Samacheer Kalvi 10th Science Chapter 15 Long Answer Questions — Nervous System with Answers

Answer:

The human brain is the most complex organ, weighing about 1.2–1.4 kg. It is protected by the bony skull (cranium) and three membranes called meninges (dura mater — outermost, arachnoid — middle, pia mater — innermost). The space between the arachnoid and pia mater is filled with cerebrospinal fluid (CSF) which cushions the brain and provides nutrients.

The brain is divided into three main regions:

1. Forebrain (Prosencephalon):

A. Cerebrum:

• The largest part of the brain, constituting about 85% of total brain weight
• Divided into two cerebral hemispheres (left and right) connected by the corpus callosum (a thick bundle of nerve fibres)
• Surface is folded into ridges (gyri) and grooves (sulci / fissures) — greatly increases surface area
• Outer layer = Cerebral Cortex (grey matter, 2–4 mm thick) — contains about 14 billion neurons
• Inner layer = white matter (myelinated nerve fibres) + basal ganglia

Lobes of Each Hemisphere:

• Frontal Lobe: Voluntary motor control, reasoning, planning, problem-solving, personality, speech (Broca's area in left hemisphere)
• Parietal Lobe: Sensory processing (touch, pain, temperature), spatial awareness
• Temporal Lobe: Hearing, language comprehension (Wernicke's area), memory
• Occipital Lobe: Visual processing

Functions of Cerebrum:

• Controls voluntary movement
• Seat of intelligence, memory, reasoning, learning, and consciousness
• Processes all sensory information from the body
• Controls speech and language

B. Thalamus:

• Acts as a relay station — all sensory information (except smell) passes through the thalamus before reaching the cerebral cortex
• Integrates sensory signals and directs them to appropriate cortical areas
• Involved in consciousness and alertness

C. Hypothalamus:

• Located below the thalamus
• Controls the pituitary gland (the "master gland") — links the nervous and endocrine systems
• Regulates body temperature (thermostat of the body), hunger, thirst, sleep-wake cycles, and water balance
• Controls autonomic nervous system functions
• Produces hormones (ADH, oxytocin) stored and released by the posterior pituitary

2. Midbrain (Mesencephalon):

• Connects the forebrain and hindbrain
• Contains nerve tracts (cerebral peduncles) connecting forebrain and hindbrain
• Corpora Quadrigemina: Four rounded bodies — superior colliculi (visual reflexes — pupil reflex, blinking) and inferior colliculi (auditory reflexes — turning head toward sound)
• Contains the reticular activating system (RAS) — maintains consciousness and arousal
• Contains the substantia nigra — dopamine-producing neurons (degeneration causes Parkinson's disease)

3. Hindbrain (Rhombencephalon):

A. Cerebellum:

• Second largest part of the brain
• Located below and behind the cerebrum
• Two hemispheres connected by the vermis
• Highly folded surface (folia) — large surface area
• Functions:
• Coordinates voluntary muscle movements for smooth, precise actions
• Maintains balance and posture (receives input from vestibular organ in inner ear)
• Controls fine motor movements (writing, playing musical instruments)
• Involved in motor learning
• Damage: ataxia (uncoordinated movements), loss of balance

B. Pons:

• Acts as a bridge between cerebellum and medulla oblongata
• Contains pneumotaxic and apneustic centres that regulate breathing rate and depth
• Contains nuclei for cranial nerves V, VI, VII, VIII
• Relays information between the cerebrum and cerebellum

C. Medulla Oblongata:

• Located at the base of the brain; continuous with the spinal cord
• Contains vital reflex centres:
• Cardiac centre: Controls heart rate
• Vasomotor centre: Regulates blood pressure (by controlling diameter of blood vessels)
• Respiratory centre: Controls breathing (rate and depth)
• Reflex centres for swallowing, coughing, sneezing, vomiting, hiccuping
• All nerve fibres crossing from the brain to the spinal cord decussate (cross over) at the medulla — explains why left brain controls right side of body

Answer:

Structure of the Spinal Cord:

The spinal cord is a cylindrical cord of nervous tissue extending from the medulla oblongata (at the base of the brain) to the level of the first lumbar vertebra (L1) in adults. It is approximately 43–45 cm long and 1–1.5 cm in diameter.

Protective Coverings:

• Enclosed within the vertebral column (backbone) — bony protection
• Surrounded by three meninges (same as brain):
• Dura mater (outer)
• Arachnoid mater (middle)
• Pia mater (inner, closely adhering to cord)
• Subarachnoid space between arachnoid and pia mater contains cerebrospinal fluid (CSF)

Internal Structure (Cross-Section): The spinal cord has a characteristic butterfly-shaped or H-shaped grey matter surrounded by white matter.

Grey Matter (inner, H-shaped):

• Contains cell bodies of neurons, unmyelinated fibres, and neuroglia
• Central canal runs through its centre, containing CSF
• Dorsal (Posterior) Horn: Contains sensory neurons — receives sensory signals from the body
• Ventral (Anterior) Horn: Contains motor neurons (alpha motor neurons) — sends motor commands to skeletal muscles
• Lateral Horn (present in thoracic and lumbar regions): Contains preganglionic neurons of the autonomic nervous system

White Matter (outer, surrounds grey matter):

• Contains myelinated nerve fibres arranged in columns (funiculi)
• Ascending tracts: Carry sensory signals upward to the brain
• Descending tracts: Carry motor commands downward from the brain

Spinal Nerves:

• 31 pairs of spinal nerves arise from the spinal cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal)
• Each spinal nerve has two roots:
• Dorsal root (posterior): Contains sensory (afferent) fibres — carries signals to the cord. Has a dorsal root ganglion (containing sensory neuron cell bodies).
• Ventral root (anterior): Contains motor (efferent) fibres — carries signals from the cord to muscles/glands.

Functions of the Spinal Cord:

Answer:

A reflex action is a rapid, automatic, involuntary response to a stimulus that occurs without conscious thought. It is the simplest form of neural response and is controlled by the spinal cord (and some brain stem reflexes are controlled by the brain).

Importance of Reflexes:

• Protect the body from harm (withdrawal from pain)
• Maintain homeostasis (pupil reflex for light adaptation)
• Free the brain from handling routine responses, allowing it to focus on complex tasks

Components of the Reflex Arc:

A reflex arc is the neural pathway involved in a reflex action. It has five components:

Example: Withdrawal Reflex (Touching a Hot Object):

1. Stimulus: Skin of finger touches a hot surface 2. Reception: Pain receptors (nociceptors) in the finger skin are activated 3. Afferent pathway: Sensory neuron carries impulse along dorsal root to the spinal cord 4. Integration: In the spinal cord, the sensory neuron synapses with an interneuron (or directly with motor neuron) 5. Efferent pathway: Motor neuron carries impulse along ventral root to the biceps muscle 6. Response: Biceps muscle contracts, pulling the hand away from the hot object

Simultaneous brain notification: While the spinal cord has already triggered the withdrawal (in milliseconds), the impulse is also sent upward through ascending tracts to the brain — this is why you feel the pain slightly after you have already withdrawn your hand.

Types of Reflexes:

Based on the nerve centre:

• Spinal reflexes: Reflex centre in the spinal cord (withdrawal reflex, knee-jerk)
• Cranial reflexes: Reflex centre in the brain stem (pupil reflex to light, blinking, salivation)

Based on neural pathway:

• Monosynaptic: Only one synapse (sensory neuron directly to motor neuron) — fastest; e.g., patellar (knee-jerk) reflex
• Polysynaptic: Multiple synapses involving interneurons; e.g., withdrawal reflex

Based on origin:

• Inborn (Unconditioned) reflexes: Present from birth; e.g., suckling, blinking
• Conditioned reflexes: Acquired through learning and experience; e.g., Pavlov's dog salivating at the sound of a bell

Answer:

Both the nervous system and endocrine system are coordination systems that help the body respond to internal and external stimuli, but they differ significantly in their mechanisms, speed, and duration of action.

Nervous System:

The nervous system uses electrical signals (nerve impulses) to communicate rapidly between body parts. Messages are transmitted through neurons via electrochemical signals.

Key features:

• Speed: Extremely fast (70–120 m/s in myelinated fibres)
• Duration: Effect is rapid and short-lived; stops as soon as the impulse stops
• Transmission medium: Nerve impulses + neurotransmitters at synapses
• Target: Specific — impulses travel along specific nerve fibres to specific muscles or glands
• Range: Local (messages go to specific target organs along nerve pathways)
• Integration centre: Brain and spinal cord
• Type of response: Precise, localised, immediate responses

Examples of nervous control:

• Muscle contraction when you pick up an object
• Pupil constriction in bright light
• Withdrawing hand from hot object (reflex)

Endocrine System:

The endocrine system uses chemical signals (hormones) secreted by ductless glands directly into the bloodstream to communicate more slowly but over a longer duration.

Key features:

• Speed: Slow (hormones travel through blood, which takes seconds to minutes)
• Duration: Effect is slow to begin but long-lasting
• Transmission medium: Blood (hormones dissolved in plasma)
• Target: Diffuse — hormones reach all body cells, but only target cells (with specific receptors) respond
• Range: Widespread (hormones circulate throughout the entire body)
• Integration centre: Hypothalamus-pituitary axis
• Type of response: Slow, widespread, prolonged responses; often regulates growth, development, and metabolism

Examples of endocrine control:

• Insulin regulates blood glucose level over hours
• Growth hormone regulates body growth over months/years
• Adrenaline prepares the body for "fight or flight" response

Comparison Table:

Integration of Both Systems:

The nervous and endocrine systems work together (neuroendocrine integration):

• The hypothalamus links both systems — it is nervous tissue but also secretes hormones
• The hypothalamus controls the pituitary gland via hormones
• The autonomic nervous system controls hormone secretion from the adrenal medulla (adrenaline and noradrenaline)
• Together they maintain homeostasis — the stable internal environment of the body

Answer:

A synapse is the junction between two neurons (or between a neuron and an effector cell). It is the site where a nerve impulse is transmitted from one neuron to the next. The synapse was described by Charles Sherrington in 1897.

Structure of a Synapse:

A typical chemical synapse has three components:

Mechanism of Synaptic Transmission:

Step 1: Arrival of Action Potential An action potential (nerve impulse) travels along the axon and reaches the pre-synaptic terminal.

Step 2: Calcium Ion Influx The depolarisation of the pre-synaptic membrane opens voltage-gated Ca2+ channels. Calcium ions rush into the pre-synaptic terminal from the extracellular fluid (where Ca2+ concentration is much higher).

Step 3: Neurotransmitter Release (Exocytosis) The influx of Ca2+ triggers synaptic vesicles to fuse with the pre-synaptic membrane. Neurotransmitter molecules are released into the synaptic cleft by exocytosis.

Step 4: Diffusion Across the Synaptic Cleft Neurotransmitter molecules diffuse across the narrow synaptic cleft and bind to specific receptor proteins on the post-synaptic membrane.

Step 5: Generation of Post-synaptic Potential Binding of neurotransmitter to receptor opens ligand-gated ion channels in the post-synaptic membrane:

• Excitatory neurotransmitters (e.g., acetylcholine at neuromuscular junction, glutamate): Open Na+ channels → Na+ flows in → membrane depolarisation → Excitatory Post-Synaptic Potential (EPSP). If sufficient EPSPs summate, a new action potential is generated.
• Inhibitory neurotransmitters (e.g., GABA): Open Cl– channels or K+ channels → hyperpolarisation → Inhibitory Post-Synaptic Potential (IPSP). This reduces the likelihood of an action potential being generated.

Step 6: Termination of Signal To prevent continuous stimulation, neurotransmitters are quickly removed from the synaptic cleft by:

• Enzymatic breakdown: Acetylcholinesterase breaks down acetylcholine into choline and acetate
• Reuptake: Neurotransmitter is taken back into the pre-synaptic terminal and repackaged into vesicles (e.g., dopamine, serotonin)
• Diffusion: Some neurotransmitters simply diffuse away from the cleft

Types of Synapses:

1. Chemical Synapse: Uses neurotransmitters (as described above). Most common in the human nervous system. Signal transmission takes about 0.5 milliseconds (synaptic delay).

2. Electrical Synapse: Gap junctions directly connect the cytoplasm of two cells. Ions flow directly. Extremely fast, bidirectional. Found in the heart (intercalated discs) and some brain areas.

Significance of Chemical Synapses:

1. Directional flow: Neurotransmitters are released only from the pre-synaptic side and bind receptors on the post-synaptic side → signals travel in one direction only 2. Integration: A neuron can receive thousands of synaptic inputs; the sum of EPSPs and IPSPs determines whether it fires 3. Plasticity: Synaptic strength can be modified (long-term potentiation / depression) — basis of learning and memory 4. Pharmacological targets: Many drugs (e.g., antidepressants, anaesthetics, nerve agents) work by modifying synaptic transmission

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