14.1 Coordination and Response Revision Notes

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.

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:

Central Nervous System (CNS): Consists of the brain and spinal cord. It acts as the "control center," processing information and deciding on a response.
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.

📊Comparison of the three neurones. Sensory neurone showing the cell body in the middle of the axon; Relay neurone showing a small, multi-polar shape; Motor neurone showing the cell body at the dendrite end with a long axon.

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:

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

📊Cross-section of the spinal cord (butterfly-shaped grey matter in the center). Show the sensory neurone entering via the dorsal root, connecting to a relay neurone in the grey matter, and the motor neurone exiting via the ventral root to a muscle.

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:

An electrical impulse arrives at the end of the presynaptic neurone.
This stimulates the vesicles to move to the cell membrane and release neurotransmitter molecules into the synaptic gap.
The neurotransmitter molecules diffuse across the gap.
The neurotransmitter molecules bind with receptor proteins on the postsynaptic neurone.
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.

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 $Distance \div Time$.

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.

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.