3 Brain and Nervous System Organization

The two divisions of the nervous system

The nervous system has two anatomical divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord; it receives and integrates information and coordinates responses. The PNS includes neural structures outside the brain and spinal cord and carries information between the CNS and the rest of the body.

Pathways between the body and the CNS

Functionally, the PNS has sensory and motor pathways. Sensory (afferent) pathways carry signals from receptors toward the CNS, while Motor (efferent) pathways carry commands from the CNS to muscles and glands.

The motor division includes the somatic nervous system, which carries sensory information from the body and controls skeletal muscles. The autonomic nervous system regulates involuntary functions through connections to smooth muscle, cardiac muscle, and glands. Its major divisions are the sympathetic and parasympathetic systems; the enteric nervous system provides extensive local control of the digestive tract.

For example, sensory signals about a hot surface can reach the spinal cord, where circuits rapidly organize a withdrawal response. The CNS can also process the information and coordinate a more deliberate response.

Major regions of the brain

The brain has four broad anatomical regions that work together rather than as isolated units. Information can pass through multiple regions as the nervous system combines input and organizes output.

The Cerebrum consists of two cerebral hemispheres, including the outer cerebral cortex and deeper structures. The cortex is divided into frontal, parietal, temporal, and occipital lobes, and the hemispheres communicate through connecting fiber pathways.

The Diencephalon includes the thalamus, which relays and processes much incoming information on its way to the cortex, and the hypothalamus, which helps regulate autonomic and endocrine functions. The Brainstem consists of the midbrain, pons, and medulla; it connects the brain with the spinal cord and contains pathways and centers important to sensory, motor, and basic bodily functions. The Cerebellum helps coordinate movement, balance, and posture.

Cell bodies and axon pathways

The arrangement of neuron cell bodies and axons differs across the nervous system. Within the CNS, collections of neuron cell bodies are organized as cortices, which are layered sheets such as the cerebral cortex, or nuclei, which are compact groups of neurons. Bundles of axons connecting CNS regions are called tracts. In the PNS, bundles of axons are nerves, while groups of neuron cell bodies are ganglia.

Gray matter is rich in neuron cell bodies and local connections. White matter is rich in axons, many of which are myelinated.

Spinal cord structure and signaling

The spinal cord is a long CNS structure within the vertebral canal. It links the brain with the body and can also process information locally, including through spinal reflex circuits. The cord is organized into segments associated with 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair.

In cross-section, gray matter lies centrally in a butterfly-like shape and is surrounded by white matter. The dorsal horns receive and process much sensory input, while the ventral horns contain somatic motor neurons. Sensory fibers enter the cord through dorsal roots, and motor fibers leave through ventral roots. The roots join to form spinal nerves carrying both sensory and motor fibers.

White-matter tracts carry signals up toward the brain or down from the brain. Local gray-matter circuits can produce rapid responses without waiting for conscious processing.

Protection and coordinated function

The brain and spinal cord are protected by the skull and vertebral column, as well as by the meninges and cerebrospinal fluid. Their regional organization supports communication, integration, and coordinated action throughout the body.