Transcript The Nervous System - Dr Rob's A

The Nervous System
The nervous system is made of:
• Interconnected neurones, specialised for rapid
transmission of impulses
• These carry impulses from receptor cells
• Neurones also carry impulses to effector cells
which carryout appropriate responses
• Simplest nervous system is made up of a
receptor, neurone and nerve endings associated
with an effector
• It can be much more complex
• Many receptors working together make up
sensory organs eg the eye
• Complex nerve pathways exist.
• Sensory neurones only carry information
from receptors to processing areas of the
nervous system
• As animals increase in complexity, so do their
nervous systems
• They develop specialised concentrations of
nerves – the Central Nervous System
(brain and spinal cord in us)
• Incoming information is processed here
• Impulses are then sent out via motor
neurones
• Neurones are individual cells each one having a
nerve fibre that carries impulses
• Nerves are bundles of nerve fibres
(axons or dendrons the name relates to direction of
impulse)
• Some nerves are exclusively motor nerves, some
sensory, some are a mix
Simple organisms
• Hydra etc have simple nerve net
• They respond to limited stimuli
• They have limited effectors
• The nerve net is made of simple nerve cells with
extensions that branch out connect up to others
in various directions.
Reflex arc
Sensory Neurone
Motor Neurone
Relay neurone
Structure and Function of Neurones
• Neurones are the basic building blocks of a
nervous system
• They have a cell body containing the nucleus etc
along with Nissl’s granules – groups of RER
and ribosomes needed to make
neurotransmitters
• Dendrites are finger like processes that
connect to neighbouring neurones
• The nerve fibre itself is a slender fibre that
carries the impulses
• Fibres that carry impulses away from the cell
body are called axons
• Those that carry impulses toward the cell body
are dendrons
• Relay neurones are found in the CNS linking
sensory and motor neurones
Myelinated Nerve Fibres
• Vertebrate neurones are associated with specialist
cells called Schwann cells
• It is a membrane that wraps round the nerve fibre
many times
• It forms a myelin sheath
• There are gaps between Schwann cells called
nodes of Ranvier
• The myelin sheath protects the nerve from
damage and speeds up impulse transmission
Myelin Sheath
Speedy Nerve Impulses
• The thicker the fibre, the quicker the impulse
• Myelinated nerves are faster than unmyelinated
• Invertebrates have no myelin sheaths and their
fibres are thin… they are slow at around 0.5ms-1
• So how do they avoid danger?
• Some invertebrates have giant axons upto 1mm
in diameter
• This allows impulses to move at 100ms-1
• Vertebrates have both myelinated and
unmylinated nerves.
• Voluntary motor neurones are myelinated
• Autonomic motor neurones eg those controlling
digestive system muscles are not
• Myelinated nerves means you don’t need giant
axons
• This saves room
• It also means that can have a more versatile
network carrying impulses at up to 120 ms-1
Investigating Nerve Impulses
• Best way is to measure the (small) electrical
changes taking place.
• Needs apparatus sensitive to these changes
• Uses micro-electrodes to record and displayed
on a screen (oscilloscope)
• Most work done using motor neurones (axons)
• Can you think why?????
Nerve Impulses
• The basis of nerve impulses is different levels of
Na+ and K+ on the inside and outside of axons
• Remember membranes are partially permeable
• It has different permeability to the 2 ions
• At rest the axon is impermeable to Na ions, but
permeable to K ions.
• It also has a very active sodium/potassium
pump
• This uses ATP to move Na+ out of the axon, K+
in
• End up with less Na+ on the inside, pumped out
but can’t get back in
• At same time, K+ gets moved in, but it diffuses
back out along the concentration gradient
• Eventually the movement of K down the conc.
gradient is stopped by the electrochemical
gradient
• The inside of the axon is left slightly –ve
compared with the outside: polarised
• This resting potential is around -70mV
Action Potential
• When a neurone is stimulated there is a rapid
change in membrane permeability to Na+
• Specific Na+ channels (sodium gates) open allowing
movement to rapidly diffuse down their conc.
gradient
• This means the potential across the membrane is
briefly reversed
• Cell becomes +ve on inside (compared to outside)
• This depolarisation last 1ms, and the difference
is about +40mV
• This is the Action Potential
• At the end of this brief
period, Na+ channels
close again
• Sodium pumps
removed the excess ions
(requires ATP)
• the membrane becomes
hyperpolarised as
voltage dependant K+
channels also open
• So more K+ move out
than should
• This is soon reversed
when they shut
All or Nothing
• There is a threshold amount of Na channels
needed to be open before the rush of Na+ in, is
more than K+ out
• When this has been reached, an action potential
will occur
• The size of the action potential is always the
same.
• This is the ‘all or nothing’ law
Weak Stimulus
• Some Sodium gates
opened
• Some Depolarisation
• Does NOT reach
Threshold
• So NO Action potential
Strong Stimulus
• Many sodium gates
open
• Enough depolarisation
to reach threshold
• Action Potential
produced
Very Strong Stimulus
• Depolarisation
reaches threshold
• Action potential
produced
• BUT the action
potential is no bigger
than before!
• The recovery time of an axon is called its
refractory period
• This depends on the sodium/potassium pump
and membrane permeability to K +
• For the first ms or so, you cant send another
impulse down the fibre
• This is the absolute refractory period
• After this there is a few ms where it can be restimulated, but it requires a much higher
stimulus
• This is the relative refractory period
• During this time, the voltage-dependent K+
channels are still open, resting potential cant be
restored until they are shut.
• Refractory periods are important in the nervous
system
• It limits rate of impulses to 500-1000 each
second
• It also ensures impulses flow in only one
direction down a nerve
• Until resting potential is restored, that section of
a fibre cannot conduct an impulse
• This means the impulse can only go forward,
never in reverse…
Neurones in Action
• In myelinated neurones it is
more complex
• Ions can only pass through
membranes at nodes of
Ranvier
• These occur every 1mm
• This means Action Potentials
can only occur at nodes
• They appear to jump from
node to node
• The effect of this is to speed
up transmission
• It is called saltatory
conduction
(from the Latin saltare – to jump)
Synapses
• Neurones need to
intercommunicate
• Receptors pass on to sensory
nerves, they relay to the CNS.
The CNS processes and pass
on to effectors via motor
neurones.
• Where 2 neurones meet they
are linked by a synapse
• Every cell in the CNS is
covered with synaptic knobs
from other cells
• Neurones don’t touch each
other, there is a gap between
• Synapses rely on movement of Ca ions
• When an impulse reaches the synaptic knob,
it increases the presynaptic membrane’s
permeability to Ca ions (Ca ion channels open)
• Ca ions move in
• The influx makes synaptic vesicles to
move to the membrane
• They fuse and release their contents
• They contain neurotransmitter (~ 3000
molecules)
• The molecules diffuse across the synaptic cleft
and bond with receptors on the post-synaptic
membrane
• This opens Na channels so there is an influx intp
the post-synaptic neurone
• This creates an excitatory post-synaptic
potential (EPSP)
• With sufficient EPSPs, the positive charge exceeds
the threshold and an Action Potential is set up.
• Once the transmitter has had its effect, it is
broken down by enzymes in the cleft so it can
react with new impulses.
• In some cases the transmitter can have opposite
effects
• Other ion channels open so the inside becomes
even more negative
• Inhibitory post-synaptic potential is set up
• It makes it less likely an AP will be set up
• IPSPs are important, for example, in how we
hear sounds
Transmitter Substances
• One of the most common neurotransmitters
found in most synapses is acetylcholine (ACh)
• It is made in the synaptic knob using the ATP
made by all the mitochondria present
• Nerves which use ACh are called cholinergic
nerves
• Cholinesterase breaks it down in the cleft
• The products are reabsorbed and recycled
• Not all nerves use ACh, some (symapthetic
nervous system) use noradrenaline – adrenergic
nerves
• Dopamine is used in the CNS
Neurones interact in a variety of complex ways…
Summation and Facilitation
• Often one synaptic knob wont release enough
transmitter to set up an AP
• If 2 or more knobs are stimulated at the same
time releasing on the same membrane, the
effects add together and an AP may be generated
• This is spatial summation
• Sometimes, if one knob isnt enough to stimulate
a response, but a second impulse from the same
one arrives soon after, it may trigger an AP
• This is called temporal summation
• This also involves facilitation
• The first impulse doesn’t trigger an AP, but it
makes it easier for (facilitates) the next one
Accommodation
• If your senses are continually triggered, you
eventually get used to it and no longer notice it.
• This is accommodation
• Essentially, what has happened is all the
neurotransmitter in a synaptic knob has been
released
• It can no longer function, it is fatigued
• A short rest and they regenerate
Sensory Systems
• Sensory receptors play a vital role in
providing an animal with information about
both its internal and external environment.
• Simple receptors are just neurones with a
dendrite sensitive to a stimulus
• This type of cell is a primary receptor
• A secondary receptor is more complicated
• Secondary receptors are made
up of one or more completely
specialised cells (NOT
neurones)
• These cells then synapse with a
normal sensory neurone
• A good example is retinal cells
in the eye
• As animals become more
complex, so do their sensory
systems
• In higher animals, many
sensory receptors come
together to make sensory
organs.
Nerves and chemicals
The Brain’s Chemical Balance
• The brain uses a number of different
neurotransmitters
• An imbalance in these transmitters can result in
mental and physical symptoms
• Treating these imbalances means getting drugs
across the blood-brain barrier
• This makes it difficult
• Drugs that affect the
brain usually work at the
synapses
• So there are a number of
stages in transmission
which they can target
Illegal Drugs and the Brain
Illegal Drugs and the Brain
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L-dopa and SSRIs can have clear benefits
They are therapeutic and legal
Others are legal and enjoyable eg caffeine
Caffeine crosses the blood brain barrier and
affects the brain in several ways
eg slows down the rate of dopamine
reabsorption in synapses
• Some drugs are used because of the impact they
have on the brain
• They are often illegal
• Ecstasy (3,4-methylenedioxy-Nmethylamphetamine or MDMA) is one
• Ecstasy acts as a stimulant raising heart rate
• It is also psychotropic
• Short term affects make people happy, sociable,
full of energy, warm and empathetic
• All by affecting serotonin synapses
• MDMA blocks serotonin reuptake so the
synapses are flooded with serotonin
• There is some evidence it makes the transport
system work in reverse so serotonin in the presynaptic knob is moved into the cleft, flooding it
• It may also affect dopamine systems leading to
the pleasure sensation
• Ecstasy causes physical changes like increased
heart rate
• It can cause problems with thermoregulation
• There may be no desire to drink so you overheat
(hyperthermia) – can lead to death
• It affects the hypothalmus so you secrete more
antidiuretic hormone.
• This stops production of urine
• If you keep drinking water to stay hydrated and
stop overheating, you can retain so much water,
osmosis destroys your cells
Sensitivity in Plants
Communication in Plants
• Specific chemicals released by plant cells are
used to carry messages
• Plants rely on these chemicals to carry messages
to different parts of their structure
• They help respond to factors like light and
gravity
• They move cell to cell and through the transport
system
Stimuli that affect plants
• Light
▫ Direction, intensity and length of time of exposure
•
•
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Gravity
Water
Temperature
Touch (in some cases)
Chemicals (in some cases)
Plant Responses
• Response to stimuli is by making or destroying
chemical messages – plant hormones
• They are produced in one part of a plant, travel
elsewhere and have an effect there.
• In animals hormones have a variety of effects
• In plants it is usually just a growth response tropism
How Plants Grow
• Define growth…
• It is brought about by:
▫ Cell division
▫ Assimilation
▫ Cell expansion
• Cell expansion most noticeable, takes up water
by osmosis
• Meristems are where most growth occur (just
behind stem & root tips).
• Meristems are also the area most sensitive to
hormones
Day length determines bud development, flowering, fruit
ripening and leaf fall. Chlorophyll formation needs light.
Without light plants die.
Sensory Systems in Plants
• Seeds of many plants will not germinate without
exposure to light
• Has been shown red light (580-660nm) is most
effective (in lettuce)
• Far red light (700-730nm) actually inhibits
germination
• It was hypothesised a plant pigment that reacts
with diff. types of light was responsible
• In 1960, it was found and isolated,
phytochrome
• Phytochrome is blue-green pigment
• It exists in 2 forms P660 (PR) and P730 (PFR)
• When one form absorbs light it is converted into
the other form.
• PR is more stable but it is PFR that is biologically
active
• Phytochromes enable plants to respond to cues
like changes in day length
• Sometimes they are excitatory
• Other times they are inhibitory
• How phytochromes influence plant responses
are not fully understood!
hytochromes and Etiolation
• In the dark plants become etiolated:
▫ Grow rapidly using up food in an attempt to reach
the light
• Plants become tall and thin, little chlorophyll
• When they reach light, growth slows and they
make more chlorophyll – survival mechanism
• In the dark there is plenty of PR but little PFR.
• PFR inhibits the lengthening of internodes and
stimulates both chlorophyll production and leaf
growth
Photoperiodism
• In the UK, daylight can vary from 9 – 15hrs
• It acts as an important cue for organisms
• One of the most obvious affected activities is
flowering in plants
• In the 1920s Garner and Allard studied a
tobacco plant (in USA)
• Most tobacco plants flower in summer, they
looked at one that flowered in December.
• They realised they were
responding to day length as a cue
• With more than 14hrs light – no
flowers
• When less than 14hrs – it
flowered.
• Called critical day length
• It has been found that plants respond
differently:
• Short day plants (SDPs) flower when days are
short
• Long day plants (LDPs) flower when days are
long
• Day Neutral Plants (DNPs) don’t care!
• Since then it has been shown it is length of
darkness, not light, that is important!!!
How the length of the dark signal is
received
• The detection of photoperiod takes place in the
leaves
• The presence of a hormone, florigen, was
hypothesised in the 1930s
• It was thought florigen was made in response to
changing levels of phytochromes and carried in
the transport system to buds.
• If the whole plant is kept in the dark
but one leaf is given usual levels of
light and dark – flowering occurs as
normal.
• If the whole plant is kept in the dark,
it doesn’t.
• Using the same set up, if the leaf that
gets normal light is removed straight
away after stimulus, plant still does
not flower
• If a number of plants are
grafted together, but only one
is exposed to correct lighting
regime, all flower!
• You can even graft a leaf that
has been exposed correctly to a
plant which hasn’t been
exposed correctly and it will
flower.
• Nobody has ever managed to isolate ‘florigen’
• It fell out of favour and was thought not to exist.
• Lately though, a particular form of mRNA has
been found to be produced in the leaf associated
to the Flowering Locus T gene.
• FTmRNA was thought not to be able to leave the
cell and therefore it could not be florigen
• But it has been shown to move via
plasmodesmata and does go to the shoot apex
activating flowering genes!
• Plants move in response to various cues:
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Light from one direction (Phototropism)
Gravity (Geotropism)
Chemicals (chemotropism)
Touch (Thigmotropism)
They all require chemical signals.
• When a seed starts to germinate, shoot and root
need to grow.
• Shoot grows up to the light
• Root grows down into the soil where water and
nutrients are.
Phototropism
• In bright, all round light, plants will grow
straight
• In unilateral light (from one side) they will bend
towards it, roots move away from it
• Shoots are positively phototropic, roots are
negatively phototropic
• How does this work?!
Went’s experiments of the 1920s
Unilateral Light and Phototropisms
• The growth substance (hormone) involved in
phototropism is called auxin
• The first auxin discovered was indoleacetic
acid (IAA)
• Auxins are now made commercially to help
gardeners make cuttings root quickly.
How Do Auxins Work in a Plant?
• The side of a plant exposed to light has less
auxin in than the darker side
• Light seems to make auxins move laterally
across the shoot
• This movement means the tip is acting as a
photoreceptor
• More hormone diffuses down the dark side of
the plant to the zone of elongation
• So the dark side grows more
• So the plant bends towards the light