Principles of cordination
There are two forms of communication in mammels, the nervous system and the hormonal system.
THE NERVOUS SYSTEM
This is when we have cells which secrete chemicals directly onto cells, in the form of nuroentransmitters. These electrical impulses gets passed from nuroene to nurone. The response is immedite, fast acting and has a very localised effect. The response is temporarily and often reversible.
THE HORMONE SYSTEM
This is when we have hormones secreted by glands, which travel in the blood plasma untill it reaches the target cell. The response takes longer, and is slower acting. However these effects are permenate and long lasting.
Although the nervous and hormonal system work in coordinating the activites of the body, this is completemented further by chemical mediators. These cells act immedietely and have effect on cells close by. They are usually released by injured or infected cells, causing arteries/arterioles to dilate causing swelling, inceased tempreture and therefore ae referred to as the 'inflamentory response'. There are two examples.
- Histimine. This is released by the white blood cells as a response to an allergen. This causes arteries to dilate. increasing the blood flow through the capilleries. This also causes itchyness, redness and swelling.
- Prostaglandis. This is released by the cell membrane of infected cells. They cause arteries to dilate and increased blood flow the capilleries. It also affects nuroetransmitters and blood pressures, therefore they affect pain sensation.
Plant growth factors
Plants dont have a nevrous system, yet they need to respond to internal and external stimuli; such as light, water, gravity. How do they do this? Plants have hormones/ growth factors which allows them to:
- Exert their influence over growth
- They are able to produce hormones in any cell, unlike oragnsisms who have certain organs to produce hormones.
- The hormones that are released by the cell are able to function on theat specific cell, unlike orgainims whose hormones can only function in target cels.
IAA is a type of plant growht factors, and work as follows.
- Cells in the tip of the shoots produce IAA which then travels down the plant.
- The sun causes IAA to move into the shade, and therefore the levels of IAA build up. More IAA causes more elongation.
- More elongation in the side in the shade. This causes the plant to bend towards the light.
This has the opposite effect in the roots, more IAA inhibits growth.
The structure of a neurone
Neurons are specailly adapted cells to be able to carry the nerve impulse from one part of the body to another. The neurones are made up of:
- Cell body. This includes the nucleous and large amount of ER. Here proteins and nuroentransmitters are produced.
- Dendron/dendrites. These are extensions of the cell body which carrys nerve impulses towards the body.
- Axon. This is a long fibre that carrys nerve impulses away from the body.
- Schwann cells. These cells wrap around the axon, providing electracal insulation and protection. They also carry out phagotysosis.
- Myelin sheath. This is a lipid coating over the axon.
- Nodes of ranvier. These are gaps in between schwann cells whcih dont have a myelin coating.
Different types of neurons
There are three types of neurones.
- Sensory neurone. These carry nerve impulses from the receptor to the intermediate/relay nurone. They have one dendron and one axon.
- Motor neurone. This carrys the nerve impulse from the intermediate/relay nurone to the effector. They have many dendrites and one long axon.
- The intermediate/relay nurone carrys the messege from the sensory neurone to the motor neuron. It has many short processes.
The nerve impulse
The nerve impulse is a self propogating wave that passes through the axon membrane. It passes by changing the potential of each membrane along the axon.
The pospholipid bilayer does not allow movements of ions across the membrane. Therefore there are intrinsic proteins which span the cell. These proteins are called ion channels, as it allows certain particles to diffuse through them. They are specif to which ions they allow, and open and close at various times. The soduim-potassuim pump is a type of protein, which actively transports soduim out of a cell and pottasuim in, vai ATP from resperiation.
The inside of a cell is very negitive and is said to be polarised (negitive) at -65mV. The axon is negitly charged relative to the outside of the membane. This is called a cells resting potentail.
Establishment of the potential difference in the a
- Soduim ions are activly removed from the axon by active transport and ATP, while pottasuim is actively transported into the axon. However for every 3 Na+ removed, two K+ is transported in.
- Therefore there is more Na+ in the tissue suronding the axon, and more K+ in the axon. Here we have a chemical gradient.
- Na+ ions begin to diffuse naturally back into the axon (from a high concentration to a low concentration),and the potassuim diffuse out the axon. However most gated Na+ channels is closed, and the K+ channels opened.
- This means that the cell is much more peremble to K+ then Na+, this makes the axon even more negitive the positively charged potassuim leave.
- The pottasuim ions leave and the outside of the axon becomes more and more positivly charged. However we reach a point where any further outward movement is prevented, as the K+ are repelled by the postiveness and are attracted to the negitive axon. (+ and + repel, while + and - attract). Here we have an electriacl gradient.
- An equilibaum is reached when there is no net movement.
The action potentail
When a stimulus comes into the body, its energy causes the Na chnnels to open, allowing an influx of ions and depolarising the cell, thus creating an action potentail. The stages are as follows:
- At resting potentail, the cell is polarised, the Na+ ions channels are closed and the K+ opened.
- The energy of the stimulus causes the Na+ channels to open, allowing ions into the axon. As they are positively charged, they change potential of the membrane.
- As sodium ions diffuse in, this causes a great influx of further ions. The axon gets deploarised as reaches +40 mV.
- The K+ channels now open and the Na close. The K+ ions can now move across their electrochemical gradient. The axon now gets repolarised.
- There is a delay in the K+ channels closing, therefore the axon gets hyperpolarised, extremely negitive.
Passage of an action potentail
An action potentail is not an electric current which flows through the membrane, rather the change of the electraicl charge of the axon membrane, stimultaes the next region of the membrane to get depolarised and transfer the action potentail. The action potentail travels from region to region of the membrane.
PASSAGE OF UNMYELINATED AXON
At resting potentail the axon is negitvly charged compared to the outside Na, and the K is greater in the axon that outside. The stimulus causes reverse cahnge of the potentail of the membrane and the influx of Na causes depolarisation. The influx stimultates the next region to be depolarised, and the behind region gets repolarised (back to normal), as the K leave the axon since the K channels have now opened, and they leave via their electrochemical gradient. This action potentail gets properganted along the axon membrane. The next region is depolarised, while the previous region has retured to its resting potentail.
PASAGE OF MYELINATED AXON
The mylein sheath acts as an insulater and des not allow ions to diffuse through the membrane, thus no potentail difference is maintained- so no action potentail is produced. Therefore the action potentil jumps from node of ranvier to node through process called saltatory conductin. An action potentail travels faster in myleinated neurone.
Factors affecting speed of action potentail
- The mylein sheath. In a myleinated sheath, the action potenatil jumps friom node to node in process of saltatory conduction. It travels at a speed of 90ms-1 as opposed to 30ms-1 in unmylinated.
- Diameter of axon. An axon with a larger diamter has less leakage, so membrane potentail is easier to maintain.
- Tempreture. The more heat, the more kinetic energy particles have and the more collsion, hence reaction. Additionally the energy for the soduim potassuim pump comes from resperaition, so more reaction means faster speed, so we need increased tempreture.
Speed of impulse
The refractory period is the period once the region of the axon membrane has been depolarised and now gets back to its normal conditions, so the ion channels are closed. It serves three purposes:
- Discrete impulses, as channels aer closed, so no impulse can be generated directly behind.
- Impulses properganted in one direction, the action potentail cant move back as the gated channels are closed.
- Limits the amount of action potentails per axon at a time
The all in nothing principle states that an action potentail will be created if the threshold value is reached, otherwise there will be mo response. The level of stimulus is called 'threshhold value', and anything below this will fail to produce a response. How can the body detect size of stimulus?
By the number of nerve impulses passing at once-the bigger the stimulus-the more impluses. And by having different neurons with different threshhold values, the body can detect the stimulus size.
Structer of synapse
A synapse is the point where the axon of the nurone meets with a denrite or effector of another neurone. Synapses transmit chemicals called nurontransmitters through the synaptic cleft.The nurone that secretes the chemical is the presynaptic nurone and end of the presynaptic axon is swelled into a knob, called the synaptic knob. Here is much ER and mitochondria needed to make the chemical, in which it is stored in vesicles.The vesicle passes the synaptic cleft and joins to the receptors of the cell membrane.
Its functions include:
- One single impulse can be sent to many neurons, causing one stimuli to have many responses.
- Or, many impulses can be sent to one nurone, so only one response is made.
The chemical is only made in the presynaptic nurone and stored in vesicles. When the action potental comes, the vesice releases the chemical which fits to the site of the post synaptic membrane.
Features of synapse
- Undirectionality. The synapse can only pass from pre to post synpaptic nurone.
- Summation. Often insuffecent amount of nuron transmitter is made to cause a response, therefore.... 1. Spatail. Many impulses from different neurons join to create action potentail 2.Temporal. A neurone will release a few amounts of chemical simultanously.
- Inhibition. This is when the chloride channels open in the postsynaptic neron and it becomes hyperpolarised, so no action potentail can get generated.
Transmission across a asynapse
A cholergernic response is when the neurontranmitter released is acetylcholine. The process is as follows:
- The action potental arrives at the end of the presynaptic nurone, causing Ca2+ channels to open.
- This causes influx of Na, so the vesicles bind with the membrane of the neurone, releasing the acetylcholine.
- This fits in the receptors of the postsynaptic nuroen, and causes Na channels to pen in the post syntic neruone. So a new action potentail is craeted.
- Acetlyholeraese breaks actelycholine into acetle and choline.
- ATP caueses acetle and choline to be put together in the synoptic knob. Soduim channels close with no acetylycholnide,