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7.5. Nervous acts

Reflex or involuntary acts

The reflexes or involuntary are fast, automatic, and are made without involving the encephalon. In a reflex act, the sensitive information only reaches the spinal cord, where the response is elaborated, so it is automatic. They are carried out when an immediate response is needed, such as in dangerous or harmful situations .

The nerve impulse follows the shortest possible path as it goes from the receiver of the stimulus to the effector following a very simple neural circuit called reflex arc, so reflex responses are very fast. The reflex arc involves, as a minimum:

  • receptor that picks up the stimulus.
  •  sensory neuron that carries information to the spinal cord.
  • One or more intercalary neurons (association neurons) in the spinal cord.
  • motor neuron that carries information from the spinal cord to the effector.
  • An effector (muscle or gland) that executes the response .

Acto reflejo

By MartaAguayo (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Activity: Reflex arc .

Activity: Reflex act .

Animation: The knee jerk reflex .

Answer in your notebook

7.9.- Why is it good that we have reflex actions?

Voluntary acts

The voluntary acts are conscious, and more elaborate than reflexes, and unlike reflex actions, participates in production the cerebral cortex.

A voluntary act has the following components:

  • receptor that receives a stimulus. The sensory nerves transmit this information to the spinal cord, where it enters through the dorsal horn (later) and up to the medulla oblongata.
  • In the medulla oblongata, the information that arrives from the receptor is transmitted to the cerebral hemisphere opposite to the side of the receptor that has captured the stimulus.
  • In the cerebral cortex, this information is made conscious and the response is elaborated, which goes through the ventral (anterior) horns of the spinal cord to the effectors.
  • The effectors, which execute the response.

Although voluntary acts are performed when an external (or internal) stimulus is received, voluntary acts can also occur in the cerebral cortex without any stimulus to trigger it.

Activity: Crossword.

Activity: Voluntary acts.

Test and video: The nervous system.

Experiment with your body: Reaction time.

Experiment with your body: Thinking takes time.


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.