As you have read, the nervous system controls both voluntary and involuntary activity. The autonomic nervous system is that part of the nervous system which concerns involuntary activity, like heartbeat, activity of the digestive organs, secretion from glands and excretion. It is controlled by the medulla oblongata. This system may be divided into the sympathetic nervous system and the parasympathetic nervous system. These systems function in a complementary fashion. While the former raises the level of an organ’s activity, say during stress or danger, the latter decreases the organ’s activity. For example, you need more oxygen during physical exercise. In response to this, the nerves of the sympathetic nervous system act to increase the rate of heartbeat to supply more oxygen. Once you are at rest ad no longer requiring extra oxygen, the parasympathetic nervous system decreases the rate back to normal.
Reflex action
Reflection action does not involve conscious thought. Rather, it is an automatic response to a stimulus requiring swift action. For example, you instantly blink when strong light is flashed in your eyes. Some other examples are a) withdrawing one’s hand instantly when it touches a hot object accidentally. b) Protecting one’s head with one’s hands or bending it down or sideways to save it form an object rushing towards it; c) adjusting one’s posture to avoid falling; and d) coughing, sneezing, yawning and hiccupping.
Reflexes do not involve the brain directly. It is the spinal cord, with the help of the peripheral nervous system, which is involved in such instant responses. The mechanism of action is different from that involved in involuntary activities controlled by the autonomic nervous system. In the case of reflex action, a message from the sensory cells that in the case of reflex action, a message from the sensory cells that have been stimulated is sent via sensory neurons to the spinal cord, which sends a message directly to the effectors organ via motor neurons. This route of transmission is called a reflex arc. As most reflex action is meant to protect us from danger, they require a shorter route of transmission.
Eye work
It is on the retina that the image of an object is formed. When you look at an object, rays of light from it are roughly focused by the cornea. The lens, which can be made thicker or thinner by the ciliary’s process, does the job of sharp focusing to form a clear image on the retina. After the image is formed, nerve impulses from the rods and cones are conveyed by the optic nerve to the brain. Here, the impulse is translated into a picture of the object as we see it.
