3.4.3 - Nerve impulses and synaptic transmission

What are nerves?

Nerve- bundle of neurones.

Describe the structure of a mylenated neurone

Neurones have a long fibre known as an axon, some axon's are insulated by a fatty sheath made out of myelin. The gaps between the sections are called the nodes of ranvier. These cells are called schwann cells.

What does it mean if an axon has shwann cells?

Presence of schwann cells means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next. This speeds up the conduction of the impulse and its transfer from one cell to another.

What is this jumping called?

The jumping is called saltatory conduction.

What are the extensions of neurone cell bodies called?

Neurone cell bodies have many extensions called dendrites. This means they connect to many other neurones and receive impulses.

What are the three types of neurones called? What to they connect to?

There are three types of neurones: sensory, relay and motor.

Sensory- receptors to the CNS

Relay- in CNS, connect sensory and motor

Motor- impulses drom CNS to effectors.

Describe the shape of a motor neurone

Motor neurones have a large cell body at one end, a nucleus in the cell body and many highly branched dendrites.

What is resting potential?

In a resting axon, the inside of the axon always has a negative potential compared to the outside- resting potential.

It is about -70mV.

What factors contribute to establishing and maintaining the resting potential?

Two factors contribute to establishing and maintaining the resting potential- the active transport of Na+ and K+ and the differential membrane permeability.

What helps maintain the resting potential?

sodium potassium pump

What do sodium-pottasium pumps do?

Sodium-pottasium pumps use ATP to actively transport 3 Na+ out and 2 K+ in. This results in a larger concentration of positive ions outside the axon than there are inside the axon, establishing an eelctrochemical gradient.

What ions can move through the protien channels and how does this affect resting potential?

The protein channels in the membrane are less permeable to sodium ions than potassium ions, to potassium ions can diffuse out faster than sodium can diffuse in, making the inside of the axon more negative.

What are action potentials caused by?

Action potentials are caused by the rapid movement of sodium ions and potassium ions across the membrane of the axon.

What happens when an action potential is stimulated?

Sodium ion channels open, sodium pass into the axon down the gradient. This reduces the potential difference as the inside of the axon becomes less negative- depolarisation.

What does depolarisation trigger?

Depolarisation triggers more channels to open allowing more sodium ions to enter and case more depolarisation. If the potential difference reaches around -50, sodium volted-gates open and many more Na+ enter causing the inside of the axon to each a potential of around 30 mV. An action potential is generated. The depolarisation of the membrane at the site of the first action potential causes the Na+ to diffuse along the axon, depolarising the membrane in the next section. This is known as condition.

What does condition trigger?

This triggers the production of another action potential in this section of the axon membrane. This allows action potentials to begin at one end and pass along the entire length of the membrane.

What happens after an action potentil happens?

After an action potential, all the sodium ion voltage gated channel proteins close and potassium ion voltage gated channel proteins open, so sodium can't diffuse in but potassium can diffuse out. This returns the potential difference to normal (-70mV), this is known as repolarstion.

What happens during repolarisation?

There is a short period of hyperpolarisation, when the potential difference is briefly more negative than the normal resting potential. This is when the K+ voltage gated channel proteins close and the Na+ ion channel proteins become responsive to depolarisation again.

What happens until the protiens are responsive to depolarisation again?

Until this occurs, the axon membrane is in a period of recovery and is unresponsive. This is known as the refractory period.

Why is the refractory period important?

It is very important and ensures that new action potentials are generated ahead rather than behind, and makes the action potential discrete and unidireactional.This also means there is a minimum time between action potentials occurring at any one place along a neurone

The length of the refractory period is key in determining the maximum frequency at which impulses can be transmitted along neurones (between 500 and 1000 per second)

What happens when the stimulus is weak?

When receptors are stimulated, they are depolarised. If the stimulus is very weak, the receptor cells won't be sufficiently depolarised and the sensory neurone will not be activated to send impulses. If the stimulus is strong enough to increase the receptor potential above the threshold, then the receptor will stimulate the sensory neurone to send impulses.

What factors affect the speed of conduction?

Presence/absence of myelin, diameter of axon and temperature.

How does the presence/absence of myelin affect the rate of conduction?

Presence/absence of myelin- slower in unmylenated as depolarisation must occur along the whole membrane of the axon. In sections of the axon that are surrounded by a myelin sheath, depolarisation can't occur as the myelin sheath stops the diffusion of sodium and potassium ions.

How does the diamter of axon of myelin affect the rate of conduction?

Diameter of axon - greater surface area, which increases the rate of diffusion

How does the temperature affect the rate of conduction?

Temperature - colder conditions can slow down the conduction as there is less kinetic energy available for the facilitated diffusion.

What is the gap between the pre and post synaptic neurone called?

The synaptic cleft separates the pre and post synaptic neurones.

What happens when an impulse arrives on the axon on the presynaptic neurone?

Electrical impulses can't jump across synapses, when an electrical impulse arrives on the axon on the presynaptic neurone, neurotransmitters are released from vesicles at the presynaptic membrane via exocytosis.

What happens next?

The neurotransmitters diffuse down the synaptic cleft and temporarily bind with the receptor molecules, which stimulates the postsynaptic neurone to generate an electrical impulse which travels down the axon of the postsynaptic nrueone. The neurotransmitters are destroyed or recycled to prevent continued stimulation.

What are cholinergic synapses?

Synapses that use acetylcoline as the neurotransmitter are described as cholinergic synapses.

Where is acetylcholine used?

Acetylcholine is a neurotransmitter used throughout the nervous system.

Describe a cholinergic synapse

When an action potential arrives at the presynaptic membrane, the membrane is depolarised, which stimulates the voltage-gates calcium ion channel protiens to open. Calcium ions diffuse down an electrochemical gradient from the tissue fluid surrounding the synapse into the cytoplasm of the presynaptic neurone. This stimulates vesicles with acetylcholine to fuse with the presynaptic membrane, the ACh diffuses along the synaptic cleft to bind to ligand-gated sodium ion channels in the postsynaptic membrane. This causes a conformational change in the receptor proteins, which causes them to open, allowing sodium ions to diffuse down the gradient in the postsynaptic membrane, causing depolarisation of the membrane. The ACh molecules are broken down and recycled by acetylcholinesterase into acetate and choline. The choline is absorbed back and binds to acetyl co enzyme A to form ACh, which is then packaged into presynaptic vesicles.

How is it ensured that synapses are unidirectional?

As the neurotransmitter is on one side and the receptors are on the other, it ensures that impulses are unidirectional.

What are the advantages of summation?

allows for the effect of a stimulus to be magnified, a combination of different stimuli can trigger a response and it avoids the nervous system being overwhelmed by impulses.

What is temporal summation?

several impulses from one neuron over time

What is spatial summation?

summing from several different presynaptic neurons

How are some neurotransmitters inhibitory?

Some neurotransmitters prevent the generation of an action potential, one way is by opening the potassium gates channels, so K+ diffuses out and Na+ enters in due to the impulse. They cancel each other out, so the threshold potential isn't reached.

Why are inhibitory synapses important?

Inhibitory synapses play a vital role, they prevent random impulses from being sent around the body and they allow for a specific pathway to be stimulated.

Inhibitory pathways can develop over time.

What are the different things drugs can do to the brain?

Drugs can have a major impact on the functioning of the brain. They can have different modes of action such as- stimulating the release of a neurotransmitter, providing the chemicals needed to symthesise neurotransmitters, acting in the same way as neurotransmitters by binding to the receptor, preventing the reuptake of neurotransmitters by the presynaptic neurone.

What are the two types of neurotransmitters?

Some neurotransmitters are excitatory and some are inhibitory.

What are excitatory neurotransmitters?

Excitatory- production of an action potential

What are inhibitory neurotransmitters?

Inhibitory - prevent the production of an action potential.

What are neuromuscular junctions?

Stirated muscle contracts when it receives an impulse from a moto neurone via the neuromuscular junction. Neuromuscular junctions are located between a neurone and a muscle cell.

Can neuromuscular junctions be inhibitory?

No only excitatory

What happens when an impulse arrives at the neuromuscular junction?

When an impulse arrives at the presynaptic membrane, calcium ions diffuse into the neurones, causing vesicles with acetylcholine to fuse with the membrane. The released ACh binds to receptor proteins on the sarcolemma. This stimulated ion channels to open, allowing sodium ions to diffuse in. This depolarises the sarcolemma, generating an action potential which passes down the T-tubules, these action potentials cause voltage gates calcium ion channel proteins in the membranes of the sarcoplasmic reticulum to open. Calcium ions diffuse out of the DR and into the sarcoplasm surrounding the myofibrils. The calcium ions bind to the troponin molecules stimulating them to change shape. This causes the troponin and tropomyosin to change position on the thin filaments. The myosin-binding sites are exposed and the muscle can start to contract.