Module 5.1.3 Neuronal communication

• What is a stimulus?


  a signal/change in the environment to which an organism responds
• What is a neurone


  Neurones are highly specialised nerve cells. Their structures allow them to transmit information over a long distances.
• Structure of a neurone- cell body


  contains nucleus surrounded by cytoplasm

  
• Why does the cell body contain lots of mitochondria and endoplasmic reticulum in its cytoplasm?


  To produce neurotransmitters (to pass signal from one neurone to the next)
• Structure of a neurone- dendrons


  Extensions of the cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve impulses towards the cell body

  
• Structure of a neurone- axons


  Carry impulses away from the cell body. Can be very long
• 3 types of neurones


  Sensory, relay and motor
• Function of sensory neurones

  To transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain
• Function of relay neurones


  To transmit impulses between neurones. e.g. sensory neurones and motor neurones.They have short axons and dendrons
• Function of motor neurones


  To transmit impulses from a relay or sensory neurone to an effector
• Pathway of neurones

  Receptor (Start)Sensory Neurones Relay Neurones Motor Neurones Effector cell (End)
• Sensory neurone structure


  Nucleus in the middle, axon in one end, dendrite at the other end, both covered in myelin sheath

  
• Motor neurone structure

  Short dendrites and long axons

  
• Relay neurone structure


  Short dendrites and short axon

  
• Function of myelin sheath and their composition


  Formed by many layers of the plasma membrane.- Acts as an insulating layer and conducts electrical impulses at a much faster speed
• What is the cell that produces plasma membranes around the axon many times

  Schwann cells
• Nodes of Ranvier


  a gap in the myelin sheath of a nerve, between adjacent Schwann cells.electrical impulse jumps from node to node transmitting impulse fasterIn non myelinated neurones it just transmits continuously along the nerve fibre so it is much slower
• Transducers


  convert stimulus energy into electrical energy (nerve impulse) producing a generator potential
• 2 main features of a receptor


  - specific to one type of stimulus- act as transducers
• What is an example of a mechanoreceptor


  Pacinian corpuscleStimulus: pressure and movementSense organ: skin
• Structure of Pacinian Corpuscle


  - end of neurone surrounded by layers of connective tissue separated by layers of gel- sodium ion channels in membranes- stretch-mediated sodium channels

  
• Stretch-mediated sodium channel


  Channels that stretch when pressure is exerted and change permeability to sodium
• How does a Pacinian Corpuscle work?


  1) At resting potential

  2) Pressure is applied, corpuscle changes shape causing the membrane surrounding neurone to stretch

  3) due to stretch, sodium ion channels open and sodium ions enter

  4) potential inside the membrane is changed. it has become DEPOLARISED causing a GENERATOR POTENTIAL

  5) this causes an ACTION POTENTIAL that passes along sensory neurone

  
• What is the term for when the cell membranes potential has changed from negative to positive


  Depolarisation
• What is resting potential

  The difference in electric charge between the inside and outside of a neuron's cell membrane-70mVNo neural impulse being trasmitted
• What is the term to be said of when the cell is at resting potential

  Polarised
• How does axon membrane maintain a resting potential


  Sodium potassium pump (3 Na+ pumped out for every 2 K+ pumped in making outside more positive)V.g. Na+ channels closed so it does not move back down the electrochemical gradientNon v.g. K+ channels open so K+ ions inside can diffuse out

  
• Stages during action potential


  Resting potential

  Depolarisation

  Repolarisation

  Hyperpolarisation

  Resting potential (repolarised)

  
• Action Potential- Stage 1 at resting potential


  the cell is at resting potential (-70mV).

  non-voltage gated K+ channels are open.

  • DEPOLARISATION- the energy of stimulus causes voltage-gated Na+ channels to open causing Na+ to diffuse into the cell down the electrochemical gradient increasing potential inside the neurone
• Action Potential- Stage 2


  The change in charge causes more sodium ion channels to open allowing more sodium ions to flow in.
• What system is the action of opening more sodium ion channels as a result of the change in charge


  Positive feedback
• Action Potential- Stage 3


  Once potential difference is at +40mV the voltage gated sodium ion channels close and voltage gated potassium ion channels open.
• Action Potential- Stage 4


  Potassium ions diffuse out of axon down their electrochemical gradient reducing potential inside the axon.
• Action Potential- Stage 5

  Lots of K+ ions diffuse out causing potential to lower than resting potential (around -80 mV). This is hyperpolarisation. It is then repolarised using the sodium-potassium pump as potassium ion channels close.
• Propagation of an action potential


  -Na+ influx causes a patch of the axonal membrane to depolarize-Local currents occur-Na+ channels toward the point of origin are inactivated and not affected by the local currents-Local currents affect adjacent areas in the forward direction-Depolarization opens voltage-gated channels and triggers an AP-Repolarization wave follows the depolarization wave

  
• Refractory period


  the time following an action potential during which a new action potential cannot be initiated

  voltage-gated sodium ions closed
• Why is a refractory period important


  Ensures that an action potential occurs in one direction only;Ensures that action potentials are separated from one another;Limits the number of action potentials
• Saltatory conduction


  Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.
• Why is saltatory conduction faster and more efficient?


  fewer ion gates are needed to send the action potential along the axon

  more energy-efficient (less ATP used because less repolarisation)
• Another factor affecting speed of an action potential


  Axon diameter- bigger axon diameter increases speed of impulse transmitted. less resistance to flow of ions in the cytoplasm