Structure of neurones
- Nerve cells are designed to respond to stimuli and transmit
information over long distances
- Neurones have 3 parts
- Cell body
- Has single nucleus
- Responsible for most of nerve cell metabolism, especially protein
synthesis
- Proteins made in cell body must be delivered to other parts of
nerve
- Axon
- Designed to transmit an electrical impulse
- Can be several meters long o Has axonal transport system for
delivering proteins to ends of cell
- Dendrites
- Receive impulses from other nerves
- In the human brain each nerve is connected to approximately 10,000
other nerves
The action potential
- Neurones transmit information as action potentials
- An action potential is a temporary change in the membrane potential
- Usually initiated in the cell body
- Travels in one direction normally
- Action potential is conducted in an all-or-none fashion
- If the stimulus is too low there is no action potential
- If the stimulus is above a threshold the action potential is always
the same size
Electrical changes during action potential
- Membrane potential depolarizes (becomes more positive)
- After the peak of the spike the membrane repolarises (becomes more
negative)
- The potential becomes more negative than the resting potential
(negative afterpotential)
- It then returns to normal
- The action potentials of most nerves last 5-10 milliseconds
- Action potentials are initiated by many different types of stimuli
- Sensory nerves respond to stimuli of many types including chemicals,
light, electricity, pressure, touch and stretch
- In the central nervous system most nerves are stimulated by chemical
activity at synapses
- Stimuli must be above a threshold level to initiate an action
potential
- After a nerve has fired there is a period of time during which it
cannot be stimulated again
- This is known as the refractory period
Biochemical changes during action potential
- The Na pump produces gradients of both Na and K ions
- Both are used to produce the action potential
- Na concentration is high on the outside the neurone and low inside
- Neurones have Na and K channels with gates that open and close in
response to the membrane voltage
- Opening gates of Na channels allows Na to rush into the cell,
carrying positive charge
- The spike of the action potential is caused by opening of Na
channels
- The membrane recovers by closing the Na channels and opening K
channels
- Two things bring the voltage back to negative values
- The Na channels close
- Potassium channels open when the voltage becomes positive
- Because K permeability is higher than in the resting state the
membrane develops a negative afterpotential
Myelin sheath
- Conduction velocity is increased by a myelin sheath
- Produced by Schwann cells in the peripheral nervous system and
oligodendrogliocytes in the central nervous system
- Multiple layers of lipid membranes are wrapped around the nerve
- Gaps are left every few millimetres and are called Nodes of Ranvier
- In a myelinated nerve the impulse jumps from node to node
- Conduction velocities for un-myelinated neurones is about 1 m/sec
- Conduction velocities for myelinated neurones is about 100 m/sec
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