Download Disorders of the Nervous System

THE NERVOUS SYSTEM
Tereza Šturmová, Jindřiška Šobáňová
The nervous system is an integrated multipurpose system made up of many parts. It
contains the higher human functions such as memory and reasoning. It controls and
coordinates all parts of the body and provides a complex communication system between the
body’s internal and external environments.
Structurally, the nervous system is composed of two main parts:
1) The central nervous system (CNS), which consists of the brain and spinal cord.
2) The peripheral nervous system (PNS), e.g. spinal and cranial nerves.
There are also two functional divisions of the nervous system as:
a) The somatic or voluntary nervous system, which is concerned with the transmission of
impulses (coded messages) to and from the non-visceral parts of the body such as skeletal
muscles, bones, joints, ligaments, skin, eyes and ears.
b) The autonomic or involuntary nervous system, which is concerned with regulation of
the activities of visceral muscles and glands.
In each division the tissues consists of specialized nerve cells called neurons, and also
of the cells called neuroglia, which provide structural support for the neurons. They have two
characteristics: irritability, which means that neurons can response to a stimulus; and
conductivity, which refers to a neuron’s ability to transmit impulses.
A neuron consists of three main parts: cell body, dendrite and axon. The cell body
contains cytoplasm and a nucleus. Extensions from the cell body are called axons, which
conduct impulses away from the cell body. Cell bodies are in the grey matter of the brain,
spinal cord and ganglia; dendrites and axons extend throughout the body.
Neurons may be classified according to their function as:
a) Motor neurons (or efferent), which carry motor impulses from the brain or spinal cord to
the muscles or organs to initiate activity,
b) Sensory neurons (or afferent), which carry sensory impulses from a body part to the brain
or spinal cord. Their dendrites have receptors for stimuli from the environment.
c) Connecting neurons (or internuncial), which transmit impulses from one part of the brain
to another.
A nerve impulse is a wave of depolarization created by a chemical inbalance.Sodium
passes through the membrane,releasing potassium.The depolarization of any part of the nerv
cell causes the depolarization of the next segment,and so on to the end of the fibre.The axon
of one neuron transmits an impulse to the dendrite of another neuron. A microscopic space
between an axon and a dendrite is called a synapse. Chemicals, which help an impulse to
cross the synapse (such as acetylcholine and catecholamine), are called neurotransmitters.
The Central Nervous System
The Brain
The human brain is a large organ comprised of billion of neurons. Total brain tissue
weight is approximately 1344 g. It is concerned with thought, memory and consciousness. It
is also concerned with a range of sensory experiences, with motor activity, the regulation of
visceral, endocrine and somatic functions and with the use of symbols and signs that underline
communication. For descriptive purposes the brain may be subdivided into the cerebrum,
basal ganglia, thalamus, hypothalamus, midbrain, pons, medulla oblongata and cerebellum.
The brain lies in the skull and is protected by bony structures, membranes and fluid.
The three membranes surrounding the brain and the spinal cord are called the meninges. They
are:
a) Dura mater – outer covering
b) Arachnoid (mater) – inner membrane
c) Pia mater – it fits around the brain and spinal cord.
The space between the arachnoid and pia mater contains cerebrospinal fluid – CSF,
which protects the organs from injury. It is formed in capillaries that filter fluid from blood
circulating in the brain and is collected in four cavities within the cerebral hemisphere called
ventricles. The average amount of cerebrospinal fluid for an adult is about 150 ml. It is a clear
and watery fluid that contains glucose, protein, and such minerals as sodium, potassium,
calcium and magnesium.
The cerebrum is the most important part of nervous system, with the areas that control
thinking, hearing, speaking, seeing and feelings. The longitudinal fissure divides the cerebrum
into right and left hemispheres, which are completely separated except where they are joined
by the corpus callosum. Other fissures separate each hemisphere into four lobes – each
controls different actions:
1. frontal lobe – voluntary muscle movement and speech
2. parietal lobe – touch, pain, temperature
3. temporal lobe (or auditory) – interpreting sounds
4. occipital lobe (or visual) – interpreting sights
The brain contains areas of tissue; the internal tissue is called white matter, and the
outer layer is known as grey matter, or the cerebral cortex. It is a highly specialised area
whose functions are not precisely understood at present. At the base of the cerebrum there is
also the olfactory (or smell) centre that receives impulses from receptors located in nose.
Deep within the white matter of each cerebral hemisphere are groups of nerve body tissues
called the basal ganglia (nuclei), which have many functions, for instance to control normal
body posture and movements.
The thalami are a pair of egg-shaped masses of grey matter at the base of each
hemisphere. They form the main relay centre for sensory impulses and cerebellar and basal
ganglia projections to the cerebral cortex.
The hypothalamus is an important grey mass, which lies beneath the thalamus. It
contains nuclei of the autonomic nervous system for the control of most of the body’s
involuntary functions as well as many aspects of emotional behaviour. It exerts a control of
vasomotor tone and heart rate, regulation of body temperature and body water. The
hypothalamus is also concerned with gastro-intestinal and feeding regulation and many other
functions.
The midbrain is a short segment of the brain that is below the thalami and the pons
where the third and fourth cranial nerves originate. It contains many nerve tracts and is a
station for sight and hearing.
The brain stem is composed from the pons varolii and the medulla oblongata, although
the midbrain can be considered as the upper part. It is composed from many conduction
pathways and autonomic centres that regulate breathing, cardiac rate, coughing, vomiting,
sneezing, etc.
The cerebellum lies beneath the posterior portion of the cerebrum from which it is
separated by a fold of meningeal membrane – dura mater. It is attached to the midbrain, pons
and medulla, and is integrated into many connective pathways for the provision of muscle
tissue coordination in the body. All sensations are relayed through the cerebellum, providing
information about muscle activity.
The spinal cord is a long, cylindrical structure within the vertebral column, continuous
with the medulla oblongata, originating at the foramen magnum of the skull and extending to
the first or second lumbar vertebra. It is completely surrounded by spinal fluid and the
meninges, and consists of an outer portion of white matter in the shape of a letter H. It serves
as the reflex centre of the body (e.g. involuntary eye blink).
The Peripheral Nervous System
A nerv is a cord like structure, usually containing bundels of conducting fibres witch
are compoust of many sensory or motor neurons as well as ganglia. Sensory neurons receive
stimuli and send impulses to the CNS; motor neurons carry messages from the CNS to
appropriate parts of the body. The nerves may be divided into two main groups: cranial and
spinal nerves. There are twelve pairs of cranial nerves; ten pairs control areas of the head, one
serves muscles in the head and one pair extends to thoracic and abdominal organs.
Thirty-one pairs of spinal nerves serve the extremities and the trunk.All spinal nerves
contain both sensory and motor fibres.There are 8 pairs of cervical nerves,12 thoracic,5
lumbar, 5 sacral, and 1 coccygeal. Injury to a spinal nerve causes a loss of sensation or loss of
movement.
The autonomic nervous system controls the visceral functions of the body. It has two
divisions:
1) The sympathetic system, which generalises psychological responses to emergency
situations like stress, pain, cold, or any threat. They mobilize the body’s resources for
defensive action that results in increased secretion of hormones adrenaline and
noradrenaline.
2) The parasympathetic system has, in general, the opposite effects; it conserves the body’s
energy and helps in the elimination of body wastes.
Disorders of the Nervous System
They are very serious and life threatening and most of them leave the patient with a
disability. The discipline dealing with the neural problems is known as neurology.
The most common disorders known are speech dysfunctions such as aphasia (loss of
ability to understand words or to use them to communicate), dysphasia (lack of ability to put
the words in order), dyslexia (inability to comprehend written words), etc.
Parkinson’s disease, which is shown by abnormal motor activities, tremors, rigidity,
slowness and limited voluntary movements that may occur due to the blockage of dopamine
secretion (it is an amino acid needed for transmission of nerve impulses in basal ganglia).
There is no cure for any from these diseases.
Mental changes include disorientation to time and place and confusion. Causes of
delirium include dysfunction of the cerebral cortex, withdrawal of alcohol or drugs and body
high temperature (usually over 40,5). If it is difficult to arouse a patient, the level of
consciousness is called stupor; a state of unconsciousness from which the patient usually
cannot be aroused is called a coma.
Multiple sclerosis is a condition in which the myelin sheath of nerves is destroyed and
white matter in the brain and spinal cord is affected. As the disease progresses, mental
changes occur and the patient may need custodial care.
Viruses or bacteria may also cause some of the disorders, for instance: meningitis,
encephalitis or poliomyelitis. All of them are manifested by high body temperature, severe
headache, nausea, vomiting and lethargy.
Meningitis is an infection of the meninges caused by streptococci, pneumococci or
staphylococci that travel to meninges via the bloodstream from another part of the body, such
as nose, throat or middle ear. Antibiotic therapy is the primary treatment.
A complication of an infection elsewhere in the body, especially influenza, chicken pox
or measles, is encephalitis (known as sleeping sickness). There is no drug to destroy the
virus, so treatment is directed towards relief the symptoms.
Poliomyelitis is a viral infection of motor neurons in the spinal cord and brain stem,
caused by three strains of viruses. It does not respond to drug therapy, but inoculation with
vaccine is effective for all three strains. Visualization of the skull, brain and vessels within the
brain is possible radiological examination. The most dangerous and painful, but very useful
technique is pneumo-encephalography. The procedure involves performing a lumbar
puncture, removing a small amount of spinal fluid and injecting an equal amount of air or
oxygen. Defects (like tumours) displace the air and are seen on the X-ray film.
An electro-encephalogram (EEG) is a recording of electrical activity in the brain;
electrodes are placed on the patient’s head and cell activity (known as brain waves) is
recording on the graph. An ultrasound study of the brain is called an echoencephalogram.