The Body’s Communication System
Neurons—bundles of which make up nerves—are the building blocks of the body’s communication system. They’re organized into networks that allow signals to move between the brain and body. These networks, composed of about 86 billion neurons, comprise the nervous system. The nervous system, in turn, has two parts: the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system.
The Central Nervous System
The central nervous system (CNS) is made up of the brain and spinal cord. Together, they form, as the name suggests, the literal center of the body’s communication system. Protective barriers surround them, including bone (skull and spine) and membraneous tissue known as meninges. Additionally, the brain and spine are suspended in cerebrospinal fluid. The CNS processes every sensation and thought that you experience. Receptors throughout the body gather sensory information and pass it on to the CNS. The CNS also sends messages to the rest of the body to control movement, actions, and responses to the environment.
The Peripheral Nervous System
The peripheral system (PNS) is composed of nerves that extend beyond the central nervous system. The neural networks that make up the PNS are actually bundles of axons from neuron cells. The nerve bundles range from relatively small to large enough for the human eye to see. The PNS is further divided into two different systems: the somatic nervous system and the autonomic nervous system.
The Somatic Nervous System
The somatic nervous system transmits sensory communications and is responsible for voluntary movement and action. It is composed of sensory (afferent) neurons and motor (efferent) neurons.
The Autonomic Nervous System
The autonomic nervous system controls involuntary functions such as your heartbeat, respiration, digestion, and blood pressure. The system is also involved in emotional responses such as sweating and crying. The autonomic nervous system is subdivided into the sympathetic nervous system and parasympathetic nervous system. The sympathetic nervous system controls the body’s response to an emergency. When the system is aroused, your heart and breathing rates increase, digestion slows or stops, your pupils dilate, and you begin to sweat. Also known as the fight-or-flight response, this system prepares your body to confront danger or avoid it. The parasympathetic nervous system balances the sympathetic system. After a crisis or danger has passed, it helps calm your body by slowing heart and breathing rates, resuming digestion, contracting your pupils, and stopping sweating.
The Endocrine System
The endocrine system is composed of glands that secrete chemical messengers known as hormones, which the bloodstream carries to organs and tissues to regulate functions such as metabolism, digestion, blood pressure, and growth. Although the endocrine system is not directly linked to the nervous system, the two interact in a number of ways. They’re linked by the hypothalamus, a tiny collection of nuclei at the base of the forebrain that controls an astonishing amount of human behavior, including emotional and stress responses. It’s also involved in basic drives such as:
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Importantly, the hypothalamus controls the pituitary gland, which in turn regulates the release of hormones from other glands in the endocrine system.
A Word From Verywell
Although the nervous and endocrine systems are separate systems, they interact in important ways to influence human behavior. They work in tandem to help people respond to the world around them and to each other.
