What is visual acuity?

Visual Acuity - The ability to tell apart points which is close together

This is a bit like the resolution of something, you talk about the resolution of an image or microscope. As far as a microscope, the resolution is the smallest distance at which two objects can be told apart.

In your eyes,  Rod cells give low visual acuity because many rods join to the same neurone which means light from two objects close together can't be told apart. In you eye, Rod cells are more sensitive, but cone cells (the other type of cells in your eyes) help you see more detail.

Why are rod cells so sensitive?

 

Check out my previous post on what photoreceptors are...


o effectively detect light, your eye has two types of photoreceptors, rod and cone cells. Rod cells are the most sensitive to light. They fire action potentials in dim light.




In your eye, many rod cells join one neurone. Essentially, this means that lots of weak signals can combine to reach the threshold and fire an action potential - in scientific terms: many weak generator potentials can combine to reach the threshold so an action potential is fired sooner.

This article specifically relates to AQA Biology Unit 5

What are photoreceptors?

Before you read this post, you might find it useful to check out my previous post on what a receptor is.


Photoreceptors are light receptors in your eye. Light enters through your pupil. Your eye cleverly adjusts the amount of light which enters the eye using the muscles of the iris.


The light rays are then focussed onto the retina (part of the eye which lines the inside of the eye). The retina contains photoreceptor cells which detect light - from here the light is pretty much turned into messages sent to your brain. One part of the retina - the fovea, contains lots of photoreceptors. Nerve impulses are sent from the photoreceptor cells to the brain via the optic nerve (a bundle of neurones).

Sickle Cell Anaemia

Sickle Cell Anaemia is a recessive genetic disorder, it's caused by a mutation in the haemoglobin gene. This causes an altered haemoglobin protein to be produced. This means the red blood cells become sickle shaped (concave) - not the typical shape you're probably aware of. The red blood cell's concave shape means they can't flow through capillaries easily like normal red blood cells can so the capillaries can become blocked - blood flow is then restricted. This can also bring up so many more issues like organ damage and periods of acute pain.

Some people can be carriers of the sickle cell gene, amazingly, carriers of the sickle cell gene are partially protected from malaria. Because of this advantageous effect, the frequency of the sickle cell gene have increased. Unfortunately this means the likelihood of inheriting two copies of the allele are increased - overall, this means sickle cell anaemia is more prevalent in these areas.