Neurons and synaptic transmission

• Neurons are cells that are specialised to carry neural information throughout the body
• Neurons can be three types: sensory neurons, relay neurons or motor neurons
• Neurons typically consist of a cell body, dendrites and an axon. Dendrites at one end of the neuron receive signals from other neurons or from sensory receptors
• Sensory neurons carry nerve impulses from sensory receptors to the spinal cord and the brain
• The term motor refers to neurons located in the CNS that project their axons outside the CNS and directly or indirectly control muscles
• Most neurons are neither sensory nor motor but lie somewhere between the sensory input and the motor output
• Once an action potential has arrived at the terminal button at the end of the axon, it needs to be transferred to another neuron or to tissue which must cross a gap between the presynaptic neuron and the postsynaptic neuron, the membrane of the postsynaptic neuron and the gap in between
• The physical gap between the pre- and postsynaptic cell membrane is known as the synaptic gap
• At the end of the axon of the nerve cell are a number of sacs known as synaptic vesicles
• Vesicles contain the chemical messengers that assist in the transfer of the impulse, the neurotransmitters. As the action potential reaches the synaptic vesicles, it causes them to release their contents through a process called exocytosis
• The released neurotransmitter diffuses across the gap between the pre- and the postsynaptic cell, where it binds to specialised receptors on the surface of the cell that recognise it and are activated by that particular neurotransmitter
• Neurotransmitters can be classified as either excitatory or inhibitory in their action which increase the likelihood that an excitatory signal is sent to the postsynaptic cell, which is then more likely to fire
• Inhibitory neurotransmitters are the nervous systems ‘off switches’, in that they decrease the likelihood of that neuron firing. Inhibitory neurotransmitters are generally responsible for calming the mind and body, inducing sleep, and filtering out unnecessary excitatory signals
• A nerve cell can receive both EPSPs and IPSPs at the same time. The likelihood of the cell firing is therefore determined by adding up the excitatory and the inhibitory synaptic input. The net result of this calculation determines whether or not the cell fires