The Nervous System

Module 2: Life Processes

Life Sciences
Matric Syllabus
Mind Action Series: Life Sciences
Textbook and Workbook

• Co-ordination is the way in which information is communicated
between the receptors of the body, the central nervous system
and the effectors to bring about appropriate responses to any
change in the environment.
• There are essentially two co-ordinating systems – the nervous
system and the endocrine system, which work together to
respond to external changes and to control conditions inside the
body.
• Co-ordination is brought about in two ways:
– The fastest method is by means of impulses travelling along nerves and is
called nervous co-ordination.
– The slower method is by means of chemicals called hormones which are
carried in the blood. This is known as chemical co-ordination.

2.2a) The Nervous System

• The nervous system is the body’s control and communication
centre.
• It provides a continuous flow of information between the
brain and the parts of the body.
• This enables the body to function in an orderly and effective
way and to perform everyday tasks such as
eating, writing, running, solving a problem and controlling
emotions such as fear.


The nervous system has three overall functions:
– Sensory function – sensory receptors detect the changes in the
external environment and the environment within the organism.
– Integrative function – the central nervous system integrates this
information.
– Motor function – effectors (muscles and glands) bring about a
response.
For example: you are riding a bicycle and see that the traffic light
has turned red (sensory function). Your central nervous system
integrates this information (red light means ‘stop’ – integrative
function), and you use your muscles to apply your brakes to stop
the bicycle (motor function).


• We live in an environment where external factors are
constantly changing. Inside our bodies, conditions are also
changing, e.g. changes in blood pressure, tension in muscles,
blood sugar levels etc.
• These internal and external changes are called stimuli.
• A stimulus is any physical or chemical change in our internal
or external environment that is capable of causing a response
in an organism.


stimulus

Sensory Function Receptors respond to stimuli

sensory

CNS processes and integrates
Integrative Function
the sensory input

motor

Effectors e.g. muscles and glands
Motor Function
make the correct response

response


The nervous system is made up of millions of special cells called
neurons that form a continuous network all over the body.
• Neurons can function for over a lifetime, given the correct
conditions.
• They cannot undergo mitosis, and thus cannot be replaced if
destroyed.
• They have a very high metabolic rate and need continuous
supplies of oxygen and glucose to survive.
In addition to neurons, there are supporting cells called
neuroglia which bind neurons together, giving them support and
helping them with their nutrition.


• A neuron has the same basic parts as most animal cells:
they have a cell body, containing a cell
membrane, cytoplasm, nucleus and cell organelles.
• In addition, neurons have long, thin fibres of cytoplasm
extending from the cell body which make it possible for
nerve impulses to be carried long distances.
• The fibres may be:
– Dendrites, which carry impulses towards the cell body.
– An axon, which carries impulses away from the cell body.


• Neurons can be classified according to their structure or
functions.
• Functionally, neurons are classified according to the direction
in which nerve impulses travel:
– Sensory neurons
– Connector neurons
– Motor neurons
• Structurally, they are divided according to the number of
fibres stretching out from the cell body:
– Multipolar neurons
– Bipolar neurons
– Unipolar neurons


• These carry impulses from receptors towards
the central nervous system (CNS), where the
sensation is interpreted. They are afferent
neurons, i.e. they take the sensation
TOWARDS the CNS.
• They are usually unipolar, i.e. they have one
fibre attached to the cell body, which then
divides.


• They are also known as relay neurons,
association neurons or interneurons.
• They are multipolar neurons with many
dendrites.
• They are found in the CNS and they make up
99% of the neurons in the body.
• They receive impulses from sensory neurons
and pass them on to other neurons.


• Motor neurons carry impulses away from
the CNS to effectors (such as muscles) to
bring about the appropriate response.
• They are therefore efferent neurons.
‘Efferent’ means away from.
• They are multipolar neurons, and are
located in the grey matter in the CNS.


Functions of parts:
Dendrites collect information from other cells and pass
it on to the cell body.
The cell body controls the metabolism of the cell.
The node of Ranvier allows rapid conduction of
impulses by forcing them to ‘jump’ from one node to
the next.
Schwann cells are wrapped around the axon several
times. Their inner layers of fatty tissue fuse to form the
myelin sheath.
The axon is extended to carry impulses away from the
cell body over long distances.
The myelin sheath forms electrical insulation around
the axon, preventing distortion of impulses from
activity in neighbouring cells.
Terminal branches carry impulses to the synaptic
knobs/end plates.
Synaptic knobs form a synapse with another neuron, a
muscle cell or a gland cell.

• Adjacent neurons do not touch each other, there is a small
gap separating them. This gap or synaptic cleft plus the
adjacent membranes, form a synapse.
• A synapse is thus the junction across which a nerve impulse
passes from an axon terminal to a neuron, a muscle cell or a
gland cell.
• In the cytoplasm of the synaptic knobs there are hundreds of
small vesicles (sacs) filled with molecules of a
neurotransmitter such as the chemical acetylcholine.


As neurons do not touch, impulses have to cross the gap (synapse)
to continue their path through the nervous system.
• When nerve impulses reach the terminal branches of axons, they
cause vesicles in the synaptic knob to fuse with the presynaptic
membrane, using energy from the mitochondria.
• The membranes then burst, releasing neurotransmitters which
diffuse across the synaptic cleft.
• The neurotransmitters become attached to neuroreceptors in
the post-synaptic membrane, which they either excite or inhibit.


• The receptors release the neurotransmitters back to the
synaptic cleft which can then be inactivated by enzymes and
reabsorbed by the pre-synaptic membrane. This is very
important as, unless the transmitter is removed from the
synaptic cleft, subsequent impulses would have no effect as the
receptors on the postsynaptic membrane would all be bound.
In this way, impulses are carried across the gap chemically, i.e. by
way of a chemical neurotransmitter. An impulse carried along a
nerve fibre is an electric impulse.
Synapses are important because: 1) they make sure that the flow
of impulses travel in one direction only, 2) they allow the dispersal
of impulses, 3) they allow unnecessary and continual stimuli to be
filtered out.

Nerve impulses reach Neurotransmitters Receptors release the
terminal branches of excite or inhibit neurotransmitters back
axons neuroreceptors into the synaptic cleft

Neurotransmitters Neurotransmitters are
Vesicles in synaptic
attach to inactivated by enzymes
knob fuse with
neuroreceptors in post- and reabsorbed by pre-
presynaptic membrane
synaptic membrane synaptic membrane

Membranes burst, Neurotransmitters Thus, impulses are
releasing diffuse across synaptic carried across the gap
neurotransmitters cleft chemically.


The human nervous system is
divide into the:
• Central Nervous System
• Brain
• Spinal cord
• Peripheral Nervous System
• Cranial nerves
• Spinal nerves


• The CNS is made up of a concentrated mass of interconnected
neurons, grouped together to form the brain and spinal cord.
• The CNS tissues are protected and nourished by three connective
tissue membranes called meninges. The space between the 2nd
and 3rd layer of membranes is filled with cerebro-spinal fluid which
acts as a cushion, protecting the CNS against shock and damage.


• The whole CNS and its meninges are enclosed within a bony
case made up of two separate structures.
– The cranium is a strong, dome-shaped structure that protects the
brain against mechanical injuries. It is made up of eight, curved skull
bones, immovably dove-tailed together by fibrous joints called
sutures.
– The vertebral column is made up of thirty-three, irregular shaped
vertebrae that surround the spinal cord and protect it against
mechanical injury.


• The brain is the enlarged, upper part of the spinal cord. It is shaped
like a large mushroom. The cap of the mushroom is the cerebrum,
and the stalk is the brain stem.


• The large cerebrum is divided into two cerebral hemispheres
by a longitudinal fissure. The hemispheres are held together by
a mass of white tissue called the corpus callosum that provides
a communication link between the cerebral hemispheres.
• Its surface is made up of a large number of folds and grooves
that enlarge the surface area so that a large number of brain
cells can fit into a small cranial cavity.
• There are four cavities known as ventricles in the centre of the
brain. They are filled with cerebro-spinal fluid.
• The outer 3mm of the cerebrum forms the cerebral
cortex, made up of grey matter. The grey matter is made up of a
collection of cell bodies of neurons. Below the grey matter is an
area of white matter, made up of a collection of myelinated
nerve fibres.

Functions of the cerebrum
The cerebral cortex is divided into three functional areas:
• The motor area in the frontal lobe is where all the voluntary
movements of the body originate.
• The sensory areas receive and interpret impulses from the
sense organs, e.g:
– Hearing, tasting and smelling in the temporal lobe
– Sight in the occipital lobe
– Skin sensations in the parietal lobe, e.g. touch, cold and hot
temperatures and pain.
• The association cortex is not involved with motor or sensory
functions and is involved in higher mental activities such as
intelligence, memory, perception, language and consciousness.

The cerebellum forms the upper part of the brain stem. It is made
up of two hemispheres and has shallow surface folds.
Functions
The cerebellum receives sensory input from the motor region of the
cerebrum, the eyes, muscle spindles & organs of balance in inner
ears.
• It processes this information and uses it to co-ordinate the
actions of the voluntary muscles so that complicated physical
actions can be performed in a smooth, controlled way.
• It is partly responsible for controlling muscle tone (correct
tension in the muscles).
• Using information from the inner ear, it helps maintain posture
and balance.

The hypothalamus is situated above the pituitary.
Functions
• Controls homeostasis, e.g. temperature, water levels, pH
levels, food intake, etc.
• Controls and drives certain behaviours, e.g. aggression, self-
defence and reproductive behaviours.
• Controls functioning of the pituitary gland.


Forms the lowest part of the brain stem.
When it leaves through the foramen magnum at the base of the
cranium, it is known as the spinal cord.
Functions
• Serves as the pathway for impulses to and from the brain.
• Acts as a reflex centre, controlling important reflexes such as
breathing, heartbeat, blood pressure, swallowing and
peristalsis.
• Also controls less important reflexes such as sneezing,
coughing, hiccupping and salivating.


• The spinal cord is an
elongated rod of nervous
tissue that extends from
the medulla oblongata
through the foramen
magnum.
• It is situated in the
vertebral canal of the
vertebral column and is
approximately 45cm long.


• A spinal cord has an H-shaped
central area of grey matter
made up of nerve cell bodies,
dendrites and synapses.
• White matter surrounds the
grey matter and is made up of
axons with myelin sheaths.
• The central canal occurs in the
centre of the grey matter. It is
filled with cerebro-spinal fluid.
The central canal runs down
from the ventricles in the brain.


• The cord is partly divided into
right and left sides by two
grooves: dorsal groove and
ventral groove.
• A pair of spinal nerves enters and
leaves the spinal cord between
successive vertebrae. Each spinal
nerve has a dorsal root that
enters the grey matter, and a
ventral root that leaves the grey
matter.
• A collection of neuron cell bodies
forms a ganglion in the dorsal
root. 2.2a) The Nervous System

The PNS consists of all the nervous tissue outside the CNS. It is
divided into two main divisions:
• Somatic nervous system that controls all voluntary muscular
movements.
• Autonomic nervous system, that controls the functioning of
involuntary muscles and glands.
The PNS is made up of nerves that link the CNS to the receptors and
effectors of the body, i.e. 43 pairs of nerves that include:
• 12 pairs of cranial nerves connected to the brain. These may be
afferent (sensory) nerves or efferent (motor) nerves or mixed
nerves made up of both sensory and motor nerves.
• 31 pairs of spinal nerves which enter and leave spinal cord
between the vertebrae (all mixed nerves).

Functions of the PNS:
• The PNS collects information from receptors and transmits
this information by way of impulses along sensory neurons to
the CNS – sensory function.
• It transmits impulses from the CNS by way of motor neurons
to effectors to bring about an appropriate response – motor
function.


This system involves receiving information from receptors and
bringing about appropriate responses by voluntary, skeletal
muscles.


A reflex action is a fast, automatic response by an effector organ, i.e.
a muscle (whether voluntary or involuntary) or a gland, to a stimulus
received by a receptor organ.
Reflex centres may be situated in the:
• Brain – controlling reflexes such as sneezing, breathing, blinking
etc.
• Spinal cord – controlling primitive reflexes such as the knee jerk
when tapped below the knee-cap.
• A reflex arc is the pathway along which nerve impulses are carried
from a receptor to an effector to bring about a reflex action.
• A reflex arc is thus the functional unit of the nervous system,
while a neuron is the structural unit of the nervous system.

Consider what happens when you prick your finger.
• The stimulus of the pin prick generates impulses in sensory nerve
endings of pain in the skin of your finger – the receptor.
• These impulses are conducted along sensory neurons to the grey
matter in the spinal cord. Here the impulses synapse with
connector neurons which serve as reflex centres.
• From the connector neurons, impulses make synaptic contact with
the motor neurons in the grey matter and travel along these
neurons to the effectors.
• The effector, the flexor muscle in the finger, brings about a
response by contracting very quickly to pull the finger away from
the pin to protect the finger.
• Impulses are also carried up the spinal cord to the cerebral cortex
(parietal lobe) where the stimulus is interpreted as pain.

• The ANS is involved in keeping a balanced internal
environment i.e. maintaining homeostasis. It responds to
sensory input from all body organs and makes the necessary
adjustments.
• The effectors in the ANS are smooth, involuntary muscle,
cardiac muscles and glands.
• There are two divisions in the ANS:
– Sympathetic nervous system – gets you ‘ready for action’
– Parasympathetic nervous system – calms you down


Body part Sympathetic system Parasympathetic system
Heart beat Strengthens and accelerates Weakens, slows down
Blood vessels of skin and Constricts arteries and raises Dilates arteries, lowers
digestive organs blood pressure blood pressure
Blood vessels of heart, lungs, Dilates arteries, allowing the Constricts arteries, reduces
skeletal muscles flow of more blood – can flow of blood
run faster
Digestive tract Slows down peristalsis Speeds up peristalsis
Urinary bladder Relaxes bladder wall Contracts bladder wall
Bladder and anal sphincters Contracts Relaxes
Muscles in bronchial tubes Muscles relax, dilating air Muscles contract, causing air
passages for easier passages to constrict
breathing


Body part Sympathetic system Parasympathetic system
Muscles of iris Radial muscles contract, dilating Circular iris muscles contract,
pupil – see better constricting pupil
Sweat glands Increases sweat to cool one down No effect

Liver Stimulates the breakdown of No effect
glycogen into glucose – use for
energy

Salivary glands Decreases secretion of saliva Increases secretion of saliva
Tear glands Little effect Increases secretion of tears
Adrenal medulla Stimulates secretion of adrenalin No effect

Kidney Decreased excretion of urine increased excretion of urine
Brain Increased concentration No effect

• The sympathetic nervous system works together with adrenalin
in emergency situations that cause stress and require the ‘fight
or flight’ state. Stimulation by these nerves results in more
oxygen, glucose and blood being sent to the skeletal
muscles, speeding up cellular respiration to release more energy
for action.
• The parasympathetic nervous system enables the body to ‘rest
and digest’, i.e. to recover from sympathetic stimulation and
return to normal.
• Most organs are innervated by both the sympathetic and
parasympathetic fibres (double innervation).
• Effects produced by each system are generally antagonistic.


Alzheimer’s disease is a progressive and degenerative disease of
the brain, which causes the deterioration of memory and
thinking skills.
Causes
Although the cause is not fully understood, it is known that
Alzheimer’s disease develops because of a complex series of
events that happen in the brain over a long period of time.
Genetic, environmental and lifestyle factors may be at fault.
Prevalence
Alzheimer’s is prevalent in older people and affects both
genders. 15% of people over the age of 65, and 50% of people
over the age of 85 develop Alzheimer’s.

Symptoms
People with Alzheimer’s show symptoms of memory loss and
changes in their thinking skills. Problems may include:
– Getting lost
– Taking a long time to do daily tasks
– Mood and personality changes
– Poor judgement
– In some cases, hallucinations.
In severe cases, the people affected by Alzheimer’s cannot
communicate properly and are dependent on others to care for
them. They may be in bed for most of the time as their body
stops functioning.


Treatment
There is no cure for Alzheimer’s. Some medicines, however, can
help maintain thinking skills and can control or improve certain
kinds of behaviour for a short period of time. These medicines,
however, cannot delay the progression of the disease.
Certain factors may reduce the risk of developing Alzheimer’s:
– Regular exercise
– Healthy diet
– Limiting alcohol consumption
– Avoiding tobacco
– Staying socially active
– Engaging in intellectually stimulating activities


Co-ordination = the way in which information is communicated between
the receptors of the body, the central nervous system and the effectors
to bring about appropriate responses to any change in the environment.
Nervous system = the body’s control and communication centre.
Synapse = the junction across which a nerve impulse passes from an
axon terminal to a neuron, a muscle cell or a gland cell.
Neurotransmitter = a chemical substance released from axon terminals
at a synapse to either stimulate or inhibit the post-synaptic membrane.
Stimulus = any physical or chemical change in our internal or external
environment that is capable of causing a response in an organism.


Cerebro-spinal fluid = a fluid formed from blood plasma in
special areas in the walls of the ventricles. It protects and
nourishes the CNS and prevents cells from drying out.


The Nervous System

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