The nervous system

Look at the image of the neurone. State which number points to the dendrites and which points to the axon, and explain the difference between the function of the former and the latter in the transmission of the nerve impulse.

Explain which three main parts a neurone has and what the general function of each one is in the transmission of the nerve impulse.

In the image of the neurone, a sheath covering sections of the axon is shown. Name it, explain its function, and reason why a disease that destroys it, such as multiple sclerosis, causes movement and sensory problems.

Analyze how two different toxins act on the neuromuscular synapse (the junction between the motor neuron and muscle fibers):

• Substance X (curare): Binds to the neurotransmitter receptors on the muscle cell and blocks them, preventing the neurotransmitter from interacting with them.

• Substance Y (nerve gas): Destroys the enzyme that breaks down the neurotransmitter after its release, causing it to remain indefinitely in the synaptic cleft.

1. a) Predict what will happen to the muscles of a person exposed to substance X and to those of a person exposed to substance Y (will the muscles remain flaccid and paralyzed, or in permanent contraction and spasm?). Justify your answer based on how chemical synapses work.
2. b) Although they act through completely opposite mechanisms on the muscle, explain why both substances cause death by asphyxiation if they affect the respiratory muscles.

Name the two main divisions of the nervous system and list at least two organs that form part of each of them.

Name the three organs that form the brain and state a characteristic function of each one.

Order the following structures of the nervous system from highest to lowest level of organisation, and explain your criterion: neurone, brain, nervous system, nervous tissue.

Explain which bony structures protect the brain and the spinal cord, and why this protection is so important in the case of the nervous system.

State whether these statements are true or false and justify your answer: a) the spinal cord is protected by the vertebral column; b) nerves only carry information from the body towards the brain; c) the cerebellum is involved in coordinating movement and balance.

The medulla oblongata is not involved in conscious decisions, and yet an injury to it is fatal. Explain which automatic functions of the body it regulates and why it is considered vital.

A motorcyclist has an accident and loses sensation and movement in his legs, but keeps those in his arms intact. Based on your knowledge of the spinal cord, reason in which part of the spinal column the injury probably occurred and why only the lower limbs were affected.

Two patients who have suffered damage to different parts of the brain arrive at the emergency room:

• Patient 1: Is unable to speak and has complete motor paralysis in their right arm and leg, but perfectly understands what they hear and maintains their balance.

• Patient 2: Speaks with complete fluency and has normal strength in all four limbs, but is unable to stand without falling and cannot coordinate simple movements, such as touching the tip of their nose with their finger.

1. a) Deduce in which cerebral hemisphere Patient 1's lesion is located and justify why the motor loss manifests specifically on the right side of the body.
2. b) Why do Patient 2's symptoms indicate that their damage is not in the motor cortex of the cerebrum? Justify what other structure of the brain is affected.

The human brain weighs approximately 1.4 kg and is suspended in cerebrospinal fluid inside the cranial cavity.

1. a) Using the principle of inertia and the anatomy of the meninges, explain what physically happens to the brain inside the skull when an athlete undergoes sudden deceleration or braking of the head ("whiplash effect"), and why this can cause a severe concussion even if there is no bone fracture.
2. b) A company designs a helmet with an undeformable steel outer shell and claims in its advertising: "Since the outer shell never deforms or breaks, the brain is 100% protected against any concussion." Scientifically refute this claim by explaining what type of protection the brain actually needs inside the helmet to reduce injuries.

At complete bed rest, a person weighing about 65 kg consumes an energy power of about 100 watts. The brain, which has a mass of barely 1.3 kg (2.0% of body mass), continuously consumes about 20 watts (20% of all the body's energy), both while awake and during deep sleep.

1. a) Calculate the relative energy consumption per kilogram of mass in the brain (watts/kg) and compare it with the relative energy consumption per kilogram of the rest of the body. How many times more metabolically active is the brain than the resting body average?
2. b) If during deep sleep we do not move voluntary muscles or receive conscious stimuli from the environment, on what cellular and physiological activities does the brain spend such an amount of energy? Explain why the brain cannot "shut down" during rest.

Explain the difference between a sensory nerve and a motor nerve, and give an example of the information each one carries.

Describe the pathway of information in a simple voluntary movement, such as writing your name on paper: which nervous centre gives the order, where the impulse travels, and which types of neurones are involved.

When you raise your hand to ask a question in class, a voluntary response is set in motion. Describe the route the information follows: which part of the brain decides the movement, which structure coordinates it so that it is precise, where the order travels down, and what finally carries it out.

After a local anaesthetic at the dentist, the area seems "numb" and you feel no pain during the filling. Explain what happens to the sensory nerves of the area while the effect lasts and why sensation takes a while to come back.

If you get a cut on your skin, the cells of the epidermis actively divide by mitosis and the wound heals in a few days. In contrast, if a person suffers trauma that severs the spinal cord, paralysis is usually permanent.

1. a) A student states: "To heal the spinal cord, it would be enough to inject substances that force surviving neurons to divide by mitosis to fill the gap, exactly like the skin does." Refute the student's idea with two biological arguments, taking into account the cellular characteristics of adult neurons and the way they process information.
2. b) In peripheral nerves (such as those going to the arm), a severed axon can slowly regenerate if the neuron's cell body survives. Why is it much harder to recover function after a spinal cord injury than after a cut to a peripheral nerve in the arm?

Explain what a reflex action is and how it differs from a voluntary action. Give an example of each and state which part of the nervous system mainly coordinates each type of response.

Explain the difference between the rapid responses coordinated directly by the spinal cord and the decisions made by the brain, and give an everyday example of each type.

Reaction time is the time that passes from when we perceive a stimulus until we respond. Explain why it increases when driving while sleepy or looking at a mobile phone, relating it to the processing of information in the nervous system.

In a neurology consultation, the patellar reflex (tap on the tendon below the kneecap) and tactile sensitivity are examined in three patients, obtaining these results:

• Patient A: Upon receiving the tap on the tendon, extends their leg automatically as normal, but states that they did not feel the touch or the tap.

• Patient B: Feels the tap completely clearly, but the leg does not move reflexively (although they can extend it if the doctor voluntarily asks them to do so).

• Patient C: Feels nothing and the leg does not move at any point either.

1. a) Based on the elements that make up a reflex arc (receptor, sensory neuron, spinal cord, motor neuron, and effector) and its connection to the brain, reasonably deduce at what point in the nervous circuit the alteration may be located for each of the three patients.
2. b) Why does a healthy person pull their foot back from a prick before becoming conscious of the pain?

A pedestrian is about to be hit by a vehicle and experiences two successive responses:

• Response 1 (at 0.15 seconds): Leaps onto the sidewalk, avoiding the impact.

• Response 2 (at 10 seconds): Their heart rate increases from 75 to 140 beats per minute, they begin to sweat profusely, and their hands tremble. This state lasts for nearly 10 minutes.

1. a) Identify which coordination system (nervous or endocrine) is primarily responsible for each of the two responses. Justify your assignment by relating the reaction time to the way the signal travels in each system.
2. b) Explain why Response 1 ceases the moment the body touches the sidewalk, whereas Response 2 takes several minutes to disappear even though the danger has already passed.