Anatomy and physiology of the nervous system (2024)

The central nervoussystem (CNS) is made up of the brain and spinal cord. The brain controlsmost body functions, including awareness, movements, sensations, thoughts,speech and memory. The spinal cord is connected to the brain at the brain stemand is covered by the vertebrae of the spine. Nerves exit the spinal cord to bothsides of the body. The spinal cord carries signals back and forth between thebrain and the nerves in the rest of the body.

The peripheralnervous system (PNS) is the part of the nervous system outside of the CNS.It is made up of nerves and ganglia that send signals to and receive signalsfrom the CNS. The PNS is divided into the somatic nervous system and theautonomic nervous system. The somatic nervous system controls body movementsthat are under our control such as walking. The autonomic nervous systemcontrols involuntary functions that the body does on its own such as breathingand digestion.

The autonomic nervous system is further divided into thesympathetic and parasympathetic nervous systems. The sympathetic nervous systemprepares the body for situations that require strength and heightened awarenessor situations that arouse fear, anger, excitement or embarrassment. This iscalled the fight-or-flight response. It causes the heart to beat faster, makesyou breathe quicker and more shallowly, dilates the pupils and increasesmetabolism. The parasympathetic nervous system has a calming effect on thebody. It returns heart rate and breathing to normal, constricts the pupils andslows down metabolism to conserve energy.

The sympathetic chain is made of the sympathetic chainganglia that run in a chain from the head to the tailbone (coccyx), along bothsides of the spine. Ganglia are clusters of nerve cell bodies. The sympatheticchain ganglia send messages to the head, neck, lower body (trunk) andextremities during the fight-or-flight response.

The sympathetic chain ganglia affect spinal nerves andnerves in the chest cavity. This helps to increase blood flow to skeletalmuscles and the brain, stimulate energy production for skeletal muscles to use,release stored fats and stimulate the sweat glands. It also increases heartrate, increases the pumping action of the heart and allows more air to moveinto the lungs.

The 3 collateral ganglia are the celiac ganglion, the superiormesenteric ganglion and the inferior mesenteric ganglion. They are in the abdomenin front of the spine. The collateral ganglia send messages to organs in theabdomen and pelvis.

The collateral ganglia affect nerves in the abdomen andpelvis. Affecting these nerves lowers blood flow to organs, lowers activity inthe digestive system, stimulates the liver to release glucose to give the bodymore energy, relaxes smooth muscle in the bladder wall and lowers urineproduction.

The adrenal medulla is located in the centre of each adrenalgland. These glands have specialized neurons. When these neurons arestimulated, they release chemical messengers (called neurotransmitters) intothe blood that act as hormones.

The adrenal medulla is involved in releasing hormone-likesubstances, such as epinephrine (adrenaline) and norepinephrine (noradrenaline),into the blood. Cells that have receptors for epinephrine or norepinephrinerespond to these substances and take part in the flight-or-fight response.