The Nervous System

The Nervous SystemThe NeuronTwo main cell types in the nervous system -> Neurons and Glial CellsNeurons transmit nerve messagesGlial Cells surround Neurons and give rise to myelin 100 million Neurons in the Human brain alone

The NeuronThe Neuron consists of three parts: Dendrites, Soma and the AxonDendrites receive information and transfer it to the somaThe Soma (or Cell Body) contains nucleus, mitochondria and other organellesThe Axon carries the message away from the soma

The NeuronThree main types of neurons in the Human body: Sensory neurons, Motor neurons, and Interneurons.Sensory Neurons carry messages to the central nervous systemMotor Neurons transmit messages from central nervous system to muscles and glandsInterneurons are only found in the central nervous system and connect neuron to neuron

The Leaky Axon

Axons contain many ion channels used to transmit an electrical signalThese work to keep internal [K+] high and [Na+] low.MyelinationThe majority of human neurons are encased in an electrically insulating material known as myelin (derived from glial cells)Myelination greatly increases the conduction velocity of a neuron by covering many ion channels, allowing the current to jump from gap to gap.This jumping process is known as Saltatory Conduction Myelination cntd...

The unmyelinated sections are known as the Nodes of RanvierThese Nodes contain a high [ ] of ion channels, allowing for a massive influx of positive current.This works to spread the signal down the neuronHow DOES This Signal Travel?At rest, the intercellular space of a neuron has a negative chargeA neuron can be stimulated to conduct a signal when it receives enough neurotransmitter from its neighbours. It responds by systematically opening ion channels, allowing an influx of Na+ ions which serves to depolarize the cell. Ion ChannelsStimulated

At rest

InactivatedThe Action Potential (AP)

AP cntd...After an AP, the cell now contains more [Na+] than [K+]. A pump known as Na/K/ATP-ase works to remove Na+ and bring in K+.As seen above, the Na+ channel is briefly inactivated, meaning the neuron cannot undergo another AP until this resting period is over

AP cntd...When this signal reaches the axon terminal, it stimulates the release of neurotransmitter molecules into the synaptic cleft.These neurotransmitters are taken up by the next neuron and the process repeats itself.

AP cntd...Once a neurotransmitter is taken up by the post-synaptic neuron, it stimulates that cells Na+ channels to open, leading to subsequent depolarization, providing that enough neurotransmitter is received.This last part is very important; stay tuned.SynapseThis is the connection between the nerve cellsThe space between is called the synaptic cleftMessages that travel inside the neuron are electrical action potential, and neurotransmitters are required to cross the synaptic cleft

NOTE: The neuron releasing neurotransmitter is referred to as pre-synaptic while the neuron taking up neurotransmitter is post-synapticNeurotransmittersVery small molecules, sometimes hormonesCross the synaptic cleft and join the receptor molecules on the next cellUsed neurotransmitters are taken back into the axon and recycled by the cell

All Or Nothing Principle

Neurons are highly branched at their terminus. One AP can thus release neurotransmitter at many synapses.In turn, one dendrite may receive input from many axons. As such, a single burst of neurotransmitter is often not enough to stimulate the post-synaptic neuron to AP.All Or Nothing cntd...If enough neurotransmitter is received, then sufficient Na+ channels will open, allowing the cell to depolarize past its threshold level and generate its own AP.If this threshold level is not met, then the neuron will NOT generate an APOnce this threshold is crossed, the neuron will ALWAYS generate an AP

All Or Nothing cntd...The post-synaptic neuron can receive enough input in one of two ways:Temporal Summation The pre-synaptic neuron generates enough APs in a given time span that they summate (ie. add together) and stimulate the post-synaptic neuron.Spatial Summation The post-synaptic neuron receives input from multiple cells which push it past the threshold, generating an AP.All Or Nothing cntd...