Overview of the Human Nervous System

The Spinal Cord

The spinal cord is the second major component of the CNS, contained within the spinal cavity and protected by the vertebrae. It serves as the primary pathway for signals traveling between the brain and the rest of the body.

Exam Tip: Remember the protective structures! The brain is protected by the skull (cranial cavity), while the spinal cord is protected by the vertebrae (spinal cavity).

The Peripheral Nervous System (PNS)

While the CNS acts as the command center, the Peripheral Nervous System (PNS) consists of the network of nerves that connects the CNS to the rest of the body. The PNS is generally less protected than the CNS. It is organized into two functional subsystems: the somatic nervous system and the autonomic nervous system.

The Somatic Nervous System

The somatic nervous system is often referred to as the voluntary system. Its primary role is to connect the brain to voluntary muscles and provide sensory feedback regarding voluntary movements.

The Autonomic Nervous System

The autonomic nervous system is known as the involuntary system because it regulates functions that occur without conscious effort, such as connecting the brain to internal organs. This system is further divided into two antagonistic (opposing) branches that work together to maintain homeostasis:

1. The Sympathetic Division: This is the "active" branch that responds to danger and stress. It triggers the "fight or flight" response, an acute stress response that causes physiological changes like increased heart rate and blood pressure to prepare the body for action.
2. The Parasympathetic Division: This branch is responsible for the "rest and digest" functions. It is active when an individual is relaxed, controlling organs at their baseline and stimulating digestion.

The Stress Response and Neuroendocrine Interaction

The nervous system does not work in isolation; it interacts closely with the endocrine system to manage physiological states, particularly during stress.

When the cerebral cortex perceives a stressful event, it signals the hypothalamus, which activates the sympathetic nervous system. This triggers the sympathetic-adrenomedullary (SAM) axis, causing the adrenal medulla to release catecholamines like epinephrine and norepinephrine.

Simultaneously, the hypothalamic-pituitary-adrenocortical (HPA) axis is activated. This involves a cascade where the hypothalamus releases corticotropin-releasing factor (CRF), prompting the pituitary gland to release adrenocorticotropic hormone (ACTH), which finally stimulates the adrenal cortex to release glucocorticoids, such as cortisol.

Clinical Note: While the "fight or flight" response is essential for survival, repeated or chronic activation of these stress systems has been linked to chronic disease.

Summary Table: Nervous System Divisions