Nervous System

Nerves and specialized cells called neurons transmit electrical and chemical signals between different parts of the body to control bodily functions.

How the Nervous System Is Organized

The nervous system senses and processes stimuli from the environment and body and responds appropriately. It provides fast, coordinated responses through interconnected cells. These cells are called neurons (nerve cells) and glia (glial cells).

Neurons are the functional units of the nervous system. The human brain contains approximately 86 billion neurons. Glia support neurons, both structurally and metabolically. Glia outnumber neurons.

Neurons generate electrical signals, called action potentials, that allow information to be quickly transmitted throughout the body. Neurons transmit this this information along thin fibers, which causes chemicals (called neurotransmitters) to be released at junctions (or synapses).

A cell that receives a signal may be excited, inhibited, or modulated in other ways. The connections between neurons form neural pathways that create larger networks that affect an individual’s perceptions and responses to stimuli. The nervous system can be broadly separated into two categories: central and peripheral. The central nervous system (CNS) consists of the brain and spinal cord. The brain receives and processes sensory information, initiates responses, stores memories, and gratis thoughts and emotions. The spinal cord conducts these signals to and from the brain and controls reflexes.

The peripheral nervous system (PNS) consists of nerves and ganglia, which are masses of tissue that contain neuron cell bodies. The PNS is the part of the nervous system found outside of the CNS. It is made up of sensory neurons and motor neurons. Sensory neurons sense information from the body and send data to the CNS for processing. Motor neurons send data to the CNS to muscles and glands, which respond accordingly. Motor neurons are divided between two systems: the somatic (or voluntary) nervous system and the automatic nervous system.

The somatic nervous system controls intentional movements. The autonomic nervous system controls involuntary responses; it is divided into the sympathetic nervous system and the parasympathetic nervous system. The sympathetic division is responsible for rapid fight-or-flight responses to stimuli. It is activated during high-stress times. The parasympathetic division manages rest-and-digest responses to low-stress stimuli.

Brain: The body’s command center and one of the largest and most complex organs. It is made up of more than 100 billion nerves that communicate with trillions of synapses (connections)

Brain stem: Composed of the midbrain, pons, and medulla oblongata, this organ controls the flow of messages between the brain and the rest of the body, including key functions like breathing and heart rate

Vagus nerve: The longest and most complex cranial nerve, which runs through the face and thorax to the abdomen. They oversee a range of critical bodily functions, including digestion, heart rate, and speech.

Brachial plexus: This group of nerves travels from the spinal cord in the neck down the arms; they control the muscles of the shoulders, elbows, wrists, and hands and provide feeling to the limbs

Spinal cord: This long, thin tube lies within protective vertebrae. Extending from the medulla oblongata in the brain stem to the lumbar region, it transmits signals from the motor cortex to the rest of the body and coordinates reflexes

Intercostal nerve: These somatic nerves rise from the anterior divisions of the thoracic nerves in the spine. They control muscle contractions and provide sensory information, including information related to touch from the skin

Iliohypogastric nerve: These nerves supply sensations to the skin in the lateral gluteal and hypogastric regions; they also supply motor signals to the internal oblique and transverse abdominal muscles

Lumbar plexus: This web of autonomic, motor, and sensory nerves is embedded in the poses major muscle, where it passes in front of the hip joint. Along with the sacral plexus, it innervates the lower extremities, glutei, and groin.

Median nerve: This main nerve is found in the front of the forearm and controls muscle movement in the forearm and thenar eminence (the ball of the thumb). This nerve controls coarse hand movements, so it is often called the “laborer’s nerve”

Ulnar nerve: Traveling from the shoulder to the wrist, this nerve is chiefly responsible for hand movements by providing nerve function

Pudendal nerve: This main nerve of the perineum carries sensation from the external genitalia and the skin around the anus and perineum. It also provides motor function to certain pelvic muscles

Sciatic nerve: Traveling down the back of the leg from the lower spine, the body’s largest nerve is formed by five nerve roots. It supplies sensation to the lower legs and the skin of the feet

Common perineal nerve: This is the smaller terminal branch of the sciatic nerve, it innervates the short head of the biceps muscle

Tibial nerve: This branch of the sciatic nerve rises from the popliteal space and follows the course of the tibial artery: it innervates the muscles of the lower legs and feet

Deep perineal nerve: It innervates the front section of the extensor muscles in the legs. It rises from the lateral side of the front of the legs, between the peroneus longs muscle and the neck of the fibula

Superficial peroneal nerve: These nerves, at the terminus of the common perineal nerve, supply the perineal longs and perineal braves, which form the lateral compartment of the lower legs. They help evert (turn out) the feet and thus act at the subtler joint (at the back of the feet)