- Recall Helmholtz found that the speed of an AP is much slower than electricity through a wire - Membrane of axon is electrically charged
- Resting potential: - 70 mV
- Action potential: a brief electrochemical event that is carried by an axon from the soma of the neuron to its terminal buttons; causes the release of a transmitter substance
reversal in the electric charge of axon
- The electric charge is caused by unequal distribution of ions in the axon and in the fluid around it - Ions: positively/negatively charged particles; produced when substances dissolve in water - Ion channel: a special protein molecule located in the membrane of a cell; controls the entry or
exit of particular ions
can open and close
membrane of axon contains Na+ and K+ channels
- Ion transporter: a special protein molecule in the membrane of a cell; actively transports ions into or out of the cell
work like pumps
need energy of the cell to move ions
- During resting, outside is more positive because there are more negative ions on inside
- At action potential, some ion channels open, allowed Na+ ions to enter, so there is a reversal of the membrane potential
- This reversal at this point causes nearby ion channels to open and so there is another reversal that the new point also, and this goes on until the terminal buttons
- Action potential is very brief
- Na+ channels close, K+ channels open, allowing K+ to exit the cell - This restores the normal electrical charge
- Eventually, ion transporters pump Na+ ions out and K+ ions back in
- All-or-none law: the principle that once an action potential is triggered in an axon, it is propagated, without getting smaller, to the end of the axon
there is no such thing as a large/small AP
the AP either happens or it doesn’t happen
- Messages are not conveyed by a single AP, but rather, the information is represented by an axon’s rate of firing
- Sensory neuron: a neuron that detects changes in the external or internal environment and sends information about these changes to the CNS
strong stimuli, like bright lights, trigger a high rate of firing in the axons of sensory neurons that receive visual information
- Motor neuron: a neuron whose terminal buttons form synapses with muscle fibres. When an AP travels down its axon, the associated muscle fibres with twitch
high rate of firing in these axons will cause strong muscular contractions
Synapses
- Synapse: the junction between the terminal button of one neuron and the membrane of a muscle fibre, a gland, or another neuron
neurons communicate with other cells by means of synapses
- Presynaptic neuron: a neuron whose terminal buttons form synapses with and excite/inhibit another neuron
releases the neurotransmitters
- Postsynaptic neuron: a neuron with which the terminal buttons of another neuron form synapses and that is excited/inhibited by that neuron
receives the message/detects the neurotransmitters - A neuron can be connected with many terminal buttons
- Let’s look at the relation between a motor neuron and a muscle
- When there is an AP in the motor neuron, all the muscle fibres that have synapses with it will contract a short twitch
- One muscle has many muscle fibres and so one muscle must be controlled by many motor neurons
- Strength of muscular contraction is proportional to rate of firing of the axons - There are two types of synapses: excitatory and inhibitory
- Excitatory synapse makes it more likely that the axons of postsynaptic neurons will fire - if there are many of these, the axon will fire at a high rate
- Inhibitory synapse makes it less likely that the axons of postsynaptic neurons will fire - if there are many of these, the axon will fire at a low rate or not at all
- Terminal buttons contain many synaptic vesicles, filled with molecules of the neurotransmitter - When AP reaches terminal button, neurotransmitters are released into the synaptic cleft: a
fluid-filled space between the presynaptic and postsynaptic membranes; the terminal button releases transmitter substance into this space
- The terminal button does not touch the postsynaptic neuron
- The neurotransmitters either excite or inhibit the postsynaptic cell, depending on the neurotransmitter receptors
- Neurotransmitter receptor: a special protein molecule located in the membrane of the postsynaptic neuron that responds to molecules of the neurotransmitter
- Neurotransmitters attach to the receptor molecules and activate them
- Then the receptor molecules produce excitatory/inhibitory effects on the postsynaptic neuron by opening ion channels
- Excite: allow sodium ions to enter - Inhibit: allow potassium ions to leave
- Recall that MS is caused by the immune system attacking the myelin sheaths
- Myasthenia gravis: disorder where the immune system attacks neurotransmitter receptors - Like the AP, the excitation/inhibition produced by a synapse is brief, they are terminated by
reuptake
- Reuptake: the process by which a terminal button retrieves the molecules of transmitter substance that it has just released; terminates the effect of the transmitter substance on the receptors of the postsynaptic neuron
- How fast the neurotransmitters are taken back determines how long the effects of the neurotransmitters on the post neuron will be
- Some drugs affect the nervous system by slowing the rate of reuptake