12.4 - Voluntary and Involuntary Actions

Comparison

• Similarity: Both involve a stimulus, impulse, neurone and effector organ.
• Voluntary action: Conscious and performed by one’s own will; involves the somatic nervous system; controlled by the cerebral cortex; involves skeletal muscles.
• Involuntary action: Automatic and unconscious; involves the autonomic nervous system; controlled by the medulla oblongata and hypothalamus; involves smooth muscles and glands.

Voluntary Actions Involving Skeletal Muscles

• Examples: Walking, talking and brushing teeth.
• Pathway: stimulus → receptor → sensory neurone → cerebrum → motor neurone → skeletal muscle → response.
• The cerebrum interprets the nerve impulse and determines the instruction sent to the effector.

Three-Neurone Reflex Arc

• A reflex action is a fast response to a stimulus without being controlled by the brain.
• A reflex arc is the nerve pathway involved in a reflex action.
• Example: Moving a finger away after it is pricked by a sharp pin.

1. A sensory receptor detects the stimulus and triggers a nerve impulse.
2. The sensory neurone transmits the impulse to the spinal cord through the dorsal root.
3. In the spinal cord, the sensory neurone transfers the impulse across a synapse to a relay neurone.
4. The relay neurone transfers the impulse across a synapse to a motor neurone.
5. The motor neurone carries the impulse through the ventral root to the effector; the muscle moves the finger quickly.

Importance of Reflex Actions

• Produce spontaneous responses without waiting for instructions from the brain.
• Prevent the extra time needed by the brain to analyse information from causing serious injury.
• Allow the brain to focus on higher-level thinking.

Two-Neurone Reflex Arc

• Example: Knee-jerk or patellar reflex; involves only a sensory neurone and a motor neurone.

1. Tapping the tendon below the kneecap stretches the quadriceps muscle and stimulates stretch receptors.
2. The sensory neurone transmits impulses directly to the motor neurone in the spinal cord.
3. The motor neurone transmits impulses to the quadriceps muscle.
4. The quadriceps muscle contracts, causing the leg to jerk forwards.