5.4 Reflex Arc

A reflex action is an immediate, automatic, and involuntary response to external and internal environmental changes. The path of the nerve impulse during reflex action is called a reflex arc. A typical reflex arc includes five fundamental parts: receptors, sensory neurons, interneuron, motor neuron, and effectors.

Definition

Reflex Action: Reflex action is an immediate, automatic, and involuntary response to external and internal environmental changes.

Reflex Arc: The path of the nerve impulse during reflex action is called a reflex arc.

Main Idea

• Receptors
• Sensory neurons
• Interneuron
• Motor neuron
• Effectors

Core Concept

• Reflex action = immediate, automatic, involuntary response
• Reflex arc = path of nerve impulse
• Five fundamental parts = receptors, sensory neurons, interneuron, motor neuron, effectors
• Example = touching a hot object → hand is rapidly removed
• Pathway = receptors → sensory neuron → interneuron → motor neuron → effector

Detailed Explanation

Reflex Action and Reflex Arc

• Reflex action is an immediate, automatic, and involuntary response to external and internal environmental changes.
• The path of the nerve impulse during reflex action is called a reflex arc.

Example of a Reflex Arc

A typical reflex arc includes five fundamental parts: receptors, sensory neurons, interneuron, motor neuron, and effectors.

Example: Touching a Hot Object

• If one unexpectedly touches a hot object, the hand is rapidly removed from the source of heat.
• Receptors in the skin of the hand are activated by the heat of the object.
• The receptors stimulate a sensory neuron leading to the spinal cord via a spinal nerve.
• The cell body of the sensory neuron is outside the cord.
• The sensory neuron enters a dorsal nerve root of the spinal cord.
• The impulse then crosses a synapse to an interneuron which lies completely within the cord.
• The impulse travels along the interneuron and then passes across a synapse to the dendrites and the cell body of a motor neuron which lies within the spinal cord.
• The motor neuron eventually branches to form synapses with several muscle cells i.e., an effector.
• The nerve impulses then move along the motor neuron to the muscles, which cause them to contract.

Five Fundamental Parts of a Reflex Arc

Summary of the Reflex Arc Pathway

• Stimulus occurs (e.g., touching a hot object).
• Receptors in the skin are activated.
• Sensory neuron carries impulse to the spinal cord.
• Sensory neuron enters the dorsal nerve root.
• Impulse crosses a synapse to an interneuron.
• Interneuron lies completely within the cord.
• Impulse travels along the interneuron.
• Impulse crosses a synapse to a motor neuron.
• Motor neuron lies within the spinal cord.
• Motor neuron branches to form synapses with muscle cells.
• Nerve impulses move along the motor neuron to the muscles.
• Muscles contract.
• Response occurs (hand is removed).

Intermediate Level

Five Fundamental Parts of a Reflex Arc

Pathway of a Reflex Arc

Example of a Reflex Arc

Characteristics of Reflex Action

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Reflex action is an immediate, automatic, and involuntary response.
• 2. The path of the nerve impulse during reflex action is called a reflex arc.
• 3. A typical reflex arc includes five fundamental parts.
• 4. The five parts are receptors, sensory neurons, interneuron, motor neuron, and effectors.
• 5. Receptors detect the stimulus.
• 6. Sensory neurons carry impulses to the CNS.
• 7. The cell body of the sensory neuron is outside the cord.
• 8. The sensory neuron enters the dorsal nerve root.
• 9. The interneuron lies completely within the cord.
• 10. The motor neuron lies within the spinal cord.
• 11. The motor neuron branches to form synapses with muscle cells.
• 12. The effector is the muscle that responds.
• 13. The response is rapid.
• 14. Reflex actions protect the body from harm.
• 15. Reflex actions do not require conscious thought.

Remember

• - Reflex action = immediate, automatic, involuntary response
• - Reflex arc = path of nerve impulse
• - Five fundamental parts = receptors, sensory neurons, interneuron, motor neuron, effectors
• - Example = touching a hot object → hand is rapidly removed
• - Pathway = receptors → sensory neuron → interneuron → motor neuron → effector
• - Sensory neuron = enters dorsal nerve root
• - Interneuron = lies completely within the cord
• - Motor neuron = lies within the spinal cord
• - Effector = muscle that responds
• - **Response** = rapid

Common Mistakes

• 1. Confusing reflex action and reflex arc — Reflex action = the response; reflex arc = the pathway.
• 2. Thinking the brain is always involved — Reflex actions can bypass the brain.
• 3. Forgetting the five parts — Receptors, sensory neurons, interneuron, motor neuron, effectors.
• 4. Mixing up sensory and motor neurons — Sensory = to CNS; motor = from CNS.
• 5. Forgetting the dorsal nerve root — Sensory neuron enters the dorsal nerve root.
• 6. Thinking the interneuron is outside the cord — The interneuron lies completely within the cord.
• 7. Mixing up the order — Receptors → sensory neuron → interneuron → motor neuron → effector.
• 8. Thinking reflexes are slow — Reflexes are very fast.
• 9. Believing reflexes require conscious thought — Reflexes are involuntary.
• 10. Forgetting the role of the effector — The effector (muscle) produces the response.

Quick Revision

Reflex action is an immediate, automatic, and involuntary response to external and internal environmental changes. The path of the nerve impulse during reflex action is called a reflex arc. A typical reflex arc includes five fundamental parts: receptors, sensory neurons, interneuron, motor neuron, and effectors. For example, if one unexpectedly touches a hot object, the hand is rapidly removed from the source of heat. Receptors in the skin of the hand are activated by the heat of the object. The receptors stimulate a sensory neuron leading to the spinal cord via a spinal nerve. The cell body of the sensory neuron is outside the cord. The sensory neuron enters a dorsal nerve root of the spinal cord. The impulse then crosses a synapse to an interneuron which lies completely within the cord. The impulse travels along the interneuron and then passes across a synapse to the dendrites and the cell body of a motor neuron which lies within the spinal cord. The motor neuron eventually branches to form synapses with several muscle cells i.e., an effector. The nerve impulses then move along the motor neuron to the muscles, which cause them to contract.

60-Second Revision

• Reflex action = immediate, automatic, involuntary response
• Reflex arc = path of nerve impulse
• Five fundamental parts = receptors, sensory neurons, interneuron, motor neuron, effectors
• Example = touching a hot object → hand is rapidly removed
• Pathway = receptors → sensory neuron → interneuron → motor neuron → effector
• Sensory neuron = enters dorsal nerve root
• Interneuron = lies completely within the cord
• Motor neuron = lies within the spinal cord
• Effector = muscle that responds
• Warning: Reflex actions are involuntary and do not require conscious thought

NEXT TOPIC

• Chapter:
• 5. Nervous System of Man

• Topic:
• 5.5 NERVE IMPULSE

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