Key Area 3(b) Neurotransmitters at synapses 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 the 2 types of signals State the structure which determines the type of signal Describe what is meant by weak stimuli State the structure which filters out weak stimuli Describe what is meant by summation

Synapse

Neurotransmitters at synapses A synapse is the region between the presynaptic neurone and the postsynaptic neurone. Presynaptic neurone Postsynaptic neurone Highlight box around synapse Direction of impulse

Synaptic Cleft The cell membranes of these neurones are very close together and separated by only a small gap known as the synaptic cleft. Neurones can also connect to muscle fibres and endocrine glands via similar synaptic clefts. Add photo from text book pg 253 fig 17.7 before the note

Neurotransmitters Neurotransmitters are chemicals which relay messages from nerve to nerve within and out with the brain. https://www.youtube.com/watch?v=p5zFgT4aofA (1.34) https://www.youtube.com/watch?v=WhowH0kb7n0 https://www.youtube.com/watch?v=xF2UFV6EKt0 Chemicals!!!!!!

Vesicles containing neurotransmitters Neurotransmitters are stored in vesicles in the presynaptic vesicle. Vesicles containing neurotransmitters Presynaptic neurone mitochondria Neurotransmitters Foldable instead of plain diagram, labels like cell division Postsynaptic neurone

Direction of impulse Neurotransmitter binding with receptor 2. The vesicles release neurotransmitters into the synaptic cleft on arrival of an impulse. They diffuse across the cleft and bind to protein receptors on the post synaptic neurone. Direction of impulse Neurotransmitters diffusing across the synaptic cleft Neurotransmitter binding with receptor

3. Once an impulse is initiated in the post synaptic neurone the neurotransmitters must be rapidly removed from the synaptic cleft. This prevents continuous stimulation of the postsynaptic neurone Neurotransmitters are removed by: Enzyme degradation – the neurotransmitter is broken down by an enzyme into non-active products. Re-uptake – the non active products are reabsorbed bu the presynaptic neurone and resynthesised into active neurotransmitters for use later.

Excitatory or Inhibiting signals The type of receptor present on the membrane of the postsynaptic neurone determine whether the signal is excitatory (stimulated) or inhibiting (stopping effect). This means that the same neurotransmitter can have more than one effect.

Acetylcholine released into the cleft between a motor neuron and a skeletal muscle fibre binds to receptors that have an excitatory effect on the muscle fibre and make it contract. Acetylcholine released into the cleft between a motor neuron and a cardiac muscle fibre binds with receptors that have an inhibitory effect. This reduces the rate and strength of contraction of cardiac muscle. Motor neurone Motor neurone Skeletal muscle fibre Cardiac muscle fibre

Filtering out weak signals The machines in a factory constantly make noise. Workers learn to filter out this noise and only respond to certain noises. Your nervous system also filters out ‘weak/background noise’….

Filtering out weak signals An impulse is only transmitted across a synapse and on through the post-synaptic neurone, IF it first brings about the release of a certain number of neurotransmitter molecules in the presynaptic neurone. A certain number of neurotransmitter molecules are needed to affect a sufficient number of receptors on the membrane of the post-synaptic neurone to transmit the impulse.

Filtering out weak signals Weak stimuli fail to generate an impulse as an insufficient number of neurotransmitters molecules have been secreted. Therefore, the synapse is able to ‘filter out’ or ignore weak stimuli.

Summation

Summation Each postsynaptic neurone is likely to be contact with many presynaptic neurones. Therefore the postsynaptic neurone may receive neurotransmitter molecules from many different neurones simultaneously or in rapid succession. Overall this may release enough neurotransmitter to generate an impulse. This is called summation - the cumulative effect of a series of weak stimuli working together to generate enough neurotransmitter to an fire impulse.