Neurons and synaptic transmission

What are neurons?

Neurons are cells specialised to carry neural information throughout the body.

What are the 3 types of neurons?

Relay, motor, sensory

What are sensory neurons?

Carry nerve impulses from sensory reception to the spinal cord and brain,

Sensory receptors are found in eyes, ears, tongue etc.

How do sensory neurons carry nerve impulses from sensory receptors to the spinal cord and brain?

Convert nerve impulses from sensory receptors into neural impulses.

When these impulses reach the brain they are translated into sensation (e.g. visual input, heat, pain etc) so that the organism can reach appropriately.

Does all sensory information travel as far as the brain?

Some neurons terminate in the spinal cord, allowing reflex actions to occur quickly without the delay of sending impulses to the brain.

What are relay neurons, and where are the found?

Relay neurons allow sensory and motor neurons to communicate.

They lie wholly in within the brain and spinal cord.

Do not have a myelin sheath

What are motor neurons and where are they found?

Conduct signals from the Central nervous system to effector organs such as muscles.

Their bodies may be in the central nervous system but they have long axons which form part of the peripheral nervous system.

Form synapses with muscles and control their contractions.

How do motor neurons control contractions?

Forms … with …

What happens when its stimulated?

What does the strength depend on?

How does a muscle relax?

Form synapses with muscles.

When stimulated the axon of a motor neurons allow sensory fires neurotransmitters, these bind to receptors on the muscles, that trigger a response and leads to muscle movements in the muscle it has formed a synapse with.

The strength of the contraction depends on the rate of firing of the axons of the motor neurons that control it.

Muscle relaxation is caused by the inhibition of the motor neuron.

Cell body

Contains the nucleus and is responsible for the metabolic activities of the neuron.

Axon

The long, thread like part of the neuron that carries electrical impulses away from the body.

Nodes of Ranvier

Gaps in the myelin sheath that facilitate faster transmission of nerve impulses through saltatory conduction.

Myelin sheath

A fatty layer that insulates the axon and speeds up the transmission of nerve impulses.

Dendrites

Branch like structures that receive electrical signals from other neurons.

Explain the process of synaptic transmission

1. Synaptic transmission is the process in which a nerve impulses (action potential) passes across the synaptic gap from the presynaptic neuron to the post synaptic neuron.

2. Information is passed down the axon of the presynaptic neuron as an electrical impulse known as an action potential.

3. Once the action potential reaches the presynaptic terminal, it triggers synaptic vesicles to move towards the presynaptic membrane.

4. Once the synaptic vesicles have fused with the pre synaptic membrane (exocytosis), they release neurotransmitters into the synaptic cleft.

5. These neurotransmitters diffuse across the synaptic cleft, towards the post synaptic membrane on the end of dendrites of the receiving neuron.

6. The neurotransmitters with then bind to the receptors found on the post synaptic cell of this neuron. - These neurotransmitters can be inhibitory or excitatory.

Why and how are neurotransmitters removed from the synapse?

Neurotransmitters unbind from receptors naturally and to prevent overstimulation they are removed in 3 ways;

1. Reuptake

2. Enzymatic breakdown

3. Diffusion away

Outline the process of re uptake

(What does it mean if reuptake is slow/fast)

The neurotransmitter is absorbed back into the pre synaptic neuron through reuptake transporters.

If reuptake is fast - The neurotransmitter spends less time in the synaptic cleft - less time to bind to recpetors - short lived signal.

If re uptake is slow - neurotransmitters remain the synaptic cleft for longer - bind multiple times - longer lasting effects on the post synaptic neuron.

Enzymatic breakdown

Enzymes in the synaptic cleft break down the neurotransmitter, making it ineffective (turns off)

Diffusion away

Neurotransmitters drift away from the synaptic cleft and are eventually broken down elsewhere.

What are exitatory neurotransmitters?

How do they have their effect?

What’s an EPSP?

Increase the likelihood that the post synaptic neuron will fire an action potential

Make the neurons charge more positive (depolarisation), bringing it close to the threshold needed to trigger an action potential.

An excitatory neurotransmitter binding with a post synaptic receptor, causes a change in the membrane of that cell, resulting in an excitatory post synaptic potential (EPSP)

What’s an inhibitory neurotransmitter?

How do they do this?

What’s an IPSP?

Decrease the likelihood of that the post synaptic neuron will fire an action potential.

They do this by making the neurons charge more negative (hyper-polarisation), making it harder for the neuron to reach the threshold needed to fire.

An inhibitory neurotransmitter binding with a postsynaptic receptor results in an inhibitory post synaptic potential (IPSP), making it less likely that the cell will fire.

What’s summation?

Since neurons receive multiple excitatory and inhibitory signals at the same time, the overall effect on the neuron depends on summation.

If excitatory signal > Inhibitory → neuron reaches threshold and fires an action potential.

If excitatory signal < inhibitory → neuron doesn’t reach threshold, becomes inhibited and doesn’t fire.

What are the 2 types of summation?

Spatial and temporal

What is spatial summation?

Multiple pre synaptic neurons release neurotransmitters at the same time onto one post synaptic neuron.

If the combined effect reaches the threshold, the post synaptic neuron fires.

What is temporal summation?

One synaptic neuron releases many neurotransmitters in a short time.

If the signal adds up quickly enough to reach the threshold, the post synaptic neuron fires.

Examples of inhibitory neurotransmitters.

Serotonin GABA

Examples of excitatory neurotransmitters

Acetylcholine, noradrenaline