THE NERVOUS SYSTEM (HK24CY005 2.1) The nervous system co-ordinates all body movement and activities, voluntary and involuntary actions. Neurology is the study of the Nervous System. Throughout the whole of the body is a vast network of nerve fibres creating the ultimate communication and messaging service. Rather like the Network Signalling System where the brain is the main controller (not the Fat Controller!), or computer, and the spinal column is the main line. The spinal nerves are branch lines with junctions at ganglions leading to the stations – the organs and glands! However, there is more than just an understanding of neurons for the Complementary Health Practitioner to appreciate about this amazing system. Many clients seek the help of a Reflexologist for pain, and pain is indeed all in the brain, as will become clear.
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THE NERVOUS SYSTEM (HK24CY005 2.1)
The nervous system co-ordinates all body movement and activities, voluntary and involuntary
actions. Neurology is the study of the Nervous System.
Throughout the whole of the body is a vast network of nerve fibres creating the ultimate
communication and messaging service. Rather like the Network Signalling System where the
brain is the main controller (not the Fat Controller!), or computer, and the spinal column is
the main line. The spinal nerves are branch lines with junctions at ganglions leading to the
stations – the organs and glands! However, there is more than just an understanding of
neurons for the Complementary Health Practitioner to appreciate about this amazing system.
Many clients seek the help of a Reflexologist for pain, and pain is indeed all in the brain, as
will become clear.
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When working on the feet, the Reflexologist has the opportunity to access over 7,500 nerve
endings all capable of responding to signals initiated by an effective thumb and finger
walking technique.
The Nervous System is split into two main parts: the Central Nervous System (CNS) and
the Peripheral Nervous System (PNS).
The Central Nervous System consists of the Brain (the Main Computer/Controller) and the
Spinal Cord (the main central line), all incoming and outgoing signals are routed through the
Central Nervous System.
The Peripheral Nervous System includes all of the branch lines (the spinal and cranial
nerves) and nerves in the rest of the system that cover actions we are not even aware of
taking place, such as the dilation and constriction of the pupils in the eye (Autonomic
Nervous System) as well as relaying nerve impulses to move skeletal muscle that we can
control (Somatic Nervous System).
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Nervous System Outline:
Central Nervous System Peripheral Nervous System
(CNS) (PNS)
Brain and Spinal Cord Cranial Nerves from Brain
Co-ordinates body functions. Spinal Nerves from Spinal Cord
Reflex actions
Autonomic Nervous System
Supplies muscles of internal organs and
glands (no voluntary control)
Somatic Nervous System
Supplies muscles of
The Skeletal System Parasympathetic nerves Sympathetic Nerves
(voluntary control) Slow down activity Speed up activity
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THE CENTRAL NERVOUS SYSTEM
The brain connects to the spinal cord which, together, makes up the Central
Nervous System (CNS), running from the neck to the hip area. The spinal cord
carries nerve messages between the brain and the body.
Protection
The cells of the nervous system are quite fragile and need extensive protection
from being crushed and/or infected by disease organisms. The brain and spinal
cord are protected by layers of connective tissue called the Meninges. The
outermost layer is a tough, translucent membrane, called the Dura Mater
(meaning tough mother). The middle layer is the Arachnoid (spider-like as it
resembles a spiders web) and the innermost layer is the Pia Mater (Pia =
delicate, as it is a thin transparent covering).
Cerebrospinal fluid (CSF) is a clear, watery liquid that surrounds the brain and spinal cord,
and is also found throughout the ventricle (brain cavities and tunnels). CSF cushions the
brain and spinal cord from jolts.
The cranium (the top of the skull) surrounds and protects the brain, whereas, the spinal cord
is surrounded by vertebrae (hollow spinal bones). Some muscles also serve to pad and support
the spine.
More subtly, blood flowing into the brain is filtered, so that many harmful chemicals cannot
enter the brain. This is called the blood-brain barrier, and it protects the brain from
chemical intrusion from the rest of the body.
Functions of the Brain
The human brain is a complex organ that allows us to think, move, feel, see, hear, taste, and
smell. It controls the body, receives, analyses and stores information (our memories). The
brain produces electrical signals which, together with chemical reactions, let the various
parts of the body communicate. Nerves send these signals throughout the body. For example,
a child touches a hot object and burns its fingers. The heat and pain are detected by sensory
nerves in the skin and this information is passed to the child’s brain. The brain stores this
information in its memory, so that when the child sees the hot object again, it will remember
the heat and pain associated with it. The brain will send out messages to the appropriate
muscles in the child’s body to move it away from the hot object.
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Size of the Human Brain
The average human brain weighs about 3 pounds (1.3kg). At birth, the human brain
weighs less than a pound (0.78-0.88 pounds or 350-400g). As a child grows, the
number of cells, within the brain, remains relatively stable, but the cells grow in size, and the
number of connections increases. The human brain reaches its full size at about 6 years of
age and constitutes about one-fiftieth of the total body weight.
Nourishment of the Brain
Although the brain is only 2% of the body's weight, it uses 20% of the oxygen supply and
gets 20% of the blood flow. Blood vessels (arteries, capillaries, and veins), supply the brain
with oxygen and nourishment, and take away wastes. If brain cells do not get oxygen for 3 to
5 minutes, they begin to die. Cerebrospinal fluid (CSF) surrounds the brain.
Composition of the Brain
The brain is located at the top of the spinal cord and fills the cranial
cavity of the skull. The surface of the brain is folded and looks a bit
like a walnut. The folds give the brain a larger surface area for
absorbing nutrients from the blood vessels on its surface. The brain
consists of 100 billion neurones (nerve cells) and 1000 billion neuroglia
(nerve tissue cells). The brain is not solid but has cavities inside it
called ventricles. These are filled with the nutrient, liquid, cerebrospinal fluid, which also
removes waste from the brain, helping to maintain the shape of the brain by providing
pressure from within (a bit like the water in a hot water bottle makes it more solid).
The brain is composed of both grey matter (40%) and white matter
(60%). Grey matter includes neuron cell bodies, dendrites (meaning
little trees – the receiving input portion of a neuron), axon terminals
(an axon is the long thin process that the nerve impulses travel along),
bundles of unmyelinated axons, and neuroglia. White matter consists
of myelinated processes of many neurons, myelin is white.
Structure of the Brain
The brain has four main regions: the Brain Stem, the
Cerebellum, the Diencephalon and the Cerebrum.
The Brain Stem – this is continuous to the spine and
consists of the:
Medulla Oblongata (known as the Medulla)
Pons Varolii (‘Pons’ means ‘bridge’ - the bridge between the spinal cord and brain)
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Midbrain (mesencephalon) - reflex centres for body movement.
The Brain Stem is essential to life, with the Medulla regulating heartbeat, breathing,
swallowing, coughing, hiccupping, vomiting, posture and balance. Ten, of the twelve pairs of
cranial nerves, originate in the brain stem, with the remaining two originating in the brain
itself. If the Medulla ceases to function, for whatever reason, then the person dies or
remains on a life support machine.
The Reticular Formation is a mass of grey matter that extends the entire length of the
brain stem. It is responsible for the motor control of visceral organs and muscle tone, as well
as consciousness and awakening from sleep.
The Cerebellum is the cauliflower-shaped region at the back of the head behind the brain
stem. Its main functions are regulating posture and balance, and co-ordinating and smoothing
complex sequences of skeletal muscular contraction.
The Diencephalon lies above the brain stem and includes the:
Epithalamus – containing the pineal gland
Thalamus – the main relay station for sensory
impulses of hearing, vision, taste, touch,
pressure, vibration, heat, cold and pain. It is
also involved in emotions and memory, cognition,
voluntary motor actions and arousal.
Hypothalamus – the main regulator of
homeostasis. Controls the pituitary gland,
synthesises oxytocin and antidiuretic hormones (see Endocrine system handout). It
regulates emotional behaviour, hunger, fullness and thirst, controls body temperature
and regulates sleep patterns.
The Cerebrum appears as a cap over the Diencephalon and fills most of the cranium. It is
divided into two halves; the right and left cerebral hemispheres. Each hemisphere of the
cerebrum is divided into four lobes that take their names from the sections of the cranium
under which they lie:
Frontal
Parietal
Temporal
Occipital
The functions of the Cerebrum include:
1. Mental activities involved in memory, language, intelligence, sense of responsibility,
thinking, reasoning, moral sense and learning.
2. Sensory perception, including perception of pain, temperature, touch, sight, hearing,
taste and smell.
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3. Initiation and control of skeletal muscle contraction.
4. It is the seat of emotions.
5. It controls consciousness.
The folded, outer layer of the cerebrum is called the Cerebral Cortex and is responsible for
integrating the sensory information into meaningful pictures, smells, tastes and sounds,
together with co-ordinating the response to these.
The Limbic System is often called the “emotional brain” because it controls the emotional
and voluntary aspects of behaviour. It is the area associated with pain, pleasure, anger, rage,
fear, sorrow, sexual feelings, affection, and memory.
Basal Ganglia are found in the cerebral hemispheres and receive information, and provide
output to the cerebral cortex, thalamus and hypothalamus. They control large automatic
movements of the skeletal muscles and help regulate muscle tone.
The Spinal Cord is continuous with the medulla oblongata and is located within the
protective “tubing” of the vertebral column. The cord passes through an outlet in the skull
and travels for approximately 42cm down the vertebral column. It is half an inch thick and
ends just above the second lumbar vertebra in a tail like structure. For protection, the cord
is bathed in cerebro-spinal fluid, and when a specimen of CSF is needed, it is taken from the
point below the end of the undivided cord, ie below the level of the second lumbar vertebra.
The spinal cord is protected by the Spinal Meninges (see protection of the brain section).
Nerve impulses are transported to and from the brain via the spinal cord. Also the spinal
cord receives and integrates information, and produces reflex actions in response to specific
changes in the environment.
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PERIPHERAL NERVOUS SYSTEM
The Peripheral Nervous System (PNS) has three main types of nerve: Cranial Nerves, Spinal
Nerves and the Autonomic Nerves. There are two types of nerve cells: neurons and neuroglia.
Neuroglia, or glial cells, perform the function of connective tissue - they support and protect
the neurons, and maintain homeostasis of the fluid that surrounds them. Neuroglia are
smaller and more numerous than neurons.
Neurons
Throughout the nervous system, information is conveyed as tiny electrical signals called
nerve impulses, or action potentials. These impulses are the same all over the body – about
100 millivolts (0.1 volts) in strength and lasting just 1 millisecond.
The information carried depends on their position in the nervous system, and frequency.
When a nerve receives enough impulses from another nerve, it fires an impulse of its own.
These impulses jump from one nerve to another at junctions known as synapses. Imagine a
firework display, with the spark from one firework igniting the touch paper of the next
firework.
These cells of the nervous system, called nerve cells or neurons (sometimes spelt with an “e”
on the end (neurone)), are specialised to carry "messages" through an electrochemical
process. The human brain has about 100 billion neurons. Neurons become excitable when
stimulated - they undergo chemical changes that produce travelling waves of electricity.
Because conduction of nerve impulses is active, energy is required, and expended by the
neurons. Therefore, thinking consumes calories!
Neurons carry messages.
Neurons come in many different shapes and sizes for example:
Multipolar (several dendrites and one axon).
Bipolar (one dendrite and one axon).
Unipolar (the axon and dendrite are fused).
Neurons are similar to other cells in the body because they:
are surrounded by a cell membrane.
have a nucleus that contains genes.
contain cytoplasm, mitochondria and other organelles
carry out basic cellular processes, such as protein synthesis and energy production.
However, neurons differ from other cells in the body because they:
have specialised extensions, called dendrites and axons. Dendrites bring information
to the cell body, and axons take information away from the cell body.
communicate with each other through an electrochemical process.
contain some specialised structures (for example, synapses) and chemicals (for
example, neurotransmitters).
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There are three main types of neurons (nerves):
Motor (Efferent) neurons - carry impulses from the brain and spinal cord to muscles,
producing movement, and to glands stimulating secretion.
Sensory (or Afferent) neurons - convey impulses that give the sensation of touch, taste and
smell etc, from the body to the brain, which then translates it into meaningful pictures,
tastes, sounds etc.
Mixed neurons - carry both of the above but, are only present in the spinal nerves.
Interneurons (or Association) neurons - carry nerve impulses from one neuron to another.
These make up the vast majority of neurons.
A Typical Neuron
Axon - the long extension of a neuron that carries nerve impulses away from the body of the
cell.
Axon terminals - the hair-like ends of the axon.
Cell body - of the neuron; it contains the nucleus (also called the soma).
Dendrites - the branching structure of a neuron that receives messages (attached to the
cell body).
Myelin sheath - the fatty substance that surrounds and protects some nerve fibres. It is
whitish in colour, giving “white matter” its name, and acts as protection for the nerve fibre
by insulating the axon, and increasing the speed of impulse transmission. If a neuron is
unmyelinated (ie, with no myelin sheath), transmission of impulses is slower - the grey matter
of the nervous system contains unmyelinated axons.
Node of Ranvier - one of the many gaps in the myelin sheath, it also increases the speed of
impulse transmission along the axon. The impulse jumps across the nodes as in the firework
analogy used earlier.
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Nucleus - the organelle, in the cell body of the neuron, which contains the genetic material
of the cell.
Schwann's cells – are cells that produce myelin in the peripheral nervous system. They are
located within the myelin sheath and are a form of neuroglia. Myelin sheaths in the Central
Nervous System are produced by oligodendrocytes, another form of neuroglia.
The Synapse (from the Greek word ‘synaptein’, to ‘join’ - pronounced
sin.aps)
Messages are passed from neuron to neuron via the synapse. Some
electrical synapses pass signals very quickly, such as in the central
nervous system, smooth muscle, viscera and cardiac muscles. They allow
for two way transmission and can synchronize the activity of a group of
neurons or muscle fibres. The other type of synapse is called a chemical
synapse, these are slower with mostly only one-way communication.
Synapses are essential for homeostasis because they allow information to be filtered and
integrated. By the time you finish this course many of your synapses will have been modified!
Some diseases and psychiatric disorders result from disruptions of synaptic communication.
Synapses are also the sites of action for many therapeutic and addictive chemicals.
When an electrical impulse arrives at a synapse it triggers the release of chemicals called
neurotransmitters. Examples of neurotransmitters include:
Acetylcholine, Dopamine and Noradrenaline (norepinephrine) – this is both a
neurotransmitter and a hormone.
They cross the gap (the synaptic cleft) between the membranes of the presynaptic (sending)
and postsynaptic (receiving) neurons, and either trigger a new impulse, or actively inhibit it
from firing. Once it has completed its action the neurotransmitter is then quickly removed
by diffusion, enzyme degradation, or it is transported back into the cells where it is
recycled. At a neuromuscular synapse, the motor end plate of the muscle fibre receives the
neurotransmitter.
The Myotatic Reflex (Reflex Action)
One of the most familiar reflexes is the
stretch reflex, also known as the knee-
jerk reflex, and the myotatic reflex. In
its simplest form it is a two-neuron loop,
one afferent neuron and one efferent
neuron.
The afferent neuron is connected to a
muscle spindle, which detects stretch in
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the muscle. The efferent neuron is the motor neuron, which causes the muscle to twitch.
This reflex can also be used to describe the action of touching something that causes pain,
for example, and withdrawing your hand before you have even thought about it. It is a reflex
that does not involve the brain and is called an Arc Reflex.
Nerve Injury and Regeneration
Peripheral nerves that have been damaged may regenerate slowly if the cell body remains
undamaged. The damaged section of fibre loses its nourishment and degenerates, leaving the
myelin sheath hollow. In the meantime, the healthy remaining fibre begins to grow along the
empty sheath at a rate of 1-2mm per day.
Natural regeneration is unlikely in the nerve fibres of the spinal cord and brain, they are too
specialised to recreate their highly developed functions. So, spinal cord injuries may lead to
paralysis, depending on the location and extent of the damage. Monoplegia is paralysis of one
limb only; diplegia – two limbs; paraplegia – both lower limbs; hemiplegia is paralysis of one
upper limb, trunk and one lower limb on the same side of the body; and quadriplegia is
paralysis of all four limbs.
Spinal shock is an immediate response to spinal injury, and includes loss of reflex action, low
blood pressure, paralysis of skeletal muscle, loss of somatic sensations and urinary bladder
dysfunction. The patient has an improved outcome if an anti-inflammatory corticosteroid
drug is administered within eight hours. Spinal shock can last from several minutes to several
months, after which reflex activity gradually returns. Recovery from a traumatic brain
injury may be quick or slow; it may be complete, partial or absent. Scientists still don’t know
how the brain heals itself.
Cranial Nerves
The cranial nerves from the brain, and the spinal nerves from the spinal cord, are the first
part of the Peripheral Nervous System to be studied.
There are twelve pairs of cranial nerves, which arise from the under-surface of the brain.
They are numbered according to where they arise in the brain, in order, from anterior to
posterior. They are named according to their distribution or function and supply the head,
neck and major organs of the body as summarised below:
1) OLFACTORY sensory nerve for smell
2) OPTIC sensory nerve for sight
3) OCULOMOTOR )
4) TROCHLEAR ) motor nerves for muscles of the eye
5) TRIGEMINAL mixed nerve, (sensory and motor) for teeth, head
and facial skin
6) ABDUCENT motor nerve for eye
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7) FACIAL mixed nerve for facial expression
8) AUDITORY sensory nerve for hearing
9) GLOSSO-PHARYNGEAL mixed nerve for taste, muscles of pharynx
10) VAGUS mixed nerve for larynx, pharynx, heart, oesophagus,