Neuronal communication

Sensory receptors are specialised cells that can detect changes in our surroundings.

Most are energy transducers that convert one form of energy to another.

Each type of transducer is adapted to detect changes in light levels, a change in pressure on the skin or one of many other energy changes.

Other receptors detect the presence of chemicals.

Each change in the environment, whether it is a change in the energy level or the presence of a new chemical, is called a stimulus.

Whatever the stimulus, the sensory receptors respond by creating a signal in the form of electrical energy- this is called a nerve impulse.

Stimulus

Sensory receptor

Energy change involved

Change in light intensity 

Rods and cones in retina

Light to electrical

Change in temperature

Temperature receptors in skin and hypothalamus

Heat to electrical

Change in pressure on skin

Pacinian corpuscles in the skin

Movement to electrical

Change in sound

Vibration receptors in the cochlea of the ear

Movement to electrical

Movement

Hair cells in inner

Movement to electrical

Change in length of muscle

Muscle spindless in skeletal muscles

Movement to electrical

Chemicals in the air

Olfactory cells in epithelium lining the nose

These receptors detect the presence of a chemical & create

Chemicals in food

Chemical receptors in taste buds on tongue

an electrical nerve impulse.

It is a pressure sensor that detects changes in pressure on the skin.

It is an oval shaped structure that consists of a series of concentric rings of connective tissue wrapped around the end of a nerve cell.

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

The corpuscle is sensitive only to changes, therefore when pressure is constant, they stop responding.

If the channel proteins are permanently open, the ions can diffuse across the membrane and will do so until their concentrations on either side of the membrane are equal. If the channels can be closed, the action of the active pumps can create a concentration gradient across the membrane. Cells associated with the nervous system have specialised channel proteins. Some of these, called sodium channels, are specific to Na+ ions. Others, called potassium channels are specific to K+ ions. These channels also possess a gate that can open or close the channel. The sodium channels are sensitive to small movements of the membrane, so when the membrane is deformed by the changing pressure the sodium channels open. This allows sodium ions to diffuse into the cell, creating a generator potential. The membranes also contain sodium/potassium pumps that actively pump sodium ions out of the cell and K+ ions into the cell. When the channel proteins are all closed, the pumps work to create a concentration gradient at a ratio of 3:2, meaning that the concentration of Na+ ions outside the cell increases, and the K+ concentration inside the cell increases. The membrane is more permeable to potassium ions, so some of these leak out of the cell. The membrane is less permeable to Na+ ions, so few of these are able to leak into the cell. The result of these ionic movements is a potential gradient across the cell membrane. The cell is negatively charged inside (-65mV) compared with outside. This negativity is also enhanced by the presence of negatively charged proteins inside the cell.

When the cell is inactive the cell membrane is said to be polarised, that is negatively charged inside compared with the outside.

A nerve impulse is created by altering the permeability of the nerve cell membrane to sodium ions. This is achieved by opening the Na+ channels.

As the channels open, the membrane permeability is increased and sodium ions can move across the membrane down their concentration gradient into the cell. The movement of ions across the membrane creates a change in the potential difference (charge) across the membrane.

The inside of the cell becomes less negative than usual. This is called depolarisation. This change in potential across a receptor membrane is often called a generator potential.

If a small stimulus is detected only a few Na+ channels will open, the larger the stimulus, the more gates that will open. If enough gates are opened and enough Na+ ions enter the cell, the potential difference across the cell membrane changes significantly and will initiate an impulse or action potential.

Once a stimulus has been detected and its energy has been converted to a depolarisation of the receptor cell membrane, the impulse must be transmitted to other parts of the body. The impulse is transmitted along neurones as an action potential. The action potential is carried as a rapid depolarisation of the membrane caused by the influx of Na+ ions.

• Motor neurones- carry an action potential from the central nervous system to an effector such as a muscle or gland.
• Sensory neurones- carry the action potential from a sensory receptor to the CNS.
• Relay neurones- connect sensory and motor neurones. 

All neurones have a similar basic structure that enables them to transmit the action potential. Neurones are specialised cells with the following features:

• Very long to carry impulse over a long distance.
• The cell surface membrane has many gated ion channels that control the entry or exit of Na+, K+ and Ca2+ ions. 
• Na and K pumps use ATP to actively transport sodium ions out of the cell and potassium ions in. 
• Neurones maintain a potential difference across their cell surface membrane.
• A cell body contains the nucleus, many mitochondria and ribosomes.
• Numerous dendrites connect to other neurones. They carry impulses towards the cell body.
• An axon carries impulses away from the cell body.
• Neurones are surrounded by a fatty layer that insulates the cell from electrical activity in other nerve cells nearby. This fatty layer is composed of Schwann cells closely associated with the neurone.

• Motor neurones have their cell body in the CNS and have a long axon that 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 the sensory and motor neurones together. They have many short dendrites and a short axon. The number of dendrites and the number of divisions of the axon is variable. Relay neurones are an essential part of the nervous system, which conduct impulses in coordinated pathways.

Around one third of the peripheral neurones in vertebrates are myelinated neurones- that is they are insulated by an individual myelin sheath.

The remainder of the peripheral neurones and the neurones found in the CNS and are not myelinated.

Most sensory and motor neurones are associated with many Schwann cells, which make up a fatty sheath called the myelin sheath. These Schwann cells are wrapped tightly around the neurone so the sheath actually consists of several layers of membrane and thin cytoplasm from the Schwann cell.

At intervals of 1-3mm along the neurone there are gaps in the myelin sheath. These are called the nodes of Ranvier. Each node is very short (about 2-3 um long).

Because the myelin sheath is tightly wrapped around the neurone it prevents the movement of ions across the neurone membranes. Therefore, movement of ions across the membranes can only occur at the nodes of Ranvier. This means that the impulse, or action potential, jumps from one node to the next, making conduction faster.

They are associated with Schwann cells, but several neurones may be enshrouded in one loosely wrapped Schwann cell.

This means that the action potential moves along the neurone in a wave rather than jumping from node to node.

Myelinated neurones can transmit an action potential much more quickly than non-myelinated neurones can. The typical speed of transmission in myelinated neurones is 100-120 ms-1, a non myelinated neurone may only reach transmission speeds of 2-20 m s -1. 

Myelinated neurones carry action potentials from sensory receptors to the CNS and from the CNS to effectors. They carry action potentials over long distances. The increased speed of transmission means that the action potential reaches the end of the neurone much more quickly. This enables a more rapid response to a stimulus.

Non-myelinated neurones tend to be shorter and carry action potentials only over a short distance. They are often used in coordinating body functions such as breathing, and the action of the digestive system. Therefore the increase speed of transmission is not so important.

When a neurone is not transmitting an action potential it is said to be at rest.

In fact, it is actively pumping ions across its cell surface membrane. Just like the sensory receptor, sodium/potassium ion pumps use ATP to pump 3 Na+ ions out of the cell for every 2 K+ ions pumped in.

The gated sodium ion channels are kept closed.

However, some K+ channels are open, and therefore the plasma membrane is more permeable to K+ ions than to Na+ ions. K+ ions tend to diffuse out of the cell.

The cell cytoplasm also contains large organic anions.

Hence, the interior of the cell is maintained at a negative potential compared with the outside. The cell membrane is said to be polarised.

The potential difference across the cell membrane is about -60mV. This is called the resting potential. 

While the neurone is at rest it maintains a concentration gradient of Na+ ions across its plasma membrane- the concentration is higher outside then inside. Equally the concentration of K+ ions is higher inside than outside. If some of the Na+ ion channels are opened, then sodium ions will quickly diffuse down their concentration gradient into the cell from the surrounding tissue fluid. This causes a depolarisation of the membrane. In the generator region of a neurone the gates channels are opened by the action of the synapse. When a few gated channels open they allow a few sodium ions into the cell and produce a small depolarisation. This is known as a generator potential. It may go no further. However, when more gated channels are opened the generator potentials are combined to produce a larger depolarisation. If the depolarisation reaches a particular magnitude it passes a threshold and will cause an action potential. Most of the Na+ channels in a neurone are opened by changes in the potential difference across the membrane- they are called voltage-gated channels. When there are sufficient generator potentials to reach the threshold potential they cause the voltage-gated channels to open.

This is an example of positive feedback- a small depolarisation of the membrane causing a change that increases the depolarisation further. The opening of voltage-gated Na+ ion channels allows a large influx of Na+ and the depolarisation reaches 40mV on the inside of the cell, when this has reached, an action potential is transmitted. All action potential have the same magnitude, therefore they are referred to asan 'all-or-nothing' response.

1. Membrane starts at its resting state (-60mV).

2. Na+ ion channels open and some sodium ions diffuse into the cell. The membrane depolarises- it becomes less negative with respect to the outside and reaches

the threshold value of -50mV.

3. Positive feedback causes nearby voltage gated sodium ion channels to open and many sodium ions flood in. As more Na+ enter, the cell becomes positively charged inside compared with outside.

4. The potential difference across the plasma membrane reaches +40mV. The inside of the cell is positive compared with the outside.

5. The Na+ channels close and K+ channels open.

6. K+ diffuse out of the cell bringing the potential difference down inside, repolarisation.

7. Potential difference overshoots slights, making the cell hyperpolarised and then original potential difference is restored.

After an action potential the sodium and potassium ions are in the wrong places.

The concentrations of these ions inside and outside the cell must be restored by the action potential of the Na/K ion pumps.

For a short time after each action potential it is impossible to stimulate the cell membrane to reach another action potential.

This is known as the refractory period and allows the cell to recover after an action potential.

It also ensures that action potentials are transmitted in only 1 direction.

The opening of Na channels at one particular point of the neurone upsets the balance of sodium and potassium ions set up by the action of the Na/K pumps. Ions are allowed to flood into the neurone causing depolarisation, this creates local currents in the cytoplasm of the neurone.Na ions begin to move along the neurone towards regions where their concentration is still lower. These local currents cause a slight depolarisation of the membrane and cause sodium ion channels further along the membrane to open (positive feedback). 

1. When an action potential occurs, the sodium ion channels open at that point in the neurone.

2. The open sodium ion channels allow Na+ ions to diffuse across the membrane from the region of higher concentration outside the neurone into the neurone. The concentration of Na+ ions inside the neurone rises at the point where the sodium ion channels are open.

3. Na+ ions continue to diffuse sideways along the neurone, away from the region of increased concentration. This movement of charged particles is a current called a local current.

4. The local current causes a slight depolarisation further along the neurone which affects the voltage-gated Na+ ion channels, causing them to open. The open channels allow rapid influx of Na+ ions causing a full depolarisation further along the neurone. The action potential has therefore moved along the neurone.

The action potential will continue to move in the same direction until it reaches the end of the neurone- it will not reverse direction, because the concentration of Na+ ions behind the action potential is still high.

The myelin sheath is an insulating layer of fatty material, composed of Schwann cells wrapped tightly around the neurone.

Sodium and potassium ions cannot diffuse through this fatty layer.

In between the Schwann cells are small gaps- the nodes of Ranvier.

Therefore, the ionic movements that create an action potential cannot occur over much of the length of the neurone: they occur only at the nodes of Ranvier.

In myelinated neurones the local currents are therefore elongated and sodium ions diffuse along the neurone from one node of Ranvier to the next. This means that the action potential appears to jump from one node to the next.

This is called saltatory conduction.

The myelin sheath means that action potentials can only occur at the gaps between the Schwann cells that make up the myelin sheath.

Effectively the action potential jumps from one node of Ranvier to the next, speeding up the transmission of the action potential along the neurone.

Myelinated neurones conduct action potentials more quickly that non-myelinated neurones.

A myelinated neurone can conduct an action potential at up to 120 m s -1.

The impulse carried by a neurone is an action potential. All action potentials are the same intensity: each one produces a depolarisation of +40mV. This is the 'all or nothing' rule.

Although the size of the action potential is unrelated to the intensity of the stimulus that caused the action potential, we can still detect stimuli of different intensities, such as loud or quiet sounds. Our brains determine the intensity of the stimulus from the frequency of action potentials arriving in the sensory region of the brain. A higher frequency of action potentials means a more intense stimulus.

When a stimulus is at higher intensity more sodium channels are opened in the sensory receptor. This produced more generator potentials. As a result there are more frequent action potentials in the sensory neurone. Therefore there are more frequent action potentials entering the CNS.

A synapse is a junction between 2 or more neurones, where one neurone can communicate with or signal to another. Between the 2 neurones is a small gap called the synaptic cleft, which is 20nm wide.

An action potential travels along the neurone as a series of ionic movements across the neurone membrane. This AP cannot bridge the gap between 2 neurones. Instead it causes the release of a neurotransmitter that diffuses across the synaptic cleft and generates a new AP in the post synaptic neurone.

Synapses that use acetylcholine as the neurotransmitter are called cholinergic synapses.

The pre-synaptic neurone ends in a swelling called the pre-synaptic bulb. This bulb contains a number of specialised features:

• many mitochondria- indicating that an active process needing ATP is involved.
• a large amount of SER which packages the neurotransmitter into vesicles.
• large numbers of vesicles containing molecules of a chemical called acetylcholine, the transmitter that will diffuse across the synaptic cleft.
• A number of voltage-gated Ca2+ ion channels on the cell surface membrane. 

The post synaptic membrane consists of specialised Na+ ion channels that respond to the neurotransmitter.

These channels consist of 5 polypeptide molecules.

2 of these have a special receptor site that is specific to acetylcholine.

The receptor sites have a shape that is complementary to the shape of the acetylcholine molecule.

When acetylcholine is present in the synaptic cleft, it binds to the 2 receptor sites and causes the sodium ion channel to open.

1. An action potential arrives at the synaptic bulb. The voltage gated Ca2+ ion channels open. Ca2+ diffuses into the synaptic bulb.

2. This causes the synaptic vesicles to move to and fuse with the pre-synaptic membrane, releasing acetylcholine by exocytosis, which then diffuses across the cleft.

3. Acetylcholine molecules bind to the receptor sites on the Na+ channels in the post-synaptic membrane and the channels open. Sodium ions diffuse across the post synaptic membrane into the post synaptic neurone.

4. A generator potential or excitatory post-synaptic potential is created.

5. If enough generator combine then the potential across the post-synaptic membrane reaches the threshold potential.

6. A new AP is created 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.

It hydrolyses the acetylcholine to ethanoic acid and choline.

This stops the transmission of signals, so that the synapse does not continue to produce action potentials.

The ethanoic acid and choline are recycled.

They re-enter the synaptic bulb by diffusing and are recombined to acteycholine using ATP from respiration in the mitochondria.

The recycled acetylcholine is stored in synaptic vesicles for future use.

Once an action potential starts, it will be conducted along the entire length of the neurone.

The action potential does not vary in size or intensity.

At the end of the neurone the pre-synaptic membrane releases neurotransmitter molecules into the synaptic cleft.

The post synaptic neurone responds to these molecules, which is an example of cell signalling.

In cholinergic synapses the signal sent to the next neurone consists of molecules of acetylcholine.

The main role of synapses is to connect 2 neurones together so that a signal can be passed from 1 to the other. However, nerve junctions can be much more complex than a simple connection between 2 neurones. Nerve junctions often involve several neurones- this could be several neurones from different places converging on one neurone, or it could be one neurone sending signals out to several neurones that diverge to different effectors.

When one AP passes down an axon to the synapse it will cause a few vesicles to move to, and fuse with, the pre-synaptic membrane. The small number of acetylcholine molecules diffusing across the cleft produced a small depolarisation. This, on its own, is not sufficient to cause an action potential in the post-synaptic neurone.

It may take several ESPSs to reach the threshold and cause an action potential. The effects of several ESPSs combine together to increase the membrane depolarisation until it reaches the threshold. This combined effect is known as summation.

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

• Several pre-synaptic neurones might converge on one post synaptic neurone, this can allow action potentials from different parts of the nervous system to contribute to generating an AP in 1 post synaptic neurone- so creating a particular response. This is spatial summation, it would be useful where several different stimuli are warning us of danger.
• The combination of several ESPSs could be prevented from producing an action potential by one ISPS.
• One pre-synaptic neurone might diverge to several post-synaptic neurones. This can allow one AP to be transmitted to several parts of the nervous system. This is useful in a reflex arc. One post-synaptic neurone elicits the response, while another informs the brain.
• Synapses ensure that APs are transmitted in the correct direction- only the pre-synaptic bulb contains vesicles of acetylcholine. Therefore, if an action potential happens to start half way along a neurone and ends at the post synaptic membrane, it will not cause a response in the next cell.
• Synapses can filter out unwanted low-level signals. If a low level stimulus creates an AP in the pre-synaptic neurone it is unlikely to pass across a synapse to the next neurone, because several vesicles of acetylcholine must be released to create an AP in the post synaptic neurone.

• Low level action potentials can be amplified by summation. If a low level stimulus is persistent it will generate several successive action potentials in the pre-synaptic neurone. The release of many vesicles of acetylcholine over a short period of time will enable the post synaptic ESPSs to combine together to produce an action potential.
• After repeated stimulation a synapse may run out of vesicles containingthe neurotransmitter. The synapse is said to be fatigued. This means the nervous system no longer responds to the stimulus- we have become habituated to it. It explains why we soon get used to a smell or a background noise. It may also help to avoid overstimulation of an effector, which could cause damage.
• The creation and strengthening of specific pathways within the nervous system is thought to be the basis of conscious thought and memory. Synaptic membranes are adaptable. In particular, the post-synaptic membrane can be made more sensitive to acetylcholine by the addition of more receptors. This means that a particular post-synaptic neurone is more likely to fire an action potential, creating a specific pathway in response to a stimulus.