Nerves

This occurs if an impulse happens:1.The membrane is at resting state; -60mV inside compared to outside. Polarised.　
2. The impulse occurs. Voltage-gated Na+ion channels open and some Na+ions diffuse in, from the tissue fluid around the neurone.This only happens if the threshold potential (-50mV) is passed ie. if the impulse is strong enough.
3.Sodium ions diffuse into the cell down their electrochemical gradient. (Electrochemical gradient means concentration and charge gradient).
4.As more Na+ions enter through the voltage-gated Na+ ion channels, the cell becomes morepositively charged, compared to outside.
5.The potential difference across the membrane reaches +40mV. The inside is now positive compared to theoutside.
6.This causes the Na+ion channels to shut, so no more Na+ ions are able to enter the cell. 
7.This action potential (+40mV) causes the voltage-gated K+ ion channels to open.K+ions diffuse out of the cell, through the voltage-gated K+ ion channels down their electrochemical gradient. This repolarises the cell
8.Too many K+ ions move out, causing the cell to become hyperpolarised (too negative) so the K+ channels close.
9.The original potential difference is restored, so the cell returns to its resting state.

Stimulus-> Receptor-> Communication pathway (cell signalling) -> Effector-> Response

A number of specialised systems are required for this pathway.

• Sensory receptor=Sensory nerve ending that respond to a stimulus in the internal or external environment of an organism and can create an action potential.
  E.g. Temperature receptors in the skin or internal receptors e.g. in the brain. When one of these receptors detects a change in the environment, it will be stimulated to send a message to an effector.
• Communication system e.g.the neuronal or hormonal systems.These are used in signalling to transmit a message from the receptor to the effector via a coordination centre, usually in the brain. When the message travels from the receptor to the coordination centre, this is known as the input. Messages from travelling from the coordination centre to the effector are called the output. 
• Effector cells are in muscles such as the liver or muscle cells, which bring about the response. 

The inside of the cell is negative relative to the outside. The cell is at -60mV and it is polarised.

1. Sodium-potassium ion channels pump 3x Na+ ions out of the cell, and 2x K+ ions into the cell at the same time.
2. This reduces the potential difference across the membrane (inside vs oiutside), as more positive charge is leaving the cell than entering.
3. There is also some K+ leakage from separate K+ ion channels, and some K+ ions diffuse out of the cell through these at the same time as the previous ions moving out of the cell. This happens slowly and therefore it does not really have an effect on the potential difference across the cell.
4. There are also some negatively charged proteins in the cytoplasm, which contribute to this negative potential difference across the membrane, between the outside and the inside of the cell.

• Myelinated neurones
  100-120ms-1
  Up to 1m transmission distance
  Fast response time
  Used in movement (very important that they ensure rapid response)
  1/3 of all neurones
  One neurone is surrounded by one tightly wrapped Schwann cell, wrapped round many times

• Non-myelinated neurones
  2-20ms-1 mm or cm transmission distance
  Slow response time
  Many neurones are surrounded by one Schwann cell
  Used in breathing and digestion (so it's not important that they ensure rapid response)
  2/3 of all neurones
  Many neurones can be enshrouded in one loosely wrapped Schwann cell

Sensory receptors are specialised cels that detect a change in environment (a stimulus). Most are energy transducers- they change one form of energy into another. Whatever the stimulus, the sensory receptors respond by creating a signal in the form of electrical energy.

Each type of transducer is adapted to detect changes in a particular form of energy e.g. change in the light energy (eyes) or sound energy (ears). Others detect pressure (skin) and chemicals (nose or tongue).

• Change in light intensity, dectected by eyes, change the energy from light to electrical
• Change in sound , detected by ears, change in energy from sound to electrical
• Change in chemicals, detected by mouth, change in energy from chemical to electrical
• Change in pressure on skin, detected by Pacinian Corpuscle, change in energy from movement to electrical
• Change in termperature, detected by temperature receptors in the skin and hypothalamus.
• Change in chemicals, detected by the olfactory cells in the epithelium lining in the nose, change in energy from chemical to electrical
• Movement, detected by hairs in the ears, energy change from movement to electrical

When pressure is put on the skin, this deforms the rings of connective tissue, which push against the nerve ending.

1. Action potential occurs, the sodium ion channels open at that point in the neurone.
2. The open sodium ion channels allowsodium ions to move into the membrane, down their electrochemical gradient.
3. The increase in sodium ions at this point causes them to diffuse into an adjacent area of the membrane.
4. This increase causes the membrane at this point to become more positively charged. This stimulates the voltage gated sodium ion channels to open, due to positive feedbck. The opening of the voltage gated sodium ion channels causes an influx of sodium ions into the neurone.
5. This increase in sodium ion concemntation causes diffusion down their electrochemical gradient to the next section of the membrane. The cycle then continues.

The action potential will continue to diffuse forwards, it is unable to diffuse backwards because the concentration of sodium ions from behind the action potential is still high.

Cholinergic synapse= A synapse that uses acetylcholine as the neurotransmitter 
Neurotransmitter= A chemical that is used as a signalling molecule between two neurones in a synapse. 

As an action potential reaches the end of a synapse, it cannot bridge the gap between two neurones. Instead, the action potential in the pre-synaptic bulb (the synaptic ending in one of the neurones) causes the release of a neurotransmitter that diffuses across the synaptic cleft (the gap between synaptic bulbs), and generates an action potential in the post-synaptic bulb of the next neurone. Synapses that use acetylcholine as its neurotransmitter are called cholinergic synapses.

Pre-synaptic bulb
The pre-synaptic neurone ends in a swelling called the pre-synaptic bulb (or pre-synaptic knob). This is specialised to do its job because of these features:
1.Lots of mitochondria, that can produce lots of ATP.
2.Lots of smooth endoplasmic reticulum, which packages the neurotransmitter (acetylcholine) into vesicles.
3.Lots of voltage-gated sodium ion channels on the plasma membrane 

1. Action potential arrives at synaptic bulb
2. Voltage gated calcium ion channels open
3. Calcium ions diffuse into synaptic bulb
4. Calcium ions cause the synaptic vesicles to move to, and fuse with, the pre-synaptic membrane
5. Acetylcholine is released by exocytosis.
6. Acetylcholine diffuses across synaptic cleft
7. Acetylcholine molecules bind to the receptor sites on the sodium ion channels in the post synaptic membrane
8. Sodium ion channels open
9. Sodium ions dioffuse across the post synaptic membrane into the post synaptic neurone
10. A generator potential is created
11. If a sufficient generator potentials combine, potential across the post synaptic membrane reaches a threshold potential
A new action potential is made in the post synaptic neurone.

If acetylcholine is left in the synaptic cleft, it will continue to open the sodium ion channels in the post synaptic membrane and will continue to cause action potentials

Acetylcholinesterase is an enzyme found in the synaptic cleft, which hydrolyses the acetylcholine to ethanoic acid and choline. Thsi stops the transmission of signals, so that the cynapse does not contibue to prdoce action potentials in the post synaptic neurone.

The ethanoic acid and choline are recycled. Thy re-enter the synaptic bulb by diffusion and are recombined to acetylcholine using ATP from respiration inb the mitochondria of the pre-synaptic bulb. The recycled acetylcholine is then stored in synpatic vesicles for future use.

Action potentials and cell signalling
As action potentials are an all-or-nothing response, they do not vary in size or intensity, only frequency. At the end of the neurone (at cholinergic synapses), the pre-synaptic cleft releases acetylcholine across the synaptic cleft. The post-synaptic cleft responds to these by opening the sodium ion channels. This is an example of cell signalling.

1. Arrival of a nerve impulse at the end of the axon of the neurone causes an influx of calcium ions and induces the vesicles to release their neurotransmitter into the space between the neurones (the synaptic cleft).
2. The acetylcholine diffuses across the synaptic cleft to receptors on the receiving membrane. Diffusion across the cleft delays the impulse transmission by about 0.5 milliseconds.
3. The neurotransmitter binds to receptors on the post-synaptic membrane.
4. Sodium ion channels in the membrane open, causing an influx of sodium ions. This response may or may not reach the threshold required to generate a nerve impulse (threshold potential).
5. The neurotransmitter is deactivated by enzymes located in the membrane. Components of the neurotransmitter are actively reabsorbed back into the synaptic bulb, recycled and repackaged. 

Synapses and nervous communication

The main role of synapses is to join 2 neurones together. However, nerve junctions can often involve several neurones converging from different receptors to one neurone, or it could be one neurone diverging to lots of effectors.

1.Transmission across a synapse occurs. Here,an action potential passes down the axon of the neurone, which causes some vesicles that contain neurotransmitter (e.g. acetylcholine) to move to, and fuse with the pre-synaptic membrane of the pre-synaptic bulb.

 2. Acetylcholine is released from the pre-synaptic bulb via exocytosis and diffuses across the synaptic cleft. 

3. As the acetylcholine is released from its vesicles in the pre-synaptic bulb and leaves to diffuse across the cleft, the pre-synaptic bulb becomes depolarised. This called an Excitatory Post-Synaptic Potential (EPSP).

4. On its own, one of these wouldn't be enough to create an action potential on the 

The opening of the sodium ion channels in the membrane of the neurone upsets the resting potential, established by the Na/K ion pumps.

When sodium ions flood in across the membrane to cause depolarisation, this creates local currents in the cytoplasm of the neurone. Sodium ions diffuse towards the regions where their concentration is lower. These local currents cause a slight depolarisation of the membrane and cause the voltage-gated sodium ion channels later on in the membrane to open (positive feedback). 

The action potential will not reverse because the concentration of the sodium ions behind the action potential is still high. 

As the Schwann cells prohibit ion movement at that section of the membrane, the sodium ions can only move in at the nodes of Ranvier. 

In myelinated neurones, the local currents are therefore longer and the sodium ions diffuse along the neurone from one node to the next. This means that the impulse (action potential) has to 'jump' from one node to the next. This is called Saltatory conduction. 

Advantages of Saltatory conduction

Transmission is much faster as diffusion isn't taking place like in the non-myelinated neurones.

All action potentials are the same size and intensity, with each producing a depolarisation of +40mV. This is the all-or-nothing rule. 

We are able to detect the intensity of different stimuli (e.g. how loud something is) by the frequency of action potentials. More action potentials = more intense stimulus.

Higher intensity stimuli means that more sodium ion channels open in the sensory receptor. This produces more generator potentials in the sensory neurone, therefore more action potentials. If there are more action potentials then there are more action potentials entering the CNS.

Post-synaptic membrane
1. Specialised sodium ion channels that can respond to the neurotransmitter.
2. A single sodium ion channel consists of 5 polypeptide molecules that join up into a little channel, 2 of these have specific receptor sites for acetylcholine.
3. When the 2 acetylcholine molecules binds, they are complementary so fit perfectly.
4. When acetylcholine is present in the synaptic cleft, it binds to the 2 receptor sites and causes the sodium ion channels to open.

Synapses and nervous communication
The main role of synapses is to connect two neurones together so that the impulse can be passed from one to the other. However, nerve junctions can be much more than just a connection from one neurone to the next. Nerve junctions often involve several neurones. This could be several neurones from different places converging to one neurone, or it could be several neurones that diverge to different effectors. 
When the action potential passes down the axon to the pre-synaptic bulb and stimulates vesicles containing acetylcholine to fuse with the plasma membrane and release acetylcholine by exocytosis. The small number of acetylcholine molecules diffuse across the synaptic cleft and bind to the specialised sodium ion channels (ligand-gated sodium ion channels) in the post-synaptic membrane. This causes a conformational shape change in the sodium ion channel and causes it to open. This means that sodium ions flood in through the post-synaptic membrane into the post-synaptic neurone, causing the membrane to become depolarised. This is called an excitatory post-synaptic potential (EPSP). On its own, one of these isn't enough to cause an action potential in the post-synaptic neurone. It may take several EPSPs for the threshold potential to be reached, so more sodium ion channels can open and more sodium ions can flood in to allow the cell to become fully depolarised. The effects of several EPSPs combining to increase membrane depolarisation until it reaches the threshold is called summation. 

Summation can occur from several action potentials in the same pre-synaptic neurone (temporal summation) or from action potentials in the arriving from several different pre-synaptic neurones (spatial summation).

Some pre-synaptic neurones can produce inhibitory post-synaptic potentials to reduce the effect of summation and prevent the action potential in the post-synaptic neurone.

• Many are very long and can transmit action potentials over  a long distance.
• Plasma membrane has gated ion channels to control movement of sodium, calcium and potassium ions
• Na/K pumps use ATP from respiration to actively transport 3Na+ out and 2K+ in to maintain a resting potential of -60mV.
• Neurones maintain a potential difference across their cell surface
• A cell body that contains many mitochondria, a nucleus and ribosomes
• Numerous dendrites to connect to other neurones
• An axon to carry the impulse away from the cell body
• A dendron to carry impulse towards the cell body
• Fatty Schwann cells to insulate the neurone for when there is an impulse. 

• Motor neurones have their cell body in the CNS and have a long axon which carries the action potential out to the effector.
• Sensory neurones have a long dendron carrying the action potential from a sensory receptor to the cell body, which is positioned just outside the CNS. They then have a short axon carrying the action potential into the CNS.
• Relay neurones connect sensory and motor neurones together. They have many short dendrites and a short axon. Relay neurones are an essential part of the nervous system, which conduct impulses in coordinated pathways.