ClassRoom Thinker — Unit 4 Biological Basis of Behaviour

01

The Biological Blueprint — From Molecule to Behaviour

The unit’s master map

🧬Genes & molecules

DNA, proteins, hormones, transmitters

🔬Cells

Neurons, glia, muscle and gland cells

🔗Circuits

Synapses and interconnected pathways

🧠Systems

Sensory, motor, autonomic, endocrine

👤Behaviour

Perception, motivation, emotion and action

Physiological psychology

Studies how biological processes—especially nervous and endocrine systems—produce and regulate behaviour and mental activity.

यह “brain causes behaviour” जैसा one-way idea नहीं; behaviour और experience भी brain को बदलते हैं।

Four rules for this unit

Function is usually distributed across networks.
Structure constrains function, but plasticity allows change.
Body and brain communicate in both directions.
Genes influence probabilities, not fixed destinies.

🎯 NET lens: When two answers look correct, prefer the one that respects a system/network rather than a simplistic one-centre explanation.

02

Sensory Systems — How Physical Energy Becomes Experience

General and special sensations, receptors and processing

🌍Stimulus

Light, sound, pressure, chemical, temperature

📡Reception

Specialized receptor detects energy

⚡Transduction

Energy becomes neural signal

🛤️Transmission

Afferent pathway carries information

🧩Perception

Brain organizes and interprets

General senses

Receptors are distributed widely: touch, pressure, vibration, temperature, pain and proprioception.

Special senses

Specialized organs: vision, audition, olfaction, gustation and vestibular equilibrium.

Transduction

Conversion of physical/chemical stimulus energy into receptor potential and then neural coding.

Receptor class	Best stimulus	Examples
Mechanoreceptors	Pressure, stretch, vibration, sound	Skin receptors, hair cells, muscle spindle
Photoreceptors	Light	Retinal rods and cones
Chemoreceptors	Molecules / chemical concentration	Taste, smell, blood CO₂/O₂ receptors
Thermoreceptors	Temperature change	Warm and cold receptors
Nociceptors	Potential/actual tissue damage	Mechanical, thermal and polymodal pain endings
Proprioceptors	Muscle length, tension, joint position	Muscle spindles, Golgi tendon organs

Sensory system	Receptor / transduction clue	Core pathway and destination
Vision	Rods: dim light; cones: colour/detail. Light hyperpolarizes photoreceptors.	Retina → optic nerve/chiasm → LGN of thalamus → primary visual cortex (occipital).
Audition	Cochlear hair cells bend with basilar-membrane movement; sound frequency is tonotopically coded.	Cochlear nerve → brainstem nuclei → inferior colliculus → MGN → auditory cortex (temporal).
Vestibular sense	Semicircular canals detect angular acceleration; otolith organs detect linear acceleration/head tilt.	Vestibular nerve → vestibular nuclei/cerebellum → eye, posture and cortical networks.
Taste	Taste receptor cells respond to sweet, sour, salty, bitter and umami chemicals.	CN VII, IX, X → nucleus of solitary tract → thalamus → insular/frontal opercular cortex.
Smell	Olfactory receptor neurons use chemical binding; their axons form CN I.	Olfactory bulb → piriform/limbic regions; unique initial cortical route without first thalamic relay.
Body senses	Skin, muscle, joint and pain receptors.	Dorsal-column system emphasizes fine touch/proprioception; spinothalamic system emphasizes pain, temperature and crude touch.

Receptor location

Exteroceptors: outside world. Interoceptors: internal organs. Proprioceptors: body position and movement.

Adaptation

Phasic receptors adapt quickly and signal change; tonic receptors adapt slowly and signal continuing state. Pain receptors adapt little—protective information must persist.

👩‍🏫 Teacher picture: Receptor is a translator, not a camera. It selects the energy it is tuned for, converts it, and the brain reconstructs meaning.

🧠 Path: Energy → Receptor → Transduction → Afferent signal → Thalamus/cortex. Olfaction is the famous exception: initial cortical access does not require a first thalamic relay.

03

Neurons & Glia — The Communicators and Their Support Crew

Structure, function and major types

🔬 Neuron anatomy map

Receive → Integrate → Conduct → Release

📥

Dendrites
Receive input from receptors/neurons.

🧮

Soma & axon hillock
Maintain cell and integrate graded potentials.

➡️

Axon
Conducts action potential away from soma.

📤

Terminal buttons
Release neurotransmitter at synapse.

Classification	Types	Meaning
By function	Sensory (afferent), motor (efferent), interneuron	To CNS, from CNS, or within CNS.
By structure	Unipolar/pseudounipolar, bipolar, multipolar	One process, two processes, or many dendrites + one axon.

Glial cell	Location	Main function
Astrocytes	CNS	Metabolic support, ion balance, synapse regulation, blood–brain barrier contribution.
Oligodendrocytes	CNS	One cell myelinates segments of multiple axons.
Schwann cells	PNS	Each cell myelinates one axon segment; supports regeneration.
Microglia	CNS	Immune surveillance, debris removal and inflammatory response.
Ependymal cells	CNS ventricles	Line ventricles; related to CSF production/circulation.
Satellite cells	PNS ganglia	Support neuronal cell bodies and regulate microenvironment.

🎯 Classic pairing: CNS myelin = oligodendrocyte; PNS myelin = Schwann cell.

04

Neural Impulse — The Electrical Story in Five Phases

Resting potential, action potential and saltatory conduction

⚡ Action-potential phase explorer

Resting · about −70 mV

Move through the phases1 / 5

Resting potential

Inside is negative (~−70 mV). Na⁺/K⁺ pump and leak channels maintain gradients.

All-or-none law

Once threshold is reached, action-potential size does not increase with stimulus strength. Stronger stimulus is coded mainly by firing rate and recruited neurons.

Refractory periods

Absolute: no second action potential. Relative: possible only with stronger stimulation. Refractoriness supports one-way propagation and limits firing rate.

Saltatory conduction

In myelinated axons, current effectively “jumps” node to node. Larger diameter and myelin increase conduction speed.

🧠 Ions: Na⁺ enters → depolarization; K⁺ leaves → repolarization. Think “Na in, K out.”

05

Synaptic Transmission — Where Electricity Becomes Chemistry

Synaptic steps, EPSPs/IPSPs and neurotransmitters

🧪 Chemical synapse switch

Excitatory mode · EPSP

Presynaptic terminal

Action potential → Ca²⁺ entry → vesicle fusion

Postsynaptic membrane

Depolarization makes firing more likely.

⚡1. Arrival

AP reaches terminal

➕2. Calcium

Voltage-gated Ca²⁺ channels open

🫧3. Release

Vesicles exocytose transmitter

🔐4. Binding

Receptors alter ion channels/signalling

♻️5. Clearing

Reuptake, enzyme or diffusion

Neurotransmitter	Major roles	High-yield association
Acetylcholine (ACh)	Neuromuscular transmission, attention, learning/memory, autonomic function	Loss of basal-forebrain cholinergic neurons in Alzheimer’s disease
Dopamine	Movement, reward learning, motivation, cognition	Nigrostriatal loss in Parkinson’s; mesolimbic dysregulation in psychosis models
Norepinephrine	Arousal, vigilance, attention, stress response	Locus coeruleus is a major brain source
Serotonin (5-HT)	Mood, sleep, appetite, impulse regulation	Raphe nuclei are major source
GABA	Main inhibitory transmitter in adult CNS	Reduces neuronal excitability
Glutamate	Main excitatory transmitter; learning and LTP	NMDA/AMPA receptors; excess can cause excitotoxicity
Endorphins / enkephalins	Endogenous opioid pain modulation and reward	Natural analgesia
Substance P	Pain transmission and inflammation	Commonly linked with nociceptive signalling

⚠️ A neurotransmitter is not simply “good” or “bad,” and most are not purely excitatory/inhibitory. Effect depends on receptor subtype, circuit and location.

06

Central Nervous System — The Command Network

Brain, spinal cord, cortex, protective layers and language areas

Nervous System

CNS
Brain + spinal cord

PNS
Nerves + ganglia

Forebrain
Cortex, thalamus, hypothalamus, basal ganglia, limbic structures

Midbrain
Orienting, movement, arousal pathways

Hindbrain
Pons, medulla, cerebellum

🧠 Cortex explorer

Tap a coloured region

Frontal lobe

Executive functions, planning, inhibition, voluntary motor control and speech production. Primary motor cortex lies in the precentral gyrus.

Broca’s area (usually left frontal) speech production से जुड़ा है।

Thalamus

Major relay/integration station for sensory and motor information to cortex; olfaction is the classic initial-relay exception.

Hypothalamus

Homeostasis, autonomic regulation, endocrine control via pituitary, hunger, thirst, temperature, circadian and reproductive functions.

Basal ganglia

Action selection, movement, habits and reinforcement learning; includes caudate, putamen and globus pallidus.

Brainstem

Midbrain, pons and medulla; arousal, cranial functions, respiration and cardiovascular regulation.

Cerebellum

Coordination, balance, timing, motor learning and error correction; also contributes to cognition.

Spinal cord

Dorsal roots/horns mainly receive sensory input; ventral roots/horns send motor output; supports reflexes.

Protection	Position/function
Dura mater	Tough outer meninx.
Arachnoid mater	Web-like middle layer; subarachnoid space contains CSF.
Pia mater	Delicate vascular layer closely follows brain surface.
CSF	Buoyancy, cushioning, chemical stability and waste transport.

🎯 Language contrast: Broca damage → nonfluent/effortful output; Wernicke damage → fluent but poorly meaningful speech and poor comprehension. Arcuate fasciculus damage is linked with conduction aphasia.

07

Peripheral Nervous System — The Body–Brain Communication Grid

Somatic, autonomic, sympathetic and parasympathetic systems

Peripheral Nervous System
12 cranial nerve pairs + 31 spinal nerve pairs

Somatic NS
Sensory input + skeletal-motor output

Autonomic NS
Viscera, smooth/cardiac muscle, glands

Sympathetic
Mobilize · thoracolumbar

Parasympathetic
Restore · craniosacral

Enteric
Intrinsic control of gut

Organ/function	Sympathetic	Parasympathetic
Pupil	Dilates	Constricts
Heart	Rate/force increase	Rate decreases
Bronchi	Dilate	Constrict toward resting state
Digestion	Inhibits activity	Promotes activity
Energy	Mobilizes glucose/fat	Supports storage and restoration

Afferent vs efferent

Afferent arrives at CNS carrying sensory information. Efferent exits CNS carrying motor/autonomic commands.

Autonomic is not “automatic only”

It is largely involuntary, yet breathing, biofeedback, attention and emotion can modulate autonomic activity.

🧠 SAME DAVE: Sensory Afferent, Motor Efferent; Dorsal Afferent, Ventral Efferent.

08

Neuroplasticity — The Nervous System Rewrites Its Own Wiring

Learning, development, injury and experience-dependent change

🔁Synaptic change

Strengthening or weakening

🌿Structural change

Spines, branches, synapses

🗺️Cortical remapping

Representations reorganize

🧠Neurogenesis

New neurons in limited adult regions

🩹Recovery

Compensation and relearning

Long-term potentiation (LTP)

Persistent increase in synaptic strength after patterned/coincident activity. Hippocampal NMDA-dependent LTP is a major learning model.

Hebb: “Cells that fire together wire together.”

Long-term depression (LTD)

Persistent reduction in synaptic strength. It is not damage; it supports updating, forgetting, fine-tuning and preventing saturation.

Experience-expectant

Brain expects common species-typical input during sensitive periods, such as patterned vision.

Experience-dependent

Individual-specific learning continues throughout life, such as new skills or vocabulary.

After injury

Diaschisis reversal, unmasking, collateral sprouting, remapping and behavioural compensation may aid recovery.

⚠️ Plasticity is not unlimited and not always beneficial. Addiction, chronic pain and maladaptive fear learning also involve plastic change.

09

Methods of Physiological Psychology — Choose the Window Carefully

Invasive and non-invasive approaches

🔬 Anatomical methods

Histology, staining, microscopy and tract tracing reveal cells, layers and connections.

Best forStructure

🧵 Degeneration methods

Trace pathways after axonal damage: anterograde/Wallerian degeneration distal to lesion; retrograde changes toward soma.

Best forConnections

✂️ Lesion methods

Aspiration, electrolytic/radiofrequency, excitotoxic or reversible chemical lesions infer necessity from loss of function.

LogicDamage → deficit

🧪 Chemical methods

Agonists, antagonists, local infusion, microdialysis and autoradiography manipulate or measure neurochemistry.

Best forMolecules

📍 Microelectrode

Intracellular/extracellular recording of single neurons or small populations; excellent temporal precision.

ResolutionCell + milliseconds

〰️ EEG / ERP

Scalp electrical activity; EEG rhythms and event-locked ERP components. Excellent timing, limited localization.

TemporalExcellent

🩻 CT

X-ray computed slices show gross anatomy, bleeding, fractures and lesions; fast but uses ionizing radiation.

MeasuresStructure

🧲 MRI

Magnetic fields and radiofrequency provide high-resolution soft-tissue anatomy without ionizing radiation.

SpatialHigh

🌈 fMRI

BOLD signal indexes blood-oxygen changes related to neural activity; indirect and temporally slower.

MeasuresFunction indirectly

☢️ PET

Radioactive tracer measures metabolism, blood flow or receptor binding; molecularly flexible but invasive tracer exposure.

StrengthMolecular imaging

🧭 MEG

Detects magnetic fields from neural currents; excellent timing and better source estimation than EEG, but costly.

TemporalExcellent

Method	Temporal resolution	Spatial resolution	Direct/indirect
EEG/ERP	Excellent (milliseconds)	Low–moderate	Electrical activity, relatively direct
fMRI	Seconds	High	Indirect haemodynamic BOLD
PET	Minutes	Moderate	Indirect tracer signal
Single-unit	Excellent	Excellent locally	Direct electrical recording

🎯 Fastest timing: EEG/ERP/MEG and electrophysiology. Best routine soft-tissue structure: MRI. Functional BOLD: fMRI.

10

Muscular & Glandular Systems — Action and Chemical Command

Muscle types, motor control and endocrine organisation

Skeletal muscle

Striated, usually voluntary; attached to skeleton. Somatic motor neuron releases ACh at neuromuscular junction.

Cardiac muscle

Striated, involuntary, branched cells with intercalated discs; intrinsic rhythm modulated by ANS.

Smooth muscle

Non-striated, involuntary; walls of organs and vessels; autonomic and hormonal control.

Motor-control terms

Motor unit: one motor neuron + all fibres it innervates. Muscle spindle: detects muscle length/stretch. Golgi tendon organ: detects tension. Agonist/antagonist: produce/opposes movement.

Endocrine vs exocrine

Endocrine glands release hormones into blood; slower, longer and widespread. Exocrine glands release through ducts, e.g., sweat and salivary glands.

Gland/organ	Key hormones	Behavioural relevance
Anterior pituitary	ACTH, TSH, GH, prolactin, LH, FSH	Tropic control, growth, reproduction and stress
Posterior pituitary	Releases hypothalamic ADH and oxytocin	Water balance, labour/lactation and social processes
Thyroid	T₃, T₄, calcitonin	Metabolic rate, development and energy
Adrenal cortex	Cortisol, aldosterone, androgens	Longer stress response, salt balance
Adrenal medulla	Epinephrine, norepinephrine	Rapid sympathetic activation
Pancreas	Insulin, glucagon	Blood-glucose regulation
Pineal	Melatonin	Circadian timing
Gonads	Testosterone, estrogens, progesterone	Reproduction and sex-related development

⚠️ Posterior pituitary stores and releases ADH/oxytocin synthesized in hypothalamic neurons; it does not synthesize them itself.

11

Hunger & Thirst — Homeostasis with Multiple Signals

Hypothalamus, peripheral hormones and fluid balance

Hunger-promoting signals

Ghrelin from stomach rises before meals. In arcuate nucleus, NPY/AgRP neurons promote eating. Lateral hypothalamic pathways participate in feeding and reward.

Satiety/energy signals

Leptin reflects adipose stores; insulin reflects energy availability; gut peptides such as CCK/PYY support satiation. POMC/CART neurons suppress intake.

Classical finding	Stimulation/lesion pattern	Modern correction
Lateral hypothalamus	Stimulation feeding; lesion aphagia	Not one simple “feeding centre”; fibres, motivation and multiple circuits are involved.
Ventromedial hypothalamus	Lesion hyperphagia/weight gain	Not one simple satiety centre; metabolic and paraventricular/arcuate circuits contribute.

Osmometric thirst

High extracellular solute concentration shrinks osmoreceptor cells; promotes drinking and ADH release.

Volumetric thirst

Loss of blood volume/pressure activates kidney renin → angiotensin II and aldosterone pathways; promotes drinking and salt appetite.

ADH

Vasopressin conserves water by increasing kidney reabsorption. Thirst and water retention work together.

👩‍🏫 Two goals: Hunger regulation defends energy availability; thirst regulation defends both cellular concentration and circulating volume.

🧠 Ghrelin growls; leptin reports long-term stores.

12

Sleep — The Brain’s Night Shift

Circadian control, NREM/REM stages and sleep architecture

🌙 Sleep-stage explorer

Wake · beta/alpha transition

Wakefulness

Alert wake shows low-amplitude fast beta activity; relaxed eyes-closed wake shows alpha rhythm.

Circadian clock

Suprachiasmatic nucleus (SCN) receives retinal light information and coordinates daily rhythms. Darkness permits pineal melatonin secretion; light suppresses it.

Sleep pressure

Homeostatic pressure rises with time awake, partly linked to adenosine, and falls during sleep. Two-process model = circadian timing + homeostatic pressure.

Night pattern

Cycles repeat about every 90 minutes. N3 is greater early in the night; REM periods lengthen toward morning.

REM paradox

Brain EEG resembles wake, vivid dreaming is common, eyes move rapidly, but skeletal muscles show near-atonia.

Disorders

Insomnia: initiation/maintenance difficulty. Narcolepsy: REM intrusion/sleepiness. Sleep apnea: repeated breathing interruption.

🎯 EEG signatures: N2 = sleep spindles + K complexes; N3 = delta; REM = low-amplitude mixed-frequency EEG + atonia.

13

Sexual Motivation — Hormones, Circuits and Context

Reproductive axis, sexual response and biopsychosocial regulation

🧠Hypothalamus

GnRH pulses

📣Anterior pituitary

LH + FSH

⚙️Gonads

Gametes + steroids

🧪Sex steroids

Testosterone, estrogen, progesterone

↩️Feedback

Regulates axis

Organizational effects

Hormonal effects during sensitive developmental periods that shape enduring neural/body organization.

Activational effects

More temporary effects of current hormone levels on already organized systems, often reversible.

1Excitement

Arousal begins

2Plateau

Arousal sustained

3Orgasm

Rhythmic response

4Resolution

Return toward baseline

↻Variation

Not everyone follows one fixed sequence

Integrated control

Hypothalamic regions, amygdala, reward circuits, spinal reflexes, autonomic activity, gonadal hormones, learning, relationship context, culture and consent all influence sexual behaviour.

Hormone व्यवहार को “force” नहीं करता; वह motivation की probability और sensitivity बदल सकता है।

⚠️ Avoid biological determinism. Human sexuality reflects biology in interaction with development, cognition, relationships, identity, culture and choice.

14

Emotion — A Distributed Brain–Body Construction

Limbic circuits, cortex, autonomic responses and hormones

Amygdala

Threat/relevance detection, fear learning, emotional salience; not a single “fear centre.”

Hypothalamus

Coordinates autonomic and endocrine expression of emotion.

Hippocampus

Context and episodic memory; helps distinguish where/when threat occurred.

Prefrontal cortex

Appraisal, inhibition, reappraisal, decision and top-down regulation.

Insula

Interoception, bodily feeling, taste/disgust and subjective awareness.

Anterior cingulate

Conflict, pain affect, motivation and regulation.

SAM axis: immediate alarm

Sympathetic activation → adrenal medulla → epinephrine + norepinephrine

Fast heart rate, pupil dilation, energy mobilization and vigilance.

HPA axis: sustained response

Hypothalamus CRH → pituitary ACTH → adrenal cortex cortisol

Cortisol mobilizes energy and changes immune/cognitive processes; chronic excess can be harmful.

Theory	Biological sequence	Exam distinction
James–Lange	Stimulus → bodily response → emotion experience	We feel emotion after perceiving bodily change.
Cannon–Bard	Brain processing → bodily response and feeling together	Simultaneous experience and arousal.
Schachter–Singer	Arousal + cognitive label/context → emotion	Two-factor account.

👩‍🏫 Better picture: Emotion is a coordinated pattern across appraisal, memory, body state, action tendency and social context—not output from a single “limbic brain.”

15

Genetics & Behaviour — Inheritance Is Influence, Not Fate

Chromosomal anomalies, twin/adoption logic and nature–nurture

Nature

Genetic variation, gene expression, prenatal biology and inherited dispositions.

×

Nurture

Family, nutrition, education, peers, stress, culture and unique experience.

🧬 Correct equation: Behavioural phenotype = genes + environment + gene–environment interplay + development + chance.

Condition	Chromosomal pattern	Core clue
Down syndrome	Trisomy 21	Extra chromosome 21; characteristic developmental and medical profile.
Klinefelter syndrome	Usually XXY	Typically male phenotype; hypogonadism, infertility and variable learning effects.
Turner syndrome	Usually monosomy X (45,X)	Typically female phenotype; short stature, gonadal dysgenesis and specific health/visuospatial risks.
Edwards syndrome	Trisomy 18	Severe developmental and medical abnormalities.
Patau syndrome	Trisomy 13	Severe congenital abnormalities and developmental impairment.

Twin studies

MZ twins share nearly all segregating DNA; DZ twins average ~50%. Greater MZ than DZ similarity supports genetic influence, assuming comparable environments.

Adoption studies

Similarity with biological relatives suggests genetic influence; similarity with adoptive relatives suggests shared environment.

Heritability (h²)

Proportion of variation in a population, under its current environments, associated with genetic differences. It is not an individual percentage.

Gene–environment correlation

Passive: parents provide genes + setting. Evocative: traits evoke reactions. Active: people select compatible environments.

G × E and epigenetics

Interaction: genetic effects differ by environment. Epigenetics: regulation of gene expression without changing DNA sequence; environment can influence these marks.

⚠️ High heritability does not mean immutability. Heritability can change across populations and environments, and it does not explain group mean differences by itself.

🧠 Anomalies: Down = 21; Klinefelter = extra X in male (XXY); Turner = missing X (XO/45,X).

16

Final Neural Circuit — Rapid Revision & Retrieval

Connect the whole unit, then answer without looking

📡Sense

Receptors transduce

⚡Signal

Neurons conduct

🔗Integrate

Synapses and CNS

🧪Regulate

ANS + endocrine

🧬Adapt

Plasticity + development

Electrical anchors

Rest −70 mV; Na⁺ in depolarizes; K⁺ out repolarizes; myelin speeds conduction.

Systems anchors

CNS = brain/spinal cord; PNS = somatic + autonomic; sympathetic mobilizes, parasympathetic restores.

Behaviour anchors

Hypothalamus coordinates homeostasis; limbic–cortical networks shape emotion; genes work through environments.

⚡ NET/JRF Retrieval Check

Score: 0 / 10

1. Which receptor primarily detects muscle stretch?

2. Rapid depolarization of an action potential mainly involves:

3. Myelin in the peripheral nervous system is produced by:

4. The main inhibitory neurotransmitter in the adult CNS is:

5. Sleep spindles and K complexes characterize:

6. BOLD signal is measured by:

7. Osmometric thirst is triggered chiefly by:

8. The HPA sequence is:

9. Turner syndrome is most commonly associated with:

10. Heritability refers to:

🧠 One-line unit: Receptors translate → neurons signal → synapses communicate → systems coordinate → hormones regulate → experience rewires → genes interact.