The Structures And Functions Of Neurons

• Neurons


  There are 100 billion neurons in the human nervous system, 80% of which are located in the brain. Transmitting signals electrically and chemically, these neurons provide the nervous system with its primary means of communication
• Types of neuron


  Three types of neurons
• Sensory neuron


  Carry messages from PNS to CNS. Has long dendrites and short axons
• Relay neurons


  Connect sensory neurons to motor/other relay neurons. Has short dendrites and axons
• Motor neurons


  Connects CNS to effectors (muscles/glands) . Has short dendrites and long axons
• Structures of a neuron

  vary in size from less than a millimetre to up to a metre long but all share a basic structure
• Cell body (Soma) and dendrites

  Includes a nucleus which contains genetic material of the cell. Has branch-like structures called dendrites that protrude from the cell body carrying nerve impulses from neighbouring neurons towards the cell body
• Axons


  Carry impulses away from the cell body down the length of the neuron; covered in a fatty layer of myelin sheath that protects it and speeds up electrical transmission of the impulse
• Myelin Sheath


  Segmented by gaps called nodes of Ranvier which speed up transmission of the impulse by forcing it to 'jump' across the gaps along the axon
• Terminal buttons at the end of the axon of Myelin Sheath

  Terminal buttons are at the end of the axon that communicate with the next neuron in the chain across a gap called the synapse
• Electrical transmission- firing of a neuron

  When a neuron is in a resting state the inside of the cell is negatively charged compared to the outside. 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
• Electrical impulses


  This creates an electrical impulse that travels down the axon towards the end of the neuron.
• Chemical transmission- synapses


  Neurons communicate with each other within groups known as neural networks and each neuron is separated from the next by a called synapses
• What the synapses include

  Synapses includes the space between them (called the synaptic cleft) as well as the presynaptic terminal and postsynaptic receptor site
• transmissions of neuron


  Signals with the neurons are transmitted electrically; however, signals between neurons are transmitted chemically by synaptic transmission
• Release of neurotransmitters


  When the electrical impulse reaches the end of the neuron (the presynaptic terminal) it triggers the release of neurotransmitters from tiny sacs called synaptic vesicles
• Neurotransmitter


  Chemical that diffuses across the synapse to the next neuron in the chain. Once neurotransmitter crosses the gap, it is taken by the postsynaptic receptor sites (dendrites of the neuron)
• Covertion


  The chemical message is converted back into an electric impulse and the process of transmission begins again in the other neuron
• Types of neurotransmitters


  Several dozen types of neurotransmitters identified in the brain, spinal cord and glands. Each neurotransmitter has its own specific molecular structure that fits perfectly into a postsynaptic receptor site- lock and key
• Excitation and inhibition

  Neurotransmitters have either excitatory or inhibitory effect on neighbouring neuron
• Serotonin


  Serotonin causes inhibition in the receiving neuron resulting the neuron in being negatively charged and less likely to fire
• Adrenaline


  Adrenaline causes excitation of the postsynaptic neuron making it positively charged and more likely to fire
• Imbalanced neurotransmitters


  Neurotransmitters can become imbalanced associated with some mental disorders. Psychoactive drugs aim to regulate neurotransmitter activity (Prozac)
• Summation


  Whether a postsynaptic neuron does fire is decided by the process of summation
• Inhibitory vs Excitatory


  If the net effect on the postsynaptic neuron is inhibitory, then it is less likely to fire and if the net effect is excitatory, it is more likely and momentarily, the inside of the postsynaptic neuron becomes positively charged
• Action potential


  Once electrical impulse is created, it travels down the neuron. Therefore, the action potential of the postsynaptic neuron is only triggered if the sum of the excitatory and inhibitory signals at any time reaches the threshold