Parts of the Nervous System

The nervous system is split into two primary components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS encompasses the brain and spinal cord, whereas the PNS acts as the connection between the CNS and the rest of the body, including muscles, organs, and sensory receptors outside the CNS.

The PNS consists of nerves made up of bundles of axons that transmit messages between the CNS and the body's extremities. It further divides into the somatic nervous system and the autonomic nervous system.

The somatic nervous system is responsible for voluntary activities, relaying sensory and motor information to and from the CNS. It comprises motor neurons (efferent fibers, moving away from CNS) that carry instructions to muscles, and sensory neurons (afferent fibers, moving toward CNS) that bring sensory input to the CNS.

The autonomic nervous system regulates internal organs and glands, operating generally outside conscious control. It has two main subdivisions: the sympathetic and parasympathetic nervous systems, which work together to maintain homeostasis (internal balance).

The sympathetic nervous system activates during stressful or high-arousal situations, triggering the 'fight or flight' response. This response involves physiological changes such as dilated pupils, increased heart rate and blood pressure, bladder relaxation, glucose release, and adrenaline surge, preparing the body to confront or escape perceived threats. While beneficial for ancestors facing physical dangers, persistent activation due to modern psychological stressors can lead to negative health consequences like heart disease and impaired immune function.

After a threat has passed, the parasympathetic nervous system takes over, restoring the body to a relaxed state. It reverses the effects of sympathetic activation, normalizing heart rate and blood pressure, constricting pupils, restoring bladder control, and storing glucose.