Neurons and synaptic transmission

What are neurons?

Specialised cells with the function of moving electrical impulses to and from the central nervous system.

How many neurons does the average human brain contain?

100 billion.

And on average, each neuron is connected to 1000 other neurons.

What are the three types of neurons?

Sensory, relay and motor neurons.

What do neurons typically consist of?

A cell body, dendrites and an axon.

Picture of dendrites

Where are dendrites located on neurons?

At the end of neurons, connected to the cell body.

What is the function of dendrites?

Dendrites on one end of the neuron receive signals from other neurons.

What is the function of the cell body?

It is the control centre of the neuron.

Picture of cell body

What happens to the impulse after being transferred to the cell body?

From the cell body, its impulse is carried along the axon, where it terminates at the axon terminal.

What is the name of the insulating layer that forms around the axon?

The myelin sheath.

Picture of myelin sheath

What is the function of the myelin sheath?

It allows nerve impulses to transmit more rapidly along the axon.

What happens if the myelin sheath is damaged?

Impulses slow down.

How can the length of a neuron vary?

From a few millimetres up to one metre.

What is the action potential?

An electrical signal.

Describe the full process of action potential.

Dendrites of neurons receive information from sensory receptors or other neurons.

This information is then passed down to the cell body and onto the axon.

Once at the axon, the information travels down its length in the form of an action potential.

What is the function of sensory neurons?

They carry nerve impulses from sensory receptors (eg vision, taste) to the spinal cord and the brain.

Where are sensory receptors found in the body?

Various locations, including the eyes, ears and skin.

How does the process of sensory neurons work?

Sensory neurons convert information from the sensory receptors into neural impulses.

When these impulses reach the brain , they are translated into different sensations (eg heat, pain) so that the organism can react appropriately.

Does all sensory information travel as far as the brain?

No.

Some neurons terminate in the spinal cord, allowing reflex actions to occur quickly.

Why does this allow reflex actions to occur quickly?

As there isn’t a delay of sending impulses to the brain.

What is the function of relay neurons?

They allow sensory and motor neurons to communicate with each other.

Where are relay neurons located?

Within the brain and spinal cord.

What is the function of motor neurons?

They conduct signals from the CNS to effector organs (eg muscles).

What is their other function?

To form synapses with muscles and control their contractions.

What is the process of how motor neurons do this?

When stimulated, the motor neuron releases neurotransmitters that bind to receptors on the muscle and triggers a response, leading to muscle movement.

When the axon of a motor neuron fires, the muscle it has formed synapses with contracts.

What does the strength of a muscle contraction depend on?

The rate of firing of the axons of motor neurons that control it.

How is muscle relaxation caused?

By inhibition of the motor neuron.

Where are motor neurons located?

Their cell bodies are in the CNS but they have long axons that form part of the PNS.

Synaptic transmission: synapse definition

The conjunction (gap) at the end of the axon of one neuron and the dendrite/cell body of another.

Why does synaptic transmission happen?

Because once an action potential arrives at the terminal button at the end of an axon, it needs to be transferred to another neuron or tissue.

How does the action potential achieve this?

It crosses the (synaptic) gap between the presynaptic and postsynaptic neuron

What does the synapse include?

The end of the presynaptic neuron, the membrane of the postsynaptic neuron and the gap inbetween.

What are synaptic vessels?

sacs located at the end of the axon of a nerve cell.

What do synaptic vesicles contain?

The chemical messengers that assist in the transfer of the impulse - the neurotransmitters.

What happens as the action potential reaches the synaptic vesicles?

It causes them release their contents.

This process is called exocytosis.

What happens to the released neurotransmitter after exocytosis?

It diffuses across the synaptic gap, where it binds to specialised receptors on the surface of the cell that are activated by that particular neurotransmitter.

What happens once the receptors have been activated?

The receptor molecules produce either excitatory or inhibitory effects on the postsynaptic neuron.

Diagram of synaptic transmission.

Explain what is mwnat by of synaptic re-uptake.

The neurotransmitter is taken up again by the presynaptic neuron, where it is stored and made available for later use.

What factor determines how prolonged the neurotransmitter’s effects will be?

How quickly the presynaptic neuron takes back the neurotransmitter from the synaptic cleft.

So the quicker it’s taken back, the shorter the effects on the postsynaptic neuron.

How do some antidepressants prolong the action of the neurotransmitter?

They inhibit the re-uptake process, leaving the neurotransmitter in the synapse for longer.

How can enzymes made by the body effect neurotransmitters?

They can ‘turn off’ the neurotransmitters after they have stimulated the postsynaptic neuron.

This makes the neurotransmitters ineffective.

Neurotransmitters: definition of neurotransmitters

Chemical messengers that carry out signals across the synaptic gap towards the receptor site on the postsynaptic cell.

What are excitatory neurotransmitters?

The nervous system’s ’on switches’. They increase likelihood that an excitatory signal is sent out to the postsynaptic cell, which is then more likely to fire.

Give two examples of excitatory neurotransmitters.

Dopamine, noradrenaline

What are inhibitory neurotransmitters?

The nervous system’s ‘off switches’, they decrease likelihood of that neuron firing.

They are responsible for calming the mind and body.

Give two examples of inhibitory neurotransmitters.

Serotonin, GABA

What does an excitatory neurotransmitter binding with a postsynaptic receptor cause?

An electrical change on the membrane of that cell, resulting in an excitatory post-synaptic potential (EPSP).

This means that the postsynaptic cell is more likely to fire.

What does an inhibitory neurotransmitter binding with a postsynaptic receptor cause?

An inhibitory postsynaptic potential (IPSP), making it less likely that the cell will fire.

Diagram of the likelihood of cells firing diagram.

How is the likelihood of EPSP or IPSP cells firing determined?

Because a nerve cell can receive both EPSPs and IPSPs at the same time, the likelihood of the cell firing is determined by adding up the excitatory and inhibitory synaptic input.

What does the net result of this summation (calculation) determine?

Whether or not the cell fires.

How can the strength of an EPSP be increased? (2 ways)

Spatial summation - a large number of EPSPs are generated at many different synapses on the same post synaptic neuron at the same time.

Temporal summation - a large number of EPSPs are generated at the same synapse by a series of high-frequency action potentials on the presynaptic neuron.

How is the rate at which a particular cell fires determined?

By what goes on in the synapses.

If excitatory synapses are more active, the cell fires at a high rate. If inhibitory synapses are more active, the cell fires at a much lower rate.