Chapter 39: The human nervous system

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Chapter 39 The human nervous system

• Leaving Certificate Biology
• Higher Level

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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

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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

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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

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The Neuron

• There are three types of neuron
• Sensory neurons (peripheral nervous system)
• Interneurons (central nervous system)
• Motor neurons (peripheral nervous system)

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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

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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

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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

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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

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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

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Synapse
Neurotransmitter
Presynaptic vesicle
Synaptic cleft
Nerve Impulse
Receptor site
Neurotransmitter reuptake
Post-synaptic cell
Pre-synaptic cell
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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

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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 51
• 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

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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

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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
• Cerebrum
• Hypothalamus
• Pituitary gland
• Cerebellum
• Medulla oblongata
• Meninges
• Spinal cord

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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
• Emotion
• Language comprehension
• Memory
• Ability to think and reason

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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)

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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)

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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

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Medulla Oblongata

• Also part of the hindbrain
• Situated just in front of the cerebellum
• Functions
• Controls unconscious bodily functions such as
• Breathing
• Heart function
• Blood vessel contractions (vasoconstriction)
• Digestion
• Swallowing
• Vomiting

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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

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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

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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

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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

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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

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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

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Parkinsons Disease

• Parkinsons 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

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Parkinsons 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

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Parkinsons Disease (cont.)

• Prevention
• Although there is no clinically-proven way to
  prevent Parkinsons 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

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Parkinsons 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

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The human senses

• Leaving Certificate Biology
• Higher Level

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The Human Senses

• 5 senses
• Vision
• Hearing
• Taste
• Smell
• Touch
• The brain is the interpreting centre for all 5
  senses

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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

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The Eye
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The Eye Structure and Function

• 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

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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

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Long-Sightedness (Hyperopia)
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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

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Correction for Long-Sightedness
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Short-Sightedness (Myopia)
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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 far-away objects
  on retina

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Correction for Short-Sightedness
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The Ear
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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)

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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

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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

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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