Animals – Nervous System

In animals, control and coordination are primarily achieved by the nervous system and the muscular system, working together.

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The nervous system is an organised network of nerve cells called neurons, specialised for receiving, transmitting, and processing information via electrical impulses. Information from the environment (stimuli) is detected by specialised tips of nerve cells called receptors, typically located in sense organs (eyes, ears, nose, tongue, skin). Different receptors detect different stimuli (e.g., gustatory receptors for taste, olfactory receptors for smell, photoreceptors for light).

Transmission of Nervous Impulses:

1. Information is acquired at the dendritic tip of a neuron.
2. This triggers a chemical reaction that generates an electrical impulse.
3. The electrical impulse travels from the dendrite to the cell body.
4. From the cell body, the impulse travels along the axon to its end (nerve ending).
5. At the axon terminal, the electrical impulse causes the release of chemicals called neurotransmitters.
6. These chemicals cross the gap between two neurons or between a neuron and another cell (like a muscle or gland cell), called a synapse.
7. Neurotransmitters stimulate the next neuron or cell, initiating a response (e.g., generating a new electrical impulse in the next neuron, causing a muscle cell to contract, or a gland to secrete).

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What Happens In Reflex Actions?

Reflex actions are sudden, involuntary responses to stimuli that occur without conscious thought or control. They are rapid reactions to potentially dangerous situations.

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Examples: pulling your hand back from a hot object, blinking your eyes in bright light, mouth watering at the sight of food.

Conscious thinking involves complex processing of information by the brain's intricate network of neurons. This processing takes time. In urgent situations (like touching a hot object), requiring a quick response, relying on conscious thought would take too long and could result in harm.

The body has evolved reflex arcs to allow for rapid responses. A reflex arc is a pathway that connects the detecting neuron (sensory neuron) directly or indirectly to the responding neuron (motor neuron) that controls the muscles.

In humans, reflex arcs are typically formed in the spinal cord. The sensory neuron (receiving input from a receptor) synapses with a relay neuron (interneuron) in the spinal cord, which then synapses with a motor neuron (sending output to a muscle). This bypasses the conscious thinking part of the brain, allowing for a much faster response. While the reflex action occurs, information about the stimulus also travels up the spinal cord to the brain, so we become aware of the event after the reflex has taken place.

Reflex arcs are efficient mechanisms for quick responses and are crucial for survival, especially in animals with less developed thinking capabilities. They remain beneficial even in complex animals for their speed.

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Human Brain

The human brain is the main coordinating centre of the body. Along with the spinal cord, it forms the central nervous system (CNS). The brain is responsible for complex processes like thinking, decision-making, voluntary actions, processing sensory information, and controlling involuntary actions.

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The brain has three major regions:

1. Forebrain: The main thinking part. Receives sensory impulses from various receptors (specialised areas for sight, hearing, smell, etc.). Integrates information from different senses and memory. Makes decisions for voluntary actions (controlled by voluntary muscles). Contains centres for sensations like hunger.
2. Midbrain and Hindbrain: Control involuntary actions (actions not under conscious control).

Hindbrain Parts and Functions:

• Cerebellum: Responsible for precision of voluntary actions (e.g., walking in a straight line, riding a bicycle, picking up a pencil) and maintaining posture and balance of the body.
• Pons and Medulla: Control various involuntary actions like blood pressure, salivation, vomiting, heart beat, and breathing.

Communication between the CNS (brain and spinal cord) and the rest of the body is handled by the peripheral nervous system (PNS), consisting of cranial nerves (from brain) and spinal nerves (from spinal cord).

Voluntary actions are consciously controlled by the forebrain. Involuntary actions (like heart beat, breathing, digestion) are controlled by the midbrain and hindbrain, often without conscious thought. Reflex actions are a distinct category, bypassing conscious processing in the brain for speed, but still involving the CNS (spinal cord).

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How Are These Tissues Protected?

The delicate and vital organs of the central nervous system are well-protected within the body.

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• The brain is protected by the skull (a bony box). It is also cushioned by a fluid-filled balloon-like structure inside the skull, which provides shock absorption.
• The spinal cord is protected by the vertebral column or backbone, a hard, bony structure running down the back.

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How Does The Nervous Tissue Cause Action?

Nervous tissue conveys decisions to muscles, which perform the final action or movement.

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Muscle cells move by changing their shape, specifically by shortening (contracting). Muscle cells contain special proteins that alter their shape and arrangement when they receive a nervous (electrical) impulse from a motor neuron at the neuromuscular junction. This change in protein structure causes the muscle cell to shorten, resulting in muscle contraction and movement.

The type of muscle (voluntary or involuntary) determines whether the action is under conscious control (forebrain) or involuntary control (midbrain/hindbrain or spinal cord reflex).

Coordination In Plants

Plants lack a nervous system and muscles, yet they respond to stimuli and exhibit coordination. Plant responses are typically slower and involve mechanisms different from those in animals.

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Plants exhibit two main types of movement:

1. Movement independent of growth (immediate response).
2. Movement due to growth (directional growth).

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Immediate Response To Stimulus

Some plant movements are rapid and do not involve growth. The folding and drooping of leaves of the 'touch-me-not' plant (Mimosa pudica) upon touch is a classic example.

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When the plant is touched, the stimulus information is communicated from the point of touch to other parts of the plant. Plants use electrical-chemical signals for cell-to-cell communication, but they lack specialised nervous tissue for rapid impulse conduction.

Movement occurs because specific plant cells change shape. Instead of contractile proteins like in animal muscles, plant cells change shape by altering the amount of water in them. When cells lose water (due to chemicals signals triggered by touch), they shrink, causing parts of the plant (like leaf stalks) to change shape and the leaves to fold or droop. When water returns, the cells regain their shape, and the leaves open up.

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Movement Due To Growth

Many plant responses involve directional growth in response to environmental stimuli. This growth is slower than immediate responses but appears as movement.

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Directional growth movements are called tropisms. Tropisms can be either towards the stimulus (positive tropism) or away from the stimulus (negative tropism).

Types of Tropisms:

• Phototropism: Growth response to light. Shoots are usually positively phototropic (grow towards light), helping leaves access sunlight for photosynthesis. Roots are usually negatively phototropic (grow away from light).
• Geotropism: Growth response to gravity (Earth's pull). Roots are usually positively geotropic (grow downwards), anchoring the plant and accessing water/minerals. Shoots are usually negatively geotropic (grow upwards).
• Hydrotropism: Growth response to water. Roots are positively hydrotropic (grow towards water sources).
• Chemotropism: Growth response to chemicals. Example: Growth of pollen tubes towards ovules during fertilisation in flowering plants is guided by chemical signals.
• Thigmotropism: Growth response to touch. Tendrils in climbing plants are thigmotropic; they grow around a support upon touch, helping the plant climb. The part of the tendril touching the support grows slower than the side away from it, causing the tendril to coil around the support.

Directional growth is controlled by plant hormones. For example, auxin, synthesised at the shoot tip, promotes cell elongation. When light comes from one side, auxin accumulates on the shaded side, stimulating cell growth there, causing the shoot to bend towards the light source.

Compared to rapid electrical impulses used in animal nervous systems, chemical communication via hormones in plants is slower but allows signals to reach many cells and can be persistent.

Other plant hormones include gibberellins (promote stem growth), cytokinins (promote cell division, abundant in fruits/seeds), and abscisic acid (inhibits growth, causes wilting of leaves).

Hormones In Animals

In addition to the nervous system, animals use chemical signals called hormones for control and coordination. This forms the endocrine system.

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Hormones are chemical messengers secreted by endocrine glands (ductless glands) directly into the bloodstream. They travel through the blood to target cells or organs in other parts of the body, where they exert specific effects.

Hormonal coordination is typically slower than nervous coordination but can produce wider-ranging and longer-lasting effects, reaching all cells in the body.

Example: Response to a Scary Situation (Fight or Flight Response): When an animal (like a squirrel) senses danger, the adrenal glands secrete the hormone adrenaline. Adrenaline prepares the body for intense physical activity (fighting or running away) by causing widespread changes:

• Heart beats faster, increasing blood supply and oxygen delivery to muscles.
• Blood flow to the digestive system and skin is reduced, diverting blood to skeletal muscles.
• Breathing rate increases, increasing oxygen intake.
• Glucose is released from the liver into the blood for quick energy.

These responses occur rapidly because adrenaline is released directly into the blood and reaches target organs quickly.

Hormones and Coordinated Growth: Hormones play a crucial role in regulating growth and development in animals, ensuring that growth occurs in a controlled and coordinated manner in specific parts of the body.

• Thyroxin: Produced by the thyroid gland, requires iodine for synthesis. Regulates carbohydrate, protein, and fat metabolism for growth and development. Iodine deficiency can lead to goitre (swollen neck).
• Growth Hormone: Secreted by the pituitary gland. Regulates overall growth and development of the body. Deficiency in childhood causes dwarfism; excess can cause gigantism.
• Sex Hormones: Testosterone (males, produced by testes) and Oestrogen (females, produced by ovaries) cause changes associated with puberty.
• Insulin: Produced by the pancreas. Regulates blood sugar levels. Insufficient insulin leads to diabetes, often treated with insulin injections.

S.No.	Hormone	Endocrine Gland	Functions
1.	Growth hormone	Pituitary gland	Stimulates growth in all organs
2.	Thyroxin	Thyroid gland	Regulates carbohydrate, protein and fat metabolism for body growth
3.	Insulin	Pancreas	Regulates blood sugar level
4.	Testosterone	Testes	Development of male sex organs, secondary sexual characteristics, etc.
5.	Oestrogen	Ovaries	Development of female sex organs, regulates menstrual cycle, etc.
6.	Adrenaline	Adrenal gland	Prepares the body for fight or flight response (increases heart rate, breathing, etc.)
7.	Releasing hormones / Inhibiting hormones	Hypothalamus	Regulates the release of hormones from the pituitary gland

Hormone secretion is regulated by feedback mechanisms. The timing and amount of hormone released are controlled by the body's needs. For example, high blood sugar levels stimulate the pancreas to release more insulin, which lowers blood sugar. As blood sugar falls, insulin secretion decreases.

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