Organism level systems Revision Notes

Myopia (Short-sightedness)

Myopia is a common eye defect where a person can see near objects clearly, but distant objects appear blurry. This happens because the light rays from distant objects are focused in front of the retina, rather than directly on it. The main causes of myopia include an eyeball that is too long, or a lens/cornea that is too curved, causing light to converge too strongly. This excessive convergence means the focal point is short. Myopia can be corrected using diverging (concave) lenses. These lenses spread out the light rays before they enter the eye, effectively increasing the focal length so that the image is formed precisely on the retina.

Hyperopia (Long-sightedness)

Hyperopia is another common eye defect where a person can see distant objects clearly, but near objects appear blurry. In this condition, the light rays from nearby objects are focused behind the retina. This typically occurs because the eyeball is too short, or the lens/cornea is not curved enough, leading to insufficient convergence of light. The focal point is too long. Hyperopia can be corrected using converging (convex) lenses. These lenses help to bend the light rays more sharply before they enter the eye, decreasing the focal length and ensuring the image is formed correctly on the retina, allowing for clear vision of close objects.

Correction Methods for Eye Defects

Both myopia and hyperopia can be corrected using various methods. The most common method involves wearing spectacles or contact lenses containing corrective lenses. Myopic individuals wear concave lenses, while hyperopic individuals wear convex lenses. Another method is refractive surgery, such as laser eye surgery (e.g., LASIK). This surgical procedure reshapes the cornea using a laser to alter its focusing power permanently. For myopia, the cornea is flattened to reduce its convergence power. For hyperopia, the cornea is made steeper to increase its convergence power. These interventions aim to ensure that light rays focus precisely on the retina.

Central Nervous System (CNS)

The Central Nervous System (CNS) is the main control center of the body. It is made up of two primary components: the brain and the spinal cord. The brain is responsible for processing sensory information, storing memories, making decisions, and initiating motor responses. It is enclosed within the protective bones of the skull. The spinal cord is a long bundle of nerve tissue that extends from the brainstem down the back, protected by the vertebrae. Its main function is to relay messages between the brain and the rest of the body. The CNS integrates all incoming information and coordinates outgoing responses.

Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS) includes all the nerves that branch out from the brain and spinal cord to other parts of the body, such as muscles and organs. It acts as a communication link, carrying signals to and from the CNS. The PNS is crucial for allowing the brain to receive sensory input from the environment and to send motor commands to muscles to perform actions. Without the PNS, information could not travel between the CNS and the rest of the body, making coordinated responses impossible.

Components of Nerves

Nerves are bundles of nerve fibres (axons) that transmit electrical impulses. These fibres are often surrounded by a fatty layer called the myelin sheath, which helps to speed up signal transmission. Nerves are found throughout the PNS and connect to the CNS. There are different types of nerves based on their function: sensory nerves carry signals from sensory receptors towards the CNS, while motor nerves carry signals from the CNS to muscles and glands, prompting a response. This two-way communication is fundamental to nervous system function.

Overall Function of the Nervous System

The nervous system's primary role is to coordinate and regulate body functions by transmitting electrical and chemical signals. It enables us to perceive stimuli, process information, and initiate appropriate responses. This includes everything from simple reflexes, like pulling your hand away from a hot object, to complex thought processes and voluntary movements. The intricate structure of the CNS and PNS working together ensures rapid and effective communication across the entire body, maintaining internal balance and allowing interaction with the external environment.

Auxins: Growth Regulation and Tropisms

Auxins are a class of plant hormones that are primarily involved in cell elongation, particularly in shoots, contributing to plant growth. They are produced predominantly in the tips of shoots and roots. A key effect of auxins is their role in tropisms: positively in phototropism (growth towards light) and negatively in gravitropism (growth against gravity in shoots, and with gravity in roots). High concentrations of auxins promote cell elongation in shoots but inhibit it in roots. Auxins also maintain apical dominance, where the main stem grows preferentially over side branches, and promote the development of lateral roots.

Gibberellins: Stem Elongation and Dormancy Breaking

Gibberellins are another group of plant hormones with significant effects on plant growth and development. Their main roles include promoting stem elongation, which can lead to taller plants, and breaking seed dormancy, allowing seeds to germinate. They are also involved in stimulating flowering and increasing the size of fruits, such as in seedless grapes. Gibberellins interact with other plant hormones to orchestrate complex developmental processes, ensuring coordinated growth throughout the plant's life cycle.

Ethene: Fruit Ripening and Senescence

Ethene (also known as ethylene) is a gaseous plant hormone that has several important functions, prominently fruit ripening. It promotes the softening, colour change, and sugar increase characteristic of ripe fruits. This property is exploited commercially to ripen fruits like bananas after transport. Additionally, ethene plays a role in the senescence of leaves, flowers, and fruits, leading to their degradation and eventual shedding. It can also induce abscission, which is the shedding of leaves, flowers, and fruits from the plant.

Limited Neuronal Regeneration

A primary reason for the difficulty in treating nervous system damage is the limited ability of neurons to regenerate. Mature neurons, particularly those in the central nervous system (brain and spinal cord), generally do not undergo cell division. This means that once they are damaged or die, they cannot be easily replaced by new neurons. In contrast, peripheral nerves have a slightly greater capacity for regeneration, but this is still a slow and often incomplete process. This lack of robust regeneration severely limits recovery from injuries like spinal cord damage or neurodegenerative diseases.

The Blood-Brain Barrier

The blood-brain barrier is a protective physiological mechanism that regulates the passage of substances from the bloodstream into the brain tissue. While vital for protecting the brain from toxins and pathogens, it also acts as a significant obstacle for drug delivery. Many potential therapeutic drugs for neurological conditions are large molecules or are not lipid-soluble, preventing them from crossing this barrier. This makes it challenging to get effective concentrations of medication to the target areas within the brain, thus limiting treatment options for conditions like brain tumours, infections, or neurodegenerative disorders.

Complexity and Interconnectedness of the Nervous System

The nervous system, especially the brain, is an incredibly complex network of billions of neurons interconnected in intricate ways. Even minor damage can disrupt extensive neural pathways and affect multiple functions. Identifying the precise location and extent of damage, as well as predicting its full impact, is often difficult. Surgical interventions carry high risks because of the danger of causing further damage to critical, adjacent areas. The interconnectedness also means that repairing one part might not fully restore function if the wider network is compromised.

Ethical Considerations and Research Limitations

Research into treatments for nervous system damage often involves complex ethical considerations, particularly concerning experimental therapies like stem cell implantation. Obtaining sufficient numbers of suitable cells for transplantation and ensuring their proper integration and function without side effects is a major challenge. Furthermore, the long recovery times and the severe impact on quality of life for patients with nervous system damage mean that even partial recovery can be slow and arduous, highlighting the profound limitations in current treatments.