Understanding the body's most complex communication network
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The Body's Command Centre
The nervous system is one of the most complex of all human body systems. More than 10 billion nerve cells operate constantly throughout the body to coordinate activities we do consciously or voluntarily, as well as those that occur unconsciously or involuntarily.
We speak, move muscles, hear, taste, see, think—our glands secrete hormones, we respond to danger, pain, temperature, and touch. We have memory, association, and discrimination. All these functions compose only a small number of the many activities controlled by our nervous system.
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How Nerve Cells Communicate
Microscopic nerve cells collected into macroscopic bundles called nerves carry electrical messages throughout the body. External stimuli, as well as internal chemicals such as acetylcholine, activate the cell membranes of nerve cells to release stored electrical energy within the cells.
This energy, when released and passed through the length of the nerve cell, is called the nervous impulse. External receptors like sense organs and internal receptors in muscles and blood vessels receive and transmit these impulses to the complex network of nerve cells in the brain and spinal cord.
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Two Major Divisions
Central Nervous System (CNS)
Consists of the brain and spinal cord—the primary control centres for processing and coordinating information.
Peripheral Nervous System (PNS)
Comprises 12 pairs of cranial nerves and 31 pairs of spinal nerves that connect the CNS to the rest of the body.
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The Autonomic Nervous System
In addition to spinal and cranial nerves, the peripheral nervous system consists of a large group of nerves that function involuntarily or automatically without conscious control. These are the nerves of the autonomic nervous system.
Sympathetic Nerves
Stimulate the body in times of stress and crisis:
- Increase heart rate and forcefulness
- Dilate airways for more oxygen
- Increase blood pressure
- Stimulate adrenal glands to secrete epinephrine
- Inhibit intestinal contractions
Parasympathetic Nerves
Act as a balance for sympathetic nerves:
- Slow down heart rate
- Contract pupils of the eye
- Lower blood pressure
- Stimulate peristalsis
- Increase secretions like saliva
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The Neuron: Individual Nerve Cell
Neurons are individual nerve cells. A stimulus triggers a wave of excitability in the dendrites, the branching fibers that receive impulses.
This electrical charge change propels the impulse along the dendrites, moving like falling dominoes. The impulse travels in one direction, reaching the cell body, which houses the nucleus.
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Neuron Components and Function
Dendrites
Branching fibres that receive the nervous impulse first and carry it towards the cell body.
Cell Body
Contains the cell nucleus and processes the incoming impulse from dendrites.
Axon
Extends from the cell body and carries the impulse away from it towards other neurons or target cells.
Terminal End Fibres
The nervous impulse passes through the axon to leave the cell via these terminal fibres.
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Myelin Sheath and White Matter
Axons are covered with a fatty tissue called myelin sheath. The myelin sheath gives a white appearance to the nerve fibre; hence the term white matter, as in parts of the spinal cord, white matter of the brain, and most peripheral nerves.
The grey matter of the brain and spinal cord refers to the collections of cell bodies and dendrites that appear grey because they are not covered by a myelin sheath. Another axon covering, called the neurilemma, is a membranous sheath outside the myelin sheath on the nerve cells of peripheral nerves.
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The Synapse and Neurotransmitters
The space between neurons through which the nervous impulse jumps from one neuron to another is called the synapse. The transfer of the impulse across the synapse depends upon the release of a chemical substance, called a neurotransmitter, by the neuron that brings the impulse to the synapse.
Acetylcholine
Key neurotransmitter for muscle activation and memory
Epinephrine
Stress response and alertness neurotransmitter
Dopamine
Regulates movement, motivation, and reward
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Nerves: Macroscopic Structures
Whereas a neuron is a microscopic structure within the nervous system, a nerve is macroscopic, able to be seen with the naked eye. A nerve consists of a bundle of dendrites and axons that travel together like strands of rope.
Afferent (Sensory) Nerves
Peripheral nerves that carry impulses to the brain and spinal cord from stimulus receptors like the skin, eye, ear, and nose.
Efferent (Motor) Nerves
Nerves that carry impulses from the CNS to organs that produce responses, such as muscles and glands.
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The Brain: Command Centre
The brain is the primary centre for regulating and coordinating body activities. In the human adult, it weighs about 3 pounds and has many different parts, all of which control different aspects of body functions.
The largest part of the brain is the cerebrum. The outer nervous tissue of the cerebrum, known as the cerebral cortex, is arranged in folds to form elevated portions known as convolutions (also called gyri) and depressions or grooves known as fissures (also called sulci).
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The Cerebrum and Its Functions
Structure
The cerebral hemispheres are the paired halves of the cerebrum. Each hemisphere is subdivided into four major lobes named for the cranial (skull) bones that overlie them.
Functions
- All thought, judgement, memory, association, and discrimination
- Reception of sensory impulses through afferent cranial nerves
- Basis for perception when registered in the cortex
- Motor impulses to muscles and glands via efferent nerves
- Production of movement and activity
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Cerebrospinal Fluid (CSF)
Within the middle region of the cerebrum are spaces, or canals, called ventricles. They contain a watery fluid that flows throughout the brain and around the spinal cord. This fluid is called cerebrospinal fluid (CSF) and protects the brain and spinal cord from shock as might a cushion.
Composition
CSF is usually clear and colourless and contains lymphocytes, sugar, chlorides, and protein.
Function
Provides cushioning protection for the brain and spinal cord against mechanical shock and injury.
Clinical Use
Spinal fluid can be withdrawn for diagnosis or relief of pressure on the brain through spinal or lumbar puncture.
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Thalamus and Hypothalamus
The two other important parts of the brain, the thalamus and hypothalamus, are located below the cerebrum.
Thalamus
A large mass of grey matter that acts as a relay centre for impulses that travel from receptors such as the eye, ear, and skin to the cerebrum. The thalamus integrates and monitors these sensory impulses, suppressing some and magnifying others. Perception of pain is controlled by this area of the brain.
Hypothalamus
Located below the thalamus, it contains neurons that control body temperature, sleep, appetite, sexual desire, and emotions such as fear and pleasure. The hypothalamus also regulates the release of hormones from the pituitary gland and integrates the activities of the sympathetic and parasympathetic nervous systems.
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Cerebellum, Pons, and Medulla Oblongata
The cerebellum, pons, and medulla oblongata lie below the posterior portion of the cerebrum and connect the cerebrum with the spinal cord. The pons and medulla are part of the brain stem.
Cerebellum
Located beneath the posterior part of the cerebrum. Its function is to aid in the coordination of voluntary movements and to maintain balance, posture, and muscular tone.
Pons
A part of the brain that literally means bridge. It contains nerve fibre tracts that connect the cerebellum and cerebrum with the rest of the brain.
Medulla Oblongata
Located at the base of the brain, connects the spinal cord with the rest of the brain. Nerve tracts cross over (decussate) in the medulla oblongata.
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Medulla Oblongata: Vital Functions
The medulla oblongata regulates critical internal activities of the body. Nerve cells that control the movement of the left side of the body are found in the right half of the cerebrum, and these axons cross over to the opposite side in the medulla oblongata before travelling down the spinal cord.
Respiratory Centre
Controls muscles of respiration in response to chemicals and other stimuli
Cardiac Centre
Slows the heart rate when the heart is beating faster
Vasomotor Centre
Affects the muscles in the walls of blood vessels, thus influencing blood pressure
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The Spinal Cord Structure
The spinal cord is a column of nervous tissue extending from the medulla oblongata to the second lumbar vertebra within the vertebral column. It ends as the cauda equina (horse's tail), a fan of nerve fibres found below the second lumbar vertebra of the spinal column.
It carries all the nerves that affect the limbs and lower part of the body, and it is the pathway for impulses going to and from the brain. A cross-section reveals an inner section of grey matter (containing cell bodies and dendrites) and an outer region of white matter (containing myelinated nerve fibre tracts).
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The Meninges: Protective Layers
The meninges are three layers of connective tissue membranes that surround the brain and spinal cord, providing essential protection.
Dura Mater
The outermost membrane of the meninges—tough and protective
Arachnoid Membrane
The second layer, loosely attached to other meninges by web-like fibres with space for fluid between
Pia Mater
The third layer, closest to the brain and spinal cord—thin and delicate
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Essential Nervous System Vocabulary
Acetylcholine
Neurotransmitter chemical released at the ends of some nerve cells
Afferent Nerves
Nerves that carry impulses toward the brain and spinal cord; sensory nerves
Efferent Nerves
Nerves that carry impulses away from the brain and spinal cord to muscles, glands, and organs; motor nerves
Blood-Brain Barrier
Blood vessels (capillaries) that selectively let certain substances enter the brain tissue and keep other substances out
Neuroglia
Cells in the nervous system that do not carry impulses but are supportive and connective in function. There are about 100 billion neuroglial cells
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More Key Terminology
Structural Terms
- Convolution: Elevated portion of the cerebral cortex; gyrus
- Fissure: Depression or groove in the surface of the cerebral cortex; sulcus
- Plexus: A large, interlacing network of nerves
- Ventricles: Reservoirs (canals) in the interior of the brain filled with cerebrospinal fluid
Functional Terms
- Receptor: An organ that receives nervous stimulation and passes it on to nerves within the body
- Stimulus: A change in the internal or external environment that can evoke a response
- Synapse: The space through which a nervous impulse is transmitted from one neuron to another
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Categories of Neurological Diseases
Neurological diseases may be classified into six major categories of disorders:
Congenital
Present from birth
Degenerative, Movement & Seizure
Progressive deterioration
Infectious
Caused by pathogens
Neoplastic
Tumour-related
Traumatic
Injury-related
Vascular
Blood vessel-related
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Congenital Disorders
Hydrocephalus
Abnormal accumulation of fluid (CSF) in the brain, causing increased pressure and potential brain damage if untreated.
Spina Bifida
Congenital defect in the spinal column due to imperfect union of vertebral parts, resulting in incomplete closure of the spine.
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Degenerative and Movement Disorders
Alzheimer's Disease
Brain disorder marked by deterioration of mental capacity (dementia) beginning in middle age
Amyotrophic Lateral Sclerosis (ALS)
Progressive disorder characterised by degeneration of motor neurons in the spinal cord and brain stem; also called Lou Gehrig's disease
Multiple Sclerosis (MS)
Destruction of the myelin sheath on neurons in the CNS and its replacement by plaques of sclerotic (hard) tissue
Parkinson's Disease
Degeneration of nerves in the brain, occurring in later life, leading to tremors, weakness of muscles, and slowness of movement
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More Degenerative Conditions
Epilepsy
Chronic disorder characterised by recurrent seizure activity
Huntington's Chorea
Hereditary nervous disorder due to degenerative changes in the cerebrum involving bizarre, abrupt, involuntary, dance-like movements
Myasthenia Gravis
Neuromuscular disorder characterised by relapsing weakness of skeletal muscles
Tourette Syndrome
Neurological disorder marked by involuntary, spasmodic, twitching movements; uncontrollable vocal sounds; and inappropriate words
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Infectious, Neoplastic, and Traumatic Disorders
Meningitis
Inflammation of the meninges; leptomeningitis
Shingles
Viral disease affecting peripheral nerves
Brain Tumours
Abnormal growths of brain tissue and meninges
Cerebral Concussion
Temporary brain dysfunction (brief loss of consciousness) after injury, usually clearing within 24 hours
Cerebral Contusion
Bruising of brain tissue as a result of direct trauma to the head; neurological deficits persist longer than 24 hours
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Cerebrovascular Accident (CVA)
Disruption in the normal blood supply to the brain. This condition, also known as stroke or cerebral infarction, is the result of a localised area of ischaemia (and ultimately infarction or necrosis) in the brain.
Thrombotic Stroke
Caused by a blood clot forming in a blood vessel in the brain
Embolic Stroke
Caused by a clot that travels from elsewhere in the body to the brain
Haemorrhagic Stroke
Caused by bleeding in or around the brain
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Laboratory Tests and Clinical Procedures
Cerebrospinal Fluid Analysis
Cell counts, bacterial smears, and cultures of samples of CSF are done when disease of the meninges or brain is suspected. Normal constituents of CSF are water, glucose, sodium, chloride, and protein. Changes in their concentrations are helpful in diagnosis of brain disease. CSF normally has more chloride and less glucose than blood.
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Imaging Procedures
Cerebral Angiography
Contrast medium is injected into an artery and X-rays are taken of the blood vessel system of the brain to diagnose vascular disease
Computed Tomography (CT)
X-rays compose a computerised cross-sectional picture of the brain and spinal cord
MRI of the Brain
Magnetic waves create images of the brain in multiple planes; excellent for viewing soft tissues
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Advanced Diagnostic Procedures
Electroencephalography (EEG)
Recording of the electrical activity of the brain. Used to demonstrate seizure activity, brain tumours, and other diseases and injuries to the brain.
Lumbar (Spinal) Puncture
CSF is withdrawn from between two lumbar vertebrae. A device can measure CSF pressure. Contrast medium or intrathecal medicines may be administered through the puncture.
PET Scan
An isotope combined with glucose is injected intravenously. Cross-sectional images show how the brain uses glucose and give information about brain function.
Stereotactic Radiosurgery
Use of a stereotactic instrument fixed onto the skull to locate targets by three-dimensional measurement. Gamma knife delivers high-energy radiation beams to treat deep brain tumours.
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The Nervous System