Summary [9700] CIE A-Level Biology Unit 15: Control and Coordination
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Course
Unit 15 - Control and Coordination (9700)
Institution
CIE
Book
Cambridge International AS and A Level Biology Coursebook with CD-ROM
This section of my summarised notes covers the topic 'Control and Coordination' in CIE 9700 Biology. These are simple, stylised note pack which are perfect for A2 biology revision. They cover the topic in the CIE syllabus with needed detail to ensure you get the perfect score.
Juggling 4 differen...
Control and Coordination
Nervous Communication
Contains 2 different sections:
⇾ Central Nervous System (CNS): brain and
spinal cord
⇾ Peripheral Nervous System (PNS): cranial and
spinal nerves
• Information is transferred in the form of nerve
impulses that are very fast and short lived
Neurones
• Sensory Neurone: from receptor to CNS
• Relay Neurone: from sensory to motor neurone through the CNS
• Motor Neurone: from the CNS to effectors (muscles or glands)
• The end of the branches of the axon have large number of mitochondria and
vesicles containing transmitter molecules responsible for nervous transmission
• Highly branched dendrites give large surface area
• Myelin Sheath: The enclosing of the Schwann cell over the axons made up of mostly lipids and
proteins that affects the speed of conduction of nervous impulses.
Reflex Arc
↬ Reflex action: the fast and automatic impulses that travel from a receptor to an effector following a
change in the external environment (stimulus).
↬ As pathway does not involve conscious regions of the brain it is helpful to prevent injuries
↬ Most use relay neurones but there can be direct links between sensory and motor neurone
Nervous Impulses
Resting potential: the potential difference (-60mV to -70mV) between inside and outside of the
axon. The inside always has a lower electrical potential (more negative) than the outside. This is
maintained by the unequal distortion of Na+ and K+ ions.
Refractory Period: period of recovery when part of the axon is unresponsive as sodium channels
are unable to open. Period length determines the maximum frequency of action potential
SxTeri Notes Page 19
, Unequal distribution “polarised membrane” is caused by:
1. Action of Na+/K+ pumps that actively expel Na+ against the concentration gradient
2. Difference in permeability of the membrane due to more K+ channels
3. Organic ions (COO-) which attract K+ and prevent diffusion out
Action Potentials
↬ Brief changes in the distribution of electrical charge
across the cell membrane caused by the rapid
movement of sodium and potassium ions in and out
of the axon
4. Once potential difference reaches the threshold
potential (-55mV) then the stimulus causes voltage-
gated Na+ channels to open causing a sudden
increase in Na+ permeability
5. Na+ ions diffuse rapidly into the axon down the
concentration gradient causing the axon inside to
become more positive (depolarisation) triggering
more Na+ channels to open
6. Potential difference of membrane revered from -70 to
+40mV
7. Na+ channels close and K+ channels open so membrane to becomes more permeable to K+ ions
8. K+ diffuses out down the concentration gradient to make the inside negative again (repolarisation)
and restored back to resting potential of -70mV
9. Hyperpolarisation: briefly the membrane become even more negative than resting potential
10.Once membrane is repolarised, the gated K+ channels close and the Na+/K+ pumps act to restore
the ion distortion by expelling Na+ and taking in K+
How action potentials carry information: The brain interprets the frequency of action potentials and
number of neurones carrying action potential to determine strength of stimulus. Strong stimulus results
in a rapid succession action potentials so more neurones are being stimulated
What affects the speed of an impulse
⇾ Presence of Myelin: Myelin allows saltatory conduction when action potential moves from node to
node. (Ion channels only present at nodes of ranvier)
⇾ Axon diameter: larger axon conducts faster due to less resistance to ion flow and increase surface
area for the exchange of ions
⇾ Distance between nodes
⇾ Body temperature: higher
temperature, higher rate of diffusion
⇾ Number of Synapses: fewer
synapses, the faster the conduction
due to time taken for diffusion
SxTeri Notes Page 20
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