1. Nervous and hormonal
communication
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2. Nervous Hormonal control
control
Type of Electrical & Chemical
transmission chemical (hormones)
Speed of Rapid Slower (except
transmission adrenaline)
Response Immediate Slow acting
Area of Very localised Widespread,
response e.g.: growth
Changes Short-term Long-term
Pathway Specific Through blood, but
(nerve cells) specific target
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3. Hormonal communication
Exocrine and endocrine glands
• Hormones are made in endocrine glands
• A gland is a groups of cells which produces and
releases one or more substances (secretion by
secretory cells directly into blood)
• ‘Endocrine’ means ‘secreting to the inside’ –
endocrine glands secrete hormones into blood
capillaries inside the gland
• ‘Exocrine ’ glands mean ‘secreting to the
outside’ – secrete substance (not hormones) into
tube or duct, along which the secretion flows
(e.g. salivary glands secrete saliva into salivary
ducts which carry the saliva into the mouth)
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4. Endocrine glands
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5. Hormones
• Relatively small molecules
• Polypeptides or proteins (insulin); steroids (testosterone)
• Transported in blood plasma in small concentrations (< few
micrograms per cm3 of blood)
• Rate of secretion is also low (few micrograms or
milligrams/day)
• Most endocrine glands can secrete hormones very quickly
when an appropriate stimulus arrives (e.g. adrenaline)
• Many hormones have a very short life in the body – they
are broken down by enzymes in the blood or cells (e.g.
insulin – 10 to 15 minutes/adrenalin – 1 to 3 minutes)
• Each hormone has a particular group of cells which it
affects (target cells )
• These cells contain receptors specific to the hormone
• The receptor for protein hormones are on the plasma
membrane
• The receptors for steroid hormones are inside the cell, in
the cytoplasm
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6. The pancreas
• Parts function as an exocrine gland while other
parts function as an endocrine gland
• Exocrine: secretion of pancreatic juice which
flows along the pancreatic duct into the
duodenum where it helps digestion
• Endocrine: carried out by groups of cells ( islets
of Langerhans )
• Islets contain 2 types of cells (hormones
involved in control of blood glucose levels):
– α cells secrete glucagon
– β cells secrete insulin
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7. • The control of blood glucose
– Carbohydrate is transported through the human bloodstream in
the form of glucose, in solution in the blood plasma
– In healthy humans, each 100cm3 of blood normally contains
between 80 and 120mg of glucose.
– If blood glucose level drops below this, then cells may run short
of glucose for respiration and be unable to carry out their normal
activities
– Very high blood glucose levels can also cause major problems
– As blood with glucose flows through the pancreas, the α and β
cells detect the raised glucose levels
– α cells stop secreting glucagon; β cells respond by secreting
insulin into blood plasma
– Insulin affects many cells, especially those in liver and muscles:
• An increased absorption of glucose from the blood into the cells
• An increase in the rate of use of glucose in respiration
• An increase in the rate at which glucose is converted into the
storage polysaccharide glycogen
– All these processes take glucose out of the blood, so lowering
blood glucose levels
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8. • A drop in blood glucose: α cells secrete
glucagon and β cells stop secretion of insulin
• Lack of insulin – stop increased uptake and
usage of glucose by liver and muscle cells
• Presence of glucagon – affects the activities of
the liver cells:
– Breakdown of glycogen to glucose
– Use of fatty acids instead of glucose as the main fuel
in respiration
– Production of glucose from other compounds such as
fats
• Liver releases glucose into blood
• Time delays in control systems results in
oscillation, where things do not stay absolutely
constant, but sometimes rise slightly above and
sometimes drop slightly below the ‘required’
level
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10. Control of insulin secretion
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11. • Diabetes mellitus (sugar diabetes)
– 2 forms:
• Juvenile-onset diabetes /insulin-dependent diabetes –
pancreas incapable of secreting sufficient insulin, possibly due to a
deficiency in the gene which codes for the production of insulin, or
because of an attack on the β cells by the person’s own immune
system
• Non-insulin-dependent diabetes – pancreas does secrete
insulin but the liver and muscle cells do not respond properly to it
(associated with obesity)
– Symptoms:
• Blood glucose levels rise and stay high
• Glucose in urine because kidney cannot reabsorb all the glucose
• Extra water and salts accompany glucose so the person feels
extremely hungry and thirsty
• The combination of dehydration, salt loss and low blood pH can
cause coma in extreme situations. Build-up of substances called
keto-acids in the blood, due to metabolism of fats and proteins as an
alternative energy source, lowers the blood pH.
• Coma may also result because of lack of glucose for respiration due
to the lack of glycogen to be mobilised. Therefore, blood glucose
levels of a person with untreated diabetes may plummet.
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12. • In insulin-dependent diabetes, regular injections
of insulin, together with controlled diet, are used
to keep blood glucose levels near normal
• In non-insulin-dependent diabetes, insulin
injections are not normally needed but control is
by diet alone
• Advantages of using genetically engineered
human insulin:
– More rapid response
– Shorter duration of response
– Less chance of an immune response to the insulin
developing
– Effective in people who have developed a tolerance
for animal-derived insulin
– More acceptable to people who feel it is unethical to
use pig or cattle insulin
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14. Nervous communication
Neurones
• aka nerve cells: cells which are specialised for the conduction of action
potentials
• Motor neurone : transmits messages from the brain or spinal cord to a
muscle or gland. Cell body of a motor neurone lies within the spinal cord or
brain.
• Dendrite /dendron : a short/long cytoplasmic process of neurone, that
conducts action potential towards the cell body
• Axon : a long cytoplasmic process of a neurone, that conducts action
potentials away from the cell body (may be extremely long); contains all
usual organelles, large number of mitchondria at the tips of terminal
branches of the axon and vesicles containing chemicals called transmitter
substances
• Schwann cells : cells which is in close association with a neurone, whose
plasma membrane wraps round and round the axon or dendron of the
neurone to form a myelin sheath (made largely of lipid and protein)
• Not all axons have myelin sheaths
• The sheath affects the speed of conduction of the nerve impulse
• Node of Ranvier : a short gap in the myelin sheath surrounding an axon
(every 1-3 mm in human neurones, 2-3 μm long)
• Sensory neurone : bring impulses from receptors to the brain or spinal
cord (one long dendron and an axon shorter than the dendron)
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17. A reflex arc
• A pathway along which impulses are carried from a
receptor to an effector, without involving ‘conscious’
regions of the brain
• Reflex action : a fast, automatic response to a
stimulus; may be innate (inborn) or learned (conditioned)
Transmission of nerve impulses
• Neurones transmit impulses as electrical signals
• Signals travel very rapidly along the plasma membrane
from one end of the cell to the other and are not a flow of
electrons like an electric current
• Signals are brief changes in the distribution of electrical
charge across the plasma membrane, caused by the
very rapid movement of sodium and potassium ions into
and out of the axon
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19. Resting potential
• In a resting axon, inside of the axon always has a slightly
negative electrical potential compared with the outside
• The difference between these potentials (potential
difference ) is around -65mV (inside lower than outside)
• Resting potential is produced and maintained by the
sodium-potassium pump in the plasma membrane of
the axon
• The process involves moving the ions against their
concentration gradients, and so use energy from the
hydrolysis of ATP
• The sodium-potassium pump removes 3 sodium ions
from the cell for every 2 potassium ions it brings into the
cell (K+ diffuses back faster than Na+)
• The result is an overall excess of positive ions outside
the membrane compared with the inside
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20. • Action potentials
– A fleeting reversal of the resting potential across the plasma membrane
of a neurone, which rapidly travels along its length, caused by changes
in permeability of the plasma membrane to Na+ and K+
– Voltage-gated channels : a protein channel through a cell membrane
that opens or closes in response to changes in electrical potential
across membrane, that allow Na+ and K+ to pass through
– The electric current used to stimulate the axon causes the opening of
the channels in the plasma membrane which allow Na+ to pass through
(they flood through the open channels)
– The high concentration of positively charged Na+ inside the axon makes
it less negative inside than it was before. The membrane is said to be
depolarised
– As Na+ continue to flood in, the inside of the axon swiftly continues to
build up positive charge, until it reaches a potential of +40mV compared
with the ouside
– At this point Na+ channels close and K+ channels open
– K+ diffuses out of the axon down concentration gradient
– The outward movement of K+ removes positive charge from inside the
axon to the outside, thus beginning to return the potential difference to
normal (repolarisation )
– So many K+ leave the axon that the potential difference across the
membrane briefly becomes even more negative than the normal resting
potential
– The potassium channels then close, and the sodium-potassium pump
begins to acts again, restoring the normal distribution of Na+ and K+
across the membrane and restoring the resting potential
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23. • Transmission of action potentials
– The function of a neurone is to transmit information
along itself
– An action potential at any point in an axon’s plasma
membrane triggers the production of an action
potential in the membrane on either side of it
– The temporary depolarisation of the membrane where
the action potential is causes a ‘local circuit’ to be set
up between the depolarised region and the resting
regions on either side of it
– Na+ flow sideways inside the axon (away from
positively charged region towards the negatively
charged regions on either sides). This depolarises
these adjoining regions and so generates an action
potential in them
– Refractory period : a period of time during which a
neurone is recovering from an action potential, and
during which another action potential cannot be
generated
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25. • How action potentials carry information
– Action potentials do not change in size (+40mV) nor
speed at which it travels (the intensity of the stimulus
which orginally generated the action potential has
absolutely no effect on the size of the action potential)
– The action potential frequency differs between a
strong and a weak stimulus (strong stimulus produces
a rapid succession of action potential and vice versa)
– A strong stimulus is likely to stimulate more neurones
than a weak stimulus
– The brain can interpret the frequency of action
potentials arriving along the axon of a sensory
neurone, and the number of neurones carrying action
potentials, to get information about the strength of the
stimulus being detected by that receptor
– The nature of the stimulus (light, heat, touch) is
deduced from the position of the sensory neurone
bringing the information (e.g. retina)
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26. • Speed of conduction
– Myelinated human neurone: 100ms-1
– Nonmyelinated neurones: 0.5ms-1
– Myelin speeds up rate by insulating axon membrane
– Na+ and K+ cannot flow through myelin sheath (not
possible for depolarisation or action potentials to
occur in parts surrounded by it – can only occur at
nodes of Ranvier)
– Saltatory conduction : conduction of an action
potential along a myelinated axon or dendron, in
which the action potential jumps from one node of
Ranvier to the next (can increase speed of
transmission by up to 50 times)
– Diameter also affects speed of transmission – thick
axons transmit action potentials faster than thin ones
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28. • What starts off an action potential?
– Wide variety of initial stimulus: electric current (light, touch,
sound, temperature or chemicals)
– Receptor cell : a cell (in sense organs) which is sensitive to a
change in the environment that may generate an action potential
as a result of a stimulus – they convert energy in one form (light,
heat or sound) into energy in an electrical impulse in a neurone
– Pacinian corpuscle : one type of receptor found in the dermis
of the skin containing an ending of a sensory neurone,
surrounded by several layers of connective tissue ( capsule ) –
ending of sensory neurone inside capsule has no myelin
– Pressure applied – capsule pressed out of shape – nerve ending
deformed – Na+ and K+ channels open – Na+ flood in/K+ flow out –
membrane depolarised – increased positive charge inside axon
(receptor potential ) – if pressure great enough, receptor
potential becomes large enough to trigger an action potential
– Below a certain threshold, the pressure stimulus only causes
local depolarisation (not action potential) and therefore no
information is transmitted to the brain
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29. Pacinian corpuscle
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30. Synapses
• Synaptic cleft : a very small gap between two
neurones at a synapse
• Synapse : a point at which two neurones meet
but do not touch; made up of the end of the
presynaptic neurone, the synaptic cleft and the
end of the postsynaptic neurone
• The mechanism of synaptic transmission
– Transmitter substance : a chemical that is
released from a presynaptic neurone when an action
potential arrives that then diffuses across the synaptic
cleft and may initiate an action potential in the
postsynaptic neurone
– Action potential arrive along plasma membrane of
presynaptic neurone – release transmitter
substance into cleft – transmitter substance molecules
diffuse across cleft (< a millisecond as distance is so
small) – sets up an action potential in the plasma
membrane of the postsynaptic neurone
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32. – The cytoplasm of the presynaptic neurone contains
vesicles of transmitter substance (>40 are known:
noradrenaline and acetylcholine – throughout
nervous system; dopamine and glutamic acid –
only in the brain)
– Cholinergic synapses : a synapse at which the
transmitter substance is acetylcholine
– In the part of the membrane of the presynaptic
neurone which is next to the synaptic cleft, the arrival
of the action potential also causes calcium
channels to open
– The action potential causes calcium ions to rush in to
the cytoplasm of the presynaptic neurone
– This influx of calcium ions causes vesicles of ACh to
move to the presynaptic membrane and fuse with it,
emptying their contents into the synaptic cleft (each
vesicle contains up to 10 000 molecules of ACh which
diffuses across the synaptic cleft in <0.5ms)
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33. – The plasma membrane of the postsynaptic neurone
contains receptor proteins which has a
complementary shape to part of the ACh molecule –
ACh temporarily binds with the receptors – shape of
protein changes – channels open and sodium ions
can pass – sodium ions rush into the cytoplasm of the
postsynaptic neurone, depolarising the membrane
and starting off action potential
– The synaptic cleft contains acetylcholinesterase which
splits each ACh into acetate and choline to stop
action potential and avoid wasting ACh
– Choline is taken back into the presynaptic neurone,
where it is combines with acetyl coenzyme A to form
ACh which is transported into the presynaptic vesicles
– Whole process takes about 5-10ms
– Between motor neurone and a muscle, the nerve
forms motor end plates and the synapse is called a
neuromuscular junction
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35. • The effects of other chemicals at synapses
– Nicotine
• Part of the molecule is similar in shape to ACh and will fit into
the ACh receptors on postsynaptic membranes
• Unlike ACh, nicotine is not rapidly broken down by enzymes
and so remains in the receptors fro longer than ACh
• A large dose of nicotine can be fatal
– Botulinum toxin
• Produced by an anaerobic bacterium which occasionally
breeds in contaminated canned food
• Prevents the release of ACh (can be fatal)
• Injections of tiny amounts of the botulinum toxin into the
muscles of eyelids that contract permenantly (cannot open)
can cause them to relax, so allowing the lids to be raised
– Organophosphorous insecticides
• Inhibits the action of acetylcholinesterase, thus allowing ACh
to cause continuous production of action potentials in the
postsynaptic membrane
• Found in flea sprays and collars for cats and dogs,
organophosphorous sheep-dip (used to combat infections by
ticks), several nerve gases also acts the same way
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36. • The roles of synapses
– Synapses slow down the rate of transmission
of a nerve impulse. So why have synapses?
• Synapses ensure one-way transmission
– Allows signals to be directed towards specific goals
rather than spreading at random
• Synapses increase the possible range of actions in
response to a stimulus
– Action potential arriving at some of these synapses will
stimulate an action potential while arriving at others will
cause the release of transmitter substances which will
actually make it more difficult to depolarise plasma
membrane and so inhibit production of action potential
– The loss of speed is more than compensated for in the
possible variety of responses which can be made but we
do have very rapid responses called reflex actions
• Synapses are involved in memory and learning
– Picturing a face from the voice
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