Chapter 6 Control and Coordination

The nervous system and endocrine (hormonal) system are two different mechanisms the body uses for control and coordination. The nervous system uses electrical impulses to transmit information rapidly through nerve cells (neurons) to specific organs or muscles. Responses are very fast but short-lived, like pulling your hand away from heat. The hormonal system uses chemical messengers called hormones, secreted by endocrine glands directly into the bloodstream. Hormonal responses are slower but longer lasting, such as the effect of adrenaline during stress. The nervous system is suited for immediate responses while hormones control processes like growth, metabolism, and reproduction that require sustained regulation. Together, they maintain the body's internal environment and coordinate responses to external stimuli effectively.

A reflex action is an automatic, rapid, involuntary response to a stimulus that does not involve conscious thinking. It is controlled by the spinal cord rather than the brain, which makes it extremely fast. The pathway of a reflex action is called a reflex arc. When you accidentally touch a hot object, sensory receptors in the skin detect the stimulus and send an impulse through sensory (afferent) neurons to the spinal cord. The spinal cord processes the signal and immediately sends a response through motor (efferent) neurons to the muscle, causing your hand to withdraw. Simultaneously, a signal is also sent to the brain for awareness. The brain becomes conscious of the event only after the reflex action has already occurred. Other examples include blinking, sneezing, and knee-jerk response when the knee is tapped.

The human body produces several important hormones through various endocrine glands. The pituitary gland, called the master gland, secretes growth hormone and controls other glands. The thyroid gland produces thyroxine which regulates metabolism; its deficiency causes goitre. The adrenal gland secretes adrenaline during emergency or fear situations, increasing heart rate and blood pressure for a fight-or-flight response. The pancreas produces insulin (lowers blood glucose) and glucagon (raises blood glucose); lack of insulin causes diabetes mellitus. The testes produce testosterone, which regulates male secondary sexual characteristics, while ovaries produce oestrogen and progesterone for female sexual development and reproductive cycle. The hypothalamus links the nervous and endocrine systems by controlling the pituitary gland.

Plants cannot move from place to place, but they respond to environmental stimuli through directional growth movements called tropic movements (tropisms). These are controlled by a plant hormone called auxin. Phototropism is the growth of a plant toward light. Auxin accumulates on the shaded side of a stem, causing those cells to elongate more, bending the plant toward light. This is why plants on windowsills grow toward sunlight. Geotropism is the response to gravity — roots grow downward (positive geotropism) and shoots grow upward (negative geotropism). Hydrotropism is growth toward water — roots grow toward moisture. Thigmotropism is the response to touch, seen in climbing plants like pea tendrils that coil around a support. These movements help plants optimise their access to light, water, and physical support.

The human brain is the main control centre of the nervous system and is divided into three main regions: the forebrain, midbrain, and hindbrain. The forebrain includes the cerebrum, which is responsible for thinking, memory, voluntary actions, reasoning, and interpreting sensory information. It is the largest and most complex part. The thalamus relays sensory information to the cerebrum, and the hypothalamus regulates body temperature, hunger, thirst, and controls the pituitary gland. The midbrain controls visual and auditory reflexes. The hindbrain includes the cerebellum, which coordinates movement and balance, the pons, which helps control breathing, and the medulla oblongata, which regulates involuntary actions like heartbeat, breathing, and blood pressure. The brain is protected by the skull and three layers of membranes called meninges.