Neurons & synaptic transmission

• Neurons

  Neurons are (nerve cells) in the nervous system. 80% are located in the brain and they transmit signals electrically and chemically.

  They underpin and provide the nervous system with means of communication.

  There are approx 100 billion of then in the human body
• Types of neurons

  There are three types of neurons

  Sensory neurons: carry message from PNS to the spinal cord and brain (CNS)

  Relay neurons: connect sensory neurons to motor neurons

  Motor neurons: carry signals from the CNS to muscles or glands

  Sensory = Long dendrites, Short axon (LS)

  Relay neurons = Short dendrite, Short axon (SS)

  Motor neurons= Short dendrites, Long axons (SL)
• Motor neurons - Carries messages (AWAY from the brain) from the CNS to effectors such as muscles and glands
• Sensory neurons - Carries messages from the PNS to the brain and spinal cord
• Relay neurons - Transfers messages from sensory neurons to other interconnecting neurons or motor neurons
• The reflex arc

  In a reflex arc, like the knee-jerk reflex, a stimulus, such as a hammer hitting the knee, is detected by sense organs in the peripheral nervous system, which conveys a message along a sensory neuron. The message reaches the central nervous system where it connects with a relay neuron. This then transfers the message to a motor neuron. This then carries the message to an effector such as a muscle, which causes the muscle to contract and, hence, the knee to move or jerk.
• Electrical transmission

  Signals within neurons are communicated electrically 

  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, 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
• Synaptic transmission - Process

  1. When a neuron is activated by a stimulus, an electrical signal called an

  action potential is sent down the end of the presynaptic neuron, arriving at the terminal button.

  2. Here at the terminal button, there are tiny sacs called vesicles which contain chemical messengers called neurotransmitters. The electrical signal causes the vesicles to release the neurotransmitters.
• Synaptic transmission - Process

  3. The neurotransmitters then travel across the synaptic cleft (this is the gap between the two neurons) to the next neuron in the chain (the postsynaptic neuron). 

  4. Once the neurotransmitter crosses the synaptic cleft, it attaches to the postsynaptic neuron receptor sites. These are located on the dendrites of the postsynaptic neuron. 
• Synaptic transmission - Process

  5. At the receptor sites on the postsynaptic neuron the chemical message is turned back into an electrical impulse which sets off again down the neuron.

  6. The leftover neurotransmitters that are left in the synaptic cleft (gap) are broken down by enzymes and reabsorbed by the presynaptic neuron, in a process called reuptake – kind of like a recycling programme!
• Electrical charge and summation

  The chemical message turning back into an electrical impulse in the postsynaptic neuron is dependent whether the neurotransmitters are excitatory and inhibitory.

  Some NRTs are excitatory such as noradrenaline, these increase the positive charge of the postsynaptic neuron, making it more likely to fire. 

  Other NRTs are inhibitory such as serotonin, these increase the negative charge of the next neuron, making it less likely to fire.
• Summation (basically maths!)

  Summation is the total accumulation of charges in a cell. 

  If there are more excitatory NRTs at the postsynaptic neuron than inhibitory NRTs, the total charge of the cell will increase past the AP threshold and will fire. 

  If there are more inhibitory NRTs than excitatory NRTs then the cell cannot fire as it won’t meet the threshold
• Adrenaline - Released by the adrenal medulla in the adrenal glands on the kidneys, involved in the regulation of fight or flight

  Acetylcholine - Generally found where a motor neuron meets a muscle, causing muscle to contract

  Dopamine - equally likely to be either inhibitory or excitatory dependent on the location and receptor
•  

  Inhibitory neurotransmitters

  Serotonin - Affects mood and social behaviour which is why it has been implicated as a cause of depression.

  GABA (Gamma-aminobutyric acid) -  Known for producing a calming effect. It's thought to play a major role in controlling anxiety, stress and fear.

  Dopamine - equally likely to be either inhibitory or excitatory dependent on the location and receptor