Neurons And Synaptic Transmission

What is a neuron?

The basic building blocks of the nervous system, neurons are nerve cells that process and transmit messages through electrical and chemical signals.

What do neurons provide the nervous system with?

Its primary means of communicaton.

What is the structure of neurons?

Neurons vary in size from less than a millimetre up to a metre long, but all share the same basic structure.

What are the components of a neuron?

• Dendrites.

• Cell body/soma.

• Axon

What is the role of dendrites?

They protrude from the cell body, and carry nerve impulses from the neighbouring neurons towards the cell body.

What is the cell body?

It includes a nucleus, which contains the genetic material of the cell.

What is the role of the axon?

The axon carries impulses away from the cell body down the length of the neuron.

What is the axon covered in?

A fatty layer of myelin sheath that protects the axon and speeds up the electrical transmission of the impulse. If the myelin sheath was continuous, it would have the reverse effect and slow down the electrical impulse.

What are the myelin sheath segmented by?

Gaps called nodes of ranvier.

What do the nodes of ranvier do?

Speed up the transmission of the impulse by forcing it to ‘jump‘ across the gaps along the axon.

What is at the end of the axon?

Terminal buttons that communicate with the next neuron in the chain across a gap known as the synapse.

What direction do electrical signals travel in neurons?

From the dendrites towards the end of the axon.

What are the three types of neurons?

• Sensory neurons.

• Relay neurons.

• Motor neurons.

What are sensory neurons?

They carry messages from the PNS to the CNS.

What is the structure of sensory neurons?

They have long dendrites and short axons.

Where are sensory neurons located?

The cell bodies of motor neurons may be in the central nervous system but they have long axons which form part of the peripheral nervous system.

What are relay neurons?

These connect the sensory neurons to the motor or other relay neurons.

What is the structure of relay neurons?

They have short dendrites and short axons.

Where are relay neurons located?

Relay neurons make up 97% of all neurons and most are found within the brain and the visual system.

What are motor neurons?

These connect the CNS to effectors such as muscles and glands.

What is the structure of motor neurons?

They have short dendrites and long axons.

Where are motor neurons located?

Sensory neurons are located outside of the CNS, in the PNS in clusters known as ganglia.

What is the charge of a neuron when it is in a resting state?

The inside of the cell is negatively charged compared to the outside.

What happens to a neuron when it is activated by a stimulus?

The inside of the cell becomes positively charged for a split second causing an action potential to occur, creating an electrical impulse that travels down the axon towards the end of the neuron.

How do neurons communicate with each other?

Within groups known a neural networks.

What is synaptic transmission?

The process by which neighbouring neurons communicate with each other by sending chemical messages across the gap (synapse) that separates them

How are neurons separated?

By an extremely tiny gap called the synapse.

How are signals within neurons transmitted?

Electrically.

How are signals between neurons transmitted?

Chemically.

What happens when the electrical impulse reaches the end of the neuron at the presynaptic terminal?

It triggers the release of neurotransmitters from tiny sacs called synaptic vesicles.

What are neurotransmitters?

Brain chemicals released from the synaptic vesicles that relay signals across the synapse from one neuron to another. They are either excitatory or inhibitory.

What do neurotransmitters do?

They diffuse across the synapse to the next neuron in the chain.

What happens once neurotransmitter diffuses across the synapse?

It is taken up by a postsynaptic receptor site on the dendrites of the next neuron. Here, the chemical message is converted back into an electrical impulse and the process of transmission begins again in this other neuron.

What is the direction of travel for neurotransmitters?

One way: from the presynaptic neuron to the postsynaptic neuron.

Why is it that neurotransmitters can only travel in one direction?

Neurotransmitters are released from the presynaptic neuron terminal and received by the postsynaptic neuron at receptor cites.

What does each neurotransmitter have?

Its own specific molecular structure that fits perfectly into a postsynaptic receptor site, similar to a lock and key.

What are the two effects that neurotransmitters can have?

Inhibitory and excitatory effects.

What is excitation, and what are examples of excitatory neurotransmitters?

When a neurotransmitter, such as adrenaline or noradrenaline, increases the positive charge of the postsynaptic neuron, increasing the likelihood that the postsynaptic neuron will pass on the electrical impulse.

What is inhibition and what are examples of inhibitory neurotransmitters?

When a neurotransmitter, such as serotonin or GABA, increases the negative charge of the postsynaptic neuron, decreasing the likelihood that the postsynaptic neuron will pass on the electrical impulse.

What is whether a postsynaptic neuron fires decided by?

The process of summation.

What is summation?

When the excitatory and inhibitory influences are summed.

What happens if the net effect on the postsynaptic neuron is inhibitory?

The postsynaptic neuron is less likely to fire

What happens if the net effect on the postsynaptic neuron is excitatory?

The postsynaptic neuron is more likely to fire, as the soma momentarily becomes positively charged.

When is the action potential of the postsynaptic neuron triggered?

Only if the sum of the excitatory and inhibitory signals at any one time reaches the threshold.