Biology · Class 9-10 · CBSE
Control and Coordination: Neurons, Reflex Arc and Phytohormones
Explore the nervous system: anatomy of a neuron, synaptic transmission, the reflex arc pathway, divisions of the human brain (cerebrum, cerebellum, medulla), plant movements (tropic vs nastic), and phytohormones.
1Structure of a Neuron and Synaptic Transmission
TheNeuronis the structural and functional unit of the nervous system. It consists of three distinct anatomical regions:Cyton (cell body)with nucleus, branchingDendritesthat receive stimuli, and a long conductingAxonending in nerve terminals.
Electrochemical Nerve Impulse Transmission Across Synapse
- 1
1. Dendritic tip detects chemical stimulus from receptor and triggers an electrical impulse.
- 2
2. Impulse travels along dendrite → cell body → conducts along axon to nerve ending.
- 3
3. At axon terminal, electrical impulse triggers release of chemical neurotransmitters (e.g. Acetylcholine) into microscopic gap (Synapse).
- 4
4. Chemical diffuses across synapse and binds to receptors on dendrite of next neuron, generating a fresh electrical impulse.
2The Reflex Arc and the Human Brain
AReflex Actionis an automatic, involuntary, and rapid response to a sensory stimulus executed without conscious thought. The structural pathway traversed by nerve impulses in a reflex action is called aReflex Arc: Receptor → Sensory Neuron → Spinal Cord (Relay Neuron) → Motor Neuron → Effector Muscle.
Divisions and Functions of the Human Brain
| Brain Region | Sub-structures | Primary Control & Functional Role |
|---|---|---|
| Forebrain | Cerebrum (largest part) | Seat of intelligence, conscious thought, memory, voluntary actions, perception of sensory signals (sight, sound, smell) |
| Midbrain | Optic & auditory centers | Controls involuntary reflex movements of head, neck, and trunk in response to visual and auditory stimuli; pupil reflexes |
| Hindbrain: Cerebellum | Posterior folded structure | Coordinates precision voluntary movements and maintains posture, equilibrium, and physical balance (e.g. walking in straight line, riding bicycle) |
| Hindbrain: Medulla Oblongata | Lower brainstem connecting to spinal cord | Controls involuntary vital life activities: heartbeat, blood pressure, salivation, vomiting, swallowing, coughing |
| Hindbrain: Pons | Transverse nerve fiber bridge | Pneumotaxic center regulating respiration rate and facial sensations |
Forebrain
- Sub-structures
- Cerebrum (largest part)
- Primary Control & Functional Role
- Seat of intelligence, conscious thought, memory, voluntary actions, perception of sensory signals (sight, sound, smell)
Midbrain
- Sub-structures
- Optic & auditory centers
- Primary Control & Functional Role
- Controls involuntary reflex movements of head, neck, and trunk in response to visual and auditory stimuli; pupil reflexes
Hindbrain: Cerebellum
- Sub-structures
- Posterior folded structure
- Primary Control & Functional Role
- Coordinates precision voluntary movements and maintains posture, equilibrium, and physical balance (e.g. walking in straight line, riding bicycle)
Hindbrain: Medulla Oblongata
- Sub-structures
- Lower brainstem connecting to spinal cord
- Primary Control & Functional Role
- Controls involuntary vital life activities: heartbeat, blood pressure, salivation, vomiting, swallowing, coughing
Hindbrain: Pons
- Sub-structures
- Transverse nerve fiber bridge
- Primary Control & Functional Role
- Pneumotaxic center regulating respiration rate and facial sensations
3Coordination in Plants: Tropisms and Phytohormones
Plant Phytohormones and Physiological Effects
| Hormone Name | Site of Synthesis | Major Physiological Function in Plants |
|---|---|---|
| Auxin | Growing shoot tips and root tips | Promotes cell elongation; mediates phototropism (bends shoots towards light) and apical dominance |
| Gibberellins | Young leaves and seeds | Promotes stem elongation, breaking seed dormancy, fruit development |
| Cytokinins | Areas of rapid cell division (fruits, seeds, root tips) | Promotes active cell division, delays leaf senescence (aging), opens stomata |
| Abscisic Acid (ABA) | Leaves and mature tissues | Stress hormone: inhibits plant growth, induces stomatal closure during drought, causes leaf fall (abscission) |
Auxin
- Site of Synthesis
- Growing shoot tips and root tips
- Major Physiological Function in Plants
- Promotes cell elongation; mediates phototropism (bends shoots towards light) and apical dominance
Gibberellins
- Site of Synthesis
- Young leaves and seeds
- Major Physiological Function in Plants
- Promotes stem elongation, breaking seed dormancy, fruit development
Cytokinins
- Site of Synthesis
- Areas of rapid cell division (fruits, seeds, root tips)
- Major Physiological Function in Plants
- Promotes active cell division, delays leaf senescence (aging), opens stomata
Abscisic Acid (ABA)
- Site of Synthesis
- Leaves and mature tissues
- Major Physiological Function in Plants
- Stress hormone: inhibits plant growth, induces stomatal closure during drought, causes leaf fall (abscission)
4Exam Corner: Tips, Mistakes & Board Questions
Critical Board Revision Takeaways
- Iodine is essential for the thyroid gland to synthesize thyroxin hormone. Deficiency of iodine in diet causes thyroid enlargement known as Goitre.
- Insulin hormone is secreted by beta cells of the pancreas; it regulates and lowers blood glucose levels. Deficiency leads to Diabetes Mellitus.
- Adrenaline (epinephrine) is the emergency / flight-or-fight hormone secreted by adrenal medulla to increase heart rate, blood pressure, and breathing rate.
Exam question · CBSE Class 10 Board
- A.Cerebrum
- B.Cerebellum
- C.Medulla
- D.Hypothalamus
- The Cerebellum (located in the hindbrain) receives sensory input from the inner ear and muscles to coordinate muscular movements, precision, and physical balance.
- Correct answer is option B (Cerebellum).
Answer: B. Cerebellum
Remember
- Information travels along neurons as anelectrical impulseand across synapses viachemical neurotransmitters.
- Reflex Arcroutes through the spinal cord for rapid protection: Receptor → Sensory → Relay → Motor → Effector.
- Cerebellumcontrols posture and balance;Medullacontrols involuntary vitals (heartbeat, BP).
- Plant growth is guided by phytohormones:Auxinpromotes growth, whereasAbscisic Acid (ABA)inhibits growth.
Check yourself
Question 1
Shoots bend towards light because auxin hormone accumulates on which side of the growing stem tip?