Nervous System and Reflex Arcs
~7 min read
- Two divisions: Central (brain + spinal cord) and peripheral (nerves).
- Reflex arc: Receptor → sensory neuron → spinal cord relay → motor neuron → effector. Bypasses brain for speed.
- Special hazards: Lead compounds harm the nervous system; brain controls voluntary action.
The nervous system enables sensing, thinking and action. CDS/OTA has asked about reflex arcs and neurotoxic pollutants.
Organisation of the Nervous System
- Central nervous system (CNS): brain + spinal cord. Processes information and initiates voluntary action.
- Peripheral nervous system (PNS): all nerves outside CNS. Carries signals to and from the CNS.
- Somatic NS: controls voluntary skeletal muscles.
- Autonomic NS: controls involuntary actions — heart rate, digestion. Has sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.
Parts of the Brain
| Part | Function |
|---|---|
| Cerebrum | Voluntary action, thinking, memory, intelligence |
| Cerebellum | Balance and coordination of voluntary movements |
| Medulla oblongata | Involuntary actions — heart rate, breathing, blood pressure |
| Hypothalamus | Hunger, thirst, body temperature; controls pituitary |
| Pons | Sleep regulation, respiration coordination |
Reflex Arc — Speed Over Thought
- A reflex action is an automatic, rapid response to a stimulus that does not require conscious thought.
- Pathway: Receptor → Sensory neuron → Relay neuron in spinal cord → Motor neuron → Effector (muscle/gland).
- The brain is informed afterwards — but the response itself bypasses the brain to save time.
- Examples: pulling hand away from a hot object, knee-jerk, blinking when something approaches the eye.
Pollutants Harming the Nervous System
- Lead compounds from leaded petrol (now phased out) and paint cause neurotoxicity — especially affects children's brain development.
- Mercury (from industrial discharge) causes Minamata disease — affects nervous system.
- Pesticides like organophosphates inhibit acetylcholinesterase — paralysing nerve signalling.
CDS/OTA PYQ Examples
Q: Match: A. CFC — 2. Ozone depletion; B. SO₂ — 1. Acid rain; C. Lead — 3. Nervous system; D. CO₂ — 4. Greenhouse effect.
(a) 4,3,1,2 (b) 4,1,3,2 (c) 2,1,3,4 (d) 2,3,1,4
Answer: (c) — CFC→ozone, SO₂→acid rain, Lead→nervous system, CO₂→greenhouse. [CDS-I 2015]
Q: Which sequence correctly represents a reflex arc?
(a) Receptor → Sensory neuron → Relay neuron in spinal cord → Brain → Motor neuron → Effector (b) Receptor → Sensory → Brain → Relay neuron → Motor → Effector (c) Receptor → Motor → Relay → Sensory → Effector (d) Receptor → Sensory neuron → Relay neuron in spinal cord → Motor neuron → Effector
Answer: (d) Receptor → Sensory → Relay neuron in spinal cord → Motor → Effector — brain is bypassed for speed. [CDS-I 2023]
Q: Which part of the brain controls involuntary actions like heartbeat and breathing?
(a) Cerebrum (b) Cerebellum (c) Medulla oblongata (d) Hypothalamus
Answer: (c) Medulla oblongata.
Q: Voluntary actions are controlled by:
(a) Cerebrum (b) Cerebellum (c) Medulla (d) Spinal cord
Answer: (a) Cerebrum — seat of voluntary action, memory and intelligence.
Drill Nervous System and Reflex Arcs for CDS/OTA
CDS/OTA-pattern items on Nervous System and Reflex Arcs with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does the reflex arc bypass the brain?
Speed. Sending the signal to the brain and back adds milliseconds — too long when withdrawing from a hot stove. The spinal cord triggers the effector first; the brain is informed milliseconds later.
What is the difference between central and peripheral nervous system?
CNS (brain + spinal cord) processes and integrates information. PNS (all nerves outside CNS) carries sensory signals to CNS and motor signals from CNS.
Why are children more vulnerable to lead poisoning?
Their developing nervous system absorbs lead more readily; even small exposure can cause irreversible IQ loss and behavioural problems.