The Nervous System and Reflex. The Eye and Light.

SECTION 1: The nervous system and reflex

When one steps on a tack, it is painful. This is so because of the activation of nociceptors in the nervous system. A type of polysynaptic reflex called a flexor or withdrawal reflex follows. This type of reflex protects the victim from the threat by causing him to immediately withdraw his leg from the stimulus as shown in the reflex arc.

The physiology of the flexor reflex

The pain receptors called nociceptors are stimulated upon an individual stepping on a tack that is a painful stimulus. Afterwards, the pain receptors respond by initiating nerve impulses that are propagated through the sensory neuron into the division of the central nervous system called the spinal cord. The spinal cord acts as an integrating center. At this point, the sensory neurons the association nerves extending to various segments of the spinal cord. Thereafter, in within the spinal cord, a cascade of neural activities take place within the spinal cord hence leading to activation of the motor neurons in various segments of the spinal cord. This activation of the associated motor neurons results in a further nerve impulse generation that gets relayed towards the axon terminals. In the axon of the motor neuron, a neurochemical substance called Acetylcholine is released at the neuromuscular junction. This effect causes the effectors that are the flexor thigh muscles to contract so fast hence, resulting in the withdrawal of the leg from the tack. The spinal cord and its segmented nervous system are heavily involved in the coordination of involuntary movements that do not involve the brain (no-brainer). These movements are so fast and automatic. This can be illustrated using the figure of a reflex arc as shown below.

Figure 1. Diagram of a reflex arc. http://what-when-how.com/neuroscience/the-spinal-cord-organization-of-the-central-nervous-system-part-6/

Section 2: The eye and light

The eye is an organ whose main function is to visualize light. Light is an external stimulus that enables one to see. Whatever one sees is usually a reflection of the light on the object. If I am a beam of light, the rays shining on the eyes of an individual will cause constriction of the pupils of both eyes. This response aims to protect the eyes from harm and it is called the light reflex. The response is under the regulation of the retina, the pretectum and the part of the central nervous system called the midbrain. In the retina is the light-sensitive photoreceptors called the rods and the cons. As light, when my rays fall on an eye from a nearby distance, the muscles of the eyes must undergo several changes in order to bring the nearby object into focus a process called accommodation ("GCSE Science/The Nervous System - Wikibooks, open books for an open world," n.d.)

When as my beams cause an image to appear blurred, the image reaches the visual cortex and through numerous cortical connections and the blurred image reaches the frontal cortex and the other located at the temporal lobe. All these interconnections aim at controlling the medial rectus muscles for converge to happen and the lens to become more round to allow my light rays to converge at the centre of the retina. In addition, the oculomotor nerve of the parasympathetic system constricts the iris to boost the resolution of the lens.

Figure 1. Image recording in the eye. https://micro.magnet.fsu.edu/optics/lightandcolor/vision.html

The outcome is to produce a clear image in the retina. A blurred image is produced when there are numerous angles to be negotiated in the cornea and focus of light happens on different points of the retina. When there is one focus on the retina, a clear image is created. The retina receives the image focused by the cornea and then converts the light stimuli into electrical impulses that are then relayed by the optic nerve to the visual cortex of the brain for proper interpretation. The cones and rods in the eyes are photosensitive receptors that are able to detect the color of my rays

References

GCSE Science/The Nervous System - Wikibooks, open books for an open world. (n.d.). Retrieved January 8, 2018, from https://en.wikibooks.org/wiki/GCSE_Science/The_Nervous_System

The Spinal Cord (Organization of the Central Nervous System) Part 6. (n.d.). Retrieved from http://what-when-how.com/neuroscience/the-spinal-cord-organization-of-the-central-nervous-system-part-6/