Full set of notes on NERVOUS SYSTEM -A2 biology unit 5 for edexcel-

Biology Edexcel Unit 5 Topic 8 nervous system 

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Biology Notes: Topic 8
Topic 8 ­ Grey Matter
The Nervous System and Nerve Impulses
All our senses, emotions, memories and thoughts are dependent on nerve impulses. The nervous system
is highly organised, receiving, processing and sending out information, as we saw with temperature and
control of heart rate.
What are nerve cells like?
A neurone is a single cell and a nerve is a more complex structure containing a bundle of the axons of
many neurones surrounded by a protective covering. The nervous system is organised as so:
Nervous system
(NS)
Consisting of sensory nerves, carrying
Consisting of the sensory information from the receptors to
brain and the Central nervous Peripheral the CNS, and motor nerves, carrying the
spinal cord system (CNS) nervous system motor commands from the CNS to the
effectors
Voluntary and stimulates Somatic Autonomic Involuntary and stimulates smooth
skeletal muscle nervous system nervous system muscle, cardiac muscle and glands.
Parasympathetic Sympathetic
nervous system nervous system
Prepares body for Prepares body for
`rest and digest' `fight or flight'
responses
There are different types of neurones but they all have the same basic characteristics. The cell body
contains the nucleus and cell organelles within the cytoplasm. There are two types of thin extensions
from the cell body:
Very fine dendrites conduct impulses towards the cell body
A single long process, the axon, transmits impulses away from the cell body
A2 BIOLOGY UNIT 5 (topic 8) EDEXCEL NOTES Page 1

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Biology Notes: Topic 8
There are three main types of neurone:
Motor neurones ­ the cell body is
always situated within the central
nervous system (CNS) and the axon
extends out, conducting impulses
from the CNS to effectors. The axons
of some motor neurones can be
extremely long, such as those that run
the full length of the leg.…read more

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Biology Notes: Topic 8
HOW THE MUSCLES OF THE IRIS RESPONDS TO LIGHT
The iris controls the size of the pupil. It contains a pair of antagonistic muscles: radical and circular
muscles. These are controlled by the autonomic nervous system. The radial muscles are like spokes of a
wheel, and are controlled by a sympathetic reflex. The circular muscles are controlled by a
parasympathetic reflex. The sympathetic reflex dilates and the parasympathetic reflex constricts the
pupil.…read more

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Biology Notes: Topic 8
How nerve cells transmit impulses
Much of the work done to establish what happens in a nerve fibre was carried out on the giant axons of
the squid. Their large size makes them easier to work with. Hodgkin, Huxley and Eccles carried out
this work in the 1940's and 1950's, and they eventually won a Nobel Prize for their efforts.
INSIDE A RESTING AXON
All cells have a potential difference across their surface membrane.…read more

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Biology Notes: Topic 8
WHAT HAPPENS WHEN A NERVE IS STIMULATED?
Neurones are electrically excitable cells, meaning that the potential difference across their cell surface
membrane changes when they are conducting an impulse.
If an electrical current above a threshold level is applied to the membrane, it causes a massive change in
the potential difference. The potential difference across the membrane is locally reversed, making the
inside of the axon positive and the outside negative. This is depolarisation.…read more

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Biology Notes: Topic 8
2. Repolarisation
After about 0.5ms, the voltage-dependent Na+ channels spontaneously close and Na+
permeability of the membrane returns to its usual very low level. Voltage-dependent K+
channels open due to the depolarisation of the membrane. As a result, potassium ions move out
of the axon, down the electrochemical gradient, and the inside of the cell once again becomes
more negative than the outside. This is the falling phase of the oscilloscope trace.
3.…read more

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Biology Notes: Topic 8
HOW IS THE IMPULSE PASSED ALONG AN AXON ?
When a neurone is stimulated, the action potential generated
does not actually travel along the axon but triggers a
sequence of action potentials along the length of the axon.
As part of the membrane becomes depolarised and repolarised, 1. At resting potential there is
it triggers another action potential. These events are repeated positive charge on the outside of
along the membrane.…read more

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Biology Notes: Topic 8
The speed of the nervous conduction is in part determined by the diameter of the axon. In general, the
wider the diameter of the axon, the faster the impulse will be . The normal axons of a squid, with a
diameter of 1-20 µm, conduct impulses at around 0.5 ms-1, whereas the giant axons, with a diameter of
1000 µm, conduct nearer to 100ms-1.…read more

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Biology Notes: Topic 8
HOW DOES THE SYNAPSE TRANSMIT AN IMPULSE ?
The arrival of an action potential at the presynaptic membrane causes the release of the
neurotransmitter into the synaptic cleft. The neurotransmitter diffuses across the gap, resulting in events
that cause the depolarisation of the postsynaptic membrane, and hence the propagation of the impulse
along the next cell. The presynaptic cell expends a considerable amount of energy to produce
neurotransmitter and put it into vesicles, ready for transport out of the cell.…read more

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Biology Notes: Topic 8
INACTIVATION OF THE NEUROTRANSMITTER
Some neurotransmitters are actively taken up by the presynaptic membrane and the molecules are used
again. Other neurotransmitters rapidly diffuse away from the synaptic cleft or they are taken up by other
cells of the nervous system. In the case of acetylcholine, a specific enzyme at the postsynaptic membrane,
acetylcholinesterase, breaks down the acetylcholine so that it can no longer bind to receptors. Some of
the breakdown products are then reabsorbed by the presynaptic membrane and reused.…read more

Comments

Divya Sareen

hey these are great notes, i was wondering if you had any notes on edexcel unit 4 biology?

trying4law

fantastic and detailed notes! thank you!

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