Download the brain

THE BRAIN
The brain contains 100,000,000,000 neurons. The grey matter (site of interneurons) is
located on the outside of the organ, with white matter (largely composed of myelinated
axons) located within. The relative location of the white and grey matter in the spinal cord is
reversed.
Brain components include:

Cerebrum: Controls all conscious actions.

Cerebellum: Coordination of complex muscular movements (eg: posture).

Hypothalamus: Temperature control, water balance, hunger etc.

Brain stem (midbrain, pons, medulla oblongata): Control of heartbeat, blood
vessels, breathing, salivation and swallowing.

Meninges: Three membranes which enclose the brain and spinal cord. Cerebrospinal
fluid between the meninges and CNS protects the brain and spinal cord from knocks. It
also circulates through the ventricles of the brain.
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The Essentials – Year 11 Biology – Book 1
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Fever
The hypothalamus raises body temperature in response to pyrogens (inc. bacterial toxins),
creating fevers during periods of infection.
QUESTION 1
Which organelles would be required to produce neurotransmitters?
_________________________________________________________________________
QUESTION 2
By what process would the neurotransmitter pass out of the axon terminal into the synapse?
Explain.
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
REFLEX ARCS
A reflex arc is a pathway from a receptor to an effector that does not involve the brain.
Instead, the involuntary response in a reflex arc is simply determined by direct connections
within the spinal cord. With less distance for impulses to travel, and few synapses involved,
the response to the stimulus is very rapid.
Stimulus  Receptor  CNS  Effector  Response
As a consequence, reflex arcs are involved in situations that could cause damage to the
individual, eg. stepping on a drawing pin. The more rapid the response, the less tissue
damage experienced.
A
B
C
D
G
F
E
Other examples of reflexes include the spontaneous withdrawal of a hand from a hot object
and the knee jerk reflex.
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The Essentials – Year 11 Biology – Book 1
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QUESTION 3
Which of the following organelles produces a neurotransmitter such as acetylcholine?
A
B
C
D
Nucleus
Mitochondria
Chloroplast
Ribosome
QUESTION 4
The part of the neuron that receives impulses is the:
A
B
C
D
Axon
Cell body
Dendrite
Myelin sheath
QUESTION 5
The key feature about the nervous system is that it is:
A
B
C
D
Long lasting
Slow
Only occurs in mammals
Fast acting
QUESTION 6
Neurotransmitter moves across the synaptic cleft by:
A
B
C
D
Diffusion
Facilitated diffusion
Osmosis
Active transport
QUESTION 7
The role of the myelin sheath is to:
A
B
C
D
Protect the cell body
Connect sensory and motor neurons
Release neurotransmitter at synapses
Increase the speed of impulses
QUESTION 8
A person has unfortunately stepped on a nail in the family garage. The response to the
stimulus would be determined by the:
A
B
C
D
Brain
PNS
Spinal cord
Hypothalamus
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The Essentials – Year 11 Biology – Book 1
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THE EYE
Label the vertebrate eye:
N
M
A
Suspensory
ligaments
L
B
K
C
J
D
E
F
I
G
H
Complete the following table of eye components:
Structure
Function
Cornea
Iris
Pupil
Ciliary Muscles
Lens
Vitreous Humour
Retina
Choroid
Sclera
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fine focussing
The shape of the lens is controlled by the ciliary muscles. The flexibility of the lens allows the
eyes to focus on both near and distant objects.
Near Vision
Distant Vision
(Campbell N A et al, 1999)
THE RETINA
The human retina contains 125 million rods and 6 million cones (70% of all sensory
receptors in the body!)
Rods:

More sensitive to light than cones, but do not distinguish colours.

Provide night vision, but only in black and white.

In humans, concentrated at peripheral regions of the retina, and totally absent from
fovea.
Cones:

3 types (‘red’, ‘green’ & ‘blue’) provide sharp colour vision in daylight.

Require more light than rods to be stimulated, so do not function at night.

Concentrated at the fovea (150,000/mm2) of the retina – some birds have over a million
cones/mm2 (eg. hawks can spot the movement of a mouse from a kilometre in the air!).
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The Trichromatic theory states that the different colours and shades we perceive are due
to the varying degree of stimulation of each cone type, i.e. the equal stimulation of all 3 types
of cones results in perception of white light.
Colour vision is found in all vertebrate classes, but not all of their species
History of Cones
In vertebrates, four different types of cones exist:

SWS1: short wavelength sensitive, i.e. sensitive to the violet region (and in some cases
UV region) of the spectrum.

SWS2: blue sensitive.

MWS: medium wavelength sensitive, in the green region.

LWS: long ...in the green/yellow to red, depending on species.

The 4 types were present in ancestral vertebrates (eg. dinosaurs).

Today, colour vision is best developed in teleost fish, birds and reptiles, each with
tetrachromatic vision. Birds and reptiles also have UV vision.

During their evolution, when they took refuge
from predators and competitors by adopting
nocturnal behaviour, placental mammals lost two
types of cones (SWS2 and MWS). Today their
colour vision is reduced to dichromacy; they
can discriminate yellow from blue, but have lost
red-green discrimination.

Primates, including humans, redeveloped a third
cone type from the duplication and modification
of the LWS gene. They now have a
trichromatic colour system, including SWS1,
M/LWS green and M/LWS red. Trichromacy is
characterised by the discrimination of red-green
colours on top of blue-yellow colours.

Australian marsupials did NOT follow the same
route as other mammals. They have three cone
types (trichromacy), but they retained an
ancestral cone type (MWS) and did not reevolve a third one from gene duplication as did
primates.
Fat-tailed Dunnart

Many marsupials (eg. fat-tailed dunnarts, honey
possums, bandicoots) also possess UV vision.

Some mammal species have lost three cone types. These mammals have
monochromatic vision, only possessing LWS cones. They have no colour vision (eg.
marine mammals and a couple of nocturnal rodents)
(Cathy Arrese, UWA)
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HORMONES AND OTHER CHEMICAL MESSENGERS
Hormones are organic molecules which are produced and secreted from one type of cell
(eg: of an endocrine gland), and travel via extracellular fluid (often via the bloodstream)
to act on specific target cells, causing profound effects in tiny quantities.
Only target cells contain the specific receptors which will bind to a particular hormone.
Although most mammalian hormones travel in the bloodstream to exert their effects in a
different area of the organism, some “chemical messengers” act on the same cell that
produced them, and others may be secreted into tissue fluid and act on cells that are
close by.

Hormones are slow acting, and yet they can exert long lasting effects.

Three main groups of hormones exist:

Proteins (peptides), eg: insulin, glucagon, adrenalin.

Steroids (lipids), eg: testosterone and oestrogen.

Amino acid derivatives, eg: thyroxin

In the case of protein/peptide or amino acid derivative hormones, the hormonesecreting cell typically releases its contents via exocytosis. Steroid hormones, and
some amino acid derivative precursor molecules, however, pass out of their hormonesecreting cells by simple diffusion.

Chemical messengers, often all loosely called hormones, are found in both plants and
animals.

Hormones attach to receptors on or in target cells via lock and key mechanisms. The
hormone attaches to a receptor site (usually a protein) that is only found in target cell
tissues. If a cell does not have the required receptor site, then the hormone will not bind
with the cell and will not affect the cell’s activity.

Receptor proteins are found in two locations. Receptors on the outside of the cell
membrane attach to hormones that are not able to pass through the cell membrane
such as polypeptide (protein) based hormones. Receptors within the cytoplasm of the
cell combine with steroid based hormones and simple amino acid hormones such as
thyroxine. These hormones have no difficulty in passing across cell membranes to
reach their receptors.
Some hormones can bind with receptors found on many different kinds of cells in the
organism, whilst others are specific, only binding with one type of cell or tissue.
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CHEMICAL NATURE OF HORMONES
Type of Hormone
Steroid
Examples
Features
Availability &
Lifespan
Testosterone,
oestrogen, cortisone,
progesterone.
Protein/Peptide
Water soluble, travel
in bloodstream;
Unable to pass
across cell
membranes.
In active form, most
are water soluble &
travel freely in
bloodstream *thyroid hormones
require a carrier
protein;
Amino Acid
Derivative
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thyroid hormones
are able to pass
across cell
membranes 
nuclear receptors;
others cannot cross
and require
membrane-bound
receptors.
Made in advance by
cell and stored in
secretory vesicles,
leaving by
exocytosis;
Short.
Made in advance by
cell and stored in
secretory vesicles.
Leave cell by
exocytosis (or by
diffusion if
precursor); *Short.
The Essentials – Year 11 Biology – Book 1
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ACTION OF WATER-SOLUBLE AND
LIPID-SOLUBLE HORMONES
Based on their mode of operation, hormones can be divided into two broad groups, i.e.
water-soluble and lipid-soluble hormones.
Water-soluble hormones include:



protein hormones (>200 amino acids in size)
peptide hormones (<200 amino acids in size) and
some amino acid derivative hormones.
These hormones dissolve readily in water, but are unable to cross membranes.
In contrast, lipid-soluble hormones, i.e.


steroid hormones and
some amino acid derivative hormones(eg: thyroxin)
are not soluble in water, and can freely pass through membranes.
In either case, a target cell must contain a protein receptor molecule for that particular
hormone.

In the case of water-soluble hormones, the receptors are located on the surface of the
cell membrane of the target cells. The hormone-receptor complex activates other
proteins within the cell (eg: G proteins) which are responsible for the cascade of
events (signal transduction) within the cell leading to the cellular response.

In the case of lipid-soluble hormones, the receptors are found within the target cell,
either within the cytosol or the nucleus. Again, the hormone binds to the waiting
receptor. The hormone-receptor complex attaches to a particular site on a chromosome
within the nucleus, leading to the cellular response.
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CELL SIGNALLING
Cell signalling occurs in the following manner:
Signal Reception
(At the cell membrane in the case of protein hormones)
Signal Transduction
(Usually a pathway of several steps, with each molecule in the
pathway bringing about a change in the next)
Cell’s Response
(Brought about by the last molecule in the pathway)
SIGNAL TRANSDUCTION
Signal transduction at the cellular level refers to the cascade of events originating outside
the cell leading to a specific cellular response.

With each step in a signal transduction pathway, the number of molecules involved is
typically amplified, i.e. many more molecules are typically involved at each subsequent
step in the pathway. The outcome is an alteration in cellular activity, which can be the
result of changes in gene expression within the cell.
(Campbell NA et al, 1999)
Signal transduction induced by a peptide hormone

The movement of signals can be simple, eg. acetylcholine acts in a simple manner on
receptor molecules in the post-synaptic membrane. Acetylcholine allows signals (in the
form of small ions, such as sodium) to be passed mainly by diffusion, either into or out
of the cell via membrane proteins. Enough ionic movement at the one time can
generate electrical impulses that travel along the nerve cell.

More complex signal transduction pathways involve many intracellular events. Signal
transduction takes place in as little time as a millisecond or as long as a few seconds.
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The Essentials – Year 11 Biology – Book 1
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FUNCTIONING OF A G PROTEIN-LINKED RECEPTOR:
If a protein or peptide hormone activates a membrane receptor.
This typically in turn activates a G protein.
The activated G protein causes a cascade of amplified changes (signal transduction).
finally leading to the cell’s response.

G protein receptor systems are very widespread and diverse in function,
eg. they are involved during
* Embryonic development
* Vision and smell reception
Note: up to 60% of all medicines used today exert their effects by influencing G protein
pathways.
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WATER SOLUBLE v. LIPID
SOLUBLE HORMONE SUMMARY
(a)
(b)
Water Soluble Hormone:

Binds to receptor in cell membrane.

Triggering a signal transduction pathway.

Leading to a change in cytoplasmic function or gene transcription.
Lipid Soluble Hormone:

Passes through cell membrane and binds to intracellular receptor (either in
cytoplasm or nucleus).

Hormone-receptor complex activates gene expression.
Response
G protein
Water soluble
hormone
Hormone binds to
receptor on cell
membrane or within
cell
Membrane
receptor
Signal transduction:
Signal detected
within cell
Lipid soluble
hormone
cascade of molecular
events linking external
signal to a specific
cellular response
Cell activates
response to
signal
Response
Cytosol/nucleus
Cytosol/nucleus
receptor
(Campbell NA et al, 2002)
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A SELECTION OF IMPORTANT ENDOCRINE HORMONES
Endocrine
Gland
Hormone(s)
Produced
Target Cells
Function/Role
Heart
Adrenal
Liver
ADH
Regulates water reabsorption
FSH
Follicle growth
Pituitary
In females: Ovulation, corpus
luteum development
LH
Most Cells
Decrease blood sugar levels
Liver and Muscle
Increase blood sugar levels
Pancreas
Thyroid
Thyroxin
Most Cells
Oestrogen
Ovary
Uterus
Progesterone
Testes
Most Cells
Regulate menstrual cycle and
secondary sexual characteristics
Regulates secondary sex
characteristics and growth
processes
QUESTION 9
Which organelle would make the hormone:
(i)
Insulin?
_____________________________________________________________________
(ii)
Testosterone?
_____________________________________________________________________
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COMPARISON OF COORDINATION SYSTEMS
Nervous System
Type of Message
Endocrine System
electrochemical
Speed of Message
slow
Duration of Response
seconds - years
Transmission
via nerves
Specificity
high (direct nerve
pathway)
Note: The endocrine and nervous systems work together in co-ordinating the
activities of multicellular organisms.
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