131674
Description
Mind Map by harry_bygraves, updated more than 1 year ago
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Created by harry_bygraves
over 11 years ago
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Nerve impulses;
action potential
- Effects of a stimulus. A nerve impulse occurs only when a neurone has a sufficiently high
stimulus. A stimulus is any disturbance in the external or internal enviroment. It may be
chemical, mechanical, thermal, or electrical, or it may be a change in the light intensity.
- When a stimulus is applied, the axon becomes depolarised; that is, the inside becomes
temporarily less negative. A sub-threshold stimulus results in a graded potential, a change
in electrical potential which is proportional to the stimulus intensity. However if the stimulus
is strong enough, an action potential occurs. there is a complete reversal of the change
across the nerve cell; the interior become postively charged relative to the outside.
- Typically, the action potential reaches a peak of about +35mV. The potential
difference then drops back down, undershoots the resting potential and finally
returns to it. The return of the potential difference towards the resting potential is
called repolarisation. The entire action potential takes about 7 milliseconds.
- The action potential results form changes in the permeability of cell
membranes to ions. at rest, the membrane is more permeable to pottasium
ions that to sodium ions because most sodium protein channels are closed.
When a stimulus is applied, the sodium channels open, sodium ions move
in, and the inside becomes more positively charged. If the stimulus reaches
the threshold level, an action potential occurs.
- When the action potential reaches its peak, the sodium channels close. Sodium
ions stop moving into the axoplasm but more potassium ion channels open and
potasium ions diffuse rapidly out. The changes cause the potential difference to
drop back down, undershoot the resting potential and finally return to it
- Transmission in an unmyelinated neurone. An individual action potential at any one point in a neurone
is a short-lived, localized event. It is transmitted along the neurone as the nerve impulse becuase it
causes a small current to flow in the axoplasm and the extracellular tissue fluid. In an unmyelinated
neurone, this local current acts as a stimulus for the next part of the nerve membrane, causing further
depolarisation, and so along the neurone. A nerve impulse is therefore transmitted as a self-propagating
wave of depolarisation with one portion of the neurone reploarising as the next polarises
- Transmisson in an myelinated neurone; saltatory conduction. In myelinated nerve fibres, the myelin
sheath acts as an effective electrical insulator. Consequently the local flow of current can only be set
up between adjacent nodes of ranvier. There is no myelin sheath at those nodes, therefore the
neurone membrane is exposed to the extracellular fluid. Also, there are many more sodium ion
channels at the nodes of ranvier than in the mylinated parts of hte neurone. The local current set up
by the depolarisation of one node depolarises the next node, and so on. The nerve impulse 'leaps'
from node to node. This type of impulse transmisson is called saltatory conduction. Saltatory
conduction allows nerve impulses to be transmitted very quickly. It is also highly efficient because
relatively few ions cross the membranes at the nodes, minimising the need for active transport
- All-or-nothing principle. This means that no
matter how long the stimulus, the size of the
action potential is always the same
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