An involuntary movement is an action that is triggered automatically, without awareness.
If the cerebrum had to process each and every piece of information that occurs every second in and around the body, there would be a major information overload. To avoid this, several operations in the human body are processed unconsciously.
When a person decides to go for a run, they do not have to make the conscious decision to contract or relax their muscles nor do they consciously control their heartbeat. Therefore, muscle contractions during a run and the heartbeat are examples of involuntary movements.
The brain stem is the command centre for involuntary actions. It receives and processes information from the sensory nerves, and sends signals to the different muscles of the body via the motor nerves.
The path of the nerve impulse during an involuntary movement is:
Sensory nerves → Brain stem → Motor nerves
A reflex, sometimes called a spinal reflex, is a rapid, involuntary reaction in response to a stimulus.
The reflex arc is the path taken by the nerve impulse during an involuntary movement.
A reflex is often considered to be a defensive reaction designed to return the body to a state of equilibrium. Since the human body must react quickly to protect itself, the information doesn’t have time to reach the cerebrum to be processed. Instead, the spinal cord receives and processes the information.
The reflex arc represents the path taken by the nerve impulse during a reflex. This is the path taken by the nerve impulse:
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The sensory organs perceive the stimulus (e.g., light, sound, touch).
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The sensory nerves transmit the information to the spinal cord.
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The spinal cord processes the information and sends a message.
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The motor nerves receive the information from the spinal cord and transmit it to the muscles.
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The muscles react to the stimulus.
The path of the nerve impulse during a reflex is the reflex arc:
A voluntary movement is a conscious action controlled by the cerebrum.
Voluntary actions involve voluntary muscles. They are executed when a person decides to make an intentional movement. These acts originate in the cerebrum, which transmits nerve impulses through neurons to the relevant muscles.
The impulses coming from the cerebrum can take two different paths.
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If the muscles that are voluntarily activated are located in the head (for example, the muscles involved in speaking), the nerve impulse leaves the cerebrum and goes directly through the motor nerves to the targeted muscle.
The path of the nerve impulse during a voluntary movement located in the head is:
Cerebrum → Motor nerves → Muscles
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If the muscles that are voluntarily activated are located below the head (for example, the muscles involved in kicking a ball), the nerve impulses start from the cerebrum, and pass through the brain stem to the spinal cord. They then travel to the muscles responsible for movement via the motor nerves.
The path of the nerve impulse during a voluntary movement located below the head is:
Cerebrum → Brain stem → Spinal cord → Motor nerves → Muscles
When the body feels a sensation, the path taken by the nerve impulse is the opposite of the one used when a nerve impulse is sent from the cerebrum to the muscles of the body.
Nerve impulses related to sensations can take two different paths.
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If the sensation is detected by an area in the head (e.g., sound), the nerve impulse originates in the sensory organ (e.g., ear). Then, it is relayed by the sensory nerve to the cerebrum, which analyzes the information.
The path of the nerve impulse when a sensation is detected in the head is:
Sensory organ → Sensory nerves → Cerebrum
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If the sensation is detected by an area below the head (e.g., holding hands), the nerve impulse starts from the sensory organ (e.g., skin) and travels through the sensory nerves. Then it travels through the spinal cord and brain stem to the cerebrum.
The path of the nerve impulse when a sensation is detected by an area below the head is:
The body feels the sensation when the information is processed by the cerebrum. The sensory organs are only able to detect the sensation.