Receptors 6.1

• A receptor is a cell that responds to stimuli by converting energy from the stimuli into an electrical impulse.
• An axon that is not transmitting impulses is at its resting potential (typically -70mv) and is negative compared to its surroundings.
• Depolarisation is the change in membrane potential from negative to positive.
• A generator potential is the depolarisation of a receptor in response to a stimuli and is caused by an influx of positive sodium ions.
• An action potential is a rapid change in charge across a membrane that occurs when a neurone fires.

  An action potential is caused by the movement of sodium and potassium ions across the cell surface membrane
• A pacinian corpuscle is an example of a receptor. They have special types of sodium ion channels in their membranes called stretch-mediated sodium channels. These channels open upon a change in shape, increasing the membranes permeability to sodium creating an action potential.

  
• The Cornea is a transparent lens that refracts light as it enters the eye
• The iris controls how much light enters the pupil
• The eye lens is a transparent disc that changes shape to focus light onto the retina
• The retina contains two types of light receptor cells.


  Rods are sensitive to light and can detect low intensity light (black and white images) Action potential is generated by break down of Rhodopsin.

  Cones requoire high intensity light to be stimulated but allow for colour vision as there are three types of cones (RGB) that are each stimulated by different wavelengths. Action potential is generated by the break down of three types (RGB) of iodopsin.
• The optic nerve is a sensory neurone that carries impulses between the eye and brain.
• The pupil is a hole that allows light to enter the eye
• Bipolar neurones act as bridges between receptors and other neurones.

  Rods are more sensitive to light as many rod cells connect to a single bipolar neurone creating retinal convergence.

  Cones are less sensitive to light as they do not express retinal convergence.
• Label the diagram of a motor neurone.

  A) Dendrite

  B) Cytoplasm

  C) Nucleus

  D) Dendron

  E) Cell surface membrane

  F) Myelin Sheath

  G) Nodes of ranvier

  H) Schawn Cell

  I) Axon

  J) Cell body

  K) Axon terminal

  
• Which axon is myelinated and which is not?

  A) Myelinated Axon

  B) Unmyelinated Axon

  
• Depolarisation:

  • Occurs when a nerve impulse opens sodium ion channels
  • The interior becomes more positive
  • Sodium ion concentration increases
  • Charge increases
• Repolarisation:

  • Occurs when the potential difference is +40
  • Sodium channels close
  • VG potassium channels open
  • Potassium ions move out
  • Charge decreases
• Hyperpolarisation:

  • Occurs when VG potassium ion channels close slower than VG sodium ion channels
  • Excess of potassium leave the neurone
  • Leads to a potential difference less than -70
  • Known as the refractory period
• Label the three parts of an action potential on the graph:

  A) Repolarisation

  B) Depolarisation

  C) Hyperpolarisation

  
• An action potential moves along the neurone as a wave of depolarisation as some sodium ions that enter the neurone diffuse sideways, increasing that charge in neighbouring regions causing VG sodium ion channels to open.
• The all or nothing principle states that all action potentials are the same size as long as the threshold is reached. A bigger stimulus will only increase AP frequency.
• Saltatory conduction is the jumping of an action potential between nodes of ranvier along a myelinated neurone. Occurs as sodium ion channels only open at the nodes of ranvier.
• The refractory period is important as it ensures action potentials are discrete so information is not blurred and ensures impulses are uni-directional.
• The factors that effect speed of transmission are:

  • Myelination
  • Neurone diameter
  • Temperature
• An action potential is proliferated across a synapse by neurotransmitters (such as acetylcholine)

  Calcium ion channels open, calcium ions move into pre synaptic neurone. Synaptic vessicles move towards and fuse with cell membrane. Neurotransmitters are released by exocytosis and bind to chemical gated sodium ion channels, opening them and creating an action potential
• Cholinergic synapses are junctions between neurones where the neurotransmitter acetylcholine is used.
• Synapses ensure transmission is unidirectional as presynaptic neurones are the only ones with synaptic vesicles and postsynaptic neurones are the only ones with chemical gated sodium ion channels.
• There are two types of synapses, these being Excitatory and Inhibitory.

  Excitatory synapses are seen as the normal and result in the depolarisation of postsynaptic neurone

  Inhibitory synapses result in the hyperpolarisation of the post synaptic membrane preventing an action potential.
• Agonists are drugs that are similar in shape to neurotransmitters and mimic their effects (e.g nicotine)

  Antagonists are drugs that are similar in shape to neurotransmitters but block the binding site
• Temporal summation is the arrival of multiple action potentials to the axon terminal in quick succession creating a cumulative effect.

  Spatial summation is the arrival of action potentials to axon terminal from multiple neurones creating a cumulative effect.
• A neuromuscular junction is a special type of synapse between a neurone and a muscle. The neurotransmitter will always be acetylcholine and the synapse is always excitatory.