Biology F214 - Nerves

Biology F214 - Nerves

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Describe the effect of myelination on the rate if

Myelinated fibres conduct mroe quickly than unmyelinated: The myelinated sheaths are made from schwann cells which are electrical insulator so imsulses cannot pass through them. Depolarisation occurs at the nodes of Ranvier so the impulse passes from one node to another - salutatory conduction

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Describe the events taking place when a neurone ch

-70mV -> 30mV

Voltage gated sodium channels open and sodium ions move into the neurone and the potential difference rises from -70mV to +30mV

+30mV -> -70mV

Potassium channels open, potassium ions move out the neurone by diffusion down the electrochemical gradient. The PD falls from 30mV to just below -70mV 

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Explain the changes in membrane potential during a

Sodium Influx (rise in graph)

Voltage gated sodium channels open, sodium ions enter the axon. Positive feedback means more sodium channels open, depolarisation occurs. PD across membrane raises to +30mV. Sodium channels close

Potassium Efflux (down gradient on graph)

Voltage gated potassium channels close and K+ ions move out the axon. Membrane is repolarised beyond -65mV - hyperpolarisation

Resting Potential

Na/K pump maintains the resting potential. The membrane is more permeable to K+ ions as there are many potassium channels open

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Sensory and motor neurones

  • Very long so transmit impulse long distances
  • Plasma membrane has gated ion channels
  • Na/K pumps use ATP to actively transpurt Na ions out and CA ions into the cell
  • Potential difference is maintained across the plasma membrane
  • Cell body contains nucleus, many mitochondria and ribosomes
  • Myelin sheath insulates axon from nearby electrical activity
  • Have numerous dendrites connected to other neurones

Motor neuroes

  • Cell body in CNS
  • Long axon carries AP to effector

Sensory Neurone

  • Long dendron carries AP from sensory receptor to cell body 
  • Cell body just out side the CNS
  • Short axon carries AP to CNS
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Resting and Action potential


  • The potential difference across the neurone cell membrane when it is at rest (not stimulated). about -60mV inside the cell conpared to outside
  • Na/K ions pumps use ATP to pump 3Na+ out, 2K+ in
  • Plasma membrane is more permeable to K+ ions so they may diffuse out
  • The cell is polarised - negative potential compared to outside
  • Na+ gated channels closed


  • A depolarisation of the cell membrane so the inside is more pos than the outside with a pd of +40mV. This is transmitted along the axon plasma membrane
  • If some of the Na+ channels open, Na+ diffuses down its gradient into the cell from tissue fluid
  • Depolarisation of the membrane: In generator region the gated channels are opened by energy changes in environment. The gates further along the neurone are opened by changed in the PD across the membrane - voltage gated channels
  • All or nothing; a small depolarisation has no effect on gated channels. But if the depolarisation reaches the threshold potential it will open nearby voltage-gated channels. 
  • This caused large influx of Na+ and depolarisation reaches +40mV, and AP, which is transmitted along the neurone
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Voltage changes

Starts at resting state: polarised. Inside cell -60mV

Na+ channels open, some ions diffuse into neurone

Membrane depolarises: becomes less neg, reaches threshold potential -50mV

Voltage-gated Na+ channels open, Na+ floods in. Cell becomes more +ve charged inside

PD across memebrane is +40mV, inside more +ve than outside

Na+ channels close, K+ channels open

Repolarisation: K+ ions diffuse out of cell, lowering PD to -ve in cell 

PD overshoots, hypoerpolarisation

Original PD is restored so cell returns to resting state

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Voltage changes


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Transmission of AP's

The opening of Na+ channels upsets the resting potential created by NaK pumps. This creartes local currants in neurone cytoplasm, Na+ channels further along open

  • Na+ channels open, Na+ diffuse into neurone
  • Localised increased concentration of Na+ inside neurone, the AP
  • Na+ diffuse along axon away fron region of high concentration
  • Na+ voltage-gates open due to movement of Na+, altering PD across membrane. AP moves along neurone as more Na+ enter

Myelin sheath

  • Insulation
  • Na+ and K+ cant diffue through schwann cells. So the ionic movements that create an AP can only occur between the schwann cells - the nodes of ranvier. 
  • AP jumps from one node to the next - saltatory conduction
  • Transmission is faster
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Synaptic knob contains:

  • Many mitochondria
  • Lots of smooth ER
  • Vesicles containing ACh - neurotransmitter
  • Voltage gated Ca2+ ions in membrane(
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  • AP arroves at synaptic knob
  • Voltage gated Ca2+ channels opel
  • Ca2+ diffuses into synaptic knob
  • Ions cause the synaptic vesicles to move to and fuse with pre-synaptic membrane
  • ACh released by exocytosis and diffuses across cleft 
  • ACh binds to receptor sites of Na+ channels in post-synaptic membrane
  • Na+ channels open, Na+ diffuese across postsynaptic membrane to postsynaptic neurone
  • A Generator potential is created
  • Of sufficient generator potentials combine, the membrane reaches threshold potential
  • A new AP is created in the potsynaptic membrane
  • Acetylcholinerase is an enzyme in the synaptic cleft, hydrolyses ACh, stops transmission of signals so synapse stops producing APs
  • They remains are diffused into pre-synaptic know and are recombined using ATP
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  • Synapses connect neurones together so a signal can be passed along
  • Several presynaptic neurones might converge to one postsynaptic neurone
  • One presynaptic neurone might diverge to several postsynaptic neurones
  • Synapses ensure signals are transmitted in one direction - ACh only in one side
  • Synapses filter out low-level signals - low-levels are unlikely to be passed on, not enough ACh released
  • Low-leved signals amplifided by summation
  • Acclimitisation - after repeated stimulation, synapse may run out of vesicles - fgatigued

A stimulus at highter ntensity produces more generator potentials. More frequent AP's in sensory neurone caused. High intensity of signals means more intense stimulus

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