Communication and Homeostasis

• What is the CNS


  Brain and spinal cord
• What is the peripheral nervous system?


  Nerves connecting the CNS to the rest of the body.
• Function of Dendrites ?


  receive signals from other cells
• Neurones carry signals from the CNS to effectors. Name 2 types of effectors.


  Glands

  muscles
• Function of Axon ?


  Transmits electrical signal
• Function of the myelin sheath ?


  Insulates neurone to speed up the impulse.
• What are the gaps found in the myelin sheath called and what is their function ?


  nodes of ranvier

  Allow ions to diffuse

  Allow for saltatory conduction ( signal jumps between gaps )
• What is myelin sheath made from ?


  Schwann cells
• Function of axon terminal?


  sends signals to other cells
• Where is the cell body found in a sensory neurone ?


  Dorsal root ganglion
• Where is cell body of an intermediate neurone located ?


  brain or spinal cord
• What is the function of intermediate neurone ?


  Transmit information around the CNS
• Name 2 structural differences between motor and sensory neurones.


  Motor neurones longer axons

  Sensory neurones have dendrons
• name 4 type of receptors

  photoreceptor

  chemoreceptor

  thermoreceptor

  mechanoreceptor
• What is the stimulus for a mechanoreceptor ?


  Pressure and movement
• Give an example of the sense organ and receptor for pressure and movement.


  Skin - pacinian corpuscle
• Transducer


  a device that converts one form of energy to another
• Where would you find pacinian corpuscles in the body ?


  most abundant in fingers, soles of feet and genitals
• What is the function of pacinian corpuscle ?


  Acts as a transducer, converting pressure into a nerve impulse.
• Where are pacinian corpuscle located ?


  the end of sensory neurone axons
• Describe the structure of pacinian corpuscle?


  Layers of membrane (lamellae) around the end of the neurone separated by a viscous gel. Stretch mediated sodium channels are found within the membranes.
• What happens when pressure is applied to the pacinian corpuscle?


  layers are distorted and the channels open allowing sodium

  ions to move down the electrochemical gradient and enter the axon.
• What happens when the sodium ions enter the axon of the sensory neurone ?


  Influx of sodium ions changes the potential of the membrane (depolarised) producing a generator potential. This causing an action potential that passes along the neurone.
• What channels are closed during resting potential ?


  Potassium and sodium voltage gated/stretch mediated
• How is resting potential maintained ?

  Sodium potassium pump pumps 3Na ions out and 2K ions into the cell
• What is the charge inside the cell at resting potential

  -70mV
• Describe what happens during depolarisation of a cell ?


  stimulus causes stretch mediated channels to open.

  Sodium ions diffuse into the cell, depolarising it.
• What happens if threshold is reached inside the neurone cell?


  The voltage gated sodium channels open and the cell reaches +40mV.
• What is the threshold level voltage ?


  -55mV
• What happens during re-polarisation ?

  Voltage gated sodium channels close and the voltage-gated potassium channels open.

  K ions diffuse out of cell restoring negative charge

  Cell experiences hyperpolarisation.
• What is hyper-polarisation ?


  The cell becomes more negative than resting potential.
• Why does the neurone cell experience hyper-polarisation?


  The potassium voltage gated channels are too slow to close.
• Describe what happens during refractory period ?


  Sodium and potassium channels are closed

  Hyperpolarisation

  The Na/K pump continues to work

  Resting potential is restored.
• What is the benefit of refractory period ?


  Ensures the reaction/impulse only flows one way.
• How does an impulse travel along the axon ?


  Action potential propagation.
• What does the action potentials create ?


  local circuit
• What are the benefits of myelination ?

  Faster conduction, as it results in longer local circuits. This means less circuits are required to reach axon terminal.
• On a myelinated neurone, why are there longer circuits?


  Action potentials only occur at node of Ranvier, due to the myelin blocking the channels . Action potential only occur when channels are present.
• How is the strength of the stimulus communicated to CNS?


  Frequency of action potentials and number of neurones involved.
• What changes the size of action potentials ?


  Nothing, they are always the same size.