BIO 203 Lecture 6: Chapter 7 continued

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Nerve impulses
Once started, action potential does not diminish
Does not vary in intensity with the strength of the stimulus that triggered it
For a brief period following an action potential, the neuron cannot be stimulated again
This is called the refractory period
It occurs because the sodium channels are closed and cannot be reopened
Synaptic Transmission
Communication between a neuron and an adjacent cell occurs by neurotransmitters
Synapse is a junction between a neuron and another cell
Synaptic Cleft
The gap between two cells
Neurotransmitters diffuse across the gap
In the case of two neurons, the presynaptic neuron sends a message to the
postsynaptic neuron
Synaptic knob
Swelling at the end of the axon of the presynaptic neuron
The nerve impulse reaches the synaptic knob of the presynaptic neuron
Calcium ions move into the synaptic knob which releases the neurotransmitter into the
synaptic cleft
The neurotransmitter diffuses across the synaptic cleft and binds with receptors on the
membrane of the postsynaptic neuron, causing an ion channel to open
At an excitatory synapse, binding of the neurotransmitter to the receptor causes
sodium channels to open, increasing the likelihood that an action potential will begin
At an inhibitory synapse, binding of the neurotransmitter to the receptor opens different
ion channels
The postsynaptic cells interior becomes more negatively charged than usual,
reducing the likelihood that an action potential will begin
A neuron may have as many as 10,000 synapses with other neurons at the same time
Some synapses have excitatory effects and some have inhibitory effects
Summation
Combined effects of excitatory and inhibitory effects at any given moment
Determines whether an action potential is generated
This level of integration provides fine control over neuronal responses
Temporary effects
Once released into a synapse, neurotransmitters are quickly removed:
Some are deactivated by enzymes
The enzyme acetylcholinesterase removes acetylcholine from synapses
Others are pumped back into the synaptic knob of the presynaptic axon
There are dozens of neurotransmitters
Some neurotransmitters produce different effects on different types of cells
Example: acetylcholine
Acts in both the PNS and the CNS
Released at every neuromuscular junction
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