PRELIMS

Human Nervous System and Brain

The nervous system coordinates body functions via electrochemical impulses. The functional unit is the Neuron (nerve cell).

0. Neuron Structure and Types

• Cell Body (Cyton/Soma): Contains nucleus; controls cell metabolism.
• Dendrites: Short, branched extensions that receive impulses and carry them towards the cell body.
• Axon: Long single extension that carries impulses away from the cell body; often covered by a Myelin Sheath (fatty, insulating layer secreted by Schwann cells) that speeds up impulse conduction.
• Synapse: Junction between two neurons where the impulse is transmitted via chemical neurotransmitters (e.g., Acetylcholine, Dopamine).
• Types of Neurons: Sensory (Afferent) — carry impulses from receptor to CNS; Motor (Efferent) — carry impulses from CNS to effector (muscle/gland); Relay/Association — connect sensory and motor neurons within the CNS.

I. Central Nervous System (CNS)

The CNS consists of the Brain and Spinal Cord, protected by the Meninges membrane and the Cranium (skull).

• Fore Brain: Sub-part: Cerebrum. Major Functions: Centre of wisdom, memory, will power, thinking, and motor knowledge.
• ****: Sub-part: Thalamus. Major Functions: Centre of pain, cold, and heat.
• ****: Sub-part: Hypothalamus. Major Functions: Centre of hunger, thirst, temperature control, love, hate, BP, and water metabolism. Controls pituitary gland.
• Mid Brain: Sub-part: Corpora Quadrigemina. Major Functions: Controls vision and hearing.
• Hind Brain: Sub-part: Cerebellum. Major Functions: Coordination of muscular movements and posture maintenance.
• ****: Sub-part: Pons Varolii. Major Functions: Bridge between brain and spinal cord.
• ****: Sub-part: Medulla Oblongata. Major Functions: Posterior-most part; controls heart rate, respiration, coughing, and sneezing.

• Spinal Cord: Centre of reflex actions (first discovered by Marshall Hall).

Reflex Arc (pathway of a reflex action, e.g., withdrawing hand from a hot object): Receptor Sensory Neuron Spinal Cord (relay/association neuron) Motor Neuron Effector (muscle). This bypasses the brain for a faster, involuntary response.

II. Peripheral Nervous System (PNS)

The PNS consists of nerves arising from the brain and spinal cord.

• Cranial Nerves: 12 pairs.
• Spinal Nerves: 31 pairs.

III. Autonomic Nervous System (ANS)

Controls internal organs and blood vessels (Concept by Langley, 1921).

1. Sympathetic: Prepares for "Fight, Flight, Fright." Increases heart rate, BP, sugar, and pupil dilation. Reduces saliva.
2. Parasympathetic: Prepares for "Rest and Joy." Widens blood vessels, increases saliva/digestion, and contracts pupil.

UPSC Relevance

Neurobiology is central to understanding human behavior, medical technology (ECG/EEG), and drug impacts.

• Brain Mapping: Identifying the Hypothalamus as the thermostat and the Cerebrum as the memory bank.
• Reflex Action: Understanding the role of the Spinal Cord in involuntary, rapid responses.
• Nervous Divisions: Differentiating between the "fight" (Sympathetic) and "rest" (Parasympathetic) responses for stress biology questions.
• Confused Pair — CNS vs PNS: CNS (Brain + Spinal Cord) is the processing/decision centre; PNS (Cranial + Spinal nerves) connects the CNS to the rest of the body. The ANS (Sympathetic/Parasympathetic) is a functional sub-part of the PNS controlling involuntary organs.
• Confused Pair — Sensory vs Motor Neuron: Sensory neurons carry impulses to the CNS (afferent); Motor neurons carry impulses away from the CNS (efferent) — remember "SAME" (Sensory-Afferent, Motor-Efferent).