Neurons and synapses

• sensory neurones carry impulses from sensory receptors to the brain and spinal cord.
• Sensory neurons are located in receptors such as the eyes, ears, skin and tongue.
• Sensory neurons have:

  • A cell body that branches off in the middle of the cell
  • A single long dendron that carries impulses to the cell body and a single long axon that carries impulses away from the cell body
• Relay neurons allow sensory and motor neurons to communicate with each other. They are entirely located within the brain and spinal cord.
• Relay neurons are short but have very highly branched axons and dendrites.
• Motor neurons conduct impulses from the CNS to effectors. They form synapses with muscles, releasing neurotransmitters which bind with receptors on the muscle, eliciting a response.
• The cell body of motor neurons lies in the CNS, but they have long axons which form the PNS. It has many highly branched dendrites extending from the cell body, providing a large surface area for the axon terminals of other neurones.
• An impulse travelling down the length of an axon is a form of electrical signal called an action potential.
• 1. an action potential reaches the axon terminal and needs to be transmitted.
  2. to do this, the action potential causes synaptic vesicles containing a neurotransmitter to be released from the presynaptic membrane by exocytosis.
  3. the neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post synaptic membrane.
  4. this either results in an excitatory or an inhibitory effect.
• The effects of a synaptic transmission are terminated in the process of re-uptake. The neurotransmitter is reabsorbed into the axon through the presynaptic membrane. Re-uptake is inhibited by SSRIs.
• An example of an excitatory neurotransmitter is acetylcholine, which increase the likelihood that an excitatory signal is sent to the postsynaptic cell. When bound to a receptor, there is an electrical change in the cell membrane, resulting in an EPSP, making the cell more likely to fire.
• Inhibitory neurotransmitters like GABA and serotonin decrease the likelihood of a post synaptic cell firing. They bind with receptors on a post synaptic cell, generating an IPSP.
• Nerves can receive IPSPs and EPSPs simultaneously. What decides the type of impulse that is transmitted is summation - the net result of adding the excitatory and inhibitory inputs.
• Spatial and temporal summation can increase the strength of EPSPs.