1. NS cells and neurotransmitters at synapses
KA 3: The cells of the nervous system and neurotransmitters at synapses
(a) Structure and function of neurones
(e) Mode of action of recreational drugs
NS cells and neurotransmitters at synapses
(d) Neurotransmitters, mood and behaviour
(b) Neurotransmitters at synapses
(c) Function of converging, diverging and reverberating pathways

2. By the end of this section you will be able to …..
Describe the role of neurotransmitters
State 3 structures with which neurones connect with and where this occurs
Describe the process of chemical transmission at a synapse using the following key words: vesicles, synaptic cleft, receptors, impulse, diffuse, nerve endings
State how neurotransmitters can be removed
Explain the need to remove these neurotransmitters
State what is meant by excitatory and inhibitory signals
Describe why weak stimuli are filtered out
Describe what is meant by summation
Chemical transmission at the synapse by neurotransmitters to include vesicles, synaptic cleft and receptors. The need for removal of neurotransmitters by enzymes or reuptake to prevent continuous stimulation of post-synaptic neurones.
. Receptors determine whether the signal is excitatory or inhibitory. Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters. Summation of a series of weak stimuli can trigger enough neurotransmitter to fire an impulse.
Neurotransmitters relay messages from nerve to nerve within and out with the brain. Neurones connect with other neurones, muscle fibres and endocrine at a synaptic cleft. Neurotransmitters are stored in vesicles and released into the cleft on arrival of an impulse. They diffuse across the cleft and bind to receptors on nerve endings.

3. Animation

4. Synapse

5. Chemical transmission at a synapse

6. Chemical transmission at a synapse

7. Chemical transmission at a synapse

8. Key terms
Synapse:
Functional region between axon ending and dendrite of the next neurone
Synaptic cleft:
Narrow space between the 2 neurones
Neurotransmitters:
Chemical which relays messages across synaptic clefts from neurone to neurone, within and outwith the brain
Neurones can also connect with muscle fibres and endocrine glands at a synaptic cleft

9. Chemical transmission at a synapse
Motor neurone
Vesicle containing neurotransmitter
Release of neurotransmitter
Synaptic cleft
Receptor
Muscle fibre
Neurotransmitters are stored in _______ and released into the cleft on arrival of an _______ . They _______ across the cleft and bind to _______ on nerve endings or muscle fibres.
vesicles
impulse
diffuse
receptors

10. The need for removal of transmitter after transmission of impulse
Neurotransmitters do not stay attached to receptors nor do they remain in the cleft.
They are either removed by enzymes or are taken back up into the pre-synaptic neurone.
This is important because it prevents continuous stimulation of post-synaptic neurones. *

11. Excitatory and Inhibitory Signals
Receptors determine whether the signal is excitatory (stimulated) or inhibitory (stopping effect).
When acetylcholine is released into this cleft and attaches to receptors on cardiac muscle fibres, an inhibitory effect is produced. The rate and strength of contraction is reduced (slows down heart beat)
When acetylcholine is released into this cleft and attaches to receptors on skeletal muscle fibres, an excitatory effect is produced and the muscle contracts.
Motor neurone
Motor neurone
Skeletal muscle fibre
Cardiac muscle fibre

12. Filtering out Weak Stimuli
A nerve impulse is only transmitted across a synapse and on through the post-synaptic neurone, IF it first brings about the release of a certain minimum number of neurotransmitter molecules.
This certain number is needed to affect the receptors on the membrane of the post-synaptic neurone.
Weak stimuli fail to do this as an insufficient number of neurotransmitters have been secreted and are therefore ‘filtered out’ by the synapse.

13. Summation
One pre-synaptic neurone is not secreting enough neurotransmitter (it is too weak)
BUT…
With two weak signals, there is enough neurotransmitter binding to receptors to trigger an impulse in the post-synaptic neurone

14. Summation
Summation is the term used to describe the many weak stimuli that come together to trigger enough neurotransmitter to fire an impulse into the post synaptic neurone.