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  • Created by: Megan
  • Created on: 11-03-14 17:52

Sensory Receptors

...are specialised to detect a stimulus and respond by producing an electrical discharge (impulse) <- Generator potential...

Mechanical - pressure or bending opens ion channels
Thermoreceptors - temperature affects an enzyme controlling an ion channel
Chemoreceptors - a chemical binds to  a receptor to open ion channels
Photoreceptors - light alters a membrane protein which control ion channels

All sensory receptors effectively open or close ion channels on membranes this changes the potential difference across membranes and is known as the receptor potential

Receptor potentials can lead to an action potential (Impulses) so sensory receptors effectively convert different forms of energy (eg light) into nerve impulses -> they are biological transducers

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Resting Potentials

All animal cell membranes contain a protein pump called Na+K+ATPase. This uses the energy from ATP to simultaneously pump 3Na+ ions out of the cell and 2K+ ions in

The normal membrane potential of nerve cells is -60mv (inside the axon) -> resting potential

When the axon is stimulated a breif reversal of the membrane potential, lasting about a millisecond is recorded -> action potential.

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  • The stimulating of electrodes cause the membrane potential to change
  • When the potential reaches -50mv, the voltage gated sodium ion channels open for 0.5mv
  • This causes sodium ions to rush in, makign the inside of the cell more positive. This phase is referred to as a depolarisation since the normal voltage polarity (negative inside) is reversed (becomes positive)


When the membrane potential reaches 0V, the potassium channels open for 0.5mv, causing potassium ions to rush out, making the inside more negative again. Since this restores the original polarity, it is called repolarisation

All or Nothing Event

The ion channels are either open or closed, there is no halfway position. This means that the action potential always reaches +40mv as it moves along the axon and it is never attenuated by long axons

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Refractory Period and Conduction

The Refractory Period

  • After an ion channel has opened, it needs a 'rest period' before it can open again. This is called the refractory period, and lasts about 2ms
  • This means that, although the action potential affects all other ion channels nearby, the upstream ion channels cannot open again since they are in the refractory period, so only the downstream chnnels open, causing the action potential to move one way along the axon

Saltatory Conduction

  • Vertebrate have a myelin sheath surrounding their neurone
  • The Na+/K+ channels are only found at the nodes of Ranvier
  • The action potential jumps from node to node (1mm), a process called Saltatory Propagation
  • This increases the speed of propagation - unmyelinated neurones speed is around 1m/s, in myelinated neurones they travel at 100m/s
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Conductance speed and frequency

Factors affecting Conductance Speed

  • Myelination
  • Temperature - higher temperatue, faster conductance
  • Diameter of the Axon -  the larger the diameter, the faster the conductance

The Frequency of Impulse Transmission

  • Action potential is all or nothing
  • The frequency of impulses conveys information about the intensity of stimulus
  • The higher the frequency of impulses, the stronger the stimulus  -> Frequency modulation
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Types of Cholinergic Synapse

...the mechanism of transmission at an excitatroy synapse

1) Excitatry Ion channel synapses - these have neuroreceptors that are sodium channels and cause depolarisation -> typical neurotransmitters - Acetylcholine & Noradrenaline

2) Inhibitatory Ion Channel Synapses - have neuroreceptors that open Chloride or Potassium channels

  • If chloride channels open, negative ions flow in causing a local hyperpolarisation, making an action potential less likely
  • These synapses inhibit an impulse form one neurone to the next ->Typical neurotransmitter - Glycine and GABA
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one neurone can have thousands of synapses on its body and dendrons. So it has many inputs but only one output -> this output is the sum of all the excitatory and inhibitatory potentials, from all that cell's synapse

Temporal Summation - it requires a series of action potential in the presynaptic neurone

Spatial Summation - several presynaptic neurones may each contribute to producing an action potential in the postsynaptic neurone

Synaptic Transmission is uni-directional - neurotransmitter only travels from the pre-synaptic membrane to the post-synaptic membrane

  • Vesicles containgin neurotransmitter only in presynaptic membrane
  • receptor proteins only present in the post synaptic membrane

As a result, transmission is one way only (uni-directional)

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