S-Cool Revision Summary

S-Cool Revision Summary


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

Examples: 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.

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.

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

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

Central nervous system comprising the brain and spinal cord.

Peripheral nervous system.

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

Conscious control

Non-conscious control

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.

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.

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.

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.

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 neurones 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...

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.