Neurons and Synaptic transmission

• There are 100 billion neurons in the nervous system, 80% of which are located in the brain. They transmit messages electrically and chemically, these neurons provide the nervous system with its primary means of communication.
• Sensory Neurons


  Sensory Neurons carry messages from the PNS to the CNS. They have long dendrites and short axons. They are located in the PNS in clusters known as ganglia.

  
• Relay Neurons


  Relay neurons connect the sensory neurons to the motor or other relay neurons. They have short dendrites and short axons. They make up 97% of all neurons and are mostly located in the brain and visual system.

  
• Motor Neuron


  Motor neurons connect the CNS to effectors such as muscles and glands. They have short dendrites and long axons. The cell bodies of motor neurons may be located in the CNS but they have long axons that form part of the PNS.

  
• Neurons vary in size from les than a millimeter to up to a metre long.
• The cell body


  the cell body (or stoma) includes a nucleus which contains the genetic material of the cell. Branch-like structures called dendrites protrude from the cell body, they carry never impulses from neighbouring cells to the cell body.
• The axon


  The axon carries the impulses away from the cell body down the length of the neuron. The axon is covered in a layer of fatty myelin sheath that protects the axon and speeds up electrical transmission of the impulse.
• Nodes of Ranvier


  The myelin sheath is segmented by gaps called the nodes of ravier, these speed up transmission by forcing it to 'jump' across the gaps along the axon.
• Terminal Buttons


  At the end of the axon are terminal buttons, these communicate with the next neuron in the chain across a gap known as the synapse.
• Electrical transmission


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


  Neurons communicate within groups known as neural networks. They are separated by gaps called synapses. Signals within neurons are transmitted electrically, but between neurons, they are transmitted chemically. When the impose reaches the end of the neuron (presynaptic terminal) it triggers the release of neurotransmitters from tiny sacs called synaptic vesicles.
• Neurotransmitters


  Brain chemicals released from synaptic vesicles that relay signals across the synapse from one neuron to another. they are divided into excitatory and inhibitory.
• Neurotransmitter function

  Once a neurotransmitter crosses the gap it is taken up by a postsynaptic receptor site on the dendrites of the next neuron. The chemical message is converted back to an electrical impulse and transmission repeats. This only happens in one direction. Each specific neurotransmitter has its own specific molecular structure that fits perfectly to a postsynaptic receptor site like a lock and key. They have specialist functions, for example, serotonin is a mood stabiliser.
• Excitation


  when a neurotransmitter such as adrenaline increases the positive charge of the post-synaptic neuron. This increases the likelihood that a post-synaptic neuron will pass on the electrical impulse.
• Inhibition


  when a neurotransmitter such as serotonin, increases the negative charge of the postsynaptic neuron. This decreases the likelihood that the postsynaptic neuron will pass on the electrical impulse.
• Summation


  Whether a postsynaptic neuron fires is decided by summation. The excitatory and inhibitory influences are summed, if the net effect on the postsynaptic neuron is inhibitory then it is less likely to fire, if it is excitatory it will more likely fire. One the 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 inhibitory and excitatory signals at any one time reaches the threshold.