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Biology A-level: Nervous and Hormonal Control

Source taken from:http://www.s-cool.co.uk/alevel/biolo...l-control.html

How hormones work

In order to protect itself the body has developed ways react to changes in its environment. For example, if you get too hot you will sweat to cool down. If light is too bright, your pupils will constrict so that your eyes are not damaged.

In order to do this, the body needs to be able to detect internal and external changes (called stimuli) and make the appropriate response.

To function effectively, there needs to be good communication inside the body.

Hormones are just one of the tools used to send messages to the various parts of the body.

(You also need to understand how nervous impulses are used to send messages within the body, and S-cool! has covered this in the nervous system section - see the nervous system topic section).

Hormones are usually small molecules made by a gland. They are secreted following a suitable stimulus and transported in the blood.

Blood carries hormones to a target organ or group of cells which will recognise the hormone (this triggers a specific chemical response when the correct receptor is activated). The behaviour of the target will then change, bringing about the right response.

Hormones need to combine with specific receptor molecules on, or in, a target cell to have an effect.

There are two structural types of hormone - protein and steroid. They have different ways of binding to and affecting a cell, and you will need to understand these for your exams.

Protein hormones

Examples of protein hormones are insulin, glucagon, and adrenaline (try and remember these).

Protein hormone molecules bind with receptors on the surface of a cell membrane. This starts off a chain reaction inside the cell.

Lets look at the protein chain reaction, step by step:

The receptor may change shape and then bind to a protein in the membrane called G protein. As a result, an enzyme called adenyl cyclase (also present in the cell membrane) increases in activity.

This triggers the conversion of ATP (adenosine tri-phosphate) into cAMP (cyclic adenosine mono-phosphate on the inner surface of the cell membrane) in the cytoplasm.

This increase in cAMP levels may activate specific enzymes called protein kinases or activate relevant genes so that the appropriate enzymes can be synthesized (see Genetic Code topic).

The particular enzyme will catalyse a reaction, e.g. in the case of glucagon and adrenaline, glycogen is broken down into glucose. In the case of insulin, glycogen is built up from glucose.

In this process the hormone is known as the first messenger, and cAMP is therefore known as a second messenger. One hormone molecule can cause many cAMP molecules to be formed. At each stage, the number of molecules involved increases so the general process is called cascade amplification.


Steroid hormones

Examples: testosterone, oestrogen.

Steroid hormones are different to protein hormones in that they cross the cell surface membrane and bind to receptors in the cytoplasm. These hormone- receptor complexes then enter the nucleus.

These complexes then bind to specific DNA sequences so that the transcription rate (this is the rate at which messenger RNA for protein synthesis is formed - see the Genetic Code topic) of the appropriate gene is increased. A protein or enzyme will be produced that will alter the cell's behavior and so bring about the desired response.

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The 'neurone'

The nervous system carries messages around the body using specialised cells called neurones. Neurones convey their 'messages' using electrical impulses.

The central nervous system and the peripheral nervous system

The nervous system (NS) is made up of two parts:

Central nervous system comprising the brain and spinal cord.

Peripheral nervous system.

A simple way of thinking about the interaction between the two systems is to imagine them as roads, and the messages as cars.

The 'car' starts off in small roads (peripheral nervous system) and heads towards to the brain. In order to get there faster it takes the motorway (central nervous system) which gets the 'car' to its final destination - the brain - very quickly.

See how this happens in the diagram below:


The somatic (voluntary) and autonomic (involuntary) nervous system

Different areas of the nervous system are used for different types of nervous reaction:

Conscious control - for example, your brain consciously deciding to move a moving skeletal muscle, and this uses the somatic/voluntary NS.

Non-conscious control - for example, your body automatically reacting to, and this uses the autonomic NS.

Generally, with the autonomic NS, if the outcome increases activity - for example, if the heart rate goes up, it involves the sympathetic NS.

If the outcome is to decrease activity - for example, if the breathing rate goes down, it involves the parasympathetic NS.


Sensory neurones and motor neurones

Receptors are cells that detect stimuli - for example, heat, pressure, light.

Sensory neurones bring impulses from receptors to the central nervous system (CNS).

From there, the impulse may pass on to a motor neurone to be taken to a muscle or gland (the effector).

Sometimes there is an intermediate neurone (also known as a 'relay' neurone) within the CNS linking the sensory neurone with the motor neurone.

To be able to understand how the impulses are transmitted through a neurone, you must first know what the cell is like at rest.

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Resting potential

In the surface membrane of a cell there are protein carriers.

These actively pump Na+ (Sodium) ions out of the cytoplasm to the outside of the cell. At the same time, K+ (Potassium) ions are pumped from the outside in.

This active pumping of Na+ and K+ ions requires energy (in the form of ATP) because the ions are being moved against their concentration gradients (from where they are at a lower concentration to where they are at a higher concentration). K+ and Na+ ions diffuse back down their concentration gradient but K+ diffuses back out of the cell faster than Na+ can diffuse back in.

This means there is a net movement of positive ions out of the cell making the inside of the cell negatively charged, relative to the outside.

This charge is the resting potential of the cell and is about -70mV.


Action potential

When a receptor is stimulated, it will create a positive environment inside the cell.

This is caused by a change in the concentrations of Na+ and K+ ions in the cell and happens in a number of steps:

There is a change in permeability (the ability of the cell membrane to let ions through it) to Na+ and K+ in the cell surface membrane at the area of stimulation, which causes Na+ channels in that area to open.

Na+ therefore floods into the cytoplasm down the concentration gradient.

As this happens the membrane depolarises (this means that the resting potential of the cells starts to decrease). If this depolarisation reaches a certain level, called the threshold level (about -55 to -50 mV), then an action potential has been generated and an impulse will be fired. If it does not reach this level, nothing will happen.

Once +40mV is reached the Na+ channels close and K+ channels open. K+ floods out of the cytoplasm so that the overall charge inside goes back down. This stage is called repolarisation.

The K+ channels then close, the sodium-potassium pump restarts, restoring the normal distribution of ions either side of the cell surface membrane and thus restoring the resting potential.

An example of an action potential being reached would be pressure receptors cells in the skin which. If your hand was squashed, the pressure receptors cells in your skin would be would be pressed out of shape (this would be the external stimuli).

In response to this the Na+ channels in that area would open up, allowing Na+ ions to flood into the cell and thus reducing the resting potential of the cells. If the resting potential of the cell drops to the threshold level, then an action potential has been generated and an impulse will be fired.

The above has only described one area of the neurone and not how the impulse is carried along the neuron, this happens by another chain reaction.

Once an impulse is made, a local current is set up between the area where there is an action potential and the resting area next to it. The flow of some Na+ sideways towards the negative area next to it causes the Na+ channels in that area to open and depolarisation to occur there. That way, the action potential is moved down the neurone.


There is a length of time called the refractory period when the resting potential is being re-established. During this time no new action potential can be generated.

In this way the action potential can only travel in one direction down the neurone because the area behind the action potential is in a state of recovery.

Saltatory conduction

Generally cells are covered in a fatty myelin sheath and therefore the Na+ and K+ cannot flow through this. This means that the ions can only flow through unprotected cell-surface membrane.

In the case of a myelinated neurone, the ions can only move in and out of the cytoplasm at the nodes of Ranvier.

Because of this, the action potential will 'jump' from one node to the next, a process called saltatory conduction, and so will travel much faster than in an unmyelinated neurone.


Other factors that affect the speed of conduction are diameter of the axon (the bigger, the faster) and temperature (up to 40°C, the higher the faster).

Action potentials themselves do not change size as they move down the neurone. All stimuli, as long as they cause the threshold level to be reached, cause an action potential of +40mV, no more or less. The speed of conduction is not altered by the intensity of the stimulus either.

If the stimulus is large, it will produce a greater frequency of impulses. Another one will very quickly follow the previous action potential (i.e. the intensity is frequency modulated).

Another consequence of an intense stimulus is that more than one neurone is likely to be affected. That way the brain, receiving more action potentials from more neurones, will interpret the stimulus as being strong.

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When an action potential reaches the end of one neurone there must be a way to start an action potential in the next neurone.

The two neurons will not be in direct contact and action potentials cannot jump across the gap, called a synapse (or synaptic cleft), so another method is employed...

Release of neurotransmitters

As you can see above, the electrical impulse cannot cross the synaptic cleft, so a chemical called a neurotransmitter is released at the end of the first neurone out of the presynaptic membrane. It diffuses across the synapse, binds with the second neurone on the postsynaptic membrane and generates an action potential.

Two examples of neurotransmitters are acetylcholine (ACL) and noradrenaline. They are synthesised in vesicles, which requires energy, so the synaptic knobs have many ATP-producing mitochondria in them.

Generation of a new action potential

As the action potential reaches the end of the first neurone, Ca2+ channels are also opened. Ca2+ flows into the cell and this induces several hundred vesicles containing the neurotransmitter to fuse with the presynaptic membrane. The neurotransmitter is released into the synaptic cleft.

The molecules of neurotransmitter bind with complementary receptors (similar to an enzyme and substrate fitting together) in the postsynaptic membrane. This makes the Na+ channels open and depolarisation occurs in the postsynaptic membrane thus starting an action potential.


To stop the neurotransmitter continually generating action potentials either the neurotransmitter is actively absorbed back into the presynaptic neurone or an enzyme is released to break it down before reabsorption.

Synapses break up the flow of action potentials and so slow down the transmission of impulses but they are useful...

they ensure that the impulses travel only in one direction.
they allow neurons to connect via neurotransmitters with many, many other neurons. This increases the range of possible responses to any particular stimulus or group of stimuli.
Many drugs act by affecting the events at synapses:

Nicotine: Lowers the threshold for activation of neurons by mimicking the action of acetylcholine on the post-synaptic membrane because it is a similar shape.
Caffeine: Causes the release of calcium ions from cell stores, thereby making firing easier.
Organophosphate insecticides: Prevent the enzyme breaking down acetylcholine after it has produced an action potential. This allows acetylcholine to produce a continuous stream of action potentials, leading to an uncoordinated response in the effectors.
Curare: (Used on the tips of arrows by some tribes) blocks the acetylcholine at the junction between neurone and muscles. This means that the victim is paralysed. Also used medically as a reversible muscle relaxant during heart surgery.