Control and Coordination — NCERT Solutions

Correct Answer: (d) All of the above

Justification: The brain performs all three functions:

• Thinking is carried out by the cerebrum (forebrain).
• Regulating heart beat and other involuntary actions are controlled by the medulla oblongata (hindbrain).
• Balancing the body and coordinating muscular movements are functions of the cerebellum (hindbrain).

Therefore, all of the above is correct.

Function of Receptors:

Receptors are specialised cells or nerve endings present in the sense organs (eyes, ears, nose, tongue, skin) that detect stimuli from the environment (light, sound, smell, taste, touch, temperature, pain, etc.) and generate nerve impulses in response. These impulses are then transmitted to the brain or spinal cord via sensory (afferent) neurons for processing and appropriate response.

Examples of receptors:

• Photoreceptors (rods and cones) in the eyes — detect light.
• Phonoreceptors in the ears — detect sound.
• Thermoreceptors in the skin — detect temperature.
• Chemoreceptors in the nose and tongue — detect smell and taste.

Problems when receptors do not work properly:

Conclusion: If receptors do not function properly, the body cannot detect changes in the environment, and appropriate responses cannot be initiated, which can be life-threatening.

Structure of a Neuron:

A neuron (nerve cell) consists of the following parts:

1. Cell body (Cyton/Soma): The main body of the neuron containing the nucleus and cytoplasm. It is the metabolic centre of the cell.

2. Dendrites: Short, branched extensions arising from the cell body. They receive nerve impulses from other neurons or receptors and carry them towards the cell body.

3. Axon: A single, long cylindrical projection arising from the cell body. It carries nerve impulses away from the cell body towards the next neuron or effector organ (muscle/gland). The axon is covered by a myelin sheath (in myelinated neurons) which insulates it and speeds up impulse transmission.

4. Axon terminals (Synaptic knobs): The branched ends of the axon that form synapses with the dendrites of the next neuron. They release neurotransmitters to pass the impulse across the synapse.

$$\text{Dendrites} \rightarrow \text{Cell Body} \rightarrow \text{Axon} \rightarrow \text{Axon Terminals} \rightarrow \text{Next Neuron}$$

[Note: Draw a neuron showing the cell body with nucleus, multiple dendrites, a long axon with myelin sheath, nodes of Ranvier, and branched axon terminals.]

Function of a Neuron:

The neuron is the structural and functional unit of the nervous system. Its primary function is to receive, process, and transmit information in the form of electrical signals (nerve impulses).

• Sensory neurons carry impulses from receptors to the brain/spinal cord.
• Motor neurons carry impulses from the brain/spinal cord to effectors (muscles/glands).
• Relay (interneurons) connect sensory and motor neurons within the brain and spinal cord.

The transmission of impulse across a synapse occurs chemically via neurotransmitters (e.g., acetylcholine), which diffuse across the synaptic gap and generate a new impulse in the next neuron.

Given/Concept: Phototropism is the directional growth movement of a plant in response to light. It is controlled by the plant hormone auxin.

Explanation of Phototropism:

1. When light falls on a plant from one side (unidirectional light), the plant detects the direction of light.

2. The plant hormone auxin (produced at the shoot tip) plays a key role. Auxin promotes cell elongation.

3. When light falls from one side:

• Auxin migrates away from the light and accumulates on the shaded (darker) side of the shoot.
• The cells on the shaded side receive more auxin and elongate more than the cells on the illuminated side.

4. Due to unequal cell elongation:

• The shaded side grows longer.
• The illuminated side grows shorter.
• This causes the shoot to bend towards the light source.

$$\text{Light} \rightarrow \text{Auxin moves to shaded side} \rightarrow \text{More elongation on shaded side} \rightarrow \text{Shoot bends towards light}$$

Result: The shoot shows positive phototropism (bends towards light), which helps the plant maximise light absorption for photosynthesis.

Note: Roots show negative phototropism (bend away from light) because roots are more sensitive to auxin — even small concentrations inhibit root cell elongation.

Given/Concept: The spinal cord serves as the pathway for nerve impulses between the brain and the rest of the body, and also coordinates reflex actions.

Answer:

In case of a spinal cord injury, the following signals will get disrupted:

1. Signals from the brain to muscles (motor signals): Voluntary motor commands sent from the brain through the spinal cord to the muscles below the injury site will be disrupted. This results in paralysis — the person loses voluntary control over muscles (e.g., legs, bladder, bowel).

2. Signals from the body to the brain (sensory signals): Sensory information (touch, pain, temperature, pressure) from body parts below the injury site cannot reach the brain. This results in loss of sensation in those regions.

3. Reflex actions: Reflex arcs that pass through the injured region of the spinal cord will also be disrupted, affecting involuntary protective responses.

Summary:

• All voluntary movements below the injury level are lost.
• All sensory perceptions (pain, touch, temperature) below the injury level are lost.
• Reflex actions coordinated by the injured spinal cord segment are disrupted.

Conclusion: A spinal cord injury can lead to partial or complete loss of motor and sensory functions below the level of injury, potentially causing paraplegia or quadriplegia depending on the location of the injury.

Given/Concept: Plants do not have a nervous system. Chemical coordination in plants is achieved through plant hormones (phytohormones).

Answer:

Chemical coordination in plants occurs through plant hormones, which are chemical substances produced in one part of the plant and transported to other parts where they regulate growth and other physiological processes.

Major Plant Hormones and their roles:

How it works:

1. A stimulus (e.g., light, gravity, touch) is perceived by the plant.
2. In response, specific hormones are synthesised and transported (through phloem or by diffusion) to the target cells.
3. The hormones cause changes in cell elongation, division, or differentiation, resulting in a coordinated response (e.g., bending towards light, root growth downward).

Conclusion: Unlike animals, plants use only chemical means (hormones) for coordination. These hormones act as messengers to regulate growth, development, and responses to environmental stimuli.

Answer:

All living organisms are made up of various organs and organ systems that must work together in a harmonious and integrated manner to maintain life. A system of control and coordination is essential for the following reasons:

1. Integration of body functions: Different organs (heart, lungs, kidneys, muscles, etc.) must work in a coordinated way. For example, when we run, the heart beats faster, breathing increases, and muscles receive more blood — all simultaneously.

2. Response to stimuli: Organisms need to detect and respond to changes in the environment (stimuli) such as light, temperature, danger, food, etc. Control systems enable quick and appropriate responses.

3. Maintenance of homeostasis: The internal environment of the body (temperature, blood sugar, water balance, pH) must be kept constant. Control mechanisms (e.g., hormonal feedback) ensure this balance.

4. Survival and protection: Quick reflex responses (e.g., withdrawing hand from a hot object) protect the organism from injury.

5. Growth and development: Hormones regulate growth, reproduction, and development at the right time and in the right proportion.

6. Coordination of voluntary actions: Complex activities like walking, writing, and speaking require precise coordination between the brain, nerves, and muscles.

Conclusion: Without a system of control and coordination, the various parts of an organism would function independently and chaotically, making survival impossible. The nervous system and endocrine system together fulfil this vital role in animals, while hormones serve this function in plants.

Difference between Involuntary Actions and Reflex Actions:

Key distinction:

• Involuntary actions are continuous background processes (e.g., the heart never stops beating).
• Reflex actions are sudden, one-time responses to a specific stimulus (e.g., you touch something hot and immediately pull your hand away).

Conclusion: Both types of actions occur without conscious thought, but reflex actions are rapid responses to stimuli coordinated by the spinal cord, while involuntary actions are ongoing processes regulated by the brain (medulla oblongata).

Comparison of Nervous and Hormonal Mechanisms:

Similarities:

1. Both are systems of control and coordination in animals.
2. Both help the organism respond to internal and external stimuli.
3. Both work together to maintain homeostasis.
4. Both involve communication between different parts of the body.

Differences:

Conclusion:
The nervous system provides rapid, precise, and short-term control (e.g., moving a muscle), while the hormonal (endocrine) system provides slower, widespread, and long-term control (e.g., regulating growth and metabolism). Both systems complement each other to achieve complete control and coordination in the body.

Difference between Movement in a Sensitive Plant and Movement in Human Legs:

Conclusion:
Movement in a sensitive plant is a non-muscular, turgor-based, non-nervous response to touch, while movement in human legs is a muscular, nerve-controlled voluntary action. The fundamental difference is that plant movement involves changes in cell water content, whereas animal movement involves muscle contraction driven by the nervous system.