Lesson starter – arrange these boxes in the correct order to show how an action potential crosses the synaptic cleft If threshold is reached then action potential is initiated   neurotransmitter is broken down by specific enzymes in the synaptic cleft Neurotransmitter binds to neuroreceptor on postsynaptic membrane Neurotransmitters are released into synaptic cleft and diffuse to postsynaptic terminal Influx of Ca2+   synaptic vesicles fuse with membrane (exocytosis) action potential reaches the presynaptic terminal Causes Na+ channels to open, and Na+ flows into postsynaptic membrane Voltage-gated Ca2+ channels open   

Solution action potential reaches the presynaptic terminal i voltage-gated Ca2+ channels open  influx of Ca2+ synaptic vesicles fuse with membrane (exocytosis)  neurotransmitters are released into synaptic cleft and diffuse to postsynaptic terminal neurotransmitter binds to neuroreceptor on postsynaptic membrane causes Na+ channels to open, and Na+ flows into postsynaptic membrane if threshold is reached then action potential is initiated neurotransmitter is broken down by specific enzymes in the synaptic cleft.

Context and exemplification Assessable learning outcomes In receptors, the energy of a stimulus is transferred into energy in an action potential in a neurone. Transmission between neurones takes place at synapses. Candidates should be able to: (a) outline the roles of sensory receptors in mammals in converting different forms of energy into nerve impulses; (b) describe, with the aid of diagrams, the structure and functions of sensory and motor neurones; (c) describe and explain how the resting potential is established and maintained; (d) describe and explain how an action potential is generated; (e) describe and explain how an action potential is transmitted in a myelinated neurone, with reference to the roles of voltage-gated sodium ion and potassium ion channels; (f) interpret graphs of the voltage changes taking place during the generation and transmission of an action potential; (g) outline the significance of the frequency of impulse transmission; (h) compare and contrast the structure and function of myelinated and non-myelinated neurones; (i) describe, with the aid of diagrams, the structure of a cholinergic synapse; (j) outline the role of neurotransmitters in the transmission of action potentials; (k) outline the roles of synapses in the nervous system.

Signals and Messages (g) outline the significance of the frequency of impulse transmission; (h) compare and contrast the structure and function of myelinated and non-myelinated neurones; A2 Biology Miss Tagore

Action Potentials and Cell Signalling The “all or nothing” response: Once an AP starts, it will continue along the axon APs do not vary in size or intensity Once an AP gets to the presynaptic knob, neurotransmitter is released into the synaptic cleft. The signal is sent to the postsynaptic neurone via the neurotransmitter acetylcholine.

Describe to the person next to you what is happening in this animation Describe to the person next to you what is happening in this animation. Use correct terminology!

Roles of synapses in the nervous system The role of the synapse is to connect two neurons together so that a signal can be passed from one to the other. Other important functions that can be performed are: Convergence Divergence Filtering unwanted signals Summation Acclimatisation Creation of specific pathways in the brain

Convergence Presynaptic neurones converge to one postsynaptic neurone. This allows signals from different parts of the NS to create the same response. E.g. different stimuli warning us of danger

Divergence One presynaptic neurone diverging to several different pathways in the NS. Useful in the reflex arc – one postsynaptic neurone initiates a response while the other informs the brain.

Your fingers are a common example of divergence in action.

Filtering unwanted signals If low level stimuli create an action potential, it is unlikely to pass across the synapse because several vesicles of Ach must be released to create an AP on the postsynaptic membrane.

Summation Low level signals can be amplified by summation. If the stimulus is persistent, it will generate several successive APs in the presynaptic neurone. Results in many ACh vesicles being released over a short period of time. Enables the postsynaptic generator potentials to combine together to produce an AP. Summation can also occur when several presynaptic neurones each release small numbers of vesicles into the one synapse.

Spatial summation Temporal summation

Acclimatisation Repeated stimulation at a synapse may result in it running out of vesicles containing acetylcholine - synapse fatigue. Synapse no longer responds to stimulus Helps avoid overstimulation of an effector (could cause damage).

Brain pathways Pathways created by synapses enable the nervous system to convey lots of different messages. We can perceive lots of different stimuli from different sense organs because our brains understand where the signals are coming from as specific receptors always connect with specific regions of the brain.

Frequency of transmission Detection of a signal informs the brain that an environmental stimulus has been “picked up” by the nervous system. This could be information about light, pressure, smell etc. Information about the intensity of these signals requires more effort… More generator potentials have to be produced to produce more action potentials in sensory neurones. When these arrive at a synapse, more vesicles containing ACh will diffuse across the synapse, thus producing a stronger signal. A higher frequencey of signals = a more intense stimulus

Myelinated and non-myelinated neurones Lower conduction of Aps (2-20m/s) Neurones tend to be shorter Carry signals over shorter distance Used in coordination of breathing and digestion (transmission speed not as important for these functions) Transmit APs much faster (100-120m/s) therefore signal reaches target faster Results in rapid response to stimulus Carry signals from receptors >CNS>effectors Carry signals over long distances

Myelinated and non-myelinated neurones Myelin sheath created by separate cells called Schwaan cells Wrap around neurone - consists of layers of membrane and thin cytoplasm from Schwaan cell Nodes of Ranvier 1-3mm along neurones Action poteitnal moves by saltatory conduction. Non-myelinated neurones still associated with schawaan cells but several neurones are loosely wrapped in one cell. Action potential moves along in a wave.

Your homework was… To read pages 16-20 Answer the textbook questions on those pages. Did you do it??

Your homework is… For Monday 30th September Complete learning package 16. Textbook pages will be scanned into the student folder and can be found at: S:\Science\Miss Tagore\Y13 Biology

Plenary Work through structured questions 1 and 2 in the A5 exam booklet you have been given. Use all your resources to help you answer these questions fully and to the best of your ability. Answers in the shared area.