The Human Eye: Structure and Function

Neurons are small in size, microscopic. They consist normally with a cell body, dendrites and an axon. However they are all differentiated in structure as they are related to their function.
Sensory Neurons – These are called afferent neurons. This means an impulse moving towards a point or the central nervous system. As the name suggests they receive their information from our sensory receptors. For example our eyes, ears, tongue and skin. The stimuli’s information is then passed onto the control centre of the cell. It then goes through the axon, till the end of the neuron is reached at the axon terminals. These electrical impulses that travel through the neuron can only flow in one way. The structure of sensory neurons include the cell body and dendrons (single dendrite) being located outside of the spinal cord located in your arms, legs and torso. Axons within a sensory neuron are short contrasting to the dendrites(extensions that conduct impulses) which are long. (WESTONE, 2015)
Diagram of a Sensory Nueron(Above) (WESTONE, 2015)
Motor Neurons – These are called efferent neurons. This means an impulse moving away from a point or to the central nervous system. This allows for responses from the brain to be sent to muscles or organs (effectors). The information first enters the motor neuron through the dendrites which is then passed onto the cell body (control centre). The impulse is then sent down though the axon until it reaches the end of the neuron called the axon terminals. The axon terminals are called motor end plates when a motor neuron connects to a muscle. Contrastingly from sensory neurons the dendrites are short and axons are normally long. The brain formulates the required response and sends a series of electron impulses through the motor neurons to the effectors to provide an appropriate response. (WESTONE, 2015)
Diagram of a Motor Nueron (left) (WESTONE, 2015)
The Myelin Sheath – These are a structure that is located within many neurons outside of the central nervous system. Specialised cells known as Schwann cells myelinate cells by tightly wrapping a sheath around the axon present in a neuron. Myelin is rich in fat and helps to create insulation for electrical activity. This insulation is what helps the impulses to travel faster. (KENT, Michael, 2000) (WESTONE, 2015)

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The Axon Terminals – These are found at the end of an axon. This structure allows the transfer of impulses from one neuron to another, without the neurons ever coming into physical contact with one another. The gap between the two neurons is known as a synapse. An electrical impulse will arrive at the end of the axon and this causes the release of neurotransmitter chemicals (stored in vesicles). The neurotransmitter chemicals cross the synaptic gap between the axons and allow the impulse to be carried on. (KENT, Michael, 2000) (WESTONE, 2015)
Role Within A Reflex Arc – A reflex arc is a neural pathway that controls an action reflex. Most sensory neurons do not pass information directly to the brain, but instead to the synapse in the spinal cord. This allows reflex actions to occur quickly using the spinal motor neurons without the delay of having to wait for an impulse from the brain. However the brain will still receive the sensory input but this will be whilst the reflex action occurs. A good example of this is when you touch something hot, the skins sensory neurons take the stimuli and using a relay neurone send the impulse to a motor neurone for an instant reflex action. The brain shortly afterwards will be alerted to the stimuli after the reflex action has occurred. (KENT, Michael, 2000) (BBC BITESIZE, 2015)
The Structure and Function of the Human Eye

Part of Eye

Description and Function

Aqueous Humour

This is a watery fluid that fills a chamber of the eye called the anterior chamber. It is located behind the cornea and in front of the lens. The fluid is a salt solution (alkaline) including sodium and chloride ions. It maintains the intraocular pressure. Provides nutrition for ocular tissues. Helps the refractive index. Immunoglobulins are present which indicate a role within the immune system.

Choroid

A layer of the eyeball between the retina and sclera. It is a quite thin and very vascularised membrane. Its function is to help prevent blurred vision due to excess light on the retina. It achieves this by its dark brown colouring and pigment which helps to absorb the excess light.

Ciliary Muscle

This is located within each of our eyes and is part of the ciliary body. It is a ring shaped muscle that contracts and relaxes to alter the curvature of the lens of the eye. For example when the ciliary muscle relaxes the lens is allowed to be flat, this enables us to focus on distant objects. Conversely when the ciliary muscle is contracted the lens is allowed to be round, this enables us to focus on objects that are close to us.

Cornea

The corneas function is to contribute to image processing by refracting light as it enters the eye. It is a strong, transparent bulge located at the front of the eye. It has a radius of ~8mm and is a non vascular structure. It is very sensitive as it has a great abundance of nerves.

Fovea

This is a small depression within the retina at the back of each eye. Large ammounts of photo-detector cells known as cones are located within the region of the Fovea. It has a slightly yellow appearance to it and is the area of greatest acuity of vision. Its function is high resolution imagery allowing us to focus on fine detail.

Hyaloid

The Hyaloid membrane is transparent and its function is to separate the retina from the vitreous humour.

Iris

This is the coloured part of our eye and it is completely variable in size. This allows the iris to adjust its size and thus the pupils size to control the amount of light entering the eye.

Lens

The lens refracts light passing through the eye. The light will already have been refracted once by the cornea. However it will focus the light into the retina. The lens can change shape according to the distance an object being focused on is. This is known as accommodation and the ciliary muscles contracting and relaxing help to achieve these adjustments and allow us to focus on objects both near and far.

Optic Nerve

The optic nerve function is to supply the brain with sensory information for processing within the brain. An optic nerve is connected to each eye and is the second cranial nerve. Each optic nerve contains roughly one million fibres, these fibres are what allow the nerve to relay all the information gathered by the eyes.

Papilla

This is where the optic nerve leaves the eye to pass on information to the brain. It is known as the blind spot of the eye.

Pupil

The pupil is present within both of our eyes and located within the centre. The pupil helps to regulate how much light enters the eye. It achieves this by the pupilary reflex. When encountered by bright light the nerves send information to the iris causing a muscular contraction that reduces the size of the pupil. Less light is then able to enter the eye, the opposite happens when there isn’t enough light. The muscles in the iris will relax allowing the pupil to increase in size. This would then allow more light to enter the eye.

Retina

The retina is located at the back of the eye and is it a surface or screen that allows an image to be formed. It also collects information and transmits it to the brain. The retina contains photosenstive cells known as rods and cones allowing for information to be converted into nerve impulses that can then be sent via the optic nerve to the brain.

Sclera

The sclera is made up of white fibrous tissues and elastic fibres. It is sometimes called the whites of your eyes. It is quite a tough substance and its firm fibrous material helps maintain the shape of the eye. It is far thicker towards the posterior of the eye.

Vitreous Humour

This a transparent substance that fills the chamber behind the lens of the eye. It is a jelly like substance and it is quite thin. It is enclosed by the hyaloid membrane. It’s function is to keep the structure and helps to keep the retina in place by pressing it against the choroid.

Zonules

The Zonules function is to attach the lens to the ciliary muscles. Primarily made of fibrillin (connective tissue).

(IVY ROSES, 2015) (KENT, Michael, 2000)
3.5
There are two types of photoreceptor cells, rods and cones, they both serve different functions.

Rods

Cones

The outer segment is rod shaped.

The outer segment is cone shaped.

Rods are distributed throughout the retina, there are roughly 109 rod cells per eye. Due to the quantity of rods they are used for peripheral vision.

Cones are mainly found within the fovea of the eye. This means that only images in the centre of the retina can be detected. There are roughly 106 rod cells per eye which is a third less than rod cells.

Rod cells offer good sensitivity and can detect a single photon of light. This makes them very useful for vision at night and in the dark.

Cones contrastingly offer quite poor sensitivity and require bright light to function. As a result they are only functional in well lit environments or the day time.

Rods only have one type, as a result they can only offer monochromatic vision.

Whereas Cones have three types, red green and blue, as a result they are responsible for allowing us to see colours and have colour vision.

Multiple rods are only connected to one bipolar cell. This means that this results in poor acuity. Rods are not effective at resolving fine detail of objects in focus.

Conversely each cone is normally connected to one bipolar cell. This results in very good acuity. The cones can be used for resolving fine detail and focusing objects in high resolution detail.

(BIOLOGY MAD, 2015)
Visual acuity is what level of detail can be seen, for example reading would require text on a page would require high acuity. For a task such as reading the cones would be responsible for the high resolution focus required. Even though we have far fewer cones than rod, we prioritise using cones as they allow for this fine detail focus and their perception of colours. We’re constantly moving our eyes so that images can be focused on a small area of the retina called fovea. It may appear that we can read many words at once but in reality we only read one word at a time. But to compensate for this our eyes move very quickly and give us the impression that we can read more. Cones offer much more clarity than rods this is due to the fact they are very densely packed at 160,000 cones per mm2. This when compared to animals such as Hawks is very low as they have 1 million cones per mm2. Hence the expressions “You see like a hawk” “Hawkeye” etc.
Cones are also responsible for colour vision. As humans we have three forms of rhodopsin that are sensitive to different parts of the visual light spectrum. Ten percent of our cones are sensitive towards red. Whereas green and blue cones are forty-five percent. This is mainly due to our evolution, being able to identify predators hiding within long grasses and jungle environments. The brain processes the light from the reaction of each of these three cones. It will compare the nerve impulses and it allows the brain to detect any colour (That we’re able to see).
In other creatures notable examples include dogs, which have two different types of cones. Green and blue. This enables dogs to see blue, green and yellows. Whereas Butterflies have five cones and can see colours that we as humans don’t know exist. The most impressive example of colour vision in the animal kingdom is the mantis shrimp. It has sixteen colour receptive cones. To put that into perspective, rainbows are made up of three colours (RGB), one can only imagine what a rainbow must appear to a mantis shrimp. (BIOLOGY MAD, 2015)
Accommodation of the Eye
Accommodation is the ability of the eye to alter focus in response to both near and far objects. To achieve this the eyes alter the shape of the lens. The cornea is responsible for the primary focus and the lens is for adjustments. To control the lenses shape the suspensory ligaments and ciliary muscles must act upon it.
The image to the left is of a distant object. The lens needs to be thin and have a long focal length. This is achieved by the ciliary muscles relaxing and allowing the suspensory ligaments to pull the lens out. This makes the lens thinner and allows us to focus on objects in the distance.
The second image to the left is of the eye focusing on an object close up. The lens is required to be thick and have a short focal length. To achieve this the ciliary muscles contract. This creates a small ring and takes the tension away from the suspensory ligaments. This makes the lens into this thick, smaller shape allowing us to see things closely in high definition. (BIOLOGY MAD, 2015)
Corneal Reflex
Also known as the blink reflex, this is the involuntary blinking of the eyelids due to a stimulus of the cornea. This can occur from light, touching, or potentially a foreign body trying to enter. This is a very fast reaction which occurs at 0.1 of a second. Its only purpose is the protection of the eyes. This is a brain stem reflex, a stimulus will affect the cornea and the nerves connected to the cornea as a sensory neurone. This information is then relayed to a motor neurone which causes the involuntary action of blinking or closing of the eyelid.
Reflexes are involuntary and most importantly rapid actions. Nervous tissue is far better suited to reflexes rather than hormones because of multiple reasons.
Type of signal, hormones are blood-borne signals where as nervous tissue transmits through electrical impulses across synapses. Hormones actions on the body are long lasting in comparison to the short lived nervous system response for a reflex. For example, holding your hand onto something hot would cause a sensory neurone to send information to a relay neurone and onto a motor neurone and the brain causing an instant muscle contraction, resulting in you moving your arm. Comparing this to the hormone Adrenaline which would give you that fight or flight boost increasing your heart rate, levels of sugar in blood, diverting blood to muscles and brain and would allow you to quickly move your hand. However the damage may have already occurred to your hand before you had time to react. This is because the speed of transmission of a hormone is comparatively slower than the very rapid and immediate nervous system. The control of a hormone is indirect as it affects the bloodstream where as the nervous system is directly controlling as it can control and activate muscles and glands. Hormones are also very widespread that affect various parts of the body, where as nervous tissue is very localised. A good example of this would be insulin and glucagon.
This is produced within the beta cells of the pancreas it is a peptide hormone. It controls our blood sugar levels by using two hormones, insulin and glucagon. The pancreas is monitoring our bodies blood sugar levels constantly and when it detects a spike, for example after eating it will stimulate the beta cells to produce insulin. The insulin aids the glucose absorption rate into cells where it can be used for cellular respiration. However when there is too much glucose it will actively convert glucose into glycogen which is how the body stores energy. This process is known as gluconeogenisis.
But this is far too general for a reflex to use. This process takes time to detect, for the hormones to be secreted into the blood stream and take time to get to areas that require remedy. This is why the rapid nature of neurones is far better suited to reflexes.
(KENT, Michael, 2000)
Bibliography
BBC BITESIZE. 2015. BBC. [online]. Available from World Wide Web: <http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/brain_mind/reflexactionsrev1.shtml>
BIOLOGY MAD. 2015. Eyes – Cones, Rods. [online]. Available from World Wide Web: <http://www.biologymad.com/nervoussystem/eyenotes.htm#rodscones>
IVY ROSES. 2015. Anatomy of Human Eye. [online]. Available from World Wide Web: <http://www.ivyroses.com/HumanBody/Eye/Anatomy_Eye.php>
KENT, Michael. 2000. Advanced Biology. Oxford, pp.182-183.
WESTONE. 2015. Structure of Neurons. [online]. Available from World Wide Web: <http://tle.westone.wa.gov.au/content/file/969144ed-0d3b-fa04-2e88-8b23de2a630c/1/human_bio_science_3b.zip/content/002_nervous_control/page_03.htm>
Word Count: 1628 (not including tables nor diagram annotations)