Communication and Homeostasis

1. Communication and Homeostasis

2. Basics • Organisms have to respond to both internal and external STIMULI that put the organism under STRESS • Organisms have a far greater chance of survival if they respond to the external and internal environment to ensure conditions are always OPTMAL • External Stimuli include – temperature, pressure, sound, sunlight and water concentration • Internal Stimuli – Oxygen and Carbon dioxide concentration, water, temperature, pH, pressure.

3. Receptors and effectors • Receptors register the change in the external or internal environment • These changes are co-ordinated if needs be • Then an effector is tasked with redressing the balance and setting things right • Example receptors – eyes, chemoreceptors • Example Effectors – muscles, pancreas

4. Two types of responses • Quick ones • Nerves and muscle twitches • Slow ones • Enzymes, steroids and hormones • One needs to be made so takes longer to get to work • Communication between cells is achieved by cell signalling – how does this work can you remember?

5. Neurones - Nerves • Quickly on your white boards • Draw out a simple nerve pathway (like from GCSE) • Name all three types of Neurone in your body • Can you label the main parts of a nerve cell?

6. Neurone Types • There are three basic types of neurones and you need to know them all and their basic structure: • Sensory Neurones – Carry information from your sense organs (eyes, ears, nose, tongue, skin) to your CNS. • Motor Neurones – Carry information from your CNS to your muscles (both conscious and unconscious) and other effector cells. • Relay Neurones – sit in your CNS and carry impulses to your motor neurones.

7. What they look like • Important words • Dendrite – collect information from neurones or sense organs • Axon – pass impulse along the neurone • Axon Terminal – leads to the synapse where neurotransmitters diffuse to depolarise the next neurone • Myelin Sheath – insulates the neurone (a bit like wire covers)

8. Pictures – can you label the bits?

9. Nerve Pathway A Cat Eyes see cat • Simple • Stimulus leads to the • Receptors • Sensory Neurone • CNS – can be spinal cord or brain • Motor Neurone • Effector • Can you Produce an example? Sensory Neurone CNSBrain Motor Neurone Kick Cat

10. Sensory Receptors • You got loads of these • The work as transducers – convert one form of energy to an electrical impulse • It all works when there is a change in potential or charge in the cell • Little bit more revision – how does electricity pass round a circuit? • What do you need to make bulbs light? • On your white boards

11. Questions • What is the function of: Motor neurones, sensory neurones, relay neurones? • Give two differences between a sensory and motor neurone • Why are sensory receptors called transducers? • Describe the pathway of nervous communication from stimulus to response. • Explain how a generator potential is produced

12. Resting potential • In an unstimulated neurone there is a difference in potential across the membrane • It is about -70mV the inside of the neurone is 65mV lower than the outside of the neurone • So inside the neurone is negative and outside positive – clear? • They found this looking at squid neurones

13. Generator and Action Potential • Okay so we now have a resting potential • But what happens next? • When a stimulus is detected there is a change in permeability of the membrane and the potential can change. • This is called a generator potential • If this gets big enough you have an action potential and the neurone fires

15. Maintaining the potential • Simple case of moving ions – any guesses as to what ions are used? • Correct – Sodium and Potassium • Sodium is high outside and low inside • The opposite is true for Potassium • How is it maintained as positive you ask – three sodiums for two potassiums (all due to the masses) • And potassium is sneaky and diffuses out quicker than sodium does (there are more channels open)

16. More Detail • First a question – what can’t pass across the cell membrane? • Charged particles, what are sodium ions? • Charged so once out they ain’t getting back in • So the Ion exchange pump moves the ions out they want out. • This maintains the difference as sodium can’t get back in. • The pump uses ATP to maintain the resting potential • What kind of gradient is this? • An electrochemical one.

18. Action Potential - Notes • Stimulus – Causes channels to open and for Sodium to rush into the cell down the electrochemical gradient – this makes the cell less negative • Depolarisation – If the potential difference reaches the threshold voltage-gated channels open and more sodium floods in (positive feedback) • Repolarisation – Voltage gated channels open and potassium leaves the cell (down the electrochemical gradient) this returns the membrane to its resting potential (negative feedback) • Hyperpolarisation – more potassium leaves the cell due to the channels remaining open. • Resting potential sodium potassium pump resumes work and ion levels are maintained • This happens in 1 millisecond

19. Detailed Action potential

20. Waves of depolarisation • All this depolarisation acts at one point on the membrane. • But during these events some sodium diffuses sideways which causes and action potential to move along the axon

21. Frequency of impulses • Once the neurone has completed the refractory period and all the channels are reset the neurone can fire again. • The action potential is always the same no matter how big the stimulus is • If the threshold is not reached it will not fire – it is an ALL OR NOTHING system • Bigger stimuli will just cause the neurone to fire more frequently but always with the same action potential if the brain receives a lot of stimulation from one source it will act accordingly • For example loud music – you will cover your ears or move away, remember this system takes milliseconds.

22. Speed of impulse • Did you know you think at the speed of light? • Three things speed up your nerve impulses • Myelin Sheaths – the nodes of Ranvier and salutatory conduction where depolarisation only happens at the nodes • Axon Diameter – the bigger the better like wires • Temperature – up to 40oC your speed increases then proteins start denaturing (shocker)

23. Describing the generation of local currents. Look at the diagram you have been given and in your own words, describe how a local current is created.

24. Describe what is different about the myelinated neurone. What effect does this have on the currents? What effect does this have on the speed of transmission? Describe saltatory conduction in your own words.

25. Cell signalling • Specific • Targeted • Controlled

26. Synapses • A bit of specialised cell signalling here. • That is why I showed the previous slide. • Synapses work by signalling the next nerve cell across the SYNAPTIC CLEFT • This is made possible due to the neurotransmitters in the AXON TERMINAL • But you only need to learn about one neurotransmitter – ACETYLCHOLINE (Ach) in CHOLINERGIC SYNAPSES

27. What happens • STEP 1 • Arrival of an action potential – when an action potential makes it to the PRESYNAPTIC NEURONE (one before the synapse) the potential causes VOLTAGE GATED CALCIUM channels open. • CALCIUM ions diffuse into the SYNAPTIC KNOB by diffusion

28. STEP 2 • The influx of CALCIUM IONs causes the vesicles containing ACh to move towards the PRESYNAPTIC MEMBRANE. • They then fuse with the membrane and the ACh is ejected. • What is this process called? • EXOCYTOSIS

29. Step 3 • ACh diffuses across the synaptic cleft and bonds to CHOLINERGIC RECEPTORS on the POSTSYNAPTIC MEMBRANE • This causes the sodium channels in the Postsynaptic membrane to open. • This causes a DEPOLARISATION and an ACTION POTENTIAL if the threshold is reached (important this)

30. Housekeeping • In the presynaptic neurone active transport pumps the calcium back out of the neurone after the ACh is released. • ACH is broken down in the synaptic cleft by ACETYLCHOLINESTERASE (AChE) the products of which are reabsorbed to make more Acetylcholine • What kind of molecule is ACH?

32. Specialist Synapses • Divergence and Convergence • Divergence – when one neurone connects to many neurones – to allow information to go to many areas • Convergence - when many neurones connect to one neurone – to allow for signals to be amplified

33. Summation • This is when we need more ACH to stimulate the neurone • Spatial Summation – when convergence leads to more than one neurone releasing neurotransmitters to illicit an action potential in the postsynaptic neurone. • This is because each presynaptic neurone releases a small amount of ACH and when they all release there is a big enough influx of sodium to make an action potential. • This allows many signals to be co-ordinated into a single response.

34. Temporal Summation • This is when there are many impulses from one or more presynaptic neurones. • This makes an action potential more likely as there is more and more ACH

35. Inhibition • Anything that blocks either the release of or reception of ACH will cause inhibition of the passage of impulses • Examples • Nicotine – mimics ACH and stimulates neurones – this causes addiction – these drugs are AGONISTS • Curare blocks the receptors in muscle cells and causes paralysis • Nerve gases inhibit AChE which stops the neurone turning of and causes a loss of control • Opiods – block the Calcium channels and stop ACH being released – so act as depressants.

36. Questions • Name three types of cells that have receptors for neurotransmitters • Describe the movement of Calcium ions in a Cholinergic Synapse both when the action potential arrives and after • Explain the purpose of synaptic divergence and convergence • Explain why impulses at a synapse are unidirectional • Try the questions on page 26 as well

37. Hormones • What do you know so far • Think of names of hormones • Where they come from • What they do • 5 minutes

38. Pathways • Similar as Neurone pathways • Stimulus – receptor – hormone – effectors – response • Example • Raised blood glucose • Receptors on Pancreas cells register the change in glucose levels • Hormone secreted from the beta cells in the Islets of Langerhans • Target cells in the liver and muscles detect glucose and convert into Glycogen (Glycogenesis) • Blood Glucose level falls as glucose is removed from the blood

39. First messenger – second messenger • Hormones are the chemical signal – it is what happens in the cells that is the real goal • When the hormone hits the target cell it leads to an enzyme in the cell creating a internal signalling molecule • The internal signal changes the action of the cell this is the second messenger – they often start a cascade of reactions within the cell

40. Adrenaline - example • First where is it made: • Adrenal glands which lie on the top of your kidneys • Two parts the Cortex and the Medulla need to concern you • Both are involved in stress responses or the so called ‘fight or flight’ response

41. What makes what • The cortex creates STEROID HORMONES • These are CORTISOL and ALDOSTERONE – stress hormones they cover short and long term responses: • Stimulates proteins and fat breakdown • Increases blood volume and pressure by increasing sodium uptake in the kidneys • Suppresses the immune system • MEDULLA makes CATECHOLAMINE HORMONES which are modified amino acids • NORADRENALINE and ADRENALINE these are again • They increase blood flow to the brain and muscles • Increase heart and breathing rate and stimulate glucose release from Glycogen • What are the possible consequences of too much Adrenaline in your system?

42. Adrenalin Pathway • ADRENALIN is a hormone that is released in response to: • Low Blood Sugar Levels • Stress – emotional and physical • It works by making your body ready for increased effort by improving blood flow and available Glucose for Respiration (GLYCOGENOLYSIS) through the breakdown of Glycogen • As in the first diagram Adrenaline is the first messenger this activates ADENYL CYCLASE • This uses ATP to activate CYCLIC AMP this starts the cascade

43. Pancreas • Involved in Glucose control • The important bits are the ISLETS OF LANGERHANS (ENDOCRINE TISSUE – HORMONE PRODUCING) • There are two types of Cells • Alpha Cells - Glucagon • Beta Cells – Insulin • What receptors are involved in the control of blood glucose

45. Now the Detail • Homeostasis • The maintenance of a constant internal environment • Such examples: • Temperature • Glucose • Water Levels • CO2 Levels • Blood Flow • Oxygen Levels What happens to your body if your Temperature gets too hot and too cold?

46. Two types of control • Positive and Negative Feedback • How is change in your body detected? • What is that information carried by? • What is the first messenger for cells released by glands?

47. Negative Feedback This leads to an oscillating picture of control – trying to maintain the Goldilocks Zone • Simples this is • Receptors monitor a particular level (temperature, glucose) • A change leads to a cascade response and it the response returns the level to normal Normal Level Change in Level Change Detected Communication Effectors Respond Level Back to Normal

48. Positive Feedback - Amplification • Where negative is about control positive is about increasing a response • In this a change is received and instead of dampening the response we get an increase • For example at the site of a cut – more platelets needed and clotting factor • Oxytocin during birth • Sex hormones do both – can you remember the way these are released.