The Nervous System
The Body's Communication Network

Human Nervous System

Contents

• What Is The Nervous System?
• What Is The Structure Of The Nervous System?
• The Central Nervous System
• The Peripheral Nervous System
• The Somatic And Autonomic Nervous Systems
• What Are Reflexes?
• Structure Of Nerve Cells
• Diseases And Disorders Of The Nervous System

Related Diagrams
The Human Body
The Female Body

The nervous system is a network of nerves that link the brain to every part of the body. The nerves carry instant messages from the brain to every muscle and organ of the body. And they send back a constant stream of information to the brain about what is going on inside and outside of the body (such as pain or the sensation of danger) so that the body can respond and remain in what is known as homeostasis: a stable, physiological state. In short, it is our body's communication highway system.

Interesting Facts
• The nervous system sends messages to the brain at 290 km per hour.
• The largest and longest nerve in the body is the sciatic nerve. It is as wide as your thumb.
• There are more than 10 billion nerve cells in the spinal cord.

The nervous system is composed of the brain, spinal cord and nerves. It is divided into two 2 parts - the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System (CNS)
The CNS is the brain and the spinal cord. It maintains all the functions that keep us alive like breathing, body temperature and appetite. It is also the source for thoughts and emotions.

Peripheral Nervous System (PNS)
The PNS is made up of the nerves that branch out from the CNS to the rest of the body. It acts as the lines of communication between the CNS and the rest of the body. Specifically there are 31 pairs of nerves that branch out from the spinal cord (spinal nerves) and 12 pairs that branch out in the head (cranial nerves). The PNS is further subdivided into the Somatic and Autonomic nervous systems.

Automatic Nervous System (ANS)
The ANS is also known as the visceral nervous system or involuntary nervous system. It controls involuntary actions like breathing, heart rate, digestion and so on.

Somatic Nervous System (SNS)
The SNS is the part of the PNS that provides sensory feedback - it is made up of nerves that connect to the skin and other sensory muscles. If you touch a hot object, the SNS will send information to the brain and the brain will return instructions to the hand muscles to pull away. All of course in the split of a second!

The CNS consists of the brain and the spinal cord.

The Brain
The brain is the organ that fills the cranium (skull). It stops developing when you reach the age of 15. It is the main mass exercising control over the body and mind and it has 3 different sections:

• the cerebrum (also known as cerebral hemispheres)
• the cerebellum
• the brain stem

Structure:
This is the largest part of the brain and is divided into 2 cerebral hemispheres, one on the right and one on the left. The outer layer is made of folds of gray matter (cell bodies). The folds increase the brain's surface area and thus the number of cell bodies. Inside the gray matter is white matter (nerve fibers). These fibers connect different parts of the brain together.
Functions:
• Controls voluntary movement (i.e. the movements we choose to make).
• Interprets and perceives conscious sensations like pain, heat and cold.
• Controls mental activity, like memory, intelligence and reasoning.

The Cerebellum

Structure:
The cerebellum is also known as the 'small brain'. Positioned in the posterior cranial fossa, behind the pons Varolii, below the cerebrum and over the medulla oblongata, it also consists of 2 hemispheres, gray matter on the surface and white matter within.
Functions:
• Coordinates muscular activity, making sure movements are smooth and precise (damage to the cerebellum results in clumsy, uncoordinated movements).
• Subconsciously controls and maintaining muscle tone and posture.
• Maintains balance and equilibrium of the body.

Hypothalamus

Structure:
Situated deep within the cerebrum at the top of the brainstem with the pituitary gland attached to its base.
Functions:
Helps with the regulation of body temperature, water balance and metabolism. Center for drives and emotions such as thirst, appetite, sex, pain and pleasure. It also regulates the pituitary gland thereby forming the main link between the nervous and endocrine systems. It secretes oxytocin and Antidiuretic Hormone (ADH) for storage in the posterior pituitary.

The Brain Stem

The brain stem consists of 3 parts: the midbrain, pons Varolii and the medulla oblongata.

Midbrain
Structure:
Lies between the cerebrum and the cerebellum and above the pons Varolii. It is about 2cm long and consists of nerve cells and fibers.
Function:
The relay station of the brain, it transmits messages to and from the spinal cord, the cerebrum and the cerebellum.

Pons Varolii
Structure:
Situated in front of the cerebellum, below the midbrain and above the medulla oblongata. It consists of nerve fibers, which bridge (hence pons, which means bridge in Latin) the gap between the 2 hemispheres of the cerebellum.
Function:
Like the midbrain, transmits messages to and from the spinal cord and cerebrum.

Medulla Oblongata
Structure:
Lowest part of the brain stem, situated above the spinal cord and below the pons Varolii. Its construction is different from the cerebrum and cerebellum with white matter on the surface and gray matter in the center. It is known as a vital center because it controls the actions of the heart and lungs (respectively the centers of the vascular and respiratory systems). It has 4 centers.
Functions:
• Cardiac center: controls rate and force of heart beat.
• Respiratory center: controls rate and depth of breathing.
• Vasomotor center: controls constriction and dilation of blood vessels.
• Reflex center: responds to irritants thus controls vomiting, coughing, sneezing and swallowing.

The Spinal Cord

The spinal cord is the other main part of the central nervous system.
Structure:
The spinal cord extends from the brain (medulla oblongata) through the spinal vertebrae (bone) ending at the first lumbar vertebra. It is about 43cm long in women and 45cm long in men. It consists of white matter on the surface and gray matter inside branching off into 31 pairs of spinal nerves and part of one cranial nerve.
Function:
The spinal cord carries motor and sensory nerve fibers along its length, sending messages to and from the body and brain.

Other important parts of the central nervous system

The Meninges

The meninges are membranes which protect the whole of the central nervous system. There are three different layers -
• Dura mater, or outer membrane: a double layer of tough, fibrous membrane: the outer layer forms the periosteum ('skin') of the skull while the inner layer, the first protective covering of the brain, continues as the spinal dura mater as far down as the sacrum.
• Arachnoid mater: a delicate membrane positioned immediately under the dura and above the pia mater. It merges with the dura mater and thus covers the spinal cord as far as the sacrum. It connects to the sub-arachnoid space, situated between the arachnoid mater and the pia mater and is filled with cerebrospinal fluid.
• Pia mater: a thin, vascular membrane which closely covers the brain, dipping into the various surface folds of the cerebrum and cerebellum, and continues along the length of the spinal cord. It supplies blood to the brain and spinal cord.

Ventricles and Cerebrospinal Fluid

There are 4 cavities inside the brain called ventricles, all containing cerebrospinal fluid. Two of these ventricles lie laterally within the cerebrum, a third lies deep inside the brain whereas the fourth, also deep inside the brain, opens into the subarachnoid space.

Cerebrospinal Fluid

Structure:
This is clear, colorless fluid, formed in special cells situated in the lining of the ventricles. It resembles blood plasma in composition, containing protein, glucose, salts, and other substances. It is secreted into the ventricles from where it circulates around the whole brain and spinal cord and is then reabsorbed into the venous sinuses of the body through the arachnoid mater.
Functions:
• Protects the brain and spinal cord, forming a cushion between the bony cavities and the nerves and acting as a shock absorber.
• Keeps the pressure around the brain and spinal cord constant.
• Transports nutrients and removes waste and toxic substances.

The peripheral nervous system (PNS) concerns all the nervous system outside the central nervous system and contains motor and sensory nerves which transmit information to and from the body and brain. It consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Cranial Nerves

Cranial nerves are divided into 12 pairs and include sensory, motor and mixed nerves. Examples of these are:

5th Trigeminal
Opthalmic: Sensory nerves supplying the lacrimal glands, conjuctiva of the eyes, eye lids, forehead, anterior part of the scalp and mucous membrane of the nose.
Maxillary: Sensory nerves supplying the lower eye lids, upper gums, upper teeth and cheeks.
Mandibular: Sensory and motor nerves. Supplying the teeth and gums of the lower jaw, ear and tongue. Motor nerves affecting the muscles for chewing.

7th Facial
Motor nerves supplying the muscles that cause facial expression and sensory nerves for taste on the tongue.

Spinal Nerves

These nerves begin in the spinal cord and supply all parts of the body not covered by the cranial nerves. They are all mixed nerves. Spinal nerves are divided into 31 pairs:

Cervical: 8 pairs
Thoracic: 12 pairs
Lumbar: 5 pairs
Sacral: 5 pairs
Coccygeal: 1 pair

The cervical and thoracic nerves are named after the vertebrae at the level at which they exit the spinal cord. The lumbar, sacral and coccygeal nerves leave the spinal cord at the level of the first lumbar vertebra and extend downwards inside the vertebral canal exiting the canal at different levels depending on their destination. All spinal nerves except the 2nd to 12th thoracic nerves branch out and regroup to form plexuses (intersecting nerves) which supply different parts of the body:

The Cervical Plexus
This contains the first four cervical nerves and supplies the muscles of the neck, shoulder and skin and includes the phrenic nerve, which sends nerve impulses to the diaphragm telling it to contract.

The Brachial Plexus
This group includes the lower four cervical nerves and the first thoracic nerve. It branches out to supply the muscles from the base of the neck to the fingertips and skin.

The Thoracic (intercostal) Nerves
The thoracic nerves supply the chest muscles and the main part of the abdominal wall.

The Lumbar Plexus
This group includes the first three lumbar nerves and part of the fourth. It supplies the skin and muscles of the lower abdomen, thighs and groin.

The Sacral Plexus
This includes the fourth and fifth lumbar nerves and first four sacral nerves. It supplies the muscles and skin of the pelvic area. The main nerve is the sciatic nerve which supplies the hamstrings, before dividing above the knee into the tibial and common peroneal nerves to supply the lower leg.

The Coccygeal Plexus
The coccygeal group forms a second small plexus on the back of the pelvic cavity, supplying the muscles and skin of the pelvic area such as the external sphincter of the anus, tissues of the perineum and the external genitalia.

The Somatic And Autonomic Nervous Systems

The motor division of the PNS is divided into the Somatic Nervous System and the Autonomic Nervous System.

Somatic Nervous System

The Somatic Nervous System conducts impulses from the CNS to the skeletal muscle fibers. This is the voluntary branch of the PNS and allows conscious control over the contraction of skeletal muscles.

Autonomic Nervous System

The Autonomic Nervous System conducts impulses from the CNS to cardiac and smooth muscles. This is an involuntary system controlled by the hypothalamus. Its nerves arise from the medulla oblongata. The ANS is further divided into Sympathetic and Parasympathetic divisions. Every organ in the body has a sympathetic and parasympathetic nerve supply with one division generating the opposite effect to the other.

Sympathetic

Structure: Consists of nerves that arise from the spinal cord at the thoracic and lumbar region, form ganglia (bundles of nerve fibers) just outside the CNS and then extend to the organ or tissue they supply.

Functions:
Prepares the body for stressful situations such as excitement or physical activity (fight or flight system). Neurones release acetylcholine and noradrenaline which have the following effects:
• accelerates action of heart, increasing rate and force of contraction.
• vasodilation of coronary arteries, increasing blood supply to the heart muscle.
• vasodilation of vessels supplying skeletal muscles, increasing oxygen and nutrient supply and waste removal.
• causes sustained contraction of the spleen, thus increasing volume of blood circulating.
• vasoconstriction of vessels that supply the digestive system and urinary system, increasing blood available for active muscles and brain.
• dilation of bronchioles, increasing volume of air that can be inspired and expired.

Parasympathetic

Structure: Consists of nerves that arise from the brain and sacral region of the spinal cord, form ganglia near to or inside the organ or tissue they supply.

Functions:
Main system in non-stressful situations and keeps normal body functions running when the body is at rest. Neurones release acetylcholine.
• Slows action of heart, decreasing rate and force of contraction
• Vasoconstriction of coronary arteries, decreasing blood supply to the heart muscle
• Vasodilation of vessels supplying the digestive system and urinary system with contraction of the bladder and rectal muscles, increasing digestion, nutrient absorption, micturition and defacation
• Constriction of the bronchi, decreasing the volume of air inspired and expired.

A reflex is the automatic (not controlled by the brain) movement produced by a sensory stimulus. It is instant and involuntary e.g. a finger touching boiling hot water will immediately move away. Several structures are involved in the production of a reflex and together they constitute the reflex arc:

• a sense organ, like the skin or the nerve endings in muscles, tendons or organs
• a sensory nerve traveling from the sensory organ
• the spinal cord
• a motor nerve starting in the spinal cord and traveling to the motor organ.

What Is A Nerve Cell?

Nerve cells are the basic unit of the system on which everything else is built. Like all cells, they have a membrane containing a nucleus and a cytoplasm but they have a particular shape: long and narrow. Some are very long (up to a meter). Nerve cells are easily damaged by toxins and lack of oxygen. Unlike other cells in the body, they are not usually replaced when they die, however, current research suggests that some may have the ability to regenerate.

The main parts of a nerve cell are:

• Cell body: the center of the neurone, with a nucleus, cytoplasm and organelles such as mitochondria.
• Dendrites: nerve fibers, like branches, which transmit nerve impulses to the cell body; most neurones have several dendrites.
• Axon: a long single nerve fiber, which transmits nerve impulses away from the cell body; neurones generally have only one axon.
• Myelin sheath: made of a white, fatty substance, this sheath covers the axon. It insulates the axon, protects it from pressure and helps speed up nerve conduction (the speed at which messages are transmitted).
• Neurilemma: a fine, delicate membrane which surrounds the myelin sheath and helps regenerate nerve cells; only found in peripheral nerves and not in the brain or spinal cord.
• Nodes of Ranvier: these are gaps in the myelin sheath along the nerve. They speed up the passage of nerve impulses along the fiber.
• End feet/axon terminals: the ends of the fibrils (tiny fibers) that make the axon are expanded and called end feet or axon terminals. They pass on the axon impulse to the dendrites of the next neurone.
• Synapse: the point where one neurone meets another. A chemical messenger fills the gap between one neurone and the next, and enables the impulse to be transmitted.

Nerve cells act as links in a chain, like relay runners, each one passing the 'baton' (information or instruction) to the next until it reaches the brain or the part of the body in question. The axon end feet of one cell are close to the dendrites of the next but they don't actually touch. The 'baton' of nerve impulses jumps across the gap via neurotransmitters, chemicals released by the nerve endings.

Collective Function
Individual neurones (a neurone is another word for nerve cell) have the same function throughout the body: to transmit information. But collectively they make up 5 different types of nerves and nervous tissue which have specific functions:
• Motor or efferent nerves: carry impulses from the brain or spinal cord to muscles or glands which then act on the information/ instruction, producing movement or a secretion.
• Sensory or afferent nerves: carry impulses from all parts of the body to the brain.
• Mixed: carry both motor and sensory nerve fibers. The only place mixed nerves are found in the body is in the brain and spinal cord as cranial and spinal nerves.
• White matter: on the inside of the brain and the outside of the spinal cord; this is made of bundles of myelinated nerve fibers (i.e. with a sheath).
• Gray matter: on the outside of the brain and inside of the spinal cord - this is made of cell bodies and unmyelinated axons and dendrites.

Tip: How do you remember which are afferent and which are efferent nerves? Efferent exit the brain, afferent arrive in the brain.

What Is A Nerve Impulse?

Nerve cells transmit and receive impulses throughout the body. Impulses do not continually run along each nerve but are created in response to internal or external stimuli - including changes in temperature, pressure or chemicals. Positively charged sodium and potassium ions are present inside and outside the cell. In a resting axon, the concentration of sodium ions is lower inside the cell than in the tissue fluid outside, but the concentration of potassium ions is higher inside than outside. This is maintained by differences in membrane permeability to these ions, and the sodium-potassium pump. The overall result is that the inside of the cell has a more negative charge than the outside.

Stimuli are detected by sensory receptors. The axon membrane becomes temporarily more permeable to sodium ions which rush in, making the inside of the cell more positive (depolarization). This electrochemical charge continues in waves along the length of the nerve cell -a nerve impulse. After it has passed the resting state is restored (repolarization).

Nerve impulses only travel in one direction. So the movement of nerve impulses in a single neurone is as follows: the impulse crosses the synapse from the end feet of cell A into the dendrites of cell B. The impulse travels from the dendrites to the cell body and then out again along the axon to cell B's end feet. It then jumps across the synapse, helped by the chemical messengers. This process continues until the impulse reaches either the brain or the muscle/organ concerned.

Neuritis
Inflammation of a nerve, caused by infection, injury, poison and so on.
Effect: Pain experienced along the length of the nerve. There may also be loss of movement in the part of the body supplied by the nerve.
Bell's Palsy
A disorder that attacks the nerves that control movement of the muscles in the face. This nerve is the 7th facial cranial nerve. Injury or infection can cause the nerve to become inflamed.
Effect: Weakness or paralysis of the face.
Ganglion
A hard, small bump above a tendon or in the capsule that encloses a joint. A ganglion lump is is a non-cancerous cyst filled with a thick, jelly-like fluid. Ganglions can develop on or beneath the surface of the skin and usually occur between the ages of 20 and 40.
Effect: Quite harmless and usually painless, but range of motion maybe be affected.
Sciatica
Pressure or damage to the sciatic nerve. The sciatic nerve starts in the lower spine and runs down the back of each leg. Common causes include slipped disk, pelvic injury or degeneration of an intervertebral disc.
Effect: Dull pain down the back, hip or one leg. Pain is often worse after standing for sitting for long periods and at night.
Parkinson's Disease
Progressive disease caused by damage to basal ganglia of the brain. This area is responsible for making a brain chemical called dopamine which helps control muscle movement. When Parkinson's occurs the brain cells responsible for dopamine production slowly die.
Effect: Causes tremors, difficulties with walking, coordination and movement; also speech and vision problems.
Multiple Sclerosis (MS, also known as disseminated sclerosis)
Autoimmune disease is caused by loss of the protective myelin sheath that surrounds nerve fibers. MS affects more women than men and strikes more often between the ages of 20 and 40.
Effect: Loss of balance, numbness, problems walking, tremors and vision problems. Symptoms can come and go, lasting for days, weeks or months at a time.
Cerebral Palsy
Damage to the brain, caused during birth or resulting from a birth defect.
Effect: Symptoms can be mild to severe and usually only affect one side of the body. There can be muscle weakness leading to an abnormal walk; speech problems (dysarthria), hearing and vision problems and seizures.
Motor Neurone Disease
A rare progressive disorder, in which the motor neurones in the body gradually deteriorate.
Effect: Weakness and wasting of the muscles, making everything more difficult from speaking and walking to breathing and swallowing. Eventually leads to complete disability and death.
Myalgic Encephalomelitis (ME)
Also known as post-viral fatigue or chronic fatigue syndrome.
Effect: Constant exhaustion, general aches and pains, headaches and dizziness, vertigo, inflammation of the brain and spinal cord.
See, what is the difference between fibromyalgia and chronic fatigue syndrome?
Stress
Stress is any factor that affects mental or physical well-being. Emotions such as anxiety, fear and other negative feelings can affect the nervous system causing increased heart rate, breathing difficulties, sleep disturbances and stomach problems. All of these physical effects are caused by the nervous system over-working in response to stress.
See: Dangers of stress.
Depression
There are various forms of depression, manic depression, seasonal affective disorder (SAD) - also known as winter depression or winter blues and postpartum depression - a form of clinical depression which can affect women after childbirth.
See: Effects of depression.
Epilepsy
A disorder of the central nervous system characterized by loss of consciousness and convulsions.
Migraine
Severe headache that usually only affects one side of the head. It is often accompanied by vomiting, visual problems, nausea and photophobia (intolerance to light).
Stroke
A stroke is where blood flow to a part of the brain is suddenly stopped. A stroke is not a heart attack, but rather a brain attack. It occurs when blood flow is suddenly reduced, either by a blockage or a rupture in an artery in the brain. Read more about strokes in women.
Transient Ischemic Attack (TIA)
TIAs are also called mini strokes. They are caused by a temporary interruption of blood flow to the brain. While the signs are similar to symptoms of stroke, a TIA does not cause permanent damage to brain cells nor does it result in disability. All signs of the TIA disappear within 24 hours. However although they may cause no harm, TIAs can recur and each time you have a TIA it increases your chance of a stroke.
Alzheimer's Disease
Alzheimer's disease is a common form of dementia in older people. It causes problems with thinking, memory and behavior. Symptoms of Alzheimer's worsen over time and interfere with daily tasks.
Concussion
Injury to the brain caused by a blow. It usually results in a loss of consciousness.
Dementia
Dementia is the loss of brain function which is caused by brain injury. Alzheimer is the most common cause although it can also be caused by other diseases such as Huntington's disease, MS and Parkinson's. It results in long-term decline in brain function beyond what might be expected from normal aging.
Meningitis
An dangerous infectious disease characterized by inflammation of the meninges (the tissues that surround the brain or spinal cord). It is usually caused by a bacterial infection. Warning signs include a sudden headache, stiff neck, fever and nausea.
Myasthenia Gravis
An autoimmune disorder it occurs when the immune system mistakenly attacks healthy tissue. The body then produces antibodies that block muscle cells from receiving messages (neurotransmitters). It causes weakness in voluntary muscles. Signs include difficulty talking, a drooping head, eyelid drooping, and chewing difficulties.
Neuropathy - Nerve Damage
The nervous system can be attacked by poorly controlled diabetes. Nerve damage is usually found in those who have diabetes the longest, typically people over 40, with poor glucose control and who smoke. Diabetic neuropathy is a disease that often leads to foot ulcerations and amputation. See diabetes complications.
Neuralgia
Various causes, usually due to irritation or damage of the nerve.
Effect: Short, sharp, burning or stabbing pain along the path of one or more nerves.
Paralysis
A loss of the ability to move a body part. It happens when something goes wrong with the way messages are passed from the brain to the affected muscles. Paralysis of both legs is called paraplegia. Paralysis of the arms and legs is quadriplegia.
Peripheral Neuropathy
Damage to nerves of the PNS system. It may be caused by a disease of the nerves or from the side effects of a systemic (whole-body) illness.
Poliomyelitis (Polio)
A highly infectious viral disease (spreads from person to person) that causes inflammation of the nerve cells of the brain stem and spinal cord. 1 in 200 infections lead to irreversible paralysis.
Spinal Cord Injury
Damage to the spinal cord. It can result from injury direct to the spinal cord or indirectly through diseases of surrounding tissue or bones.
Spina Bifida
Spina bifida is a birth defect where the spinal cord fails to develop properly while the baby is still growing in the womb. It is one of the most common types of neural tube defects.

What It Links To

Nervous system links to:

All systems: Nerves from the central nervous system control and receive information from every body system.
Muscular system: Muscles require a nerve impulse to contract.
Skeletal system: Muscle contraction (caused by nerve impulses) produces movement in the skeleton.
Circulatory system: Nerves control the heart rate.
Respiratory system: Nerves control the process of respiration.
Endocrine system: Works closely with the endocrine (hormone) system to maintain homeostasis — balance in the body.
Skin system: The skin contains a variety of nerve endings, at different levels in the layers

Summary

The nervous system: has two parts, the central and peripheral (including autonomic) nervous systems informs and warns the body of environmental changes, sensations, pain and danger and initiates responses to stimuli.

Other Useful Guides

Latest female health statistics: Life expectancy and how is health measured?
Main causes of death in women: Conditions and diseases that are life threatening.
Development of the female body: How the body grows through puberty.
Women's health books: Popular health books and resources.
Hospital departments explained: How to find your way around a hospital.

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