Coordination and response

1. Overview

Coordination and response is the process by which organisms detect changes in their internal and external environment and make appropriate responses to survive. In mammals, this is primarily managed by the nervous system, which uses rapid electrical impulses to communicate between different parts of the body, ensuring all systems function together effectively.

Key Definitions

• Stimulus: A change in the environment (internal or external) that is detected by a receptor.
• Receptor: A cell or organ that detects a stimulus (e.g., light receptors in the eye).
• Effector: A muscle or a gland that produces a response to a stimulus.
• Central Nervous System (CNS): The part of the nervous system consisting of the brain and spinal cord.
• Peripheral Nervous System (PNS): All the nerves outside the CNS that connect it to the rest of the body.
• Neurone: A specialized cell (nerve cell) that transmits electrical impulses.
• Synapse: A junction between two neurones.
• Reflex Action: A means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors.

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Core Content

The Mammalian Nervous System

The nervous system is responsible for the coordination and regulation of body functions. It is divided into two main parts:

1. Central Nervous System (CNS): Consists of the brain and spinal cord. It acts as the "control center," processing information and deciding on a response.
2. Peripheral Nervous System (PNS): Consists of nerves that branch out from the CNS to the sense organs and effectors.

Neurones and Electrical Impulses

Information travels through the nervous system as electrical impulses along specialized cells called neurones.

• Sensory Neurone: Carries impulses from the receptor to the CNS.
  • Structure: Has a long dendron and a cell body located on a side branch of the axon.
• Relay Neurone: Located entirely within the CNS. It connects sensory neurones to motor neurones.
  • Structure: Short, highly branched cells.
• Motor Neurone: Carries impulses from the CNS to an effector (muscle or gland).
  • Structure: Large cell body at one end with long axon leading to the effector.

The Reflex Arc

A reflex arc is the pathway an impulse travels during a reflex action. Because it often bypasses the conscious parts of the brain, the response is much faster.

The sequence of a reflex arc:

1. Stimulus (e.g., a hot plate)
2. Receptor (e.g., temperature receptors in the skin) detect the stimulus.
3. Sensory neurone transmits the impulse to the spinal cord.
4. Relay neurone passes the impulse across the spinal cord.
5. Motor neurone transmits the impulse to the effector.
6. Effector (e.g., biceps muscle) contracts.
7. Response (e.g., arm pulls away).

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Extended Content (Extended Only)

Synaptic Transmission

A synapse is the microscopic gap between two neurones. Since electrical impulses cannot jump across this gap, the signal is converted into a chemical one.

Structure of a Synapse:

• Presynaptic neurone: The neurone bringing the impulse. It contains vesicles filled with neurotransmitter molecules.
• Synaptic gap: The physical space between the neurones.
• Postsynaptic neurone: The neurone receiving the signal. Its membrane contains specific receptor proteins.

Step-by-Step Process at the Synapse:

1. An electrical impulse arrives at the end of the presynaptic neurone.
2. This stimulates the vesicles to move to the cell membrane and release neurotransmitter molecules into the synaptic gap.
3. The neurotransmitter molecules diffuse across the gap.
4. The neurotransmitter molecules bind with receptor proteins on the postsynaptic neurone.
5. This binding triggers a new electrical impulse in the next neurone.

Direction of Impulses

Synapses ensure that impulses travel in one direction only. This is because:

• Neurotransmitter vesicles are only present in the presynaptic neurone.
• Receptor proteins are only present on the membrane of the postsynaptic neurone.

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Key Equations

There are no specific mathematical equations for this topic. However, you should be able to interpret data regarding:

• Reaction time: The time taken between a stimulus and a response.
• Speed of impulse: Calculated as $Distance \div Time$.

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Common Mistakes to Avoid

• ❌ Wrong: Thinking that the impulse "jumps" across the synapse.
• ✅ Right: The impulse stops; chemicals (neurotransmitters) diffuse across the gap to start a new impulse.
• ❌ Wrong: Confusing the CNS and PNS.
• ✅ Right: Remember CNS is "Central" (middle of the body - brain/spine), PNS is "Peripheral" (outer edges/limbs).
• ❌ Wrong: Saying the message "travels through the nerves."
• ✅ Right: Use the biological term: Electrical impulses travel along neurones.

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Exam Tips

• Command Word - "State": If a question asks you to "State the components of the CNS," keep it brief: "The brain and spinal cord."
• Command Word - "Describe": When describing a reflex arc, ensure you list the components in the correct order (Receptor → Sensory → Relay → Motor → Effector).
• Labeling Diagrams: You are frequently asked to identify neurones. Look for the cell body. If the cell body is in the middle of the neurone, it is Sensory. If it is at the end, it is Motor.
• Real-World Contexts: Be prepared to apply the reflex arc to scenarios like the "knee-jerk reflex" or "withdrawing a hand from a sharp object."
• Extended Curriculum: In synapse questions, always mention diffusion. It is the specific process by which neurotransmitters move across the gap.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [6 marks]

Question:

(a) Define the term reflex action. [2]

(b) Describe the roles of the sensory neurone and the motor neurone in a reflex arc. [4]

Worked Solution:

(a)

1. A reflex action is an involuntary and rapid response to a stimulus.

How to earn full marks:

• Must include both "involuntary" AND "rapid" for full marks.
• "Automatic" can be used instead of "involuntary".

(b)

1. The sensory neurone transmits the impulse from the receptor to the central nervous system (CNS).
2. The motor neurone transmits the impulse from the CNS to the effector (muscle or gland).
3. The sensory neurone carries the electrical impulse to the relay neurone in the spinal cord.
4. The motor neurone causes the effector to carry out the response.

How to earn full marks:

• 1 mark for sensory neurone -> receptor to CNS
• 1 mark for motor neurone -> CNS to effector
• 1 mark for mention of a specific location of sensory neurone
• 1 mark for mention of effector response

Common Pitfall: Students often forget to include both "involuntary" and "rapid" in their definition of a reflex action. Also, make sure to specify the direction of impulse transmission for both sensory and motor neurones.

Exam-Style Question 2 — Short Answer [5 marks]

Question:

(a) State two components of the central nervous system (CNS). [2]

(b) Describe how an impulse travels across a synapse. [3]

Worked Solution:

(a)

1. The brain is a component of the CNS.
2. The spinal cord is a component of the CNS.

How to earn full marks:

• 1 mark for stating "brain"
• 1 mark for stating "spinal cord"

(b)

1. An impulse causes the release of neurotransmitter molecules from vesicles into the synaptic gap.
2. The neurotransmitter molecules diffuse across the synaptic gap.
3. The neurotransmitter molecules bind with receptor proteins on the next neurone, stimulating an impulse.

How to earn full marks:

• 1 mark for neurotransmitter release from vesicles
• 1 mark for diffusion across the gap
• 1 mark for binding to receptors on the next neurone, causing an impulse

Common Pitfall: Students sometimes confuse the roles of neurotransmitters and vesicles. Remember that neurotransmitters are the chemicals that carry the signal, and vesicles are the sacs that store and release them.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A scientist is investigating the effect of a new drug on reaction time. He asks participants to press a button as quickly as possible when they see a light flash. He measures their reaction time before and after taking the drug. The table shows the results for one participant:

Measurement	Reaction Time (s)
Before Drug	0.25
After Drug	0.38

(a) Calculate the percentage increase in reaction time after taking the drug. [3]

(b) Suggest how the drug might be affecting the nervous system to cause this increase in reaction time. [5]

Worked Solution:

(a)

1. Calculate the increase in reaction time. $0.38 - 0.25 = 0.13$ The difference in reaction time is 0.13 seconds.
2. Calculate the percentage increase. $\frac{0.13}{0.25} \times 100 = 52$ Percentage increase is the increase divided by the original time, multiplied by 100.

How to earn full marks:

• 1 mark for calculating the difference (0.13)
• 1 mark for correct use of percentage formula (increase/original * 100)
• 1 mark for correct answer with unit: $\boxed{52 %}$

(b)

1. The drug might slow down the transmission of impulses across synapses.
2. This could be because the drug reduces the amount of neurotransmitter released.
3. Or the drug could block the receptor proteins on the next neurone.
4. Another possibility is that the drug slows down the speed of impulse transmission along the neurones.
5. The drug could also affect the relay neurone in the spinal cord, making the processing slower.

How to earn full marks:

• 1 mark for suggesting the drug affects synaptic transmission.
• 1 mark for explaining how the drug reduces neurotransmitter release.
• 1 mark for explaining how the drug blocks receptor proteins.
• 1 mark for suggesting slower impulse transmission along neurones.
• 1 mark for suggesting the drug affects the relay neurone.

Common Pitfall: When calculating percentage increase, students sometimes divide by the new value instead of the original value. Also, remember to include the percentage sign (%) in your final answer.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A person touches a hot pan and quickly pulls their hand away. This is a reflex action.

(a) Draw a labelled diagram of the reflex arc involved in this situation, including the receptor, sensory neurone, relay neurone, motor neurone, and effector. [5]

(b) Explain why reflex actions are important for survival. [4]

Worked Solution:

(a)

How to earn full marks:

• 1 mark for correctly labelled receptor.
• 1 mark for correctly labelled sensory neurone (with correct direction of impulse).
• 1 mark for correctly labelled relay neurone.
• 1 mark for correctly labelled motor neurone (with correct direction of impulse).
• 1 mark for correctly labelled effector.
• 1 mark for correctly labelled spinal cord.
• 1 mark for correctly labelled synapse between the sensory and relay neurone.
• 1 mark for correctly labelled synapse between the relay and motor neurone.

(b)

1. Reflex actions are fast.
2. This allows for a quick response to dangerous stimuli.
3. This prevents or minimizes damage to the body.
4. They are automatic, so do not require conscious thought, freeing up the brain for other tasks.

How to earn full marks:

• 1 mark for stating that reflexes are fast.
• 1 mark for explaining that this allows for a quick response to dangerous stimuli.
• 1 mark for explaining that this prevents damage.
• 1 mark for stating that reflexes are automatic and do not require conscious thought.

Common Pitfall: When drawing the reflex arc, students sometimes forget to include the direction of the impulse on the sensory and motor neurones. Also, make sure to label all the key components clearly.