What do cells need?

1. What do cells need? External Environment Internal environment • -needs to be highly regulated • This is done by receiving information in the form of signals and responding to these signals • -Homeostasis is the process of maintaining a relatively stable internal environment Extracellular( tissue fluid and blood plasma) Factors are constantly changing -temperature -light intensity -gases -nutrients -water -challenges from other organisms cell cell cell Internal environment cell cell

2. Detecting and responding to signals When a change occurs homeostasis mechanism acts to restore the the normal state Receptor Stimulus Transmission by nerves -physical, chemical, internal or external Receive the signals Communication systems -nervous and endocrine systems Feedback-the stimulus is changed because of the response Transmission by nerves or hormones The response Effectors

3. Types of signals -physical stimuli -light -heat -touch -pheromones -hormones -nutrient molecules -neurotransmitters -electrical signals

4. What are communication systems? • Signals are transmitted to target cells via: • -Nervous system • Endocrine system

5. Neuron structure • Neurons are the basic unit of the nervous system. • They join up together to form ‘nerves’. • Neurons contain Nerve body: containing nucleusAxon: thin wire like structure that carries signalsMyelin sheath: Fatty covering over the axon increasing signal speed and reducing risk of signal lossDendrites & Axon terminals: Points of connection to other neurons

6. Neurons receive, conduct and transmit electrical signals.

8. 3 main types of nerve cells sensory neurone inter neurone motor neurone • 90% of our neurons are interneurons

9. Receptor types • Chemoreceptor Detect chemical stimulus: taste, smell, co2 levels, blood glucose levels • Mechanoreceptors Detect changes in pressure, touch, balance • Photoreceptors Detect changes in light • Thermoreceptors Detect changes in temperature • Pain receptors Free nerve endings in the skin

10. Signal Transmission • Messages travel within the neuron as an electrical action potential.

11. How Do Neurons Operate? • Neuron at Rest  Resting Potential • Occurs when the neuron is at rest. • A condition where the outside of the membrane is positively(+) charged compared to the inside which is negatively(-)charged. • Neuron is said to be polarized. • Neuron has a voltage difference of -70 mV

12. The action potential is an electrical event occurring when a stimulus of sufficient intensity is applied to a neuron

13. Action Potential Action potentials occur in two stages: • Depolarisation Sodium moves inside the cell causing the inside of the membrane more positive than the outside. • RepolarisationPotassium ions flow out of the cell, restoring the resting potential net charges.

15. Resting Potential • A nerve cell has an electrical potential, or voltage, across its cell membrane of approximately 70 millivolts (mV). • The specific ions that are responsible for the charge difference are Na,K negatively-charged protein molecules and Cl.

16. Threshold • When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold.

18. Refractory Period • Brief period of time between the triggering of an impulse and when it is available for another. • NO NEW action potentials can be created during this time.

19. Conduction Velocity: • impulses typically travel along neurons at a speed of anywhere from 1 to 120 meters per second • the speed of conduction can be influenced by: • the diameter of a fiber • the presence or absence of myelin • Neurons with myelin (or myelinated neurons) conduct impulses much faster than those without myelin.

22. All or None Response • The strength of the response is not dependent upon the stimulus. • An axon cannot send a mild or strong response. It either responds or does not!!!

23. Action potential Extension

24. Action potential(Extension) • All cells have a membrane potential because on the inside of the membrane there are more potassium ions, and on the outside there are more sodium ions.  • The potential is maintained by a biological mechanism called the sodium potassium pump. • The membrane potential difference is about 70 mV, with the outside being regarded as being at 0 and the inside being at -70 mV.  The negative ions are carried on large organic ions that cannot cross the cell membrane.  When a cell membrane on a nerve cell is stimulated, it suddenly becomes permeable to sodium ions which diffuse through, attracted by the negative charge.   • The potential rises initially to 0 millivolts (depolarisation)  • and then to +30 mV (reverse polarisation). • Then the membrane becomes impermeable to sodium ions and they are trapped within the nerve cell. • Potassium ions diffuse out of the membrane which restores the potential (repolarisation). • The whole process takes about 2 ms. • Then the potassium ions are pumped out, a process taking about 50 ms.

25. Pre and post synaptic neuron The neuron beforea synapse is called the pre synaptic neuron. The neuron after a synapse is called the post synaptic neuron.

26. What happens at the synapse? The synapse is the connection between nerve cells The synapse consists of: The presynaptic terminal at the end of an axon. This contains tiny vesicles which contain neurotransmitters - the small molecules which carry the nerve impulse from the sending neuron to the receiving neuron. The synaptic cleft - a gap between the two neurons across which the neurotransmitters migrate. The postsynaptic terminal usually in the dendrites of receiving neurons. This contains receiving sites for the neurotransmitters.

27. The synaptic cleft • At this point, the electrical signals stimulate the release of a chemical substances known as a neurotransmitter, which crosses the cleft • The neurotransmitter acts like a little key, and the receptor site like a little lock.  When they meet, they open a passage way for ions, which then change the balance of ions on the outside and the inside of the next neuron.  And the whole process starts all over again.

28. The electrical message travelling along a nerve has to cross the synapse as a chemical message

29. Neurohormone • A neurohormone is any hormone produced and released by neurons • Examples of neurohormones are TRH (Thyrotropin releasing hormone)

30. Neural circuits • The inter-connections between neurons are called neural circuits. Neural circuits are of 4 types: • diverging, • converging, • reverberating and • parallel

31. Diverging circuits • In diverging circuits, an incoming fibre triggers responses in ever-increasing numbers of neurons along in the circuit. Thus, diverging circuits are often amplifying circuits.

32. converging circuit • In a converging circuit, the pool receives inputs from several presynaptic neurons, and the circuit has a funneling, or concentrating, effect. Incoming stimuli may converge from one area. 

33. Reverberating • In reverberating, or oscillating, circuits, the incoming signal travels through a chain of neurons, each of which makes collateral synapses with neurons in a previous part of the pathway.  • Impulses are sent through circuit again and again by collateral synapses.

34. Parallel /after-discharge circuits • Impulses reach the output cell at different times, creating a burst of impulses called an after-discharge that lasts 15 ms or more after the initial input has ended. This type of circuit has no positive feedback, and once all the neurons have fired, circuit activity ends. Parallel after-discharge circuits may be involved in complex exacting types of mental processing.

35. Out of the four types of neural circuits diverging ,converging reverberating and parallel after-discharge which is likely to fire the longest after a stimulus ceases?

36. Types of neurotransmitters: • 1- Excitatory - neurotransmitters that make membrane potential less negative &, therefore, tend to 'excite' or stimulate the postsynaptic membrane • 2 - Inhibitory - neurotransmitters that make membrane potential more negative

37. Convergent circuit with excitatory and inhibitory connections • Inputs from receptors summate to determine output of circuit