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Sensory Receptors+ Motor Neurones

Sensory receptors= ENERGY TRANSDUCERS 

Light sensitive cells in eye-- light intensity + wavelengths of light 

Olfactory cells in nose- volatile chemicals 

Tastebuds- soluble chemicals 

Pressure receptors on skin- pressure on skin 


  • both long to transmit AP over distance
  • CSM has gated ion channels for Na, K or Ca
  • Na/K pumps- use ATP to actively transport Na out
  • Myelinated sheath- made of schwann cells- nodes of ranvier 
  • both have cell body 
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Sensory+Motor contrast

  • Sensory - receptor-CNS, motor CNS-effector
  • Sensory- cell body outside CNS, motor, cell body inside CNS
  • Sensory - short axon, motor- long axon 
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Resting potentials+action potentials

Resting potential= -60mV- 3 Na pumped out, 2 K diffuse in - polarised- cytoplasm also contains anions (neg charged ions) 

Action potential= at rest, Na+ kept closed- if some open, Na will diffuse quickly down concentration gradient- opened by energy changes in environment - gates further along opened by the change in p.d. - voltage gated channels. 

- once generator potentials are large enough to reach threshold potential (-40mV) - causes large influx of sodium ions as the gates open -depolarisation to +40mV= action potential 

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Ionic movements

  • resting potential- -60mV - 3 Na+ out, 2 K in 
  • some sodium channels open due to change in environement- sodium diffuses in 
  • causes a depolarisation, and if large enough, reaches threshold pot. to cause depolarisation (-40mV)
  • causes voltage gated channels to open- detect change in p.d.-large influx of sodium diffuses in = +40mv= ACTION POTENTIAL
  • K+ channels open and Na+ channels close - hyperpolarisation where it overshoots too negatively 
  • original p.d. restored 

REFRACTORY PERIOD- imposs to restimulate the membrane - allows cell to recover and for signal to be transmitted in ONE DIRECTION
Key words: 
resting potential, polarised, sodium ion channels, generator potential, threshold potential, voltage gated ion channels, all or nothing, depolarisaiton, hyperpolarisation, 

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Transmission of action potentials


  • AP causes sodium channels to open
  • sodium diffuses in down conc gradient 
  • conc rises at the point where the channels opened
  • causes sodium to diffuse sideways in the neurone - LOCAL CURRENT 

Voltage gated sodium ion channels: 

  • decrease in p.d causes the voltage gates to open - allowing AP to move along 

Myelin sheath: 

  • fatty sheath- non conducting of Na+ and K+- keeps AP in neurone 
  • nodes of ranvier, gaps between the schwannn cells- allow saltatory conduction - ionic exchanges happen here - local currents elongated + sodium ions diffuse in at nodes of ranvier 
  • Myelinated much faster than non-myelinated- 120 m/s

Local currents- sideways- p.d. decrease- v. gated channels- saltatory conduction 

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Nerve junctions

CHOLINERGIC SYNAPSES- use acetylcholine as NT 

presynaptic knob:

  • mitochondria
  • vesicles containing acetylcholine 
  • voltage gated calcium ion channels in membrane 

postsynaptic knob:

receptors complementary to acetylcholine - once binded causes sodium channels to open
Transmission across synapse:

  • AP arrives at synpatic knob - calcium voltage gated channels open- calcium diffuses in 
  • causes vesicles to fuse to membrane- exocytosis- acetycholine diffuses across synaptic cleft 
  • bind w receptors on post synaptic membrane- cause sodium channels to open
  • generator potential created- threshold- voltage gated channels open etc... AP created
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Role of acetylcholinesterase

In synaptic cleft - hydrolyses acetylcholine into choline and ethanoic acid-stops transmission +synpase doesnt produce more action potentials . Products diffuse in and packaged to form acetylcholine again - uses ATP- recycled. 

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Signals and messages

The roles of synapses:

  • several presynpatic converge into one postsynaptic- different parts create the same response - useful in danger 
  • one pre synpatic diverge into many post synpatic- allows to be transmitted to different parts of the NS- eg reflex arc - informs the brain + elicits response 
  • Allow transmission in one direction 
  • filter out low level signals 
  • or low level signals amplified by summation -when low level persistent generates successive AP's in presynaptic - release of many vesicles=generator potential in postsynaptic
  • acclimatisation- run out of vesicles- means u get used to bad smells 

Frequency of transmission - 
generator potentials = more action potentials = more vesicles released= higher frequency of AP's in postsynaptic = more intense stimulus 

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Myelinated vs non myelinated


  • Quicker- 120m/s compared to 20m/s 
  • more rapid response to stimuli
  • longer than non-myelinated- but they dont need to be long - in digestive system + breathing 
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