Neurons and synaptic transmission

• How many neurons are estimated to be in the nervous system?

  100 billion
• What percentage of neurons are located in the brain?

  80%
• How do neurons help the nervous system communicate?

  By transmitting signals electrically and chemically
• What are the main components of a neuron?

  • Cell body (soma) with a nucleus
  • Dendrites (branch-like structures that protrude from the cell body)
  • Axon
  • Myelin sheath (fatty layer that covers the axon)
  • Nodes of Ranvier (gaps in between the myelin sheath)
  • Terminal buttons (at the end of the axon)
• What is the function of dendrites in a neuron?

  To carry nerve impulses to the cell body from neighboring neurons
• What is the role of the axon in a neuron?

  To carry impulses away from the cell body down the neuron
• What is the function of the myelin sheath?

  It protects the axon and speeds up transmission
• What are nodes of Ranvier and their function?

  They are gaps in the myelin sheath that speed up transmission by allowing impulses to jump across the gaps along the axon
• What are terminal buttons?

  They are at the end of the axon and communicate with the next neuron across the synapse
• What is the function of sensory neurons?

  They carry messages from the peripheral nervous system (PNS) to the central nervous system (CNS)
• What is the structure of sensory neurons?

  They have long dendrites and short axons
• What is the function of relay neurons?

  They connect sensory neurons to motor neurons or other relay neurons
• What is the structure of relay neurons?

  They have short dendrites and short axons
• What is the function of motor neurons?

  They connect the CNS to effectors such as muscles and glands
• What is the structure of motor neurons?

  They have short dendrites and long axons
• What is the charge of a neuron when it is resting?

  The inside of the cell is negatively charged
• What happens when a neuron is activated by a stimulus?

  The inside of the cell becomes positively charged for a split second, causing an action potential to occur
• What does an action potential create?

  An electrical impulse that travels down the axon
• How do neurons communicate within groups?(synaptic transmission)


  They communicate within neural networks
• What is a synapse?

  A gap that separates each neuron
• How are signals transmitted within neurons?

  Signals are transmitted electrically
• How are signals transmitted between neurons?

  Signals are transmitted chemically across synapses
• What happens to neurotransmitters after they are released?

  They diffuse across the synapse + fit into postsynaptic receptor sites (like lock and key) - each neurotransmitter has a specific molecular structure that fits perfectly into the site

  
  

  e.g. Acetycholine (ACh) is found when a motor neuron meets a muscle and it will release causing the muscle to contract
• What effect can neurotransmitters have on the neighbouring neuron?

  They can have either an excitation or inhibition effect
• What does excitation do to the likelihood of a neuron firing?

  It makes it more likely to fire
• How does adrenaline affect the post-synaptic neuron?

  It increases its positive charge, making it more likely to fire
• What does inhibition do to the likelihood of a neuron firing?

  It makes it less likely to fire
• What does the neurotransmitter, serotonin, do in the neuron?

  Causes inhibition, making the neuron negatively charged, less likely to fire
• Label the parts of each neurone.

  diagram:

  A) Receptor cell

  B) myelin sheath

  C) axon

  D) cell body

  E) axon

  F) cell body

  G) dendrite

  H) pre-synaptic terminal

  I) node of Ranvier

  J) myelin

  K) axon

  L) cell body

  
• What is the process of synaptic transmission?


  • Action potential (impulse) travels down axon electrically
  • Action potential reaches presynaptic terminal where neurotransmitters are held in synaptic vesicles (sacs)
  • The electrical impulse then triggers neurotransmitters to be released
  • The neurotransmitters chemically diffuse across the synapse and are received by postsynaptic receptor sites on the dendrites of the next neuron
  • The chemical message is converted back to an electrical impulse in the new neuron where the process begins again
• What is excitation in neuron?


  Makes it more likely that the receiving neuron will fire, increased positive charge
• What does inhibition do?


  Makes the receiving neuron less likely to fire, becomes negatively charged
• What is summation?


  Process that determines whether or not an action potential is triggered, based on the combined effects of excitatory and inhibitory signals of the neurotransmitters
• Why do neurons only transmit info in one direction at a synapse?

  • Synaptic vesicles containing the neurotransmitter are only present on the presynaptic membrane
  • Receptors for the neurotransmitters are only present on the postsynaptic membrane
  • The binding of the neurotransmitter to the receptor enables info to be transmitted
  • Diffusion of neurotransmitters mean they can only go from high to low concentration