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

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

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

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

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

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

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

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.

A nerve is a bundle of axons from numerous neurons encased in a protective sheath.

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

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

Neurons are the building blocks of the nervous system and transmit information throughout the body.

The path that an impulse takes in a reflex action is called a reflex arc.

The neurons which carry impulses from the brain or spinal cord to the body parts are called motor or efferent neurons.

The neurons which carry impulses from the body parts to the spinal cord or the brain are called sensory or afferent neurons. For example, the optic nerve of the eye.

A stimulus is an agent or a sudden change in the external or internal environment that changes an organism or body parts. e.g., light, sound, heat, pain, hunger.

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.

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.

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).

Hormones are chemical messengers created by the endocrine glands and transported through the bloodstream to govern body functions such as development, metabolism, reproduction and mood by interacting with specific target cells.

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

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

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

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.

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 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 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.

It is an automatic/quick/immediate, involuntary action in the body brought about by a stimulus.

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.

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.

• The human eye works like a camera, forming a real and inverted image on the retina, which is light-sensitive.
• The cornea allows light to enter the eye and performs most of the refraction, while the lens fine‑tunes the focus.
• The iris controls the pupil, regulating the amount of light entering the eye—contracting in bright light and widening in dim light.
• The power of accommodation is the ability of the eye lens to change its focal length by altering its curvature using the ciliary muscles.
• For a normal eye, the near point is 25 cm and the far point is at infinity.

1. The ear has three main parts: outer ear, middle ear, and inner ear.
2. The outer ear includes the pinna and auditory canal, ending at the eardrum (tympanum).
3. 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.
4. 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.
5. The inner ear includes the cochlea (with the organ of Corti for hearing) and the semicircular canals for dynamic balance.
6. The vestibule (utriculus and sacculus) contains sensory cells responsible for static balance when the body is stationary.

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.

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

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

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.