Revision: Control and Co-ordination Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

Effectors are muscles or glands that produce a response by contracting or secreting substances when stimulated by nerve impulses.

Receptors are specialised sensory cells that receive stimuli and convert them into nerve impulses.

A response is the reaction or change in an organism produced as a result of a stimulus.

A nerve impulse is an electrical disturbance that travels along a neuron in response to a stimulus.

A neuron is a specialised cell that forms the structural and functional unit of the nervous system and is responsible for transmitting nerve impulses.

A stimulus is any sudden change in the internal or external environment that brings about a response in an organism.

The structural and functional unit of nervous system is called neuron.

Nodes of Ranvier are gaps present at intervals in the myelin sheath that help in faster transmission of nerve impulses.

Axon terminals are closely placed near the dendrites of another one or more neurons but are not connected. Such gaps in between are called synaptic clefts.

Neurolemma is the outermost thin protective sheath covering the myelin sheath of a nerve fibre.

Synapse (syn: together, apse: gap/window) is the point of contact between the terminal branches of the axon of a neuron with the dendrites of another neuron separated by a fine gap.

or

The junction between two nerve cells having a minute gap called synaptic cleft through which nerve impulse is transmitted by neurotransmitters is called synapse.

The polarised state is when the outer side of a nerve fibre carries a positive charge due to more Na⁺ ions outside the axon membrane in the resting condition.

The sodium pump is a cellular mechanism that uses energy from ATP to actively transport sodium ions out of the axon, helping restore the resting state after depolarisation.

The brain is protected by 3 membranous coverings called meninges (meninx: membrane) which continue backwards on the spinal cord.

Inflammation of the meninges is called meningitis.

• It is an automatic/quick/immediate, involuntary action in the body brought about by a stimulus.
• Involuntary functioning or movement of any organ or part of the body in response to peripheral nervous stimulation is called reflex action.

Reflex action is an automatic/quick/immediate, involuntary action in the body brought about by a stimulus. This mechanism prevents injury to the body by triggering a rapid and automatic response.

Adaptation is the process by which the human eye adjusts to changes in light intensity.

1. Light Adaptation: When a person moves from a dark environment to a brightly lit area (e.g., stepping out of a cinema hall in the afternoon), they initially experience a dazzling effect. After a few seconds, the eyes adjust to the brightness. This process is called light adaptation.
2. Dark Adaptation: When a person enters a dark area from a brightly lit environment (e.g., entering a cinema hall), they initially struggle to see clearly. Gradually, their vision improves as the eyes adapt to the darkness. This process is called dark adaptation.

Light-sensitive cells in the retina that work in dim light and contain the pigment rhodopsin; they do not detect colour.

The tough, white outer layer of the eyeball made of fibrous tissue, visible as the white part of the eye.

The transparent, bulged-out front part of the sclera that covers the coloured part of the eye and allows light to enter.

The middle, dark-pigmented layer of the eye rich in blood vessels; it nourishes the eye and prevents light scattering.

A muscular part of the choroid layer that changes the shape of the lens to focus light on the retina.

The circular opening in the centre of the iris that regulates the amount of light entering the eye.

The innermost light-sensitive layer of the eye containing rod and cone cells that detect light and colour.

Light-sensitive cells in the retina responsible for colour vision and bright light, containing the pigment iodopsin.

The coloured part of the eye (extension of choroid) that controls the size of the pupil using circular and radial muscles.

The middle ear contains three tiny bones malleus, incus and stapes or hammer, anvil and stirrup in popular terms and a eustachian tube which connects the cavity of the middle ear with the throat. The three bones are collectively called the ear ossicles ( osseus: bone, ossicle : little bone).

Hormone is a molecule that is created and secreted in very minute quantities into the blood stream by an endocrine gland or a specialized nerve cell that governs the growth or functioning of a specific tissue organ in a distant area of the body, e.g., insulin

The endocrine system is defined as a group of organs performing an overall common function. The endocrine glands secrete certain chemicals called hormones.

A group of glands and glandular cells that secrete hormones to regulate and coordinate various functions in the body through chemical control.

Endocrine glands secrete certain chemical known as hormones.

or

Hormone is a secretion from some glandular part of the body, which is poured directly into blood and which acts on the target organs or cells of the same individual, bringing about coordination between distant parts of the body.

Myxoedema is a condition that affects an adult if his thyroid does not function properly; in this condition, the person becomes sluggish with swelling of the face and hands.

• Glands that discharge secretions into ducts are known as exocrine glands.
• Sebaceous glands in the skin, salivary glands in the buccal cavity, etc. are examples of exocrine glands.

The small intercellular space of about 20–30 nm present between presynaptic and postsynaptic neurons is called synaptic cleft.

The process by which a nerve impulse is conducted from the presynaptic neuron to the postsynaptic neuron by neurotransmitters is called synaptic transmission.

The type of synapse in which transmission of impulse occurs directly through a narrow gap by electrical current is called electrical synapse.

The type of synapse in which transmission of impulse occurs through chemical neurotransmitters across a wider synaptic gap is called chemical synapse.

• Coordination is the process by which two or more organs interact and complement each other's functions.
• The nervous system provides an organised network of point-to-point connections for quick coordination throughout the body.
• The nervous system controls the body using a network of electrically conducting cells called neurons (nerve cells).
• Neural coordination is rapid and specific — it allows fast responses to stimuli by transmitting electrical signals along neurons to target organs.

• Hydra has the most primitive diffused nervous system — first animal group with a true nervous system. Belongs to Cnidaria.
• Nerve cells scattered throughout body, interconnected by synapses forming a nerve net/plexus. Sensory cells present but no sense organs.
• Two nerve nets in mesoglea — one towards epidermis, one towards gastrodermis.
• Nerve impulse has no polarity — travels in any direction from any point. No specialised sensory or motor nerves.
• Any point of stimulation causes movement of body/tentacles e.g. catching prey. Also seen in Ctenophora and gut of higher animals.

• Planaria (phylum Platyhelminthes) is the most primitive animal with a Central Nervous System (CNS), located on the ventral side of the body.
• CNS consists of cephalic ganglion (inverted U-shaped brain-like structure) in the head region with 9 branches on each side, and a pair of Ventral Nerve Cords (VNC).
• VNC are interconnected by transverse commissures in a ladder-like manner.
• Peripheral nerve plexus arises laterally from VNC. PNS includes sensory cells arranged in lateral cords and single sensory cells scattered in the body.
• A pair of photosensory eyes is located on the dorsal side of the brain. Represents evolution from diffused (Hydra) → centralised nervous system (Planaria).

• Neuron is the structural & functional unit of the nervous system. Three parts — Cyton (cell body with nucleus, no centrosome so cannot divide), Dendrites (receive impulses toward cell body), Axon (transmits impulses away from cell body).
• Axon is covered by myelin sheath interrupted by Nodes of Ranvier. Ends in terminal bulbs that release neurotransmitters across a synaptic cleft to the next neuron's dendrite.
• Key properties — Excitability (perceive stimulus) | Conductivity (transmit excitation).
• Stimulus types — Threshold (minimum effective) | Subliminal (no effect) | Supraliminal (strong, same as threshold) | Summation effect (weak stimuli added together produce an impulse).
• All or None Law — impulse conducted fully or not at all. Refractory period — the nerve cannot respond to a second stimulus. Synaptic delay — 0.3–0.5 ms to cross a synapse. Synaptic fatigue — halt due to neurotransmitter exhaustion.
• Velocity is higher in long, thick, myelinated nerves; higher in homeotherms; faster in voluntary fibres than autonomic nerves.

• Synapse = gap junction between two nerve cells, allowing impulse transmission via a neurotransmitter bridge.
• Two types — Electrical (direct, via gap junctions, cardiac/smooth muscles) | Chemical (via neurotransmitters, in PNS, CNS, neuromuscular junction).
• Chemical Synapse Steps — Impulse arrives → Ca²⁺ influx → synaptic vesicles release neurotransmitters (exocytosis) → bind postsynaptic receptors → excitatory/inhibitory response → cholinesterase destroys neurotransmitter → ready for new impulse.
• Chemical Synapse — Gap = 20–40 nm, Unidirectional, Slower.
• Electrical Synapse — Gap = 3.8 nm, Bidirectional, Faster, Found where the quickest response is needed (e.g. defence reflexes).

• Nerve impulse is unidirectional — received by dendrites → transmitted via axon to next neuron.
• Polarisation (Resting State) — Na⁺ high outside, K⁺ high inside. Inside = negative. Resting potential = −70 mV. Both Na⁺/K⁺ channels closed.
• Depolarisation (Action Potential) — Stimulus → Na⁺ channels open → Na⁺ rushes in → inside becomes positive.
• Repolarisation — Na⁺ influx stops → K⁺ flows out → inside becomes negative again → resting potential restored.
• Nerve impulse = wave of depolarisation along the neuron. During the refractory period, the nerve cannot respond to a new stimulus until repolarisation is complete.

• The CNS consists of two main parts — the brain, which is enclosed within the cranium of the skull, and the spinal cord, which occupies the vertebral canal of the vertebral column.
• The brain and spinal cord are surrounded and protected by three layers of membranes collectively called meninges.
• The outermost layer, Dura mater, is a tough, non-vascular, fibrous membrane attached to the inner side of the cranium.
• The middle layer, Arachnoid mater, is made of connective tissue with a web-like appearance and is separated from the dura mater by the subdural space filled with serous fluid.
• The innermost layer, Pia mater, is a delicate and highly vascular membrane that remains in close contact with the CNS, supplying it with blood.

• Brain weighs ~1.35 kg (2% body weight, 80% water), protected in cranium, uses over 25% of body oxygen.
• Meninges & CSF — Three membranes: Dura mater → Arachnoid → Pia mater. CSF between them cushions the brain and fills the ventricles and the spinal canal.
• Forebrain — Cerebrum (85% brain mass; 2 hemispheres; 4 lobes — frontal, parietal, temporal, occipital); Diencephalon (contains thalamus and hypothalamus — regulates homeostasis).
• Midbrain — Has corpora quadrigemina (superior colliculi = visual; inferior colliculi = auditory reflexes) and red nucleus (controls posture and motor coordination).
• Hindbrain — Pons (connects brain parts); Cerebellum (balance, posture, muscle tone); Medulla Oblongata (controls heartbeat, respiration, and reflex actions like coughing, sneezing).

• Spinal nerves are mixed nerves — they contain both sensory and motor nerve fibres, formed inside the neural canal of the vertebral column.
• Two Roots — Dorsal (posterior) root carries sensory nerve from dorsal root ganglion; Ventral (anterior) root gives out motor nerve.
• Mixed Nerve Formation — Dorsal sensory + Ventral motor nerves together form the mixed spinal nerve, emerging from both sides of the spinal cord through the intervertebral foramen.
• Three Branches — Ramus dorsalis (supplies dorsal skin/muscles); Ramus ventralis (largest; supplies lateral and anterior organs/muscles); Ramus communicans (smallest; T1 to L3; joins sympathetic ganglion).
• The dorsal root ganglion contains sensory neuron cell bodies, while motor neuron cell bodies are located in the spinal cord itself.

• Voluntary actions are performed consciously, while involuntary actions (reflexes) occur automatically without conscious control.
• A stimulus is any change in the environment that initiates a reflex action.
• Reflex actions are quick, automatic responses to stimuli and do not require conscious thought.
• Examples of reflexes include withdrawal of the hand from a hot object, shivering in cold, and sweating in heat.
• All reflexes are triggered by sensory stimulation and result in either muscular movement or glandular secretion.

• ANS has two divisions — Sympathetic and Parasympathetic nervous system, which work opposite to each other.
• Sympathetic NS — Thoraco-lumbar outflow; originates from the thoracic and lumbar regions of the spinal cord. Has 22 pairs of ganglia along the spinal cord.
• Parasympathetic NS — Cranio-sacral outflow; consists of branches from cranial nerves (III, VII, IX, X), sacral (II, III), and spinal (IV) nerves. Ganglia are located near or within effector organs.
• Neurotransmitters — Sympathetic uses Adrenaline and Noradrenaline (adrenergic fibres); Parasympathetic uses Acetylcholine (cholinergic fibres).
• Functions — Sympathetic controls emergency responses (fight or flight), stimulates most organs. Parasympathetic counters the sympathetic, normalises activities and inhibits most organs.

• Specialised structures that receive stimuli and generate action potentials (impulses) sent to the brain for processing and interpretation.
• Two main types: Exteroceptors (receive external stimuli) and Interoceptors (receive internal stimuli).
• Exteroceptors — Phonoreceptors (sound); Photoreceptors (vision); Statoreceptors (balance); Thermoreceptors (heat/cold); Mechanoreceptors (touch/pain); Chemoreceptors (taste and smell).
• Interoceptors — Enteroceptors (hunger, thirst, internal pain); Proprioceptors (joint/muscle/tendon movement); Baroreceptors (sense blood pressure; restore homeostasis).
• Each receptor responds only to a specific stimulus — this is called receptor specificity.

1. The eyes are a pair of sensory organs located in the skull orbit and are protected by fat, bony sockets, eyelids, eyelashes, eyebrows, and tear glands.
2. Eye movements are controlled by six sets of muscles that help in proper vision and coordination.
3. The human eye works like a camera, forming a real and inverted image on the light-sensitive retina.
4. The cornea and lens focus light onto the retina, while the iris regulates the amount of light entering by controlling the pupil size.
5. The eye shows accommodation, where the lens changes its curvature to focus on near and distant objects; the near point is about 25 cm and the far point is infinity.

• The eyeball has three layers: outer sclera (protective), middle choroid (vascular and pigmented), and inner retina (sensory layer).
• The sclera is tough and white, giving shape to the eye; its front transparent part forms the cornea, which helps in focusing light.
• The choroid contains blood vessels and pigment, nourishes the eye, and prevents internal reflection; it forms the ciliary body and iris.
• The iris controls the size of the pupil to regulate light entry; the pupil dilates in dim light and constricts in bright light.
• The lens, along with the cornea, focuses light onto the retina to form a clear image.
• The retina contains rod cells (for dim light) and cone cells (for bright light and colour); the fovea provides sharp vision, while the blind spot lacks photoreceptors.
• Light stimulates rods and cones, generating nerve impulses that pass through bipolar and ganglion cells to the optic nerve, which carries them to the brain for image perception.

• The ear has three main parts: outer ear, middle ear, and inner ear.
• The outer ear includes the pinna and auditory canal, ending at the eardrum (tympanum).
• The middle ear contains three tiny bones — malleus (hammer), incus (anvil), and stapes (stirrup) — called ossicles, and is connected to the throat by the eustachian tube.
• The stirrup fits on the oval window, and a second membrane-covered opening called the round window separates the middle ear from the inner ear.
• The inner ear includes the cochlea (with the organ of Corti for hearing) and the semicircular canals for dynamic balance.
• The vestibule (utriculus and sacculus) contains sensory cells responsible for static balance when the body is stationary.

• The inner ear consists of the cochlea (for hearing) and vestibular apparatus (for balance), including semicircular canals, utricle, and saccule.
• The semicircular canals (with cristae) detect dynamic balance, while the utricle and saccule (with maculae and otoliths) detect static balance and head position.
• Sound waves are collected by the pinna, causing the eardrum to vibrate, and vibrations are transmitted through the ear ossicles to the cochlea.
• Vibrations in the cochlea create waves in the endolymph, leading to movement of the basilar membrane and stimulation of hair cells.
• Hair cells generate nerve impulses that are carried by the auditory nerve to the brain, where the sound is interpreted.

• Two types — Psychological (mental, affects mood/thinking/behaviour) and Neurological (damage/malfunction of the nervous system).
• Psychological Disorders — Examples: ADHD, Anxiety, Depression, Bipolar Disorder, ASD, Intellectual Disability.
• Neurological Disorders — Examples: Alzheimer's and Parkinson's disease.
• Parkinson's Disease — Degeneration of dopamine-producing neurons in the CNS. Symptoms: tremors, stiffness, difficulty in walking and balance.
• Alzheimer's Disease — Common dementia; memory and cognitive decline. Caused by loss of neurons and amyloid protein accumulation in the CNS.

• Endocrine system controls body activities through chemical messengers called hormones, which are released directly into the bloodstream (no ducts).
• Hormones act as intercellular messengers — they inhibit, stimulate, or modify specific body processes like growth and development, and act only on specific target organs.
• Types of Hormones — Steroid (aldosterone, sex corticoids); Amine (thyroxine, adrenaline); Peptide (ADH, GnRH); Protein (TSH, FSH); Fatty acid derivatives (prostaglandins).
• Endocrine glands are ductless glands — they secrete hormones directly into the blood. Examples: thyroid, pituitary, pancreas, hypothalamus, adrenal glands.
• All endocrine glands work in a coordinated manner, activating each other to bring about overall chemical coordination in the body.

1. Hormones are chemical secretions by endocrine glands that regulate body activities.
2. Body activities must be timed and coordinated so processes (like digestive secretions) occur only when needed.
3. Besides the nervous system, hormones help regulate complex body functions by acting as chemical messengers.
4. Hormones are released into the bloodstream and carried to all parts of the body, but act only on specific target organs or cells.
5. Endocrine glands are ductless glands that secrete hormones directly into the blood; some hormones are also produced by organs with other primary functions (e.g., stomach, duodenum).

• Hormones bind to specific receptors in target tissues only, forming a hormone-receptor complex → causes biochemical changes.
• Two receptor types — Membrane-bound (cell surface) and Intracellular (inside cell/nucleus).
• Membrane Receptor — Water-soluble hormones bind surface receptor → adenylate cyclase → ATP converts to cAMP (2nd messenger) → enzyme activation. Other 2nd messengers: Ca²⁺, cGMP, IP₃.
• Intracellular Receptor — Lipid-soluble hormones cross membrane → bind cytoplasm receptor → complex enters nucleus → activates DNA → mRNA → protein synthesis.
• Membrane receptor = faster, short-lived; Intracellular receptor = slower, long-lasting.

• Controls Pituitary — Hypothalamus regulates the anterior pituitary via Releasing (RH) and Inhibiting (IH) hormones.
• Oxytocin — Stored in posterior pituitary; causes uterus contractions during childbirth and milk ejection.
• ADH (Vasopressin) — Stored in the posterior pituitary; promotes water reabsorption in the kidneys, reducing urine loss.
• GnRH — Stimulates the pituitary to release gonadotropins (reproductive hormones).
• Somatostatin — Inhibits the release of growth hormone from the pituitary.

• Pineal gland is a small endocrine gland located on the roof of diencephalon, between the two cerebral hemispheres, on the dorsal side of the forebrain.
• It consists of neuroglia and secretory cells called pinealocytes.
• It secretes Melatonin — an amine hormone derived from tryptophan, also called the sleep hormone.
• Melatonin regulates the body's biological clock (circadian rhythm) — the 24-hour diurnal rhythm, including the sleep-wake cycle and body temperature.
• It also influences pigmentation, metabolism, defense capability, and reproductive/menstrual cycle.

1. The thyroid gland is a butterfly-shaped gland located in the neck and secretes thyroxine and calcitonin.

2. Thyroxine controls basal metabolic rate, influences growth, mental development, bone formation, and body temperature.

3. Hypothyroidism may cause:

• Simple goitre (due to iodine deficiency),
• Cretinism (dwarfism and mental retardation in children),
• Myxoedema (sluggishness and swelling in adults).

4. Hyperthyroidism leads to exophthalmic goitre with symptoms like protruding eyes, neck swelling, high metabolism, rapid heartbeat, and breathlessness.

5. Use of iodised salt helps prevent thyroid-related issues caused by iodine deficiency.

• Four glands on posterior surface of thyroid; secrete peptide hormone Parathormone (PTH) / Collip's hormone.
• PTH Function — Increases blood Ca²⁺ by bone resorption, renal Ca²⁺ reabsorption, and intestinal Ca²⁺ absorption.
• PTH and Calcitonin (TCT) are antagonistic — together maintain calcium-phosphate balance. PTH is under feedback control of blood Ca²⁺.
• Hyposecretion → low Ca²⁺ → muscle twitch/spasm = Hypocalcaemic Tetany.
• Hypersecretion → excess bone demineralisation → softening/fracture = Osteoporosis (common in post-menopausal women).

• Located in the upper thorax, dorsal side of the heart; soft, pinkish, bilobed mass of lymphoid tissue.
• Prominent at birth but gradually atrophies in adults — hence called a temporary gland.
• Secretes hormone Thymosin, which plays a key role in the development of the immune system.
• Thymosin promotes differentiation and maturation of T-lymphocytes → provides cell-mediated immunity.
• Also promotes production of antibodies → provides humoral immunity.

• Pancreas functions both as an exocrine gland (secreting pancreatic juice into the duodenum) and an endocrine gland (secreting hormones into the blood).
• The Islets of Langerhans contain beta cells (secrete insulin) and alpha cells (secrete glucagon), which regulate blood sugar levels.
• Insulin lowers blood glucose by promoting glucose uptake by cells and storing excess glucose as glycogen in the liver and muscles.
• Under-secretion of insulin causes diabetes mellitus (hyperglycemia), with symptoms like excessive urination, thirst, weakness, and sugar in urine.
• Glucagon raises blood sugar by converting glycogen to glucose in the liver; overdose of insulin can lead to hypoglycemia or insulin shock.

• Gonads = Testes (male) and Ovaries (female); both secrete steroid hormones.
• Testes → Androgens (Testosterone) — Develops male accessory organs (epididymis, vas deferens, prostate, etc.); essential for spermatogenesis.
• Ovaries → Oestrogen — Develops female secondary sex organs and follicles; stimulates GnRH release before ovulation; involved in mammary gland development.
• Ovaries → Progesterone — Maintains pregnancy; thickens the uterus wall; stimulates alveoli formation; inhibits oestrogen release.
• Ovaries → Inhibin (inhibits FSH secretion) & Relaxin (at end of pregnancy — dilates cervix, increases flexibility of pubic symphysis).

• Placenta — Secretes Oestrogens, Progesterone, HCG (Human Chorionic Gonadotropin) and HCS (Human Chorionic Somatotropin) to maintain pregnancy; acts on ovaries, mammary glands and uterus.
• Heart — Secretes ANF/Atriopetin (Atrial Natriuretic Factor) — dilates blood vessels, lowers blood pressure, and maintains fluid balance; targets blood vessels and kidneys.
• Kidney — Secretes Erythropoietin (stimulates RBC production in bone marrow); Urotensin (vasoconstriction of major arteries); Calcitriol (aids calcium and phosphorus absorption in small intestine).
• Gastrointestinal Tract — Gastrin (stimulates HCl and pepsinogen/digestive enzymes in the stomach); Secretin (stimulates pancreatic juice and water/bicarbonate ions); CCK (stimulates pancreatic enzymes and bile from the gall bladder); GIP (inhibits gastric secretion and motility).
• Adipose Tissue — Secretes Leptin — suppresses appetite by acting on the brain.

1. Nerve Impulse
   A nerve impulse is a wave of bioelectrical (electrochemical) disturbance that travels along the neuron membrane.
2. Resting (Polarized) State
   In a resting neuron, the membrane is polarized with excess Na⁺ outside and K⁺ inside, creating a resting potential of about –70 mV.
3. Sodium–Potassium Pump
   Active transport of Na⁺ out and K⁺ into the neuron against their gradients using ATP energy is called the Na⁺/K⁺ exchange pump.
4. Role of Ion Channels
   Voltage-gated Na⁺ and K⁺ channels regulate membrane permeability and enable change from resting potential to active potential during impulse conduction.