Transmission of nerve impulses
- Created by: sierrah
- Created on: 11-04-14 02:25
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- Transmission of nerve impulses
- Resting membrane potential
- There is a difference between the charges on the inside and outside of a neuron
- The membrane of a neuron at rest is said to be polarised
- ICF has proteins (neg) and potassium, overall negative charge
- ECF has sodium and chloride, overall positive charge
- RMP is -70mV, potential of inside is less than out
- Maintaining the RMP
- The membrane of a neuron is selectively permeable - ions can pass in and out of cell
- Potassium and sodium diffuse across the membrane using channel proteins - limited bc these are gated
- Sodium-potassium pump works against concentration gradient to keep membrane polarized
- Uses ATP - 3NA+ leave ICF, 2K+ leave ECF
- Interior of cell remind negative due to the proteins that cannot leave
- Generating an action potential
- The depolarization and repolarisation of a neural membrane is caused by a change in permeability of the membrane to sodium
- 4 main steps: Stimulus and slight depolarization, rapid depolarization, repolarisationand hyperpolarisation and RMP
- AP 1 - stimulus and slight depolarisation
- 3: Sodium spreads along inside of membrane (assisted by proteins)
- 2: Channel opens
- 4: Slight depolarization (-70 to -65mV)
- This slight depolarisation triggers the rapid depolarization
- Depolarisation only occurs at a certain threshold as this makes the membrane more permeable to sodium
- 1: stimulus - acetylcholine bonds to a gated channel protein for sodium
- AP 2 - Rapid depolarisation
- 3: Sodium floods the cell
- 2: Lots of sodium gates open
- 4: Membrane potential changes (-65 to -35mV)
- 1: Enough sodium enters the cell to cause rapid depolarization
- AP 3 - repolarisation
- 1: Change in membrane potential causes the gates to close
- 2: Potassium gates open
- 3: Potassium diffuses from ICF to ECF
- 4: Repolarisation occurs as the inside becomes more negative and the outside become more positive
- AP 4 - hyperpolarisation and RMP
- 1: sodium gates close
- 2: potassium gates close - this slow response causes hyperpolarization
- 3: Correct charges across the membrane exist but there is now more potassium outside the cell than in
- 4: the sodium-potassium pump works to restore this
- Conduction along an unmyelinated fibre
- Fibres that don't have a fatty myelin sheath
- Depolarisation of one area of neural membrane causes a local current in neighbouringareas
- Process repeats itself along the length of the membrane
- Action potential moves along the membrane away from point of stimulation (like domino line)
- If stimulus occurs in middle of fibre impulses travel in both directions away from point of stimulation (impulses usually occur at end of fibre)
- Nerve impulse prevented from going backwards bc of refractory period
- Refractory period is time in which another action potential cannot be generated due to imbalance of ions
- Need ion concentrations to return to normal before another action potential is generated
- Speed of transmission is 2m/sec
- Conduction along a myelinated fibre
- Fibres covered in myelin sheath
- Nodes of rangier at intervals
- Myelin sheath insulates fibres from extracellular fluid so no ions can flow between ICF and ECF and an action potential cannot form
- Action potential 'jumps' from one node of Ranvier to another as myelin is absent from the nodes
- This jumping is known as saltatory conduction - significantly speeds up impulse transmission
- Speed of transmission along myelinated fibres is 140m/sec
- Transmission across a synapse
- Neuratransmitters diffuse across gap
- Resting membrane potential
- If stimulus occurs in middle of fibre impulses travel in both directions away from point of stimulation (impulses usually occur at end of fibre)
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