How is the transmission of an electrical impulse down a neuron generated?
By the movement of Na+ and K+ ions
What is resting potential?
- a resting neuron has a net positive charge on the outside and a net negative charge on the inside
- the potential difference across the membrane is -70mv
- resting potential results from accumulation of Na+ ions on the outside of the membrane
Describe excitation
Na+ channels in the membrane open and Na+ rushes into the cell via facilitated diffusion
(Ligand-gated ion channel transport)
-this causes a reversal of charge (depolarization)
Describe action potential
Upon depolarization, the potential difference across the membrane changes to between +30 and +40mV
(Voltage gated transport)
-action potential moves down neuron like a wave by triggering the opening of adjacent Na+ channels
Describe repolarization
SOPI
After the action potential has moved through an area, sodium-potassium pumps use ATP to move Na+ ions out of the cell and K+ ions in
-pump moves ions 3:2 (Na+ : K+)
-as a result, resting potential is reestablished (+O-I, -70mV)
Refractory period
Nerve cell cannot fire again until resting potential has been reestablished (1-10 ms)
-ensures that signal travels in 1 direction (gates close)
-myelinated cells transmit impulses faster because the action potential jumps between the nodes of Ranvier
All or nothing
Transmission of an impulse down a neuron is an all or nothing event
Minimum level of a stimulus required to produce an impulse is called the threshold level (-55mV)
- managed by axon hillock
- intensity of all generated impulses same despite discrepancies in stimuli
- larger stimuli will have more frequent impulses than smaller stimuli
What are synapses?
Small spaces between neurons or effectors (muscles/glands)
Describe synaptic transmission
Neurotransmitters (NT) are used to transmit impulses across a synapse
- Axon terminal of the presynaptic neuron contains vessels that hold NT and upon stimulation release them
- NT diffuse across synapse and initiate an impulse in the postsynaptic neurons
2 types of synapses
- Electrical
-very rare
Presynaptic and postsynaptic cells are in direct contact
-allows current to flow through instantly - Chemical
- make up majority of synapses
- delays transmission as NT is released, diffusion, and binding of NT across synapse
- allows neurons to receive input from hundreds to thousands at the same time
- action potential reaches axon terminal
- Ca2+ gates open and flow into cytosol
- triggers Pr in synaptic vesicles to fuse w cell membrane
- NT is released through exocytosis
- NT binds to postsynaptic receptor, opening ion channels for AP to continue
