyramids travel toward the CNS with sensory information from the skin, muscles, and tendons and travel away from the CNS with motor information to control the body’s muscles in the periphery.
The spinal cord is the central processing and relay station that transmits both sensory and motor information. It receives and projects information via peripheral nerves and ascending and descending tracts. Afferent sensory fibers enter the spinal cord through the dorsal roots of spinal nerves, and efferent motor fibers leave by way of ventral roots. Sensory signals travel via ascending tracts to the brain, while motor signals travel to the PNS via descending tracts. Vertebrae, three layers of meninges, and cerebrospinal fluid protect the spinal cord. The spinal column is divided into four sections: cervical, thoracic, lumbar, and sacral.
See also: Afferent Tracts; Brain Anatomy; Brainstem; Cerebral Cortex; Cranial Nerves; Peripheral Nervous System; Somatosensory Cortex; Somatosensory System
Kiernan, John A. (2009). Barr’s the human nervous system: An anatomical viewpoint (9th ed.). Baltimore, MD: Lippincott Williams & Wilkins.
Noback, Charles R., N. L. Strominger, R. J. Demarest, & D. A. Ruggiero. (2005). The human nervous system structure and function (6th ed.). Totowa, NJ: Humana Press.