5.1.3 - neuronal communication

• Features common to all sensory receptors are tha they act as energy transducers which establish a generator potential. They all respond to a specific stimuli.
• Pacinian corpuscle is a single nerve fibre surrounded by layers of connective tissue which are separated by viscous gel and contained by a capsule. There are Stretch-mediated Na+ channels on plasma membrane Capillary runs along base layer of tissue.
• Pacinian corpuscle responds to pressure.
• in pacinian corpuscle:

  pressure deforms membrane causing stretch-mediated Na+ ions channels to open.

  if influx of Na+ raises membrane to threshold potential, a generator potential is produced.

  action potential moves along sensory neurone.
• All neurones have:

  • cell body - contains organelles and high proportion of RER
  • dendrons - branch into dendrites which carry impulses towards the cell body
  • axon - long, unbranched fibre carries nerve impulses away from cell body
• Structure and function of sensory neurone:

  usually unipolar. Transmits impulses from receptors to CNS.

  Cell body is in the middle of the axon, has a long axon with axon terminals at one end and dendrites at the other.
• structure and function of relay neurone:

  usually bipolar. Transmits impulses between sensory and motor neurones.

  highly branched axon terminals and highly branched dendrites at the other end.
• structure and function of a motor neurone:

  usually multipolar. transmits impulses from relay neurones to the CNS then to effectors.

  Has dendrites at one end and axon terminals at the other. The celL body is at the end of the axon.
• additional features of a myelinated neurone are:

  Schwann cells

  myelin sheath

  nodes of ranvier
• schwann cells wrap around the axon many times
• myelin sheath is made from myelin rich membranes of Schwann cells
• nodes of ranvier are very short gaps between neighbouring schwann cells where there is no myelin sheath
• Schwann cells are involved in

  • electrical insulation
  • phagocytosis
  • nerve regeneration
• myelinated axons conduct impulses faster than non-myelinated axons.
• Myelin axons conduct impulses faster because of saltatory conduction. Where the impulse jumps from one node of ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator. So the impulse does not travel along the whole axon length.
• myelinated neurones are most of the neurones in the central and peripheral nervous systems.
• non-myelinated neurones are group C nerve fibres involved in transmitting secondary pain
• Resting potential is the potential difference (voltage) across neurone membranes when they are at rest (not stimulated).
• resting potential in humans is -70mV
• Resting potential is established as the neurone membrane is more permeable to K+ than Na+.

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

  Establishes an electrochemical gradient: cell contents are more negative than extracellular environment.
• the stages in generating an action potential are:

  • depolarisation
  • repolarisation
  • hyperpolarisation
  • return to resting potential
• During depolarisation:

  stimulus - facilitated diffusion of Na+ into cell down electrochemical gradient through sodium channels.

  Potential difference across membrane becomes more positive.

  If membrane reaches threshold potential (-50mV) the voltage gated sodium ion channels open. (Positive feedback mechanism)

  significant influx of Na+ ions reverses potential difference to +40mV
• the threshold potential of the cell is -50mV
• the action potential of a cell is +40mV
• During repolarisation:

  voltage gated sodium ion channels close and voltage gated potassium ion channels open.

  Facilitated diffusion of K+ ions out of the cell down their electrochemical gradient.

  Potential difference across membrane becomes more negative.
• during hyperpolarisation:

  Overshoot 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 potassium ion channels close and sodium potassium pump reestablishes the resting potential.
• Neurones are specialised cells that pass on nerve impulses
• There are three types of neurones:

  1. Sensory neurones - pass impulses from receptors to CNS
  2. motor neurones - pass impulses from CNS to effectors
  3. relay neurones - connect sensory and motor neurones
• axon terminals in neurones form synapses with other neurones or muscles
• When a stimulus is supplied, it is picked up by a receptor which converts the stimulus into an impulse. Once it reaches a neurone it triggers an action potential causing a movement.
• a nerve is connected to a receptor at one end and an effector at the other end
• Nerves are made up of a series of neurones - charge moves down the axon of each neurone in turn and this charge always travels away from the receptor and Towards the effector.
• a nerve impulse is movement of charge moving through the neurone cytoplasm
• A nerve impulse is always positive - enabled by K+ ions and Na+ ions
• there are more K+ ions inside the axon than outside the axon
• there are more Na+ ions outside the axon than inside the axon
• the outside of the axon is more positively charged than the inside of the axon
• Na+ and K+ move through the axon membrane through specific channel proteins for facilitated diffusion. either sodium ion channels or potassium ion channels.
• movement of ions is determined by their electrochemical gradient. when one side of the membrane is more positively charged than the other it causes ions to move from an area of more positive charge to an area of less positive charge
• sodium and potassium ion channels are always open