6.2 - Nervous Coordination

• Describe the 3 structures that make up a Motor neuron

  1. Cell body:contains organelles & a high proportion of RER

  2. Dendrons:branch into dendrites which carry impulses towards cell body

  3. Axon:long, unbranched fibre carries nerve impulses away from cell body

  
• Describe the additional 3 structures found in a Myelinated motor neuron

  1. Schwann cells:wrap around the axon many times

  2. Myelin sheath:made from myelin-rich membranes of schwann cells

  3. Nodes of Ranvier:very short gaps between neighbouring schwann cells where there is no myelin sheath

  
• What 3 processes are Schwann cells involved in?

  1. Electrical insulation

  2. Phagocytosis

  3. Nerve regeneration
• how does an Action potential pass along an unmyelinated neuron?

  1. Stimulus leads to influx of Na+ ions, first section of membrane depolarises

  2. Local electrical currents cause sodium voltage-gated channels further along the membane to open, while the section of membrane behind begins to repolarise

  
  

  3. Sequential wave of depolarisation.
• Why do 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(voltage) across neuron membrane whennot stimulated

  (-50 to -90mV, usually around -70mV in humans)
• How is Resting potential established?

  1. Membrane is more permeable K+than Na+

  2. Sodium-potassium pump actively transports 3Na+ out of cell, and 2K+ into cell

  3. This establishes an electrochemical gradient, cell contents more negative than extracellular environment
• 4 Stages in generating an action potential

  1. Depolarisation

  2. Repolarisation

  3. Hyperpolarisation

  4. Return to resting potential

  
• What occurs during Depolarisation?

  1. Stimulus > facilitated diffusion of Na+ ions into cell down electrochemical gradient

  
  

  2. Potential difference accross membrane becomes more positive

  
  

  3. if membrane reaches threshold potential (-50mV), voltage-gated Na+ channels open.

  
  

  4. Significant influx of Na+ ions reverses potential difference to +40mV
• What occurs during Repolarisation

  1. Voltage-gatedNa+ channels closeand Voltage-gatedK+ channels open

  2.Faciliated diffusion of K+ ionsout of cell down their electrochemical gradient

  3.Potential differenceaccross membrane becomes more negative
• What occurs during Hyperpolarisation

  1.Overshootwhen K+ ions diffuse out,potential difference becomes more negativethat resting potential

  2.Refractory period:no stimulus is large enough to raise membrane potential to threshold.

  3. Voltage-gatedK+ channels closeandsodium-potassium pump re-establishes resting potential.
• Explain the Importance of refractory period

  No action potential can be generated in hyperpolarised sections of the membrane:

  
  

  - ensures Unidirectional impulse

  - ensures discrete impulses

  - limits frequency of impulse transmission
• what is the 'All or nothing principle'?

  Any stimulusthat causes the membrane toreach thresholdpotential will generate an action potential, andall action potentials have the same magnitude.
• Name 3 Factors affecting speed of conductance

  1. Myelin sheath

  2. Axon diameter

  3. Temperature
• How does axon diameter affect speed of conduction?

  Greater Diameter = Faster

  - Less resistance to flow of ions (depolarisation & repolarisation)- less leakage of ions (easier to maintain membrane potentials)

  
• How does Temperature affect speed of conduction?

  Higher temperature = Faster

  - faster rate of diffusion (depolarisation & repolarisation)- faster rate of respiration (enzyme-controlled) = more ATP for active transport to re-establish resting potential

  Temperature too high = membrane proteins denature
• Statistical test for speed of conductance

  Student's t-test (comparing means of continuous data)

  
• Units for maximum frequency of impulse conduction


  Hz
• How does 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?

  1. elecrical impulses cannot travel over junction between neurons

  2. neurotransmitters send imulses between neurons to effectors

  3. New impulses can be initiated in several different neurons for multiple simultaneous responses
• Structure of a synapse

  1.Presynaptic neuronends insynaptic knob:contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter

  2.Synaptic cleft:20-30nm gap between neurons

  3.Postsynaptic neuron: has complimentary receptors to neurotransmitter (ligand-gated Na+ channels)

  
• Outline what happens in a Presynaptic neuron during action potential transmission

  1. Wave of Depolarisation travels down causing voltage-gated Ca2+ channels open

  
  

  2. vesicles move towards and fuse with Presynaptic membrane

  
  

  3. Exocytosis of neurotransmitter into synaptic cleft

  
• How do Neurotransmitters cross the synaptic cleft?

  via Simple diffusion
• What occurs in the Postsynaptic neuron during action potential transmission?

  1. Neurotransmitter binds to specific receptor on its membrane

  
  

  2. ligand-gated Na+ channels open

  
  

  3. If influx of Na+ ions raises membrane to threshold potential then action potential is generated

  
• Explain why synaptic transmission is unidirectional

  Vesicles of neurotransmitter only in presynaptic neuron & complementary receptors are only in postsynaptic membrane

  
  

  Therefore impulse always travels presynaptic > postsynaptic
• Define Summation & name 2 types

  Summation = neurotransmitter fromseveral sub-thresholdimpulsesaccumulatesto generate action potential

  2 Types:-Temporalsummation-Spatialsummation

  
• What is the difference between Spatial and Temporal summation?


  Spatial:Multiplepresynaptic neurons release neurotransmitter

  Temporal:Onepresynaptic neuron releases neurotransmitter several times in quick succession

  
• What are Cholinergic synapses?

  Use acetylcholine as neurotransmitter, excitatory or inhibitory, located at:

  
  

  - Motor end plate (muscle contraction)

  - Preganglionic nuerons (excitation)

  - Parasympathetic postganglionic nuerons (inhibition of heart or breathing rate)

  
• What happens to Acetylcholine from synaptic cleft?

  1.Hydrolysisby acetylcholinesterase (AChE) into acetyl and choline

  2. Acetyl & cholinediffuse backinto presynaptic membrane

  3. ATP is used toreformacetylcholinefor storagein vesicles
• Explain 2 important functions of acetylcholinesterase (AChE)

  1.Prevents overstimulationof skeletal muscle cells2.Enables recyclingof acetyl and choline
• What happens in an Inhibitory synapse?

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

  
  

  2. Cl- moves in, and K+ moves out via facilitatd diffusion

  
  

  3. Potential difference becomes more negative: hyperpolarisation

  
• Descrube the structure of a neuromuscular junction

  Synaptic cleft between presynaptic neuron and a skeletal muscle cell

  
• Contrast a cholinergic synapse and a neuromuscular junction

  1. Postsynaptic cell:Cholinergc = anotherneuronNeuromuscular =skeletal muscle cell

  2. AChE locationCholinergc =synaptic cleftNeuromuscular =Postsynaptic membrane

  3. Action potentialCholinergc =new a.pproducedNeuromuscular =end of neural pathway

  4. ResponseCholinergc =bothexcitatory or inhibitoryNeuromuscular = alwaysexcitatory

  5. Neurons involvedCholinergc =motor, sensory or relayNeuromuscular =only motor
• How might drugs increase synaptic transmission?

  1. Inhibit AChE

  2. mimic shape of neurotransmitter
• How might drugs decrease synaptic transmission?

  1. Inhibit release of neurotransmitter.

  2. Decrease permeability of postsynaptic membrane to ions.

  3. Hyperpolarise postsynaptic membrane.
• At rest, are the charge across a neurone membrane the same?

  No, there are more positive charges outside compare to inside the neurone.
• Define resting potential.


  The potential difference across a neurone membrane when it is at rest.
• What is the value of the resting potential for a neurone?

  About -70mV.
• What causes the resting potential across a neurone membrane?

  Na⁺-K⁺ pumps move 3 sodium ions out of the cell for every 2 potassium into the cell

  
  

  Na⁺ ions can't move back in, but K⁺ ions can move back out of the neurone using potassium ion channels

  
  

  There is more positive charge outside of the neurone compared to inside it, which causes there to be a resting potential

  
• What sort of transport is involved in potassium ion channels?

  Facilitated diffusion