12.4 Voluntary and Involuntary Actions
Learn how the human body produces conscious voluntary actions and automatic involuntary actions, the pathways of reflex arcs, and why reflex responses are essential for survival.
Learning Objectives
- Compare and contrast voluntary actions and involuntary actions.
- Describe the information pathway of a voluntary action involving skeletal muscles.
- Define a reflex action and a reflex arc.
- Trace the transmission of nerve impulses in a three-neurone reflex arc and a two-neurone reflex arc.
- Explain the importance of reflex actions to humans.
Comparison of Three-Neurone and Two-Neurone Reflex Arcs
A side-by-side flow diagram showing the impulse pathways of the withdrawal reflex and the knee jerk reflex.
Three-neurone arc
Withdrawal reflex: sensory, relay and motor neurones.
Two-neurone arc
Knee jerk reflex: sensory and motor neurones only.
Spinal cord
Both reflex arcs are integrated in the spinal cord, not the brain.
Short Explanation
Voluntary and involuntary actions
Voluntary actions are conscious actions done on our own will, controlled by the cerebral cortex through the somatic nervous system. Involuntary actions happen automatically without conscious control, regulated by the medulla oblongata and hypothalamus through the autonomous nervous system.
Voluntary action pathway
Stimulus → receptor → sensory neurone → cerebrum → relay neurone → motor neurone → effector (skeletal muscle) → response.
Reflex action and reflex arc
A reflex action is a fast response to a stimulus without being controlled by the brain. The nerve pathway involved is called a reflex arc.
Three-neurone reflex arc
Finger prick → sensory receptor → sensory neurone → dorsal root ganglion → dorsal root → spinal cord grey matter → relay neurone → motor neurone → ventral root → effector muscle contracts.
Two-neurone reflex arc
Knee tap → stretch receptor → sensory neurone → dorsal root → motor neurone → ventral root → quadriceps muscle contracts → leg jerks forward.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What are the control centres for voluntary actions and involuntary actions?
Why does a knee jerk reflex involve only two neurones instead of three?
State two reasons why reflex actions are important to humans.
Common Mistakes
- Confusing the two types of reflex arcs: assuming all reflex arcs include a relay neurone. The knee jerk reflex is a two-neurone reflex arc consisting only of a sensory neurone and a motor neurone.
- Misidentifying roots in the spinal cord: sensory neurones enter via the dorsal root, while motor neurones exit via the ventral root.
- Assuming reflex actions are voluntary because skeletal muscles are involved. Even though skeletal muscles act as effectors in withdrawal and knee jerk reflexes, the response is completely involuntary and bypasses conscious control by the brain.
Concept Misunderstandings
The brain is completely unaware of reflex actions.
Although reflex actions are triggered and completed at the level of the spinal cord without waiting for brain instructions, sensory impulses are simultaneously relayed to the brain, which is why a person feels pain shortly after the reflex response has already occurred.
Involuntary actions only involve internal organs like the heart and stomach.
While internal involuntary actions involve smooth muscles, cardiac muscles, and glands, involuntary reflex actions also involve skeletal muscles, as seen in the knee jerk and withdrawal reflexes.
Summary
- Voluntary actions are conscious, deliberate actions coordinated by the cerebral cortex and the somatic nervous system using skeletal muscles.
- Involuntary actions occur automatically without conscious awareness, coordinated by the medulla oblongata and hypothalamus via the autonomous nervous system.
- A reflex action is an immediate, automatic response to a stimulus that does not require conscious direction from the brain.
- Three-neurone reflex arc: Stimulus → Receptor → Sensory neurone → Relay neurone (spinal cord) → Motor neurone → Effector (skeletal muscle).
- Two-neurone reflex arc: Stimulus → Stretch receptor → Sensory neurone → Motor neurone (spinal cord) → Effector (quadriceps muscle).
- Reflexes protect the body from trauma and harm while allowing the brain to concentrate on higher-order cognitive processing.
Short Activity
Objective: compare voluntary and involuntary actions, reflex arcs, and their importance.
A. Objective Quiz
Which of the following correctly pairs the type of action with its controlling centre and effector?
What is the correct sequence of impulse transmission in the knee jerk reflex arc?
B. Fill in the Blanks
In a three-neurone reflex arc, the cell body of the sensory neurone is located in the .
Reflex actions such as the reflex protect the retina from being damaged by bright light.
C. Matching / Drag and Drop
Drag each action or component to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Feature / Reflex to Its Correct Description
Write the correct feature or reflex next to each description.
| Correct Description | Feature / Reflex |
|---|---|
| 1. Action that is consciously controlled by the cerebral cortex and involves the somatic nervous system | |
| 2. Automatic action controlled by the medulla oblongata and hypothalamus without conscious awareness | |
| 3. Nerve pathway involving sensory, relay, and motor neurones, such as moving a finger from a sharp pin | |
| 4. Nerve pathway that connects a sensory neurone directly to a motor neurone in the knee jerk reflex |
Answer choices: Voluntary action • Involuntary action • Three-neurone reflex arc • Two-neurone reflex arc
