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The Nervous System

The human brain is remarkable. In comparison to other animals, the brain is seven times bigger than it should be for the size of the body. During evolution, it enlarged in a rather short amount of time allowing humans to surpass other primates.

Researchers now have insight into the human advantage and ways to optimize the brain. We will explore these findings as well as the intricate functions of the nervous system.

The Human Advantage

The human advantage is not due to the size of the brain; it is the fact that humans have more neurons that carry and process information in a relatively small cerebral cortex (outer surface of the brain) than any other animal.

Thanks to the invention of cooking, humans were able to ingest a greater number of calories and nutrients in a short time. This lead to the rise in the human’s ability to reason, discover patterns, develop technology, and expand language.

The brain has approximately 86 billion neurons and is part of a complex, sophisticated system. The nervous system controls and coordinates all bodily functions and activities—every heartbeat, breath, movement, and thought.

Cells of the Nervous System

Fresh neurons arise

Call it neurogenesis

New tricks for old brains

The Little Book of Neuroscience Haikus by Eric H. Chudler, PhD

For many years it was believed that nerve cells in the adult brain, once damaged or dead, did not replace themselves. Researchers, however, have found that neurogenesis, the growth and development of new neurons, is found in adults in several areas of the brain.

There are two broad categories of cells within the nervous system: neurons and glia.

  • Neurons are known as the working units that process information and carry electrochemical impulses, which are electric and chemical messages. The chemical messages are known as neurotransmitters (e.g., serotonin, dopamine, and glutamate).
  • Glial cells provide metabolic and mechanical support to the neurons as well as produce myelin (definition below).

The neuronal cell structure, or soma, includes a nucleus, cytoplasm, and cytoplasmic organelles. The nucleus contains deoxyribonucleic acid (DNA) and data that is necessary for growth, metabolism, and repair. Cytoplasm, also called protoplasm, is the colorless substance that fills the cell, which includes all the chemicals and parts necessary for the cell to function. The cytoplasmic organelles are known as “little organs” in the cytoplasm; each type of organelle has a specific role.

Figure 1 typical neuron structure

Figure 1: Neuron: Typical Structure. (© User: Ajimonthomas)/Wikimedia Commons/CC BY-SA 4.0)

The cell membrane surrounds each neuron and separates the contents of the cell from its surrounding environment. It controls what enters and leaves the cell and responds to signals from the environment.

Dendrites are the short, branched segments on the receiving end of the nerve cell.

The axon is the threadlike part of the nerve cell that extends from the cell body to meet and deliver impulses to another nerve cell. The length of an axon can range from a fraction of an inch to several feet.

Myelin is a fatty white sheath that surrounds the axon and some nerve cells. It forms an electrically insulating layer that is essential for the quick transmission of impulses along nerve cells.

Synapses are tiny gaps between neurons where chemical and electrical signals move from one neuron to another.

Organization and Functions of the Nervous System

Figure 2 components of the nervous system

Figure 2: Components of the Nervous System. (© Jenna Fair/Wikimedia Commons/CC-BY-3.0)

The Central Nervous System

The organs of the central nervous system include the brain and the spinal cord. Although recognized as two separate organs, the brain and spinal cord are continuous at the foramen magnum, which is a hole at the base of the skull forming the passage through which the spinal cord exits the cranial vault.

Because they are vital to human survival, they are encased in bone for protection. The brain sits in the cranial vault (the space within the skull), and the spinal cord is located in the vertebral column within the vertebral canal (spinal canal).

In addition to bone, meninges, or connective tissue, and cerebrospinal fluid surround and protect the membranes of the CNS. The three layers of meninges include the dura mater (outer), arachnoid (middle), and pia mater (innermost).

Figure 3 Meninges

Figure 3: Meninges. (User: Mysid/Creative Commons/CC0 1.0)

Functions of the Brain

Nobody realizes that some people expend tremendous energy merely to be normal.

Albert Camus, Notebooks 1942-1951

Neurons need oxygen, glucose, and stimulation to function optimally. For example, if you have ever gone too long without eating, you may have felt sluggish from low blood glucose levels. The neurons in the brain have a high energy demand and consume approximately 5.6 mg of glucose per 100 g of human brain tissue per minute—approximately 25% of the body’s energy budget.

The brain divides into left and right cerebral hemispheres, which are joined by and communicated through the corpus callosum (figure 4).

Figure 4 Corpus callosum

Figure 4: Corpus Callosum. (© User: Was a bee/Wikimedia Commons/CC BY-SA 2.1)

In neuroscience, the cerebrum and cerebral cortex are practically, but not precisely, the same thing.

There is a slight difference:

  • The cerebrum is the entire top part of the brain including the white matter, which is mainly myelinated axons.
  • The cerebral cortex technically refers to the highly wrinkled outer surface, or the gray matter, which contains numerous cell bodies and few myelinated axons.

The brain consists of three basic units: forebrain, midbrain, and hindbrain. Figure 5 illustrates each unit in a developing embryo.

Figure 5 six week embryo brain

Figure 5: Six-Week Embryo Brain. (User: Kurzon/Wikimedia Commons/CC0 1.0)


The forebrain, also called prosencephalon, separates into the telencephalon and diencephalon, which are interconnected with the limbic system.

The telencephalon, or cerebrum, includes the cerebral cortex, which subdivides into four lobes (figure 6):

Lobe Basic Function
Frontal Responsible for planning, reasoning, parts of speech, problem-solving, movement, and perception
Parietal Integrates sensory information, which includes spatial orientation, movement, recognition, and perception of stimuli
Occipital Involves visual processing
Temporal Responsible for recognition and perception of auditory stimuli, speech, and memory

Figure 6 Brain lobes

Figure 6: Brain Lobes. (© BruceBlaus/Wikimedia Commons/CC BY-SA 3.0)

Parts of the diencephalon are as follows:

Diancephalon parts and definitions
Structure Basic Function
Thalamus Relays sensory and motor signals to the cerebral cortex, regulates consciousness, sleep, and alertness
Hypothalamus Coordinates both the ANS and the activity of the pituitary—it controls temperature, hunger, thirst, sleep, and emotional and motivational behavior (the hypothalamus includes the *posterior pituitary gland*, which secretes oxytocin and vasopressin
Epithalamus Links the nervous system to the endocrine system through the pituitary gland—it controls body temperature, thirst, hunger, fatigue, sleep, and circadian rhythm
Pineal body Small endocrine gland that is a part of the epithalamus that produces melatonin to modulate sleep patterns, regulate new bone metabolism, and protect against neurodegeneration; it may also influence drug metabolism
Subthalamus Plays a role in motor control and is interconnected with the basal ganglia (described below)

Figure 7 diencephalon

Figre 7: The Diencephalon. (© User: Openstax/Creative Commons/CC BY-SA 3.0)

Other major components located within and at the base of the cerebral cortex are as follows:

Structure Location Basic Function
Cingulate cortex Medial aspect of the cerebral cortex, above the corpus callosum Responsible for emotion formation and processing, memory, learning, and links behavioral outcomes to motivation
Amygdala Base of the cerebral cortex Involved with memory, emotional reactions, and decision making
Hippocampus Base of the cerebral cortex Promotes short- and long-term memory, spatial memory, and is the center of mood and emotion—it is also a unique structure where new neurons can be generated
Olfactory bulb Base of the cerebral cortex Transmits smell information from the nose to the brain
Basal ganglia Base of the cerebral cortex Involves voluntary motor movement, procedural learning, and routine behaviors or habits (such as eye movements, teeth grinding, emotion, and cognition)

The midbrain, or mesencephalon, is a section of the brainstem that forms the primary link between the forebrain and the hindbrain. It controls motor and visual functions, auditory responses, and sleep/wake arousal. It is made up of the tectumtegmentumcerebral aqueduct, and cerebral peduncles.


The hindbrain, or rhombencephalon, occupies the posterior region of the brain and is a part of the brainstem. The hindbrain consists of the myelencephalon and metencephalon.

  • The metencephalon portion of the hindbrain includes the cerebellum (Latin for “little brain”), pons, and reticular formation (located throughout the brainstem and cerebral cortex). The pons and the cerebellum are responsible for the control of movement, balance, and transmitting sensory information. The reticular formationis responsible for sleep and consciousness.
  • The myelencephalon is also called the medulla oblongata, or the medulla. It deals with the vital autonomic functions of the body, such as digestion, respiration, and heart rate. Meditation may influence the medulla and the immune system.

Figure 8 Brain Stem

Figure 8: The Brainstem. (© User: Openstax/Creative Commons/CC BY-SA 3.0)

Cranial Nerves

Twelve pairs of cranial nerves emerge directly from the brain; 10 of the 12 originate from the brainstem, and two nerves arise from the cerebrum. All of these nerves, except for the vagus nerve, pass through the foramina of the skull. A very small 13th cranial nerve, known as the terminal nerve, was identified more than a century ago; however, more research is needed to determine its role in humans.

Figure 9 cranial nerves

Figure 9: The Cranial Nerves. (© User: CFCF/Wikimedia Commons/CC-BY-4.0)

# Name Basic Function
I Olfactory Sense of smell
II Optic Sense of sight
III Oculomotor Controls motor movement of the eye and eyelid
IV Trochlear Controls an extraocular muscle that assists with visual tracking
V Trigeminal Responsible for motor innervation to muscles of mastication (chewing) and sensory innervation of the face
VI Abducens Eyeball movement
VII Facial Muscles of the face (facial expression) and taste
VIII Vestibulocochlear Hearing and balance (sense of body position)
IX Glossopharyngeal Swallowing and taste
X Vagus Gut (gastrointestinal tract), heart, and larynx
XI Accessory Sternocleidomastoid and trapezius muscles
XII Hypoglossal Tongue muscles

Functions of the Spinal Cord

Downward from the medulla oblongata is the spinal cord, a long white tube that is made up of nervous tissue and support cells. The spinal cord divides into 31 segments with each giving rise to a pair of spinal nerves (peripheral nerves).

  • 8 cervical (C) nerves – in the neck and upper spinal cord
  • 12 thoracic (T) nerves – chest region
  • 5 lumbar (L) nerves – middle to lower back
  • 5 sacral (S) nerves – lower back
  • 1 coccygeal (Cx) nerve – low back

Figure 10 anatomy of the spine

Figure 10: Anatomy of the Spine. (© University of Wisconsin School of Medicine and Public Health/No alterations made to image)

Three primary functions of the spinal cord include:

  • Carrying information – The spinal cord sends signals to and from the brain.
  • Coordinating reflexes – The spinal cord can coordinate reflexes on its own, which makes it responsible for quick, coordinated responses to stimuli. The thermal grill illusion is an excellent example this. It is an experiment that elicits an ice-like burning feeling—without causing harm—by using warm and cool metal bars. When a person places their hand on the metal bars, the pain signals quickly tell the spinal cord to react without knowing what it’s reacting to.
  • Controlling reflexes – The spinal cord controls reflex responses that are typically involuntary. These reactions are sudden and are an automatic response.

The Peripheral Nervous System

The peripheral nervous system (PNS) consists of the nerves and ganglia. Nerves are bundles of nerve fibers, similar to how muscles are bundles of muscle fibersGangliaare clusters of nerves cells.

The spinal and cranial nerves extend from the CNS to peripheral organs. The basic constituents of the neural circuits within the PNS include afferent (sensory) neurons, efferent (motor) neurons, and interneurons (relay) neurons.

The PNS further subdivides into the somatic nervous system, autonomic nervous system, and the visceral nervous system. Further discussion will focus on the somatic and autonomic divisions of the peripheral nervous system.

Figure 11 somatic autonomic enteric structures

Figure 11: Somatic-Autonomic-Enteric Structures. (© User: CFCF/Wikimedia Commons/CC-BY-4.0)

Functions of the Somatic and Autonomic Nervous Systems

The somatic nervous system (SoNS), also called the voluntary nervous system, permits conscious control of the skeletal muscles and consists of both afferent nerves and efferent nerves. The SoNS includes spinal nerves, cranial nerves, and association nerves.

The autonomic nervous system (ANS), or involuntary nervous system, supplies motor impulses to the cardiac muscle, smooth muscle, and to the glandular epithelium (one or more cells that can produce and secrete a particular product). The ANS is only in connection with the efferent side of the nervous system.

The ANS further subdivides into the sympathetic, parasympathetic, and enteric nervous systems. The sympathetic and parasympathetic branches interact to maintain cardiovascular activity within an optimal range and permit reactions to changes in internal and external conditions.

  • The sympathetic nervous system initiates the “fight-or-flight” mechanism when it detects danger or a threat. This threat may include internal stressors (e.g., chronic stress or anxiety) or external stressors (e.g., physical injury or environmental pollutant). A redirection of energy takes place when the sympathetic nervous system is activated; digestion is put on hold, the heart rate and breathing rate escalate, pupils dilate, and there is an increase in the production of saliva and sweat.
  • When the parasympathetic nervous system takes over, the opposite effect occurs; digestion restarts and breathing and pupils return to normal.
  • The enteric nervous system, also known as the intrinsic nervous system, is a separate nervous system for the gastrointestinal system, which to a great extent, autonomously controls bowel motility. The gut-brain axis (GBA) consists of a bidirectional connection between the enteric and central nervous systems. The GBA links cognitive and emotional centers of the brain with peripheral intestinal functions. Recent advances in research describe the importance of the gut microbiotain influencing these interactions.

Figure 12 the autonomic nervous system

Figure 12: The Autonomic Nervous System. (User: Geo-Science-International/Wikimedia Commons/CC0 1.0)

Signs and Symptoms of Nervous System Disorders

The brain gives clues about its state of health. The following are the most common key indicators of a nervous system disorder; however, each person may experience symptoms differently.

  • A headache that changes or is different or persistent
  • Blood pressure changes
  • Slurred speech
  • Sudden loss of sight or double vision
  • Impaired mental ability
  • Memory loss
  • Lack of coordination
  • Muscle rigidity and wasting
  • Weakness or loss of muscle strength
  • Loss of feeling or tingling
  • Back pain that radiates to the feet or another part of the body
  • Tremors and seizures
  • A nervous system disorder may resemble other medical conditions or problems

Disorders of the Nervous System

If not quickly and appropriately treated, a disease or illness that involves the nervous system may have severe consequences. As you review the list below you will find that many disorders fit into several categories.

Nerve and Blood Flow Disruption (Blood Supply to the Brain: Video Tutorial)
Structural Disorders
Functional Difficulties, Degeneration, and Autoimmune

Optimize Your Brain and Prevent Injury

By following the guidelines below, people can help keep their nervous system healthy. Always speak to a healthcare practitioner before starting any new exercise, regime, or diet.

Please contact us at to reach the author or recommend other ailments or brain-health tips that interest you.

Sarah Gehrke, MSN, RN Written by Sarah Gehrke, MSN, RN and last updated Nov 5, 2017

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