Chapter Notes

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.

ANIMALS - NERVOUS SYSTEM

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.

How the Nervous System Works

1. Detecting Information: Information from the environment is detected by specialised nerve cell tips called receptors. These are located in our sense organs.

   • Gustatory receptors in the tongue detect taste.
   • Olfactory receptors in the nose detect smell.

2. 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:

   • Dendrite: Acquires the information.
   • Cell Body: The impulse travels through the cell body.
   • Axon: The impulse travels down the axon to its end.

3. 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.

What happens in Reflex Actions?

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.

• It starts with a receptor detecting a stimulus (e.g., heat on the skin).
• A sensory neuron sends the signal to the spinal cord.
• In the spinal cord, the signal is passed directly to a motor neuron (often via an interneuron).
• The motor neuron carries the signal to an effector (a muscle or gland), which carries out the response (e.g., the muscle contracts to pull the hand away).

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.

Human Brain

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.

Major Parts of the Brain and Their Functions

• Fore-brain: This is the main thinking part of the brain.

  • It has specialised areas for receiving sensory information like sight, hearing, and smell.
  • Association areas interpret this sensory information by combining it with other information and memories.
  • Based on this interpretation, a decision is made, and motor areas send instructions to voluntary muscles (like leg muscles) to act.
  • It also has centres for sensations like hunger.

• Mid-brain and Hind-brain: These regions control many involuntary actions—movements we do not consciously control.

  • Cerebellum (part of the hind-brain): This is responsible for the precision of voluntary actions and for maintaining the body's posture and balance. Activities like walking in a straight line, riding a bicycle, or picking up a pencil are possible because of the cerebellum.
  • Medulla (part of the hind-brain): This controls essential involuntary actions such as blood pressure, salivation, and vomiting. Other involuntary actions like heartbeat and breathing are also controlled by the hind-brain and mid-brain.

How are these Tissues protected?

The central nervous system is vital and delicate, so it is well-protected.

• The brain is housed inside the skull, a bony box. Inside the skull, the brain is further protected by a fluid-filled balloon that acts as a shock absorber.
• The spinal cord is protected by the vertebral column, or backbone.

How does the Nervous Tissue cause Action?

When a decision to act is made, the nervous system sends an electrical impulse to the appropriate muscle tissue.

• Muscle cells contain special proteins.
• When a nerve impulse reaches a muscle cell, these proteins change their shape and arrangement.
• This change causes the muscle cell to shorten.
• The collective shortening of many muscle cells results in muscle contraction and movement.

COORDINATION IN PLANTS

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.

Immediate Response to Stimulus (Growth-Independent)

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.

• This movement does not involve growth.
• The plant uses electrical-chemical signals to transmit information from cell to cell.
• The movement happens because specialised cells at the base of the leaves change shape by rapidly gaining or losing water, causing them to swell or shrink.

Movement Due to Growth (Tropic Movements)

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).

• Phototropism: Movement in response to light.
  • Shoots grow towards light (positive phototropism).
  • Roots grow away from light (negative phototropism).
• Geotropism: Movement in response to gravity.
  • Roots grow downwards, towards gravity (positive geotropism).
  • Shoots grow upwards, away from gravity (negative geotropism).
• Hydrotropism: Movement in response to water (e.g., roots growing towards a water source).
• Chemotropism: Movement in response to chemical stimuli (e.g., the growth of a pollen tube towards the ovule).
• Thigmotropism: Directional growth in response to touch. For example, the tendrils of a pea plant are sensitive to touch. When a tendril touches a support, the side away from the support grows faster, causing the tendril to circle around and cling to the object.

Plant Hormones

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.

• Auxin: A growth-promoting hormone synthesised at the shoot tip. It helps cells to grow longer. In phototropism, when light comes from one side, auxin diffuses to the shady side of the shoot. The higher concentration of auxin on the shady side causes those cells to elongate more, making the plant bend towards the light.
• Gibberellins: Help in the growth of the stem, similar to auxins.
• Cytokinins: Promote cell division. They are found in high concentrations in areas of rapid cell division, like fruits and seeds.
• Abscisic acid (ABA): A growth-inhibiting hormone. Its effects include the wilting of leaves.

HORMONES IN ANIMALS

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.

Key Hormones and Their Functions

• 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:

  • Increasing heart rate to supply more oxygen to muscles.
  • Increasing breathing rate.
  • Diverting blood from the digestive system and skin to the skeletal muscles.

• 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.

Feedback Mechanism

The body must secrete hormones in precise quantities. The timing and amount of hormone release are regulated by feedback mechanisms.