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The Nervous System and Nerve
Impulses
Section 8.1
Specification-topic 8
3 Describe the structure and function of sensory, relay and motor
neurones including the role of Schwann cells and myelination.
4 Describe how a nerve impulse (action potential) is conducted
along an axon including changes in membrane permeability to
sodium and potassium ions and the role of the nodes of Ranvier.
5 Describe the structure and function of synapses, including the role
of neurotransmitters, such as acetylcholine.
7 Explain how the nervous systems of organisms can cause
effectors to respond as exemplified by pupil dilation and contraction.
8 Compare mechanisms of coordination in plants and animals,ie
nervous and hormonal, including the role of IAA in phototropism
(details of individual mammalian hormones are not required).
stimulus→ receptor →sensory neurone→ processor →motor neurone→ effector →response
Nervous System: Structures, types of neuron
page 197
Structures, types of Neurones
1. Identify the neurons as- motor,
sensory, or relay (interneurone or
connector neurone).
1. Which neurons are part of the
peripheral nervous system, central
nervous system or both?
2. Draw arrows to show the direction
of the electrical impulse
3. Label the following on the
diagrams - cell body, axon,
dendrites, nucleus, myelin sheath,
terminal branches, nucleus of the
Schwann cell, node of Ranvier.
Name parts of a neuron and describe their function
• Cell body– contains nucleus and cell organelles
• Nucleus– contains genetic material
• Dendrites – conduct electrical impulses towards the cell body
• Axons– conduct electrical impulses away from the cell body
• Terminal branches– form synapses with other neurons
• Schwann cell– make the myelin sheath
• Myelin sheath– fatty insulating layer
• Nodes of Ranvier– parts of neuron with no myelin sheath, Electrical impulses jump
from one node to the next
Q8.1 Draw up a table comparing the structure and location of motor, relay and
sensory neurones. Use figure 8.5 AND 8.6 to help
Motor
Relay
Sensory
General
Cells have cell body, short
Cells have cell body, short
Cells have cell body, short
structure
_________ and long
_________ and
________ and long___________.
__________.
____________ axon.
Location of
Cell body and dendrites
Cell bodies __________
Cell body and dendrites
cell body
________ CNS, axons
CNS.
__________ CNS. Cell body is in
relative to
__________ CNS.
_________________ at entrance
CNS
route to the spinal cord.
Dendrites
Dendrites synapse with
Dendrites synapse with other
Dendrites synapse with other
___________________________
neurones in ____________.
neurones in _____________. ________
.
Axons
Axons synapse with
______________________
Axons synapse with
______________________
_________________.
_________.
Function
Conduct impulse to an
______________________
______________
Connect ________________
with appropriate
_________________.
Axons synapse with
___________________________
________.
Conduct impulse to the
_____________.
General
structure
Location of
cell body
relative to
CNS
Dendrites
Axons
Function
Motor
Relay
Sensory
Cells have cell
body at one end,
short dendrites and
long axon.
Cell body and
dendrites inside
CNS, axons outside
CNS.
Cells have cell
body, short
dendrites and short
or long axon.
Cell bodies inside
CNS.
Cells have cell body
on a side branch,
long dendrites and
shorter axon.
Cell body and
dendrites outside
CNS. Cell body is in
dorsal root ganglia
at entrance route to
the spinal cord.
Dendrites synapse
with other neurones
in CNS.
Axons synapse with
effector cells
(muscles and
glands).
Conduct impulse
from the CNS to an
effector (muscle or
gland).
Dendrites synapse Dendrites synapse
with other neurones with sensory
in CNS.
receptor cells.
Axons synapse with
Axons synapse with
other neurones in
other neurones.
CNS.
Connect sensory
neurones with
appropriate motor
neurone.
Conduct impulse
from the receptors
to the CNS.
Grey matter = numerous cell bodies and few myelinated axons
White matter = very few cell bodies and mainly long-range
myelinated axon
REFLEX ARC
1. Define a reflex– rapid involuntary response to stimuli important for protection
and survival
2. What is the advantage of a reflex pathway
– important for protection and survival
3. Label the reflex arc. Add the central canal, spinal nervesdorsal and ventral routes, and dorsal route ganglion
4. Use arrows to show the direction of the electrical impulse.
5. Draw an arrow to label ONE synapse.
6. Describe the role of the
a. Receptor
– receptors detect the stimuli (the stronger the stimulus the greater the
change in polarity across the membranes).
b. Effector
– Effectors carry out the response (e.g. muscles or glands)
7.
Use page 198 of your textbook to label the following diagram. Identify
the receptor, effector, sensory neuron, relay neurons, motor neurons, stimulus,
response
8.
As well as synapsing with relay neurons in the reflex arc, sensory
neurons synapse with neurons in the brain. What is the significance of this?
• two relay neurons synapsing with two motor neurons- one stimulating the triceps
and one stimulating the biceps- remember that they are antagonistic
• Synapse to the brain so that you become aware of the stimulus (e.g. pain) and can
then use voluntary nervous system to respond in some way. In addition we can
remember what caused the pain and avoid that particular situation that caused
the pain.
Nervous System: Control of pupil size
9. Label the radial and circular muscles in the
diagram above.
10. Which pupil is constricted? Dilated?
11. Annotate the diagram above to show when
a muscle is contracted (shortened) or when it is
relaxed.
Nervous System: The papillary reflex
12. Colour in the sensory neurone in red and the
motor neurons in green.
13. Label the retina and the iris muscles. Which is
the receptor? Effector?
14. Answer Q8.5- 8.8.
The Resting Potential
• Why was a squid neuron used
to measure nerve impulses?
• When no nerve impulses
were initiated the voltmeter
measured -70mV.
• This is the potential
difference – what does this
mean?
Resting potential
Nerve impulse - Important terms to
understand
• Potential difference– the difference in charge (measured in voltage).
• Polarised (membrane) (adjective)
– uneven distribution of charge across a membrane
resulting in a potential difference
• Resting potential
– the potential difference across a membrane at rest
due to the uneven distribution of charge.
Nerve impulse - Important terms
to understand
• Action potential– the large change in potential difference across the
membrane, the nerve impulse
• Voltage gated channels– Channel proteins in the membrane which open
due to a change in voltage across the membrane
(i.e. depolarisation)
Nervous System: Ion Channels
Label the1.
2.
3.
4.
5.
6.
sodium/potassium pump
K+ channels,
inside the cell,
outside the cell,
K+ concentration gradient
electrical gradient
SNAB Interactiv tutorial
Activity 8.2- slide 5-10
Describe the structure of an axon
membrane and explain how each part
contributes to the resting potential.
• Na+/K+ pump– pumps 3 Na ions out for every 2 K ions it pumps into the axon
• K+ channels– Are open. K diffuses out down the concentration gradient
resulting in increased negative charge inside the axon, so
some K are attracted back in – moving down the electrical
gradient. Eventually a electrochemical equilibrium is reached
(no gradient)- -70mv and there is no net movement of K ions
• Voltage gated Na+ channels – most are closed.
• Voltage gated K+ channels – closed
The Action Potential
http://highered.mheducation.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html
SNAB
Interactive
tutorial
Activity 8.2slide 1- 4
Nerve Impulse
•
•
•
•
•
•
Depolarisation
– Reversal of the potential difference across the membrane, the inside of the
axon becomes positive compared to the outside.
– E.g. -70mV becomes +40mV
Repolarisation
– The return of the potential difference to the resting potential.
– E.g. +40 mV back to -70mV
Hyperpolarisation
– When the potential difference becomes more negative on the inside than a
the normal resting potential.
– E.g. -70mV to -80mV
Concentration gradient
– A difference in concentration
Electrical gradient
– A difference in the distribution of electrical charge
Electrochemical gradient
– A difference in concentration and the distribution of electrical charge
Action Potential
Axon Membrane Summary
Membrane permeability
Neurons, like all cells, maintain different
concentrations of certain ions across their cell
membranes. Neurons pump out positively charged
_ sodium ___ ions. In addition, they pump in
positively charged __ potassium _ ions . Thus there is
a high concentration of sodium ions present
_ outside _ the neuron, and a high concentration of
potassium ions _ inside ___. The neuronal membrane
also contains specialized proteins called _ protein channels
_, which form pores in the membrane that are
selectively permeable to particular ions. Thus _sodium channels
__ allow sodium ions through the membrane while
potassium channels allow potassium ions through.
The resting potential
Under resting conditions, the membrane is _ more
__
permeable to potassium ions than to sodium ions. So there is a
slow _ outward _leak of potassium ions that is _ greater_ than
the inward leak of sodium ions. This means that the membrane
has a charge on the inside face that is _ negative ___ relative to
the outside, as more positively charged ions flow out of the
neuron than flow in. In addition many K+ ions which have diffused
out due to the _concentration _ gradient will be _ attracted _
back in by the _ negative __ charge inside the cell. K+ ions are
moving due to the _electrochemical_ gradient not just the
concentration gradient. The electrochemical equilibrium which is
in place results in the resting potential of__-70 _ mV.
Activity 8.02b The Action Potential
SNAB Interactiv
tutorial Activity
8.2- slide 12-14
The Action Potential- threshold
Once the stimulation reaches at least the _threshold_ level the voltage gated sodium
channels change _shape___ , open and positively charged sodium ions diffuse _into___
the neuron and make the membrane potential _less negative_____. This change in
membrane potential causes more sodium channels to open eventually resulting in
the inside becoming momentarily _positive_____ in comparison to the outside- the
cell is said to be_depolarised________ This is an example of _positive__
feedback.
The sodium channels _close_ spontaneously 0.5ms after opening. Depolarisation
leads to the _opening_ of the voltage gated potassium channels, allowing potassium
ions to _leave_the cell. Thus, there is first an influx of sodium ions (leading to
massive deplorisation) followed by a rapid efflux of potassium ions from the
neuron (leading to_repolarisation_). This repolarisation causes the potassium ion
channels to _close_. However they respond rather slowly so K+ continue to
diffuse out of the cell even after the membrane is completely repolarised (back to
the resting potential). This results in _hyperpolarisation_ Excess ions are
subsequently pumped in/out of the neuron by the _Na+/K+ pump_so that the
_resting potential_ is restored.
This temporary switch in membrane potential(depolariation,
repolarisation) is called the_action potential_. The cycle of
_depolarisation_and repolarization is extremely rapid, taking only
about 2 milli-seconds (0.002 seconds) and thus allows neurons to
fire action potentials in rapid bursts, a common feature in neuronal
communication. An action potential will only be propagated if the
initial depolarisation reaches a minimum _threshold value____.
After the sodium ion channels close they are _inactivated for a
short period of time. This means that no change in voltage can
stimulate them to open. This time is know as the
_refractory_period. This period has TWO consequences _Nerve impulses can pass in only one direction_______
 _There is an upper limit to the frequency of nerve impulses
Refractory period
• Short period of time
after an action
potential during which
this part of the
membrane cannot
respond to further
stimulation
• This is either because
depolarisation is
happening or because
the voltage gated
sodium channels are
inactive (unable to
open)
Compare the actions done with
these pieces of equipment
Which is an all-or-nothing response?
TRANSMISSION OF THE NERVE IMPULSE
• Conduction of
the impulse
along the axon
OR
• Propagation of
the impulse
along the axon
• Which direction
is the impulse
travelling?
 Transmission/conduction of an axon
potential is unidirectional because of the
refractory
__________period. A change in voltage
sodium channels
leads to the opening of the ___________
and _ Na+ ___ ions diffuse. After these
channels open they become _inactive ___ for
a certain time. During this time a voltage
change will _
NOT
__ cause them to open.
 Axon potentials can be transmitted /conducted
along myelinated and non-myelinated axons.
o Non-myelinated axons: Depolarisation in one
part of the axon sets up _
local currents
__-
positive
where positive ions flow from ___________
to
_
negative
__ regions (NO movement across
local currents
membranes here). These ____________
stimulate voltage gated sodium ion channels in
the neighbouring part of the membrane to
open
action potential
_______. This results in an ___________ if
threshold
the ___________
value of depolarisation is
exceeded.
Myelinated axons: The myelin sheath
electrical insulator
acts as an _____________.
Action
potentials cannot form where myelin is
present. Therefore they can only form
Node of Ranvier
at the _____________where there is
no myelin. Therefore local circuits can
node and a
form between one ___
neighbouring one and as a result the
node
action potential jumps from one ______
increases
to another. This _________
the rate of
conduction because action potentials do
not have to be formed at every point
along the axon. Saltatory conduction
Speed of impulse depends on 1. diameter of neuron
2. myelination
Action potential Crash Course
https://www.youtube.com/watch?v=OZG8M_ldA1M
Transmission of the impulse
across the synapse
Different types of synapses
Red synapse = inhibitory synapse
Green synapse = excitatory synapse
Inhibitory and excitatory synapses- all synapsing to
ONE post-synaptic neurone
Excitatory Synapses
ONE action potential arriving from ONE pre-synaptic
neurone is not enough to produce an action
potential in the post synaptic neurone.
Does not reach the threshold level of depolarisation
across the post-synaptic membrane.
Spatial summation – impulses from several neurones
arrive at the post synaptic neurone
Temporal summation – several impulses arrive from
one neurone in a short space of time.
Q 8.14
a). Spatial summation: a larger insect means more impulses from several
neurons arrive at the synapse and are more likely to trigger an action
potential in the postsynaptic membrane
b) Temporal summation: With a moving butterfly several impulses are
conducted along one neurone to the synapse making it more likely for an
action potential to be set up in the postsynaptic membrane
Inhibitory Synapses
Inhibitory Synapses
– What happens at an inhibitory synapse?
Chloride and potassium channels open causing a more
negative potential difference (hyperpolarisation)depolarisation above the threshold is less likely
The Synapse Crash Course
https://www.youtube.com/watch?v=VitFvNvRIIY
Q 8.15 - see page 200
• Constriction of pupilsInhibitory synapses connected to radial
muscles resulting in them relaxing
Excitatory synapses connected to circular
muscles stimulating contraction
• Dilation of pupilsInhibitory synapses connected to circular
muscles stimulating contraction
Excitatory synapses connected to radial
muscles resulting in them relaxing
Q8.4 Make a list of key words to distinguish the features of:
a nervous and b hormonal control.
Nervous
neurones
impulses
electrical (and chemical)
action potentials
synapses
rapid
short-term
specific
response local
muscle cell or gland
Hormonal
blood
chemical
slow
long-term
widespread
target cells
receptors
Nervous vs. Hormonal control
Nervous
Faster response
Localised response
Short term response
Specific cells receive signal
Electrical impulses and
chemical
neurotransmitters
Neurons conduct impulses
Hormonal
Slower response
Widespread response
Long term response
All cells receive hormone,
but only target cells are
affected.
Chemicals
Blood
Hormonal Response- pages 213-214
– Hormones work by altering the metabolism of a target
cell
– Target cells have receptors for this specific hormone
– The hormone may alter an enzyme’s activity or may
turn on a gene
– Eg. Testosterone, a steroid hormone
• Produced in testes and transported in blood
• Target cells - male sex organs, skin and muscle cells
• Effect - bind to androgen receptors, modify gene expression
to alter development of cell
Two types of hormone
1. Polypeptide hormones - bind with a
receptor in the cell membrane. This activates
a second messenger within the cell (cAMP).
cAMP activates enzymes or transcription
factors. Eg. Insulin, adrenalin.
2. Steroid hormones - diffuse across the cell
membrane and bind with a receptor in the
cytoplasm. This hormone receptor complex
acts as a transcription factor and a gene is
activated. Eg. Oestrogen, testosterone.
Coordination in plants
•
•
•
•
Plants do not have a nervous system.
They do coordinate activities and respond to stimuli.
This occurs via plant growth substances (hormones).
Tropisms are directional growth responses to stimuli
such as light and gravity.
• Plants carry out phototropism and geotropism.
• Some plants also carry out other tropisms – eg.
Thigmotropism, hydrotropism
Tropisms
Phototropism is contolled by
the shoot tip
Auxins
• Synthesised in actively
growing tissue –
meristems
• Bind with receptors on
the membranes of cells
• Actives a secondary
messenger
• Changes gene expression
• Can be used
commercially as
– Rooting powders
– Weed killers
– Produce fruits without
pollination
Coordination in plants
• Read pages 214-215 and answer Q 8.18-8.20 using the
diagram below.
Q8.5 Draw up a table that summarises the similarities and the differences between the action of hormones in animals
and growth substances in plants.
Similarities
Differences
Both hormones and plant growth
Hormones are produced by
substances are chemical _____________.
_____________ _____________ whereas
plant growth substances are produced by
dividing cells (____________).
Some hormones and some plant growth
Hormones are transported in the
substances bring about long-term changes
_________________ whereas plant
through control of g______________ and
growth substances move from cell to cell
d_____________. (Other hormones and
or in ________________.
plant growth substances bring about rapid
changes.)
Some hormones and some plant growth
substances affect ______________
expression, Other hormones and plant
growth substances have direct effects, for
example on e__________ or membrane
properties.
Q8.5 Draw up a table that summarises the similarities and the differences between the
action of hormones in animals and growth substances in plants.
Similarities
Both hormones and plant growth
substances are chemical messengers.
Some hormones and some plant growth
substances bring about long-term
changes through control of growth and
development. (Other hormones and
plant growth substances bring about
rapid changes.)
Some hormones and some plant growth
substances affect gene expression,
Other hormones and plant growth
substances have direct effects, for
example on enzymes or membrane
properties.
Differences
Hormones are produced by endocrine
glands whereas plant growth
substances are produced by dividing
cells (meristems).
Hormones are transported in the blood
whereas plant growth substances move
from cell to cell or in vascular tissue.