1. Today you will:
Define threshold and use it to explain the all-or-none response
Describe a synapse
Explain how chemical transmitters aid in transmission of information
Describe the function of acetylcholine and noradrenalin

2. Threshold Levels and All-or-None Response
The threshold level is the minimum level of a
stimulus required to produce a response. A lower
level has no effect.
Once a threshold is reached, all neurons will fire
(all-or-none response). An increase in stimulus does
not change the reaction.
Two things happen:
The frequency of the stimulus increases with increased intensity of the
stimulus. The brain senses the increased frequency and responds
Appropriately.
Not all neurons have the same threshold level. If the stimulus is not very
intense, only a few neurons will fire. However, if the stimulus is intense.
then many more neurons will fire. The brain senses this and responds
appropriately.

3. Transmission of Nerve Impulses
There is no direct contact between nerves.
Synapse – a gap between the end of the axon of one
neuron and dendrite or cell body of the next neuron.
Chemical transmitters allow contact across the synaptic
gap by simple diffusion. The space between neurons
is small (20nm), but diffusion still takes a relatively long
time. If many neurons are involved the nervous response
is significantly slower.
A single neuron may branch many times at its end;
a synapse rarely involves just two neurons.

4. One-way Transmission
Impulses travel in one direction only across a synaptic gap.
Pre-Synaptic Neuron Post-Synaptic Neuron

5. Steps in the Transmission
Synaptic Vesicles
Release transmitter substances into the synaptic gap following
stimulation by an impulse
Transmitter Substances
Diffuse across the synaptic gap
Attach to receptor sites on postsynaptic neuron, depolarizing it and
therefore initiating an action potential

6. Transmitter Substances
Neurotransmitters can be either excitatory or inhibitory
Excitatory – work by opening sodium ions channels on the
postsynaptic neuron, causing an action potential
Inhibitory – work by opening more potassium ion channels which
make the neuron even more negative; it is said to be hyperpolarized and can’t
transmit impulses.
Some neurons can’t transmit enough neurotransmitter to create an action
potential in the next neuron. However, if more than one neuron fires at
the same time, enough chemical may be present to cause the depolarization
in the postsynaptic neuron. This is called summation.

7. Acetylcholine Noradrenalin
Released at synapse of the CNS, ANS, and nerve and skeletal muscle
Shortly after it is released it is destroyed by cholinesterase
(acetylcholinesterase), an enzyme. It is released into the synaptic gap
To prevent continuous stimulation of the dendrite. After Ach is destroyed,
the sodium gates close and the neuron begins recovery. If
cholinesterase wasn’t released, it would cause the dendrites of the
adjacent neuron to be continually stimulated and would lead to
uncontrollable tremors, spasms, and perhaps death.
Noradrenalin
Released by the ANS

8. Read questions 418-425 in your text book.
Complete:
#5-7 on page 420
The case study on page 424
#1-8 on page 425