Neurons and synaptic transmission

• Neurons are specialised cells that make up the nervous system and communicate with one another through electrical impulses.
• Neurotransmitters

  Chemical messengers that transmit signals between neurons
• Nerves carry messages between different parts of the body and transmit signals to and from the central nervous system (CNS).
• Motor neurons carry messages from the central nervous system to the effectors.
• Briefly outline the process of synaptic transmission (2)

  • Neurotransmitters are released from vesicles into the synaptic cleft.
  • They bind to receptors on the postsynaptic neuron, triggering an action potential.
• The sensory neurons carry information (electrical impulses) towards the brain.
• Relay neurons connect sensory neurons to motor neurons and are only found in the brain, spinal cord or visual system.
• The relay neuron consists of 3 parts: dendrites, axon and the cell body.

  
• The sensory neuron consists of a cell body in the middle of the axon, dendrites, a receptor cell and an axon.

  
• The motor neuron consists of a cell body, dendrites and axon.

  
• Describe the structure and function of a neuron (6)

  Structure

  • consists of a cell body (soma) containing a nucleus with genetic information
  • branch-like dendrites extend from the cell body
  • dendrites can receive information from other neurons
  • axons carry messages (action potential) away from the cell body

  Function

  • enables communication within the nervous system
  • transmit electrical impulses throughout the body
• Explain the process of synaptic transmission (4)

  • electrical impulses reach the axon terminal
  • this triggers the release of neurotransmitters from vesicles in pre-synaptic neuron
  • neurotransmitters diffuse across the synaptic cleft
  • neurotransmitters bind to receptors on the postsynaptic membrane
• Synaptic transmission is unidirectional because receptors for neurotransmitters are found only on the postsynaptic membrane and vesicles containing neurotransmitters are only found in the presynpatic neuron.
• Explain why neurons can only transmit information in one direction at the synapse (3)

  • synaptic vesicles containing the neurotransmitter are only present on the presynaptic neuron
  • receptors for neurotransmitters are only present on the postsynaptic membrane
  • diffusion of neurotransmitters means they can only go from high to low concentration, so can only travel from presynaptic to postsynaptic membrane
• Briefly outline how excitation and inhibition are involved in synaptic transmission (4)

  • neurotransmitters can be excitatory and inhibitory
  • if the neurotransmitter is excitatory then the postsynaptic neuron is more likely to fire an impulse
  • if the neurotransmitter is inhibitory then the postsynaptic neuron is less likely to fire an impulse
  • the excitatory and inhibitory influences are summed, if the net effect on the postsynaptic neuron is inhibitory, the neuron will be less likely to fire if the net effect is excitatory, the neuron will be more likely to fire.
• Excitatory neurotransmitters increase the likelihood that a neuron will fire an action potential (electrical signal).

  • They do this by increasing the positive charge inside the postsynaptic neuron, making it more likely to reach the threshold for firing.
• Inhibitory neurotransmitters decrease the likelihood of a neuron firing an action potential.

  • They do this by increasing the negative charge inside the postsynaptic neuron, making it less likely to reach the threshold for firing.
• Summation is the net effect of both excitatory and inhibitory neurotransmitters on the post-synaptic neuron to determine whether an action potential will be generated.
• Two types of summation are:

  1. Spatial Summation: When multiple presynaptic neurons release neurotransmitters at the same time, their combined effect can either increase (excitation) or decrease (inhibition) the likelihood of the postsynaptic neuron firing.
  2. Temporal Summation: When one presynaptic neuron releases neurotransmitters repeatedly in a short period, the combined effect increases the likelihood of the postsynaptic neuron reaching the threshold to fire.