Neural Control and Vision: Brain and Eye Functions

The Cerebral Cortex and Synaptic Transmission

The cerebral cortex is the highest level of the brain, responsible for controlling the activities of other neurons at lower levels. This demonstrates the hierarchical nature of our nervous system. For instance, while the medulla oblongata (often referred to as 'the bulb' in older texts) controls automatic breathing movements, it also sends signals to the cerebral cortex. This explains why, when we enter water, we can consciously hold our breath, as the cerebral cortex can override the medulla's automatic control.

Understanding the Synapse

A synapse is the specialized space between two neurons that a nerve impulse must traverse to pass from one to another. This crucial process is known as synaptic transmission. Each synapse comprises three main elements:

• Presynaptic Neuron: The neuron transmitting the signal.
• Postsynaptic Neuron: The neuron receiving the signal.
• Synaptic Cleft: The tiny gap between the presynaptic and postsynaptic neurons.

For the nerve impulse to cross this synaptic cleft, the presence of neurotransmitter vesicles is essential. These vesicles are located within the axons of the presynaptic neuron. When a nerve impulse arrives, the axon releases these neurotransmitters into the synaptic cleft. The neurotransmitters then bind to specific receptors on the dendrite or cell body of the postsynaptic neuron, thereby transmitting the signal and continuing the nerve impulse.

The Eye: Structure, Function, and Common Conditions

The eye is a remarkable organ that perceives light and color. It is housed within a protective cavity of the skull called the orbital cavity. The eye has a spherical shape and contains two types of fluid that give it its form:

• Vitreous Humor: A clear, jelly-like substance filling the space behind the lens.
• Aqueous Humor: A clear, watery fluid filling the space between the cornea and the lens.

The eye consists of several crucial membranes and structures:

• Sclera: The outermost, tough, fibrous, white, and opaque layer, providing protection and maintaining the eye's shape. The lacrimal gland, associated with the eye, secretes tears that lubricate the eye and possess antibacterial properties.
• Choroid: A thin membrane rich in blood vessels, located between the sclera and the retina. It nourishes the eye and includes the iris, which controls the size of the pupil.
• Retina: The innermost membrane of the eye, containing specialized photoreceptor cells:
  • Rods: Highly sensitive to light, responsible for black-and-white vision and vision in low-light conditions.
  • Cones: Responsible for color vision and detailed vision in bright light.

Common Ocular Abnormalities

Several refractive errors can affect vision:

• Myopia (Nearsightedness): Distant objects appear blurred, while nearby objects are clear. This occurs when the eyeball is too long or the cornea is excessively curved, causing light to focus in front of the retina.
• Hyperopia (Farsightedness): Distant vision is normal, but nearby objects appear blurred. This occurs when the eyeball is too short or the cornea is too flat, causing light to focus behind the retina.
• Astigmatism: All images appear blurry or distorted. This is due to the cornea having an irregular curvature in different directions, preventing light from focusing evenly on the retina.

The most common technique used to correct these refractive abnormalities is laser eye surgery. This method can be effective for myopia under 15 diopters, hyperopia less than 6 diopters, and for certain levels of astigmatism.