Coordination and response: the nervous system

What you'll learn

This revision guide covers coordination and response through the nervous system, a fundamental topic in CIE IGCSE Biology. You'll learn how organisms detect and respond to changes in their environment using electrical impulses transmitted through specialised cells. This includes the structure and function of neurones, how impulses travel across synapses, reflex actions, and the roles of the central and peripheral nervous systems.

Key terms and definitions

Receptor — a specialised cell that detects stimuli (changes in the environment) and converts them into electrical impulses in sensory neurones.

Effector — a muscle or gland that produces a response when stimulated by motor neurones.

Synapse — a junction between two neurones where chemical transmission of impulses occurs, involving neurotransmitter molecules diffusing across a microscopic gap.

Reflex arc — the pathway taken by nerve impulses in a reflex action, involving receptor, sensory neurone, relay neurone, motor neurone and effector.

Central nervous system (CNS) — the brain and spinal cord, which coordinate responses by processing information from receptors.

Neurone — a specialised nerve cell adapted to transmit electrical impulses rapidly around the body.

Stimulus — a change in the environment (internal or external) that is detected by receptors and may lead to a response.

Reflex action — a rapid, automatic response to a stimulus that does not involve conscious thought, providing protection from harm.

Core concepts

The need for coordination and response

Living organisms must detect and respond to changes in their environment to survive. These changes (stimuli) include:

• Light intensity and direction
• Temperature changes
• Sound vibrations
• Chemicals (taste and smell)
• Touch and pressure
• Pain

The nervous system provides rapid, precise responses to stimuli through electrical impulses. This system works alongside the hormonal system (covered separately), but the nervous system is much faster, acting in milliseconds rather than seconds or minutes.

Sense organs contain groups of receptor cells that respond to specific stimuli:

• Eyes contain light receptors (photoreceptors in the retina)
• Ears contain sound receptors and balance receptors
• Nose contains chemical receptors for smell
• Tongue contains chemical receptors for taste
• Skin contains receptors for touch, pressure, pain and temperature

Structure and function of neurones

Neurones are specialised cells adapted for rapid transmission of electrical impulses called nerve impulses. There are three types, each with distinct functions:

Sensory neurones

• Carry impulses from receptors to the CNS
• Cell body located in the middle of the axon
• Long dendron carries impulses from receptors toward the cell body
• Shorter axon carries impulses from cell body to CNS

Relay neurones (intermediate neurones)

• Found entirely within the CNS (brain and spinal cord)
• Connect sensory neurones to motor neurones
• Have many short dendrites and short axons
• Allow processing and coordination of responses

Motor neurones

• Carry impulses from the CNS to effectors (muscles and glands)
• Cell body at one end of the neurone (in CNS)
• Long axon carries impulses to effectors
• Axon terminals connect to effector cells

Key adaptations of neurones:

• Long axons/dendrons transmit impulses over long distances without loss of signal
• Many dendrites increase surface area for connections with other neurones
• Myelin sheath (fatty insulating layer) surrounds some axons, speeding up transmission by forcing impulses to jump between gaps
• Cell body contains many mitochondria to provide energy (ATP) for active transport of ions during impulse transmission
• Branched endings (dendrites at one end, axon terminals at the other) enable connections with multiple cells

How nerve impulses are transmitted

A nerve impulse is an electrical signal that travels along the membrane of a neurone. The process involves:

1. At rest, the neurone membrane maintains a potential difference (voltage) by actively pumping sodium ions out
2. When stimulated, the membrane becomes permeable to sodium ions
3. Sodium ions flood in, reversing the potential difference
4. This change triggers the same process in the adjacent region of membrane
5. The impulse propagates along the entire length of the neurone
6. The membrane returns to its resting state after the impulse passes

The speed of transmission is increased by:

• Thicker axons (less resistance)
• Myelin sheath insulation (causing impulses to jump between nodes)
• Higher temperature (up to approximately 40°C)

Synapses and neurotransmission

A synapse is the junction between two neurones (or between a neurone and an effector). Impulses cannot jump directly across the gap, so chemical transmission is required.

Synaptic transmission sequence:

1. Electrical impulse reaches the end of the pre-synaptic neurone
2. Impulse triggers vesicles containing neurotransmitter chemicals to move to the pre-synaptic membrane
3. Vesicles fuse with membrane and release neurotransmitter into the synaptic gap (cleft)
4. Neurotransmitter molecules diffuse across the narrow gap (approximately 20 nanometres)
5. Neurotransmitter binds to specific receptor proteins on the post-synaptic membrane
6. This binding triggers a new electrical impulse in the post-synaptic neurone
7. Enzymes in the synapse break down neurotransmitter to prevent continuous stimulation

Important features of synapses:

• Ensure impulses travel in one direction only (vesicles only present on pre-synaptic side)
• Allow one neurone to connect with many others (divergence)
• Allow multiple neurones to converge on one neurone
• Can filter out weak stimuli (threshold must be reached)
• Are sites where drugs and toxins can interfere with transmission

Reflex actions and reflex arcs

Reflex actions are rapid, automatic responses that protect the body from harm. They do not involve conscious decision-making by the brain, making them extremely fast.

Examples of reflex actions:

• Withdrawing hand from hot object
• Blinking when object approaches eye
• Pupil constriction in bright light
• Knee-jerk reflex when tendon tapped
• Dropping hot object

The reflex arc is the neural pathway followed during a reflex:

Stimulus → Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector → Response

The withdrawal reflex (detailed example)

1. Stimulus: Hand touches hot object
2. Receptor: Temperature and pain receptors in skin detect heat
3. Sensory neurone: Impulse travels from receptor to spinal cord
4. Relay neurone: In spinal cord, impulse passes across synapse to relay neurone, then to motor neurone
5. Motor neurone: Impulse travels from spinal cord to arm muscles
6. Effector: Biceps muscle in arm
7. Response: Muscle contracts, pulling hand away from heat source

Key features of reflex arcs:

• Bypass the brain (processed in spinal cord only) — enabling faster response
• Involve a maximum of three neurones (sensory, relay, motor)
• Involve at least two synapses in the simplest reflexes
• Are involuntary (cannot be consciously controlled)
• Protect body from damage
• Though processed in the spinal cord, sensory impulses also reach the brain, which is why you feel pain shortly after withdrawing your hand

The central nervous system

The CNS consists of the brain and spinal cord. It coordinates responses by receiving information from receptors, processing it, and sending instructions to effectors.

The brain

The brain is the most complex organ in the body, containing approximately 86 billion neurones. Key regions include:

• Cerebrum (cerebral hemispheres): Controls voluntary actions, memory, intelligence, language, personality and conscious thought
• Cerebellum: Coordinates muscle movement and balance
• Medulla (medulla oblongata): Controls unconscious activities like heart rate, breathing rate and peristalsis
• Hypothalamus: Monitors and regulates body temperature and water balance

The spinal cord

• Protected by the vertebral column (backbone)
• Connects brain to peripheral nerves throughout the body
• Contains relay neurones that coordinate reflex actions
• Contains ascending tracts (carrying sensory information to brain) and descending tracts (carrying motor commands from brain)

Voluntary and involuntary actions

Voluntary actions:

• Require conscious thought and decision-making
• Involve the cerebrum (conscious part of brain)
• Examples: writing, speaking, walking, picking up an object
• Slower than reflex actions
• Can be learned and improved with practice

Involuntary actions:

• Automatic, do not require conscious thought
• Include reflex actions (rapid, protective)
• Include autonomic functions controlled by medulla (breathing, heartbeat)
• Examples: pupil reflex, heart beating, peristalsis
• Cannot be voluntarily controlled (or only partially)

Worked examples

Example 1: Describing a reflex arc pathway

Question: A person accidentally touches a sharp pin. Describe the pathway of the reflex arc that causes them to pull their hand away. [6 marks]

Mark scheme answer:

1. Sharp pin is the stimulus [1]
2. Detected by pain/pressure receptors in the skin/finger [1]
3. Sensory neurone carries impulse to the spinal cord/CNS [1]
4. In the spinal cord, impulse passes across synapse to relay neurone, then across another synapse to motor neurone [1]
5. Motor neurone carries impulse to effector (muscle in arm/biceps) [1]
6. Muscle contracts causing hand to withdraw/move away [1]

Examiner note: Use correct sequence and biological terminology. Each component must be named precisely.

Example 2: Explaining synaptic transmission

Question: Explain how an impulse is transmitted across a synapse. [5 marks]

Mark scheme answer:

1. Electrical impulse arrives at the end of the pre-synaptic neurone [1]
2. Causes vesicles containing neurotransmitter to move to/fuse with the pre-synaptic membrane [1]
3. Neurotransmitter released into the synaptic gap/cleft [1]
4. Neurotransmitter diffuses across the gap [1]
5. Binds to receptor proteins on the post-synaptic membrane [1]
6. Triggers a new electrical impulse in the post-synaptic neurone [1]

(Any 5 points for 5 marks)

Examiner note: "Explain" requires a sequence of causation. Show what happens and why each step leads to the next.

Example 3: Comparing neurone types

Question: State two differences between the structure of a sensory neurone and a motor neurone. [2 marks]

Mark scheme answer:

Any two from:

• Sensory neurone has cell body in the middle of the axon; motor neurone has cell body at one end [1]
• Sensory neurone has a long dendron; motor neurone does not [1]
• Motor neurone has long axon from CNS to effector; sensory neurone has shorter axon [1]
• Sensory neurone connects to receptors; motor neurone connects to effectors [1]

Examiner note: "State" requires factual points without explanation. Must identify structural differences, not functional ones.

Common mistakes and how to avoid them

• Confusing neurones with nerves: A nerve is a bundle of hundreds or thousands of neurones (axons/dendrons) wrapped in connective tissue. A neurone is a single cell. Always refer to sensory and motor neurones, not sensory and motor nerves.

• Incorrect synapse descriptions: Students often write that "the impulse jumps across the synapse" or "chemicals cross the synapse." Be precise: neurotransmitter molecules diffuse across; electrical impulses travel along neurones but not across gaps.

• Mixing up stimulus and response: The stimulus is what is detected (e.g., heat, light); the response is what the organism does (e.g., muscle contraction, moving away). The hot object is the stimulus, not "touching" it.

• Wrong reflex arc sequence: The correct order is always: receptor → sensory neurone → relay neurone (in CNS) → motor neurone → effector. Do not miss out the relay neurone or place components in wrong order.

• Vague effector descriptions: "Muscle" alone is often insufficient. Specify which muscle where possible (e.g., biceps muscle in arm, iris muscles in eye). Always state what the effector does (contracts/relaxes for muscle; secretes for gland).

• Confusing electrical and chemical transmission: Impulses are electrical along neurones; transmission is chemical across synapses. Be clear which type you are describing.

Exam technique for "Coordination and response: the nervous system"

• Command words matter: "Describe" requires a sequential account without explanation; "Explain" requires causes/reasons linked together; "Compare" needs similarities and/or differences stated explicitly; "State" needs brief factual answers only.

• Reflex arc questions: Always give the full sequence in order. Name each component precisely (receptor, sensory neurone, relay neurone, motor neurone, effector). State what the effector does and what response occurs. Expect 5-6 marks for complete reflex arc descriptions.

• Annotated diagrams: If asked to label a neurone diagram, use precise terminology (axon, dendrite, cell body, myelin sheath, nucleus). Draw label lines to exact structures with a ruler. If asked to "annotate," add brief notes explaining function as well as labelling.

• Synapse questions: Follow the sequence logically. Use correct terminology: vesicles, neurotransmitter, diffusion, receptors, pre-synaptic and post-synaptic membranes. Typically worth 4-5 marks for full descriptions.

Quick revision summary

The nervous system coordinates rapid responses to stimuli using electrical impulses transmitted by neurones. Receptors detect changes and send impulses via sensory neurones to the CNS. The brain and spinal cord process information and send impulses via motor neurones to effectors (muscles and glands). Synapses use chemical neurotransmitters to pass impulses between neurones. Reflex arcs enable automatic protective responses without conscious thought, following the pathway: receptor → sensory → relay → motor neurone → effector. Voluntary actions involve the cerebrum; involuntary actions are controlled automatically by the medulla or spinal cord.