Control and Coordination
Living organisms respond to changes in their environment. This response often involves movement. Some movements are related to growth, like a seedling pushing through the soil, while others are not, such as a cat running or a person swinging. These movements are not random; they are carefully controlled and coordinated responses to specific events, or stimuli, in the environment. To achieve this, multicellular organisms have specialised systems for control and coordination.
In animals, control and coordination are primarily managed by the nervous system and the muscular system. The nervous system is a complex network of nerve cells, or neurons, that transmit information as electrical signals.
Detecting Information: Information from the environment is detected by specialised nerve cell tips called receptors. These are located in our sense organs.
Transmitting Information: When a receptor detects a stimulus (like touching a hot object), it triggers a chemical reaction that creates an electrical impulse. This impulse travels along a neuron in a specific path:
Passing the Signal: At the end of the axon, the electrical impulse causes the release of chemicals. These chemicals cross a tiny gap, called a synapse, and start a similar electrical impulse in the dendrite of the next neuron. This process continues, allowing the signal to travel throughout the body. A similar synapse, called a neuromuscular junction, allows impulses to be delivered from neurons to muscle cells or glands, causing them to act.
A reflex action is a sudden, involuntary action in response to a stimulus. We perform these actions without thinking, like pulling our hand away from a flame or jumping out of the path of a bus. These quick responses are crucial for survival in dangerous situations.
Thinking is a complex process involving many neurons in the brain. If we had to think about the danger of touching a flame, our hand could be severely burnt by the time we decided to move it. Reflex actions provide a shortcut.
The pathway taken by the nerve impulses in a reflex action is called the reflex arc.
Reflex arcs are formed in the spinal cord, allowing for an extremely fast response. While the reflex action is happening, the information also continues to the brain, so we become aware of the event after the action has already been taken.
While the spinal cord handles reflex actions, the brain is the main coordinating centre of the body. The brain and spinal cord together make up the central nervous system (CNS). The CNS receives and integrates information from all parts of the body. The peripheral nervous system (PNS), consisting of nerves extending from the brain and spinal cord, facilitates communication between the CNS and the rest of the body.
The brain allows us to think and perform voluntary actions, which are based on conscious decisions, like writing or moving a chair. It has three major regions: the fore-brain, mid-brain, and hind-brain.
Fore-brain: This is the main thinking part of the brain.
Mid-brain and Hind-brain: These regions control many involuntary actions—movements we do not consciously control.
The central nervous system is vital and delicate, so it is well-protected.
When a decision to act is made, the nervous system sends an electrical impulse to the appropriate muscle tissue.
Plants do not have a nervous system or muscles, but they still respond to stimuli. They show two main types of movement: one independent of growth and one dependent on growth.
Some plants, like the sensitive plant (chhui-mui or Mimosa), respond very quickly to touch. When its leaves are touched, they fold up and droop.
Most plant movements are slow, directional movements caused by growth in response to an environmental trigger. These are called tropic movements or tropisms. The movement can be towards the stimulus (positive tropism) or away from it (negative tropism).
The slow, growth-related movements in plants are controlled by chemical compounds called plant hormones (or phytohormones). These are synthesised in one part of the plant and diffuse to another part to influence growth.
While the nervous system provides rapid, targeted control, it has limitations. Its signals only reach cells connected by nervous tissue, and neurons need time to reset after transmitting an impulse. For widespread, sustained responses, animals use a second system of control: chemical communication via hormones.
The endocrine system consists of glands that secrete hormones directly into the bloodstream. The blood carries these hormones to all parts of the body, allowing them to act on specific target organs or tissues.
Adrenaline (from Adrenal Glands): This is the "fight or flight" hormone, released in scary or stressful situations. It prepares the body for intense activity by:
Thyroxin (from Thyroid Gland): Regulates the metabolism of carbohydrates, proteins, and fats to ensure balanced growth. Iodine is essential for the production of thyroxin. A deficiency of iodine can lead to a condition called goitre, which causes a swollen neck.
Growth Hormone (from Pituitary Gland): As its name suggests, it regulates the growth and development of the body. A deficiency during childhood can lead to dwarfism.
Testosterone (in males, from Testes) and Oestrogen (in females, from Ovaries): These sex hormones are responsible for the physical changes that occur during puberty.
Insulin (from Pancreas): Regulates blood sugar levels. If the pancreas does not produce enough insulin, blood sugar levels rise, causing diabetes. Diabetic patients may need to take injections of insulin to manage their condition.
The body must secrete hormones in precise quantities. The timing and amount of hormone release are regulated by feedback mechanisms.
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