The nervous system is the body’s control and communication network. It comprises the central nervous system (CNS)—the brain and spinal cord—and the peripheral nervous system (PNS), a network of nerves throughout the body. The CNS processes, interprets, and responds to sensory information. The PNS relays information between the brain and the rest of the body, including organs, muscles, and glands.
This system transmits electrical impulses across neurons to coordinate bodily functions. These signals direct everything from conscious thought and movement to involuntary processes like breathing and digestion. It continuously monitors internal and external environments, deciding on appropriate responses to maintain bodily function.
Directing Movement and Processing Sensation
The nervous system directly controls voluntary movements via the somatic nervous system, a component of the PNS. Motor neurons from the brain and spinal cord send signals to skeletal muscles, initiating contractions for actions like walking or lifting. This control allows for complex, coordinated physical activities. The brain’s cerebral cortex plans and coordinates these movements, with signals descending to lower motor neurons that directly connect to muscle fibers.
Beyond movement, the nervous system gathers sensory information from the environment and body. Sensory neurons in the skin detect stimuli like touch, temperature, and pain. These signals travel along sensory nerves to the spinal cord and brain, interpreted in areas like the somatosensory cortex for sensation perception. The skeletal system also provides sensory input through proprioception, the sense of body position and movement. Receptors in muscles and joints send signals to the nervous system, enabling awareness of limb placement and body orientation without visual cues.
Governing Internal Organ Functions
The autonomic nervous system (ANS), a PNS subdivision, regulates involuntary internal organ functions without conscious thought. The ANS has two main branches: sympathetic and parasympathetic, which often exert opposing effects to maintain homeostasis. The sympathetic system prepares the body for “fight or flight” responses, while the parasympathetic system promotes “rest and digest” activities.
The nervous system influences the circulatory system. The ANS regulates heart rate and blood pressure, with the sympathetic system generally increasing both and the parasympathetic system decreasing them. Baroreceptors in blood vessels detect blood pressure changes, relaying this to the brainstem, which adjusts heart rate and vessel constriction to maintain stable blood pressure. Similarly, the respiratory system is under nervous system control, with brainstem centers regulating breathing rate and depth. These centers respond to chemical signals, like carbon dioxide levels, to adjust ventilation and ensure gas exchange.
The digestive system also relies on nervous system coordination. While the gastrointestinal tract has its own enteric nervous system, the CNS provides extrinsic inputs regulating digestion, nutrient absorption, and waste elimination. The parasympathetic system typically stimulates digestive processes, including motility and secretion, while the sympathetic system generally inhibits them.
The urinary system’s functions, including bladder control and urination, are also governed by nervous system interactions. Signals from the bladder inform the brain about its fullness, coordinating relaxation and contraction of bladder muscles and sphincters to allow or inhibit urination. The reproductive system’s functions, such as sexual response, are also influenced by the nervous system, with both sympathetic and parasympathetic divisions regulating these activities.
Orchestrating Chemical Balance
The nervous and endocrine systems are linked, forming a neuroendocrine axis that regulates the body’s chemical balance. The hypothalamus, a brain region, acts as a bridge, receiving signals and translating them into hormonal directives. It produces releasing and inhibiting hormones that control the pituitary gland, often called the “master gland.” The pituitary, at the base of the brain, releases hormones regulating other endocrine glands, including the adrenal and thyroid.
For example, the hypothalamus produces corticotropin-releasing hormone (CRH), stimulating the pituitary to release adrenocorticotropic hormone (ACTH), triggering cortisol release from the adrenal glands, a stress response hormone. Thyroid-stimulating hormone (TSH) from the pituitary, regulated by hypothalamic hormones, controls thyroid hormone production, influencing metabolism and energy levels. Hormones released by the endocrine system can impact nervous system function. Hormonal fluctuations can affect mood, cognition, memory, and even sleep regulation. For instance, estrogen influences memory and mood, while cortisol levels, especially if chronically elevated, can impair cognitive function. This feedback loop between the nervous and endocrine systems maintains the body’s chemical equilibrium and adaptation.
Mediating Defense and Repair
The nervous system also plays a role in the body’s defense and repair through interactions with the immune system. This communication is bidirectional; each system can influence the other. Stress signals from the nervous system can modulate immune responses, enhancing or suppressing them depending on context. For example, neurons can release neurotransmitters like norepinephrine, influencing immune cell production and inflammatory mediators.
Neuroinflammation, a localized inflammatory response in the brain or spinal cord, highlights direct interaction between nervous system components and immune cells. Glial cells and neurons can produce immune factors and respond to inflammatory signals. Immune cells can also secrete chemical signals, including cytokines, that activate or influence neurons, leading to behavioral changes like fever or lethargy during infection. This interplay helps the body mount defenses against pathogens and initiate tissue repair following injury or infection.