Nerves

• Structure of a motor neuron

  A) Dendrites

  B) Cell body

  C) Axon

  D) Schwann's cells

  E) Myelin Sheath

  F) Node of Ranvier

  G) Axon Terminals

  
• The peripheral nervous system includes all sensory neurons, motor neurons and sense organs
• The parasympathetic and sympathetic nervous system are antagonistic- they don't turn on and off
• Schwann's cells wrap around the axon
• The cytoplasm of the schwann's cells is flattened which creates many layers
• The schwann's cells form a lipid rich insulating layer called a myelin sheath
• 4 proteins of the Node of Ranvier

  • Na+/K+ pump
  • Na+/K+ leakage channel
  • Na+ voltage gated channel
  • K+ voltage gated channel
• When a neurone is not conducting an impulse, it is 'resting'
• The neurone has a resting potential of -65mv
• Two steps to maintain this gradient:

  • The sodium potassium pumps use ATP to transport sodium ions out and potassium ions in
  • 3 sodium ions move out and 2 potassium ions move in
• Both the sodium ions and the potassium ions create an electrochemical gradient
• While resting, the outside of the neurone will be positively charged relative to the inside
• Voltage gated channels are both closed during a resting potential
• The 'gates' only open when the potential difference across a membrane reaches a specific value
• When a stimulus reaches a resting neurone, it causes a slight depolarisation
• Not every stimulus causes an action potential- if there isn't enough information, it will just go away and nothing will happen
• Depolarisation causes the sodium gated channels to open at the point of stimulation
• Sodium ions move through the channel into the membrane causing a rapid influx of sodium ions
• If a threshold value is reached, even more sodium ions channels open
• Sodium channels opening as a consequence of a threshold value being reached is an example of positive feedback
• Sodium ions diffuse along an electrochemical gradient
• This influx of sodium ions causes a potential difference across the membrane, which is called depolarisation
• The influx of so many sodium ions causes the inside of the axon to now be positively charged relative to the outside- the potential difference is +30mv
• The sodium voltage gated channels close as soon as the potential difference inside the axon hits +30mv
• Repolarisation happens as potassium ions move out of the inside of the axon
• Potassium ions move out of the axon through potassium voltage gated channels via facilitated diffusion
• Potassium ions move out of the axon along both a concentration and an electrical gradient
• The potassium channels stay open after repolarisation, causing hyperpolarisation which is where the axon has a potential difference of -80mv
• Hyperpolarisation is essential in preventing damage to the neurone from overstimulation
• Absolute refractory period- no action potential can be generated
• Relative refractory period- action potential can only be generated if the stimulus is very large
• Hyperpolarisation makes it so the neurone can only work in one direction
• Hyperpolarisation ensures that nervous impulses are discrete (separate) which allows the body to distinguish what is happening
• Action potential- the whole cycle of depolarisation and repolarisation, takes about 3ms
• Action potential

  A) Action potential

  B) Na+ ions in

  C) K+ ions out

  D) Threshold

  E) Stimulus

  F) Hyperpolarisation

  G) Resting state

  H) Repolarisation

  I) Depolarisation

  
• The stronger the stimulus, the greater the frequency of action potentials
• The magnitude of action potential will never change- this is the all or nothing principle
• Factors affecting nervous transmission speed

  • Myelination
  • Body temperature
  • Diameter of the axon
• Myelination- depolarisation only occurs at the Nodes of Ranvier as the nervous impulse 'jumps' from node to node. This is saltatory propogation
• Saltatory propagation- nervous impulses 'jumping' from node to node