Neuronal Communication

• Draw a table showing 7 different types of receptor and the stimulus that they detect

  Mechanoreceptor - pressure and movementChemoreceptor - chemicals (volatile - smell, in solution - taste)Thermoreceptor - heatPhotoreceptor - lightOsmoreceptor - water potentialNociceptors - pain
• Define the terms "sensory receptor", "transducer" and "stimulus".

  Sensory receptor - specialised cell which detects a stimulusTransducer - converter of one energy form to anotherStimulus - detectable change in external or internal environment of an organism
• State 3 characteristics of sensory receptors and for each explain why they are important.


  Transducers - stimulus -> electrical impulseSpecific - impulse doesn't say what stimulus is, where it comes from indicates what it isSensitive
• Draw and label a diagram showing the structure of a Pacinian corpuscle

  
• Explain how a Pacinian corpuscle converts mechanical pressure into a nerve impulse

  Pressure acts as stimulus causing membrane of neurone to stretch. This widens Na+ channels allowing Na+ to diffuse into the neurone, depolarising the membrane and causing a generato potential. This, in turn, creates and action potential which passes along the neurone. The viscous gel slowly move out of the way if pressure is sustained allowing the membrane to return to normal; it only detects a change, does not continuously send an impulse.
• Outline the steps in a stimulus-response pathway and identify the role of the sensory, relay, and motor neurones in this pathway

  StimulusReceptorSensory neuroneRelay neurone(s)Motor neuroneEffectorResponse
• Draw and label diagrams of a motor neurone, a relay neurone, and a sensory neurone

  
• Define the terms "dendrite", "dendron", "axon" and "axon terminal".

  Dendrite - short branched extension of neurone which receives impulses from other cells/receptorsDendron - from dendrites to cell bodyAxon - extension of neurone that carries impulses away from cell bodyAxon terminal - ends of axons which make synaptic connections to other cells
• Define the terms "myelinated neurone", "myelin sheath", "Schwann cell" and "node of Ranvier"

  Myelinated neurone - neurone with axon covered in myelin sheathMyelin sheath - membrane rich in lipid which surrounds axon of some neurones speeding up impulse transmissionSchwann cell - cell that spirally wraps around an axon of the peripheral nervous system to form the myelin sheathNode of Ranvier - a gap in the myelin sheath of a neurone between adjacent Schwann cells
• Draw and label a cross section diagram to show the nature of the myelin sheath

  
• Describe and explain the advantage of myelination

  Myelination insulates the axon allowing impulses to be conducted at much higher speeds
• Explain why some neurones are myelinated and others are unmyelinated

  Some neeurones are unmyelinated as distance of transmission is very short.
• Draw a table to compare the structure and function of motor neurones, relay neurones and sensory neurones

  All have cell body, axon, axon terminalsSensory neurone:Has a long dendron, no axon hillock, short axon, cell body on stalk separate from conduction, cell body in dorsal ganglion (just outside CNS), myelinated, axon terminals are in CNS, communication towards CNS, receive connections from receptor cells/stimulus directly to specialised endings, make connections to relay neuronesMotor neurone:No dendron, many short dendrites, has an axon hillock, long axon, cell body at one end of cell - dendrites directly off body, cell body in the CNS, myelinated, dendrites and cell body in CNS, communication away from CNS, receive from relay neurones, connect to effectorsRelay neurone:Has a short dendron, no axon hillock, short axon, cell body in middle of cell, cell body in CNS, no myelination, all parts on cell in CNS, communication through CNS, receive from sensory neurones (or other relay), connections to motor neurones (or other relay)
• Explain the role of sodium ions, potassium ions, organic anions, the sodium/potassium ion pump and potassium ion channels in establishing and maintaining the resting potential

  Resting potential: -70mVMore organic anions inside than outsideNa+/K+ pumps, 3Na+ out for every 2K+ inMembrane permeable to K+ but not Na+
• Explain why a neurone is active when it is said to be resting

  Pump is active transport of ions therefore active processes are taking place even if no impulse is being sent.
• Define the terms "resting potential" and "potential difference"

  Resting potential - the potential difference across the membrane of the axon of a neurone at restPotential difference - the difference of electrical potential between two points. (Difference in charge - simplification for biology)
• Define the terms "voltage-gated channel", "threshold potential", "action potential" and "nerve impulse"

  Voltage-gated channel - ion channels that are activated by changes in the electrical membrane potential near the channel, regulating opening and closing by changing the shape of the protein.Threshold potential - the critical level to which a membrane must be depolarised to initiate an action potentialAction potential - the change in potential difference across the neurone membrane of the axon when stimulatedNerve impulse - a signal transmitted along a nerve fibre. It consists of a wave of electrical depolarisation that reverses the potential difference across the nerve cell membranes
• Define the terms "polarised", "depolarisation", "repolarisation" and "hyperpolarisation".

  Polarised - a membrane is polarised if there is a potential difference across itDepolarisation - a change in potential difference from negative to positive across the membrane of a neuroneRepolarisation - a change in potential difference from positive back to negative across the membrane of a neuroneHyperpolarisation - the inside of the axon becoming more negative (compared to outside) than its normal resting state
• Describe how the initial depolarisation of the membrane occurs and what is required for it to lead to an action potential

  Energy of stimulus triggers some Na+ channels to open allowing Na+ to diffuse into axon making inside less negative compared to outside. If this reaches the threshold potential, the change in charge causes more voltage-gated Na+ ion channels to open leading to an action potential.
• Explain the role of positive feedback in the depolarisation stage of the action potential


  The change in charge caused by Na+ ions diffusing through channels causes more channels to open and more diffusion of Na+ ions ...positive feedback!
• Draw, label and annotate a graph of an action potential occurring over time to show the different stages of an action potential and what happens at each stage.

  
• Define the term "refractory period" and explain its importance for the conduction of the action potential

  Refractory period - a period immediately following stimulation during which a nerve or muscle is unresponsive to further stimulationDuring this time Na+ channels remain closed preventing any movement of Na+ into axon. Important as it prevents propagation of an action potential backwards - makes sure impulses are unidirectional. Also ensures action potentials do not overlap but exist as discrete impulses.
• Describe the role of the sodium/potassium pump after an action potential has occurred

  Pump restores resting potential conditions after an action potential.
• Define the terms "local circuits" and "saltatory conduction"

  Local circuits - the depolarisation of a small region of plasma membrane creates a circuit of Na+ concentration gradients with neighbouring regions, causing depolarisationSaltatory conduction - form of nerve impulse conduction in which the action potential 'jumps' from one node pf Ranvier to the next rather than travelling the entire length of the nerve fibre.
• Describe and explain how an action potential is transmitted along an unmyelinated axon

  A stimulus causes an influx of Na+ triggering an action potential and depolarisation of the area. The local circuit that this establishes triggers opening of voltage-gated Na+ channels further along axon which depolarises that section. This continues along the membrane while behind the action potential the membrane is repolarised by outward movement of K+ etc.
• Describe and explain how an action potential is transmitted along a myelinated axon

  Depolarisation can only occur at nodes of Ranvier so longer local circuits arise and the action potential 'jumps' from one to the next.
• Describe and explain how the transmission of an action potential along a myelinated axon is different to transmission along a non-myelinated one

  Due to the longer local circuits, fewer action potentials occur in a myelinated neurone. Every time ion channels open and diffusion takes place it takes time so reducing the number of places this happens means myelinated neurones conduct more quickly.
• Describe how a graph of an action potential over time (with time on the x-axis) looks different to a graph of an action potential travelling along an axon from left to right (with distance along the axon on the x-axis)

  The two graphs are the reverse of each other. The regions at the start of the axon have completed more of the action potential than furhter along so they are 'ahead' in terms of the 'time' axis of the action potential graph.
• Explain what is meant by the "all-or-nothing" response of neurones.

  The strength of the response to a stimulus is independent of the strength of the stimulus. If the threshold is reached a full action potential is triggered. If threshold is not reached, no action potential occurs.
• Describe what information the frequency of impulse transmission transmits to other neurones

  The higher the frequency of the impulse the stronger the stimulus. Stronger stimulus triggers more action potentials per unit time.
• Explain why the "all-or-nothing" response of neurones means that information must be transmitted by the frequency of impulse transmission

  The 'strength' of an action potential cannot change so the only variable to convey differing strengths of stimulus is the frequency at which action potentials are sent.
• Explain how the nervous system encodes the nature of the information being transmitted (i.e. the type of stimulus - e.g. light or sound).

  The type of stimulus is conveyed by the location it is coming from. All action potentials are the same but those from thermoreceptors convey heat etc.
• Define terms "neurotransmitter", "synapse", "cholinergic synapse", "synaptic knob", "presynaptic membrane", "synaptic cleft", "postsynaptic membrane", "acetylcholine", and "acetylcholinesterase".


  Neurotransmitter - chemical involved in communication across a synapse between adjacent neurones or a neurone and a muscle cellSynapse - the junction (small gap) between 2 neurones or a neurone and an effectorCholinergic synapse - a synapse that uses acetylcholine as its neurotransmitterSynaptic knob/axon terminal - the somewhat enlarged, often club shaped endings by which axons make synaptic contacts with other nerve cells or with effector cellsPresynaptic membrane - the cell surface membrane of an axon terminal that faces the receiving cellSynaptic cleft - the space between neurones across which a nerve impulse is transmitted by the diffusion of neurotransmitterPostsynaptic membrane - the membrane of the post synaptic neurone that receives a signal from the presynaptic cell Acetylcholine - a compound which functions as a neurotransmitter in cholinergic synapsesAcetylcholinesterase - as enzyme that causes rapid hydrolysis of acetylcholine into choline and acetate
• Draw, label and annotate a diagram to show the structures present in a cholinergic synaptic - for each structure explain what it is for.

  
• Describe the structure and function of the sodium channels on the post synaptic membrane.

  Channel and receptor are same protein. Made from 5 polypeptide chains. Complementary binding sites for acetylcholine. Channel can be open or closed (gated). Open when acetylcholine binds so Na+ diffused in leading to depolarisation and an action potential.
• Describe the sequences of events that occur at a synapse that can result in an action potential being generated in the post-synaptic neurone.

  Action potential arrives at axon terminalAxon terminals of presynaptic neurone depolariseVoltage gated Ca2+ channels openConcentration of Ca2+ inside axon terminal increases as Ca2+ diffuses in down electrochemical gradientVesicles containing neurotransmitter - acetylcholine - move to the presynaptic membrane, fuse and release acetylcholine by exocytosisAcetylcholine diffuses across synaptic cleftAcetylcholine binds to receptor protein on the postsynaptic membraneNa+ channels open and Na+ diffuses in down its electrochemical gradientIf threshold is reached an action potential occurs
• Describe the role of acetylcholinesterase and explain how acetylcholine is recycled


  Acetylcholinesterase hydrolyses acetylcholine into choline and acetate. This means the neurotransmitter isn't left in the synaptic cleft to trigger more action potentials. Instead the choline diffuses back across the synaptic cleft and is reabsorbed by the presynaptic neurone where it is then recycled into more acetylcholine and packaged into vesicles for the next action potential.
• Explain why synaptic transmission is very energy demanding

  ATP needed to synthesise acetylcholine, for reabsorption and exocytosis.
• Explain why synapses are unidirectional

  Only presynaptic neurone has Ca2+ channels, neurotransmitter production and vesicles while only the post synaptic neurone has the receptors
• Define the terms "summation", "temporal summation" and "spatial summation".

  Summation - the process by which a sequence of stimuli that are individually inadequate to produce a response are cumulatively able to induce a nerve impulse. From textbook: Build up of neurotransmitter in a synapse to sufficient levels to trigger an action potentialSpatial summation - mechanism of eliciting an action potential in a neurone with input from multiple presynaptic cells. It is the algebraic summing of potentials from different areas of input, usually on the dendrites.Temporal summation - high frequency of action potentials in presynaptic neurone causes neurotransmitter and therefore Na+ and depolarisation to build up before it can repolarise, triggering an action potential in a postsynaptic neurone.