Revision: Human Anatomy and Physiology >> The Nervous System Biology (English Medium) ICSE Class 10 CISCE

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.

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

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.

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

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.

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.

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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 aggregates of the cytons or cell bodies of nerve cells from which the nerve fibres may arise or enter into are called ganglia.

Nerve is a bundle of nerve fibres (axons) of separate neurons, enclosed in a tubular sheath.

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.

The cerebrum is divided into two halves called the right and left cerebral hemispheres.

White matter is the inner region of the cerebrum that consists mainly of myelinated axons (nerve fibres).

The subconscious or unconscious mind is a part of the cerebrum where past experiences are stored and may be recalled during dreams or hypnosis.

The corpus callosum is a thick band of nerve fibres that connects the two cerebral hemispheres and allows communication between them.

Sulci are the grooves or depressions between the gyri on the surface of the cerebrum.

Gyri are the raised folds or ridges on the surface of the cerebrum.

The outer portion (cortex) of the cerebrum contains cell bodies of the neurons and, being grayish in colour, is called the gray matter.

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.

• 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.

An involuntary, spontaneous automatic response brought about due to a previously learned experience is called a conditioned reflex.

A reflex which develops during lifetime due to experience or learning is called a conditioned (acquired) reflex.

A reflex in which no previous experience or learning is required and is inherited from the parents is called a natural (inborn) reflex.

A reflex arc is the shortest route that can be taken by an impulse from a receptor to an effector.

• Receptors present in sense organs detect changes in the environment and send signals to the brain through neurons.
• A neuron has three parts — dendrites (receive signals), cell body (processes signals), and axon (transmits signals) — which work together to carry electrical impulses.
• The electrical impulse always travels in one fixed direction: dendrite → cell body → axon → axon terminal, ensuring a controlled flow of information.
• At the synapse, special chemicals called neurotransmitters are released to pass the impulse from one neuron to the next, or to a muscle or gland.
• The neuromuscular junction is where a nerve meets a muscle fibre; when the impulse reaches this point, it causes the muscle to contract and produce a response.

• 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.

• 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).

• 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).

1. The cerebrum is the largest part of the brain and is divided into right and left hemispheres.
2. Its surface is highly convoluted with gyri (ridges) and sulci (grooves), increasing the surface area for more nerve cells.
3. The outer cortex consists of gray matter (cell bodies of neurons), while the inner portion contains white matter (nerve fibres).
4. The corpus callosum connects the two cerebral hemispheres and transfers information between them.
5. The cerebrum is the seat of intelligence, reasoning, memory, consciousness, and voluntary actions, and also stores subconscious experiences.

1. The cerebellum is located at the base of the brain beneath the cerebrum.
2. It has an outer gray matter cortex and inner white matter, with a tree-like appearance in section.
3. It lacks convolutions but has numerous furrows.
4. Its main function is to maintain balance and coordinate muscular movements.
5. It does not initiate movement, but ensures timing and coordination of muscle actions; alcohol affects its function, causing unsteady movements.

1. It is the lowest part of the brain, connecting the brain to the spinal cord.
2. It controls involuntary activities such as heartbeat, breathing, and peristalsis.
3. Damage to the medulla is usually fatal due to its role in vital functions.

• 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.

• 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.

• 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.