neurons and synaptic transmission

How many neurons (nerve cells) are there in the human nervous system

100 billion

How many of the neurons in the human nervous system are located in the brain

80%

How do neurons provide the nervous system with its primary means of communication

By transmitting signals electrically and chemically

How many types of neurons are there

3

What are the different types of neurons

Sensory, relay and motor neurons

What is the size of a neuron like

They vary in size from less than a millimetre to up to a metre long

What do all neurons have in common

The same basic structure

What does the cell body include

A nucleus

What does a nucleus contain

The genetic material of the cell

What are dendrites

Branchlike structures that protrude from the cell body

What do dendrites do

Carry nerve impulses from neighbouring neurons towards the cell body

What does the axon do

Carry the impulses away from the cell body down the length of the neuron

What is the axon covered in

A fatty layer of myelin sheath

What does the myelin sheath do

Protects the axon and speeds up electrical transmission of the impulse

What would happen if the myelin sheath was continuous

The role of the myelin sheath would have a reverse effect and slow down the electrical impulse

What are nodes of ranvier

Gaps that segment the myelin sheath

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

Where are terminal buttons located

At the end of the axon

What do terminal buttons do

They communicate with the next neuron in the chain across a gap known as the synapse

Where are the cell bodies of motor neurons located

In the central nervous system

What do axons of motor neurons form

Part of the peripheral nervous system

Where are sensory neurons located

Outside of the central nervous system, in the peripheral nervous system in clusters known as ganglia

What percentage of all neurons do relay neurons make up

97%

Where are most relay neurons found

Within the brain and the visual system

What is electrical transmission

The firing of a neuron

What happens when a neuron is in a resting state

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

What happens 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

What does an action potential create

An electrical impulse that travels down the axon towards the end of the neuron

How do neurons communicate

Within groups known as neural networks.

What is each neuron separated from the next by

An extremely tiny gap called the synapse

How are signals within neurons transmitted

Electrically

How are signals between neurons transmitted

Chemically across the synapse

What happens when the electrical impulse reaches the end of the neuron

It triggers the release of neurotransmitter from tiny sacs called synaptic vesicles

What is the presynaptic terminal

The end of the neuron

What are neurotransmitters

Chemicals that diffuse across the synapse to the next neuron in the chain

What happens to a neurotransmitter once it crosses the synapse

It is taken up by a postsynaptic receptor site on the dendrites of the next neuron

What do axons do

Take signals to the synapse

What do dendrites do

Take signals away from the synapse

What happens at the postsynaptic receptor sites on dendrites of the next neuron

The chemical message is converted back into an electrical impulse and the process of transmission begins again in this other neuron

What direction is the travel of neurotransmitters

One-way

Why is the travel of neurotransmitters one-way

Because neurotransmitters are released from the presynaptic neuron terminal and received by the postsynaptic neuron (at the receptor sites).

Where have several dozen types of neurotransmitters been identified

In the brain (as well as in the spinal cord and some glands).

What is different about each neurotransmitter

Each has its own specific molecular structure that fits perfectly into a postsynaptic receptor site, similar to a lock and key

What's an example of neurotransmitters having specialist functions

Acetylcholine (ACh) is found at each point where a motor neuron meets a muscle, and upon its release, it will cause muscles to contract

What type of functions do neurotransmitters have

Specialist functions

What effect can neurotransmitters have on the neighbouring neuron

An excitatory or inhibitory effect

What effect does serotonin have

It causes inhibition in the receiving neuron, resulting in the neuron becoming more negatively charged and less likely to fire

What effect does adrenaline have

It causes excitation of the postsynaptic neuron by increasing its positive charge and making it more likely to fire

What is adrenaline

An element of the stress response which is both a hormone and a neurotransmitter

What decides whether a postsynaptic neuron fires

A process of summation

How are the excitatory and inhibitory influences summed

If the net effect on the postsynaptic neuron is inhibitory then the postsynaptic neuron is less likely to fire.

What happens if the net effect is excitatory

It is more likely to fire, i.e. the inside of the postsynaptic neuron momentarily becomes positively charged

What happens once the electrical impulse is created

It travels down the neuron

How 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.