Neuronal Communication

• Why do multicellular organisms need communication systems?


  (1) They have differentiated cells and organs

  (2) To coordinate activities of different organs

  (3) To respond to changes in the environment

  (4) To esnure conditions are optimal for survival

  
• What type of changes do animals and plants need to respond to in the internal environment?


  Blood glucose concentration

  Internal temperature

  Water potential

  Cell pH
• What type of changes do animals and plants need to respond to in the external environment?


  Humidity

  External temperature

  Light intensity

  New/sudden sounds
• Cell signalling


  Cells communicate with each other using signal molecules. Some signals take place inside cells, while other signals carry messages from oen cell to another.

  Messenger molecules often bind to receptors that have complementary shape to the messenger molecule.

  
• What is a sensory receptor?


  Specialised nerve cells that respond to a stimulus in the internal/external environment of an organism. They can create action potentials (and most are energy transducers).

  
• What 3 distinct parts do neurones have?


  Cell body

  Axon

  Dendrite
• What is a transducer?


  Converts one form of energy into another
• Type of sensory receptor: mechanoreceptor


  Stimulus: Pressure and movement

  Example of receptor: Pacinian corpuscle (detects pressure)

  Example of sense organ: Skin
• Type of sensory receptor: chemoreceptor


  Stimulus: Chemicals

  Example of receptor: Olfactory receptor (detects smells)

  Example of sense organ: Nose
• Type of sensory receptor: thermoreceptor


  Stimulus: Heat

  Example of receptor: end-bulbs of Krause

  Example of sense organ: Tongue
• Type of sensory receptor: photoreceptors


  Stimulus: Light

  Example of receptor: Cone cell (detects different light wavelengths)

  Example of sense organ: Eye
• What is a pacinian corpuscle?


  A specific sensory receptor in the skin that detects mechanical pressure.

  The corpuscle is an oval-shaped structure

  
• How does the Pacinian corpuscle work


  1) At resting potential, the stretch-mediated sodium ion channels in the sensory neurone's membrane are too narrow to allow sodium ions to pass through them

  2) When pressure is applied to the Pacinian corpuscle, the corpuscle changes shape. This causes the membrane surrounding its neurone to stretch

  3) When the membrane stretches, the sodium ion channels present widen. Sodium ions can now diffuse into the neurone

  4) The influx of positive sodium ions changes the potential of the membrane - it becomes depolarised. This results in generator potential.

  5) In turn, the generator potential creates an action potential that passes along the sensory neurone. The action potential will then be transmitted along neurones to the CNS.

  
• Cell body


  The largest part of a neurone - it contains the nucleus, cytoplasm with large amounts of RER and mitochondria

  
• Dendrites


  Thin strands of cytoplasm that receive impulses and conduct them towards the cell body

  
• Axon


  Longer cytoplasmic extension that carries nerve impulses away from the cell body

  
• Myelin sheath


  A layer of fatty tissue that segmentally encases the axons of some neurons (provides electrical insulation). It allows impulses to transmit quickly and efficiently along the nerve cells.

  
• Nodes of ranvier


  Small, unmyelinated gaps along the axon where there are spaces between the Schwann cells.

  The gaps allow saltatory conduction (where action potentials 'jump' from one node to the next)

  
• Give adaptations of neurons


  Maintains potential difference across membrane

  Plasma membrane with many gated ion channel proteins

  Plasma membrane with many sodium/potassium ion pumps
• What is the structure of a sensory neurone?


  It has long dendrites and short axons

  
• What is the function of sensory neurones?


  Transmits impulses from sensory receptors to relay neurones in the CNS

  
• What is the structure of a relay neurone?


  It has many short dendrites and axons. They have their cell bodies within the CNS

  
• What is the function of relay neurones?


  Connects motor and sensory neurones.

  
• What is the structure of a motor neurone?


  They have short dendrites and long (normally one) axons

  
• What is the function of motor neurones?


  Transmits impulses from the CNS out to effectors (muscles/glands)

  
• How does the nervous response pathway follow?


  Receptor --> sensory neurone --> relay neurone --> motor neurone --> effector cell
• Difference in structure between a myelinated and unmyelinated neurone


  Myelinated neurones have many layers of plasma membrane. Schwann cells produce these layers of membrane by growing around the axon many times.

  Whereas non-myelinated neurones are also associated with Schwann cells, but several neurones may be enshrouded in one loosely wrapped Schwann cell
• Difference in function between a myelinated and unmyelinated neurone


  Myelinated neurone --> Can transmit an action potential much more quickly than non-myelinated neurones can

  Unmyelinated neurone --> Action potentials move along the neurone in a wave rather than jumping from node to node as seen in a myelinated neurone. They are carried over a short distance and transmission is much slower
• Define resting potential


  The charge difference across the membrane when a neuron is not being fired (-70mV)

  
• Define action potential


  A brief reversal of the potential across the membrane of a neurone causing a peak of +40mV

  
• Define depolarisation


  The process by which the membrane potential of a neurone becomes less negative, leading to an action potential

  
• Define repolarisation


  The process by which the membrane potential of a neurone returns to its resting state after an action potential

  
• Define hyperpolarisation


  A change in a cell membrane's potential that makes it more negative. A hyperpolarized neurone is less likely to induce an action potential

  
• How is a resting potential established and maintained?

  The resting potential is maintained by a sodium-potassium pump (specific intrinsic protein).

  - Sodium ions (Na+) are actively transported out of the axon, whereas potassium ions (K+) are actively transported into the axon by the sodium-potassium pump. For every 3 sodium ions that are pumped out, 2 potassium ions are pumped in

  - As a result, there are more sodium ions outside the membrane than inside the axon cytoplasm, whereas there are more potassium ions inside the cytoplasm than outside the axon. Therefore, sodium ions diffuse back into the axon down its electrochemical gradient

  - However, most of the 'gated' sodium ion channels are closed (preventing the movement of sodium ions), whereas many potassium ion channels are open, therefore allowing potassium ions to diffuse out of the axon. Therefore, there are more positively charged ions outside the axon than inside the cell.

  - This creates a resting potential across the membrane of -70mV

  
• Explain how an action potential is generated


  At resting potential, the membrane is at rest and polarised at around -70 mV.

  - A stimulus triggers voltage-gated Na+ channels open, so more Na+ diffuse into the axon (down the electrochemical gradient) making the inside less negative - this is an example of positive feedback

  - When the potential difference across the membrane reaches a threshold of around -55mV, depolarisation is triggered and more Na+ channels open causing an influx of Na+

  - After the potential difference has reached +40mV, Na+ channels close and K+ channels open, so K+ diffuse out of the axon (down the electrochemical gradient) and the membrane starts repolarising (known as negative feedback). This returns potential difference to normal -70mV

  - Potassium ion channels are slow to close and as a result too many potassium ions diffuse out of the neurone causing a short period of hyperpolarisation. The axon becomes more negative than its normal resting state - this is known as hyperpolarisation.

  - The voltage-gated potassium ion channels now close. The sodium-potassium pump causes sodium ions to move out of the cell, and potassium ions to move in. The axon returns to its resting potential (refractory period).

  
• What is the all-or-nothing principle?


  -> Once the threshold potential is reached, an action potential is always triggered, regardless of the strength of the stimulus

  -> Without reaching the threshold potential, no action potential is initiated.

  -> Action potentials are always the same size - A stronger stimulus doesn't increase the size of the action potential, but it does increase the frequency of action potentials generated.
• What are the factors affecting speed of transmission?

  Myelination --> Enables saltatory conduction and so myelinated neurones transmit impulses faster than non-myelinated neurones

  Axon diameter --> A large axon diameter means that there is less resistance to ion flow, so the wave of depolarisation travels faster along the axon. Broader axons transmit impulses faster

  Temperature --> High temperatures accelerate the diffusion of ions, leading to faster depolarisation and impulse transmission. However, temperatures above 40°C can cause proteins to denature
• Explain the propagation of action potentials in a myelinated neurone


  1) The opening of Na+ channels results in local depolarisation, allowing positive ions to spread sideways

  2) Adjacent voltage-gated Na+ channels open in response to this change and this action leads to depolarisation of nearby membrane areas

  3) As each patch of membrane activates the next, an advancing wave is formed

  4) Areas of the membrane that have just experienced depolarisation are in the refractory period and remain unresponsive while they repolarise (K+ exits the axon and Na+ channels are closed)

  5) This ensures that the wave moves in one direction, preventing the backward flow of the nerve impulse.

  
  

  Once triggered, an action potential self-propagates through local currents along the axon without any decrease in size.

  
• Why does the action potential not reverse direction?


  The concentration of sodium ions behind the action potential is still high
• What is a synapse?


  A junction between 2 neurones