2-1 Overview of Brain Function and Structure

The brain’s primary function is to produce behavior, or movement, in a perceptual world the brain constructs. This perceptual world is ever-changing. To adapt, the brain must also change, a property referred to as neuroplasticity.

To study how the nervous system functions, we abandon the anatomical divisions between the central nervous system and the peripheral nervous system to focus instead on function—on how the CNS interacts with the divisions of the PNS: the somatic, autonomic, and enteric nervous systems.

2-2 The Nervous System’s Evolutionary Development

The vertebrate nervous system evolved from a relatively simple structure mediating reflexlike behaviors to the complex human brain mediating advanced cognitive processes. To allow for more complex behavior in an increasingly sophisticated perceptual world, primitive forms have not been replaced but rather have been adapted and modified as new structures have evolved.

The principles of nervous system organization and function generalize across the three vertebrate brain regions—hindbrain, midbrain, and forebrain—leading to multiple levels of functioning. The evolution of neural levels of control thus adds flexibility to behavioral control.

2-3 Central Nervous System: Mediating Behavior

The CNS includes the brain and the spinal cord. The spinal cord can perceive sensations from the skin and muscles and produce movements independent of the brain. The brain can be divided into the brainstem and forebrain, each made up of hundreds of parts. The brainstem both directs movements and constructs a sensory world through its connections with the sensory systems, spinal cord, and forebrain. The forebrain modifies and elaborates basic sensory and motor functions; regulates cognitive activity, including thought and memory; and ultimately controls movement. The most elaborate parts of the brain, the cerebral cortex and cerebellum, grow disproportionately large in the human brain.

2-4 Somatic Nervous System: Transmitting Information

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The SNS consists of two sets of spinal nerves that enter and leave the spinal column, connecting with muscles, skin, and joints in the body, and the cranial nerves that link the facial muscles and some internal organs to the brain. Both sets of SNS nerves are symmetrical: one set controls each side of the body. Some cranial nerves are sensory, some are motor, and some combine both functions. The spinal cord acts as a minibrain for the peripheral (spinal) nerves that enter and leave its five segments. Each spinal segment works independently, although CNS fibers interconnect them and coordinate their activities.

2-5 Autonomic and Enteric Nervous Systems: Visceral Functions

The ANS controls the body’s glands and internal organs and operates largely outside conscious awareness. Its sympathetic (arousing) and parasympathetic (calming) divisions work in opposition. The parasympathetic division directs the organs to rest and digest, whereas the sympathetic division prepares for fight or flight.

The ENS controls the gut over its entire length, from the esophagus to the colon, interacting with the brain via the ANS. ENS activity can affect our behavior and mental state. In turn, the ENS is affected by the microbiome, the roughly 100 trillion bacteria that inhabit our gut.

2-6 Ten Principles of Nervous System Function

Ten principles listed in the right column below form the basis for discussions throughout this book. Understanding them fully will enhance your study of brain and behavior.

Ten Principles of Nervous System Function