You are viewing a free preview of this lesson.
Subscribe to unlock all 9 lessons in this course and every other course on LearningBro.
A single-celled organism such as an Amoeba can sense its surroundings and respond all by itself, because everything it needs is contained in one cell. A human being is built from trillions of cells, and those cells cannot each act independently — they must be coordinated so that the whole body responds in a sensible, joined-up way. This lesson opens Topic B3 (Organism-level systems) of OCR Gateway Science A by looking at the first of the body's two great coordinating systems: the nervous system. You will follow the pathway that links a change in the environment to a response, meet the central and peripheral parts of the nervous system, study the three types of neurone, and finish with a required practical that measures reaction time.
By the end of this lesson you should be able to describe the stimulus → receptor → coordinator → effector → response pathway, describe the structure and function of sensory, relay and motor neurones, explain how an electrical impulse passes along a neurone, and describe how reaction time can be measured.
Living things must constantly detect and react to changes in their surroundings in order to survive. A change that an organism can detect and respond to is called a stimulus (plural: stimuli) — for example a bright light, a loud sound, a rise in temperature or the smell of food.
In a large, complex animal, the cell that detects a stimulus is usually nowhere near the cell that needs to respond to it. If you touch something hot with a fingertip, the receptor cells are in your skin but the muscle that pulls your hand away is in your arm. Something has to carry information rapidly from one to the other and make sure the response is the right one. That job of detecting stimuli and producing a coordinated response is called coordination, and in animals it is carried out by the nervous system and the endocrine (hormonal) system working together.
The nervous system allows an organism to:
Exam Tip: A stimulus is the change, not the object. "A hot pan" is not a stimulus; the rise in temperature detected by your skin is. Read these questions carefully — OCR often asks you to name the stimulus precisely.
Every nervous response, no matter how simple or complex, follows the same five-stage pathway. Learning this sequence is the single most useful thing in the lesson, because almost every question in this part of B3 is built on it.
flowchart LR
A["Stimulus<br/>(a change)"] --> B["Receptor<br/>(detects the change)"]
B --> C["Coordinator<br/>(CNS: brain / spinal cord)"]
C --> D["Effector<br/>(muscle or gland)"]
D --> E["Response<br/>(action taken)"]
Information passes along this pathway carried by neurones (nerve cells) as tiny electrical impulses. The receptor converts the energy of the stimulus into an electrical impulse, and effectors turn that impulse back into an action.
Exam Tip: Remember that an effector is "a muscle or a gland". Students often write only "a muscle". A question about sweating, releasing a hormone or producing saliva needs the answer gland.
The human nervous system is divided into two parts.
The central nervous system (CNS) is made up of the brain and the spinal cord. It is the body's processing centre: it receives information from receptors all over the body, coordinates a response, and sends instructions out to effectors.
The peripheral nervous system (PNS) is made up of all the nerves that carry information to and from the CNS. A nerve is a bundle of many neurones wrapped together. The PNS connects the CNS to every receptor and every effector in the body.
flowchart TD
A["Nervous system"] --> B["Central nervous system (CNS)<br/>brain + spinal cord"]
A --> C["Peripheral nervous system (PNS)<br/>nerves to and from the CNS"]
B --> D["Processes information<br/>and coordinates responses"]
C --> E["Carries impulses between<br/>receptors / effectors and the CNS"]
The flow of information is therefore: receptor → (PNS neurone carrying the impulse in) → CNS → (PNS neurone carrying the impulse out) → effector. Neurones that carry impulses towards the CNS are sensory neurones; those that carry impulses away from the CNS to effectors are motor neurones.
A neurone is a specialised cell adapted to carry electrical impulses quickly over long distances. There are three types, each with a slightly different job and shape.
| Neurone | Carries impulses... | Key feature |
|---|---|---|
| Sensory neurone | From receptors to the CNS | Long dendron bringing the impulse in; cell body off to one side |
| Relay neurone | Within the CNS (links sensory to motor) | Short; many connections; found in the brain and spinal cord |
| Motor neurone | From the CNS to effectors (muscles/glands) | Long axon carrying the impulse out to the effector |
All neurones share the same basic parts:
The diagram below shows a motor neurone, the type you are most often asked to label.
Key:
| # | Part | Function |
|---|---|---|
| 1 | Dendrites | Receive impulses from other neurones |
| 2 | Cell body (with nucleus) | Controls the neurone |
| 3 | Axon | Carries the impulse away from the cell body |
| 4 | Myelin sheath | Insulates the axon and speeds up the impulse |
| 5 | Nerve endings | Pass the impulse on to the effector (here, a muscle) |
A sensory neurone has a similar set of parts but its cell body lies on a small side-branch part-way along, and it carries the impulse the other way — from a receptor into the CNS. A relay neurone is short, lies entirely within the CNS, and has many connections so it can link incoming sensory neurones to outgoing motor neurones.
Exam Tip: Neurones are adapted cells. Two adaptations score marks again and again: they are very long (so they carry impulses over large distances), and they have an insulating myelin sheath (which speeds the impulse up). Mentioning both is worth easy marks in a "how is a neurone adapted?" question.
When a receptor detects a stimulus it sets off an electrical impulse that travels along the neurone. You do not need the detailed biophysics for GCSE, but you should be able to describe the journey clearly:
Electrical transmission along a neurone is extremely fast, which is exactly why the nervous system is the body's system for rapid responses. This speed is one of the key differences between nervous control and the slower, longer-lasting hormonal control you will meet later in B3.
It is worth being clear about the two ends of the pathway.
A receptor detects a stimulus. Receptors are often grouped together in sense organs: the eye (light), the ear (sound and balance), the skin (touch, pressure, temperature, pain), the tongue (taste) and the nose (smell). Each kind of receptor responds to one kind of stimulus and converts it into an electrical impulse.
An effector carries out the response. There are only two kinds:
Reaction time is the time taken to respond to a stimulus. It is usually very short — a fraction of a second — because the nervous pathway is so fast. Reaction time can be increased (made slower) by factors such as tiredness, age, distractions, the drug alcohol, and by the brain having to make a more complicated decision. It can sometimes be shortened a little by practice or by caffeine.
A simple and reliable way to measure reaction time in the laboratory is the ruler-drop test. The further the ruler falls before it is caught, the longer the reaction time.
flowchart TD
A["Person rests their forearm on a desk<br/>with hand over the edge"] --> B["Partner holds a ruler vertically,<br/>zero mark level with the open fingers"]
B --> C["Partner releases the ruler without warning"]
C --> D["Person catches the ruler<br/>as quickly as they can"]
D --> E["Record the distance the ruler fell<br/>(reading at the top of the thumb)"]
E --> F["Repeat several times and take a mean;<br/>convert distance to reaction time"]
Key points the exam rewards:
Two students do the ruler-drop test. Student A catches the ruler after it falls a mean of 12 cm; student B after a mean of 18 cm. Which student has the faster reaction time, and what could explain the difference?
Step 1 — interpret the distances. A shorter distance means the ruler was caught sooner, so the reaction time was shorter.
Step 2 — compare: student A's ruler fell 12 cm and student B's fell 18 cm, so student A has the faster (shorter) reaction time.
Step 3 — suggest a reason for the difference. Student B might be more tired, more distracted, less practised, or older; any factor that increases reaction time would lengthen the distance fallen.
Answer: Student A has the faster reaction time, because their ruler fell the shorter distance. The difference could be due to tiredness, distraction or practice.
Exam Tip: In the ruler-drop test, the variable you actually measure is the distance fallen, which is then used to work out reaction time. If asked to convert distance to time you would be given a conversion table or chart — you are not expected to use the equation of motion in a biology exam.
The table below shows example readings (illustrative, not real data) for one student over five trials.
| Trial | Distance ruler fell (cm) |
|---|---|
| 1 | 14 |
| 2 | 13 |
| 3 | 19 |
| 4 | 12 |
| 5 | 13 |
Trial 3 (19 cm) stands out as an anomaly — perhaps the student was distracted — so it should be ignored when calculating the mean. The mean of the remaining four readings is:
414+13+12+13=452=13 cm
So the student's typical catch distance is about 13 cm.
| Misconception | The correct idea |
|---|---|
| "The stimulus is the object (e.g. the hot pan)" | The stimulus is the change detected (e.g. the rise in temperature) |
| "An effector is always a muscle" | An effector is a muscle or a gland |
| "Nerves and neurones are the same thing" | A neurone is a single nerve cell; a nerve is a bundle of many neurones |
| "The CNS is the brain only" | The CNS is the brain and the spinal cord |
| "Impulses travel along neurones as chemicals" | Impulses travel along a neurone as electrical signals; chemicals are only used at the synapse between neurones |
| "The myelin sheath carries the impulse" | The myelin sheath insulates the axon and speeds up the impulse; the axon carries it |
| "A shorter ruler-drop distance means a slower reaction" | A shorter distance means a faster (shorter) reaction time |
Question (6 marks): A person touches a sharp pin with their finger and quickly moves their hand away. Describe how the nervous system detects this stimulus and produces the response.
Mid-band response: "The pin is the stimulus. Receptors in the skin detect it and send a message to the brain. The brain sends a message to the muscle in the arm, which moves the hand away."
Examiner-style commentary: This captures the broad sequence and correctly identifies skin receptors, a coordinating centre and a muscle effector. It is held back by loose language ("a message") and by saying "the brain" rather than the CNS/spinal cord. To climb a band, use the terms electrical impulse, sensory and motor neurone, and effector.
Stronger response: "The stimulus is the sharp pin. Receptors in the skin detect it and an electrical impulse passes along a sensory neurone to the central nervous system. The CNS then sends an impulse along a motor neurone to an effector, which is a muscle in the arm. The muscle contracts and moves the hand away from the pin."
Examiner-style commentary: A clear, well-ordered answer using the correct neurone names and the term effector, with the impulse described as electrical. To reach the top band, name the stimulus precisely (the pressure/pain of the pin), state that the receptor converts the stimulus into the impulse, and refer to the full stimulus → receptor → coordinator → effector → response pathway.
Top-band response: "The stimulus is the pressure and pain of the sharp pin on the skin. Receptor cells in the skin detect this change and convert it into an electrical impulse. The impulse travels along the axon of a sensory neurone to the central nervous system (the coordinator), which processes the information. The CNS then sends an electrical impulse out along a motor neurone to an effector — a muscle in the arm. The muscle contracts, producing the response of pulling the hand away. The whole pathway is stimulus → receptor → coordinator → effector → response, and because the impulses are electrical the response is extremely rapid."
Examiner-style commentary: Full marks. The answer names the stimulus precisely, states that the receptor converts it into an electrical impulse, uses the correct neurone types and the term effector, gives the full five-stage pathway, and links the speed of the response to the electrical nature of the impulse — exactly the depth and precision expected.
This content is aligned with OCR Gateway Science A GCSE Biology (J247), Topic B3 Organism-level systems. Refer to the official OCR specification document for the exact wording.