Chapter 39: The human
nervous system
Leaving Certificate Biology
Higher Level
The Nervous System
• The nervous system has three overlapping
functions:
– Sense stimuli
– Integration
– Motor responses
• The nervous system consists of the:
– Central Nervous System
• Consists of the brain and spinal cord
– Peripheral Nervous System
• Consists of all nerves that are found outside the
central nervous system
The Nervous System
• The central nervous system and the
peripheral nervous system both consist of
nerve cells
• Nerve cells are the fundamental structural
and functional units of the nervous system
• There are many different types of nerve cell
– an important one being the neuron
• The neuron is a specialised nerve cell that generates
and transmits electrical nerve impulses
The Neuron – Structure and Function
• Structure of a neuron:
– Dendrites
• Receive impulses from other cells or stimuli
– Cell body
• Contains nucleus and synthesises neurotransmitter
• Directs incoming impulses into the axon(s)
– Axon
• Conducts impulses away from cell body towards
another nerve cell/tissue/organ
– Myelin sheath (Schwann cells)
– Neurotransmitter vesicles
• Contain neurotransmitter chemicals that transmit
impulses from one neuron to another
The Neuron
• There are three types of neuron:
– Sensory neurons (peripheral nervous system)
– Interneurons (central nervous system)
– Motor neurons (peripheral nervous system)
Sensory Neurons
• Sensory neurons (peripheral nervous
system)
– Sense external and/or internal stimuli
– Carry messages towards the central nervous
system
– Sensory neurons then synapse with
interneurons in the central nervous system
Interneurons
• Interneurons (central nervous system)
– Interneurons are the most numerous type of
neuron found in the human body
– Interneurons receive messages from sensory
neurons and other interneurons in the brain
– Interneurons integrate messages received
and relay them onto motor neurons
Motor Neurons
• Motor neurons (peripheral nervous
system)
– Cause/effect a response (e.g in a muscle)
following a message from the interneurons of
the central nervous system
– Carry messages away from the central
nervous system
The Nerve Impulse
• The nerve impulse is an electrical signal
that passes through neurons and along
axons at great speed (up to 150 m/s)
• The conduction of electrical impulses
through neurons and along axons involves
the movement of ions across the cell
membrane of the neuron
• Eventually the impulse will reach the end
of the axon and is passed onto another
cell at a region called the synapse
Synapse
• A synapse is a specialised junction
between either two neurons or a
neuron and a target cell (e.g. a muscle
cell) that is adapted to allow the
transfer of an electrical impulse
– The end of the axon of the neuron is called
the axon terminal or synaptic bulb and
contains many small vesicles containing
chemicals called neurotransmitter
Synapse
Neurotransmitter
Presynaptic vesicle
Synaptic
cleft
Nerve Impulse
Neurotransmitter
reuptake
Pre-synaptic cell
Receptor
site
Post-synaptic cell
Synapse
• When the electrical impulse enters the axon
terminal from the axon it stimulates many of
the vesicles to move towards and fuse with
the cell membrane
• The neurotransmitter is released into the
synaptic cleft and binds to receptors on the
post-synaptic cell causing ions to rush in
thereby setting up a new electrical impulse
• Neurotransmitter chemicals are quickly
degraded by enzymes in the synaptic cleft or
are taken up by the surrounding nerve cells
The Nervous System
• Not all nerve cells are neurons – there are
other nerve cells called glial cells
• Glial cells outnumber neurons in the
nervous system 5:1
• An example of a glial cell is the Schwann
cell
– Schwann cells wrap around axons of sensory
neurons, interneurons and motor neurons
thereby forming the myelin sheath
Schwann Cells and the Myelin Sheath
• One Schwann cell wraps its cell
membrane around the axon of a neuron
many times forming a layer of myelin
• Cell membranes are made from
phospholipids – which are poor conductors
of electricity
• Myelin, therefore, provides electrical
insulation so that the electric current in the
axon is not lost to the surrounding tissue
• The myelin sheath helps to maintain the
strength and speeds up the impulse
The Central Nervous System
• Brain
– (Contains ~ 1000 billion nerve cells)
– (Has ~ 100 trillion synapses)
– Consists of mainly (glial cells) and interneurons
– Consists of the following main structures:
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Cerebrum
Hypothalamus
Pituitary gland
Cerebellum
Medulla oblongata
Meninges
• Spinal cord
Cerebrum
• Composed of two cerebral hemispheres
• Has many functions depending on area
• Involved in:
– Sensing stimuli (touch, taste, smell, hearing,
vision)
– Sending motor commands (movement)
– Higher brain functions such as:
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Emotion
Language comprehension
Memory
Ability to think and reason
Hypothalamus
• Lies just above pituitary
• Functions:
– Controls the pituitary gland
– Controls hunger, thirst, body
temperature, metabolic rate and
biorhythms (the ways the body responds
to day and night)
Pituitary Gland
• Lies just below hypothalamus
• Functions:
– It is the ‘master’ endocrine gland as it
releases hormones that control the
functions of other endocrine glands (e.g.
testes, ovaries, adrenals, thyroid)
Cerebellum
• Also known as the hindbrain because
it is the rear-most structure of the brain
• Functions:
– Controls the co-ordination of the skeletal
muscles
– Important in balance
– Important in hand-eye co-ordination
Medulla Oblongata
• Also part of the hindbrain
• Situated just in front of the cerebellum
• Functions:
– Controls unconscious bodily functions such as:
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Breathing
Heart function
Blood vessel contractions (vasoconstriction)
Digestion
Swallowing
Vomiting
Meninges
• Composed of highly specialised cells that do not
divide
• Composed of three membranes of strong
connective tissue just outside the brain and spinal
cord
• Inflammation of meninges is called meningitis and
can be caused by either a virus or a bacterium
• Functions:
– Protects the delicate tissues of the brain and spinal cord
(brain and spinal cord are the best protected organs in
the body with a covering of bone, a central watery
cushion called cerebrospinal fluid and the meninges
Spinal Cord
• Composed of an outer area called the white
matter (mostly glial cells), an inner area
called grey matter (mostly neurons), and the
central canal
• Protected by the meninges and 33 vertebrae
• (Approx same width as your little finger)
• (40 cm long)
• Functions:
– Carries messages to and from the brain
– Reflex centre
Reflex
• A reflex is a very fast, automatic
(unconscious), and pre-determined spinal
cord response to a stimulus
– e.g. pulling hand away from hot object
– Carried out by a reflex arc
– The advantage of reflex arcs is that they can
protect the body from harm
Reflex Arc
• A reflex arc is a special nerve pathway that
carries out the automatic spinal cord
response to a stimulus
– Composed of:
• Sensory neuron (peripheral nervous system) – its
cell body is always located in a dorsal root ganglion
just outside the spinal cord
• Interneuron (central nervous system) – located
entirely within the spinal cord
• Motor neuron (peripheral nervous system) – its cell
body is always located just inside the spinal cord
Mechanism of the Reflex Arc
• Pain and temperature receptors at endings
of sensory neurons in the skin are stimulated
and generate nerve impulses
• Nerve impulse travels the through the
dendrite to the cell body of the sensory
neuron located in the dorsal root ganglion
and then travels the short section of axon of
the sensory neuron into the central nervous
system (spinal cord)
• The sensory neuron synapses with a number
of interneurons
Mechanism of the Reflex Arc (cont.)
• Some interneurons carry impulse directly to
cell bodies of motor neurons located in the
spinal cord whereas others carry impulses to
the brain
• The stimulated motor neurons carry impulses
from spinal cord along the ventral root nerve
to the effector(s), in this case, muscle(s)
• Muscle(s) is/are stimulated and response
(muscular contraction) is carried out
• A pain sensation will be felt as the impulses
reach the brain
Parkinson’s Disease
• Parkinson’s Disease is a continuous,
uncontrollable shaking or tremor of the
body and limbs caused by lack of the
neurotransmitter, dopamine, in a specific
area of the brain
Parkinson’s Disease (cont.)
• Causes:
– Lack of dopamine in the brain
– Slow, progressive, irreversible death of dopaminergic
neurons
– Dopamine controls muscular contractions, but without it
movement becomes uncontrollable
– Cause of death of dopaminergic neurons is unknown but
is thought to be due to one or a combination of the
following:
• Exposure to pesticides
• Exposure to environmental pollutants
• Untreated allergies that affect the sinuses over many
years
Parkinson’s Disease (cont.)
• Prevention:
– Although there is no clinically-proven way to
prevent Parkinson’s disease, avoiding pesticide
exposure and environmental pollutants and
treating allergies that affect the sinuses (e.g.
hayfever) may be preventative measures that
may reduce chances of developing this disease
Parkinson’s Disease (cont.)
• Treatments:
– Administering drugs that mimic the effect of
dopamine in the brain, such as L-dopa
– Deep brain stimulation (DBS), which involves
insertion of electrodes into the brain that are able
to control muscle contractions
• However both of these treatments eventually become
ineffective over time as the brain gets used to the
treatments
The human senses
Leaving Certificate Biology
Higher Level
The Human Senses
• 5 senses:
– Vision
– Hearing
– Taste
– Smell
– Touch
• The brain is the interpreting centre for all 5
senses
The Human Senses
• The 5 senses contain receptors:
– Photoreceptors – rods and cones in eye
– Mechanoreceptors – hearing, balance, touch
– Chemoreceptors – taste and smell
– Thermoreceptors – respond to temp changes
The Eye
The Eye – Structure and Function
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Tear gland: production of tears
Eyelids: protection of eye and keeping eye moist
Conjunctiva: thin membrane protecting sclera
Aqueous humour: maintains shape of eye
Cornea: transparent part of sclera allows light in
Pupil: opening in iris allows light in
Lens: focuses light onto retina
Eyelashes: prevent foreign bodies entering eye
Iris: controls amount of light entering eye
Suspensory ligament: holds lens in place
The Eye – Structure and Function
• Ciliary muscle: surrounds lens and controls shape
(accommodation) of lens
• Sclera: white area; tough protective covering of eye
• Choroid: contains blood vessels (nourishes eye) and
melanin (absorbs light)
• Retina: contains sensory cells (rods and cones)
• Vitreous humour: viscous solution - maintains shape of eye
• Fovea: area of retina containing only cones (gives sharpest
vision) where image is focused
• Blind spot: point where all neurons exit the eye – no
rods/cones are situated here
• Optic nerve: nerve containing all the nerves from the retina
– carries sensory messages to the brain
• External muscles: help move the eye in various directions
Long-Sightedness (Hyperopia)
Long-Sightedness (Hyperopia)
• Long-sightedness means you can see
far-away objects clearly, but close objects
are blurred
• Cause: either eye-ball is too short or the
focusing elements are too weak
• Correction: convex lens is place in front
of eye and is used to focus images of near
objects on retina – e.g. for reading
Correction for Long-Sightedness
Short-Sightedness (Myopia)
Short-Sightedness (Myopia)
• Short-sightedness means you can see
near objects clearly but far-away objects
appear blurred
• Cause: either the eye-ball is too long or
the focusing elements are too strong
• Correction: concave lens placed in front
of eye and is used to focus images of faraway objects on retina
Correction for Short-Sightedness
The Ear
The Ear – Structure and Function
• Pinna: channels sound waves into ear
• Auditory canal: carries sound waves to
eardrum
• Eardrum (tympanic membrane): collects
sound waves by vibrating
• Ossicles: hammer (malleus), anvil (incus),
stirrup (stapes) – amplify and transfer
vibrations from eardrum to inner ear (oval
window)
The Ear – Structure and Function
• Eustachian tube: equalises pressure on
either side of the eardrum
• Cochlea: spiral tube that converts the
vibrations from the ossicles to pressure
waves in the fluid (lymph) of the cochlea
that cause microscopic hairs on sensory
cells to move and this sets up electrical
impulses which travel to brain via the
auditory nerve
The Ear and Balance
• The semicircular canals are part of the
vestibular apparatus that is responsible for
balance
• The canals are filled with lymph that
moves around the tubes as the head
moves that stimulates receptors
• When receptors in canals detect
movement an electrical impulse is
produced that is sent to brain via
vestibular nerve
Taste and Touch
• Taste receptors are located in the taste
buds of the tongue that respond to 4
tastes: sweet (tip of tongue); sour and salt
(sides of tongue); bitter (back of tongue)
• Receptors that sense pressure and
temperature are located all over skin
• Olfactory neurons are located in the nasal
cavity and respond to approx. 50 different
chemicals by producing electrical impulses
that are sent to the brain in response to the
presence of these chemicals