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A2 Biology Revision: CNS
Coordination and Control of Animals Booklet
Neurones:
Neurones are specialised cells of the nervous system that are designed to carry nerve
impulses to and from the brain
Motor Neurone:
A motor neurone’s cell body is at the beginning of the neurone, at the beginning of
the AXON (important in distinguishing the difference between neurones)
Structure and Function:
Cell body: Contains most of the cytoplasm and organelles of the nerve cell (inc
...
Created by SCHWANN CELLS
NODES OF RANVIER: Gaps in the myelin sheath which allow the nerve impulse to
speed up by jumping from node to node
SYNAPTIC BULBS: Junctions between two neurones where the impulse can pass from
neurone to neurone across the gap called a SYNAPSE
Sensory Neurone:
Similar in structure to a motor neurone
Role is to carry nerve impulses from receptor towards the brain/spinal cord
The cell body is located IN THE CENTRE OF THE AXON
How a neurone creates a nerve impulse:
Charged ions are present in the cytoplasm of the cell and the surrounding fluid
There are different amounts of charges inside and outside the cell
The inside of the cell membrane is NEGATIVELY charged in respect to the outside
The difference in charge creates what is referred to as RESTING POTENTIAL and the
membrane is said to be POLARISED (outside more positive than inside, inside
negative in respect to outside, difference in polarities)
RESTING POTENTIAL= POLARISED MEMBRANE!!!

Resting potential (Rest Pot) created by uneven distribution of Sodium and Potassium
ions
Two features of the cell membrane bring about the uneven distribution of Na and K
ions:
-

Sodium-Potassium Pumps
Diffusion of Na and K ions across the membrane

Sodium Potassium Pumps: active transport systems that continuously pump sodium
out and potassium in across the membrane
NA and K can’t diffuse through the phospholipids of the CSM but they can diffuse
through the ion channels in special channel proteins in the membrane
There are special channel proteins for each SPECIFIC ION and they can be opened or
closed

Potassium ions open, sodium closed

Action Potentials:
When an AXON is stimulated, RESTING POTENTIAL is REVERSED!!!!!
Resting pot= round -70 mV
The stimulation of the axon means inside of membrane is more positive than outside
(round +30 or40mV or so)
The membrane is DEPOLARISED
RESTING= POLARISED

AXON STIMULATION= DEPOLARISED

This happens because, as the impulse passes, the membrane suddenly becomes
more permeable to SODIUM IONS than to POTASSIUM IONS
The SODIUM channels open and the POTASSIUM channels close
SODIUM diffuses in quicker than the potassium diffuse out
Makes the inside of the membrane more positive than the outside
For every ATP hydrolysed, 3 sodium diffuse in, 2 potassium diffuse out (reverse true
if at resting)

This opening of sodium channels and depolarisation of the membrane is known as an
ACTION POTENTIAL
After an impulse passes a particular point on the membrane the original resting
potential is re-established by the POTASSIUM CHANNELS OPENING and the SODIUM
CHANNELS closing
SODIUM moves in during depolarisation as the electrical concentration gradient
makes it do that basically, inside negative, outside positive, positive to negative
charges

Important Features of the Nerve Iqmpulse:
All or Nothing Nature:
This concerns the strength of the stimulus and how it affects the action potential
produced
If the strength of the stimulus is below THRESHOLD VALUE no action potential will be
produced
If the strength of the stimulus reaches THRESHOLD VALUE then an action potential
will be produced
This is all or nothing; if it doesn’t reach it will not happen
Any further increase past the threshold does not increase the size of the action
potential- action potentials are a fixed size of about +30mV
A HIGH INTENSITY does however cause a number of action potentials to fire off
RAPIDLY in QUICK SUCCESSION
HIGH INTENSITY STIMULUS= HIGH FREQUENCY OF IMPULSES
High intensity of pain for example causes high frequency of impulses to be sent to
the brain

Refractory Period:
Once an impulse has passed to a particular point in the AXON the axon membrane AT
THAT POINT enters ABSOLUTE REFRACTORY PERIOD of about 1 millisecond during
which the membrane is completely incapable of further action potentials no matter
how strong the stimulus

This happens after an IMPULSE PASSES, the axon needs time for REPOLARISATION
(the setting up of resting potential again) before a FURTHER action potential can
happen
Followed by RELATIVE REFRACTORY PERIOD
Lasts about 3 milliseconds which another impulse can be generated in but only if the
stimulus is very strong

There is a refractory period for 2 reasons:
-

Impulse direction
Impulse frequency

Direction: nerve impulses can only travel in a forward direction
...

Frequency: anther impulse cannot pass until after the refractory, this limits the rate
of impulses along the axon to about a maximum of about 1000 a second

Speed of Impulses along the axon:
Nerves carry vital information and the speed at which an impulse moves can be vital
in dangerous situations where an organism must react quickly to survive
The axon is adapted to speed up impulses

Axon Diameter:
Larger axon diameter= faster impulses
The speed depends on the RESISTANCE offered by the AXON
THICKER axons have less resistance
Giant axons found in primitive animals, many invertebrates, such as insects and
worms

TEMPERATURE:
Impulses go faster as the temperature increases up to around 40 degrees, after that
the proteins start to denature

MYELIN SHEATH:
Not all axons have myelin sheaths
Invertebrates are mostly non, vertebrates mostly have myelinated
PRESENCE SPEEDS UP IMPULSES
Myelin is a lipid material made by the SCHWANN cells
Wrapped in layers around the axon except in gaps known as NODES OF RANVIER
where it is missing
Because DEPOLARISATIONS only happen at these GAPS the impulse can jump from
NODE TO NODE along the axon
This is known as SALTATORY CONDUCTION
Saltatory is much faster than faster than normal conduction and so speeds up
impulse transmission
Saltatory conductions are much quicker than GIANT axons

Nervous System Breakdown AQA
The graph:
There are 5 points on the graph:
1)
2)
3)
4)
5)

Stimulus
Depolarisation
Repolarisation
Hyperpolarisation
Resting Potential



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