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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…

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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…

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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…

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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…

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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…

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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…

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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…

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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…

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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…

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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,…

Comments

Divya Sareen

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

trying4law

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fantastic and detailed notes! thank you!

Issycswahn

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Legend notes

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