Nervous System and Reflex Arcs

Why the Nervous System Scores Easy Marks

Control and coordination is one of the most frequently tested areas in CDS General Science, and like digestion it needs no numbers. The exam loves single-fact questions: which part of the brain controls balance, what a synapse is, the longest nerve in the body, or the correct order of a reflex arc. If your facts are crisp, these are seconds-per-mark questions.

The smartest way to learn this chapter is to picture the nervous system as the body’s electrical wiring and command centre. Sense organs pick up a stimulus, nerves carry the signal as an impulse, the brain or spinal cord processes it and sends back an instruction, and a muscle or gland responds. Once you fix this stimulus → response flow and the station at each step, the objective options almost sort themselves.

This chapter links cleanly to others you are revising — sense organs, endocrine glands and hormones, and muscle physiology — so the effort here pays off across the biology section.

What Control and Coordination Means

Coordination is the working together of the various organs of the body in a proper, controlled manner to produce a suitable response to a stimulus. In animals this control is achieved by two systems acting together:

• Nervous system — gives a fast, short-lived electrical response through nerves.
• Endocrine (hormonal) system — gives a slower, longer-lasting chemical response through hormones in the blood.

A stimulus is any change in the environment (light, heat, sound, touch, smell) to which the body reacts. The reaction produced is called the response. The nervous system is what lets you respond almost instantly — pulling your hand back, blinking, or balancing on one foot.

The Neuron: Structural Unit of the Nervous System

The neuron (nerve cell) is the structural and functional unit of the nervous system. It is the longest cell in the human body and is specialised to receive and transmit electrical impulses. A neuron has three main parts:

• Cell body (cyton) — contains the nucleus and cytoplasm; it is where information is received.
• Dendrites — short, branched projections that receive impulses and carry them towards the cell body.
• Axon — a single long fibre that carries the impulse away from the cell body towards the next cell.

Many axons are covered by a fatty myelin sheath, which insulates the fibre and speeds up impulse conduction. A bundle of many axons bound together forms a nerve.

The Nerve Impulse and the Synapse

A nerve impulse is an electrical and chemical signal that travels along a neuron. Within a single neuron the message is mainly electrical, but the message must often jump from one neuron to the next.

The tiny gap between the axon of one neuron and the dendrite of the next is called a synapse. At the synapse the electrical impulse cannot jump the gap directly, so it is converted into a chemical signal:

• The axon ending releases chemicals called neurotransmitters (for example, acetylcholine).
• These chemicals diffuse across the synaptic gap and start a fresh impulse in the next neuron’s dendrite.

A similar junction between a neuron and a muscle is called a neuromuscular junction, where the signal makes the muscle contract.

Three types of neurons

Neurons are also classified by the job they do in carrying the signal, and CDS often tests which type does what:

• Sensory (afferent) neurons — carry impulses from receptors (sense organs) to the central nervous system.
• Motor (efferent) neurons — carry impulses from the central nervous system to effectors (muscles and glands).
• Relay (interneurons / association) neurons — lie within the brain or spinal cord and connect sensory neurons to motor neurons.

Organisation: CNS and PNS

The whole nervous system is divided into two parts. Learn this split clearly because many questions hinge on it:

• Central Nervous System (CNS) — the brain and the spinal cord. It is the main processing and control centre.
• Peripheral Nervous System (PNS) — all the nerves that branch out from the CNS to the rest of the body.

The PNS is itself divided into the somatic nervous system (controls voluntary actions of skeletal muscles) and the autonomic nervous system, which controls involuntary actions of internal organs. The autonomic system has two opposing halves:

• Sympathetic — prepares the body for emergencies (“fight or flight”); raises heartbeat, dilates pupils.
• Parasympathetic — calms the body and conserves energy (“rest and digest”); slows heartbeat.

Parts of the Brain and Their Functions

The brain is protected by the skull and by three membranes called meninges, and cushioned by cerebrospinal fluid. It has three major regions:

Forebrain (Cerebrum)

• The largest part of the brain; the seat of intelligence, memory, thinking, will and voluntary actions.
• It also receives and interprets sensory information (sight, sound, smell, touch).

Midbrain

• A small connecting region that controls certain reflexes of the eye and relays signals between the forebrain and hindbrain.

Hindbrain

• Cerebellum — controls balance, posture and coordination of muscular activity. Damage here causes unsteady, jerky movement.
• Medulla oblongata — controls involuntary vital actions such as heartbeat, breathing, blood pressure, vomiting, coughing and sneezing.
• Pons — relays signals and helps regulate breathing.

The Spinal Cord

The spinal cord is a long cord of nervous tissue that runs from the medulla oblongata down through the protective backbone (vertebral column). Like the brain it is wrapped in meninges and bathed in cerebrospinal fluid.

It has two main jobs:

• It acts as the main pathway carrying impulses to and from the brain.
• It is the centre for spinal reflexes — quick, automatic responses that do not need the brain to act first.

This second role is the key to understanding reflex actions, which we look at next.

Reflex Action and the Reflex Arc

A reflex action is a sudden, rapid, automatic and involuntary response to a stimulus, in which the brain is not directly involved at the moment of action. Examples: pulling your hand off a hot pan, blinking when something nears the eye, the knee-jerk, sneezing, and the watering of the mouth at the smell of food.

The pathway taken by the nerve impulse during a reflex action is called the reflex arc. Because it bypasses conscious thought, the response is extremely fast. The reflex arc has five steps, always in this order:

1. Receptor — the sense organ (e.g. skin) detects the stimulus.
2. Sensory neuron — carries the impulse to the spinal cord.
3. Relay neuron (in the spinal cord) — connects sensory to motor neuron.
4. Motor neuron — carries the impulse out to the effector.
5. Effector — the muscle or gland that produces the response.

Worked Example: Touching a Hot Plate

Let us trace exactly what happens when your finger accidentally touches a very hot plate and you jerk it away.

This is why the action feels ‘automatic’. The full conscious sensation of pain registers in the cerebrum a fraction of a second after the protective movement is already complete. This stepwise tracing is exactly how CDS frames its tougher application questions.

Common Mistakes to Avoid

A few errors cost candidates marks year after year. Fix them now:

• Brain vs spinal cord in reflexes: reflexes are controlled by the spinal cord; the brain only registers them afterwards.
• Cerebrum vs cerebellum: the cerebrum handles thinking and memory; the cerebellum handles balance and coordination. Do not swap them.
• Sensory vs motor direction: sensory neurons go to the CNS; motor neurons go away from it to muscles.
• Dendrite vs axon: dendrites receive (bring impulses in); the axon transmits (sends impulses out).
• Synapse signal: across a synapse the signal is chemical (neurotransmitter), not a direct electrical jump.

Previous-Year Style Question

Notice how the question rewards the three-fact habit — structure, location and single main function. A candidate who only learned “the brain controls movement” could easily pick a wrong distractor here.

Quick Revision

Revise this recap the night before the exam and you will handle almost every control-and-coordination question the CDS paper throws at you.