Learn Unit 5: Human Physiology with Free Lessons & Tips

             +-------------------+
             |                   |
             |    Endocrine      |
             |      Glands       |
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |  Pituitary Gland  |
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |   Thyroid Gland   |
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |  Parathyroid Gland|
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |   Adrenal Gland   |
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |    Pancreas       |
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |     Pineal Gland   |
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |    Thymus Gland   |
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |   Ovaries (Female)|
             |                   |
             +---------+---------+
                       |
             +---------+---------+
             |                   |
             |   Testes (Male)   |
             |                   |
             +---------+---------+

Please note that the diagram represents the approximate location of the endocrine glands in the human body. The actual position and size of these glands may vary slightly from person to person.

Our perception of color is facilitated by specialized cells in the retina of our eyes called cones. Cones are sensitive to different wavelengths of light and enable us to perceive colors. There are three types of cones, each sensitive to specific ranges of wavelengths corresponding roughly to red, green, and blue light. When light enters the eye, it is absorbed by these cones, and the information is processed by the brain to generate our perception of color.

The part of our body responsible for maintaining balance is the vestibular system, which is located in the inner ear. Specifically, the semicircular canals and otolith organs within the inner ear detect changes in head position and movement. This information is sent to the brain, where it is processed to help us maintain our balance and sense our orientation in space.

The eye regulates the amount of light that falls on the retina primarily through the action of the iris and the pupil. The iris is a circular muscle surrounding the pupil, and it can contract or relax to adjust the size of the pupil. In bright conditions, the iris contracts, making the pupil smaller to reduce the amount of light entering the eye. In dim conditions, the iris relaxes, allowing the pupil to dilate and let in more light. This process, known as pupillary reflex, helps regulate the amount of light reaching the retina to optimize visual sensitivity in different lighting conditions.

The suprachiasmatic nucleus (SCN) of the hypothalamus acts as the master clock or central pacemaker of the circadian rhythm in humans. The SCN is a small group of cells located above the optic chiasm and receives direct input from the retina, allowing it to synchronize with the light-dark cycle of the external environment. The SCN generates endogenous rhythms with a period close to 24 hours, which serve to regulate various physiological and behavioral processes, including sleep-wake cycles, hormone secretion, body temperature, and metabolism. The SCN coordinates the timing of these circadian rhythms throughout the body, helping to maintain internal synchrony and adapt to environmental changes.

Afferent neurons, also known as sensory neurons, transmit sensory information from sensory receptors (such as those in the skin, eyes, ears, and internal organs) to the central nervous system (CNS). These neurons detect external stimuli (e.g., light, sound, touch, taste, smell) and internal stimuli (e.g., temperature, pain, pressure) and convey this information to the CNS for processing and interpretation. Efferent neurons, also known as motor neurons, transmit motor commands from the CNS to muscles and glands, initiating appropriate responses. These neurons carry signals that control muscle contraction (motor output) and glandular secretion in response to stimuli. In summary, afferent neurons transmit sensory information to the CNS, while efferent neurons transmit motor commands from the CNS to muscles and glands.