Neurons and Excitable Membranes

1. what are neurons: - nerve cells that transmit electrical impulses rapidly around the body so that the organisms can respond to changes in it internal and external environment 2. describe a sensory neuron: - transmit impulses from a sensory receptor cell to a relay, motor neuron or the brain - have one dendron and one axon 3. describe a relay neuron: - transmit impulses between neurons - have many short axons and dendrons 4. describe a motor neuron: - transmit impulses from relay/sensory neuron to effector - have one long axon and many short dendrites 5. what is the cell body or soma: - nucleus surrounded by cytoplasm - includes endoplasmic reticulum and mitochondria which are involved in neurotransmitter production 6. what are the dendrons: - short extensions from cell body - divide into dendrites - responsible for transmitting electrical impulses towards cell body 7. what are axons: - singular, elongated nerve fibres - transmit impulses away from cell body - cylindrical in shape consisting of narrow region of cytoplasm surrounded by plasma membrane 8. how does the electrical info flow in a neuron: dendrites -> soma -> relay -> motor -> target cell 9. what is the myelin sheath: - covers axon of some neurons - made of layers of plasma membrane - layers produced by Schwann cells - cells grow around axon and each time they do, a double layer of phospholipid bilayer is laid down - acts as insulating layer - allows neurons to conduct impulse faster 10. what are nodes of ranvier: - gap between each Schwann cell - therefore impulse jumps from one node to the next - allows transmission to be faster 1/6 11. what is the axon hillock: - part of the neuron, between the cell body and the axon. - acts as an administrator, sums up total signals received - if the sum exceeds the limiting threshold, the action potential is triggered resulting in transmission of the signal through the axon away from the soma 12. what is the resting potential of neurons: -65mv - difference in charge between inside and outside of axon 13. how is the resting potential maintained: - 3 sodium ions actively move out of axon while 2 potassium ions actively move in axon through sodium-potassium ATPase pump (energy received from the hydrolysis of ATP to ADP and phosphate) - to balance charge, sodium ions diffuse in (through sodium gated channels) and potassium ions diffuse out (through potassium gated channels) down the electrochemical gradient - however most sodium ion gated channels are closed so sodium ions can't diffuse in but potassium ions can - this results in more positive charge outside than inside causing resting potential 14. how does charge spread across the membrane: - part of membrane is depolarised (change in potential difference from negative to positive) - charge flows away from that point along the membrane - rate of propagation along the membrane is dependent upon how much resistance to flow there is for the charge 15. explain resistance to electrical charge flow: - small diameter axons have high resistance therefore slower conduction rates - large diameter axons have low resistance therefore faster conduction rates 16. how is the rate of propagation speeded up: - by myelination (Schwann cells in PNS) (oligodendrocytes in CNS) - depolarisation can only occur at nodes of ranvier therefore action potential jumps from one node to next - this process is called saltatory conduction - every time channels open and ions move to depolarise the axon, it takes time, so reducing the number of places where this happens speeds up the propagation 17. what are the functions of dendrites: - receive info via dendritic tree - each input changes membrane potential of the dendrite and the electrical charge difference is conveyed down the dendrite to the soma 18. what are the functions of the soma: - receives and aggregates the impulses from the dendritic tree and passes it on to the axon 19. describe the axoplasmic transport of compounds to terminal: - this is the transport of neurotransmitter made in the soma to the axon terminal where it communicates with target cell 2/6 20. describe a multipolar neuron: - one axon and many dendrites 21. describe a bipolar neuron: - Axon and single dendrite on opposite ends of the soma 22. describe a pseudounipolar neuron: The cell body is off to one side of the axon 23. describe a unipolar neuron: Neurons in which dendritic and axonal processes are continuous and the soma lies off to one side 24. explain the classification of axons: - A: mean that the axon is myelinated - ±, ², ³,donates ´: descending diameters of axon - C: means unmyelinated axon 25. what is a neurotransmitter: - chemical messenger - released by neuron terminal, crosses synaptic cleft, binds to protein receptor on postsynaptic membrane of the target cell types: excitatory, inhibitory 26. explain excitatory neurotransmitters: - result in the depolarisation of postsynaptic neuron - if threshold is reached in postsynaptic membrane, action potential is triggered - sodium ions causes depolarisation - more positive than -65mv - an excitatory postsynaptic potential (EPSP) 3/6 27. explain inhibitory neurotransmitters: - results in hyperpolarization of postsynaptic membrane preventing action potential being triggered - potassium ions cause hyperpolarization - more -ve than -65mv - an inhibitory postsynaptic potential (IPSP) 28. what is a synapse: Gap between neurons 29. What is a presynaptic neuron?: - neuron along which the impulse has arrived 30. what is the synaptic knob: - end of presynaptic neuron containing mitochondria and endoplasmic reticulum to enable it to make neurotransmitter 31. what are synaptic vesicles: - vesicles containing neurotransmitter - vesicles fuse with presynaptic membrane and releases neurotransmitter into synaptic cleft 32. what is the synaptic cleft: - gap separating axon of one neuron to dendrite of the next 33. what are neurotransmitter receptors: - receptors on postsynaptic membrane 34. what is the postsynaptic neuron: - neuron that receives neurotransmitter 35. transmission of impulses across synapse (noradrenaline): - action potential reaches end of presynaptic neuron - depolarisation of presynaptic membrane causes calcium ions gated channels to open - calcium ions diffuse into presynaptic knob - causes synaptic vesicles containing neurotransmitter to fuse with presynaptic membrane - neurotransmitter is released into synaptic cleft by exocytosis - neurotransmitter diffuses across cleft and binds with its specific receptor molecule on the postsynaptic membrane - causes sodium ion channels to open and sodium ions to diffuse into the postsynaptic neuron - this triggers an action potential and the impulse is propagated along the postsynaptic neuron 36. how are synapses switched off (cholinergic synapse): - acetylcholine is hydrolysed by acetylcholinesterase (enzyme) to give choline and ethanoic acid - the products are taken back into the presynaptic knob to be reformed to acetylcholine 37. explain the concept of ions having two driving forces: - conc gradient force and electrical gradient force - sodium will want to move inside cell down conc gradient and because the positive ions are attracted to the negative charge of the inside of the cell - potassium will want to move out down its conc gradient however will also want to 4/6 stay on the inside as it is attracted to the negative charge - to overcome this, there is a specific charge across the membrane at which there is no drive for movement of an ion 38. what is the equilibrium potential of an ion: - when they membrane potential is equivalent to the equilibrium potential for the ion, there is no drive for that ion to move across the membrane 39. how is the equilibrium potential calculated: Nernst equation 40. how is the equilibrium potential equation simplified: - the universal gas constant and faraday constant values are always constant - in humans the temperature is usually constant. therefore RT/zF is equal to 61.5 Eion = 61.5 log ([ion]o/[ion]i) 41. how do we work out the membrane potential of a neuron at any given time: goldman equation RT/F is simplified like the nernst equation to give 61.5 42. what can cause changes to the membrane potential: - changes in [ion]o [ion]i - changes in Pna+, Pk+, other ions - charge spread along membrane - external electrical activity/currents - other cellular events 43. what is an action potential dependent on: - voltage dependent (activated,fast) Na+ channels (INa+ = sodium current) - voltage dependent (activated) K+ channels (IK+ = potassium current) - depolarisation to threshold 44. explain the all or nothing principle: - once electrical impulse reaches threshold it fires. - no matter how large the stimulus is the same sized action potential is always triggered - however the larger the stimulus, the more frequent the action potential 45. explain the action potential process: 1. neuron at resting potential of -70mV. due to the potassium leak current 2.depolarisation happens to the threshold causing the fast sodium channels to open causing sodium ions to diffuse into axon don the electrochemical gradient. this make inside of neuron less negative 5/6 3. this creates an increased permeability for sodium and sodium current. this depolarises the membrane towards the equilibrium potential for sodium. the sodium channels close and the potassium channels open 4. this causes an increase in potassium permeability, creating a potassium current which causes a rapid hyperpolarisation towards the equilibrium potential for potassium. then the potassium channels close 5. there are other potassium channels open which are responsible for the after hyperpolarization (AHP) that close slowly explaining the slow repolarization up to the resting potential 6. returns to resting potential (use graph in notes to further understand) 46. what is the refractory period: - a period of inactivity after a neuron has fired - there are two types: absolute and relative 47. Explain absolute refractory period: - the ion channels are either busy creating a action potential or are in the deactivated state (state where they aren't able to open) 48. explain the relative refractory period: - when membrane is able to generate an action potential however the membrane is hyperpolarized - a stronger stimulus is needed to reach the threshold 49. explain action potential propagation: - charge spreads along axon - charge flows unidirectionally in the direction of the target cells - why?: because it starts at axon hillock so can only flow one way - if the action potential starts in the middle of the neuron it can move both ways however it will be impossible to go backwards as that piece of the membrane will be in refractory period 50. how do axons code for stimulus strength: - stronger stimulus' will be able to start a new action potential during the relative refractory period - weaker stimulus' wont be able to start a new action potential during the refractory period but will have to wait until the membrane potential is back at -65mv - therefore stronger stimulus' will be more frequent than weaker ones - axons code for stimulus strength by frequency of action potentials. 6/6