B14 - Coordination and Response

Overview

Coordination ensures organism survival. Requires communication systems to detect stimulus and initiate response.

• Detection: Stimulus received by Receptors.
• Processing: Signal processed in Central Nervous System (CNS) or Endocrine system.
• Response: Action initiated by Effectors.

Neuron Structure

• Definition: Neuron, or nerve cell, transmits electrical signals throughout the body.
• Structure Components: Neurons generally consist of three main parts:
  1. Cell Body (Soma): Contains the nucleus and organelles; maintains the cell’s life functions.
  2. Axon: Long, slender projection that transmits signals away from the cell body toward other neurons or effectors. Axons can be covered in a fatty sheath called myelin, which speeds up signal transmission significantly (like insulation on wires).
  3. Dendrites: Shorter, branched extensions that receive signals from other neurons and transmit impulses toward the cell body.
• Function Summary: Dendrite $\rightarrow$ Cell Body $\rightarrow$ Axon. (Reception $\rightarrow$ Processing $\rightarrow$ Transmission).

Deep Dive: Myelin Sheath

The axon is often covered by a fatty sheath called myelin. Its primary function is to act like electrical insulation on a wire. This greatly increases the speed of the action potential (electrical impulse) traveling down the axon, making the nervous system respond much faster than it otherwise would. Without myelin, signal transmission would be slow and energy inefficient.

Synapse Structure

• Vesicles: Contain neurotransmitters
• Synaptic Gap: Space between neurons
• Receptor Proteins: Bind neurotransmitters to transmit signal

Events at Synapse:

1. Impulse reaches presynaptic neuron
2. Neurotransmitter release from vesicles
3. Diffusion across synaptic gap
4. Binding to receptor proteins
5. New impulse in next neuron

Nervous Coordination

Mechanism: Electrical impulse transmission via neurons. Very rapid response time.

Pathway (Reflex Arc): Receptor $\rightarrow$ Sensory Neuron $\rightarrow$ CNS $\rightarrow$ Motor Neuron $\rightarrow$ Effector.

• Neuron Action Potential: Electrical charge (depolarisation/repolarisation) moves along axon in directional flow.
• Synapse Gap: Chemical transmission. Neurotransmitters cross gap to excite or inhibit next neuron.
• Key Concept: Reflexes: Automatic, immediate protective actions. CNS intercepts signal quickly.

Hormonal Coordination (Endocrine System / Homeostasis)

Definition: Maintenance of stable internal conditions despite external changes.

Examples:

• Blood glucose regulation (insulin/glucagon)
• Body temperature (sweating/shivering)
• Water balance (ADH hormone)

Mechanism: Chemical messengers (Hormones) travel via blood. Regulates long-term balance and processes. Slower action, but sustained effect.

• Feedback Loop: Primary control structure. All major systems rely on Negative Feedback to return body to set point (homeostasis).

Blood Glucose Homeostasis

Hormones Involved: Insulin, Glucagon, GLP-2 (Released by pancreatic Islets of Langerhans).

• Low Blood Glucose ($\downarrow$ Glu): Hypothalamus detects low glucose $\rightarrow$ release of Glucagon and potentially $GLP-2$; stimulates liver to break down stored glycogen (glycogenolysis) and synthesize glucose (gluconeogenesis) $\rightarrow \uparrow$ blood glucose.
• High Blood Glucose ($\uparrow$ Glu): Pancreas detects high glucose $\rightarrow$ releases Insulin $\rightarrow$ facilitates uptake of glucose by body cells; liver converts excess glucose into glycogen storage $\rightarrow \downarrow$ blood glucose.

Thermo-regulation

Process: Maintaining stable core temperature. Fluctuation destabilizes homeostasis. Hypothalamus acts as primary monitoring center.

• Cooling Responses: Sweating and vasodilation (blood vessels near skin surface expand, releasing heat).
• Warming Responses: Shivering/Vasoconstriction (reducing blood flow to outer skin layers to conserve heat).

Stress Response

• Input: Danger/Stress.
• Immediate Action: Adrenaline release rapidly (increased heart rate, respiration rate).
• Sustained Action: Cortisol release maintains response over long period.

Plant Coordination

Lacks nervous system; uses chemical signals and structural changes.

Tropisms

Directional growth responses to stimuli.

• Phototropism: Growth toward light via differential auxin concentration.
• Gravitropism: Reaction to gravity.

Nastic Movements

Non-directional movement regardless of stimulus angle (e.g., leaf folding upon touch).

Nervous vs Hormonal vs Plant Control

Feature	Nervous System	Hormonal System	Plant Tropisms
Messenger	Electrical/Chemical Impulse	Chemical (Hormones)	Chemical (Auxin)
Speed	Very Fast	Slow	Growth Rate Dependent
Duration	Brief/Immediate	Long-Lasting	Sustained Growth
Example	Reflex Arc	Blood Glucose Regulation	Phototropism

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Sense Organs

Organ	Stimulus Detected
Eye	Light
Ear	Sound waves
Skin	Touch, pressure, temperature
Nose	Chemicals (smell)
Tongue	Chemicals (taste)

Eye Structures

Structure	Function
Cornea	Refracts light
Iris	Controls pupil size
Pupil	Opening for light entry
Lens	Focuses light on retina
Retina	Contains photoreceptor cells
Optic nerve	Transmits impulses to brain
Blind spot	No photoreceptors

Pupil Reflex

• Stimulus: Bright light
• Response: Pupil constricts (pupil reflex)
• Pathway: Retina $\rightarrow$ optic nerve $\rightarrow$ brain $\rightarrow$ oculomotor nerve $\rightarrow$ iris muscles
• Purpose: Protect retina from excessive light

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Glossary

Term	Definition
Receptor	Structure that detects stimulus
Sensory neuron	Carries impulses from receptor to CNS
Relay neuron	Connects sensory and motor neurons
Motor neuron	Carries impulses to effector
Effector	Muscle or gland that responds
Synapse	Junction between neurons
Neurotransmitter	Chemical messenger at synapse
Hormone	Chemical messenger in blood
Negative feedback	Response reduces deviation
Homeostasis	Stable internal conditions
Tropism	Directional growth response
Auxin	Plant hormone
Phototropism	Growth toward light
Gravitropism	Growth response to gravity