Art & science life sciences: 20
Nervous system: part 1
Farley A et al (2014) Nervous system: part 1.
Nursing Standard. 28, 31, 46-51. Date of submission: March 30 2012: date of acceptance: November 5 2012.
Abstract
This article, which forms part of the life sciences series and is the
first of three articles on the nervous system, explores the major
divisions of the nervous system and their functions. The basic
structure of a nerve cell is described, and generation and conduction
of nerve impulses is discussed. Blood supply to the brain is also
covered. The second article will examine the central nervous system
(CNS) in greater detail, including protection of the CNS, and the
structure and function of the cerebral cortex and cerebellum. The
third article will examine the peripheral nervous system and the
autonomic nervous system, and provides an overview of some of the
disorders of the nervous system.
Authors
Alistair Farley, Retired, was lecturer in nursing, School of Nursing
and Midwifery, University of Dundee, Dundee, Scotland.
Carolyn Johnstone, Lecturer in nursing,
School of Nursing and Midwifery, University of Dundee.
Charles Hendry, Retired, was senior lecturer,
School of Nursing and Midwifery, University of Dundee.
Ella McLafferty, Retired, was senior lecturer,
School of Nursing and Midwifery, University of Dundee.
Correspondence to: c.c.johnstone@dundee.ac.uk
Keywords
Autonomic nervous system, central nervous system,
nervous system, neurone, peripheral nervous system, stroke
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HOMEOSTASIS IS VITAL to human wellbeing
and health, and is maintained through the
combined action of the nervous and endocrine
systems. The nervous system monitors and
responds to changes in the internal and
external environment. It is also responsible for
perception, behaviour and memory, and initiates
all voluntary movements. The nervous system
46 april 2 :: vol 28 no 31 :: 2014
includes all of the neural tissue in the body.
Neural tissue carries information from one
region of the body to another. Integration and
co-ordination occurs within the brain and spinal
cord, which form the central nervous system
(CNS). The peripheral nervous system includes
all of the neural tissue outside the CNS (Tortora
and Derrickson 2013).
Brain
The adult brain contains almost 100 billion nerve
cells or neurones (Tortora and Derrickson 2013).
Neuroglia (glial cells) make up the remaining 90%
of nervous tissue located in the brain (Thibodeau
and Patton 2010). There are six major divisions
in the adult brain: the medulla oblongata, pons,
midbrain, cerebellum, diencephalon and cerebrum.
The medulla, pons and midbrain are often referred
to as the brain stem (Thibodeau and Patton 2010).
The cerebrum is the structure most frequently
mentioned when referring to the brain. The
diencephalon is situated between the cerebrum
and the midbrain and consists of the thalamus,
hypothalamus, optic chiasma, pineal gland and
other small structures.
Spinal cord
The spinal cord is continuous with the brain
stem and exits the cranium at the foramen
magnum of the occipital bone. It extends to the
lower border of the first lumbar vertebra.
It aids homeostasis by providing rapid reflexive
responses to many stimuli (Tortora and
Derrickson 2013). The spinal cord carries
sensory information in the form of nerve
impulses to the brain and motor responses
away from the brain.
Peripheral nervous system
The brain and spinal cord form the CNS. Cranial
nerves communicate directly with the brain
and spinal nerves communicate with the spinal
cord. The peripheral nervous system consists of
all nervous tissue outside the CNS. The nervous
system has a number of subdivisions as shown in
Figure 1.
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Somatic nervous system
Sensory and motor nerves, also known as afferent
and efferent nerves, communicate with the CNS.
Motor nerves communicate with skeletal muscle
only and are under voluntary control. Sensory nerves
carry information to the brain and spinal cord from
somatic receptors found throughout the body.
Autonomic nervous system
The activity of the autonomic nervous system
(ANS) is not under conscious or voluntary
control. The ANS is divided into two parts:
the sympathetic division and parasympathetic
division. The ANS regulates temperature, heart
rate, blood pressure and blood glucose.
protein synthesis and maintaining the health
of the cell. Cell bodies are found mainly in the
CNS packed together forming tissue known
as grey matter. They are responsible for the
analysis, integration and storage of information.
The soma does not contain a centrosome and,
therefore, neurones cannot undergo mitosis
(Thibodeau and Patton 2010). It is important to
appreciate that this means damaged brain tissue,
for example which occurs following stroke,
cannot be replaced.
FIGURE 2
Structure of a myelinated neurone
Enteric nervous system
Nucleus
A further subdivision of the peripheral nervous
system has been identified and is known as the
enteric nervous system (ENS). The ENS is the
intrinsic nervous system of the gastrointestinal
tract. It has been referred to as the ‘brain of the
gut’ (Tortora and Derrickson 2013) and exercises
local control over gastrointestinal function.
Cell body
Axon
Neurilemma
Neurones
Dendrites
Neurones are the functional units of the nervous
system. They are composed of three parts: the
cell body, dendrites and axon (Thibodeau and
Patton 2010). The dendrites and axon may be
referred to as nerve fibres (Figure 2).
A neurone consists of a cell body, also known
as a soma or perikaryon, containing a nucleus
surrounded by cytoplasm. It is responsible for
Nucleus of
Schwann cell
PETER LAMB
Cell body
Nodes of Ranvier
Synaptic
knob
Myelin sheath
Axon terminal
FIGURE 1
Subdivisions of the nervous system
Central nervous system
Peripheral nervous system
Sensory (afferent) nerves
Motor (efferent) nerves
Somatic nerves
Brain and spinal cord
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Autonomic nerves
Sympathetic nerves
Parasympathetic nerves
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Art & science life sciences: 20
Dendrites
Dendrites are branching cytoplasmic projections
of the cell body. They receive information and
direct it into the cell body. They are often highly
branched and may account for most of the total
surface area of the neurone.
Axon
The axon is a single cytoplasmic projection of
the cell body and directs information away from
the cell body. It can vary in length from less than
one millimetre to more than one metre. The
axon may or may not have collateral branches.
At its distal tip, the axon gives rise to several
finer terminal branches, the axon terminal
or telodendria. When gathered together into
TABLE 1
Structural classification of neurones
Structural classification
Characteristics
Multipolar
Contains several dendrites
communicating directly with the cell
body, and a single axon. Most neurones
in the central nervous system are
multipolar.
Bipolar
Contains one main dendrite and one
axon. These neurones are found in the
retina of the eye, inner ear and olfactory
area of the brain.
Unipolar
Contains dendrites and one axon fused
together forming a single process
extending from the cell body. These are
sensory neurones.
Anaxonic
Small neurones in which the dendrites
and axon are indistinguishable.
(Thibodeau and Patton 2010, Tortora and Derrickson 2013)
TABLE 2
Functional classification of neurones
Functional classification
Characteristics
Sensory or afferent
neurones
Transmit nerve impulses to the central
nervous system (CNS) via cranial or
spinal nerves. Monitor the internal and
external environment and convey this
information to the CNS.
Motor or efferent neurones
Transmit nerve impulses away from the
CNS to muscles and glands, bringing
about an action, for example muscular
contraction or glandular secretion.
Interneurons or association
neurones
Found mainly in the CNS and are
responsible for linking sensory and
motor neurones.
(Thibodeau and Patton 2010, Tortora and Derrickson 2013)
48 april 2 :: vol 28 no 31 :: 2014
bundles, dendrites and axons form white matter
and are responsible for the transmission of nerve
impulses. The white appearance is the result of
the presence of myelin surrounding the axon
(Tortora and Derrickson 2013).
Structural classification of neurones
Structurally, neurones are classified according to
the number of processes extending from the cell
body (Tortora and Derrickson 2013). General
categories include multipolar, bipolar, unipolar
and anaxonic neurones (Table 1). Neurones may
also be classified according to function (Table 2).
Nerve impulse transmission
To be effective, nerves must communicate with
one another and with target tissues. Information
must be conveyed to the CNS via sensory or
afferent neurones, and the CNS must process
this information before initiating a response via
motor or efferent neurones. Information can be
conveyed via electrical and/or chemical means.
Nerve cells are unique within the body in that
they generate and conduct signals called nerve
impulses (Thibodeau and Patton 2010). These
nerve impulses are electrical in nature and are
often referred to as action potentials.
Transmission of the impulse or action potential
occurs as a result of the movement of ions across
the nerve cell membrane (Waugh and Grant
2010). Differences in electrical charge exist on
either side of the cell membrane. This is called the
potential difference and is the result of unequal
distribution of potassium ions and sodium ions on
either side of the membrane. In the resting state,
the charge on the inside of the cell membrane is
negative, while the charge on the outside of the
cell is positive (Waugh and Grant 2010). Within
the cell, the potassium level is 148mmol/L and
the sodium level is 10mmol/L, whereas outside
the cell the potassium level is 5mmol/L and
the sodium level is 142mmol/L. It is important
to note that substances tend to move down a
concentration gradient by diffusion. At rest, the
cell membrane is impermeable to sodium ions,
however potassium ions diffuse slowly out of the
cell through open potassium channels. These
channels are protein structures located within the
phospholipid bilayer of the cell membrane, with
a central pore or channel through which ions can
pass. As the positively charged potassium leaves
the cell, the inside of the cell carries a greater
negative charge (Seeley et al 2008).
There is an overall difference in charge on
either side of the membrane – positive outside
and negative inside. This difference is known as
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Synapse
Impulses are conducted from one neurone to
another cell across a type of junction known as
a synapse. This other cell can be either another
neurone or an effector cell or organ such as a
muscle or gland. At a synapse, the activity of one
neurone affects the membrane characteristics of
another cell. This is dependent on the presence
of chemicals known as neurotransmitters.
Communication usually occurs from the
presynaptic neurone to the postsynaptic neurone
or effector cell (Figure 3).
A synapse where a neurone communicates with
another cell type represents a neuroeffector
junction. The presynaptic membrane and
the postsynaptic membrane are separated
by a narrow gap known as the synaptic cleft
(Figure 3). The diffusion of a neurotransmitter
across this cleft accounts for the observed
synaptic delay in the transmission of the impulse
(Jenkins and Tortora 2013).
When the nerve impulse reaches the end of the
axon, it divides and enters the small branches
terminating in synaptic knobs (Waugh and
Grant 2010). This impulse causes the release of
a neurotransmitter from the presynaptic neurone
into the synaptic gap. The neurotransmitter
diffuses across the gap to the postsynaptic
FIGURE 3
A synapse
Presynaptic neurone
Dendrites
Nucleus
Axon
Cell body
Synaptic knobs
Axon
Postsynaptic
neurone
Presynaptic
neurone
Vesicles containing
neurotransmitter
Synaptic
knob
Synaptic
cleft
Dendrite
Postsynaptic
neurone
PETER LAMB
the resting potential or membrane potential. In
this state, the membrane is said to be polarised.
The membrane potential is typically -70mV,
the minus sign indicates that the inside of the
cell has a negative charge relative to the outside
(Tortora and Derrickson 2013). If a stimulus
of adequate strength is applied to a polarised
membrane, the membrane depolarises – the
electrical charge across the membrane moves
towards a positive value of >0mV – and once
the events of depolarisation have occurred, an
action potential (nerve impulse) is initiated.
The threshold at which depolarisation occurs
completely is -55mV. If this value is not reached
then depolarisation cannot proceed and an action
potential is not initiated. However, if it is reached
then depolarisation is an all or nothing event and
the membrane potential is +30mV (Tortora and
Derrickson 2013).
The action potential is conducted along the
length of the axon in a segmental wave-like
fashion. By the time the impulse has travelled
from one part of the axon membrane to the
adjacent part, the previous part becomes
repolarised – its resting potential is restored.
Until repolarisation occurs, the neurone cannot
conduct another impulse; this is known as the
refractory period (Waugh and Grant 2010).
During repolarisation, potassium leaves the cell
returning the membrane potential to its resting
state. It should be noted that at this point the
distribution of potassium and sodium across
the cell membrane is not in balance, such that
there is an excess of sodium inside the cell and an
excess of potassium outside the cell. To restore
the appropriate ionic balance on either side
of the cell membrane, the nerve cell is actively
transporting ions across its membrane using a
mechanism known as the sodium-potassium
pump. Sodium ions are transported out of the
cell, while potassium ions are transported into the
cell (Waugh and Grant 2010).
Large axons and those of the peripheral nerves
in the body are surrounded by a myelin sheath
(Waugh and Grant 2010). The myelin sheath
prevents leakage of electrical charge from the
axon and conducts the impulse more efficiently.
Between the segments of the myelin sheath,
unmyelinated gaps called nodes of Ranvier can
be found. At these nodes, depolarisation can
occur. When an impulse is conducted along
a myelinated sheath, it moves rapidly from
one node to another through surrounding
extracellular fluid. This speeds up the rate
of nerve impulse conduction, compared to
unmyelinated neurones, and is referred to as
saltatory conduction (Waugh and Grant 2010).
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Art & science life sciences: 20
neurone or effector muscle, where it binds to
receptor molecules on the postsynaptic membrane
(Seeley et al 2008). The effect of this may be
excitatory or inhibitory depending on the type
of neurotransmitter and the type of postsynaptic
membrane (Seeley et al 2008). Box 1 provides
some examples of neurotransmitters.
Subsequently, neurotransmitters can be
broken down by the action of enzymes,
diffuse away from the synapse or be transported
back into the presynaptic neurone (reuptake).
The phenomenon of reuptake has led to the
development of a class of drugs known as
selective serotonin re-uptake inhibitors.
BOX 1
Examples of neurotransmitters
FIGURE 4
Circle of Willis
Anterior
communicating
artery
Internal
carotid
artery
Basilar
artery
Posterior
communicating
artery
Vertebral
artery
Spinal cord
50 april 2 :: vol 28 no 31 :: 2014
Neurones have a high demand for adenosine
triphosphate to support synthetic and active
transport activities. They obtain energy through
aerobic breakdown of glucose and do not
maintain glycogen reserves. As a result, these
cells are completely dependent on the oxygen
and glucose delivered by the circulation and
thus any interruption in the circulatory supply
may damage or destroy neurones (Jenkins and
Tortora 2013).
The brain demands a constant supply of oxygen
and glucose. It receives about 15% of cardiac
output (750mL/minute) (Waugh and Grant
2010). Blood reaches the brain via two internal
carotid arteries and two vertebral arteries.
Within the brain, these arteries form the circle
of Willis (Figure 4). This circle helps to ensure
that no part of the brain receives an inadequate
supply of blood because the circulation can
reach any part of the brain from this arrangement
of blood vessels. Posteriorly, the right and left
vertebral arteries originate from the subclavian
arteries. Once in the skull, on the underside
of the brain, they unite to form the basilar
artery. Anteriorly, the right and left internal
carotids arise from the common carotid arteries.
As they enter the skull they become the anterior
cerebral arteries. These arteries are linked by
the anterior communicating artery. The anterior
cerebral arteries unite with the basilar artery
via the posterior communicating artery forming
the circle of Willis. Arteries arise from this circle
and serve the brain (Waugh and Grant 2010).
The main veins returning blood from the brain
are the internal jugular veins draining into the
subclavian veins (Waugh and Grant 2010).
Stroke
PETER LAMB
Posterior
cerebral
artery
Neurones and metabolic processes
Blood supply to the brain
 Acetylcholine.
 Dopamine.
 Adrenaline (epinephrine).
 Gamma aminobutyric acid.
 Glycine.
 Histamine.
 Noradrenaline (norepinephrine).
 Serotonin.
Anterior
cerebral
artery
These drugs are used in the treatment
of depression and include citalopram, fluoxetine
and paroxetine (White and Clare 2009).
A common neurological condition resulting
from an abnormality of cerebral circulation
is stroke. Lim et al (2007) defined stroke as ‘a
sudden onset of a focal neurological deficit that
persists for more than 24 hours.’ Stroke results
from an interruption in blood supply to a part
of the brain. This may be caused by occlusion or
rupture of a blood vessel within the brain. This
results in damage and/or death to an area of the
brain and if extensive, can result in death of the
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individual. As a consequence, the person may
be left with motor, sensory or language deficits
and higher brain dysfunction (Lim et al 2007).
Each year in the UK, about 150,000 people
have a stroke, with about 25% of these being
under the age of 65 (NHS Choices 2012). It is
the third most common cause of death in the
Western world, following heart disease and
dementia (Lim et al 2007, Office for National
Statistics 2013).
Damage to and/or death of areas of the brain
can be reduced by recognising the features of
stroke and responding promptly. Healthcare
workers have a role in increasing public
awareness of stroke and helping to reduce risk
factors, such as those listed in Box 2. The
acronym FAST is currently recommended in
the UK for first responders to stroke (Stroke
Association 2014):
Facial weakness: can the person smile? Has his
or her mouth or eye drooped?
Arm weakness: can the person raise both arms?
Speech problems: can the person speak clearly
and understand what you say?
Time to call 999.
POINTS FOR PRACTICE
 Identify risk factors for stroke that might be
modifiable. Outline ways in which the risk of stroke
may be reduced.
 Referring to Box 1, identify the actions of the named
neurotransmitters.
 Using resources of your choice, identify tools that
may be used when conducting a neurological
examination.
GLOSSARY
Axon terminal
The end structure of the axon; axon terminals are
separated from neighbouring neurones by the synapse.
Neuroglia
Also known as glial cells; non-neuronal cells that
provide support, protection and insulation for neurones.
Homeostasis
The mechanisms by which a stable internal
environment is maintained within the body.
Neurotransmitter
Chemicals that transmit signals from a neurone to
another neurone or target cell across a synapse.
Sodium-potassium pump
An active transport mechanism that maintains the
correct balance of sodium and potassium on either
side of the cell membrane.
BOX 2
Risk factors for stroke
Conclusion
 Age.
 Gender.
 Race.
 Heredity.
 Hypertension.
 Smoking.
 Diabetes.
 Hyperlipidaemia.
 Atrial fibrillation.
 Obesity.
 High alcohol consumption.
Nurses need to have an understanding of the
structure and function of the nervous system to
provide appropriate care for patients who may be
experiencing neurological deficits. While many
patients with such deficits will be cared for in
specialised units, some may receive care in general
medical and surgical wards. Similarly, community
nursing staff are increasingly caring for patients with
long-term neurological deficits. The second article in
this series examines the CNS in greater detail NS
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