Neural Control & Coordination Notes

1. Neural System – Introduction & Origin

Neural system origin: Ectoderm
First nerve cells (neurons) appear in: Coelenterata (Hydra) — apolar type neurons (no defined poles)
Cephalisation (concentration of nervous tissue in head) first seen in: Platyhelminthes (flatworms — first invertebrates)
In Hydra: nervous system = simple network of neurons
In insects: better developed brain is present with ganglia and neural circuits
All animals: neural system composed of highly specialised cells called neurons — detect, receive and transmit different kinds of stimuli
Vertebrates have the most developed neural system

2. Neuron – Structural & Functional Unit

Parts of a Neuron

Part	Details
Cell Body (Cyton/Soma)	Contains nucleus, Nissl's granules (rough ER), mitochondria. Grey coloured. Centre of metabolic activity.
Dendron / Dendrites	Short branched processes arising from cell body (A–B branches). Receive impulses from other neurons and bring them TO the cell body. Multiple per neuron.
Axon	Single long process arising from axon hillock. Carries impulses AWAY from cell body. Ends in synaptic knob (terminal knob / terminal bouton). Contains synaptic vesicles with neurotransmitters.

Nissl's Granules

Present in: Cyton (cell body) and Dendron
Absent in: Axon, Axon hillock
Composed of: Rough ER + Ribosomes → site of protein synthesis
Also called Chromatophilic substance

Myelin Sheath

Insulating layer around the axon
Formed by: Schwann cells in PNS | Oligodendrocytes in CNS
Composed of: Phospholipids (sphingolipids) — [PNS] [CNS + ANS]
Neurilemma = outer covering formed by Schwann cells (outermost covering of axon in PNS)
Nodes of Ranvier = gaps between adjacent Schwann cells on the axon — present ONLY in PNS myelinated fibres

Feature	Myelinated Fibre	Non-myelinated Fibre
Myelin sheath	Present	Absent
Nodes of Ranvier	Present (many)	Absent
Speed of conduction	Fast (saltatory conduction)	Slow
Myelin formed by	Oligodendrocytes (CNS) / Schwann cells (PNS)	Schwann cells (no wrapping)

Saltatory conduction = impulse jumps from one Node of Ranvier to the next (saltare = to jump in Latin) → much faster than continuous conduction in non-myelinated fibres.

Types of Neurons

Type	Structure	Location
Non-polar (Apolar)	No distinct axon or dendrites — all processes similar	Coelenterata (Hydra)
Unipolar	Cyton + one process (axon only — no dendrites)	Early embryonic stage
Bipolar	Single dendrite + single axon	Nose and eyes (olfactory and retinal neurons)
Multipolar	Single axon + multiple dendrites	Most neurons in body (brain, spinal cord)
Pseudo-unipolar	Sensory neuron — single process splits near cell body into central and peripheral processes	Dorsal Root Ganglia (skin receptors → spinal cord)

Nerve Fibre Coverings

Endoneurium → covers individual nerve fibre
Perineurium → covers bundle of nerve fibres (fascicle)
Epineurium → covers entire nerve (group of bundles)

Q

What are Nissl's granules? Where are they present and absent in a neuron?

Ans: Nissl's granules = Rough ER + ribosomes — site of protein synthesis in neuron

Nissl's granules are clumps of rough endoplasmic reticulum (rER) with ribosomes found in neurons.
They appear as dark-staining granules under microscope (named after Franz Nissl).
Present in: Cell body (Cyton) + Dendron (dendrites) — these regions need protein synthesis for maintaining neuron structure and function.
Absent in: Axon and Axon hillock — these regions do not have rER because they are specialized for conducting impulses, not synthesizing proteins.
Function: Protein synthesis for neurotransmitters, membrane repair, enzymes, etc.

Q

What is the difference between Nodes of Ranvier and Neurilemma?

Ans:

Nodes of Ranvier: Gaps between adjacent Schwann cells along a myelinated axon where the axon is exposed (no myelin). Impulse jumps from node to node (saltatory conduction) → faster conduction. Present ONLY in PNS myelinated nerve fibres.

Neurilemma: The outermost covering of the axon in PNS formed by the plasma membrane of Schwann cells. Important for nerve regeneration after injury — provides a tube for the axon to regrow. Not present in CNS (where oligodendrocytes form myelin — this is why CNS nerves regenerate poorly after injury).

3. Conduction of Nerve Impulse (Electrochemical Process)

Ion Channels on Neural Membrane

Type	Opens when	Examples
Voltage-Gated Channels (VGC)	Change in membrane potential	Na⁺, K⁺, Ca²⁺ VGC
Ligand-Gated Channels (LGC)	Specific ligand/chemical binds	Na⁺, K⁺ LGC at synapse
Mechanically-Gated Channels (MGC)	Mechanical stimulation	Na⁺ channels in touch receptors

K⁺ channels (leaky) are always open (responsible for resting potential). Na⁺ voltage-gated channels are closed at rest.

Phase 1: Polarization (Resting Condition)

Resting Membrane Potential = −70 mV (inside negative relative to outside)
Outside: High Na⁺, Low K⁺ | Inside: Low Na⁺, High K⁺
K⁺ channels open → K⁺ leaks out → inside becomes more negative
Na⁺ channels closed
Na⁺-K⁺ pump active: pumps 3 Na⁺ OUT and 2 K⁺ IN (using ATP) → maintains concentration gradient
Membrane is in polarized state (charged)

Phase 2: Depolarization (Action Potential — Stimulation)

When nerve is stimulated → voltage-gated Na⁺ channels open → large influx of Na⁺ into cell
Inside becomes positive → membrane potential reaches +30 mV
This is action potential / depolarization
Na⁺-K⁺ pump: temporarily stops/slows
Impulse direction: unidirectional (Dendrite → Cyton → Axon → Dendrite of next neuron)

Phase 3: Repolarization

Na⁺ VGC close → K⁺ VGC open → K⁺ rushes out → inside becomes negative again
Membrane potential returns to −70 mV
Na⁺-K⁺ pump turns back on (Active/ON)

Phase 4: Hyperpolarization (After-hyperpolarization)

K⁺ channels remain briefly open even after −70 mV is reached
Extra K⁺ exits → potential goes below −70 mV (e.g., −90 mV)
Then slowly returns to −70 mV (resting state)
Na⁺-K⁺ pump: Active/ON throughout

Direction of conduction (in terms of charge):
Positive charge → Negative charge (from depolarized region to polarized region)
In axon A→B: depolarization moves from A to B
Return (repolarization) moves in opposite direction (B→A)

Q

What is resting membrane potential? How is it maintained?

Ans: −70 mV, maintained by Na⁺-K⁺ pump and selective ion permeability

Resting membrane potential = −70 mV (inside negative relative to outside).
How maintained:
1. K⁺ leaky channels are always open → K⁺ leaks out → inside becomes negative
2. Na⁺ channels are closed at rest → Na⁺ cannot enter
3. Na⁺-K⁺ pump (ATPase): actively pumps 3 Na⁺ OUT and 2 K⁺ IN per cycle → maintains high Na⁺ outside and high K⁺ inside
4. Large negatively charged proteins are trapped inside (cannot cross membrane)
Net result: Outside has excess positive charge (Na⁺) and inside has excess negative charge → resting membrane potential of −70 mV.

Q

What is threshold potential? What happens when stimulus is below threshold?

Ans: Threshold = minimum stimulus to generate action potential (all-or-none law)

Threshold potential is the minimum level of depolarization needed to trigger a full action potential (approximately −55 mV for most neurons).
All-or-none law: If stimulus is below threshold → subthreshold stimulus → no action potential generated (nothing happens). If stimulus reaches or exceeds threshold → full action potential is generated regardless of stimulus strength.
This means nerve impulses are all-or-none events — there is no "partial" impulse. The strength of sensation is coded by the FREQUENCY of impulses, not the amplitude.

4. Transmission of Impulse – Synapse (Chemical Process)

Synapse

Junction between two neurons where impulse is transmitted from one neuron to the next.

Feature	Electrical Synapse	Chemical Synapse
Gap	Very narrow (2 nm) — almost no gap (gap junctions)	Wide synaptic cleft (20–50 nm)
Neurotransmitter	Not required	Required (released from synaptic vesicles)
Direction	Bidirectional	Unidirectional (pre→post)
Speed	Fastest	Slower than electrical
Synaptic fatigue/delay	No	Yes (synaptic delay)
Found in	Cardiac muscle, some brain regions	Most synapses in body

Mechanism of Chemical Synapse

Step 1: Action potential reaches pre-synaptic knob
Step 2: Ca²⁺ VGC open → Ca²⁺ enters pre-synaptic terminal
Step 3: Synaptic vesicles fuse with pre-synaptic membrane → Acetylcholine (ACh) released into synaptic cleft
Step 4: ACh binds with Na⁺ LGC (ligand-gated channels) on post-synaptic membrane
Step 5: Na⁺ enters post-synaptic cell → Depolarization → EPSP (Excitatory Post Synaptic Potential)
Step 6: ACh broken down by enzyme Acetylcholinesterase (AChE) → Choline + Acetate
Step 7: Choline transported back → re-synthesized into Acetylcholine (responsible for repolarization / removal of signal)

Mitochondria in synaptic knob → provide ATP for neurotransmitter synthesis and vesicle recycling.

Types of Neurotransmitters

Type	Effect	Examples	Opens which channel
Excitatory (EPSP)	Depolarization → impulse generation	Acetylcholine, Epinephrine, Norepinephrine, Glutamate, Serotonin (in some)	Na⁺ channels (LGC)
Inhibitory (IPSP)	Hyperpolarization → inhibits impulse	GABA (gamma-aminobutyric acid), Glycine	Cl⁻ channels (LGC)

Q

What is the role of Acetylcholinesterase (AChE) at a synapse?

Ans: AChE breaks down ACh in the synaptic cleft to terminate the signal.

After ACh binds to post-synaptic receptors and generates an EPSP (excitatory post-synaptic potential), the signal must be terminated otherwise the post-synaptic membrane would remain continuously depolarized.
AChE (Acetylcholinesterase) is an enzyme located in the synaptic cleft that rapidly breaks down ACh into Choline + Acetate.
Choline is then transported back into the pre-synaptic terminal where it is re-acetylated (using Acetyl-CoA from mitochondria) to form new ACh → recycled into synaptic vesicles.
Drugs that inhibit AChE (e.g., nerve agents, some insecticides like organophosphates) → ACh accumulates → continuous nerve stimulation → muscle spasm/paralysis.

Q

What is GABA? What is its effect on the post-synaptic membrane?

Ans: GABA = inhibitory neurotransmitter → causes hyperpolarization (IPSP)

GABA (Gamma-Aminobutyric Acid) is the main inhibitory neurotransmitter in the CNS.
When GABA is released, it binds to Cl⁻ LGC (ligand-gated Cl⁻ channels) on the post-synaptic membrane → Cl⁻ enters the cell → inside becomes MORE negative → hyperpolarization.
This produces an IPSP (Inhibitory Post-Synaptic Potential) — makes it harder to reach threshold → inhibits impulse generation.
GABA is important for sleep, reducing anxiety, muscle relaxation. Drugs like benzodiazepines (e.g., diazepam/Valium) enhance GABA activity → calming, anti-anxiety effect.

5. Human Neural System

Human Neural System = CNS (Central Nervous System) + PNS (Peripheral Nervous System)
CNS = Brain + Spinal Cord
PNS = Cranial nerves (12 pairs) + Spinal nerves (31 pairs)

Division	Sub-division	Controls
PNS → SNS (Somatic Nervous System)	Afferent (Sensory) neurons	Carry impulses FROM sense organs TO CNS
PNS → SNS (Somatic Nervous System)	Efferent (Motor) neurons	Carry impulses FROM CNS TO voluntary organs (skeletal muscles)
PNS → ANS (Autonomic Nervous System)	Sympathetic NS (Emergency "fight or flight")	Involuntary organs — increases heart rate, dilates pupil, etc.
PNS → ANS (Autonomic Nervous System)	Parasympathetic NS (Normal/rest "rest and digest")	Involuntary organs — decreases heart rate, constricts pupil, etc.

Visceral afferent nerve = carries sensory impulses from visceral organs (internal organs) to CNS

6. Human Brain

Protection of Brain

Brain protected by: Skull (bone) + Meninges (3 membranous layers)
Meninges (3 layers): Duramater (outer, tough) → Arachnoid (middle) → Piamater (inner, highly vascular, thin)
Piamater is in direct contact with cerebrum
CSF (Cerebrospinal Fluid) fills the space between arachnoid and piamater (subarachnoid space) — acts as shock absorber

Divisions of Brain

Division	Parts
Forebrain (Prosencephalon)	Telencephalon: Cerebrum + Olfactory lobes + Hippocampus + Limbic system + Amygdala Diencephalon: Thalamus + Hypothalamus + Epithalamus (Pineal gland)
Midbrain (Mesencephalon / Metencephalon)	Cerebral peduncles + Corpora Quadrigemina (2 Superior Colliculi + 2 Inferior Colliculi)
Hindbrain (Rhombencephalon)	Myelencephalon: Medulla Oblongata Metencephalon: Pons + Cerebellum

Cerebrum — Largest Part of Brain

Largest part of brain — divided into two cerebral hemispheres
Each hemisphere has outer cortex (grey matter — cell bodies) and inner medulla (white matter — myelinated fibres)
Cortex highly folded → increases surface area (convolutions)
Corpus Callosum: Thick bundle of neurons connecting and coordinating BOTH hemispheres — present ONLY in mammalian brain

Lobes of Cerebral Hemisphere

Lobe	Location	Function
Frontal Lobe	Front (anterior)	Motor area (voluntary movement of skeletal muscles — movements of limbs, tongue etc.) \| Motor speech area \| Memory, association
Parietal Lobe	Middle	Sensory area (touch, pain, temperature, pressure) \| Gustatory area (taste)
Temporal Lobe	Sides	Auditory area (hearing) \| Olfaction (smell — Wernicke's area for speech comprehension)
Occipital Lobe	Posterior	Visual area (sight/vision)

Central fissure = Fissure of Rolando (between frontal and parietal lobes) | Lateral fissure = Sylvian fissure | Parieto-occipital fissure separates parietal and occipital lobes

Diencephalon

Part	Function
Thalamus	Relay centre for sensory and motor signals. Coordination between sensory and motor control. All sensory impulses (except smell) pass through thalamus.
Hypothalamus	Controls thermoregulation, hunger, thirst, sleep, sexual behaviour. Controls pituitary gland (involuntary gland). Contains neurosecretory cells that secrete hypothalamic hormones. Controls autonomic NS.
Epithalamus	Contains Pineal gland (secretes melatonin — controls circadian rhythm, sleep-wake cycle)

Limbic System: Inner parts of cerebral hemispheres + Amygdala + Hippocampus + Hypothalamus + Thalamus = complex structure called Limbic Lobe/System. Involved in regulation of sexual behaviour, expression of emotions (excitement, pleasure, rage, fear), motivation, olfaction.

Midbrain

Located between thalamus/hypothalamus (forebrain) and pons (hindbrain)
Canal = Cerebral aqueduct passes through midbrain
Cerebral peduncles: Two thick bundles of neurons connecting fore brain and hindbrain
Corpora Quadrigemina (dorsal): 4 rounded swellings
— 2 Superior Colliculi (visual reflex — blink reflex to bright light)
— 2 Inferior Colliculi (auditory reflex — turning head towards sound)

Hindbrain

Part	Function
Medulla Oblongata	Controls involuntary actions: heartbeat, blood pressure, respiration, swallowing, vomiting. Contains pneumotaxic and apneustic centres for breathing control.
Pons	Relay between cerebellum and cerebrum. Contains pneumotaxic centre (controls breathing rate).
Cerebellum	Second largest part of brain. Controls body balance and coordination of body movements. Has cortex (grey) and medulla (white). Divided into two cerebellar hemispheres. Has vermis (worm-like structure connecting both hemispheres).

⚡ Brain Stem = Midbrain + Pons + Medulla Oblongata (NOT cerebellum)

Position of grey and white matter in CNS:
Brain → grey matter OUTSIDE (cortex), white matter INSIDE (medulla)
Spinal cord → grey matter INSIDE (H-shaped), white matter OUTSIDE (funiculi)

Q

What is Corpus Callosum? In which organisms is it found?

Ans: Corpus Callosum — thick bundle of neurons connecting both cerebral hemispheres. Found only in mammals.

Corpus Callosum is the largest white matter structure in the brain — a broad, thick band of approximately 200–250 million nerve fibres connecting the right and left cerebral hemispheres.
It allows communication and coordination between both hemispheres — when you use one hand, the other hemisphere also gets information via corpus callosum.
Has three parts: Genu (anterior) + Body + Splenium (posterior).
Found ONLY in mammalian brain — not present in non-mammalian vertebrates (fish, amphibians, reptiles, birds).

Q

Which part of the brain controls: (a) body balance (b) heartbeat (c) vision (d) memory (e) hearing?

Ans:

(a) Body balance → Cerebellum
(b) Heartbeat → Medulla Oblongata (controls involuntary actions)
(c) Vision → Occipital lobe of cerebrum (visual area)
(d) Memory → Frontal lobe of cerebrum (association area) + Hippocampus (limbic system)
(e) Hearing → Temporal lobe of cerebrum (auditory area)

7. Spinal Cord

Extends from Foramen Magnum (base of skull) downward
Provides neural connections for upper and lower limbs
Ends as Conus Medullaris — tapers off
Cauda Equina: Bunch of nerve roots below conus medullaris (looks like horse's tail)
Filum Terminale: Non-nervous terminal part (piamater extension)

Transverse Section of Spinal Cord

Grey matter: H-shaped (butterfly-shaped) — INSIDE — contains cell bodies of neurons
White matter: OUTSIDE — contains myelinated fibres (funiculi)
Dorsal horn: Contains sensory (afferent) neurons — Pseudo-unipolar — brings impulse from receptor to spinal cord
Ventral horn: Contains motor (efferent) neurons — multipolar — carries impulse from spinal cord to effector (muscle)
Interneurons: Connect sensory and motor neurons within spinal cord (in grey matter)
Dorsal Root Ganglion (DRG): Cell bodies of sensory neurons lie here (outside spinal cord)

In spinal cord: GREY matter is INSIDE (H-shaped), WHITE matter is OUTSIDE — OPPOSITE to brain (where grey cortex is outside, white medulla is inside).

Q

Where are the cell bodies of sensory neurons found? Where are those of motor neurons?

Ans: Sensory = Dorsal Root Ganglion | Motor = Ventral horn of spinal cord

Sensory (afferent) neurons: These are pseudo-unipolar neurons. Their cell bodies are located in the Dorsal Root Ganglion (DRG) — a ganglion located just outside the spinal cord on the dorsal side. Their peripheral processes go to receptors (in skin etc.) and central processes enter the dorsal horn of spinal cord.
Motor (efferent) neurons: These are multipolar neurons. Their cell bodies are located in the ventral horn (anterior horn) of the spinal cord grey matter. Their axons exit via ventral root and go to effectors (muscles).

8. Reflex Action & Reflex Arc

Reflex Action

Definition: Automatic, rapid, spontaneous and involuntary action which is controlled by the CNS
Cerebral reflex: controlled by Brain (e.g., salivation, blinking, sneezing, coughing, vomiting)
Spinal reflex: controlled by Spinal cord (e.g., withdrawal of body part on touching hot object)

Feature	Cerebral Reflex	Spinal Reflex
Control by	Brain	Spinal cord
Speed	Fast	Fastest
Examples	Salivation, blinking, smell, sneezing, coughing, vomiting	Withdrawal reflex (hand from hot object), chemical/mechanical stimulation

Conditional vs Unconditional Reflex

Conditional Reflex	Unconditional Reflex
Studied by Pavlov (conditioned salivation)	Normal innate reflex
Salivation at sight/smell of food (conditioned)	Salivation on taste of food (unconditioned)
Acquired through learning/experience	Inborn, present from birth
e.g., Cycling, playing musical instruments	e.g., Withdrawal reflex, blinking

Reflex Arc

Path followed by impulse for completion of reflex action:
Receptor → Sensory Neuron → CNS (Interneuron) → Motor Neuron → Effector (Muscle/Gland)

Type	Monosynaptic Reflex	Polysynaptic Reflex
Synapses	Single synapse between sensory and motor neuron	Two or more synapses through interneurons
Interneurons	Absent	Present
Speed	Fastest	Slower
Examples	Knee jerk, elbow jerk, ankle reflex (stretch reflex)	Withdrawal reflex (hand from hot object)

Q

What is the difference between monosynaptic and polysynaptic reflex arc? Give examples.

Ans:

Monosynaptic Reflex Arc:
Only ONE synapse present — directly between the sensory neuron and motor neuron. No interneurons involved.
Pathway: Receptor → Sensory neuron → (1 synapse) → Motor neuron → Effector
Examples: Knee jerk reflex (patellar reflex), elbow jerk, ankle reflex — all are stretch reflexes
Speed: Fastest — because only one synapse = minimum synaptic delay

Polysynaptic Reflex Arc:
Two or more synapses — sensory neuron → interneuron(s) → motor neuron
Pathway: Receptor → Sensory neuron → Interneuron → Motor neuron → Effector
Examples: Withdrawal reflex (jerking hand away from hot object)
Speed: Slower than monosynaptic due to multiple synaptic delays

9. Sense Organs

Olfactory Receptors (Nose)

Olfactory epithelium present in roof of nasal cavity
Olfactory lobe = part of brain that processes smell signals
Olfactory receptor cells → Olfactory nerve → Olfactory bulb → Olfactory tract → Olfactory lobe
Bowman's glands = mucus glands in olfactory epithelium
Supporting cells present around receptor cells
Basal cells = replace old olfactory receptor cells (sensory in nature)

Gustatory Receptors (Tongue)

Taste receptors / Taste buds present on epithelium of tongue
Each taste bud has microvilli (increase surface area)
Gustatory receptor cells → Facial nerve (VII) / Glossopharyngeal nerve (IX) / Vagus (X) → Brain
Basal cells → convert into gustatory receptor cells

Human Eye — Structure

Layer	Parts & Details
Fibrous tunica (outer)	Sclera (tough, white, avascular, modified to form cornea anteriorly) + Cornea (transparent, modified sclera, first refractive surface)
Uvea / Vascular tunica (middle)	Choroid (vascular, black/blue/brown pigmented, reduces internal reflection) + Ciliary body (has ciliary muscles — control lens curvature for accommodation) + Iris (coloured part — controls pupil diameter)
Retina (inner/photo-sensory tunica)	Contains photoreceptors (rods and cones). Three layers: Ganglionic cells (inner) → Bipolar cells (middle) → Photoreceptor cells (outer/outermost)

Important parts of the eye:
Lens: Biconvex, transparent, held by suspensory ligaments. Changes shape for near/far vision (accommodation by ciliary muscles).
Pupil: Opening in iris that controls amount of light entering eye. Iris muscles — Radial muscles (dilate pupil — sympathetic) | Circular muscles (constrict pupil — parasympathetic).
Aqueous humor: Fluid in anterior chamber (between cornea and lens) — contains nutrients.
Vitreous humor: Jelly-like fluid in posterior chamber (behind lens) — maintains eye shape.
Yellow spot (Macula lutea): Area of highest visual acuity — contains only cones. Central pit = Fovea centralis — highest resolution (no rods, only cones).
Blind spot: Where optic nerve exits — no rods or cones — no vision here.

Rods vs Cones

Feature	Rods	Cones
Number	~100–120 million	~6 million
Active in	Dim light (scotopic vision)	Bright light (photopic vision)
Image produced	Black and white	Coloured
Sensitivity	High	Low
Visual acuity	Low	High
Photopigment	Rhodopsin (Visual Purple)	Iodopsin (Visual Violet)
Pigment composition	Opsin protein + Retinal (Vit A aldehyde)	3 types of opsin + Retinal → Red, Green, Blue cones

Colour blindness = deficiency/absence of one or more types of cones. Night blindness (Nyctalopia) = deficiency of Vitamin A → less rhodopsin → rods don't function properly → cannot see in dim light.

Mechanism of Vision (Scotopic Vision in Dim Light)

Rhodopsin (opsin + retinal — in dark) → Light hits → Retinal changes shape (cis → trans) → Opsin changes → Photoreceptor cell hyperpolarizes (Na⁺ channels close) → Bleaching of rhodopsin
↓
Signal: Photoreceptor cells → Bipolar cells → Ganglionic cells → Optic nerve → Occipital lobe

Human Ear — Structure & Hearing

Ear functions: Hearing (20–20,000 Hz) + Balancing (vestibular apparatus)

Part	Components	Function
External Ear	Pinna + Auditory canal + Ear wax glands + Tympanum (eardrum)	Collects and transmits sound waves to middle ear
Middle Ear	3 ossicles: Malleus (hammer) + Incus (anvil) + Stapes (stirrup) \| Eustachian tube	Transmit and amplify sound vibrations from tympanum to oval window. Eustachian tube = connects to pharynx, maintains equal pressure on both sides of tympanum.
Internal Ear	Bony labyrinth (filled with Perilymph) containing Membranous labyrinth (filled with Endolymph)	Cochlea = hearing \| Vestibular apparatus = balancing

Mechanism of Hearing

Sound waves → Pinna → Auditory canal → Tympanum vibrates → Malleus → Incus → Stapes → Oval window → Perilymph vibrates in Scala vestibuli → Reissner's membrane vibrates → Endolymph vibrates → Basilar membrane vibrates → Organ of Corti (sensory hair cells on basilar membrane) excited → Tectorial membrane (above) bends hair cells → Nerve impulse generated → Cochlear nerve → Auditory area of temporal lobe

Vestibular Apparatus (Balancing)

Structure	Sensory organ	Detects
3 Semicircular canals (at X, Y, Z axes)	Crista (in ampulla)	Rotational acceleration / Dynamic balance
Utricle + Saccule (otolith organs)	Macula	Linear acceleration + Static balance + Gravitational pull

Q

What is rhodopsin? How is it related to Vitamin A?

Ans: Rhodopsin = visual pigment in rods made of opsin protein + retinal (Vit A derivative)

Rhodopsin (also called Visual Purple) is the photosensitive pigment present in rod cells of the retina.
Composition: Opsin protein + Retinal (11-cis retinal = aldehyde form of Vitamin A)
Mechanism: In dark, rhodopsin is intact. When light hits → 11-cis retinal converts to all-trans retinal (changes shape) → opsin changes conformation → signal generated → rod cell hyperpolarizes → impulse to brain.
Vitamin A connection: Retinal is derived from Vitamin A. If Vitamin A is deficient → insufficient retinal → less rhodopsin → rod cells cannot function properly → Night blindness (Nyctalopia) — cannot see in dim light.
Cones use iodopsin (Visual Violet) with three types of opsins for red, green, blue colour detection.

Q

What is the function of the Eustachian tube?

Ans: Connects middle ear to pharynx, equalizes pressure on both sides of tympanum.

The Eustachian tube (auditory tube) connects the middle ear cavity to the nasopharynx.
Function: Maintains equal air pressure on both sides of the tympanic membrane (eardrum). When you swallow or yawn → Eustachian tube opens briefly → pressure equalizes.
If pressure is unequal (e.g., during rapid altitude change in airplane or deep sea diving) → tympanum cannot vibrate properly → muffled hearing, pain, or even rupture. Swallowing helps equalize by opening the tube.
It also drains mucus from the middle ear → if blocked (e.g., during a cold) → fluid accumulates → middle ear infection (otitis media).

10. Quick Revision — All Key Points

Key Fact	Detail
Neural system origin	Ectoderm
First neurons in evolution	Coelenterata (Hydra) — apolar neurons
Cephalisation first in	Platyhelminthes
Myelin in PNS formed by	Schwann cells
Myelin in CNS formed by	Oligodendrocytes
Nodes of Ranvier present in	Myelinated PNS fibres only
Neurilemma formed by	Outer covering of Schwann cells (PNS only)
Nissl's granules absent in	Axon + Axon hillock
Resting membrane potential	−70 mV (inside negative)
Action potential peak	+30 mV
Na⁺-K⁺ pump ratio	3 Na⁺ out : 2 K⁺ in
Impulse direction (in axon)	Unidirectional (anterograde)
Synaptic cleft (electrical)	~2 nm
Synaptic cleft (chemical)	20–50 nm
ACh broken down by	Acetylcholinesterase (AChE)
GABA effect	Inhibitory — opens Cl⁻ channels → IPSP
Cranial nerves	12 pairs
Spinal nerves	31 pairs
Brain meninges (outer→inner)	Duramater → Arachnoid → Piamater
Brain stem	Midbrain + Pons + Medulla Oblongata
Corpus Callosum	Only in mammals — connects both cerebral hemispheres
Superior Colliculi	Visual reflex (blink)
Inferior Colliculi	Auditory reflex
Blind spot	Where optic nerve exits — no rods or cones
Fovea centralis	Highest visual acuity — only cones — no rods
Rods pigment	Rhodopsin (Vit A derivative)
Cones pigment	Iodopsin (3 types — R, G, B)
Night blindness cause	Vitamin A deficiency → less rhodopsin
Ear ossicles	Malleus → Incus → Stapes (Stapes contacts oval window)
Eustachian tube connects to	Pharynx — equalizes pressure in middle ear
Balance organ (rotation)	Crista in semicircular canals
Balance organ (linear/gravity)	Macula in utricle and saccule
Organ of Corti	On basilar membrane — actual hearing organ
Monosynaptic reflex example	Knee jerk (patellar reflex)
Polysynaptic reflex example	Withdrawal reflex

Neural Control & Coordination – Complete Notes