Nervous coordination

• Myelinated motor neurone

  
• Axon


  long fibre which carries nerve impulse along the motor neurone away from the cell body
• Dendrites


  Extension of the cell body that allows the neurons to connect to other neurones to recieve and transmit impulses, this forms a continuous network for quick impulse transmission
• Myelin sheath (schwann cells)

  schwann cells wrap around the axon to produce myelin and form the myelin sheath
• Nodes of Ranvier

  gaps of unmyelinated sections along the axon between schwann cells which causes the electrical impulse to jump from one node to the next, speeds up the conduction of the impulse
• Saltatory conduction


  the jumping of an electrical impulse between nodes of ranvier

  this speeds the conduction of the impulse and transfer between neurones
• resting potential


  the difference between electrical charge inside and outside of the neurone

  -70mV
• why is the resting potential of a neurone -70mV

  as there are more positive ions outside than inside
• how is the negative resting potential established and maintained?

  As the sodium potassium pump actively transports 3Na+ ions out and 2K+ ions in

  this establishes an electrochemical gradient

  the membrane is more permeable to K+ as there are more K+ channels than Na+ channels
• action potential


  a brief change in the distribution of electrical charge across the cell surface membrane of an axon
• What happens when an action potential is stimulated?

  Na+ channels in the axon membrane opens

  Na+ pass into the axon down the electrochemical gradient

  This reduced the potential difference across the axon membrane as the axon becomes less negative

  depolarisationoccurs
• Depolarisation and how it generates an action potential

  occurs when the threshold potential is reached (-55mV)

  causes more sodium voltage gated channels to open and increases the axon membranes permeability to Na+

  this causes the potential difference to reach 40mV

  sodium voltage gated channels close
• Repolarisation

  K+ voltage gated channels open (membrane becomes more permeable to K+)

  K+ diffuses out of the membrane and lowers the potential difference back to -70mV
• Refractory period

  when hyperpolarisation occurs (membrane potential of -80mV)

  membrane is unresponsive and unable to be depolarised

  this ensures than new action potential are generated and that the impulses travel in one direction
• what happens when a receptor is stimulated?

  it is depolarised
• all-or-nothing principle


  an impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
• how does the strength of a stimulus affect the receptor potential

  -a strong stimulus generates a high FREQUENCY of impulses in the sensory neurone

  -a weak stimulus generates a low FREQUENCY of impulses

  -a very weak stimulus doesn't generate any impulses
• Factors that affect the speed of impulse conduction

  -myelination

  -diameter of the axon

  -temperature
• How does myelination affect the speed of impulse conduction?

  increases with increased myelination

  as depolarisation cannot occur in the sections of the axon that are insulated by the myelin sheath

  saltatory conduction occurs which allows the impulse to travel much faster
• How does diameter affect the speed of impulse conduction?

  thicker diameter = increases impulse conduction

  as there is a greater SA for diffusion to occur, faster rate of depolarisation so action potentials can be generated faster
• How does temperature affect the speed of impulse conduction?

  increased temperature increases the conduction of impulses as there is a faster rate of facilitated diffusion

  at very high temperatures membrane proteins can denature
• maximum frequency of impulses equation

  Time/ duration of refractory period
• synapse


  gaps between neurones

  junction between the axon of one neurons and the dendrites of another
• structure of a synapse

  -presynaptic neurone

  -post synaptic neurone

  -synaptic cleft
• synaptic transmission using acetylcholine (detailed answer)


  -action potential arrives at the presynaptic membrane, causing depolarisation of the neurone

  -stimulates calcium voltage gated channels to open

  calcium ions diffuse down their electrochemical gradient into the presynaptic neurone

  -This causes vesicles containing acetylcholine to fuse with the presynaptic membrane and release ACh into the synaptic cleft

  -ACh diffuses across the synaptic cleft and binds to ligand-ated sodium ion channels in the postsynaptic membrane

  -this causes Na+ voltage gated channels to open which allows Na+ ions to diffuse down an electrochemical gradient into the postsynaptic neurone

  -Na+ causes depolarisation of the postsynaptic membrane, this re-starts the electrical impulse

  -ACh is broken down and recycled, this closes the Na+ VG channels and prevents the postsynaptic membrane from being permanently depolarised

  -acetycholinesterase catalyses the hydrolysis of ACh into acetate and choline

  -choline is reabsorbed into the presynaptic membrane and reacts with acetyl coenzyme A to from ACh which can be used when another action potential arrives
• what causes the Ca2+ voltage gated channels to open

  the arrival of an action potential at the presynaptic membrane
• what causes vesicles containing ACh to fuse with the membrane?

  Ca2+
• what causes the depolarisation of the postsynaptic membrane

  Na+
• why is ACh released from the receptor in the postsynaptic membrane?

  so that the Na+ voltage gated channels close and the postsynaptic membrane isnt permenantly depolarised
• what is ACh hydrolysed by?

  Acetylcholinesterase
• How do synapses ensure the one-way transmission of impulses?

  As the neurotransmitter is only released from the presynaptic membrane and there are only complementary receptors on the postsynaptic membrane

  this prevents impulses from traveling the wrong way
• synaptic summation


  multiple sub-threshold impulses are added together to build up enough neurotransmitter in the synapse to meet the threshold and generate an action potential
• Temporal summation


  multiple impulses arrive in quick succession, lots of neurotransmitter released into the synaptic cleft, this generates enough synaptic cleft

  
• Spatial summation


  multiple impulses from different synaptic knobs arrive at the same synaptic cleft

  each neurone releases neurotransmitter, this increases the amount of neurotransmitter in the cleft

  this builds up enough neurotransmitter to meet the threshold

  
• what inhibits synaptic transmission?

  some neurotransmitters prevent the generation of an action potential
• how do some neurotransmitters prevent the generation of an action potential?

  by opening K+ vg channels so that K+ can diffuse out, when Na+ enters the effect is cancelled out by K+ so depolarisation doesn't occur and the threshold is not met and so the generator potential does not occur
• neuromuscular junction


  gap between motor neurone and muscle cell, muscle contracts when it receives an impulse from a motor neurone via the neuromuscular junction