Nervous coordination

• Describe the general structure of a motor neuron

  Cell body = contains organelles and high proportion of RER

  Dendrons = branch into dendrites which carry impulses towards cell body

  Axon = long, unbranched fibre carries nerve impulses away from cell body
• Describe the additional features of a mylelinated motor neuron

  Schwann cells = wrap around axon many times

  Mylein sheath = made from myelin rich membranes of schwann cells

  Nodes of Ranvier = very short gaps between neighbouring schwann cells where there is no myelin sheath
• Name three processes schwann cells are involved in

  ~Electrical insulation

  ~Phagocytosis

  ~Nerve regeneration
• How does an action potential pass along an unmyelinated neuron

  ~Stimulus leads to influx of Na+ ions. First section of membrane depolarises

  ~Local electrical currents cause sodium voltage gated channels further along membrane to open. Meanwhile, the section behind begins to repolarise

  ~Sequential wave of depolarisation
• Explain why myelinated axons conduct impulses faster than unmyelinated axons

  ~Saltatory conduction = impulse jumps from one node of ranvier to another

  ~Depolarisation cannot occur where myelin sheath acts as electrical insulator

  ~So impulse does not travel along whole axon length
• What is resting potential

  Potential difference across neuron membrane when not stimulated
• How is resting potential established

  ~Membrane is more permeable to K+ than Na+

  ~Sodium potassium pump actively transports 3Na+ out of cell and 2K+ into cell

  ~Establishes electrochemical gradient = cell contents more negative than extracellular environment
• Name the stages in generating an action potential

  ~Depolarisation

  ~Repolarisation

  ~Hyperpolarisation

  ~Return to resting potential
• What happens during depolarisation

  ~Stimulus = facilitated diffusion of Na+ ions into cell down electrochemical gradient

  ~Potential difference across membrane becomes more postive

  ~If membrane reached threshold potential voltage gated Na+ channels open

  ~Significant influx of Na+ ions reverses potential difference
• What happens during repolarisation

  ~Voltage gated Na+ channels close and voltage gated K+ channels open

  ~Facilitated difusion of K+ ions out of cell down their electrochemical gradient

  ~Potential difference across membrane becomes more negative
• What happens during hyperpolarisation

  ~Overhoot when K+ ions diffuse out = potential difference becomes more negative than resting potential

  ~Refractory period = no stimulus is large enough to raise membrane potential to threshold

  ~Voltage gated K+ channels close and sodium potassium pump re-establishes resting potential
• Explain the importance of the refractory period

  ~No action potential can be generated in hyperpolarised sections of membrane

  ~Ensures unidirectional impulse

  ~Ensures discrete impulses

  ~Limits frequency of impulse transmission
• What is the all or nothing priciple

  ~Any stimulus that causes the membrane to reach threshold potential will generate an action potential

  ~All action potentials have same magnitude
• Name the factors that affect the speed of conductance

  ~Myelin sheath

  ~Axon diameter

  ~Temperature
• How does axon diameter affect the speed of conductance

  ~Greater diameter = faster

  ~Less resistance to flow of ions (depolarisation and repolarisation)

  ~Less leakage of ions
• How does temperature affect speed of conductance

  ~Higher temperature = faster

  ~Faster rate of diffusion

  ~Faster rate of respiration = more ATP for active transport to re-establish resting potential

  ~Temperature too high = membrane proteins denature
• Suggest an appropriate statistical test to determine whether a factor has a significant effect on the speed of conductance

  Student's t-test
• How can an organism detect the strength of a stimulus

  Larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulses
• What is the function of synapses

  ~Electrical impulse cannot travel over junction between neurones

  ~Neurotransmitters send impulses between neurons/ from neurones to effectors

  ~New impulses can be initiated in several different neurons for multiple simultaneous responses
• Describe the structure of a synapse

  ~Presynaptic neuron ends in synaptic knob = contains lots of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter

  ~Synaptic cleft = 20-30 nm gap between neurons

  ~Postsynaptic neuron = has complementary receptors to neurotransmitter
• Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another

  ~Wave of depolarisation travels down presynaptic neuron, causing voltage gated Ca2+ channels to open

  ~Vesicles move towards and fuse with presynaptic membrane

  ~Exocytosis of neurotransmitter into synaptic cleft
• How do neurotransmitters cross the synaptic cleft

  Via simple diffusion
• Outline what happens in the postsynaptic neuron when an action potential is transmitted from one neuron to another

  ~Neurotransmitter binds to specific receptor on postsynaptic membrane

  ~Ligand gated Na+ channels open

  ~If inlfux of Na+ ions raises membrane to threshold potential, action potential is generated
• Explain why synaptic transmission is uniderectional

  ~Only presynaptic neuron contains vesicles of neurotransmitter and only postsynaptic membrane has complementary receptors

  ~So impulse always travels presynaptic - postsynaptic
• Define summation and name the two types

  ~Neurotransmitter from several sub threshold impulses accumulates to generate action potential

  ~Temporal summation

  ~Spatial summation
• What is the difference between temporal and spatial summation

  Temporal = one presynaptic neuron releases neurotransmitter several times in quick succession

  Spatial = multiple presynaptic neurons release neurotransmitter
• What are cholinergic synapses

  ~Use acetylcholine as primary neurotransmitter

  ~Excitatory or inhibitory

  ~Motor end plate

  ~Preganglionic neurons

  ~Parasympathetic postganglionic neurones
• What happens to acetylcholine from the synaptic cleft


  ~Hydrolysis into acetyl and choline by acetylcholinesterase

  ~Acetyl and choline diffuse back into presynaptic membrane

  ~ATP is used to reform acetylcholine for storage in vesicles
• Explain the importance of AChE

  ~Prevents overstimulation of skeletal muscle cells

  ~Enables acetyl and choline to be recycled
• What happens in an inhibitory synapse

  ~Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane and triggers K+ channels to open

  ~Cl- moves in and K+ moves out via facilitated diffusion

  ~Potential diffusion becomes more negative = hyperpolarisation
• Describe the structure of a neuromuscular junction

  Synaptic cleft between a presynaptic neuron and a skeletal muscle cell
• How might drugs increase synaptic transmission

  ~Inhibit AChE

  ~Mimic shape of neurotransmitter
• How might drugs decrease synaptic transmission

  ~Inhibit release of neurotransmitter

  ~Decrease permeability of postsynaptic membrane to ions

  ~Hyperpolarise postsynaptic membrane