The structure of a myelinated motor neurone. The establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the movement of sodium and potassium ions. Changes in membrane permeability lead to depolarisation and the generation of an action potential. The all-or-nothing principle. The passage of an action potential along non-myelinated and myelinated axons, resulting in nerve impulses. The nature and importance of the refractory period in producing discrete impulses. Factors affecting the speed of conductance: myelination and saltatory conduction; axon diameter; temperature.

Structure

A motor neurone is a nerve that carries an impulse for a response from the CNS to an effector.

A mylinated neurone is one which is partially covered by a myelin sheaths. A myelin sheath is actually the membrane of a Schwann cell- the cell wraps itself around the axon of a neurone many times creating the sheath. The sheaths are not conductive because they contain the lipid myelin and therefore keep a signal inside the axon.

In between the myelin sheaths are nodes of Ranvier. These are gaps which are not insulated by the myelin allow signals to leave the axon, how ever they are attracted back in further along- at the next node of Ranvier. They therefore speed up a signal by letting it jump sections- called saltatory conduction.

Signals arrive at the dendrites and are transmitted at the axon terminals.

Resting potential maintenance

When the neurone is not transmitting a signal, it is in a state called resting potential (also called being polarised). In this state the potential difference (between the charges inside and outside the axon) is around -65 mV.

This is maintained by moving positive ions out of the axon- making the outside more positive than the inside. The ions that are moved out are sodium ions (I remember this because the symbol for sodium is Na+ which I pretend stands for Not Allowed (in the axon)).

To move these ions out there is a protein embedded in the membrane which transports 3 Na+ out at a time; as it does this, it also moves potassium ions (K+) in. Potassium ions are positive, however, because only two are moved in for every three Na+ moved out it means that the overall charge stays more positive outside than inside.

There are other ion channels in the membrane, ones K+ and Na+ separately. Many more of the K+ channels are open during resting potential making the membrane around 100x more permeable to potassium. Sometimes potassium moves out through the channel because it is moving down the concentration gradient away from the high concentration of K+ ions inside the axon. Sometimes it is in through the channel because out side is positive which repels its positive charge. The inward and outward movements find an equilibrium.

Action potential

Many of the sodium ion channels are voltage gated and will open due to a positive charge in the axon. A stimulus or signal from a synapse will cause sodium to be released into the axon at the dendrites this increases the positivity, causing the voltage gated sodium channels around to open and the charge inside to be more positive than the outside (known as depolarisation). These then make the area near them more positive and cause the voltage gated sodium channels to open further along the axon (represented by the channel after the dotted line). In this way a wave of positivity moves down the nerve.

When the potential difference of the axon is 40mV, the sodium channels close, and the potassium channels open.

1. Depolarisation occurs as Na+ channels open, it moves in down its chemical and electrical gradients
2. When the mV reaches 40 the Na+ gates close stopping further influx, the K+ gates open
3. Too much K+ leaves the axon, making the potential difference more negative that normal (when it reaches -80 the K+ gates close)
4. K+ moves back in down the chemical and electrical gradients through permanently open K+ channels (then resting potential maintenance resumes)

The all or nothing principle

The potential difference must reach a certain voltage before a voltage gated ion channel will open. Voltage gated sodium ion channels in the axon hillock open at -55mV. These channels opening causes an action potential in the axon.

So if there is enough Na+ coming from the dendrites to make the potential difference -55mV, the channels will open and a action potential will start. If there are some Na+ coming from the dendrites, but not enough to make the potential difference -55mV then no action potential will be started and no signal will be transmitted.

Passage of a signal in a mylinated neurone

 

Localised circuits form between areas where there is an action potential and areas where there is not. In a unmylinated neurone this happens with an adjacent area. In a mylinated neurone the majority of the axon is insulated by the fat, so circuits cannot form with adjacent areas, however there are gaps (nodes of Ranvier) every 1-3mms and circuits form between these areas- this allows the signal to jump between the nodes. Know as salutatory conduction this speeds up the passage of a signal.

 

Refractory period

 

A circuit does not form to both sides of an action potential- it only forms in front of it, not backwards. The reason for this is because in an area where there has just been, there is hyperpolarization, meaning the inside of the axon has an even lower potential difference than usual. So the positive ions, which would usually be enough to open ion channels can't bring the potential difference up high enough to. This is beneficial because it means that signals only travel in one direction down a neurone.

 

The refectory period stops new signals forming immediately in the same way, which enables the body to transmit discrete signals

Speed of signals

 

An increase in temperature increases energy. This means that ions move more and so diffuse more quickly- speeding up an action potential. Also ion pumps work using active transport using ATP which is created during respiration; so enzymes are involved and they work faster with heat until they denature.

 

A larger axon has a smaller surface area ratio- this means that comparatively less ions are lost through leaky channels, speeding up the signal.

 

If the axon is mylinated (which it is in humans) then there is salutatory conduction which speeds up the signal.

Nerve cells pass electrical impulses along their length. They stimulate their target cells by secreting chemical neurotransmitters directly on to them. This results in rapid, short-lived and localised responses. Mammalian hormones are substances that stimulate their target cells via the blood system. This results in slow, long-lasting and widespread responses. Histamine and prostaglandins are local chemical mediators released by some mammalian cells and affect only cells in their immediate vicinity. In flowering plants, specific growth factors diffuse from growing regions to other tissues. They regulate growth in response to directional stimuli. The role of indoleacetic acid (IAA) in controlling tropisms in flowering plants.

There are some chemical mediators which are released at the site which needs a response. In animals these are histamine (involved in immune and allergic response) and prostaglandins (involved in response to injury and illness).

In plants chemicals used to control growth, in terms of direction, are released from the growing root or shoot and diffuse down the chemical gradient to other parts of the plant. IAA is an example of this: it is released from the growing shoot and causes cells to elongate, however it is no present in direct sunlight, which means that the side of a plant with the most sun will not grow in comparison to the rest of the plant, curving it towards the light.

The basic structure of a Pacinian corpuscle as an example of a receptor. The creation of a generator potential on stimulation. The Pacinian corpuscle should be used as an example to illustrate the following. • Receptors only respond to specific stimuli • Stimulation of receptor membranes produces deformation of stretch-mediated sodium channels, leading to the establishment of a generator potential. Differences in sensitivity and visual acuity as explained by differences in the distribution of rods and cones and the connections they make in the optic nerve.

Pacinian corpuscle

Below is an image of a pancinian corpuscle. This is a receptor for pressure, found deep in the skin. It is made up of layers of connective tissue filled with gel and at the center is the beginning of a sensory neurone.

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When pressure is applied the layers of tissue compress and distort the neurone. This causes ion channels that are set in the membrane of the nerves axon to stretch, making them big enough for sodium ions to move through. For this reason they are called stretch-mediated ion channels.

When a nerve is at rest, the inside of the axon is negative, and the outside is positive. But as the positive sodium ions move in they change the charge, so the inside is more positive than the outside. This is known as a generator potential, and it will go on to create an action potential (the signal that sends messages to the CNS).

Rod and cone cells

In the retina of the eye there are different receptors for light. Both transduce light energy into an electrical impulse by the breaking up of a protein. However, rod cells will respond to any wavelength of light, but there are three different types of cone cells, each only responding to specific wavelengths (red, green and blue); meaning that rod cells see light and dark, and cone cells see colour.

Another difference is that each cone cell has its own bipolar neurone that connects it to a sensory neurone, but rod cells have to share. There are three rod cells to each bipolar neurone which means it is easier to reach the threshold for a generator potential, as the reception of signals in all three cells can be added together (retinal convergence). This means that rod cells see better when light is low.

Because there are three rod cells to each sensory neurone, the brain cannot tell which rod cell is being stimulated, so the image is not as sharp. Whereas cone cells each have their own attachment so visual acuity is high.

The role of chemoreceptors and pressure receptors, the autonomic nervous system and effectors in controlling heart rate.

The autonomic nervous system carries messages around without concious input. One example being that heart rate is controlled in this way, without having to think about it.

The autonomic nervous system has two different roles:

• To stimulate, fight or flight,(the sympathetic nervous system): increase blood pressure, increase cardiac functions, dilate pupils, increase ventilation, relax the bladder
• To inhibit and slow down, rest and digest (the parasympathetic nervous system): the opposite of above.



In the control of heart rate, chemical and pressure receptors in the carotid artery and aorta send signals to the medulla oblongata in the brain. This is the area in control of heart rate and has a center for increasing and a center for decreasing heart rate.

The events leading to a change in heart rate go as follows:

• If the chemo receptors sense a low PH level in the blood, it means there is a high concentration of CO2.
• They send signals to the center for increasing heart rate through sensory neurones.
• A sympathetic nerve then takes a signal from this center to the SAN which increases the heart rate.
•  Blood flows through the lung more frequently and therefore more CO2 is removed.
• As the PH in the blood returns to normal.
• signals are sent from the chemo receptors that the PH is normal, to the center for decreasing heart rate, which sends a signal through a parasympathetic nerve to the SAN to decrease heart rate to normal.

In a similar way if blood pressure is too high a signal will be sent with the response of a decreased heart rate by the parasympathetic nerves and if it is too low the sympathetic nerves will be used to increase it.

Organisms increase their chance of survival by responding to changes in their environment. Tropisms as responses to directional stimuli that can maintain the roots and shoots of flowering plants in a favourable environment. Taxes and kineses as simple responses that can maintain a mobile organism in a favourable environment. A simple reflex arc involving three neurones. The importance of simple reflexes in avoiding damage to the body.

Organisms move away from things that make them more less likely to be able to reproduce, and toward things that make the more likely to reproduce. This means they will be moving away from things that will kill them and towards things that will protect them, help them to grow, get energy or find a mate.

These movements can happen in three different ways:

• Growing towards or away from something (tropism)
• Moving towards or away from something (taxis)
• Speeding up or slowing down movement and amount of direction changes (kinesis) to make an organism more likely to get out of or stay in an environment

Positive means towards and negative means away from.

In plant roots we see positive hydro-tropism (growth towards water) and positive gravi-tropism (growth towards gravity). In plant shoots we see negative gravi-tropism (growth away from gravity) and positive photo-tropism (growth towards light).

In wood lice we see kinesis in woodlice when they speed up and change direction more rapidly in arid conditions, but slow down and change direction less in humid conditions (because they need the water in the air so they don't dehydrate).

In mice we see positive thigmo-taxis, this is where they will move towards the edge of something.

These movements are all responses to stimuli, caused by a coordinator (such as the brain) which decides a response. The sequence goes stimulus, receptor, coordinator, effector, response.