Chapter 23: Control and Coordination

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VARDAAN COMET

1. The Neuron (Nerve Cell) & Impulse Transmission

The neuron is the structural and functional unit of the nervous system specialized to transmit electrical impulses.

Figure 23.1: Structure of a Neuron

Core Anatomical Parts:

Dendrites: Branched terminal fibers that receive external stimuli and generate an electrical impulse.
Cyton (Cell Body): Contains the nucleus, cytoplasm, and Nissl's granules; integrates incoming electrical signals.
Axon: A long single fiber carrying the electrical impulse away from the cyton toward target cells.
Myelin Sheath & Neurilemma: Insulating lipid layer around axon preventing leakage of impulses.
Nodes of Ranvier: Unmyelinated periodic gaps along axon enabling rapid jump conduction (saltatory conduction).
Synapse & Neurotransmitters: Microscopic gap ($20\text{--}30\text{ nm}$) between terminal bulb of one axon and dendrite of the next. Arrival of impulse causes exocytosis of neurotransmitter (e.g., Acetylcholine) which diffuses across the synaptic cleft. Transmission is strictly unidirectional.

Neuron Type	Direction of Transmission	Location
Sensory (Afferent)	Receptor $\longrightarrow$ Central Nervous System (CNS)	Sense organs to spinal cord/brain
Motor (Efferent)	Central Nervous System (CNS) $\longrightarrow$ Effector Organ	Brain/spinal cord to muscles or glands
Association (Relay)	Interconnects Sensory and Motor Neurons	Within gray matter of CNS

2. Central Nervous System: Brain & Spinal Cord

Figure 23.2: Longitudinal Section of Human Brain

Brain Protection & Enclosures:

Cranium: Hard bony skull protecting delicate neural tissue from external physical trauma.
Meninges (Three Layers): Outer tough Dura mater, middle web-like Arachnoid, inner highly vascular Pia mater. Meningitis is the infectious inflammation of these membranes.
Cerebrospinal Fluid (CSF): Shock-absorbing fluid in subarachnoid space and central canal; provides mechanical cushioning, buoyancy, and nutrient/gas exchange.

Brain Region	Structural Features	Key Physiological Functions
Cerebrum (Forebrain)	Largest part; two hemispheres connected by Corpus Callosum; heavily convoluted (gyri and sulci). Outer = Gray matter (cell bodies); Inner = White matter (axons).	Seat of intelligence, reasoning, memory, planning, sensory perception, consciousness, and all voluntary motor decisions.
Cerebellum (Hindbrain)	Located beneath posterior cerebrum; has fine furrows; no gyri.	Coordinates voluntary muscle contractions, timing of movement, posture, and body balance. Impaired by alcohol consumption.
Medulla Oblongata (Hindbrain)	Base of brain connecting to spinal cord.	Controls involuntary vital visceral functions: heartbeat, breathing rate, blood pressure, peristalsis, swallowing, coughing. Damage is fatal.

Spinal Cord Architecture:

Extends downward from medulla inside the neural canal of the vertebral column.
Reversed arrangement: Inner 'H'-shaped gray matter (cell bodies) and outer white matter (longitudinal axon tracts).
Functions: Controls all spinal reflex actions below the neck; conducts sensory inputs up to brain and motor commands down to effectors.

3. Reflex Action & The Reflex Arc

A reflex action is a sudden, automatic, spontaneous, and involuntary response to a sensory stimulus controlled at the level of the spinal cord without initial brain involvement.

Figure 23.3: Reflex Arc: Pathway of Nerve Impulse in a Spinal Reflex

The Five Sequential Steps of a Reflex Arc:

Receptor: Detects environmental stimulus (e.g., pain/heat receptor in finger skin).
Sensory (Afferent) Neuron: Transmits action potential along dorsal root into the spinal cord.
Interneuron (Relay Neuron): Evaluates input in the spinal cord gray matter and relays impulse directly to motor neuron.
Motor (Efferent) Neuron: Carries impulse through ventral root out to effector organ.
Effector: Muscle contracts (e.g., biceps withdraw hand) or gland secretes.

Natural (Inborn) Reflex	Conditioned (Acquired) Reflex
Inherited from birth; wired genetically without learning.	Acquired during lifetime through repetitive training/experience.
Spinal or brainstem level; no cerebral learning required.	Requires cerebral memory association and regular reinforcement.
Examples: Blinking, knee-jerk, pupil constriction in light, withdrawing hand from hot pan.	Examples: Salivation at smell of favourite food (Pavlov), playing guitar, cycling, applying brakes.

4. Peripheral Nervous System (PNS)

Somatic Nervous System (SNS): Regulates voluntary skeletal muscles. Contains 12 pairs of cranial nerves (from brain) and 31 pairs of spinal nerves (all mixed nerves from spinal cord).
Autonomic Nervous System (ANS): Regulates involuntary visceral organs and smooth muscles via two opposing/complementary divisions:

Target Organ	Sympathetic Division ("Fight or Flight")	Parasympathetic Division ("Rest and Digest")
Heart Rate	Accelerates rate and cardiac output.	Decelerates heart rate back to resting.
Pupils	Dilates pupils (mydriasis).	Constricts pupils (miosis).
Bronchioles	Dilates airways for maximum oxygen intake.	Constricts airways to normal caliber.
Digestion & Salivation	Inhibits peristalsis and salivary secretion.	Stimulates digestive enzymes and salivation.
Blood Glucose	Stimulates glycogen breakdown into glucose.	Promotes glucose uptake and storage as glycogen.

5. Human Endocrine System & Hormones

Hormones are chemical messengers synthesized by ductless (endocrine) glands, secreted directly into blood, and transported to act on specific target organs.

Figure 23.4: Complete Human Endocrine System

Figure 23.5: Close-Up Diagram of Endocrine Glands

Gland & Location	Hormone Secreted	Target & Function	Clinical Deficiency / Hyperactivity
Pituitary Gland (Base of brain; Master Gland)	• Growth Hormone (GH / STH) • Gonad Stimulating Hormone (GSH)	• Promotes bone elongation & body growth. • Stimulates gonads to produce sex hormones at puberty.	• Dwarfism: Childhood GH deficiency. • Gigantism: Childhood GH hypersecretion. • Cushing's Disease: Hyperactivity causing hirsutism & sterility.
Thyroid Gland (Anterior neck)	Thyroxine ($T_4$) (Requires dietary Iodine)	Regulates Basal Metabolic Rate (BMR), carbohydrate/protein/fat metabolism, and tissue growth.	• Simple Goitre: Swelling of neck gland due to iodine deficiency. • Cretinism: Infantile thyroid deficiency causing dwarfism and mental retardation.
Pancreas (Islets of Langerhans)	• Insulin ($\beta$-cells) • Glucagon ($\alpha$-cells)	• Insulin lowers blood glucose levels. • Glucagon raises blood glucose levels.	• Diabetes Mellitus: Insulin hyposecretion resulting in hyperglycemia and glucose in urine.
Adrenal Gland (Atop each kidney)	Adrenaline (Epinephrine)	"Emergency hormone"; prepares body for fight-or-flight; elevates heart rate and blood sugar.	Impaired stress adaptation and blood pressure regulation.
Testes (Male)	Testosterone	Sperm production; male secondary sexual characteristics (deep voice, facial hair).	Failure of sexual maturation and spermatogenesis.
Ovaries (Female)	Estrogen & Progesterone	Ovum development, female secondary traits, menstrual cycle regulation, pregnancy maintenance.	Menstrual disruption and infertility.

6. Plant Coordination: Phytohormones & Tropisms

The 5 Major Phytohormones:

Auxins: Shoot/root tip elongation; apical dominance; phototropic curvature.
Gibberellins: Stem elongation (bolting); breaks seed dormancy; fruit growth.
Cytokinins: Promotes active cell division (cytokinesis); delays leaf senescence.
Abscisic Acid (ABA): Stress hormone; induces stomatal closure during drought; leaf abscission.
Ethylene: Gaseous hormone promoting commercial fruit ripening.

Figure 23.6: Phototropism: Shoot Bending Toward Light Due to Auxin Redistribution

Figure 23.7: Geotropism: Positive Geotropism in Roots, Negative Geotropism in Shoots

Tropic Movement	Stimulus	Positive Response	Negative Response
Phototropism	Unidirectional Light	Stem shoots bend toward light	Roots grow away from light
Geotropism	Gravity	Primary roots grow downward into soil	Shoots grow upward against gravity
Hydrotropism	Water / Moisture	Roots grow toward water sources	-
Chemotropism	Chemical Gradient	Pollen tube grows toward ovule micropyle	-
Thigmotropism	Physical Touch	Tendrils coil around support	-

7. Summary & Direct Exam Points

Synapse Unidirectionality: Neurotransmitters are located only in presynaptic axon terminals, while receptors exist only on postsynaptic dendrites.
Vital Centers in Medulla: Injury to medulla oblongata is instantly fatal because it controls autonomous respiratory rhythmicity and cardiac pacemakers.
Alcohol and Ataxia: Alcohol disrupts the cerebellum, impairing voluntary muscular coordination and balance, causing staggering gait.
Iodine and Goitre: Iodine is needed to make thyroxine. Deficiency causes compensatory enlargement of the thyroid gland (simple goitre).
Insulin Action: Secreted by pancreas $\beta$-cells; facilitates cellular uptake of glucose from blood. Deficiency causes Diabetes Mellitus.
Adrenaline Surge: The emergency hormone produced by adrenal glands during danger to prepare the body for fight or flight.